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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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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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

• How to Read Pump Curves
• Download - Short Training Guide on Pumps
• Pump Guide
• Download - Troubleshooting Guide
• Download - Modern Methods of Identifying problems

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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Deaerators - Basic Understanding

Deaerators are simple mechanical devices that remove dissolved gases from boiler feed water using steam stripping. De-aeration protects the steam system from the effects of corrosive gases. It accomplishes this by reducing the concentration of dissolved oxygen and carbon dioxide to a level where corrosion is minimized.

A dissolved oxygen level of 5 parts per billion (ppb) or lower is needed to prevent corrosion in most medium or high pressure (>200 pounds per square inch) boilers. While oxygen concentrations of up to 43 ppb may be tolerated in low-pressure boilers, equipment life is extended at little or no cost by limiting the oxygen concentration to 5 ppb. Dissolved carbon dioxide is essentially completely removed by the de-aerator.

Please understand that the main function of a de-aerator is to remove “Dissolved” gases not free air or free oxygen. The most important gases are oxygen & CO2.

How They Work
The design of an effective de-aeration system depends upon the amount of gases to be removed and the final oxygen gas concentration desired. This in turn depends upon the ratio of boiler feed water makeup to returned condensate and the operating pressure of the de-aerator.

Deaerators use steam to heat the water to the full saturation temperature corresponding to the steam pressure in the de-aerator and to scrub out and carry away dissolved gases. Steam flow may be parallel, cross, or counter to the water flow. The de-aerator consists of a de-aeration section, a storage tank, and a vent.

In the de-aeration section, steam bubbles through the water, both heating and agitating it. Steam is cooled by incoming water and condensed at the vent condenser. Non-condensable gases and some steam are released through the vent. Steam provided to the de-aerator provides physical stripping action and heats the mixture of returned condensate and boiler feed water makeup to saturation temperature. Most of the steam will condense, but a small fraction (usually 5% to 14%) must be vented to accommodate the stripping requirements.

Normal design practice is to calculate the steam required for heating and then make sure that the flow is sufficient for stripping as well. If the condensates return rate is high (>80%) and the condensate pressure is high in comparison to the de-aerator pressure, then very little steam is needed for heating and provisions may be made for condensing the surplus flash steam

De-aerator Steam Consumption
The de-aerator steam consumption is equal to the steam required to heat incoming water to its saturation temperature, plus the amount vented with the non-condensable gases, less any flashed steam from hot condensate or steam losses through failed traps.

The heat balance calculation is made with the incoming water at its lowest expected temperature. The vent rate is a function of de-aerator type, size (rated feed water capacity), and the amount of makeup water. The operating vent rate is at its maximum with the introduction of cold, oxygen-rich makeup water.

The de-aerator section and storage tank and all piping conveying hot water or steam should be adequately insulated to prevent the condensation of steam and loss of heat. This will reduce the steam consumption in de-aerator which is an additional cost to increase the life of equipment.

Sudden increases in free or “flash” steam can cause a spike in de-aerator vessel pressure, resulting in re-oxygenation of the feed water. A dedicated pressure-regulating valve should be provided to maintain the de-aerator at a constant pressure. This also helps in reducing steam consumption.

Additional Benefits
Deaerators provide the water storage capacity and the net positive suction head necessary at the boiler feed pump inlet. Returned condensate is mixed with makeup water within the de-aerator. Operating temperatures range from 215° to more than 350°F, which reduces the thermal shock on downstream preheating equipment and the boiler.

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