NPSH stands for Net Positive Suction Head, a critical hydraulic factor in pumping systems. NPSH is crucial for submersible pump applications, preventing cavitation, and ensuring smooth performance and improved longevity. Unnati Pump focuses deeply on hydraulic designs to make the pumps operate safely within the required NPSH limits, even in adverse situations. So, in this article, we are discussing the importance of NPSH in submersible pumps and the optimal performance of these pumps due to proper system design.
What Is NPSH in A Submersible Pump?
Net Positive Suction Head (NPSH) is the measure of the pressure exerted for suction at a submersible pump, to prevent the liquid from vaporizing. When the liquid enters the pump, it does not form vapor bubbles due to this pressure. NPSH is the difference between the absolute pressure of the liquid for suction and the vapor pressure formed while pumping. It is represented as meters (m) or feet(ft), which are the units of length.
Two types of NPSH are usually available, which are as follows –
- NPSH Available (NPSHa): The actual pressure head available at the pump for suction from the system.
- NPSH Required (NPSHr): The minimum pressure head needed to operate the pump without cavitation, as per the instructions of the manufacturer.
NPSHa should always be more than NPSHr to ensure safe and smooth operation of the submersible pump.
Why Is NPSH Important for Submersible Pumps?
Submersible pumps operate when fully submerged in the liquid. Unnati Pump designs these pumps with a positive suction head that reduces suction problems. However, NPSH is also very important for these pumps. So, the NPSH condition should be maintained very carefully, to avoid the following issues in the submersible pumps –
- Cavitation can damage impellers
- Reduction in the liquid flow
- Decrease in pump head pressure
- Causes excessive vibration and noise in the pump
- Breakdown of bearings and their protective seals
- Consumption of more energy
NPSH-related problems can be caused due to improper depth of installation, losses in the suction system, and high fluid temperature.
What is Cavitation in Submersible Pumps?
Cavitation occurs when the pressure at the entry point of the pump decreases below the vapor pressure of the fluid, resulting in the formation of vapor bubbles. When these bubbles move inside the pump, they may collapse and cause an explosion. Even if cavitation is audible in some submersible pumps, the following long-term ill effects can be seen –
- Severe erosion of the impeller surfaces
- Decrease in hydraulic efficiency
- Slow damage in the structure of the pump
The maintenance of enough NPSHa is the only way to prevent cavitation in submersible pumps. Usually, NPSHa is higher in submersible pumps compared to surface-mounted pumps.
Factors That Affect NPSHa in Submersible Pumps
A few factors directly affect NPSHa in all submersible pump systems, which are listed below –
- Submersion depth – When a submersible pump is placed at a greater depth, it experiences higher static pressure at the suction inlet of the pump, resulting in more NPSHa. If the submergence depth is lower or the fluid level is fluctuating, NPSHa will decrease to some extent.
- Fluid temperature – If the fluid is hotter, the chance of vapor formation becomes higher. In this situation, NPSHa decreases rapidly due to high vapor pressure, making the submersible pump more vulnerable to cavitation.
- Properties of the fluid – NPSHa is directly affected by various properties of the fluid in which the pump is submerged. Density of the fluid, the presence of different gases dissolved in the fluid, and vapor pressure are the properties responsible for increasing or decreasing NPSHa.
- Atmospheric pressure – NPSHa becomes lower in high altitudes due to the lower atmospheric pressure there. So, the installation of submersible pumps is a problem in hilly areas, and pumps should be specially designed to function there.
- Flow rate of fluid – When the flow rate of the fluid is higher, NPSHa also increases due to the greater friction of the fluid particles with the pump surfaces.
- Loss of pressure at the inlets – If the inlets of a submersible pump are partially blocked, NPSHa drops below the required limit. However, Unnati Pump carefully designs submersible pumps with minimal suction piping, strainers, and screens to avoid any risk of blockage.
The Relation between NPSHr and the Submersible Pump Design
NPSH Required can be checked while testing a submersible pump. It is determined by the design of the pump, speed, and geometry of the impellers. If NPSHr is lower, it ensures better suction capability of the submersible pump. Therefore, Unnati Pump constructs its submersible pumps with the following qualities –
- Optimized design of the impeller eye
- Unobstructed passages for the smooth flow of fluids
- Less chance of turbulence at the inlet zones
These qualities ensure that minimum NPSHr is maintained, which allow the pumps to function safely even in adverse suction conditions.
A Few NPSH Advantages Experienced by Submersible Pumps
Several inbuilt benefits are experienced by the users of submersible pumps owing to the NPSH of the pumps, which are as follows –
- Positive suction head because of submergence
- Elimination of long suction pipelines
- Lesser risk of air entrainment
- Stable inlet pressure conditions
Due to these advantages, submersible pumps can be used for all applications, like borewells, sewage systems, sumps, irrigation of fields, and transfer of industrial fluids.
A Few Tips to Maintain Adequate NPSH in Submersible Pumps
Users need to follow these practices to ensure the required NPSH in their submersible pumps, which are mentioned below –
- Make sure of maintaining the adequate minimum submergence at all water levels for the operation of the pump
- Prevent clogging or restrictions at the inlets
- Select submersible pumps where NPSHr is less than the expected NPSHa
- Follow all instructions of the manufacturer for the installation of the pump
Conclusion
It is essential to understand the phenomenon of NPSH before buying and using a submersible pump. Submersible pumps work well due to the positive suction conditions. But the submergence depth, inlet pressure, and fluid temperature play a crucial role in determining NPSH. So, people need to buy pumps with low NPSHr and carefully evaluate NPSHa to ensure good performance and high longevity of submersible pumps.