4 replies to this topic

[ayan_dg]

System resistance curve as I undestand is the curve generated when you plot head loss in Y- axis for different flow rates plotted in X axis. For a series circuit it is ok but how to draw a system resistence curve for circuits in parallel. Say three pipe line originating from pump discharge header & going to three different destinations.

[Art Montemayor]

Ayan:

I gather from the way you describe your question that you either have not been taught the reason and background of the Pump System Curve or you have not understood what you have been taught. I have always known the Pump System Curve by that name and not by what you name it. But the name means nothing as long as we are addressing the same principle and tool. I believe you have a problem only because you don’t have the background or full understanding of what the curve is meant to do or describe and how it can help you when you have a pumping definition and specification problem.

The first thing to start with is the Pump Performance Curve. This is the heart of the whole matter when it comes to a centrifugal pump application. This curve identifies the operating characteristics of the specific centrifugal pump and is a plot of the total developed head in feet of liquid (on the ordinate) against the pump’s rated volumetric flow capacity (on the abscissa). You should already be very familiar with the centrifugal pump’s Performance Curve before you proceed because this is the basis of what you are going to do. The need to identify where on your pump’s Performance Curve the actual system will be is the problem to resolve. In other words, once the pump is put into operation, where will the pump’s developed head and flow rate be on its Performance Curve. Obviously, the more resistance you impose on the pump (such as more piping length, smaller diameter piping, more fittings, control valves, block valves, etc.), the more will be the required developed head. This relationship is different for all pumping systems, so you must model (or simulate) your pumping system’s behavior by drawing (or plotting) a System Curve on top (overlaid) of your specific pump’s Performance Curve. The point at which the two intersect is where the pump will be operating in that system. Now that you know the background of what is needed or required, you can now also use your common sense and basically understand what you have to do in order to describe (or simulate) what your pump has to do.

Since you are going to OVERLAY the System Curve on the Performance Curve, it is common sense to understand that you are plotting the feet of total developed head against the pump’s flow rate. That means your units must be the same and consistent – in meaning and in definition.

Common sense also tells you that you must plot the total developed head (TDH) that the pumps “sees”. That means that the downstream number of parallel runs on the pumps discharge mean nothing to the pump. It is only the TDH that the pump is interested in. Therefore, you must calculate the TDH that the pump requires to pump a variety of flow rates through the multiple downstream piping – whatever it may be or whatever it looks like. You have to deal with the problem this way because what you plot on the Performance Curve must comply with the spirit of what it is supposed to represent. Obviously, if you have parallel piping, you have to back-calculate the pressure drop to the point where all the piping becomes one, common, pump discharge pipe which leads to the pump’s discharge nozzle.

That is how you should be organizing and calculating this problem in order to come up with the table of TDHs and related flow rates. If you want to get fancy and academic you can regress the calculated results into an equation and plot that on the Performance Curve. Most practicing engineers don’t do that. They simply plot the various points and draw a smooth curve between them to identify where the two curves intersect.

I think this answers your basic question.

[ayan_dg]

Thanks Art for your reply.
I have a small doubt in it which I think you can clear.
You said
" is a plot of the total developed head in feet of liquid (on the ordinate) against the pump’s rated volumetric flow capacity (on the abscissa). "

When you say the total developed head do you mean to say discharge pressure of the pump minus suction pressure of the pump expressed in m of water column ?

For example a pump discharge pressure (as read by a PG ) is 8 kg/cm2 g and suction pressure is 2 kg/cm2 g . The pump suction and discharge PG are considered at the same height. Then for water expressed in m of water column is
8 kg/cm2 g= 80 m of water
2 kg/cm2g = 20 m of water

Hence total developed head = (80 -20 ) = 60 m of water

Is my calculation right ?

[fallah]

Ayan:
Obviously, if you have parallel piping, you have to back-calculate the pressure drop to the point where all the piping becomes one, common, pump discharge pipe which leads to the pump’s discharge nozzle.

With the permission from Mr. Art for adding small point:
Max. back calculated TDH among all branches up to common point is the TDH value for branched part of discharge line which considered in calculating the TDH of the pumping system.
Hope correct if needed.

Regards

[fallah]

If Velocity Head Difference is neglected, your calculation is right.

Regards