One of the most critical factors in choosing a centrifugal pump model is the net positive suction head (NPSH) margin. Properly calculating the NPSH available (NPSHa) and maintaining sufficient margin relative to the NPSH required (NPSHr) helps prevent damaging pump cavitation and ensure a successful pumping operation.
Cavitation occurs when the pressure anywhere inside the pump is less than the vapor pressure of the liquid. It may cause noisy pump operation, capacity loss, reduced efficiency, damage to the impeller, vibration that leads to damage of the shaft, mechanical seals and bearings and eventual pump failure.
There are two main types of centrifugal pump cavitation: suction and discharge cavitation. NPSH issues typically lead to suction cavitation, which affects the area around the pump’s impeller eye.
How to Determine NPSH Margin
NPSH margin is the difference between these two values:
NPSHa: the stagnation pressure available over the liquid’s vapor pressure at the pump’s suction port
NPSHr: the minimum pressure at a specified rate of flow and speed for a specified pumped liquid that causes a 3% drop in the pumped head
The NPSHr is provided by the pump manufacturer. When there is zero margin between NPSHa and NPSHr, the pump undergoes significant cavitation that results in loss of performance on top of pump damage.
There are ways to characterize NPSHr at different instances of cavitation, such as 0% or 1% head drop, but the Hydraulic Institute Standard guidelines use a 3% head drop criterion when defining NPSH margin with the goal of preventing damaging cavitation as opposed to entirely eliminating cavitation.
How to Calculate NPSHa
The basic formula for calculating NPSHa is this:
Site atmosphere pressure (if the suction source is open to atmospheric pressure, or absolute suction pressure if it is not) + static head (positive for flooded suction applications and negative for suction lift applications) - friction + velocity head at suction port - vapor pressure at the pumping temperature.
In certain applications, such as positive displacement pumping that involves sudden acceleration of liquid, an acceleration head component must also be factored in.
Since NPSH margins are typically tight, a small miscalculation could lead to big problems with the pump and pump setup.
Factors That Can Affect NPSHa
Some factors play a larger role in certain systems and a smaller role in others. Factors include:
Atmospheric pressure: Changes in elevation and weather can affect atmospheric pressure. Lower pressure decreases NPSHa.
Static head: An increase in static head (the height of the fluid above the pump’s centerline) results in higher NPSHa. In suction lift applications, care must be taken to ensure the NPSHa calculation is valid for all operating conditions.
Fluid velocity: Velocity depends on the flow rate and the diameter of the suction pipe. A higher velocity leads to higher friction loss and a lower NPSHa. In addition, the NPSHr characteristics of centrifugal pumps typically indicate an increase in NPSHr with an increase in the flow rate.
Liquid temperature: Changes in the temperature of the liquid affect the vapor pressure. For water, a higher-temperature application will lead to a lower NPSHa.
The length of piping and number of fittings: These affect friction losses and reduce the NPSHa.
Suction lift: When the liquid level is below the pump centerline, the pump must work harder to draw the liquid up. Too much suction lift decreases NPSHa and can lead to cavitation.
Cavitation history: Pump damage from previous cavitation can affect the NPSHa and should be repaired to prevent further cavitation damage.
Liquid properties: Viscosity, specific gravity and other liquid properties may increase friction losses and affect NPSHa. Entrapped gas in the liquid could also affect the level of cavitation damage.
Regular monitoring of pumping operations and adjustments to the pump setup may be necessary, especially if there are changes to the system or operating conditions. A temporary equipment vendor with expertise in pumps and pump systems can help companies anticipate and respond to issues.
Ensuring a Sufficient NPSH Margin
NPSHa should exceed NPSHr by a certain margin. Failing to maintain a sufficient NPSH margin is a common mistake. Looking at published suction lift curves is not a substitute for calculating the NPSH margin.
The recommended margin varies based on the application, the liquid and properties of the pump. ANSI/HI 9.6.1-2017 Rotodynamic Pumps – Guideline for NPSH Margin outlines the recommended margin for different applications and pump designs.
A general rule of thumb is to choose a pump with a positive margin of at least 3.3 feet or 10% of NPSHa (whichever is greater) to allow for variations in operating conditions and prevent the pump from operating too close to its NPSH limit. However, rules of thumb are just that. The NPSH margin could be higher if the duty point is outside the preferred operating range. A professional engineer can help contractors determine the optimal margin and size of a pump and design a pumping system accordingly.
Tips for Improving NPSH Margin
Any changes to the operational environment or pump system can change the NPSH. Consider these adjustments if the NPSH margin is insufficient:
Operating the pump at lower speed might lower NPSHr for certain pump models. For temporary pumping systems, operating two pumps instead of one can lower the NPSHr by reducing the pump speed and the flow per pump.
Reduce friction by increasing the diameter of the suction piping or using shorter pipes and hoses with minimal fittings. Fittings such as tees, elbows and foot valves should generally be avoided in suction piping designs. Replace corroded pipes and hoses.
Reduce static head by surcharging the liquid level in the suction source or benching down the pumps.
Monitor and regularly clean the suction screen or strainer to keep debris out of the suction line and prevent head loss due to clogging.
Use a submersible pump or other intake pump when the suction lift is too high for an above-ground pump to operate without cavitating.