selection of booster pump in order to provide NPSHr for main pump

[mirghaffari]

hi every body,
we want to select the booster pump to supply the required NPSH for main pump.

main pump:

NPSHa = ATM
NPSHr = 5bar
suction pressure = ATM
Discharge pressure = 37 bar
what shall i considere to select the booster pump for this application?
1- i think that the flow of booster pump shall be equal to main pump, what a bout the head of booster pump?
2- when these two pump operate in series, can we sure that they work in compatible with other?
3- is it possible we select the screw pump as booster pump since our main pump is centrifugal?

regards,

 

[micalbrch]

To be safe you should ask the main pump's manufacturer.

Do not mix up pressure and NPSH. That are two different things.

 

[JJPellin]

It is not meaningful to state that NPSHa is atmosheric. NPSHa is in units of feet (or meters). The units are absolute pressure converted to head. To say it is atmosheric suggests 32 feet (about 10 meters) which is quite a lot. NPSHr of 5 bar is also not meaninghful. The units are not correct. Do you mean to suggest that the pump has a net positive suction head requirement of about 50 meters? This seems unlikely.

Johnny Pellin

 

[mirghaffari]

thank you JJpellin and you are right, this is my mistake, the units shall be changed to meter. but my problem is provide the sufficent head for our main pumps in oil pipeline, in our basic design we have a booster pump with 5 bar discharge to meet our requirment for main pumps.
first of all i want to know what shoud i considere to select the booster pump? flow,head,speed,power and ....?
becuse of low NPSHa for bosster pumps we have a option to select screw pump as bosster pump. do you recommend screw type pump as a bosster while our main pump is cenrifugal?

 

[shahyar]

The critical case is usually pumps in parallel. For pumps in series you only need to make sure they have the same flowrate. The discharge pressure for a booster pump is usually assumed as seven times of NPSHR(in gauge) or 50 psig. I have never heard about any limitation on impeller speed.
You can also use a PD pump(e.g. Screw type) as booster pump, as long as considering its pulsation. It means you need to check with manufacturer to see the minimum discharge pressure.
Hope this help

By the way, don't forget to put PSV upstream of your main pump, and its set pressure should be minimum of yield pressures of either of two pumps' casings.

 

[Pumpsonly]

A screw pump or any P must not be used up stream of a centrifugal pump. A screw pump will have no or very little flow variation in changing differential or down stream pressure pressure. Where else a centrifugal pump can have very high flow variation with small changes in differential or down stream pressure.
The screw / PD pump will behave like a throttling valve and cause cavitation when the CP is trying to push more flow through the pipe line when there is a drop in down stream pressure.

The booster pump must be sized to have the same capacity range as the main pump with the discharge pressure/ head to provide adequate NPSHA to the main pump + the head losses on the piping between the 2 pumps.

 

[mirghaffari]

thanks shahyar and pumpsonly,
i have a same idea as pumpsonly.it is very difficalt to use PD pumps as booster. but in case of emergensi, can we use screw pump with considering the by-pass flow for screw pump?. for example when main pump work in low flow,we by pass some flow of booster pump to downstram? do you think any more criteria or problem that my be happen during operation?

 

[shahyar]

I am sorry!I made a mistake, Pumponly is right, PD pumps can not be used as booster pump for a centrifugal pump.
I mixed up because the other way around is right, I mean a centrifugal pump as booster pump for a PD pump(Ref: Karasski section 3.81).

Sorry again for misdirecting.

 

[mirghaffari]

thanks a lot,
i think the only problem is flowrate when we use PD pumps as a bosster pump. with considering the by-pass line from dischare of PD pump to tank, can we solve this problem? is there any more problem except flowrate?

thank again

 

[Pumpsonly]

mirghffari
re your last posting, you will nave to over size the PD pump to cater for the maximum possible flow of the CP.Why are you so insisting of using a screw pump? Some Progressing Cavity (PC) pump sales guy might have told you a PC pump can be used a booster because of the low NPSHr.

 

[mirghaffari]

ok thanks for your help