NPSHr Is Different for Positive Displacement Pumps

Email Received

Subject: NPSHr in Positive Displacement (PD) Pumps

Professor Elsey, first I want to thank you for your teachings and the quality of your articles. I am a mechanical engineer, and I have a question regarding the net positive suction head required (NPSHr) for positive displacement pumps. I haven't seen or found any information from the manufacturer that provides this data. So, how can cavitation be checked in these types of pumps if I can't compare the NPSHr with the net positive suction head available (NPSHa)?

Response

Dear (name withheld),

Thank you for the nice compliment, and I am glad that you find my articles informative. First, I must point out that I am not a “professor,” but I am a teacher, and I was, am and always will be a student. Your question is excellent, one that most people overlook.

NPSHr as it pertains to positive displacement (PD) pumps is one of those subjects that usually gets ignored until there is a problem in the field. The short answer to your question is that NPSHr for PD pumps is a complex variable, strongly dependent on the pump’s field installation position and as a function of the flow rate (pump speed).

The typical field problem will be due to suction side conditions caused by unanticipated friction losses, acceleration head and/or acoustic resonance issues in the piping. The friction issues are usually due to the pipe geometry, e.g., diameter too small, length too long or configuration too torturous. Other problems may be due to the traditional vapor pressure or suction pressure (head) deficiencies that occur on many pump systems (insufficient NPSHa resulting in cavitation).

You do not mention the manufacturer or type of PD pump in question. I am most familiar with piston and plunger type PD pumps (power pumps) so I will focus there, but my comments apply in general across the spectrum of PD pump types (gear, lobe, vane, diaphragm, screw, progressive cavity and, to a lesser degree, peristaltic).

The pump manufacturer should follow industry standards for testing. Hydraulic Institute (HI) and American National Standards Institute (ANSI) have a standard that both defines and explains the procedures, tests and formulas for determination of NPSHr for PD pumps. You may also reference American Petroleum Institute (API) 674 and International Organization for Standardization (ISO)13710.

If the manufacturer of the pump in question is a legitimate OEM, they will have documentation pertaining to the NPSHr for the pump in question. The manufacturer may not publish the data in public forums, but the engineering department will have the applicable pump test results. You will need to contact them directly and request the information.

PD Pumps Are Different

It is important to understand that NPSHr is different for a PD pump than for a centrifugal pump. For starters, the NPSHr for PD pumps is designated as net positive inlet pressure required (NPIPr). NPIPr is measured or calculated similarly to NPSHr, but the acceleration head for the system is also taken into consideration.

When testing a centrifugal pump for NPSHr, the NPSHa is reduced while simultaneously maintaining the flow rate (Q) at some fixed value, and the total dynamic head (H) is monitored to watch for a 3% head drop due to cavitation. The 3% head drop is the marker for NPSH3, also normally designated as NPSHr. However, when testing a PD pump, that centrifugal pump technique will not work because the head (pressure) developed by the PD pump is a different function. Consequently, the designated cavitation marker for PD pumps is a 3% volumetric drop in flow rate (Q) as opposed to the 3% head drop in centrifugal pumps.

As a side note, most commercial manufacturers use the 3% drop because anything less is very difficult to distinguish. Some purists will push for markers at a 1% drop, which is a path littered with good intentions but fraught with issues of inaccuracy. It is important to realize that in both PD and centrifugal pumps, the 3% drop (either head for centrifugals or volume for PDs) means the pump is already cavitating prior to that point.

Further

PD pump manufacturers are less concerned about NPSHr for PD pumps tested in the factory as compared to centrifugal pumps, because NPIPr (NPSHr) changes so much in the field due to the suction piping geometry, pump speed, liquid viscosity, specific gravity and acceleration head—not to dismiss any of the normal considerations (factors) as evidenced in the NPSHa formula. A published NPSHr curve by the PD pump manufacturer is only accurate for the conditions under which it was tested in the factory; the results will be different in the field.

Even taking acceleration head (expressed as Ha or h acceleration) into consideration does not always predict the true NPIPr, so the manufacturer will often integrate a term known as net positive suction head field (NPSHf).

NPSHf will also account for the power density index (PDI) and suction valve pressure drop, all in conjunction with the other remaining standard variables in the NPIPr/NPSHr computation (see NPSHa formula in Image 1).

More and more, pump manufacturers will rely on computational fluid dynamics (CFD) for the actual NPIPr number and/or measure it in the field (empirically) under actual and true conditions.

Always keep in mind that NPSHa/r calculations are not exact or fixed and will always be changing. I hope this information helps you to better understand the subject. I wish you the best of luck in your endeavor.