VFD Pumps
VFD Pumps
(OP)
Hi guys, can nay one help me , that I have pumping station for large irrigation system controlled by VFD with 7.5 bar designed pressure, but i am going to reduced the pressure at 5 bar from 7.5, due to some problems, will any impact on flow and velocity and will i get the same flow as on 7.5 bar





RE: VFD Pumps
I don't think that really matters much, though. If you reduce the pressure, the flow rate will go down. Of course, flow rate is
proportional to velocity (Q=V/A).
If you're interested in the actual amount of flow, that you get, you'll probably want to do a flow-network calculation. Look into the Darcy–Weisbach equation for doing that sort of calculation.
Here is a link to a famous paper by Lewis Moody that discusses how to determine friction factors for pipes :
Friction Factors for Pipe Flow, L.F.Moody
The Moody chart is used often in these sorts of calculations. Losses due to bends, sudden expansions, sudden contractions, etc can be found in Crane Technical Paper No. 410 (or other places).
Interestingly enough, fluid flow calculations have been of interest for a very long time. The Romans had methods of estimating the required sizes of aqueducts (mostly based on experience). I suppose you don't go through the effort of building an a 30 mile long aqueduct without knowing if it's going to work, eh?
RE: VFD Pumps
RE: VFD Pumps
Ted
RE: VFD Pumps
Pressure and velocity are not inversely proportional. The pressure drop is roughly proportional to the velocity squared.
See the Darcy-Weisbach equation: http://en.
To know the true change in flow rate, however, you'll want to do a flow network analysis, considering all of the minor losses (bends, valves, etc) and how the flow will branch through different pipes. There's software available to do this (Flowmaster, etc), or you can write your own.
RE: VFD Pumps
RE: VFD Pumps
RE: VFD Pumps
RE: VFD Pumps
how are you reducing the head? By changing the system or by changing the controller pressure setpoint?
Ted
RE: VFD Pumps
If something changes about the system (part of the irrigation system is turned off, for example) the flow rate will change, however, the controller should maintain a constant pressure at the pump outlet. This is desirable if you're turning parts of the system off and on.
If you had a speed controller and the pump ran at a constant speed (assuming a positive displacement pump), the flow rate would be constant. However if something changed downstream, the pressure at the exit to the pump would change (up to a limit, of course - eventually you break a pipe or stall the pump).
So, I suppose I'm not sure exactly what you're asking. A pump will generally either supply a pressure or a flow rate. It sounds as though your setup has a computer-controlled supply pressure.
If your pump is supplying water at a given pressure, the downstream piping will determine how much water is flowing. You can determine this with a flow-network analysis. Software is available for this sort of calculation. Here's a software package that offers a free demo: http://www.aft.com/products/fathom/
RE: VFD Pumps
@ Flash3780:.... I agree with your second paragraph thats correct. and VFD also controlling the speed of pump, it maintain the speed according to demand flow.
thanks for software.
RE: VFD Pumps
Ted
RE: VFD Pumps
RE: VFD Pumps
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.)
RE: VFD Pumps
In this case, the pressure at the pump will rise if a restriction is added downstream (ex. if a branch of your irrigation system is turned off). To quantify this restriction, you'll have to do a flow network analysis.
As was mentioned above, if you're trying to predict the relationship between pump speed, flow, and pressure differential, the pump vendor should be able to provide you with that information.