Sanitary Centrifugal Pumps Can Operate Effectively with Less Than a Full Atmosphere of Suction Pressure, but They Must Maintain Their Prime
We’ve talked plenty about sanitary pumps in this blog in the past. One topic we’ve touched on, but haven’t explicitly detailed, is NPSHA. Today’s post will focus on what NPSHA is and how it applies to your sanitary pumping application.
To begin, NPSHA stands for Net Positive Suction Head Available. NPSHA should not be confused for NPSHR, which stands for Net Positive Suction Head Required. NPSHA is a measure that corresponds to the level of pressure at the sanitary pump suction. The high the pressure the gauge at the pump suction reads, the higher the NPSHA, and the better the pump will operate. This pressure can be easily measured with a gauge at the pump inlet. Remember, most pressure gauges scales to atmospheric pressure, meaning they read zero when there is no pressure other than atmosphere (14.7 psi). In most cases, we can assume gravity will give us 34 ft of head.
The most important part of NPSHA is the head component. We’ve talked about fluid head in the past, and for NPSHA, the component of total head we’re most interested in is static head. The static head is the distance between the fluid level and the inlet of the pump. Generally speaking, the greater the static head, the greater the NPSHA.
After we know what atmosphere is giving us and how much static head we have, we need to net out or system losses. This is mainly friction loss. We’ll lose head as the fluid flows or is restricted through the system. The most common friction loss adders include, elbows, valves, and strainers.
So once you know what your NPSHA is, how do you know if you have enough? Well, that is where NPSHR comes back in. The pump manufacturer tests the pump under various suction head conditions and provides a requirement or NPSHR for each flow condition on the pump curve. All we have to do is check our NPSHA against our NPSHR and we’ll know if we have enough. Most pumps can operate with a suction pressure that is below atmospheric (below 34 ft or 14.7 PSIA). In these situations, however, it is very important to keep the suction line primed through the use of foot or check valves.
So what happens if in your pump application you don’t have enough suction head? What do you do? Well first, it will be pretty apparent, especially with centrifugal pumps, because you won’t be getting the flow you expect out of it. You may also have pump cavitation. It will sound like marbles are going through you pump.
So how do we fix it? Well, one factor that is commonly overlooked is supply tank level control. You may start out with a full vessel and have plenty of NPSHA, but as fluid is moved the level in the tank decreases, our NPSHA will also fall. To solve this, we can control the tank level with either point or continuous level sensors, or we can raise the height of the tank so that we’ll have enough suction head even at low tank levels. Another solution would be to lower the level of the tank inlet relative to the tank fluid level. You could also pressurize the vessel to juice your NPSHA.
Another issue we see often is locating elbows or fittings too close to the inlet of the pump. Fittings, such as 45’s, 90’s, and valves will all detract from the NPSHA. We recommend minimizing the number of fittings leading up to the pump. We also want to locate the pump as close as possible to the tank, avoiding long straight lengths leading up to the pump. We also want our suction line to be as large as possible. This is why the inlet of a sanitary pump is usually larger than the discharge.
So there you have it- a detailed overview of most everything you’d want to know about NPSHA in your sanitary application and how you can fix the problem. Remember, if you’re experiencing noisy operation, capacity loss, or pitting, check to make sure you’re supplying adequate pressure to the inlet pump. If you have any additional questions about your next sanitary pump application, contact a Holland Sales Engineer today.