March 25, 2008

Pressure Drop in the BFW System - Study more, Save More

The objective of this study was to carry out a pressure drop survey in the BFW system to identify the areas where we can reduce the pressure drop, which will help in reducing the BFW pump discharge pressure so as to conserve MP steam in the turbines.

Existing System
There are two turbine driven pumps & one motor driven BFW pump in ammonia plant out of which motor driven pump remains standby on AUTO mode for an emergency use. Each pump is having a normal capacity of ~200 m3/hr. The steam turbine drives for both the pumps are of condensing types and use MP steam.

These BFW pumps take suction (approx. flow ~295 m3/hr) from ammonia plant de-aerator at ~120°C & ~1.5 kg/cm2g. The discharge of these pumps at ~128.3 kg/cm2g pressure goes to the first set of BFW pre-heaters E-307A/B where it is preheated to a temperature of 165°C by utilizing the waste heat from process gas.

The preheated BFW at the exit of E-307 A/B goes to the next series of heat exchangers E-211A/B for further preheating by the process gas. The provision of a bypass through a temperature control valve TV-84 is made for controlling the final temperature of BFW after both sets of pre-heaters i.e. after E-307 A/B & E-211 A/B. The temperature achieved after E-211 A/B is ~176°C at present load.

The BFW preheated in these two sets of pre-heaters is now divided in two parts. One part of ~130 m3/hr goes to the synthesis loop BFW pre-heater E-502 where, it is preheated to ~273°C temperature by using the waste heat of reactor effluent. After preheating of this BFW, part of it (~65 m3/hr) is used in the adjacent loop boiler E-501 for generating high-pressure steam & balance (~65 m3/hr) is returned back to the front end.

The remaining part of main BFW stream goes to another set of pre-heater E-210 A/B in the process gas circuit, where it is preheated to a temperature of ~ 274°C. The exit of E-210 A/B is then mixed with the return stream from synthesis loop. The combined BFW stream is passed to the front-end boiler drum B-201 through a flow control valve FV-42.

The flow controller FV-42 is also utilized for controlling the level of boiler drum B-201.

Pressure Survey
We collected the plant data for BFW flow, pressures at different locations in the BFW circuit, temperatures. The Isometric data were utilized for evaluating the calculated pressure drop in the system & was compared with the actual one in the plant at existing load.

The results are as below

It is evident from the above data that the pressure drop evaluation was matching with the actual plant condition.

Actions Taken
Based on the above study, it was found that the excessive pressure drop of 8.9 kg/cm2a was there in the flow control valve FV-42 (present valve opening was ~70%) in the front end. So, it was decided to reduce the speed of the BFW pumps in such a way that the valve opening remains at around 85% which is necessary for a better operability & control on the BFW flow & boiler drum level. In this condition the pressure drop across the control valve was ~4.0 kg/cm2a with a pump discharge pressure of 123.5 kg/cm2g against the present value of 128.3 kg/cm2g.

Further reduction in the speed or consequently, in the pump discharge pressure was not justified due to improper control of boiler drum levels on account of any process variation upstream or downstream in the system.

This simple exercise is just to indicate that if you wish you can identify opportunities for energy saving anywhere in the plant. This exercise resulted in a saving of ~0.7 TPH of MP steam used for turbines which was equal to Rs. 40 Lac/Year OR ~100,000$/year.

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March 20, 2008

pH-phosphate Co-ordinated Graph

During one of the studies of boiler pH vs phosphate concentration I found that the problem of data plotting takes lot of time manually & chances of error are also there. For a clear picture of the system co-ordinated pH-phosphate diagram is must.

Therefore I developed the Excel sheet for the same. In our case we were having 3 boilers B-601, B-605 & B-606. From a standard curve I derived the correlation & put the data in Excel sheet called "Data". Now you can put your pH against phosphate concentration in the corresponding cell. For example say for B-605 you need to put pH on F column as 9.70 against phosphate value of 3.56 in row no 260.

This diagram is useful in identifying the effectiveness of your boiler pH control program. After getting this curve you can have an analysis of Caustic gauging zone, Or Acidic Zone or localized attack / proper system.

Using this sheet we maintained very well pH control system.

So Use this file & improve your boilers health.

To Download the file , just click .... pH-Phosphate Co-ordinated Diagram

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March 19, 2008

Positive Displacement Vs Centrifugal Pumps - How to Select

When to use a centrifugal or a Positive Displacement pump (PD) is not always a clear choice. To make a good choice between these pump types it is important to understand that the two types of pumps behave very differently. By looking at the performance chart below you can see just how different they are. The centrifugal has varying flow depending on pressure or head, whereas the PD pump has more or less constant flow regardless of pressure.

Another major difference between the pump types is the effect viscosity has on the capacity of the pump. You will notice in the flow rate chart how the centrifugal pump losses flow as the viscosity goes up but the PD pump actually increases flow. This is because the higher viscosity liquids fill in the clearances of the pump causing a higher volumetric efficiency.

This chart shows only the effect of viscosity on the pump flow. Remember, when there is a viscosity change there is also greater line loss in the system. This means you will also have to calculate the change in pump flow from the first chart for pressure changes.

The pumps behave very differently when considering mechanical efficiency as well. By looking at the efficiency chart to the left you can see the impact of pressure changes on the pumps efficiency. Changes in pressure have little effect on the PD pump but a dramatic one on the centrifugal.

Another consideration is NPSHr. In a centrifugal the NPSHr varies as a function of flow, which is determined by pressure. In a PD pump NPSHr varies as a function of flow which is determined by speed. The lower the speed of a PD pump, the lower the NPSHr.

Another thing to keep in mind when comparing the two types of pumps is that a centrifugal pump does best in the center of the curve. As you move either to the left or right, additional considerations come into play. If you move far enough to the left or right pump life is reduced due to either shaft deflection or increased cavatation. With a PD pump you can operate the pump on any point of the curve. In fact the volumetric efficiency as a percent actually improves at the high speed part of the curve. This is because the volumetric efficiency is affected by slip, which is essentially constant. At low speed the percentage of slip is higher than at high speed.

The data presented in these charts is the actual data for a specific application. The centrifugal was picked at its Best Efficiency Point (BEP) and the PD pump (Internal Gear) selected to match the flow, viscosity, and pressure. Different applications will have different curves and efficiencies. These curves are presented as an example of the type of performance behavior between the two different principles.

  1. The most obvious reason to use a PD pump is when you have a high viscosity application. It is common knowledge that a centrifugal becomes very inefficient at even modest viscosity. However, there are many other reasons to select a PD pump over a centrifugal other than high viscosity. In fact PD pumps are very commonly used on thin liquids like ammonia and solvents.
  2. A simple rule of thumb is you should consider using a PD pump whenever you might be operating a centrifugal at other than at the BEP. Of course the further away from the BEP you get the more likely a PD pump will be a better choice. This can typically happen at low flow conditions, modest to high head conditions, or any type of elevated viscosity. As you can see from the efficiency curve it takes more horsepower to operate a centrifugal outside of its BEP. This horsepower has a cost, the initial cost of the larger motor plus a higher life cycle cost in energy consumed. Many times the PD pump will have a lower initial cost as well as a lower operating cost.

  3. Another reason to use a PD pump would be if the application has variable pressure conditions. A centrifugal pump will “walk” up and down the curve which can cause process problems. A PD pump will give near constant flow that makes it possible to match the flow to the process requirements. The desire to have constant flow is the reason that a PD pump is the pump of choice for metering applications.

  4. Obviously, if there is changing viscosity in the application the PD pump is the best choice. As can be seen from the charts, viscosity has a major impact on the centrifugals performance. Even a small change in viscosity, like 200-400 SSU, has a large impact on the centrifugal.

  5. PD pumps generally can produce more pressure than centrifugals. This will depend on the design of each pump but pressures of 250 psi (580 feet) are not unusual for a PD pump with some models going to over 1000 psi (2,300 feet). This is a significant difference between the two principles. The capability for a PD pump to produce pressure is so great that some type of system overpressure protection is required.

  6. Generally speaking pumps tend to shear liquids more as speed is increased and the centrifugal is a high speed pump. This makes the PD pump better able to handle shear sensitive liquids. Shear rates in PD pumps vary by design but they are generally low shear devices, especially at low speeds. Internal gear pumps, for example, have been used to pump very shear sensitive liquids. It is important to contact the manufacturer for specific information on shear rates and application recommendations.

  7. By their nature, PD pumps create a vacuum on the suction side so they are capable of creating a suction lift. The standard ANSI centrifugal does not create a vacuum so it can not lift liquid into the suction port. There are self-priming centrifugal designs that can lift liquid an average of 15 feet. This corresponds to a vacuum of 13” hg. Wetted PD pumps (a pump that is not full of liquid but with some liquid in it) can often reach vacuums of 25 to 28” hg. So a PD pump is the logical choice when there is a suction lift required.

  8. As mentioned earlier, PD pumps tend to run at lower speeds than centrifugals. This will have an impact on seal life, so PD seals tend to last longer than seals in centrifugal pumps. In addition, to assure adequate seal life a centrifugal will typically require one of the seal flush plans. A PD pump, because of its lower shaft speed typically does not need an external flush plan. Also, generally speaking, low speed mechanical devices tend to operate longer than high speed mechanical devices.
  9. At certain combinations of flow and pressure centrifugals are inherently inefficient, due to the design of the impeller and the short radius turn the flow must make. These applications are generally under 100 GPM but particularly under 50 GPM. A PD pump, by contrast, is very well suited for low flow conditions. Centrifugals, by contrast, tend to do very well in high flow conditions.
Hopefully, these rules of thumb will make it easier to determine the proper pump principle for your pumping applications.

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March 12, 2008

Bonus: Other problems identified

During the toubleshooting of ammonia synthesis loop pressure I found that there was some problem across the gas-gas exchanger bcoz its calculated temperature was not matching with the measured temperature. So we checked the Temp gauge at the first place & found it faulty. It was replaced & again temp mismatch was observed but this time difference was lower.

Therefore it was decided to measure the ammonia concentration in the hot gas at the exit for any leakage confirmation. Upon analysis it was found that the exchanger was leaking. It gave an opportunity prior to a major failure & major downtime to rectify the problem in a planned shutdown. On opening it was found that there was minor leakage from the seal packings.

The above probelm also contributed slightly to higher loop pressure due to higher ammonia concentration at the inlet of reactor compared to other plants.

After rectifying this we again found that actual ammonia at reactor inlet was still high. So another simulation was done. This time ammonia liquid separator was the culprit. The VLE data was indicating much lower ammonia concentration than actual one. So after few data points it was decided to check its demister pad for physical condition. On inspection it was found totally damaged & therefore ammonia carryover was taking place.

The rectification lead to a increase of 6% in the capacity at the same loop pressure.

Thus finally this program proved its value to the management and investment they made by sparing me for a month or so to prepare it. I thank again Mr. Pankaj Shah for giving me this opportunity.

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Ethanol: How far, How Close

In recent years, everybody is dreaming about one cheaper & cleaner fuel ethanol. Though the cleanliness over its total life cycle is still a debatable point along with energy efficiency debates. I am considering few Nos which were presented in some othe blog for my visitors. In this we will see the total requirement & growth of Ethanol as fuel & as industry demand.

What is the total Oil demand of the world as whole. It is ~90 Million barrels/day, which gives us an annual demand of 32850 Million barrels/year. On the other hand world Ethanol production in 2005 was ~328 million barrels/day. Wonderfully it is only ~1% of the demand.

The rise in ethanol capcity in recent past has been ~15%/year which has caused a food grain price rise of 20-40% in last 3 years worldwide. So can you imagine the scenario if we go for even 20% replacement of oil - foregt about 100%. This is the scenario even if I delete all industrial demand of ethanol.

Add one more interesting factor that Ethanol's energy content is only 60% of the Gasoline / Oil content for the same mass as Ethanol is having 76 KBTU/Gallon while gasoline has 125KBTU/Gallon. So net equivalence is only 0.6% of current demand.

Energy production wise, I have already written my previous post
Ethanol - Facts & Common Sense the lowest positive enrgy production says it produces 134 units of energy for every 100 Unit input. So we are basically generating (Even if I consider this report as 100% correct) than we are effective only 34/100 = 34%. While it is given as 74 units / 100 unit for gasoline. So it is having a net impact of 50% higher energy gain. Even if I consider this as a fact than also ethanol goes back to 0.9% equivalent to Oil demand.

Whereas actual fact is something different, as Oil production consumes ~1 Unit of energy for every 5 units of energy. Just a commonsense man ( No oil company will be able to make profit if it would have been consuming more than its content bcoz Oil is sold on the basis of its energy content not based on quantity). So Ethanol production itself is much more energy in-efficient.
Given this scenario, when food prices are rising steeply & grain shortage is there, I dont see even it is catching to 10% of the world oil demand in next 5 - 10 years period and if it does then be prepared for a world where no food will be available as currently the growth rate of agriculture sector in terms of produtivity is even less than 10% whereas we are talking about 100% rise every year at current levels.

How can you claim that Ethanol is a substitute for Oil......NO.....WAY. Forget it...

Moral - Never focus your strategy based on this wonderfully misleading fact which is fooling the globe.....

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Ammonia Synthesis loop pressure - Mystery solved

This post onwards, my postings are not in actual order of occurence. The next job I did was the problem solving of higher loop pressure in ammonia plant synthesis loop. The loop pressure was running by 10% higher value i.e. ~220 Ata compared to 200 Ata in other similar size, identical, same technology plants of same vintage and company was unable to identify the reason for last 12 years as it was a problem since commissioning.
When I evaluated this system I used the ammonia sysnthesis converter simulation program developed in C++. The major first difference identified was that the overall conversion of reactants into ammonia was higher than other companies (We compared our data with identical plants). The reason of this difference was identified that the second bed in the converter was giving mor econversion & first bed was giving less compared to others.
Ammonia converter S-200 radial flow type Haldor Topsoe make has two beds with one interbed exchanger. Feed gas first goes to the shell annular space rises up providing cooling to the sheel & then enters to interbed exchanger & is preheated to the desired temp range. Then it goes to first bed radially exits from it at higher temp & goes to interbed exchanger shell side for cooling. Relatively cold gas enters to second bed & exits from the bottom.
Now on identification of above reason of higher overall conversion, less conversion in first bed, higher conversion in second bed was sufficient for identifying the problem area.
On simulation again with the actual conditions, we found that some amount of additional gas is required to enter to second bed directly wihtout passing thru first bed. At this point came the idea of some internal bypass or leakge in the exchanger first which was ruled out as it will result opposite to the actual phenomenon. It will lead to higher conversion in first bed.
So finally upon detailed analysis of reactor internals drawing for few days, the only possible way of gas flow to second bed was identified thry bottom support plate sealing.
Based on this it was decided to open the reactor in next turnaround but it was openend earlier due to basket failure which was also an unique phenomenon in this particular reactor. Such failures are not common but it was happening in my company for 4-5 th time.
Surprisingly it was found on inspection of plate that tightening bolts were loose & they were only hand fitted & probably engineer missed the step. This expalined both the reasons of high loop pressure & repeated failure of converter internal pipe. SO COSTLY..........MISTAKE..........A good learning for all of us.....................

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March 11, 2008

100 Posts - 75 Readers & More......

This blog was conceptualized to share the information for young process engineers who need guidance at each step in the early career of their professional life. Since May2007 I started active blogging. Now we had 100 Posts in 10 Months & 75 readers which is a very good growth specially when your focus area is so specific. I am not blogging for my own thoughts, day to day happening like my personal diary but this blog is meant for purely technical & useful learnings.

As promised earlier, I am now launching a page on my detailed work in Fertilizer industry where from I started my Career. On this page I'll be posting my jobs one by one as & when I get time.

I hope for your continued support in bringing more & more friends to encourage sharing of knowledge among a community of true professionals.

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Compressors Performance Evaluation

Happier with the performance of my Ammonia plant simulation package, I went on to write performance evaluation program, General in nature - means a program which can be used for any gas or gaseous mixture, any centrifugal compressor without any limitation on discharge pressure etc, considering real gas. It was writtten in C++. It took me ~25 Days to compile & do the total testing of the program.
This program uses generalized co-relations of Equation of states (EOS) from thermodynamics. So you can input any gas mixture in the basic program. The only basic information needed is Tc, Pc, Vc & omega of the gases handled. No other compressor evaluation program uses these thermodynamic properties to my knwoledge till date.
For example in case of ammonia synthesis compressor N2, H2, Ammonia, Methane & Argon properties are used. The compressor operates from 26 Ata to ~220 Ata in 4 different stages. The program is tested for the entire range of operation. Only Suction & Discharge pressure & temperatures are required for efficiency calculation and flow is required for power calculation.
It also calculates critical properties of mixture Tcm, Pcm & Vcm and Entropy & Compressibility Z at inlet & discharge conditions. Power & Polytropic head is also calculated.

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P&ID - learning small things for big projects

Normally a project passes through various phases e.g. basic engineering, detail engineering, equipment fabrication, erection, commissioning etc. The conceptual idea of business from the process development cell is converted to executable documents during basic engineering.

This is the step where we need P&ID or piping and Instrumentation diagram

P&ID is one of the important drawing representing all the engineering info required to build a process plant.

What is considered in a good P&ID?

Various aspects are considered during the preparation of P&ID which are:
  1. Plant Operation

  2. Plant Hardware

  3. Equipments connectivity

  4. Operational controls & instruments

  5. Safety controls & instruments

  6. Instruments interlocks

  7. Complete Piping information

  8. other details

Steps for P&ID Preparation
  1. Show all equipments with necessary piping to carry out the process
  2. Show all connecting process piping necessary to carry out the process
  3. Show all other pipings required for auxiliaries
  4. Show all required valves and major fittings
  5. Show all required instruments
  6. Mark size, fluid code, material code & identification numbers of all pipe lines
  7. Mark interlock numbers as per interlock description
  8. Review P & ID considering all operational, startup/shutdown, safety, maintenance & aesthetic aspects.

There are specified formats for each kind of equipments, Instruments etc to be filled in as data sheets in the Basic Engg Package. So BEP includes
  1. P&IDs
  2. Line List with operating & Design conditions, each line is represented with Size, No, Area code, MOC, Insulation etc.
  3. Equipment List
  4. Instrument List
  5. Data sheet for each equipments called PDS (Process Data Sheet) depending on the type of equipment specific formats are to be used
  6. Tie In points list which is required for the marking of point where from process or utility is connected in the existing systems

This BEP package is sufficient for converting & implementing the technology in actual process plant.

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March 05, 2008

The Blog I love most in Chemical Space

This is a notable blog on chemistry. Since long I was searching for a single good blog on chemistry which are useful for others. Thanks to Michelle who is a professor of chemistry at Bryn Mawr College. You can read her Blog Here.......The Culture of Chemistry......Its my first preferred blog on the subject.

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March 03, 2008

New Idea for better & low cost Insulation

Common problem of carrying out calculation of optimum insulation thickness, Yet loosing lot of energy in the form of heat loss or cold loss, need re-work on insulation due to damage over a certain period of 2-3 years during annual turnaround etc. etc..............

Poor process engineers. Need survey of thickness, need good maintenance & monitoring program....lots of cost items..& still not satisifed.

After so many years of struggle, I thought about this Idea which is very cost effective & higher savings for lifetime. No need of re-work etc. No maintenance cost.

Interesting.........Ya....Very good business idea is my input.

The idea came to my mind when I was looking for a cold insulation in ASU unit for liquid oxygen lines & found that best insulation also need of lot of investment still we would be incurring >2% losses in our system.

Therefore I thought if I can utilize some kind of vacuum on the surface of process pipes, the losses can be reduced to minimum due to poor conductivity of heat in vacuum.

Now the next question was how to design such a system which can be easily implemented from mechanical erection point of view and can be maintained without much problems.

So I prepared a sketch & its implementation strategy. The sketch is attached below for giving you exact idea how it will be done.

There is inner pipe which is carrying process fluid say it is 4" in size. Then next higher size pipe of 6" can be considered for insulation & it can be any cheapest material available e.g. CS etc. irrespective of the type of fluid handled. So the difference in ID is 2" therefore put some bars welded on the inner pipe of slightly smaller size of the difference in the radius of these two pipes. May be 0.8 Or 0.9" in this case. The allowance depends on the linearity of your pipe layout & size but should be sufficient to push bigger size pipe over the main process pipe.

Therefore, it also requires that this insulation is done in many smaller parts instead of doing it in lengths. Weld it at the ends to the process pipes with some reducers of B to S ratio (B is bigger pipe ID & S is smaller pipe ID.

Keep a tapping from the bigger pipe to vacuum system with PI for indication & combine all such smaller segments to one single vacuum system which will not be very large & high capacity due to practically nil losses in welded construction.

That's It.......................

  1. So Simple,

  2. One time construction,

  3. No change, No maintenance,

  4. Limited running cost due to only intermittent running of Vacuum system, leakage thru PI tappings only,

  5. Very low energy losses,

  6. Very low payback period

  7. No fire hazards due to enclosed vacuum,

  8. Immediate knowledge of system leak - saving costly products / raw materials loss,

  9. Less effluent cost,

  10. Need less safety instruments,

  11. Easy construction in grass root projects,

  12. No special material of separate services,

  13. No expert needed, In-house team can do it,

  14. ....................You can fill more benefits here if you can identify them.............

Hope You Enjoyed it........

If somebody need any guidance to use it for business purpose I can support the design with very nominal charges for my time only.

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