Understanding NPSH in Submersible Pumps: A Comprehensive Guide

NPSH stands for Net Positive Suction Head, a crucial parameter in the operation of submersible pumps and other types of pumps. NPSH is defined as the difference between the pressure at the suction port of the pump and the vapor pressure of the liquid being pumped. This difference is typically expressed in meters or feet of liquid.

Key Components: NPSH Available and NPSH Required

There are two key components of NPSH that every pump designer or operator must consider:

NPSH Available (NPSHa): This is the actual NPSH available to the pump, which takes into account the pressure conditions of the fluid source—such as the height of the liquid above the pump’s atmospheric pressure—any friction losses in the suction line, and the vapor pressure of the liquid. NPSH Required (NPSHr): This is the minimum NPSH that the pump needs to operate without cavitation, which can cause damage to the pump and reduce its efficiency. NPSHr is determined by the pump manufacturer and varies with the pump design and operating conditions.

Importance of NPSH:

Preventing Cavitation: Ensuring that NPSHa is greater than NPSHr is critical to avoid cavitation. Cavitation occurs when the pressure in the pump drops below the vapor pressure of the liquid, leading to vapor bubbles forming and collapsing within the pump. This can cause significant damage and reduce overall efficiency. Pump Performance: Adequate NPSH is essential for maintaining optimal pump performance and longevity. It ensures that the pump operates efficiently and reliably under various conditions.

Calculating NPSH Available (NPSHa)

To calculate NPSHA, follow these steps:

Take the source pressure. Add the atmospheric pressure. Subtract the losses from friction within the pipeline. Subtract the vapor pressure of the fluid.

The result will give you the NPSHA or Net Positive Suction Head Available of your system.

NPSH in Submersible Pumps

Submersible pumps are designed to be submerged in a liquid source, such as a well. The static height of the water in a well is usually greater than the vapor pressure of the liquid at the pump impeller, making cavitation unlikely. Therefore, the NPSH concept is less meaningful for submersible pumps in this context.

In suction pumps, however, the value of Hs (height of suction) is often a negative number, making NPSH lose its significance. This is particularly true when dealing with jet pumps on the surface, where a positive Hs (height from the foot valve to the pump impeller) is necessary to prime the pump correctly.

Priming a Jet Pump

Jet pumps, often used in deep wells, require careful priming to ensure proper operation. The process involves the following steps:

Unscrew a pipe plug on the casing (varies between pump models and brands). Pour water into the pump casing and down into the pipes entering the well. Listen for gurgling sounds, which indicate the presence of air. Essential tip: At times, you may need to run the pump for a very short time and repeat the priming process to ensure a solid column of water between the foot valve and the pump. Screw in the pipe plug and check the sound of the pump to ensure it is not cavitating.

Recognizing the sound changes is crucial for diagnosing issues with the pump. If the pump is cavitating, you may hear little knocking sounds as if a bunch of Santa's elves were hammering on the pump casing. A good sound is a smooth, higher-pitched movement of water into the pressure tank, indicated by a rising pressure gauge.

Conclusion

Understanding NPSH is vital for the installation and operation of pumps, especially submersible pumps and jet pumps. Proper NPSH ensures that the pump operates efficiently and reliably, preventing cavitation and maintaining optimal performance.