godpneuma, Have you tried setting up a data table in Excel and used the "chart function" to setup the graph and then done curve fit in the "chart fuction"?

Hope this helps.
saxon

you will need more data than just the TDH

the three points should bracket the point of maximum efficiency of the pump.  If you are specifiying the pump, you should also bracket the minimum, maximum and average design flows expected in your system.  Therefore, assuming you are pumping into a transmission main which is interconnected with your distribution system,

Point 1 - low demand, high system pressure
Point 2 - average demand, average system pressure
Point 3 - high demand, low system pressure

So...the total dynamic head is the Head from the High system demand to the low system demand?  

And the three points are really the controlling factor...instead of the TDH?

the total dynamic head (TDH) is the sum of the following:

HL = static head
Hf = friction head loss
Hv = velocity head

in order to specify a pump, you need to know the complete range of conditions that the pump will operate in.  Normally a system curve(s) is generated for the pumping system being studied.  This defines the full operating range of the pump.  As stated before, you need to specify points on the curve to define the range of pressures and flow rates that the pump will be operating at.

I think what you are asking is what is the TDH rating on the pump mean.

The pump should have a plate stating the TDH at a particular flow.  This is the design point of the pump.  It can produce x gpm at the stated TDH.  This was estimated by overlaying the system curve on the pump curve as CVG was stating.

A pump curve is typically generated using the pump, a pressure gauge, a flow meter, and a throttling valve.  The pump is run against a closed valve where flow is zero and the pressure read.  This will give you one point on the curve.  By the way, the pressure gauge is on the pump side of the valve.

The valve is open allowing flow.  Again the pressure and the flow are read.  This is another point.

This is repeated until several points are generated and a curve is then drawn.

What CVG was trying to say in his first post is you want one point near zero flow, one near the midpoint of the flow, and one near the high end of the pumps capacity.

If you have the pump manufacturer and model number, you can probably obtain the curve from a local sales rep or possibly the internet.  Otherwise you might have to generate one in the field.

For a radial pump (specific speed in the region of 25) the closed valve head can be estimated as TDH x 1.25.

You could obtain a group of pump curves from a supplier of similar pump models and extract a typical pump curve shape -above and below TDH.

If you really want to go into it, a whole range of dynamically typical pump curves have been produced for a whole range of specific speeds - "Pump Characteristics for Transient Flow" ARD Thorley and A Chaudry - Pressure Surges and Fluid Transients Conference 1996 - BHR Group.
Obtain paper from BHR (British Hydraulic Research)
Search for BHR in the internet.