Air Fed Double Diapragm Pumps

[DonyWane]

If I have one double diapragm pump on a closed loop, I know that I can simply take the pump curves for the pump, use the air pressure curve that corresponds to my air pressure, and find the flow at any given frictional resistance.

Here's the question (somewhat spurred by previous discussion concerning pumps in series): What if I add a second air fed double diaphragm pump in series with the first? The resulting pressure obviously isn't purely additive because the outlet pressure can never exceed the set pressure on the air (correct?), even if the pumps are in series. Any suggestions on how one would calculate the resulting flow for a given frictional resistance?

Also, although pump spacing (distance between each) doesn't matter on centripetal pumps - does it have an impact on double diaphragm pump systems?

 

[abcmex]

My opinion:
1. The pressure will add? When the suction pressure of the is higher than the comp. air the pump will probably freeze. Fluid may flow still but no function will be achived by the pump.
2. Is the location important? If the distance (pressure drop) is big enough the second pump could work.

I hope it helps
Rgs

 

[DLANDISSR]

DonnyWane:

Right on abcmex!
The pressure will always be additive with pumps in series.
The problem with diaphragm pumps is that since the pumping is not continuous but cyclic the cycle of each pumps many not be in sync with eachother. The suction pressure of the second pump will not be constant. To avoid this problem a pulastion dampner could be added between the pumps.

If the pressure at the inlet of the second pump above the air pressure it will not pump.

Since the pressure at the inlet of the second pump is higher than the suction at the inlet of the first pump the second pump will not cycle as fast.

In Centrifugal pumps the flow varies with the TDH across the pump. With air powered diaphragm pumps the flow varies with the difference between the average pressure across the pump and the air pressure. Viz; given the same pump if the average pressure is 20 psig and the air pressure is 100psig and the average pressure is changed to 50 psig with the air pressure at 100psig the pump will cycle slower with the increased pressure and therefore will pump a lower volume.

Putting two air powered diaphragm pumps in series is not a good application.

 

[zdas04]

You can buy double-diaphram pumps with different sized diaphrams between the two chambers. This will allow higher discharge in one stage if you can find a ratio that gets you to the discharge pressure you want (e.g., if the driver diaphram is, say, twice the size of the pump diaphram then the discharge pressure can reach almost twice supply-air pressure).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[DonyWane]

I was under the impression that the maximum discharge pressure in an AODD pump was the air pressure. Your comment about differnt sized diaphragms does make some snese - I had never thought of that arrangement before. But - wouldn't the second stroke be decreased in pressure by half?

Also, for a standard AODD pump, if the pump curve dictates that the discharge pressure wil be 50 psig - the discharge pressure will be 50 psig regardless of whether the supply pressure is 5, 10, or 15 psig. I know that if the pump is forced to lift, this will cause less flow at a given discharge pressure, but positive supply pressure doesn't necessarily mean a higher discharge pressure. Is this a misunderstanding?

Thanks!

 

[kcj]

yes, dischargre pressure will be max of 50 psi inthat case. What previous posts refer to is that if inlet is >50 psi, the outlet is not 95, but 55. The second pump would stall in the retracted position, the water goes on through the checks, but the air diaphragm cannot push against the >50 psi fluid pressure.

Centrifugal pumps are based on dynamics, motion and weight of fluid. AAOD are based on statics, balance of air & fluid forces.

k