NPSH and TDH
NPSH and TDH
(OP)
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.
Please clarify! Thanks a bunch
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.
Please clarify! Thanks a bunch





RE: NPSH and TDH
static head = discharge elevation - suction elevation
Suction lift has to be added to the total dynamic head.
RE: NPSH and TDH
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.
RE: NPSH and TDH
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?
Thanks for the input so far... anyone else with an opinion?
RE: NPSH and TDH
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.
RE: NPSH and TDH
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.
RE: NPSH and TDH
From the Pump Handbook by Karassik et al:
RE: NPSH and TDH
RE: NPSH and TDH
TDH = Total Dynamic Head. If you call it total discharge head, in a centrifugal pump you are by definition incorrectly inferring what’s going on at the discharge. You will be ignoring what energy is available (or not available) on the suction side of the pump, energy that does not need to be imparted by the pump to the system.
Another way to look at it is total "differential" head. That is, what does the PUMP have to impart to the system? When you specify TDH, the pump manufacturer looks at it as if you already accounting for what's going on on the suction side.
No one has mentioned this, but that the above discussions apply to centrifugal pumps. Things are different for positive displacement pumps.
RE: NPSH and TDH
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So as per my understanding the suction conditions do not apply to the pump manufacturers TDH value."
This is incorect, when a centrifugal pump is tested in accordance with test code, whether it is the US, European or other, the inlet conditions are also included it the HQ curve, as either an addition or a subtraction to the measured discharge head.
Quoting from Cameron:
" Total system head (H) - formally total dynamic head - is the total discharge head (hd) minus the total suction head(hs) if positive or plus if a suction lift: H = hd -hs (head)or H = hd+hs (lift). -------"
Naresuan University
Phitsanulok
Thailand
RE: NPSH and TDH