combining pumps 3

[mectarek]

we have at my work place pumps that have different outlet pressure, 11 bars and 8 bars pumps

I want to know if there is a methode to combine the pumps together (parallel) in the same pipeline

 

[bimr]

You don't give a lot to go on.

Assuming that you are referring to centrifugal pumps, the head/ capacity curve for a centrifugal pump is supplied by the pump manufacturer. The curve describes the relationship between the head and capacity of the particular pump. As you look at the curve, note that the BEP (best efficiency point) is located somewhere between 80% and 85% of the shut off or maximum head. To maximize the life of the pump you should operate the pump as close to the BEP as you can.

If the pumps are running at the same time, the capacities are additive at the same head. The resultant curve gives a new intersection point on the system curve for the combined capacity.

With parallel pumps with different pressures, you will not be operating the pumps at the BEP. Running different size pumps in parallel is probably not a good idea because the larger pump will cause the smaller pump to be throttled forcing the smaller pump to run too far off of the BEP. This can cause shaft deflection and possible premature bearing and seal failure.

Refer to the attached article.

http://www.mcnallyinstitute.com/15-html/15-01.htm

 

[LittleInch]

Bimr is correct and you don't give a lot of details about your system.

However in direct answer to your question the answer is yes, but would require your 11 bar pump to be throttled to reduce its pressure to something similar to the 8 bar pump. Due to the extra friction effects, it is likely that neither pump will operate at its"rated" flow, but by some experiment by reducing the pressure from the higher pressure unit, you will find the optimum flow which allows both pumps to work to their best efficiency and capacity.

Give us some more details about the pumps and system and you'll get a lot more back. Details such as flow, power, pipe size, static h head, flow diagram, things like that...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mectarek]

LittleInch

How can the 11 bar pump be throttled to reduce its pressure to be close to 8 bar?..I'll provide more details as soon as possible

 

[LittleInch]

By inserting a control valve on the discharge pipe and then closing it gradually.

More details would be good.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[JohnGP]

mectarek,
Throttling as LittleInch suggests is intended to reduce the pressure after the control valve to a value matching the discharge pressure of the "8 Bar" pump at its operating point. Depending on your actual requirement, "control valve" can be anything from a full control valve with the ability to automatically adjust the downstream pressure of the "11 bar" pump, or a manual valve you can adjust and leave, or even a restriction orifice if the pump doesn't have other duties to perform.

 

[Artisi]

The performance curves for both pumps would be a good place to start with an analysis of your inquiry.

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.)

 

[BigInch]

Everybody is seeing this from the typical perspective.

If each pump had a BEP at the resultant individual flows in each pump, of the combined flow through the system, then each pump would operate at their individual BEP.
In other words...
If the designer had enough forethought to simulate these pumps using curves without the BEP points marked, s/he would see the flow in each pump and the head produced by each pump and could then order each pump with the exact BEPs corresponding to the head and flow seen in each pump during the simulation.

Independent events are seldomly independent.

 

[JohnGP]

No, I took it that they had these two pumps already and had this duty that neither pump could handle alone, and someone said "Let's put the two pumps in parallel!". Or the duty changed, and they had another pump. Not sure, OP could explain maybe.

 

[LittleInch]

The phrase "we have" implies existing units, but as usual we have baout 10% of the rquired information so wait for the OP to come back with some more info.

It is going to be very unlikely that either pump will be at the BEP or indeed the "rated" flow as most of these options ignore the increased head loss through the system. Some person sees a "500" gpm pump and a "400" gpm pump and thinks Ah ha I can get 900 gpm.... Errr

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mectarek]

JohnGP,LittleInch
yeah that's exactly true. We already have those pumps and we have a duty that neither pump could handle alone

I apologize for the lack of information. I can't provide them now. maybe in two or three days.
and thank you for the help

 

[DubMac]

For starters, would be helpful to know wherther they are centrifugals or positive displacement pumps (recips/rotaries). If you can't find pump curves, the rated duty points would work.

 

[BigInch]

OK, my comment was meant to note that where the BEP points are make no difference to the hydraulics.

Independent events are seldomly independent.

 

[Artisi]

This same question seems to come up on weekly basis, nearly always with scant information. A quick search in Eng-Tips would probably answer 99% of the questions or else a search on the internet where it is well documented over and over.

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.)

 

[DubMac]

We may have to start asking each other questions.

 

[Artisi]

Well hopefully they will be of interest.

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.)