Energy Loss in Vane Pump and Gear Pump

GA Dave. You clearly know what you're talking about. The issue of comparative efficiency is hard to score on small PD pumps by simply extracting numbers from advertised performance curves.

Capacity/pressure curves are based on displacement and leakage through pump clearances due to high pressure and low viscosity. The displacement is a cut and dry figure dased on the geometry of the pumping elements.

The leakage (often referred to as "slip") shown on the advertized performance curves is largely a marketing decision based on trading off the performance to be delivered against the warranty the manufacturer is willing to offer.

On the other hand the power requirements, especially on smaller pumps driven by fractional HP motors, also has some fairy tale aspects inasmuch as the marketing folks are well aware of the liberal service factors in many of the small motors and will rate them accordingly especially if they supply the motor with the pump.

Ed Weldon

Thanks Dave for sharing your valuable exp, vane pumps have wear compensation for wear of vanes, but gear pump does not have any thing like that. Once The gears wear away, the slip (leakage) will exponentially increase. That leads to low volumetric efficiency. When it comes to Gear pumps, relative speed between the rotating components (idler and driving gear) is less. So that accounts to less shear drag losses. After studying all these things in research papers published in SAE by well founded companies, i get confused looking at the results published by these companies itself. there seems no relationship between the theory and practical. I think Ed Weldon is partially right, companies will never reveal the true performance graphs. Some kind of marketing is also involved.

I was looking for that efficiency figure as OP was looking for. A liquid that is pumped to a height of h meters and has a mass m kg and gravitational acceleration being g meters / sec^2. Potential energy = mgh. Now we need to look at electrical energy input in kilowatt hours to pump up m Kg. Thus efficiency is (mgh/ kwh)*100.

I think the suggested equation gives overall efficiency- but does anyone have figures? I too interested to learn / understand.

What you have done is straight away to the point. yes thats correct in a simple way. there are many factors on which pump efficiency depends on. generally we do that calculations (Good estimation) for centrifugal pumps used in domestic applications. But when it comes to do with other industrial pumps, its gets little complicated. taking into consideration of pumps ability to suck the fluid into the pumping chamber(volumetric efficiency), and ability to push the liquid (mechanical efficiency) with much losses. There are many types of pumps. Each pumps principle is different. selection of the pumps depends on applications and other factors like maintenance, cost, performance requirements. Regards Vishwanath

the losses are due to dynamics and don't have that much to do with the principles. The gear pump (lobe pump) has a rotating body within a close fitting casing. All the interfaces are lubricated by the fluid in the small gaps, like sliding a plate on an oily surface. That generates a fairly large drag but slippage (backwards leakage) is far less than a vane pump (propeller). The vane pump has good clearance and the fluid is not greatly stressed or squeezed but a propeller is not very efficient. The centrifugal pump is the process pump of choice for water and similar fluids. They are theoretically better but have limited pressure increments. Higher pressures are developed by multiple stages.

Trevor,

You are right about some gear pumps, External gear pumps, and Bi-Wing rotor pumps do have a large gear or rotor "has a rotating body within a close fitting casing" which will account for drag. Slippage depends on viscosity, and pressure of liquid being pumped. Lobe, and External Gear pumps however have minimal close clearance points. so drag should be less.

A vane pump does not have a propeller, however a rotor, with vanes which slide back and forth while the rotor rotates. Vane pump clearances are fairly tight on the sides of the rotor, and at the top or off set where the cavity disapears. Vanes in theory ride on a film close to the casing. With ubricating like fluids this appears to be so, lighter liquids however do have contact. The vanes eventually do wear, and can be changed restoring the pump to as new performance.

Centrifugals are great, however applications where NPSHr limitations, variable or linear flow control requirements, and tank, tank car, or truck unloading will almost always better suit a PD Pump. Higher pressure applications will also be an energy saver using a PD pump.

All pumps should be application driven, with energy consumption in mind. However many sales are based on equipment price today with little regard for energy, or maintanance costs tomorrow.

As usual the devil's in the details. Good/bad are relative terms. My background is with hydraulics. If you want to compare you must select the exact things you want to compare. Generalities are just that. The ones you have stated are somewhat accurate.

As others have stated vane pumps can compensate for wear. I would never call their volumetric efficiency "good" - "Stably over time" is probably a better term. I have seen volumetric efficiency vary on new units considerably between brands. A gear pump can have significantly better volumetric efficiency than some vane pumps again dependant on brand, design,viscosity, driven speed, and operating pressure.

Zhock, You are very much right. I think that makes sense. The final efficiency depends on many factors. I think when talking about pumps, a particular model may be better than other than type of pumps. once again which may depend on the brand. Any field data you could share with us for pressure, flow and motor power consumed? I referred some companies published data but that does not seem to be reflecting the trend.

Quite right, Dave of Montreal. My mistake. I'm a process man and not much of a fan of either vane,gear or lobe pumps. I do like the Wankel engine though, now that's the way to drive a vane pump.

I have zero knowledge in this field. Are there no piston based pumps? I can imagine that with piston based pumps- the fuild output will not be smooth. But will it be more efficient?

I have been trying to understand why pumps are known to be efficient. Hence I was trying to read the discussion on vane pumps, propeller pumps etc - is it slippage of fluid backwards - OR WHAT ELSE?

Piston products are significantly more efficient volumetrically. Leakage paths vary between construction types as well as with the same old variables rotational speed, pressure, viscosity. The output does have pressure ripple which again varies with construction type and has other variables like how many pistons in the construction some have as low as three most small to mid size axial piston pumps have 7-9 pumping pistons. Ripple is not usually a factor in most systems as system capacitance smooths them out. Accumulators on the outlet of the system can virtual eliminate it as they act as large capacitors if it is an issue.

It is typically not the most energy efficient process for power transmission though a well designed system can make huge differences. It is not just pump losses that must be considered but the entire circuit. Pressure losses in valves, and fluid conductors as well as the actuator can be very significant. On servo controlled systems upwards of 50% of input horsepower can be lost. It is typically the application the best suits hydraulic drive due to some particular advantage of hydraulics that make it the best choice not the overall efficiency.

The efficiency is a combination of the mechanical efficiency and the volumetric efficiency.

Mechanical efficiency is the sum total of resistance to the rotation of the pump due to rolling and sliding friction between the various elements moving within the pump. Viscosity and speed dependent. I would imagine that as some of the elements can be changing direction there are inertial forces that must be overcome.

Volumetric Efficiency is the sum total of loss from the hypothetical amount a pump can pump. I am talking positive displacement pumps here. Each pump has a displacement or volume / revolution. Typically measured in cubic inches per revolution or more and more commonly in cc/rev. Multiply by driven speed and you have flow rate. Of course nothing in life is perfect leakage occors between areas of high to lower relative pressures. As pumps are moving the sealing elements are often metal to metal clearances. The paths vary with the type of construction. The total leakage varies with the old variables of pressure,viscosity and driven speed. Variable pumps will also consume a portion of the output flow to control their position which contributes to loss in overall volumetric efficiency.

Multiply Mechanical X Volumetric and you have Overall efficiency.

90% X 90% = %81 Overall Efficiency would not be an abnormal type of result.

Thank you very much for an elaborate response.

While in discussion with a few people from the utility company, I came to know that the agricultural pump sets (open or bore - well type) are inefficient. So let us find a solution. Beyond these words- no one seems to know where to put his finger on. Hence my interest i this thread. As everyone knows due to global warming and need to reduce carbon foot print, there is a need to focus on efficiency. There are also some pumps supplied with Solar photovoltaic systems with electronic drivers. These also seem to be equally inefficient. I understand that the electric motor by itself is efficient !!!

So by the end the thread- hope I will get some answers.

Thank you so much for that very elaborate explanation.

GA Ed. If I were making a comparison, I would select two viscosity ranges.

38 SSU's very common diesel fuel viscosity, with a common system pressure of 80 or so psi. I would also select 750 SSU's many liquids out there around this point, Resins, lubes Etc. at about 100 psi, just to keep the vane pumps in the mix.

I would select three major manufacturers of industrial process pumps, Blackmer (Vane), Viking (Internal Gear), and Roper (External gear).

Ask for performance curves at these the designated operating conditions. Evaluate the BHP required, check out the NPSHr, and if you want some fun add a centrifugal pump to the mix, Watch the HP requirement on that one?

As Ed Said "The subject of this thread is essentially a comparison of two types of rotary pumps" piston pumps do not apply, nor do hydraulic pumps as they are rarely used in other than high pressure applications.

Dave

It would be difficult to compare industrial/process type vane and gear pumps as nobody (as far as I know) makes both types for similar applications. Be wary of published curves if you want accurate figures for absorbed power. They generally take the worst case from a bunch of tests, and then add a safety factor. This ensures that motors should never be undersized, but it doesn't give an accurate figure. The only way to know for sure is to test it yourself. Also, different gear pump manufacturers use different standard (and special) clearances.

Returning to the origins of the thread, it is true to say that " a Vane pump has good volumetric efficiency but poor mechanical efficiency. Gear pump on other side has got good mechanical eff but poor volumetric eff", but this is a generalization. The good volumetric efficiency of the vane pump is due to close clearances, and a good seal between the vane tip and the body, so very little slip. Mechanical efficiency is not so good, mostly due to frictional losses from the vanes. There is some variation with different pressures/viscosities, but not so much. With a gear pump the volumetric efficiency is a function of clearances, pressure and viscosity, and performance/efficiency can vary enormously as these are changed. A gear pump correctly applied on a reasonably viscous fluid can have a very flat curve (flow proportional to speed / high volumetric efficiency), and compared alongside a similar vane pump it would be smaller and absorb less power (I know this from experience). But if you put the same pumps in a similar installation but with lower viscosity and/or higher pressure, the vane pump will perform similarly whereas the gear pump might not perform at all as volumetric efficiency can drop to 0%.

To summarize, with greater variation in gear pump types & designs (internal, external, gerotor, process, lubeoil, etc), different clearances and greater performance variations under different conditions than the vane pump, it difficult to know what you are comparing when looking at published curves, and you can only generalize.

Wondering about your pump experiences, you say "It would be difficult to compare industrial/process type vane and gear pumps as nobody (as far as I know) makes both types for similar applications." How pumps from Blackmer, Corken, Foster, Rotant, Viking, Tuthill, Spectrom Gear, Gorman Rupp? I've left out quite a few.

You say to "Be wary of published curves if you want accurate figures for absorbed power. They generally take the worst case from a bunch of tests, and then add a safety factor."

I can tell you first hand that Two of the PD Manufacturers I have been associated with over the years, one Vane, and one Gear, have done extensive laboratory testing, dealing with pressure loss, and NPSH, so they could prepare their curves, NPSHr data, as well as pressure loss data for high viscoties, try to find data for 250,000 SSU's if you find information it probably comes from a PD pump company.

Quality manufacturers do not want to dole out bad information, nor do they want their product to be seen as a horsepower hog, or do they want to be uncompetitive.

You are right when you say "different gear pump manufacturers use different standard (and special) clearances." you will not see the same clearances on a gear pump handling 28 ssu's, as you would find on a gear pump handling 250,000 ssu's. By the way a Vane pump handling 100,000 ssu's will also have extra clearances.

As far as "the gear pump might not perform at all as volumetric efficiency can drop to 0%." for this statement to come true, the pump would have to be completely worn out, or be handling a light solvent, after being set up to handle very high viscosities.

Briefly, I started out many moons ago in centrif process pumps (LaBour), then small peristaltic pumps (Autoclude), then gear pumps (Tuthill) and currently small gear pumps (Fluidotech).

You are quite right here – "Quality manufacturers do not want to dole out bad information, nor do they want their product to be seen as a horsepower hog, or do they want to be uncompetitive." – this is the crux of the biscuit. Some manufacturers (particularly the Germans) are very good at providing comprehensive and accurate technical data, others sadly are not, and the problem is knowing which you are getting. When you are putting together a large proposal it is irritating to find you are uncompetitive because your competitor is using smaller motors, and particularly when you cannot get a true figure for absorbed power (even as an employee, albeit remote from the factory). If you are dealing with an engineering contractor, they want a true figure so they can apply their own safety margin (whether duty + x%, end of curve, @ relief valve pressure, or whatever), otherwise the safety factors stack up and motors are oversized. Some manufacturers will play safe, knowing that a generous safety margin will avoid problems from undersized motors being specified by distributors and sales engineers.

There are also other factors in pump selection that can impact on your competetiveness. If your NPSHr curves and noise level data have excessive safety margins you could find yourself unnecessarily specifying a larger, slower-running pump & motor than your competitor.

Regarding clearances, you will always have special clearances for certain applications, eg chocolate, but you will also find that one manufacturer might use closer clearances to improve volumetric efficiency for a certain range of pump sizes / liquid viscosities if this gives them a commercial advantage in the markets that are most important to them. Another might go for larger clearances across the board so that their 'standard' pumps will cover a wider range in their biggest markets. It may or may not be useful to know this sort of thing, but it is not easy to find out.

Regarding the final point, in most cases if the volumetric efficiency drops to 0% it is because the pump has been wrongly specified (you wouldn't use a process type internal gear pump for pumping water, and yes, I have had it requested!). However, I have worked with very small external gear pumps that are designed to handle very low viscosities (down to 0.3cps) and which have very close clearances (talking several microns, or tenths of thou). I have often had applications involving low-viscosity solvents at low-flow and high pressure, where for various reasons (size, noise, pulsation, material compatibility, etc.) a gear pump has been the only option, and this is a pig of an application. Turning down the speed to reach down to a low flowrate, the curve can drop of very rapidly, and you can go from duty flow to zero flow in the space of a few rpm. What this means is that the gear pump is not a 'true' positive displacement pump, as flow is not proportional to speed.

Holzfeller, you said right. That is because, the applications these types of pumps put to are different. So no manufacturer would want to design the two pumps for same application for some obvious reasons. Thats why we do not get to see same manufacturer having both pumps in similar applications. the trend remains that vane pumps are preferred in think oil applications. But the gear pumps (specially Internal gear design) are used in high viscous oils as they better handle it by its inherent design principle of working. So when i see the performance curves, i can not compare the two types of pumps as oils pumped in are different. A direct comparison is only possible if same oil and pumps of similar dimensions and clearances are tested on rig. That data is usually not available to the public even if reliable companies have tested their products. After all what is important for a buyer is that if pump suits/ function to his requirements. Keeping this in view, companies publish the performance graphs after adding a factor of safety. that will mask the true picture of the whole discussion we are doing here. What we are interested is a direct comparison with true scenario.

For the larger industrial/process types of vane pump and gear pump indeed a direct comparison is not possible, and you would rarely, if ever, see these pump types competing with each other. As you say, they are each best suited to different applications. You would never use a vane pump on chocolate or bitumen, or a gear pump for vapour recovery. On the very small (OEM type) pumps there can be an element of crossover. In a recent case we installed a vane pump in place of an external gear pump (for cost-reduction purposes). The pumps were of similar size and performance, however, the vane pump needed a larger motor due to torque requirements, and absorbed power was much higher. This meant the current draw was greater than what could be supplied by the power supply in the machine, so we had to revert to gear pumps.

I am sure that each of us learnt great deal of stuff from this thread. Atleast I learnt great tips, inside stuff, technical issues and practical knowledge from you all. I thank each one of you for taking time to share valuable information. Kudos to all! Best regards Vishwa