Diaphragm pump maximum discharge pressure 2

[chemks2012]

Hello all,

I have one query on diaphragm pump and wondering if you could help me.

Please see the attached diagram.

We have a closed tank containing refrigerant at 30degC is being pumped to another tank via chiller [chiller is located just after the pump but chiller has not been shown on the attached diagram].

At 30degC, the vapour pressure of refrigerant is 5barg which I have shown as vessel pressure.

Query:

What would be the maximum pressure at the discharge of a pump? Will it be 7barg + 5barg = 12barg [ignore the suction line pressure loss] OR will it be just 7barg as the discharge pressure is limited by air compressor?

Your help would be highly appreciated.

Thanks
KS

 

[Compositepro]

It depends on the specific design of the pump. Most likely the output pressure is limited to the drive air pressure. But it is very easy to design in pressure intensification. Also many air driven diaphragm pumps are designed so the inlet pressure must not be above ambient pressure or the diaphragms will rapidly fatigue reversal of the flex of the diaphragm.

 

[chemks2012]

Compositepro,
Thanks very much for your reply.
Do you mean that, I can ask supplier to supply the diaphragm pump which has discharge pressure limited by the supply air pressure?
Is this is the reason why diaphragm pump curve has Discharge Pressure on Y-Axis and not the differential pressure unlike centrifugal pump?

Regards, KS

 

[Compositepro]

If air is directly driving the pumping diaphragm, then fluid inlet pressure will not add to the outlet pressure. This is the most common case. However, it is possible to have the air drive a piston or second diaphragm, which is linked to the pumping diaphragm by a rod. Then it is possible that the fluid inlet pressure is on one side of the diaphragm while the outlet pressure is on the other. In this case the inlet pressure would add to the outlet pressure. If the piston is a different area than the pumping diaphragm, then pressure or volume amplification is possible.

 

[chemks2012]

Compositepro
Thank you so much for your reply.
Will find out more from supplier.
Cheers

 

[StoneCold]

chemks2012
I don't know much about what you are trying to do, but my gut feeling is that an air operated diaphragm pump is not a good pump for this application. We use AODD pumps all the time, but we pump liquids not condensed gases, and usually at much lower pressures 1-2 bar total pressure. I think you need a tighter pump design. Like a mag drive centrifugal pump or rotary vane pump if you like. This looks like the wrong application for this pump to me.

Regards
StoneCold

 

[Compositepro]

I believe that the most common type of pump for liquid refrigerant would be a gear pump.

 

[chemks2012]

StoneCold,
Thank you very much for your reply. Agree with you. We address this to client but client is very confident that diaphragm pump would be OK based on their past experience. In fact diaphragm will pump refrigerant to the process vessel via chiller. Please note that chiller is located at the downstream of diaphragm pump. Chiller will reduce the refrigerant temperature from 30degC to -30degC. Also rotary pump will then pump refrigerant from chiller to process vessel. [PS chiller and rotary pump are not shown in that sketch].

Compositepro,

Thanks very much for your reply. I agree with your provious post wherein you said that most likely is that the discharge pressure is limited by the air pressure. But when you calculate what air pressure you need for diaphragm pump, do we not always calculate just like centrifugal pump i.e. differential pressure required [total discharge pressure required - NPSHa] and hence the air pressure/flow required? If this is true are we not saying, vessel pressure will affect the discharge pressure of pump?

What i believe happening here is...

NPSHa = liquid head + pressure above liquid - friction loss - vapour pressure
NPSHa = liquid head + 5barg - friction loss - 5barg
NPSHa = liquid head - friction loss

Therefore, the net effect of that vapour pressure on the pump is negligible and hence the major driving force is air in this case?