There is a discussion between myself and another engineer as to the actual definition of TDH (total dynamic head), being the head condition that is specified to the pump manufacturer for the duty point.  Assume that this is a suction lift pump (so that both suction and discharge lifts are +), so it was my assumption that friction and minor losses and the suction lift were used for the NPSH calculation.  Then once it was determined that the NPSHr was met, then a sepetate dynamic discharge head is calculated also for static, friction and minor losses.  Now when spec-ing the pump, are the Suction and Discharge static + losses added to get the Total Dynamic Head OR do you use the Total Dynamic Discharge Head when specifying the duty point?  I assumed that the Suction calcs were only used for NPSH and not TDH.  My understanding has always been that TDH stands for Total Discharge Head and is the total of the static head, friction head, and pressure heads. static head = max. height of liquid being pumped (pump centerline to liquid level taking into consideration siphoning effects.friction head = losses due to pipe and fitting resistance.pressure head = any pressurized vessels on the discharge side of pump.So as per my understanding the suction conditions do not apply to the pump manufacturers TDH value.I have thought the same as CPMech up until i was challenged recently and I could not prove it otherwise.  Text books show Total Dynamic Head, but as we all know, text book -> real world isn't always applicable.  Don't get me wrong, I understand what each of them are and what the difference is, but when you are writing a spec to a pump manufacturer, what do they need to get you the right pump?  A example in the Forged Steel Valves book Cameron Hydraulic Data can give you good answer. Page 3-9.For a suction lift, you have to add it to the TDH; for a suction level higher than the pump center line, you have to take away the suction head; for a suction level higher than discharge level, fluid can gravity flow...I guess everyone knows this. Best to leave it at TDH=Total Dynamic Head. This must include the "Suction" as well as the "Discharge" conditions in order to specify to a pump manufacturer the required duty. To me, "Total Discharge Head" is just referring to conditions on the discharge side of the pump.The amount of Suction Head or Suction Lift obviously plays a part in the available NPSH, but is also a factor in the differential head that the pump has to accommodate.

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