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Parallel Pump Installation: Estimating Friction Head

10/13/2011 4:01 AM

Hello Colleagues,

For 2 identical pumps in parallel, in estimating the total Head of a pumping system which includes both static height and frictional losses in the pipes the flowrate is a key factor in the Hazen-Williams formula for estimating friction head.

The question is, in estimating the Total suction head and Total discharge head, which flowrate will be applied, is it the normal flow of each pump or the numerical addition of the flow of both pumps?

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#1

Re: Parallel Pump Installation: Estimating Friction Head

10/13/2011 4:19 AM

You must use the sum of flows from both pumps, but they will shift to the left on their respective curves so that that the total flow is somewhat less than twice the flow of each pump alone.

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#5
In reply to #1

Re: Parallel Pump Installation: Estimating Friction Head

10/13/2011 3:04 PM

sounded familiar, had to search

but it was in series instead of parallel

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#7
In reply to #1

Re: Parallel Pump Installation: Estimating Friction Head

10/14/2011 5:20 AM

GA

I would add: For the suction side, the flowrate of individual pump will be used since there must be individual pipe runs (in most cases) to prevent suction problems between the pumps, when running in parallel. Also, Check valves will be added on the discharge side (Normally), for each pump ...

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#2

Re: Parallel Pump Installation: Estimating Friction Head

10/13/2011 8:23 AM

Yink....

The answer to your question is: It depends on how the plant owner is going to operate the pumps

It has been my experience that many, many installations with two pumps are configured for operation of one pump at a time.

This is also called an "installed spare" configuration.

The pumps are typically designated as "A" and "B".

Remember that the System Curve, for the piping system will be completely different for single versus double pump operation.

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#3

Re: Parallel Pump Installation: Estimating Friction Head

10/13/2011 10:34 AM

Tornado is correct, you need to use a little less than the sum of the flowrates from both pumps to estimate flow. You can look at the system entirely independently of the pump(s) when calculating the friction losses and total head. It doesn't matter if it is one pump, two pumps or multiple pumps, a given system will have given friction losses, depending on flowrate and viscosity.

However, bear in mind that you won't know the flowrate until you know the TDH, and you won't know the TDH until you know the flowrate. You should put together a system curve by calculating losses and TDH at several flowrates, and then match this to the composite performance curve (which also has to be estimated) to find the duty point on the curve(s).

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#4

Re: Parallel Pump Installation: Estimating Friction Head

10/13/2011 10:48 AM

You first need to make a system curve - (total heads at different flow-rates)

Take any lowish flow and calculate your total head.

Now repeat the operation by incrementing the flow.

Plot this graph.

Now double the flow of the pump (or add when different) at various total heads.

Superimpose on the system curve.

Where the two graphs cross is your duty point. (Magic)

Or go to my profile and get yourself a demo. (One can mix and match any pump setup)

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#6

Re: Parallel Pump Installation: Estimating Friction Head

10/13/2011 11:23 PM

1. What is the desired flowrate?Add some capacity (20%?) for future demand. Use this as the basis of design. Regardless whether you are running one pump or more.

2. If the two pump are existing pumps - need to determine the line size and piping configuration that meet the design flowrate. Need to develop system characteristic and pump characteristic.

3. If pumps are new and not yet selected - Use optimum pipe diameter or the maximum allowable velocity, determine the line size. From the line size and piping confuguration determine the head required and select the right pumps that can deliver the required flowrate.

Hope the above helps.

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#8

Re: Parallel Pump Installation: Estimating Friction Head

10/14/2011 5:36 AM

I think some more details are required in your quiry to reply you correctly. The reply of Tornado is correct. When two pumps are running and discharging through a single commen discharge line, due to higher flow the friction losses will be more as compared to the case of flow from one pump.The increased friction loss at higher flow will reduced the total flow of two pumps in parallel.

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#9

Re: Parallel Pump Installation: Estimating Friction Head

10/14/2011 11:17 AM

Thanks all,

The pumping system actually consists of three pumps and the operation philosophy is 2 pumps running with the third as a spare. The drivers are diesel engines for firewater service

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#10

Re: Parallel Pump Installation: Estimating Friction Head

10/16/2011 7:48 PM

You may have to spilt the system into sections and nodes for the longest route, calculate the friction head for each section between nodes and then add them all up for the system friction resistance.

Now, if the section between nodes is for single pump use single pump flow, and if the section is the combined header then use combined capacity of both the pumps.

You will come up with a system resistance curve.

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Hendrik (1); Holzfeller (1); LAA_Lucke (1); MJCronin (1); Mukesh0861 (1); phoenix911 (1); sabriahmad (1); sv13 (1); Tornado (1); Yinka Akinkunmi (1)

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