NPSH and why its important

Learn the lesson before a catastrophe happens.

NPSH is arguably one of the least understood operational and pumping application issues in the fluid pumping industry and why NCS Fluid Handling Systems firmly believes that our engineering solutions and “Ahead of the Curve Training” will assist to select the correct pump for the job and prevent premature pump or engine failures or both!

It is not difficult to calculate Net Positive Suction Head typically referred to as NPSH, but it is very important to successful pump selection and efficient long-term operations of fluid pumping application and the engineered design of a system design. Proper selection of the right pumps and long-term operation of a water pumping systems must include NPSH calculation and should be calculated during the design of all pumping systems or revisions to existing systems.

We will provide some industry definitions, clarity on exactly what NPSH is, the impacts on pumps and systems when there is insufficient NPSH, and what considerations can be considered when NPSH is a limiting factor. We will also provide the contact number of our innovation center team so that we can share how to calculate NPSH or book an “Ahead of the Curve” training session.

Industry Definitions – Learn them and understand them

Net Positive Suction Head (NPSH) – The measurement of liquid pressure at the pump end of the suction system and is the measure of the pressure experienced by a fluid on the suction side of a centrifugal pump. ... NPSH is defined as the total head of fluid at the center line of the impeller less the fluid's vapour pressure.

Net Positive Suction Head Required (NPSHr) – Amount of atmospheric pressure required to move liquid through the suction side of the pump. NPSHr is directly related to pump selection and designed application.

Net Positive Suction Head Available (NPSHa) – The difference between standard atmospheric pressure and the combination of atmospheric pressure at elevation, total dynamic suction lift, vapor pressure, and safety factor. NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump supplier. NPSHA MUST be greater than NPSHR for the pump system to operate without cavitating

Atmospheric Pressure (Ambient) – Atmospheric pressure, also known as barometric pressure, is the pressure within the atmosphere of Earth. In short, the Earth's atmospheric pressure at sea level is approximately 1 atm and adjustments for elevational change are needed from this basis.

Standard Atmospheric Pressure – The weight of atmosphere at sea level under normal atmospheric conditions is 14.7 PSI.

Total Dynamic Suction Lift (TDSL) – total equivalent height that a fluid is to be pumped, considering friction losses in the pipe and fittings. This equation can be derived from Bernoulli's Equation. TDH is simply the pressure head difference between the inlet and outlet of the pump, when measured at the same elevation and with inlet and outlet of equal diameter. On a suction lift, the total dynamic suction lift is calculated by adding the static suction lift plus the friction loss at flow rate. The value of any total dynamic suction lift or total dynamic suction head of a system is the suction gauge reading, while the pump is operating.

 

Vapor Pressure – The pressure at which a liquid will vaporize, temperature does play into this equation. An ineffective pump design or improper selection can contribute adversely during operation to a rise in temperature.

Specific Gravity – The weight of any liquid relative to that of water.

Safety Factor – This value is used in engineered design and pump selection. This Safety factor considers fluctuations in atmospheric pressure as a part of the NPSH calculation

So, What Is NPSH?

NPSH is the amount of atmospheric pressure at the pump end of the suction system, including the pump. The NPSH calculation is not difficult and should never be overlooked as it could be time well spent avoiding a costly failure, that is not an operational fault.

The force of this pressure is equal to 14.7 PSI, 33.9 feet of water, 10.3 meters of water, and 29.9 inches of mercury. This is the same principle that causes a pressure reading and reflects change in atmospheric pressure in a barometer.

One most first understand that this external force helps push water up the suction pipe during priming phase of pumping through to pumping operations and that it varies given elevation and atmospherics changes. With this understood a calculation to ensure this force during pumping operations is sufficient to adequately supply liquid to the pump end. Given normal atmospheric conditions, which equals 33.9 feet of water, the value must be adjusted relative to the specific gravity of the liquid being handled by the pump. Once this pressure is determined then the consideration must be made to adjust for the pump location, temperature, fluid being pumped, and the overall configuration of the pumping design or system. These deductions for elevation adjustment, vapor pressure, total suction lift, and the safety factor are used to determine the value for Net Positive Suction Head Available. Then the last adjustment for Net Positive Suction Head Required can be deducted. The result of this calculation completes the Net Positive Suction Head. It is important to note that the NPSH value must be greater than or equal to zero for the pumps and system to function properly. The negative result of a value less than zero is cavitation within the pump and premature failure of the pump, engine and other pumping systems will result. A common misunderstanding is that cavitation may have something to do with the pump not priming, cavitation only refers to the dynamic operational condition the pump experiences. It is only when there is a reduction due to elevation that results in a negative number that will cause the pump not to prime. The engineers at NCS Fluid Handling Systems can provide the details around the failure and the result of the pump being placed at an elevation higher than atmospheric pressure and therefore not being able to support the required static suction lift. It is also important to note that a larger calculated number above zero does not mean a better operating pumping system, It simply means that there is enough atmospheric pressure available to push fluid into the pump and maintain that amount of available fluid supply during pumping operations.

Water pumping systems are dynamic and Net Positive Suction Head is often calculated during the design phases of a pump and system and NPSH may be dismissed upon completion of the design. An important rule to remember is to confirm once the system is placed and that the NPSH calculation will need to be checked if speed changes due to an increased flow requirement or a pumping system suction piping revision have been made. The increase in speed will increase the velocity of the liquid in the suction pipe and the increase in velocity will increase friction losses. Therefore, the total dynamic suction lift will increase, and this additional flow rate will also increase the NPSHr deduction. Check your math and get the correct answers from pumping professionals like NCS Fluid Handling Systems. Call 1-888-618-7867 or Tech Services directly at 604-679-0427.

www.ncsfluidsystems.ca get you directly to the solutions.