NPSH (UNI EN 12845 ) is a key consideration when seeking to prevent cavitation in the inlet of main pumps. In this response, I summarize the requirements and explain how to correctly interpret the terms “required NPSH” and “available NPSH.”
Table of Contents
Why Does UNI EN 12845 Mention NPSH?
The stated objective is to prevent dangerous cavitation conditions from developing in the inlets of the main pumps. Cavitation is treated as a risk to be prevented as early as the design phase, by adjusting both the configuration of the inlet and verifying the NPSH conditions.
In other words: it’s not enough just to “have a pump”; the installation must also allow for proper operation without causing unwanted phenomena in the inlet.
Requirements for the inlet piping (Section 10.6.2.1)
The requirement stated above is clear and practical: the pump’s inlet line must be connected to a straight or tapered pipe that is at least twice the diameter in length.
In addition, the inlet piping (including valves and fittings) must be designed to ensure a minimum NPSH, taking into account:
- maximum required flow rate;
- maximum water temperature (with the restriction that the water temperature must not exceed 40 °C).
Finally, installation guidelines are provided to minimize typical inlet problems: the pipes must be horizontal or have a continuous upward slope toward the pump, in order to prevent the formation of air pockets.
NPSH UNI EN 12845: Definition and What It Represents
NPSH stands for Net Positive Suction Head. In the Italian text, it is defined as “assoluta altezza di carico netto.”
The operational meaning is: NPSH represents the total head at the pump inlet, measured relative to a reference plane, with two conceptual corrections:
- it increases by an amount corresponding to the atmospheric pressure;
- it decreases by an amount equal to the vapor pressure.
For a horizontal pump, the reference plane is clearly defined: it passes through the pump’s axis of rotation. The NPSH value is expressed in meters.
Absolute pressures: a key factor not to be overlooked
One point that is often overlooked but has been made clear is this: when discussing NPSH, one must refer to absolute pressures, not relative pressures.
This step serves to avoid ambiguity when calculating and comparing values. If inconsistent units are used, the final check may appear formally “correct” but be conceptually incorrect. The guidance here is clear: NPSH must be treated in absolute terms.
Required NPSH vs. Available NPSH: What’s the Difference?
The text distinguishes between two concepts, both of which are necessary:
- Required NPSH (NPSHr): This is the value specified by the manufacturer to ensure proper pump operation.
- Available NPSH (NPSHd): This is the value determined by the installation conditions and must be calculated by the system designer.
This distinction clearly “divides” the responsibilities: the manufacturer provides the machine as specified, while the designer must verify that the installation (piping, valves, fittings, flow rate, and temperature conditions) ensures sufficient NPSH.
The verification required at design Q (anti-cavitation condition)
To prevent cavitation, the requirement is stated as a check against the design Q:
NPSHd – NPSHr > 1
The interpretation is straightforward: the available NPSH must exceed the required NPSH, maintaining a margin greater than 1 (expressed in meters, consistent with the unit of measurement for NPSH).
In practice, UNI EN 12845 does not merely “recommend” caution: it requires an explicit verification, taking into account the design flow rate and the conditions that affect NPSH availability (including water temperature).
Common mistakes to avoid (in line with the requirements listed)
Without introducing any external elements, the text highlights a number of practical considerations:
- to overlook the fact that the test must be performed at the maximum required flow rate;
- Do not take into account the maximum water temperature (and the 40 °C limit);
- design the inlet system without including valves and fittings in the verification process;
- Lay the pipe in a way that allows for air pockets, rather than laying it horizontally or with a continuous upward slope toward the pump;
- confusing absolute and relative pressures when discussing NPSH.
Conclusion
NPSH according to UNI EN 12845 essentially means designing the pump inlet system so thatthe available NPSH (calculated by the designer) exceedsthe required NPSH (specified by the manufacturer), with the NPSHd verification – NPSHr > 1 at the design flow rate (Q), while also adhering to construction and piping installation criteria to minimize risks such as air pockets and cavitation.
