Pumps facilitate the movement of liquids by forming a low-pressure area at the pump inlet, which enables fluids to be forced inside by atmospheric or head pressure. In terms of pump performance, it is important to consider the physical limit that external pressure places on how high fluids can be lifted by the pump. This limitation is taken into consideration by Net Positive Suction Head (NPSH), a term that describes the difference between pump suction pressure and vapor pressure. The NPSH is the most important element to consider in a pumping system.

NPSH involves two parts: net positive suction head available (NPSHa) and net positive suction head required (NPSHr). 

• NPSHa refers to the NPSH available at the inlet of the pump and is calculated based on several variables related to the specific system. 
• NPSHr, which is supplied by the pump manufacturer, is the NPSH necessary for the pump to operate without experiencing cavitation. Cavitation occurs when bubbles rapidly form at the pump’s inlet, then abruptly collapse to create a shockwave. It is crucial to avoid cavitation as it can cause permanent damage to the pump.  

Calculating Net Positive Suction Head

Calculating NPSH is essential in order to prevent cavitation, improve efficiency, and ensure optimal pump performance. When selecting the most suitable pump for a particular application, it is important to make sure that the NPSHa is greater than the NPSHr to avoid cavitation. In other words, the system must have a greater amount of suction-side pressure available than the amount required by the pump.

NPSHa is calculated using this formula:

NPSHa = Ha ± Hz – Hf + Hv – Hvp

• Ha: Absolute pressure, typically atmospheric pressure, being exerted on the liquid’s surface
• Hz: Distance between the liquid surface within the tank and the pump centerline
• Hf: Losses due to friction in the suction piping
• Hv: Velocity head at the pump suction port
• Hvp: Absolute vapor pressure of the liquid at pumping temperature

Using Net Positive Suction Head to Select a Pump  

As mentioned, the most important factor to consider when selecting a pump is the NPSH. The margin of error between your calculated NPSHa and the manufacturer-provided NPSHr should be 10% or greater to ensure that cavitation is avoided. Other factors to consider when selecting a pump include:

• Temperature. The higher the liquid temperature, the greater its vapor pressure. This has a significant effect on the NPSH equation, causing the NPSHa to decrease. It is important to consider this, especially if the pump is being used in high-temperature applications.
• Suction-specific speed (Nss). This single value is calculated from three factors: the amount of head generated by a pump, the pump operation speed, and the amount of NPSH necessary for pump operation. High Nss values could indicate that a pump is prone to cavitation.
• Distance between pump and suction source. Due to its effect on the NPSHa, the distance between the pump and the suction source should be considered.
• Pump design. Pump design, age, and the rotational speed of its impellers will all influence the NPSH required for the pump to operate properly.

Net Positive Suction Head Solutions by Thermal Fluid Systems, Inc.

Thermal Fluid Systems, Inc. is a thermal fluid heater and hot oil system supplier with 40 years of experience in the field. Our extensive experience allows us to be involved in every step of our customer’s projects, from design and engineering to fabrication and after-sales support. By closely communicating with each customer to understand their specific project challenges and requirements, we can recommend and install the pump that is best suited for a particular application. Our goal is to save our customers from the stress of having to worry about the expensive repairs or unexpected downtime that can come with selecting the wrong pump for a system. 

Additionally, we also perform system and control upgrades, provide onsite-technical support, and offer an extensive inventory of replacement parts. To learn more about our many services and capabilities, please contact us today or fill out a request for a quote.