VFD Pumps

[Fezi4u]

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

 

[flash3780]

I'm not going to lie, I had to google VFD. I assume you're referring to a Variable-frequency drive and not a Vacuum fluorescent display or Vicarius Filii Dei.

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?

 

[Fezi4u]

ya its variable frequency drive which connected with PLC, i have made a max pressure set point 5 bar instead of designed 7.5 bar, i think to cover up the flow, velocity may increase and that will be dangerous for pipes, as pressure is inversely proportion to velocity and flow is directly proportional to Velocity

 

[hydtools]

I would suggest that flow will go down. Reducing the setpoint pressure will cause the drive to reduce speed to reduce flow to reduce system pressure to the new, lower pressure setpoint.

Ted

 

[flash3780]

Had to Google PLC. I assume you mean programmable logic controller and not product life cycle, powerline communication, Palestinian Legislative Council, or Presbyterian Ladies' College.

Pressure and velocity are not inversely proportional. The pressure drop is roughly proportional to the velocity squared.

See the Darcy-Weisbach equation:

http://en.wikipedia.org/wiki/Darcy–Weisbach_equation

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.

 

[Fezi4u]

I simply calculated the flow vs head from friction loss chart given by pipe manufacturer at longest point not branches, it shows 6.5 bar, but the flow meter showing the same as design flow rate at the discharge of pump line. the problem is that since the system is partially in operation so i am afraid that i will not get the max flow at the longest point, but VFD has to keep the constant pressure in the pipe i think so.

 

[flash3780]

The controller will likely maintain the pressure at the supply until the pump runs out of oomph (max flow) no matter what's going on downstream. So, less resistance to flow would equal more flow for the same head pressure and vice versa.

 

[Fezi4u]

lets keep away the VFD for a moment, and considering simple star delta connection as i know if the water has to be pumped at more than the designed head, definitely the flow will be less because pump has to apply more efforts. m i rite? but since the VFD has been introduced, save the energy and start the pump very softly as well, so here we have reduced the head means flow should be more.

 

[hydtools]

Fezi4u,
how are you reducing the head? By changing the system or by changing the controller pressure setpoint?

Ted

 

[flash3780]

I don't want to speak out of my area of expertise, so I'll stick with pumps and pipe flow. If the controller is working properly it will adjust the pump speed to maintain a constant pressure at the exit to the pump (until a limit is reached, say if the motor has a max speed, etc.).

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/

 

[Fezi4u]

@ Hydtools:... i am changing the control set point, system will remains same.

@ 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.

 

[hydtools]

At system pressures below 5 bar you will get the same performance as with the higher 7.5 bar setpoint. At system pressure of 5 bar the pump control will adjust flow to maintain the 5 bar setting.

Ted

 

[IFRs]

If this is a centrifugal pump, and perhaps others, the head (pressure) and flow are related and graphically represented by a characteristic curve for the pump. There will be a different but similarly shaped curve for each motor speed. You can get these curves from the pump manufacturer. Typically as head (pressure) goes up flow goes down. I'm probably way off base but I'd get the curves from the manufacturer.

 

[Artisi]

You need to draw a H/Q system curve for the installation across a variable speed pump curve to properly establish what is going to happen as you reduce speed, otherwise you are just guessing an outcome for a reduction in speed.

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.)

 

[flash3780]

it maintain the speed according to demand flow.

So, the controller is maintaining a flow rate?

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.