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# Flow rate changes ?

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## Flow rate changes ?

(OP)
Assume we have a pump that discharges to two branches , branch 1 and branch 2 which are in parallel . branch 1 has total head of 5 bar and needs flowrate = 10 m3/h , while branch 2 has total head of 2 bar and needs flowrate = 20 m3/h ,, so we choose a pump of H= 5 bars and Q= 30 m3/h ... we know that branch 1 is the critical user as it needs more head ... Now when the pump starts , the 5 bars goes to branch 1 and reached the end user , but the 5 bars that get in branch 2 reaches the end user 3 bars as 2 bars is lost to over come head and losses as I said above .. the end user at branch 2 needs pressure only of 1 bar so we installed a restricted orfice to decrease the pressure from 3 bars to 1 bar ,, The question is does this decrease in pressure affects either the flow rate in branch 2 which is 20 m3/h ,, or even does it affects the pump curve ? ( take in consideration that branch 2 is not the critical path or not the path of the largest head).

### RE: Flow rate changes ?

Are you and anber attending the same class at university?

Look at the responses to his many posts and then come back to us.

Actually so long at the pressure drops from one end ( the pump) to the end user remain constant it doesn't really matter in that branch where you take the pressure drop.

In your case it is either 2 bar pressure drop at the start of the branch or 2 bar at the end of the branch.

so long as your pressure restriction doesn't change the flow rate then it should all even out.

This is based on the very limited data you've given us....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Flow rate changes ?

(OP)
who is anber ? I am a graduate ,, so you didn't answer my question about does it affects pump curve ? Also my question is does it affects flow rate and your answer is "as along as the orfice doesn't change flowrate" , so I really don't know wheither it changes flow rate or not

### RE: Flow rate changes ?

The pump curve is a characteristic of the pump, at a given RPM, only.

The pump's operating point lies, by definition, at the intersection of the pump curve and the system curve.

You need to generate the system curves for your system, in as many configurations as you wish to evaluate, and then, perhaps, the solution to your problem will become clear.

For N branches running in parallel, it may be helpful to compute a Cv for each branch, as if it were running alone, for an arbitrary flow, preferably not far from the flow you wish to occur in that branch.

You may combine an arbitrary number of branch Cvs to get a system Cv, provided that the branches are all running full in all conditions under study.

The usual definition for Cv, which goes something like

#### Quote:

The flow coefficient, or Cv, is a universal capacity index and is simply defined as “the number of US gallons of water per minute at 60F that will flow through a valve with a pressure drop of one psi. Sometimes “pressure drop” is described as pressure differential or “delta P”.
... is useless and misleading, because 1 is its own square root.

Remember that arbitrary flow, above? Calculate the pressure drop for that flow for each branch, then divide the flow by the square root of the pressure drop to compute a Cv for each branch.

I.e., Cv is NOT dimensionless; it has a dimension of gpm/sqrt(psi) .

Once you get a _system_ Cv, you can use it to plot the system curve, computing the system pressure drop for various pump flows, and superimpose it on the pump curve to get your operating points.

Mike Halloran
Pembroke Pines, FL, USA

### RE: Flow rate changes ?

anber is someone asking very similar questions in this forum - look at the other posts on the first page.

What you haven't expressed is what happens when the user in branch 2 receives fluid at 3 bar? Does the user drop 2 bar to get it down to the 1 he needs?

If so then installing an orifice which drops exactly 2 bar at the start of the branch will be the same as dropping 2 bar at the end of the branch, so no change in flow

As nothing has changed the pump will still deliver 5 bar at 30m3/h.

a pump curve will not change, but your position on it changes with flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Flow rate changes ?

Are they going through different types of valves? (reduced bore, vs full bore)?

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