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)

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 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.