I know TDH is Total discharge head minus total suction head (plus if it is suction lift).My feeling is that if you calculate the TDH then you can determine the flow rate from the pump curve.
For a centrifugal pump, the pump curve (head versus flow rate) defines where the pump will operate.  So, yes, if you calculate the TDH for the pump, the curve will tell you what the flow rate through the pump is.This assumes the pump is properly installed and is in the same condition as when the curve was originally generated (if it's a certified curve or at least, same condition as the generic pump used to generate the curve).
 

Assuming the pump installation is designed to insure that the required flowrate can be delivered to the pump suction then the actual flowrate is determined by:The intersection of the pump curve (for the impeller size installed) and the fire pumps system curve.  The system curve must be calculated - you must count all fittings and line lengths and compute the system resistance K. For the system curve, the TDH REQUIRED INCREASES with higher flowrates - while for the pump curve, the AVAILABLE TDH DECREASES with higher flowrates.Again, this is where the pump would like to operate; but it will do so ONLY IF THE NPSH is adequate.  The more you learn, the less you are certain of.
 

2011-08-31

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