December 29, 2008

Hydrogen Economy: Dream or Future

Hydrogen powered automobiles are a recurrent idea these days. Prototype hydrogen-powered cars are debuting in the United States, Japan and Europe. Two major issues are driving the Hydrogen Dream – Global Warming and Over-dependence on imported oil.

The idea of hydrogen economy is to utilize locally available and renewable energy sources, such as wind, solar and biomass to produce hydrogen gas. Hydrogen can then be used in fuel cells to produce electricity without producing pollution; replacing gasoline and petroleum with hydrogen would decrease the consumption of petroleum and also reduce the carbon-dioxide emissions.

BUT.. is it realistic to think that inexpensive, pollution-free hydrogen energy will be the fuel of the future?

First of all hydrogen is not at all a primary source of energy. It is always chemically bound in organic compounds or water. We require some other source like fossil fuel, nuclear reactors, hydroelectric dams, geothermal wells, wind turbines or solar panels to break the chemical bonds and hence produce free hydrogen. So basically hydrogen economy is incomplete without a primary energy source.

Well we can use electrical energy too for electrolysis of water (direct current + water = hydrogen + oxygen). But this process is highly inefficient. Only 45% of the initial energy is captured in this process.

Then there are numerous other problems attached to Hydrogen. Storing hydrogen is perhaps the greatest hurdle. Compressed hydrogen gas is the only viable approach. Carbon-fiber tanks can hold hydrogen at 10,000 psi. However, thee tanks hold only one-eighth the energy of a gasoline tank of equal size.

Moreover, compressed gas at 10,000 psi can be extremely dangerous. The energy released by the sudden rupture of a 10,000 psi tank holding 6 kg of hydrogen is equivalent to 50 sticks of dynamite!!

Even liquid hydrogen ( at -250 °C) has only one-fourth of the energy per unit volume of gasoline.

To add to the problems, hydrogen is odorless and invisible and has a wide range of flammability.

So, how realistic is the vision of the Hydrogen economy ?? And when will it occur ?? I guess some fundamental breakthroughs are necessary to make hydrogen economy a reality but one more thing.. Is Hydrogen economy the right goal ??

CHEW ON THIS TOO: - Recent Developments
Recently I came across an article - “Hydrogen Balls: a safe fuel of future”. This was about hydrogen powered cars. Thought of mentioning it here.

Hydrogen gas is stored in small balls – “ping pong” balls to overcome the risk of fire and explosion. Lass Stenmark, Uppsala University says, “By storing the gas in round, spherical form, it can withstand twice the pressure that a cylindrical form can. If the car crashes and tank breaks, the hydrogen-filled balls would just spread out and roll away, and the gas from any broken balls would just simply seep out and disappear into the atmosphere without causing harm”

Sounds simple and exciting, lets wait and see the application part !!


By Associate Writer - Nidhi Garg

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December 24, 2008

Understanding Steam Flow Meters

Energy has always been a concern for all producers / manufacturers & those who cared for it - survived all seasons, balance were extinguished by the nature. This is the law of survival.

Currently, rising prices of crude oil has affected each & every producer in many areas & therefore it is essential to understand every minute aspect of energy saving.
(Though by the time it reaches you Crude has already come down to <40$ from 140$, but doesnt matter as Energy is always beneficial for the company, for the nation, & for the Globe).


The past few years have brought a great deal of attention to energy production and use. The rising price of crude oil has affected many areas of our economy, including by such impacts as higher gasoline prices, rising jet fuel prices, and increased heating oil costs.

Steam flow measurement is important in this context because steam is widely used as a source of power in the production of electricity. In today’s power plants, heat is obtained from multiple sources. These include fossil fuels, such as coal, natural gas, or oil, as well as other sources of heat, including nuclear fuel, waste fuels, solar energy, and geothermal energy. The heat energy in these sources of heat is released, either through combustion or through a similar process. The heat is transferred to water, which generates steam. In many cases, this steam is then used to drive an electric generator. The system used to generate the steam is called a boiler.

As the costs of energy rise, many companies are looking to increase
efficiencies in their energy and power generation. As a result, more attention is being paid to steam flow measurement.

Steam flow measurement accounts for about 6 percent of the revenues from flowmeters sold in today’s $4.5 billion flowmeter market. While as many as seven different types of flowmeters can be used to measure steam flow, the vast majority of steam flow measurements are made with differential-pressure (DP) and vortex flowmeters.

1. Differential-Pressure Flowmeters
DP flow transmitters together with primary elements account for well over half of the flowmeters sold for steam flow applications. DP flow transmitters sold for steam flow have many of the same advantages and disadvantages of DP flow transmitters sold to measure liquid and gas flow. Disadvantages include pressure drop, interference with the flowstream, and wear over time. DP flow transmitters also have limited rangeability and are reliant on the square-root method for calculating flow.

On the one hand, DP flow transmitters offer some pretty compelling advantages when it comes to steam flow measurement. For example, they are relatively inexpensive and offer straightforward installation. DP flow is also the most studied and best understood method of flow measurement, and multivariable DP flow transmitters are capable of measuring more than one process variable, such as differential pressure, process pressure, and temperature, which in turn enables the measurement of mass flow.

The effectiveness of a DP flow transmitter for steam flow depends on the primary element used. Flow nozzles are well suited for steam flow because they can handle the high temperatures and pressures that accompany steam flows. Orifice plates are widely used to measure steam flow. In fact, all the primary elements, including Pitot tubes, Venturi tubes, and wedge elements can be used for steam flow measurement.

2. Vortex Flowmeters
Vortex flowmeters have some advantages over other types of new-technology flowmeters when it comes to measuring gas and steam flow. Gas flow measurement is still a relatively new application for Coriolis meters, and the use of Coriolis meters to measure steam flow is just beginning to occur. While ultrasonic meters have been used for a number of years to measure gas flow, steam flow is a very new application for them. Magnetic flowmeters cannot be used to measure either gas flow or steam flow. Multivariable differential-pressure flowmeters can be used to measure liquid, gas, and steam. However, most multivariable DP flowmeters have substantially greater pressure drop than vortex meters, due to the presence of a primary element.

Steam is the most difficult fluid to measure. This is due to the high pressure and high temperature of steam and because the measurement parameters vary with the type of steam. Main types of steam include wet steam, saturated steam, and superheated steam. Steam is often measured in process plants and for power generation. In addition to their ability to tolerate high process temperatures and pressures, vortex meters offer wide rangeability, which enables the measurement of steam flow at varying velocities. In process and power plants, steam is often measured coming from a boiler.

3. Variable-Area Flowmeters
Variable-area meters, also called rotameters, have a limited use for steam applications. One reason is that many of them do not have an output signal and must be manually read, although some companies have developed variable-area meters with an output signal. Most process and power plants today are highly automated and would find a flowmeter that has to be read manually of limited value.

Another reason for the limited use of variable-area flowmeters is that they have low accuracy. Many variable-area meters have accuracy levels in the +/-5 to +/-10 percent range, which is generally not suitable for measurements in process and power plants. If end-users want to go to a low-cost meter, they will be more likely to try a DP flowmeter than a variable-area meter.

Variable-area meters do have the advantage of being low cost though, so they are a good fit for noncritical measurements where high accuracy is not a requirement.

4. Target Flowmeters
While target flowmeters can be used for liquid, gas, and steam applications, they are predominantly used to measure steam flow. Target meters can measure both superheated and saturated steam. While the target is subject to wear, this can be compensated for with recalibration. The target meter has no moving parts, and it can withstand pressures up to 15,000 PSIG and temperatures up to 500 F. Target meters can fit into almost any line size above 3/8”.

Even though target meters were developed after 1950, they are classified as a traditional-technology flowmeter, due to their primarily mechanical means of construction. It is not obvious why more suppliers have not chosen to offer this meter, because, in many ways, the target meter is most like a vortex meter. Target meters rely on a body inserted into the flowstream, not unlike the bluff body that characterizes vortex meters. However, instead of counting vortices generated by the bluff body, like a vortex meter, a target meter measures the force exerted on the target by the fluid.

5. Turbine Flowmeters
Turbine flowmeters are widely used for liquid and gas applications, but they have very limited use for steam flow measurement. One major issue has to do with the effect of condensation on measurement accuracy. When condensation occurs in a flowing stream, the fluid becomes a mixture of steam and water, which creates two-phase flow. The condensate can harm the blades, and interfere with measurement accuracy.

Turbine flowmeters do have an advantage since they can perform better at low flowrates than vortex flowmeters. They also have a good turndown ratio (some suppliers claim a turndown of 25-to-1). The most favorable condition for steam flow measurement by turbine meters is measurement of dry steam. However, because steam is so readily influenced by changes in temperature and pressure, steam flow conditions can change very quickly. Steam is most stable at the boiler, but cold spots can occur in the line, bringing about condensation.

6. Coriolis Flowmeters
Coriolis flowmeters can be classified as an emerging technology for steam flow measurement. Like turbine meters, Coriolis meters have difficulty handling condensation in steam. When steam condenses, the water droplets create two-phase flow, a mixture of steam and water. The presence of water interferes with the accuracy of the flow measurement.

Coriolis flowmeters can work to measure flow with dry steam. However, because steam is easily affected by variations in temperature and pressure, steam-flow
conditions can change very quickly. There remain some technical challenges for Coriolis suppliers to resolve before Coriolis meters can be a major part of the solution for steam flow measurement.

7. Ultrasonic Flowmeters
Ultrasonic flowmeters have many advantages over both traditional and new-technology flowmeters. They are minimally invasive, have high accuracy, cause little-to-no pressure drop, and they have no moving parts. However, there are some technical limitations that make steam flow a difficult application for ultrasonic flowmeters.

One limitation of ultrasonic flowmeters applies to clamp-on flowmeters. The speed with which an ultrasonic wave travels through metal may be different than the speed of the wave through steam. This can interfere with the accurate calculation of the flowrate of the steam. In addition, the exact thickness of the pipe is not always known, either because there is buildup or deposits on the pipe wall or simply because this is an unknown variable. This pipe-thickness issue applies to all clamp-on flowmeters, whether they are measuring liquid, gas, or steam.

A limitation of spoolpiece ultrasonic flowmeters is that the transducers can become overheated due to the temperature of the steam. If this happens, it can ruin the transducers. While some ultrasonic flowmeter suppliers have found a way to successfully deal with this issue, it remains an important technical challenge.

Vortex Offers a Safety Net
Some suppliers of turbine, Coriolis, and ultrasonic technologies for steam flow measurement have added vortex meters to their product line so as to have an alternative technology to offer end-users who may have difficulty measuring steam with a given technology. The function that vortex meters have as a kind of back-up meter, or safety net, for companies offering difficult-to-apply technologies is likely to grow as end-users increase their willingness to try new technologies for measuring steam.

Vortex flowmeters are very reliable when used to measure steam flow, they cause minimal pressure drop, and they can handle high pressures and temperatures. The ability of vortex meters to measure steam flow will encourage suppliers of alternative technologies to further develop and promote their alternative technologies for steam flow measurement, knowing that they can always fall back on their vortex meters for difficult applications.

So choose your meter carefully. You can share your experience for the benefit of all in this area..based on the reasons you selected the particular type of flow meter.

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December 17, 2008

Quick Tips for Saving in Pumps

If you want to get some quick energy saving tips follow these guidelines to identfiy opportunities in CW pumping, Or may be in all pumps with slight care.

This is the method which also guarantee you minimum energy saving achievable in pumps. The actual savings will be higher always depending on various other factors. I used this method recently & saved ~15% power consumption in CW pumps i.e. ~600 kW out of 4000 kW load resulting in huge savings with a payback of merely 4 months.


Guidelines
  • First of all make a list of all pumps with design flow & head in two different columns in an Excel sheet. Say column A with flow & Column B with head

  • Now measure the power consumption in the field & put it in Column C.

  • Now in Column D put calculated power consumption with efficiency available for new pump & high efficiency motors.

  • New pump efficiency for CW flow of more than 300 M3/hr at 40 m head can be considered ~80% which can go as high as 85% for bigger size pumps beyond 1000 M3/hr.

  • High efficiency motor can have efficiency in the range of 94%+for motors of >100 kW, which can be 96%+ for bigger motors >300 kW

  • After getting the final figure of power consumption for a new pump, you can put the saving potential in the next column E.

  • Put the investment required for motor & pump in next column F judiciously (you can ask me for indian cost roughly) Or get it from some vendor

  • Put payback or ROI in the next column G


You are through now. Suggest the replacement to your management in a nice presentable format for approval. Mind it that this method gives you minimum guaranteed savings from these pumps because,

  • This method is not considering any flow & head measurement therefore, you get minimum savings in case your pump is not delivering the design or considered flow. Anyway it can not deliver more than design without compromising on head.

  • The chances of errors are very less as you are measuring only power which is measured reliably & more accurately.

  • You do not have to deal with plant people convincing them about the accuracy of flow measurement. Because flow is the most troublesome area where mostly people do not believe (Or they dont want to) about large drops.

  • This gives you option of selecting the latest & most efficient equipment in the market.


Don't you believe on this?

I Said......I have already saved 15% power i.e. ~600 kW.

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December 10, 2008

Equal Percentage Valves - Opening Vs Flow

In case of equal percentage valves, the flow varies according to the following definition.

'Any % change in the opening from its current existing value changes the flow by same percentage of its current value at current opening.'

Means if current opening is say 10% & u change it to 15% than the change in opening from its current value (of 10%) is 50%, so the flow will also increase by 50% of its existing value at 10% opening. This means if flow was say 20% of the total range (Or capacity of the valve) at 10% opening it will become 30% (50% higher compared to 20%) of the total range.

So how to calculate it????

Now you know the definition so you can generate an equation which gives you all the values where 0% opening or lift is 0% flow while 100% lift is 100% flow.

So the curve for these valves look like this.




Long back I generated an equation from a general curve for an equal percentage valve. Its fixed for all kinds of valves from any manufacturer generally. There may be minor variations in case of specially designed valves otherwise it is same.

The equation is.

% Flow is = .06 + 0.49 * X -.019 * X^2 +.0005 * X^3 - 0.0000502 * X^4 + 0.0000000242 * X^5

Where X is % Lift or opening , X = 60 for 60% opening & not 0.6

Hope it is useful for all.

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December 04, 2008

Pinch Technology: Some Basics


Most Industrial processes involve heat transfer from one process stream to another process stream (interchanging) or from a utility stream to a process stream. In the present energy crisis scenario, target in any industrial process is MER – Maximum Energy Recovery or Minimum Energy Requirements.



In the early days of pinch technology, the main emphasis was on heat exchanger network (HEN) design. Today pinch technology stretches far wider in the fields of overall process improvement and utility system design

Pinch Technology analyses process utilities (particularly energy and water) to find optimum way to use them, resulting in financial savings. It does this by making an inventory of all producers and consumers of these utilities and then systematically designing an optimal scheme of utility exchange between them. Energy and water re-use are at the heart of pinch technology.

With the application of pinch technology, both capital investment and operating cost can be reduced. Emissions can be minimized and throughput maximized.

What is Pinch Technology?
The term “Pinch Technology” was introduced by Linhoff and Vredeveld to represent a new set of thermodynamically based methods that guarantee minimum energy levels in design of heat exchanger networks. It also ensures minimizing the capital costs and fewer emissions.

Basis of Pinch Analysis

Pinch Technology represents a simple methodology for systematically analyzing chemical processes and the surrounding utility systems with the help of the First and the Second Law of Thermodynamics.

1st Law of Thermodynamics: Provides the energy equation for calculating the enthalpy changes (ΔH) in the streams passing through a heat exchanger.

2nd Law of Thermodynamics: Determines the direction of heat flow. That is, the heat energy may only flow in the direction of hot to cold. This prohibits ‘temperature crossovers’ of the hot and cold stream profiles through the exchanger unit.

In practice the hot stream can only be cooled to a temperature defined by the ‘temperature approach’ of the heat exchanger. The temperature approach is the minimum allowable temperature difference (ΔTmin) in the stream temperature profiles, for the heat exchanger unit. The temperature level at which ΔTmin is observed in the process is referred to as the “pinch point” or “pinch condition”. The pinch defines the minimum driving force allowed in the exchanger unit. Thus, the prime objective of pinch analysis is to achieve financial savings by better process heat integration (maximizing process-to-process heat recovery and reducing the external utility loads).



Pinch principle

The point where ΔTmin occurs is known as the “Pinch”. Once the pinch has been identified, it is possible to consider the process as two separate systems: one above and one below the pinch as shown in the figure.
The system above the pinch requires a heat input and is therefore a net heat sink. Below the pinch, the system rejects heat and so is a net heat source



To summarize, the understanding of the pinch gives three rules that must be obeyed in order to achieve the minimum energy targets for a process:

  • Heat must not be transferred across the pinch, i.e., no temperature crossovers.
  • There must be no external cooling above the pinch.
  • There must be no external heating below the pinch.


JUST FOR FUN & General Knowledge
After so much of pinch technology I would like to mention a few more pinches here :)
Ever heard about Network Pinch, Hydrogen Pinch, Water Pinch or Z Pinch?

NETWORK PINCH:When optimizing energy consumption in an existing industrial process, a number of practical constraints must be recognized. Traditional Pinch Technology focuses on new network designs. Network Pinch addresses the additional constraints in problems associated with existing facilities.

HYDROGEN PINCH:The Pinch Technology approach applied to Hydrogen management is called Hydrogen Pinch. Hydrogen pinch enables a designer to set target for the minimum hydrogen plant production and/or imports without the need for any process design.

WATER PINCH:This is a systematic technique for analyzing water networks and reducing water costs for processes. It uses advanced algorithms to identify and optimize the best water reuse, regeneration, and effluent treatment opportunities. It has also helped to reduce losses of both feedstock and valuable products in effluent streams.

Z PINCH:In fusion power research, the Z-pinch, or zeta pinch is a type of plasma confinement system that uses an electric current in the plasma to generate a magnetic field that compresses it (Pinch). The name refers to the direction of the earliest experimental devices in England, where the current flowed down a vertical quartz tube, the Z-axis on a normal mathematical diagram.

By Associate Writer - Nidhi Garg

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November 05, 2008

Find out Pump Head without curve

You are handling lot many centrifugal pumps & is always struggling for design data which may or may not be available specially in case of vintage units its very difficult to recollect them.

Even in newer plants if you do not have all procedures in place for documents handling you might face the same problem. What to do in such a case? Boss is not ready to listen any excuse.....You know that there is something wrong which can be improved and therefore you get a good recognition.......But

Here is the answer....

Few rules are there based on data analysed by experts in this field which are generally true for conventional systems. Such rules can be utilized when you do not have any other source to know them...at least an initial guess is must.

Rule-1
Shut-off head can be calculated by obtaining the square of impeller dia in inches. This gives you shut off head in feet. That means if your dia is 15" then 15 x 15 = 225 feet will be the shut off head for water.

There are other conditions also in this rule.
1. This rule is applicable at 1800 RPM.
2. This rule is applicable for water so apply density correction for other fluids.
3. Its generally accurate by ~95%.
4. Its a guide line only.
5. Its applicable to 85% population with 15% being exceptions in design.
6. The rule do not apply to PD pumps.

What is the shutoff head? The shutoff head is the beginning of the pump curve. It represents maximum elevation (in feet or meters) at zero flow. The performance curve proceeds to and ends at a point called maximum flow at zero elevation.

Now how can you use this information. Say you measure the performance of a pump you have collected all the data but you dont have pump curve. But you know from maintenance person that impeller dia is 15". So you know that this can develop 225 feet of water head.

Let us say fluid is also different say alcohol where SG = 0.8
So you know that 225 feet is = 225 * .3048 meters of water = 68.6 meter
Now fluid is alcohol so new head shall be = 68.6 * 0.8 = 54.9 meter

So the differential pressure must be 5.49 Kg/cm2.

No....No....No you are ......wrong....This is shut off differential head i.e. zero flow. So as a thumb rule it must be ~85% of shut off head i.e. 5.49 * 0.85 = 4.66 Kg/cm2. So if your actual differential is significantly lower than you can say that the pump is having something wrong with it.

What is that something......Well you yourself have to identify it.????

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October 28, 2008

Type of Valves - Ball Valve

In the last article I discussed brief & important things about Globe Valve. Today I will discuss some basics of Ball Valve.



The Ball valves as name suggests is having a ball type spherical body. They are stop valves that use a ball to stop or start the flow of fluid. The ball performs the same function as the disk in the globe valve.

Ball Valves
This rotational-motion valve uses a ball-shaped disk with a hole bored through to stop or start fluid flow. When the valve handle is turned to the open position, the
ball is rotated so that the hole lines up with the valve body’s inlet and outlet.

When the ball is rotated so the hole is perpendicular to the flow, the valve is closed. Because the ball moves across the seats with a wiping motion, ball valves can handle fluids with suspended solids.

When the valve is shut, which requires only a 90-degree rotation of the handwheel for most valves, the ball is rotated so which is divided in two parts by a baffle. Flow direction is steeply changed in this type of valve so the controlling of flow is better by the movement of restriction element.

Most ball valves are of the quick-acting type (requiring only a 90-degree turn to operate the valve either completely open or closed), but many are planetary gear operated. This type of gearing allows the use of a relatively small handwheel and operating force to operate a fairly large valve. The gearing does, however, increase the operating time for the valve. Some ball valves contain a swing check located within the ball to give the valve a check valve feature. Ball valves are normally found in the following systems aboard ship: seawater, sanitary, trim and drain, air, hydraulic, and oil transfer.

Ball valves are available in Venturi, reduced and full-port patterns. The full-port pattern has a ball with a bore equal to the inside diameter of pipe. Most ball valves instead have a reduced bore with a Venturi shaped flow passage of about three quarters the nominal valve size.

CONCLUSION
Uses a large lever to turn a ball that closes or opens the flow with one quick quarter turn.

Ball Valves are mainly used in fluids containing solids.

Ball Valves have lower pressure drops.

Are the standard for natural and LP gas, replacing the older plug valves that were traditionally used as gas valves. Now Ball Valves are standards for gas service also replacing Plug Valves.

Available in either metal or plastic, threaded or non-threaded types.

Ball valves with double-stem seals provide greater durability.

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October 21, 2008

Vacuum Pumps - Dry Vs Liquid Ring

Vacuum is any system of reduced
pressure, relative to local (typically atmospheric) pressure. Achieved with a pump, vacuum systems are commonly used to

• Remove excess air and its constituents.
• Remove excess reactants or unwanted byproducts.
• Reduce the boiling point.
• Dry solute material.
• Create a pressure differential for initiating transport of material

Liquid-ring and dry pumps offer the most advantages for the chemical process industries (CPI). Both of these pump types have bearings sealed off from the pumping chamber and do not require any internal lubrication because the rotors do not contact the housing. Both, when employing a coolant system, prevent the coolant from contacting the process fluid and causing contamination, and both use mechanical shaft
seals for containment.

Liquid Ring Pumps
In the cylindrical body of the pump, a sealant fluid under centrifugal force forms a ring against the inside of the casing (See Figure below).



The source of that force is a multi-bladed impeller whose shaft is mounted so as to be eccentric to the ring of liquid. Because of this eccentricity, the pockets bounded
by adjacent impeller blades (also called buckets) and the ring increase in size on the inlet side of the pump, and the resulting suction continually draws gas out of the vessel being evacuated. As the blades rotate toward the discharge side of the
pump, the pockets decrease in size, and the evacuated gas is compressed, enabling its discharge.

The ring of liquid not only acts as a seal; it also absorbs the heat of compression, friction and condensation. Popular liquid choices include water, ethylene glycol, mineral oil and organic solvents.

Advantages
  1. Simpler design; employs only one rotating assembly.

  2. Can be fabricated from any castable metal.

  3. Little increase in the temperature of the discharged gas.

  4. No damage from liquid or small particulates in the process fluid.

  5. Maintenance and rebuilding are simple.

  6. Slow rotational speed (1,800 rpm or less), maximizing operating life.

  7. Can use any type of liquid for the sealant fluid in situations where mingling
    with the process vapor is permissible.

  8. No lubricating liquid in the vacuum chamber to be contaminated.

  9. Accommodation of both condensable vapors and noncondensables, while operating as both a vacuum pump and condenser.

Disadvantages
  1. Mixing of the evacuated gas with the sealing fluid.

  2. Risk of cavitation requires a portion of process load to be noncondensable under operating conditions.

  3. High power requirement to form and maintain the liquid ring, resulting in large motors. Therefore consume more energy.

  4. Achievable vacuum is limited by the vapor pressure of sealant fluid at the operating temperature. Hence require Chilled water & hence operating cost is higher than dry pumps.


Screw Pump
Rotary-screw pumps dominate as dry pumps in the CPI, particularly in larger-size pump applications. Other type of dry pumps are Rotary-claw and rotary-lobe.

Two long helical rotors in parallel rotate in opposite directions without touching, synchronized by helical timing gears. Gas flow moves axially along the screw without any internal compression from suction to discharge. Pockets of gas are trapped with in the convolutions of the rotors and the casing, and transported to the discharge.




Compression occurs at the discharge port, where the trapped gas must be discharged
against atmospheric pressure. Each convolution of the rotor acts similarly to a stage in series with the one behind it; at least three convoluted gas pockets in the rotor are required to achieve acceptable vacuum levels.

See the figure & saving nos in my previous article - Guaranteed Energy Saving in Vacuum Pump.

Advantages
  1. Rugged rotor design, constructed of sturdy cast or ductile iron without any flimsy rotating components.

  2. Noncontact design facilitated by timing gears.

  3. High rotational speed reduces the ratio of gas slip to displacement, increases net pumping capacity and reduces ultimate pressure.

  4. Multiple staging provides inlet pressures below 1-mm Hg absolute while discharging to atmosphere.

  5. No contamination of evacuated gas & so process vapors can be recovered thru downstream condensation. Very cost effective in such situations.

  6. Due to lack of condensation, pump can be fabricated of standard, inexpensive cast iron.





Disadvantages
  1. Cannot handle particulate matter, nor large slugs of liquid.

  2. May discharge gases at high temperatures.

  3. Most difficult to repair or rebuild.

  4. May require a gas purge for cooling, or to protect the bearings and seals from the process gas.

  5. Due to high operating temperatures,some process gases may polymerize.


Source: CHE Fact Sheet

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October 14, 2008

Type of Valves - Globe

In the last article I discussed brief & important things about Gate Valve. Today I will discuss some basics of Globe Valve.



The Globe valves as name suggests is having a globe type spherical body which is divided in two parts by a baffle. Flow direction is steeply changed in this type of valve so the controlling of flow is better by the movement of restriction element.

Globe Valves
A Globe valve is a type of valve used for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body. Like Gate valve which is the most common for isolation type service, Globe valves are most common for flow regulation service.

Globe Valves are used when a valve must be opened and closed frequently under high water pressure. They are used to control volume of flow. These valves have two chambers with a partition between them for passage of water that must change course several times from port to port.

Globe Valves should not be used in water supply lines for occasional shut-off purposes.

CONCLUSION
Globe valves are not designed for general shut off conditions.

Globe valves are used for controlling of flow by reducing the pressure.

Angle Valve is a kind of globe valve which changes the flow direction by 90° thus eliminates need of elbow & conventional globe valve combination.

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October 07, 2008

Type of Valves - Gate Valve

It is very difficult for fresh engineers to understand the use of different kind of valves according to the process requirement. In which condition we should use a particular type of valve? What are the differences in each one of them.

To simplify the matter, I have compiled some information from different resources, which can be used by all of you.

Although many different types of valves are used to control the flow of fluids, the basic valve types can be divided into two general groups: stop valves and check valves.

Besides the basic types of valves, many special valves, which cannot really be classified as either stop valves or check valves, are found in the engineering spaces. Many of these valves serve to control the pressure of fluids and are known as pressure-control valves. Other valves are identified by names that indicate their general function, such as thermostatic recirculating valves.

Here I am discussing few basic types of stop valves and check valves.

Gate Valves
Gate valves are by far the most widely used in industrial piping. That's because most valves are needed as stop valves - to fully shut off or fully turn on flow - the only job for which gate valves are recommended.

Gate valves are used when a straight-line flow of fluid and minimum restric­tion is desired. The gate is usually wedge shaped. When the valve is wide open, the gate is fully drawn up into the valve, leaving an opening for flow through the valve the same size as the pipe in which the valve is installed. Therefore, there is little pressure drop or flow restriction through the valve.



Gate valves are not suitable for throttling purposes since the control of flow would be difficult due to valve design and since the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized, gate valves should not be used for throttling.

Gate valves provide optimum performance in conditions where high flow efficiency, tight shut off and long service is required.

Gate Valves are designed to operate fully open or fully closed. Because they operate slowly they prevent fluid hammer, which is detrimental to piping systems. There is very little pressure loss through a gate valve. In the fully closed position, gate valves provide a positive seal under pressure.

A gate valve usually requires more turns - more work - to open it fully. Also, unlike many globe valves, the volume of flow through the valve is not in direct relation to number of turns of handwheel.

CONCLUSION
Gate valves, while not designed for throttling or too frequent operation are generally ideal for services requiring full flow or no flow.

Gate valves are not designed for throttling.

In a slightly opened position high-velocity flow will cause wire drawing and erosion of seating surfaces in gate valves.

Repeated movement of disc near point of closure under high-pressure flow may gall or score seating surfaces on downstream side.

Slightly opened disc in turbulent flow may cause troublesome vibration and chattering.

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September 30, 2008

Just Time Pass.....

Just Time Pass....
Some fun facts from another blog called tellmewhyfacts

  1. Venus is the only planet that rotates clockwise.

  2. Elephants are the only animals that cannot jump.

  3. Google receives about 200 million search queries each day.

  4. The first owner of the Marlboro company died of lung cancer.

  5. Russia has the most movie theaters in the world.

  6. Canada is an Indian word meaning "Big Village"

  7. Pearls melt in vinegar.

  8. The Eiffel Tower has 1792 steps.

  9. Each king in a deck of playing cards represents a great king from history. Spades - King David, Clubs - Alexander the Great, Hearts - Charlemagne and Diamonds -
    Julius Caesar.

  10. If a statue in the park of a person on a horse has both front legs in the air, the person died in battle; if the horse has one leg front leg in the air, the person died as a result of wounds received in battle; if the horse has all 4 legs on the ground, the person died of natural causes.

  11. Honey is the only food that does not spoil. Honey found in the tombs of Egyptian pharaohs has been tasted by archaeologists and found edible.

  12. Guinness Book Of Records holds the record for being the book most stolen from Public Libraries.

  13. Donald Duck comics were banned in Finland because he didn't wear pants.

  14. If you can see a rainbow you must have your back to the sun.

  15. There are only four words in the English language which end in '-dous': tremendous, horrendous, stupendous, and hazardous

  16. The colder the room you sleep in, the better the chances are that you'll have a
    bad dream.

  17. No piece of paper can be folded more than 7 times.

  18. The term Cop comes from Constable on Patrol, which is a term used in England.

  19. More steel in the US is used to make bottle caps than to manufacture automobile
    bodies.

  20. Humans and dolphins are the only species that have sex for pleasure.


  21. Last one added from my side for my Indian friends.

  22. There are only two Hindi words which end on -dook; Sandook & Bandook. - Tell me the third one if you Can


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September 23, 2008

Process Checklist for Energy Saving in Distillation

Distillation operations have been branded as high energy users. An estimate says 3-5% of the total energy used in the United States was for distillation.

Therefore it is necessary for process engineers to have a ready check list for energy saving in distillation columns. In this post we are suggesting a list for process side. This does not include general recommendations for utilities.

Here is the Process Check List.
  1. What is present energy usage per unit of production? Keep electrical energy and mechanical energy separate from heat energy. Keep separate usage of steam at different pressures. Don't forget to include energy used for auxiliaries such as
    instrument air, vacuum pumps or cooling water.

  2. What is present energy usage per unit of production by competitors? If lower usage, why?

  3. Are foremen and operators sufficiently skilled and trained to incorporate energy conservation techniques in their job duties?

  4. Can product purity be lowered, thereby reducing column reflux and reboiler heat duty?

  5. Can column operating pressure be allowed to float with ambient conditions with subsequent reduction in reflux and reboiler heat duty?

  6. When column is operated at production rates below the economic production rate, is reflux flow reduced to maintain same reflux ratio and lower reboiler heat duty?

  7. Can circulation in the main process e.g. in absorber - stripper combination be reduced.

  8. Is feed stream at optimum temperature to maximize separation and reduce column heat duty requirements.

  9. Is location of feed to column at optimum position?

  10. Can column internals be replaced with more efficient design?

  11. Can additional trays be installed in stripping or rectification sections of column to improve separation, thereby reducing heat duty requirements?

  12. Can an impurity be withdrawn from column to improve separation?

  13. Can the technique of pasteurization be used on existing column, thereby eliminating further separation operations?

  14. Is it economically feasible to convert column operation to a heat pump or vapor recompression cycle?

  15. Can control system be updated to automatic control with an economic payout and subsequent savings in energy requirements?

  16. Can steam supply pressure be allowed to float ?

  17. Are exchangers used in heat recovery maximizing the available heat recovery, considering economics? Will additional exchangers or revisions in the flow scheme improve heat recovery?

  18. Has a procedure been developed for deciding when to remove exchangers that are used in heat recovery service for cleaning? Procedure is based upon economics.

  19. Can excess heat from other process units within the plant be used in this process unit?

  20. Is heat being recovered from column pumparounds at the highest temperature level? Are pumparounds at best tray locations for heat recovery and column separation?

  21. If column operates at high overhead temperature, has heat recovery from overhead being maximized by using two stage condensation and cooling?

  22. Can control valves be resized for less rangeability, but still maintain required system control? Benefits are reduced pressure drop and power loss across the new, larger control valve.

  23. a. If production is to be increased by installing another column, can present column be cascaded with new column to save energy?

  24. Can columns in the distillation train be resequenced to improve separation, production and reduce energy usage?

  25. Can a vacuum pump replace a steam ejector at an energy saving and economic benefit?

You can add more if you have done something else.

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September 16, 2008

Excel Sheet For Steam Leak Calculation

One of the most talked about energy wasters is steam leakage from "bad" steam traps and leaking fittings. Steam traps are blamed for being inefficient or worn out and causing as much as 10% of the generated heat from steam to be lost. Is this true or just a sales method to sell more traps? It turns out that steam leaks cause a significant energy loss.


In general you can find many references where steam losses from different leak sizes at different steam pressures are listed. An example shows that a 1-inch union was found leaking at a loss of $3000 per year. The repair cost was $50 or a six day payout. So the point is whatever be the size of leaks or whatever be the pressure of steam the payback is generally in days compared to high fuel cost now a days.

If condensate is recovered, leaking traps can cause an excessive return temperature and cause failure of the condensate return pumps. Severe water hammer can occur
as hot steam contacts condensate that has cooled below the temperature of the steam

The leakage rate can be easily calculated by using general orifice equation.

Q = K x d^2 x H^0.5

Example template sheet in Excel is attached herewith for your easy use. In the said example steam of 34 bar can have a leakage rate of 17.0 Kg/hr from 1 mm hole. This is equal to Rs. 95000 / year in fuel loss. Normal repair cost can be only few thousand Rs. resulting in payback of few days/months. Therefore a regular survey of such leakage systems is necessary for every process plant.

Download Excel Sheet Here

The following steps are recommended for saving energy in your steam condensate distribution system and starting an effective steam energy management program:
  • Develop an estimate of the cost of steam leaks based upon your plant costs similar to other articles or references. A method for demonstrating visually to plant people what these losses are can be made.

  • Run a survey, recording all leaks, size, cost, and location.

  • Check the operation of all installed disc traps used for drips and steam tracing. If found leaking, consider replacing with a more efficient type trap. Before replacing, check installation design and confirm trap size (not over or undersized).

  • Check installation and operation of steam traps used on equipment using the sound detection method, the pyrometer method, or the glove method. The installation should be checked for proper trapping. Items checked include strainer, check valve, back pressure, orifice, and inert gas venting. Improper venting can cause a severe reduction in heat transfer rate.

  • Check vent valves on steam jacketed equipment and kettles for proper operation (removal of inerts without steam loss).

  • Start a preventative maintenance program to maintain the steam distribution system in excellent condition. If manpower is not available in maintenance, you can have the operating people maintain a simple log for their area of responsibility.

  • Steam trap manufacturers will be happy to furnish information to assist in your energy saving program to reduce steam losses, but use your own economic costs to decide whether to replace, repair, or redesign the system.

  • It is always beneficial to collect condensate at different pressures in different flash drums to recover flash steam
  • .
  • Also connect each high pressure drum liquid to subsequent low pressure drum

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September 09, 2008

Expansion Tank Sizing for Thermic Fluids

The design of the expansion tank in a liquid phase heat transfer system using Thermic fluids or other organic fluids is an important parameter in the total system’s successful operation.


It is important to design an expansion tank correctly as it can result in lower maintenance, lesser failure, low downtime, higher onstream factor etc.

Purpose
The main function of the expansion tank in a heat transfer system is to provide
for fluid expansion, which can be greater than 25% of its original volume depending on the fluid used and the operating temperature.

Since the tank is usually installed at the highest point in the system, it also can serve as the main venting point of the system for excess levels of low boilers and moisture which may accumulate in the heat transfer fluid. The highest point installation also creates positive head pressure to the pump’s inlet, providing flooded pump suction with uninterrupted flow of fluid to the user station. A simplified drawing showing a suggested positioning of the expansion tank in a heat transfer system is given below.

Sizing
The expansion tank should be sized so that it is 25% full at ambient temperature and 75% full at normal operating temperature. This sizing should cause positive fluid pressure to the pump’s suction side during system startup and should minimize the
vapor space in the tank during normal operation.

Fluid expansion between two temperatures can be calculated by dividing the fluid’s density at the lower temperature by the density of the fluid at the higher temperature — i.e., the density of Therminol® 66 at 40°F is 8.47 lb./gal. and changes to 6.72 lb./gal. at 600°F. Thus the expansion of Therminol 66 is 8.47/6.72 = 126% of the original volume at 40°F when heated to 600°F.

Therefore, an expansion tank for a 1,000-gallon Therminol 66 system operating between 40°F and 600°F should be sized for 260 gallons of expansion. Since this expansion represents 50% of the tank volume (the volume between 25% and 75% full), the expansion tank should be 520 gallons in size.

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September 02, 2008

Easily calculate LMTD orally

You may feel frustrated when somebody suddenly ask you about LMTD for the given heat exchange condition. That time you feel screamed if you do not have calculator with you.

Also, when you just begin using your calculator somebody answered the LMTD value and you feel surprised & wonder how he did it so quickly?

Now you can also do it.

LMTD can easily be calculated manually by the difference of airthematic means of fluid temperatures. Got it or Not....NO Problem. Here is an Example.

Suppose Hot fluid is entering at ~140°C & leaving at ~110°C. So its Airthmatic mean is 125°C.

Now if Cold fluid is entering at 40°C & leaving at 60°C its mean (average) is 50°C

Now the difference of two mean i.e. 125 - 50 is 75°C.
So your LMTD should be ~75°C.

Let us check it with conventional formula.
Hot side difference = 140 - 60 = 80°C
Cold side diff = 110 - 40 = 70°C

So LMTD = 10 / Ln(80/70) = 74.88°C

So an error of <0.2%. Usually it is less than 1%.

Hope all of you find it useful.

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August 26, 2008

Thermic Fluid Vs Steam for Heating

Recently I came across an article on above topic suggesting the use of thermic fluids in place of steam as heating medium even for lower temperature range where usually steam is employed. The article was from Hydrocarbon processing Oct'1991 issue.

The article suggest that we should consider thermic fluids in place of steam for new projects as they are more efficient & cost effective.

Do you agree with this?

I also doubt, so I decided to do some calculations as they are presented in the paper.

Let us assume that we need steam for a process heating load of 1,000,000 Kcal/hr i.e. 1.0 Million Kcal/Hr or 1.0 Gcal/Hr. Steam temperature required is 200°C.

So the corresponding pressure & other parameters are given below in the table.



In the above table it is clear that though the fired heater duty requirement is more for boiler, the efficiency is ultimately playing the role in higher fuel consumption for thermic fluid heaters. Even if we consider other system components the cost of operation will be higher for TF heaters as shown below.



The said article also shows that area requirement for TF based exchangers for process heating is almost similar due to gain in LMTD, but if I consider reasonable temperatures then the following table suggest that area requirement increases by at least 2 fold.



The other factors which should be considered in the selection between the two are.

  • Higher Flow rates are required for TF (as shown you need only 2000 Kg / Hr steam but 66 M3/hr TF) resulting in higher power consumption in pumps.

  • Need more heating fuel due to low efficiency of fired heaters @60% compared to large boilers having efficiency range from 88 - 92%

  • Steam can be economical after utilizing its pressure energy for power generation which has conversion efficiency of ~94%. In such cases even if we consider the overall steam generation station efficiency as said in the article, it will be economical

  • Generally heat transfer coefficients are large for phase change compared to sensible heat transfer & hence you need more surface area for TF heaters

  • Practically fouling is the biggest problem for TF heaters as they degrade easily due to temperature fluctuations. This is rare with steam

  • Careful design & selection of TF is needed for each application. Sometimes they can be dangerous & hazardous also

  • For large heating loads steam is more economical compared to TF as shown above

  • TF may be economical at very high temperature requirements >250°C where steam system hardware becomes uneconomical or where steam facility is a limitation.

  • Capital cost wise you need to replace TF after a specified time limit which add to your capital cost

  • TF system can not be used where live steam can be used.

  • In the process where cooling & heating cycles are required its very complex & response time is very low. It is always better to use steam in such cases.

  • Safety is an issue with the use of TF as in case of leaks they easily absorbed by insulation & cause sudden fire due to higher temperatures.

  • Since only sensible heat transfer is there & because of low specific heat of TF, the temperature control in the narrow range is quite difficult.

The above discussion suggest that one should very carefully consider the option of using TF system in place of steam system. In fact, the pressurized hot water system is more safer & better if high temperature is required. So always compare TF with HP hot water system as benchmark before suggesting any TF.

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August 19, 2008

Flow Device Upstream or Downstream of Control Valve

Often when you prepare a P&ID, a question is raised by my young engineers where to place any flow orifice or any other flow measuring device when there is a control valve in the system. And of course, Why????

What do you think? Should it be upstream or downstrem of control valve.
The answer depends on many factors e.g. the condition of fluid, type of fluid, operating parameters, purpose of flow mesurement etc. but most practical situations permit it upstream of the control valves.

The first important thing is that what type of flow meter you are going to use & what are the possible conditions of flow upstream as well as downstream of control valve.

It is easier to put it in upstream of any type of control valve because of two simple reasons. One is that you always know the flow condition (whether it is single phase or two phase flow) at upstream but you never know all possible variations downstream of the control valve.

Second thing is that due to variations in flow conditions your flow element may not be suitable or may give you erratic results due to larger variations in P & T parameters compared to updtream condition which is almost fixed & can be envisaged for different scanrios.

If we list these possibilities then probably it will be a list like this.
  • Variation in pressure downstream of CV may change FE reading, may lead to phase change, may lead to flashing etc.

  • Downstream of CV may have problems of full bore flow also in case of extreme limit operation

  • Downstream of CV may have expansion & sudden cooling in case of vapor as well as liquids e.g. in Cryo processes

  • Downstream of CV may have vibrations problem & therefore type of meter is important (Vortex meters may not work)

  • In case of compressible fluids the low pressure side may or may not have large flow element requirement. As density goes down DP across FE goes down & may result in higher bore size which may lead to larger inaccuracies (Depending on beta ratio).


You may also add more reason to the above list.

The evolution of good practices is a result of large experience of many & every process engineer over a period of time. There is no written rule for them, but logical common sense is required. To develop the habit of good reasoning I ask lot of step by step questions to my team engineers to give them right direction so that they themselves feel the practical situation & learn these small things.

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August 12, 2008

EconoFrost - Save Energy in Super Stores

This post is based on the suggestions from our vistior Samantha, who pointed out this source. So I thought to share it with all of you.

EconoFrost are nothing but reflective night covers made of woven aluminium for display boxes/cases with commercial refrigeration system in Super Stores. What is the need?



The need is based on the cold losses & heat gain from ambient condition during hot summer nights in these stores. If some cover is avaialble which can easily reflect the heat back to atmosphere, the load on thses refrigerated machines can be reduced during off-peak hours & night. This will definitely result in Energy Saving.

EconoFrost is a cover made by weaving of aluminium wires. Aluminium is having very high reflectivity compared to other metals & is easily available. Thus it can save ~37 - 50% on energy bill. The woven fabric reflects 70% of the heat that would normally enter open refrigerated display cases. The woven pattern of the aluminum refrigeration blinds disperses reflected heat in multiple directions, effectively maintaining optimum, even temperatures throughout the refrigerated display case

ECONOFROST insulating night covers also protect all heat-sensitive merchandise from exposure to radiation during closed hours. UV radiation from supermarket lighting penetrates the surfaces of refrigerated products, heating them and causing premature decay and discoloration. Enclosing your refrigeration display cases with ECONOFROST night curtains extends the shelf life of your merchandise and reduces produce shrinkage.

Finally good option to save energy in Super stores where lot of saving potential exist.

EconoFrost is a product from Market Group Ventures Canada. Source: Their website.

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August 07, 2008

Chemical Professionals among Top-100 Science Blogs

Congratulations to all the readers.

Chemical Professionals appeared on top-100-cutting-edge-science-blogs on X-ray technicians website. Click here to check it out.

The important thing is that 'Chemical Professionals' is the only blog related to chemical engineering among top 100 useful sites. This has been possible only with you overwhelming support & interaction of more than 450 readers by now in different bots & feed aggregators.

Thanks for your support.

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August 03, 2008

Adjustable Speed Application - Free Excel Calculator

Most pumps operating today are selected to meet a maximum system demand, or potential future demands. This means that most pumps are oversized, rarely operating at their full design capacity. In addition, pumps are often installed in systems with multiple operating points that coincide with process requirements.



A throttling valve is usually employed when the process flow requirement is less than the flow at the pumping system’s natural operating point. Throttling valves control flow by increasing the system’s backpressure or resistance to flow. This increase in pressure or head requirements shifts the pump’s operating point to the left along its performance curve, and, typically, away from its best efficiency point. The result is a loss in efficiency.

Adjustable Speed drives (ASDs) provide an efficient flow control alternative by varying a pump’s rotational speed. These drives are broadly classified as mechanical (fluid or eddy current) drives and variable frequency drives (VFDs).

Related References



Today, the VFD is the most frequently specified type of ASD, and pulse-width-modulated VFDs are the most commonly used. In centrifugal applications with no static lift, system power requirements vary with the cube of the pump speed. Small decreases in speed or flow can significantly reduce energy use. For example, reducing the speed (flow) by 20% can reduce input power requirements by approximately 50%.

In addition to energy savings, VFDs offer precise speed control and a soft-starting capability. Soft-starting reduces thermal and mechanical stresses on windings, couplings, and belts. Also, VFDs reduce voltage fluctuations that can occur in starting up large motors. Induction motors with across-the-line starting draw as much as six times the full-load current during start-up. During acceleration, a VFD-controlled motor’s locked rotor current is limited to one and one-half times the full load current. Operating at reduced speeds results in other benefit, as well, such as lower bearing loads, reduced shaft deflection, and lower maintenance costs. Estimating Performance We can use the affinity laws to predict the performance of a centrifugal pump with little or no static head at any speed, if we know the pump’s performance at its normal operating point. The affinity law equations are as follows:

Q1 / Q2 = N1 / N2


H1 / H2 = (N1 / N2)^2


P1 / P2 = (N1 / N2)^3




The affinity laws show that the pump head decreases significantly when the pump speed is reduced to match system flow requirements (see figure). Pump shaft horsepower requirements vary as the product of head and flow or as the cube of the pump’s speed ratio. Note, however, that the affinity laws will not provide accurate results for systems with static head. In that case, constructing a system curve to calculate new duty points is essential.

The Issue
We operate a pump with VFD, the significant drop in flow rate comes with significant drop in head also whereas in actual operation sometimes system curves are changing which results in lesser drop in head compared to calculated from affinity laws. In such cases, the VFD alone can not provide the desired head & flow combination becasue speed variation alone can not change system curve.

So Be Careful while selecting or recommending VFD. DO NOT CONSIDER IT without the knowledge of system curve. In such case, we need to consider other alternate also e.g. combination of conventional throttling with VFD.

A Very useful & versatile ASD Calculator for estimating Energy Savings from ASD based on running hours & requirement is Available Here FREE.

This calculator is available free from owners website & we do not guarantee or owe any responsibility for the ocnsequences out of its use.

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July 28, 2008

Distillation, Absorption & LLE - 3-in-1 Software - Available for Free

ChemSep is a program for performing multicomponent separation process calculations. ChemSep is designed for use in courses on thermodynamics and/or separation processes and features an easy to use interface for Windows, equilibrium and rate-based column models, integrated graphics (with GNUplot), and export capabilities (e.g. to Excel, Word, and html).



ChemSep integrates flash, the classic equilibrium stage column model and a nonequilibrium or rate-based column model in one easy to use program. ChemSep-Lite (a version with some limitations) is available free - Download from Original Site.

Following are the case studies presented on ChemSep site.



You can either download above PDF or can download ChemSep simulation files also from ChemSep Site which can be used with ChemSep Lite.

ChemSep is CAPE open Compliant that means you can use these simulation files with other programs e.g. ASPEN. It is also integrated with Sulzer's Column Rating tool SULCOL.




Download ChemSep Guide Book Here.

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July 22, 2008

Guaranteed Energy Saving in Vacuum System

Steam Ejectors are the widely used equipments for vacuum generation till the recent developments of liquid ring & dry pumps. Also, the reason for preferable use of ejectors was availability of steam at different levels, low maintenance, non availability of other systems etc. with major one being the only system suitable for very high vacuum.



However, the major disadvantage of using ejectors is - higher steam consumption. As we go for higher vacuum, the steam consumption goes up exponentially & since the cost of fuel is going up like a rocket in the recent past, the alternative solutions are more benefitial.

Here is a proven method which is used by us for sometime & is working fine.

Related References



The option is to use dry vacuum pumps which are capable of running dry e.g. Screw or Scroll. The picture of a typical pump from M/s. Kinny is given above. Here are the two main parts as photographed from an actual installation.










The other advantage of these pumps is to recover process fluids. These pumps discharge non-condensables & vapor load at ambient pressure so the benefit is that you can boil your fluid at low temperature under vacuum & can recover them at atmospheric pressure with probably cooling water.

Thus it can be used for multipurpose depending your system. I am using them for both. In one of my process I have to remove methanol under vacuum So I used dry screw pump there & then recovered methanol at the downstream with cooling water condensation. Boiling was done using low temperature hot water at 80°C. However, the vapor load should be low in such cases.

This example is given for a 1 torr vacuum in the system. Kindly do some financial calculation before making any decision in your plant.

Calculate following.

  • Steam Running Cost Including boiler efficiency

  • Cooling water cost.

  • Cooling water Pumping cost

  • Condensate Pumping cost

  • Any other direct & indirect cost


Now the installation cost for steam ejectors was ~Rs.12 Lac while the cost of vacuum pump was only Rs. 16 Lac, but the saving was Rs. 10 Lac/year which paid back my investment in less than 6 months.

So now you need to assess your current systems & implement this sure shot energy saving plan.

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July 16, 2008

High Efficiency Boiler Fans - Amazing Potential of Energy Saving

The fans help burn the fuels efficiently and complete combustion. The combustion air requirement from under the grate is met by supply air from forced draft fans (FD fans), and the combustion product from furnace is exhausted by induced draft fan (ID fan). The secondary air fan (SA fan) supplies air over the grate to create turbulence over fuel.

For spreading fuel over the grate, and the burning fuel in suspension the pneumatic spreader is used, and pneumatic spreader fan (PS fan) plays important role in spreading and burning fuel efficiently. The balanced drought system, thus, is very essential for economic operations of boiler



Related References


For a particular specified duty, there is only one diameter of impeller at specified speed, which can give best efficiency, provided optimised inlet diameter and good blading is provided. Low lead edge angle and long smooth passage of air from inlet to outlet of impeller, without separation of flow in two blades will provide highest possible efficiencies. It is therefore work of the designer to:
  • Establish best diameter of wheel that is capable of highest efficiency possible at specified speed and duty.
  • Establish optimum impeller inlet diameter at specified duty and speed.
  • Select blade angles and number of blades to convert dynamic energy into static energy, without separation of flow and avoiding shock conversions.

The performance curves of laboratory tested high efficiency fan are presented below.


The features are:
  • maximum efficiency is 88%.
  • Optimal operating range is between 50 to 75% of free delivery.
  • Maximum BHP occurs at highest efficiency point, making it possible to use small motors.
  • Maximum noise level occurs at minimum efficiency.

Following are the tips for maintaing good fan system.
  • Use correct belt tensions as per manufacturers recommendations only. This will not only increase life of belts, it as well saves energy. Use of belt tension meter is recommended.
  • Excess tightening of adaptor sleeve also heats up bearings causing more wear, and more power consumption. Use of filler gauges while tightening adaptor sleeve is recommended.
  • Fill grease into bearing with correct quantity and quality recommended. It is general observation that plummer blocks are filled with grease fully which overheats bearings.


Conclusion
Now try to see the nos & utility of these considerations which are posted on this "Chemical Professionals' Blog. The example is given just for Cement Industry which may be the largest user of fans & blowers.

The World Cement production capacity is ~2250 Million TPA. The average power consumption in cement industry for fans is ~75 kWh/Te of Cement produced. Which means the total power consumption in fans in cement industry alone is ~ 168750 Million Units per year.

If we assume current efficiency of 80% (Best possible) & replace it with high efficiency fan of 88% than there will be a reduction of 10% in the power consumption. Thus we can save ~16875 Million Units per year.

At an average price of 12 Cents/unit it becomes ~2000 Million USD saving per year.....WOW.......

Wait Wait.........More savings are available

For each unit of power there is an average carbon credit of 0.01 Te, which means you can save another 1650 Million USD / year at minimum 10$/Te of credit rate.

So total potential is ~$3650 Million / year just by single change across the world. Avoid Global Warming as a BONUS.......

Enjoy Reading.......your favorite Blog - Chemical Professionals

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July 10, 2008

Save Power or Increase Capacity of your Air Compressors.

Have you ever noticed that most of the package air compressor units face a typical problem of higher air temperature from interstage coolers? Yes. Go & Check it. You might be having very high approach to equilibrium in case of these inter stage coolers. Why?


The simple reason is that they have a basic design flaw of using a cross flow exchanger in place of true counter current exchanger. These cross flow exchangers have low pressure drop compared to counter current & therefore save on compressor sizing if its a turnkey package unit.

So What?

Till date there is no guideline or formula in any of the heat transfer books for what should be the flow configuration in a given exchanger. This can come either from FE to some extent or by CFD analysis only.

Related References


As said above, there is no guideline or formula which can tell you that whether water flow should be from top or from bottom but this depends on common sense & logical thinking of design engineers.

One good example is as below.
It was a interstage cooler of air compressor in which air is in cross flow pattern and coolign water flow was in two pass in the tubes. So basically one side was counter current & one was co-current but not in real sense. The Sketch is given below.




We were facing the problem of high temperature in this exchanger since commissioning. Based on my previous experience I decided to inspect it for the same flaw ( Actually I got it from experience from a lube oil cooler of big size compressor of 14 MW load - Yes you got it right it was form Fertilizer unit)

On inspection we found above configuration. In this case air exit temperature can not go below CW outlet temperature. So if exchanger is designed for 33° to 43°C CW temperatures then air temperature will always be expected to go beyond 43 + 5° (Expected approach OR ATE) = 48°C. While in true counter current it can be 33 + 5° = 38°C. So there is a huge possible gap in air cooling from 38 to 48°C.

In our case the air temperature were further high at 52°C.

Therefore, based on my previous experience I suggested to swap the CW connections to make it like



In this case, after changing the CW connection, we were able to bring down the air temperatures by 6 - 9°C in different exchangers raising the capacity of air compressor by 10% or so due to cummulative effect in 3 stages.

So, try to identify the configuration of CW inlet & outlet viz-a-viz process inlet & outlet & then you can improve the cooling just by making them right.

Hope you will find some of them in your plant as well.

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July 04, 2008

Pump Efficiency - Quick Way to Calculate

Pump efficiency have always been a problem to determine, especially when no pump information is available. Here is a small equation which can be used to estimate an approximate value without power measurement.


I see this equation more useful for getting the flow information which is generally not available while head & power can be measured.

The formula is useful to make decisions for improvements in pumping system.

The equation is

Eff = 80 - 0.2855 * F + 0.000378 * F * G - 0.000000238 * F * G^2 + 0.000539 * F^2 - 0.0000000639 * (F^2) * G + 0.0000000004 * (F^2) * (G^2)

Where F = Developed head in Feet - F should be 50 to 300 feet
G = Flow in GPM - G should be 100 to 1000 GPM

So if you know flow & head, you can calculate approx efficiency & then can confirm this with power consumption. OR if you dont know flow then use iteration method to calculate flow based on actual power consumption.

Thus it is very useful for all process engineers who need to improve their pumping system. This formula can definitely guide them in making proper decision on replacing or repairing the pumps.

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June 28, 2008

Mist Eliminator / Demister - Sizing & Efficiency Calculation

Mist Eliminators or Demister Pads are very common & important things for process engineers but they are often neglected. Here is some input from my side theory as well as practical knowledge & gains from them.





The importance of a demister pad comes from the efficiency of separation & its impact on either product recovery or the indirect impact on downstream processes. Usually later is more important than direct impacts & therefore they are generally neglected peace of equipments.

The separation of entrained liquid droplets from a vapor / gas stream is called mist elimination & the equipment used for the purpose is called De-mister. Generally, in a manufacturing plant, process engineers or production engineers do not focus on the benefits which can be derived from the improvement in demister pad or mist eliminators.

For example, suppose your outlet gas goes to a reactor with VLE amount of component i.e. Ammonia. Now if ammonia from the separator goes up due to mist carry over or poor efficinecy of de-mister pad then the conversion in the reactor will go down, resulting in overall impact on synthesis loop pressure & energy. Also the production will go down.

Similarly, if a separator in acidic gas service do not work properly the downstream equipment will corrode easily. That's why we need efficient mist separation.

What is mist.......? Mist is the fine droplets of liquids of various sizes. They may vary from 0.5 micron to few 100 of microns in size. For example, good spray system generally generate 20 - 1000 micron size droplets, while columns & tray do have ~8 - 100 micron size drops. Saturated Vapors generally have finest size of droplets depending on their velocity at generating point.

Sizing
Generally we use Souder's equation as used for phase separator Or for knock out drums. That is

Vd = k x [ (L-G)/G ]^0.5

L & G are liquid & gas densities.

where k is the important part & is called the capacity design factor. It depends on type of de-mister pad. Selection of a too low or too high k is always have a negative impact in case of demisters as the efficiency greatly depends on velocities.

In case of lower velocities, droplets have low momentum to get path impingement & coalescene & therefore avoid capture into bigger drops & thus escape from the pad. At higher velocities the vapors have sufficient kinetic energy to re-entrain them. Therefore, correct range of k selection is necessary.

Based on past experiences & designs a value of k = 0.42 is most suitable for many applications. So after choosing k get the design velocity & then find out the diameter of separator. Now for predicting efficiency of de-mister pad, calculate K inertial parameter as below

K = [ (L - G)/ Vd^2 ] / ( 9 x mu x D)

L & G are liquid & gas density
Vd - velocity of gas calculated above
D - Diameter of pad

Now use following curve to get E factor for above K value.


Now calculate specific area of pad as below

A = Specific Area x Thickness x 0.67 / PI()

Now calculate % Efficiency as below

Eff = 100 - 100 / e ^ ( 0.213 x A x E )

In the next part I will consider pressure drop calculation.

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June 23, 2008

Impact of Specific Gravity on Pumps

The pump performance or characteristic curve is often referred to as a water performance curve (see Figure 1). Water is naturally the fluid of choice for testing pumps because of its availability. The question is: can the water performance curve be used for other fluids? The answer is YES with care.



The pump manufacturers will sometimes identify the unit of the total head axis as feet of water, or sometimes there will be a notice somewhere on the chart saying: tested with water at 70ºF. This is the manufacturer’s way of telling the user that the pump was tested with water and that the power curves are ONLY valid for water, but careful there are other pitfalls.

Related References


Here is the question that you need to think about. Say that you measure the shut-off head of a pump with water, will you get a different value if the fluid is a dense salt solution (assuming the viscosity is the same)? The answer is NO. Why....Because pump is going to develop the same head.



Head is what?.... Height of Liquid.....So Height of liquid column will remain same whether it is water or salt solution but the pressure exerted by the head will be different. Head remains same is the typical property of a centrifugal pump....Keep this always in mind whenever you are dealing with pumps. They are constant head machines.

Head = Pressure / SG

So,

Pressure / SG = Constant

So,

P1/SG1 = P2/SG2

Thus developed pressure will be = curve pressure x SG / SG of water (1.0)

Here the curve pressure is the developed pressure for water, SG is the specific gravity of fluid in question & SG of water is 1.0 at test conditions or curve conditions.

Impact on Power

Power = Q (M3/Hr) x H (Head in meters) x SG / 367.2 / Efficiency of pump

So the power will be different because of SG term in the equation. So in case of different fluid do not consider power from pump curve it is better to calculate from above formula...OR if you consider curve power correct it for SG change.

Next I am going to cover the impact of viscosity on pumping.

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