NPSH - Priming - Diaphragm vs Lobe

[DrMarvin]

Hi Folks,

I have the following situation and was expecting maybe you can help me out.
I have to pump a viscous fluid (1000cps) out of a 200L drum and up to a mixing reactor. I have been discussing with my partners with regards to the ability of a air operated diaphragm pump and a lobe PD pump to extract the fluid in a condition of suction lift. What would be the parameter I need to evaluate to know which pump will better pull (or will not) the liquid from dry condition?
Some of my collegues tell me I need to calculate NPSHa vs NPSHr. I think that calculation will tell me about operating condition once the pump is filled with liquid; but will not tell me about the ability to pull from dry condition. Am I correct? (i.e. a centrifugal pump may have an NSPHr of 5ft; be installed 1ft above water and will never start pumping on its own; unless I fill it with water...)
The manual of the Wilden pump I'm evaluating quotes the MAX SUCTION LIFT (dry and wet; 11.5ft and 30.8ft respectively) What is this? Does it mean that: geometric suction lift+friction loss+vapor pressure < max suction lift?

Thanks for your help in advance.
Best regards
DrMarvin

 

[Artisi]

Dr ???
First up you need to understand that pumps don't "pull" - a pump unit is capable of lowering the pressure in the inlet side / pipe and atmospheric pressure then forces the liquor into the pump.

I would do a search on the net for drum transfer pumps to start with.

 

[DrMarvin]

I know pumps don't "pull" liquid; it was just a figure of speech...sorry. With regards to your suggestion, I know for sure that pneumatic diaphragm pumps can transfer liquid from a drum starting from dry condition...what I would like to understand is what makes a pneumatic pump capable of doing that, while a centrifugal pump is not? Thanks

 

[Artisi]

Diaphragm pumps are capable of pumping air which allows them to "self-prime" ie, exhaust any air within the inlet line and pump casing lowering the pressure on the inlet side, this allows atmospheric to force product into the pump whereas a centrifugal pump, specifically on a suction lift condition needs to be charged with liquor before pumping can commence.

 

[powerplantpumps]

HI, I WANT TO CALCULATE THE npsh AVAILABLE FOR A CONDENSATE EXTRACTION PUMP. BUT I M FACING LOT OF DIFFICULTIES IN HOW TO APPROACH TO IT AS IT WORKS UNDER VACCUM.

 

[TenPenny]

NPSHa for a system under a vacuum is no different from NPSHa under a pressurized system. You have to work with absolute pressure, so it makes no difference to the calculations.

 

[Artisi]

powerplantpumps (Mechanical) 16 Jun 08 12:00
HI, I WANT TO CALCULATE THE npsh AVAILABLE FOR A CONDENSATE EXTRACTION PUMP. BUT I M FACING LOT OF DIFFICULTIES IN HOW TO APPROACH TO IT AS IT WORKS UNDER VACCUM

There is no need to shout - we can all hear you ok and it's ok to use lowercase type - PLUS start a new thread for a new question - that's if you want answers to your question.

 

[HEC]

DrMarvin
The diaphragm pump is a positive displacement pump as is the rotary lobe pump. Positive displacement implies that there is a cavity that positivly expands and contracts to move the fluid through the pump. A centrifugal pump on the other hand does not have such cavities. Centrifugal pumps (and fans) rely on throwing the fluid out into a volute using a spinning disc (very crude description).

Back to the OP; A diaphragm pump generates the suction by expanding a cavity that is sealed using a non return valve. The pressure stroke is generated in a similar fashion by compressing a cavity using a non return valve. A rotary lobe pump on the other hand moves fluid around the outside of the "gear mesh". "Suction" is generated by the opening gear mesh and the "pressure" discharge is generated by the closing or meshing gears. In the case of the diaphragm pump back flow is prevented by a non return valve in the case of the lobe pump it is the mesh clearances of the gears. Intuitivly the meshing gears will allow air (vapour) to pass back through them and therefore limit the amount of "suction" it can apply. Hence the diaphragm pump will be more likely to self prime.

In relation to the fluid you are trying to lift or prime if the suction pressure drops low enough it will boil and only vapour will be drawn to the pump. Unless the pump can pump the boiled vapour away faster than it is being generated the pump will not prime regardless of the pump type.
Regards

Mark Hutton