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Pump Nozzle Sizing and Proper Pump Selection

Pump Nozzle Sizing and Proper Pump Selection

Pump Nozzle Sizing and Proper Pump Selection

I have reviewed a number of forum discussions and posted guidelines regarding pump selection and pump nozzle sizing, and most appear to indicate that pump nozzle sizing is solely a product of the pump manufacturer's design and need not be considered in the system design and/or calculations for system TDH. However, perhaps this is a misunderstanding on my end.

Should not a reduction at the pump nozzle (assuming the system piping is larger diameter than the pump nozzle, which is typical) be specifically considered in system NPSHa calculations, considering pressure loss resulting from a sudden contraction at the pump and/or pressure loses from the increased pipeline velocity local to the pump connection may reduce NPSHa to a point where the pump is not usable due to cavitation (NPSHa < NPSHr)? In particular I would expect this evaluation would be essential when the pumped media has a high viscosity and sudden contractions could have large effects to the calculated NPSHa.

RE: Pump Nozzle Sizing and Proper Pump Selection

The manufacturer's curves will take into account anything that comes on the pump casing. You need not consider these as part of you system.

I used to count sand. Now I don't count at all.

RE: Pump Nozzle Sizing and Proper Pump Selection


I apologize as my question may not have been clear. I am not referring to any nozzle reduction on the pump body itself as it is understood those loses should be accounted for in the pump curve. I am referring to reductions made immediately upstream of the pump casing in order to connect to the pump inlet. Typically system piping will be larger diameter than the connection size of the pump, and an eccentric reducer would be used to reduce the diameter in order to connect to the pump inlet. I have found that including the loses through the reducer can occasionally precluded the use of certain pumps due to their connections being too small and resulting in too large a reduction in NSPHa. I have seen this most often with high viscosity oil transfer systems our company has designed where, for example, there might be a 2" supply pipe and a reduction to a 1" pump inlet connection would work, but reduction to a 3/4" pump connection would reduce NPSHa too much to be workable. So we needed to be careful to evaluate the pump connection size in conjunction with the pump characteristics/curve to select the proper pump.

RE: Pump Nozzle Sizing and Proper Pump Selection

IMO, for the most of the time, we are dealing with the pumps which are designed in a general condition of the "typical" viscosity range fluids. The pump for the high viscosity fluid may require a special considerations in the design and sizing. It's important to communicate with the manufacturers with the correct process and operation conditions for the proper pump selected for the service.

RE: Pump Nozzle Sizing and Proper Pump Selection

If a pump comes with a 1" inlet, then in most cases the testing was done with a reasonable length of 1" line feeding the pump. If your system has a 2" supply line and you reduce to 1" a few feet before the pump you are no worse off than the standard pump conditions.
There are exceptions, such as MK commented about high viscosity fluids, similar would be very low vapor pressure fluids. These will be much more sensitive.

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P.E. Metallurgy, Plymouth Tube

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