Requierd NPSH for Diaphragm pumps

[Sabergg1981]

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

 

[EngMecBra]

Sabergg1981,

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.

 

[micalbrch]

Typical NPSHR values for diaphragm pumps are around 5 m (+/-). An NPSHA of 1 m is definitely too low, especially when you consider that NPSHA must be higher than the NPSHR of the pump. What kind of fluid do you want to pump?

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.

 

[Artisi]

I would suggest that EngMecBra's comment was more than likely a leading question endeavouring to gleam a little more information from the OP rather than a statement of fact. But anyway it would be interesting to know why only 1m NPSHa

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.)

 

[Sabergg1981]

1m NPSH a is based on process dicipline calculations. Before challenging with them i want to know is there any diaphragm pumps with required NPSH less than 1 m?
Pumping fluid is hydrocarbon condensate and these pumps are downstream of a 3 phase separator.
Thanka

 

[Artisi]

Seems you may have your answer - NO. confirmed by micalbrch, Milton Roy and Lewa.
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.)

 

[LittleInch]

I don't know of any type of pump that can handle 1m NPSHA. Pumping condensate like this needs a can pump or a deep pit to locate your pump.

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

 

[moltenmetal]

I positively disagree. There are MANY hydraulic diaphragm pumps which have NPSHr far less than 5 metres! And we're frequently in the position of having to move fluids with only 4-6' of NPSHa.

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.

 

[LittleInch]

It (increased depth ot the pump inlet) might not be the only way, but it's reliable and normally fairly cheap. Handling condensate straight out of a separator is not easy as it is very volatile and flashes very easily, but doesn't always re-combine when pressurised. It would be nice to know what the inlet pressure is to this thing.

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

 

[moltenmetal]

Sorry LittleInch, but even in a new design situation, raising the tank or dropping the pump may not be a feasible or inexpensive solution. In our modular systems, it's frequently an enormous headache.

 

[LittleInch]

There's no need to be apologetic, in your case the height solution isn't available so then you move onto other systems - we have no idea what the OP's situation is - my post was just to point out that there were other solutions available, but I wasn't aware of units that could handle this sort of volatile fluid at that low an NPSH.

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

 

[Sabergg1981]

I should mention our suction pressure is about 20 barg.
@ 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?

 

[Pumpsonly]

Use a VS6 pump as a NPSHA booster to the diaphragm pump. Lewa and Milton Roy do not made hydraulic driven pump. They are motor driven using hydraulic oil between the diaphragm and plunger to transmit the pressure to the pumped fluid.
Suction Acceleration head can only be calculated with the pump stroke length, Number of heads and stroke per minute known.

 

[micalbrch]

I think what OP means with "hydraulically driven" is "hydraulically actuated" with a liquid (oil or an emulsion) between piston/plunger and diaphragm.

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.