If you put too much NPSH on an AODD, it fails prematurely, in my operating experience. Typically you're limited to 10 psig or so. You can put a regulator on the inlet to limit it but I'm still not sure if an AODD makes sense.
This sounds like a potentially dangerous application of an air operated double diaphragm pump to me. If the diaphragm ruptures, the propane (or whatever the liquefied gas is, I'm assuming it's flammable, toxic, or Oxygen displacing creating a personnel hazard), may leak at a relatively high rate. Exactly what is being pumped here? Might I suggest you consider a Blackmer, Dover, or similar rotary sliding vane pump (I'm not pushing a specific brand here), e.g. a liquid propane pump? These are available with air-driven motors and are relatively inexpensive due to the large number in service for LPG delivery trucks and gas grill/forklift/LPG cylinder filling stations. Also, they will be rated to several hundred psi, unlike typical air pumps, which are usually good to only 125 psi or so for a standard model. Also, I would consider using LP hose, grounded, and use ACME LP threaded fittings and NOT cam & groove hose connections -- use whichever fittings/hoses are recommended for that LPG. At least, this is what I preferred using when pumping flammable liquefied gases in specialty chemical manufacture. When propane leaks, it's scary. If you're pumping what I think you are, that would be safer. You cannot afford a hose end ejection in this case, especially if this is a large pump. The Blackmer can leak at the seal but in my opinion that's typically a lot lower leak rate than what you may see out the muffler of an AODD. I'd also make sure air line has a check valve in it to prevent this process fluid from entering the compressed air system and contaminating it, which might also be devastating. Keep in mind that, as a pump's inlet pressure limit is approached, diaphragm life is drastically reduced. Inversion of the diaphragm can occur at 25-40 psig, even with rubber diaphragms and about 4.3 psig with PTFE diaphragms without back-up diaphragms. Yamada recommends limiting inlet pressure to a max of 15% of discharge pressure in their excellent Yamada AODD Engineering Handbook, which is available free online. A suction dampener can help with these problems but with the limited information thus far provided, this still seems like it may not be a good application of an AODD.
As for actually answering the question, the deadhead pressure will be approximately equal to the air pressure; see a pump curve for the specific AODD. Some specialized ones have a booster arrangement, and can do, say, 3:1 ratio, but that is an exotic pump. That is, specialized pumps boost the air pressure, say, from 30 psi, to 90 psi. Not that this does NOT mean the process side is boosted. If you watch a youtube video of a Wilden or other pump, you'll see why. In the course of a cycle, the fluid is drawn in, stopped, and accelerated out through discharge check valve. That is where acceleration head also enters into calculations for PD pumps.
The reason a centrifugal pump boosts pressure, is the fluid is not stopped and reversed, as it is in a piston or diaphgram pump. And yes, there are [rotary] PD pumps that boost pressure, e.g. iquiflo gear pumps, that is why their specs, will typically indicate both a max differential AND a max discharge pressure; the casing can only take so much pressure.
In light of all these issues, a diaphragm monitor in conjunction with an automated shutoff valve may be advisable depending upon the liquid being pumped depending on the specifics of the application, i.e. IF an AODD is appropriate.
I believe gravity flow would occur through the idle AODD at high suction pressures. I don't know what the internal check valves crack at so not sure how much flow & at what pressure it starts? That is something I've not seen but would expect.
Edited by fseipel, 20 July 2014 - 05:25 PM.
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