4 replies to this topic

[teson1]

Dear all,
I hope you can help me with following question regarding bellows or diaphragm pumps:

My application:
I want to predict the flow rate in a system discharging 96% sulfuric acid. Fluid is driven by a air-operated bellows pump.
I have calculated system pressure losses for the piping system with sulfuric acid, and plan to determine flow rate as the flow where system losses equal pump curve.
Manufacturer publishes pump performance curve as a graph Pressure vs Flow (for water) for a given compressed air pressure.

My question:
-> Do I have to correct published pump performance curves of an air-operated bellows (or diaphragm) pump for density (or viscosity?) if a fluid with properties differing from water is used? And how?

In particular, I wonder wether I should rescale the pump curve for the higher density of sulfuric acid (1.8g/cm3)? But maybe the high viscosity of sulf (25 cP) would have an impacy as well?

If I have not been clear, please see attached sketch.

Thank you in advance for your advice.

Attached Files

•  sketch.GIF   2.76KB   50 downloads

[Art Montemayor]

Teson:

I'm confused about your diaphragm type of pump and the performance curve you supply.

A diaphragm type of pump is a positive displacement type of pump (as differentiated from a dynamic -centrifugal - type of pump) and its performance curve should be approximately a horizontal line on the head versus flow rate graph. Your performance curve seems to be similar to a centrifugal pump's curve.

Am I wrong or am I thinking of the wrong type of pump?

For example, a diaphragm pump uses air to actuate a diaphragm that displaces a set quantity of liquid. The suction and discharge actions are regulated by check valves - which are sometimes nothing more than plastic or stainless steel spheres seating on a teflon seat. The pump action should be similar to a conventional piston pump - except that instead of a plunger, you have a diaphragm. The use of a diaphragm avoids having to use a packing gland on a plunger or piston and suffering the normal packing leakage as well as wear and tear.

For an example, go to: http://www.wildenpum...ail.cfm?pid=140

Note that these type of diaphragm pumps are described as pumping capacity versus air consumption - not in the usual Total Developed Head versus capacity flow rate method.

[teson1]

Dear Art,
thanks a lot for your feed-back on my question.

I have attached the pump performance curve of one of the pumps under consideration. Please see attachment "Pump performance curve".
Here's also a link to the product sheet of the pump:
http://www.entegrisf...ault.asp?G=1627

Shape of the pump performance curves is indeed kind of parabolic, with discharge pressure decreasing with increasing flow for a given air operating pressure.

I have some thoughts on the reason for this, and ideas on how to correct. However, i am not at all sure wether these ideas are correct and relevant. Also, I do not want to focus the discussion too much onto this aspect, so I have placed these considerations below, in PS.

Ideally I would appreciate if you, or other readers of this forum, would have any first hand experience with calculations regarding air operated pneumatic pumps, or could provide your thoughts on how to improve accuracy of my calculations.

Your advice is highly appreciated.
Thank you.

PS: My interpretation for the discharge pressure for the pumps reduces with increasing flow rate so far has been that there is pressure loss due to dynamic head, and maybe due to the check valves. This would account for a parabolic shape in the performance curves. I have sketched this idea in attachment "sketch 2".
However, I have too little understanding on the real loss mechanisms in such a pump to judge wether this is a relevant interpretation. Also, shape does only fit a second order polynomial for low operating pressures. For high operating pressures shape is rather first order or less.
I could imagine some other loss mechanisms, e.g. due to losses in the compressed air lines, maybe some adiabatic effects, ect.
Also, and particularly, I wonder wether such approach would improve the accuracy of the model in a sensible way. Or would I be as well (or better) off by just using the published curve??

PPS: Please note that I have simplified my problem in my first message. In fact I'll have a rough measurement of the flow rate in the equipment with water, to which I could adjust the model (pressure loss coefficients ect.). Then I want to predict flow with other fluids (sulf among other) and study the impact that system component changes (fittings, pumps, additional components, ect.) have on flow rate. So what I am interrested in, is that the change from water to other chemicals is modeled as accurately as possible.
In my current calculations the difference in flow I get with corrected and non-corrected pump curve for sulf is 10% - this difference would be relevant for my application!
That's why I'm so trying to understand the impact fluid properties have on pump performance.
I guess nothing beats the experiment, but problem is I won't be able to do the experiment with sulf...

PPPS: Art, I have greatly profited from some of the documents you have supplied via this forum -thank you very much for that as well.

PPPPS: I think I'll stop with the PS now ;-)

Attached Files

•  Pump_performance_curve.GIF   19.12KB   32 downloads
•  sketch_2.GIF   3.47KB   32 downloads

[Art Montemayor]

Teson:

I appreciate you giving us a website for the referenced pump’s manufacturer. It is difficult to respond to your specific question because there isn’t just ONE type of diaphragm or bellows pump. It helps also to know the origin of your question in order to address it accordingly. For example, have you already bought and installed the pump? Or are you contemplating using the pump or evaluating its application? I’m going to guess that you are evaluating it as an acid pump in your project and you want to know if you will be able to calibrate it for sulfuric acid service as opposed to water service – possibly in a chemical engineering lab assignment. If you already owned the pump, you could do the normal, expected thing and simply call the manufacturer to obtain an explanation in detail. Allow me first to define some aspects of this type of pump, in order to facilitate my final explanation.

According to Wikipedia, the free encyclopedia, a diaphragm pump is a positive displacement pump that uses a combination of the reciprocating action of a rubber or teflon diaphragm and suitable non-return check valves to pump a fluid. Sometimes this type of pump is also called a membrane pump.

There are three main types of diaphragm pumps:

1. In the first type, the diaphragm is sealed with one side in the fluid to be pumped, and the other in air or hydraulic fluid. The diaphragm is flexed, causing the volume of the pump chamber to increase and decrease. A pair of non-return check valves prevents reverse flow of the fluid. This is typical of a positive-displacement pump;
2. As described above, the second type of diaphragm pump works with volumetric positive displacement, but differs in that the prime mover of the diaphragm is neither oil nor air; but is electro-mechanical, working through a crank or geared motor drive. This method flexes the diaphragm through simple mechanical action, and one side of the diaphragm is open to air;
3. The third type of diaphragm pump has one or more unsealed diaphragms with the fluid to be pumped on both sides. The diaphragm(s) again are flexed, causing the volume to change.

When the volume of a chamber of either type of pump is increased (the diaphragm moving up), the pressure decreases, and fluid is drawn into the chamber. When the chamber pressure later increases from decreased volume (the diaphragm moving down), the fluid previously drawn in is forced out. Finally, the diaphragm moving up once again draws fluid into the chamber, completing the cycle.

Some Applications Are:

1. Diaphragm pumps have good suction lift characteristics, some are low pressure pumps with low flow rates; others are capable of higher flows rates, dependent on the effective working diameter of the diaphragm and its stroke length. They can handle sludges and slurries with a good amount of grit and solid content.
2. Diaphragm pumps have good dry running characteristics.
3. Diaphragm pumps are low-shear pumps.
4. Diaphragm pumps can be used to make artificial hearts.
5. Diaphragm pumps can be up to 97% efficient.
6. Diaphragm pumps have good self priming capabilities.”

Note that two of the most important applications for this type of pump are missing from the above listing:

1. The pumping of hazardous and dangerous fluids – such as acids – in order to avoid any leaking glands or packing on rotating shafts or reciprocating plungers;
2. The pumping of fluids (especially explosive or combustible ones) in a highly dangerous location where any spark or ignition point has to be avoided; the use of air or hydraulic power makes this pump inherently spark-proof.

To accurately predict how your pump is going to perform, you must know which type of diaphragm pump you are dealing with. Note that your selection is probably type #1 listed above. This type of pump has the pneumatic fluid acting directly as the mechanism that contributes the impulse force directly. This is a big and important factor because it means you are using a compressible fluid to do the “pushing” – which implies that the compressible fluid will undergo some compression as it tries to force a NON-compressible fluid to move. This should explain why this “positive-displacement pump does not have a flat-line type of performance curve (where the capacity is constant, regardless of the pump’s head – well, almost constant).

Type #2 is one where the driving mechanism is mechanical – a cam, eccentric, or a piston. This version of the diaphragm pump is really a positive displacement type and its performance curve is almost a flat-line type. This is to be expected. The use of a mechanical force means that the displacement is positive, un-yielding, and the same – every stroke.

Regardless of which type of displacement you have, your process fluid (the sulfuric acid) will not vary in density as it goes through the pump. Only the liquid volume will vary, depending on the head it has to overcome. Obviously, since you have air being applied at a fixed pressure in the pump’s driving chamber, the acid being pumped cannot go quicker as its head is increased and approaches the same value of “head” (or pressure) as the driving air. Therefore, your pump’s capacity will diminish as the head is increased. But since the density remains constant (as well as viscosity), you should have no worries. The main problem you will face with your type #1 pump is that the displacement is varying in accordance with the head variance. This is one of the bad features of this type of pump. It is not an accurate-feed type of pump. If you require constant, accurate fluid delivery for varying head and capacities, then this is not the type of pump to use.

You can check out my comments with the manufacturer in order to make sure you have 100% correct understanding of what the pump really can do – and can’t do.

I hope I succeeded in answering your question but also in explaining as to why the pump behaves in the way that it does.

[teson1]

Dear Art,
thank you, again, for your time spent to help me in my problem.
I should probably have stated my problem in fullest detail earlier. Will do next time.

I didn't do that, because, frankly, I believed that "That must be a dumb beginner question that you find answered in any advanced engineering book" (to which I unfortunately have no access). But I get the impression that impact of fluid parameters on pump curves is actually not relevant to design questions with air-operated diaphragm and bellows pumps.
I have previously found no reference in an extensive web search either.

I think I'll stick with the default performance curve for my calculations.

Probably good enough for practical purposes, and who knows if any correction (if applicable) would give a more accurate prediction of real world performance.

Thank you and best regards.

PS: Pump is indeed of type 1. Actually an air operated doubble bellows pump. I don't have the pump.