Requierd NPSH for Diaphragm pumps
Requierd NPSH for Diaphragm pumps
(OP)
Hi everybody
I have a little problem with required NPSH of hydraulic driven diaphragm pump. Based on pump calculations available NPSH for this type is about 1m for Q=10m3/hr & delta p=90 bar. Searching milton roy and lewa products i couldnt find suitable pump for my application. Can anyone help me? Are diaphraghm pump suitable for low NPSH application? What kind of pump are suitable for low NPSH application in PD pump category?
Thanks
I have a little problem with required NPSH of hydraulic driven diaphragm pump. Based on pump calculations available NPSH for this type is about 1m for Q=10m3/hr & delta p=90 bar. Searching milton roy and lewa products i couldnt find suitable pump for my application. Can anyone help me? Are diaphraghm pump suitable for low NPSH application? What kind of pump are suitable for low NPSH application in PD pump category?
Thanks





RE: Requierd NPSH for Diaphragm pumps
In this case I think you should increase your available NPSH, once you couldn't find suitable pumps for this application.
But why your available NPSH is so low ?? If we consider suction from an atmospheric tank, for example, you have round 10m of available NPSH.
RE: Requierd NPSH for Diaphragm pumps
EngMecBra: Your Statement "If we consider suction from an atmospheric tank, for example, you have round 10m of available NPSH" is only valid for water at 20 °C, flooded suction, no altitude and a very short suction line with no bends, valves etc.
RE: Requierd NPSH for Diaphragm pumps
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Requierd NPSH for Diaphragm pumps
Pumping fluid is hydrocarbon condensate and these pumps are downstream of a 3 phase separator.
Thanka
RE: Requierd NPSH for Diaphragm pumps
But you can keep trying - you never know - guess there are other manufacturers.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Requierd NPSH for Diaphragm pumps
The process guys are probably correct, the liquid will be at vapour pressure at the liquid level in the separator so your nozzle is probably about 1m below and a bit of friction losses in the nozzle and a few elbows.
Your only option is to create head by lowering the inlet of your pump.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Requierd NPSH for Diaphragm pumps
The NPSHr for a positive displacement pump such as a hydraulic diaphragm pump is mostly associated with accelerating the fluid into the suction on each suction stroke- an "acceleration head" as some call it. The consequences of inadequate suction head are capacity loss rather than cavitation-related damage except in extreme cases or very large pumps. A pulsation dampener located immediately at the suction of the pump can solve the acceleration head problem if designed properly.
Also, there are numerous ways to solve the problem of inadequate NPSHa, and not all of them involve putting the pump in a pit! The most obvious, especially if the stream is of a low flow, is to reduce the temperature and hence reduce the vapour pressure of the flowing fluid. Given that p* is a strong function of temperature, you don't have to drop temperature much below saturation to get a fair bit of NPSHa.
RE: Requierd NPSH for Diaphragm pumps
The OP needs to compare all the available soultions and not get too fixed on one particualr option before the feasibility and cost of them is known.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Requierd NPSH for Diaphragm pumps
RE: Requierd NPSH for Diaphragm pumps
I would still like to see who makes pumps able to handle such low NPSH for future designs so that they can be considered as a option. Do you have any links or suppliers who make this sort of unit or does it mostly need a damper as well?
I've managed to use various pumps with NPSHR of 2-3m, but not 1, so any info gratefully received.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Requierd NPSH for Diaphragm pumps
@ moltenmetal
Based on Exxon mobile practices our process guys consider acceleration head to be zero for diaphragm pumps.(?)consider that liquid will be at vapor pressure at liquid level.
In this case in first step we don't interest to change NPSHa, we want to know is there any diaphragm pump with NPSHr less than 1m?
A hydraulic driven? Or air operated diaphragm one? Margine of NPSHr for diaphragm pump? 5m(+|-)?
In second step, we think to change of NPSHa by solution such as move to can type pump or lowering suction nozzle putting inside a pit. You should also know our condensate is sour and it probably May cause a HSE problem.
Is there any can type pump with Q=10m3/hr and delta p= 90 bar?
RE: Requierd NPSH for Diaphragm pumps
Suction Acceleration head can only be calculated with the pump stroke length, Number of heads and stroke per minute known.
RE: Requierd NPSH for Diaphragm pumps
The acceleration head can be neglected with an appropriately sized pulsation dampener on the suction side as already mentioned by moltonmetal.
Lewa and Milton Roy are definitely a good choice although bloody expensive for 10 m³/h and 90 bar as they are more specialized in metering pumps. GEHO and Abel Pumps cover these diaphragm pump sizes better but I'm sure they will refuse to bid for 1 m NPSHA. And GEHO is more a mining pump supplier.