Understanding the Relationship Between NPSHpump, Safety Margin and NPSHplant

For trouble-free operation of a centrifugal pump in the facility, the NPSHplant (Net Positive Suction Head available at the plant) must be equal to or greater than the NPSHPump (Net Positive Suction Head required by the pump). The values of NPSH3%=f(Q) are shown in the sales characteristic curves (referred to as ‘characteristic curves’, going forward). This article will examine NPSH3%= f(Q) curves for selected manufacturers of standardized single-stage volute casing pumps with axial inlet. This pump type is significant for this study because of its widespread use in both industrial plants and ships. The presented investigation aims to determine whether the specialized literature and technical documentation provided by manufacturers deliver information about the necessary safety margin on NPSH3% or not.

By Dipl.-Ing. Jürgen H. Timcke, Mechanical Engineer.

Regarding the NPSH3% of the pump, it is important to note that the most commonly used cavitation criterion is NPSH3(=NPSH3%), not because it holds exceptional technical relevance, but because it is straightforward to measure. As a result of this, the term NPSH appears in pump specifications of numerous manufacturers without explicitly indicating that it specifically refers to NPSH3.1

When looking at NPSH3%, NPSH safety margin, NPSHPump, and NPSHPlant, the relevant specialized literature either lacks accurate information or presents inconsistent information on these matters. Consequently, it becomes interesting to determine whether the manufacturers’ technical documentation provides answers to the following questions:

  1. What information is provided in the specialized technical documentation?
  2. Do the details in the manufacturers’ technical documentation consistently align, or is it conflicting?
  3. Do varying conditions exist for the required minimum NPSHPlant that must to be met?
  4. Are the differences between the calculated NPSHPlant values using distinct calculation methods substantial or negligible?

Breaking it Down

When looking at example 1, the term ‘NPSH3% + 0.5 meters’ from equation 2 into equation 1 results in equation 3:

NPSHPlant = NPSHPump + safety margin = NPSH3% + safety summand + safety margin = NPSH3% + 0.5 meters + 0.5 meters = NPSH3% + 1 meter (Equation 3)

NPSH3% = f(Q), NPSH3% + 0.5 [m] = f(Q), and R0.5 = f(Q)

To further understand this, the characteristic curves from two different standardized sizes of randomly chosen manufacturers were utilized. These sizes are compliant with the criterion of having nearly the same specific speed nq at n=2900[1/min]. The selected size for this example is > 80-65-160 (ISO 2858) and >125- 100-260 (EN 733)

Figure 1: Size 80-65-160 NPSH3%, NPSH3%+0.5[m] and R0.5 all presented as function of Q.
Figure 2: Size 125-100-260 NPSH3%, NPSH3% + 0.5 [m] and R0.5 all presented as function of Q

The curves of NPSH3%= f(Q) were extracted from their characteristic curves and presented in Figures 1 and 2. Additionally, the curves of NPSH3% + 0.5[m] = f(Q) and R0.5 = f(Q) are depicted. While NPSH3% =f(Q) curves are familiar, the term R0.5 refers to the safety summand of “S0.5 = 0.5 [m] expressed as a percentage of NPSH3% [m]”. It is calculated as follows:

R0.5 = S0.5 [m] / (NPSH3% [m] / 100 [%]) = S0.5 [m] • 100 [%] / NPSH3% [m] (Equation 4).  The dimension of R0.5 is [m • % / m] = [%]. As NPSH3%=f(Q) increases, the value of R0.5 consistently decreases.

Given this information, why does the smallest NPSH3% value correspond to the highest R0.5 value, and conversely, why does the largest NPSH3% value correspond to the lowest R0.5 value? This can seem counterintuitive. Perhaps attempting to reverse this might seem more logical.

Figure 3: Dimensionless presentations. Full lines: size 80-65-160. Broken lines size 125-100-260.

Figure 3 presents NPSH3% =f (Q) and R0.5=f(Q) from Figures 1 and 2. The curves of NPSH3% =f(Q/QBEP) as well as R0.5=f (Q/QBEP) present good correspondence, particularly within the range of Q/QBEP [-] = 0.7 to 1.3. These requirements represent the part-load and overload ranges for efficient operation of the discussed standard pump models.

The diagrams in Figure 4 offer a comparison of the tendencies of the curves for the two sizes relative to Q/QBEP, within the part load and overload ranges.

Figure 4: Comparison of Figure 1 and Figure 2.

*** Read Part Two of this article in the  December 2023 issue of Pump Engineer. ***

About the Author

Dipl.-Ing. Jürgen H. Timcke studied mechanical engineering at the University of Applied Sciences in Karlsruhe. He has 40 years’ experience in the field of centrifugal pumps and has gained a thorough understanding of the pump industry. In the last 30 years, he has been a Manager of the Development, Design and Testing at a number of international and well-known pump companies. In addition to his professional activities he was also a regular lecturer at the University of Applied Sciences in Konstanz. As an expert in his field he was elected a member of the AMERICAN SOCIETY OF NAVAL ENGINEERS. Other articles by Timcke can be found at: www.juergen-h-timcke.ch


  1. Gülich, Johann Friedrich, Dr.-Ing. Centrifugal Pumps. Springer-Verlag Berlin Heidelberg 2008
  2. Sulzer Centrifugal Pump Handbook; First edition, May 1986; Sulzer Brothers Limited, Winterthur, Switzerland
  3. Timcke, Jürgen H. Das Konzept der Wassernormpumpe EN 733: Eine kritische Betrachtung auf der Basis mehrerer Baugrössen führender deutscher Hersteller. The program of the water standard pump EN 733:

a critical examination on the basis of several sizes of leading German manufacturers Industriepumpen + Kompressoren Zeitschrift für die Praxis der Pumpen- und Kompressorentechnik Vulkan-Verlag, Essen. Part 1: September 2000, pages 180/185. Part 2: June 2001, pages 108/112

  1. Europump. NPSH bei Kreiselpumpen NPSH at centrifugal pumps Maschinenbau-Verlag GmbH, 1974 Frankfurt/Main-Niederrad
  2. Wagner, Walter Kreiselpumpen und Kreiselpumpenanlagen Centrifugal pumps and centrifugal pumps plants Erste Auflage 1994, Vogel Verlag und Druck KG, Würzburg
  3. Technical documentations of several centrifugal pumps manufacturers
  4. Technical documentations from the archive of the author
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Shopia Ketheeswararajah is a feature editor contributing to Pump Engineer, Stainless steel World Americas, Hose and Coupling World, and other related print & online media.