Centrifugal Pump Discharge Pressure

[JLooking]

Hi all, I would love it if someone could give me a couple of pointers. I'm selecting a centrifugal pump for a pump skid. The pump discharges at about 100 psi (70 meters of head)from a 1 1/2" outlet. I then need to reduce up to a 3" line size. Does the change in line size (1 1/2" to 3") affect the pressure, i.e. if I neglect losses due to friction and change in pipe diameters, will the change in line size noticeably change the pressure? The reducer is placed fairly close to the discharge side of the pump. Sorry if its a silly question, I'm a bit of a pump novice.

Regards JL

 

[JLooking]

Oh,

Water (S.G. 1.0) at ambient temperature,

 

[BigInch]

The pump discharges at 70 meters of head, at a particular flowrate. If your piping can match that flowrate, with 70 meters of head (100 psi) as its inlet pressure, the pump won't care what diameter it is.

Designing the downstream pipeline will require that you to size its diameter(s), using its required length, using that same pump flowrate, an inlet pressure of 100 psig, and some currently unknown outlet pressure at the end of the pipeline. Make the diameter is large enough that the outlet pressure is at least above zero psig, and also high enough to do whatever you need to do with the fluid after it leaves the pipeline. For example, if you need to fill a tank to a level 10 meters above the pipeline outlet, your outlet pressure must be at least equal to 1 barg.

Independent events are seldomly independent.

 

[Latexman]

Pop your data into the Bernoulli equation (google that) and find out how much the pressure increases.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[JohnGP]

JLooking,
You're looking at it too narrowly - you need to examine the complete system to see what effect individual components have.

The pump will "move" out along its Head/Flow curve for its given impeller diameter and speed until it finds an operating point dictated by the system. This point is where the system resistance curve meets the pump curve. You need to develop a system resistance curve, which will include static head, frictional losses, and any pressure requirement at the outlet. You can change size and type of fittings and pipe size to generate further system resistance curves to your heart's content, until you determine where you want to operate, or to select the pump with the best fit.
Cheers,
John

 

[LittleInch]

JL, it is interesting that you all you talk about is the pump and not the system it is feeding.

What you need to be able to do is say "I have a requirement for 100psig into my system at x gpm or m3/hr which I need a pump for.

You appear to be doing this the other way around, which is not the best way.

In answer to your question, it will make a very small difference in pressure due to velocity head, but unless the requirement for 100 psi is based on 1 1/2 " pipe and you're replacing it with 3" pipe, you won't see any difference in pressure for a flat straight pipe.

My motto: Learn something new every day

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

 

[Artisi]

Just adding to JohnGP closing comment " select the pump with the best fit" -- together with the most economical pipline / pump operating costs

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]

L" I think I did mention the system, especially the length.

JLooking, Replacing a 1.5 inch diameter with a 3" could make quite a bit of difference, esp. if the length is long. If the length is short, maybe not so much. A 3" line has a much larger flow area, so velocity is reduced. Reduced velocity means reduced friction and that usually means less head required at the pump. Your pump discharge pressure may drop and flowrate may increase. It all depends on the length of the 3" line and any elevation difference, along and at the end of the line, and any backpressure there from a partially closed nozzel, or valve, or high water level.

Independent events are seldomly independent.

 

[Artisi]

friction loss changes as D1/D2^5 (use actual diameters not nominal sizes)

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]

BI, I didn't say you didn't mention the system, but we're all now guessing about what JL's system design is or isn't.

So come on JL - is your system fixed and you need 100 psig to get the flow you want and you were just asking a straightforward question or are you going around changing things??

My motto: Learn something new every day

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

 

[JLooking]

Thanks for the responses. I'd have given more information about the system but its one of those cases where no one knows (more precisely, its going on a pumping skid and each site could be different). The discharge could go to a 6" or 3" line and the distance of this could be close or a long way away. That scenario could change over time. I initially looked at using a VFD but am now looking at a soft start with a throttling valve. The only parameter I've been given is that the pressure can't exceed 55 psig when it hits the line it feeds into (3" or 6", long way away or close by). Its unlikely that any elevation exists on the discharge side. The feed is fed from another low pressure pump at around 600 - 800 L/min. Anyway, the original question was just a curiosity. I assume as you step up a size the velocity slows and pressure increases, which may affect the required throttling, and maybe the throttling valve may be better placed in the 3" line over the 1/5" discharge (assuming I reduce to the 3" line on the skid).
Thanks guys