Rethinking NPSH

Net positive suction head (NPSH) and its two main components—NPSH_R and NPSH_A—are an often misunderstood mystery to a large percentage of people in the pump industry. I have studied and catalogued more than 150 technical articles on NPSH in the last 40 years, and most have begun with comments about the complexity of the topic. A common statement in the pump industry is that 80 percent of all pump problems are on the suction side of the pump. I would state that, with the exception of operating the pump away from the best efficiency point (BEP), the percentage is much higher.

Rethinking the Concept

The responsibility and purpose of the centrifugal pump is to receive the liquid that the suction system delivers and move it downstream. The suction-side system, if properly designed and operated, delivers the fluid to the pump. The pump does not reach upstream and retrieve the fluid, nor is it capable of doing so. The common misconception is that the pump will "suck" the fluid from the suction system into the pump. Perhaps if liquids had tensile strength characteristics, that could be remotely possible (I acquiesce that the impeller does create a small differential pressure at the eye), but the suction-side system must have adequate energy to deliver the fluid to the pump. Using the analogy of a cellphone, if the suction-side system does not have enough "signal strength" (bars of energy), then the "call" will be dropped or be of poor quality—in other words, the pump will cavitate.

Suction Pressure

One of the most common errors I witness is confusing suction pressure with net positive suction head available (NPSH_A). Even people with decades of pump experience and education seem to make this mistake. A common comment is, "I do not need to calculate NPSHA because I have 135 psig of suction pressure." What they fail to understand is that the temperature of the fluid in this case is 350 degrees F. (Please assume water as the fluid for all examples in this article.) The formula for NPSH_A indicates that 100 percent of the negative head caused by the vapor pressure of the 350 degree fluid negates the positive head contributed by the pressure of 135 psig. After accounting for the losses that result from friction head, the only positive head available to make up the remaining energy (bars of signal strength) is the static head. Static head is the energy (bars) contributed by the elevation of the fluid over the centerline of the impeller. (Note: This article does not account for velocity head because of the fractional contribution and, in this case, flooded suction.) Pump users must also remember that NPSH is not pressure. Pressure is a force, but head is an energy level, and the suction pressure is only one of numerous components in the total makeup of NPSH. Another comment I often hear in the field is, "I do not need to calculate the NPSH_A because I have a flooded suction." Again, these individuals are not taking the negative factors of friction and vapor pressure into account.

Submergence

Submergence is the vertical distance from the top surface of the fluid to the centerline of the pump intake line. Submergence is applicable to both flooded and lift situations. If the submergence is not positively sufficient, then the velocity of the fluid in the suction line will create a vortex. The captured air will be ingested into the pump. Centrifugal pumps are not designed to pump (or compress) air, and the average centrifugal pump will drop performance quickly even with small amounts of entrained air. While certain designs, such as recessed impeller pumps, can handle up to 24 percent entrainment, just 12 percent will stall most pumps. This is vital because many people in the field confuse cavitation with air binding/entrainment. Every pump suction-side installation has a minimum submergence below which air will be ingested. The flow rate for a pipe of a given size, geometry and material makeup has a corresponding fluid velocity. The resultant velocity corresponds with an amount of required submergence (distance) to prevent the formation of a vortex. Keep in mind that just because you cannot see the vortex with the naked eye does not mean the vortex phenomenon is absent.

Vacuum

At the bottom section of most steam condensers is a collection area, usually a tank-shaped reservoir for the condensate commonly known as the hot well. In these applications, end users commonly make errors determining the correct absolute pressure when making the NPSH_A calculations. Pumps are subject to vacuum on the suction side in many other instances as well. The error is in the assumption that the vacuum level is equal to the absolute pressure. Consider a condenser with a vacuum level of 28 inches of mercury (Hg). Inexperienced users might incorrectly assume that they need to convert the vacuum level to a corresponding head, which they determine is the absolute value (see Equation 1). In reality, the actual absolute pressure is the difference between the existing vacuum and what would be the perfect vacuum or zero absolute pressure. Think about it as how much pressure remains if the vacuum is at some level, X, (as in this case of 28 inches Hg). A perfect vacuum would be 14.69 (atmospheric pressure at sea level) x 2.31 (the conversion factor) = 33.933 (rounded to 34 feet). Equation 1 ( Incorrect approach) 28 in/Hg vacuum x 1.135 conversion, in/Hg to feet of water = 31.78 feet \ At sea level, the atmospheric pressure typically supports a mercury column not more than 29.92 inches high. Therefore, the standard for atmospheric pressure at sea level is 29.92 inches Hg, which translates to an absolute pressure of 14.69 psia, which is usually rounded to 14.7 psia. So the true absolute pressure (to be converted to head) is really the difference between the two (see Equation 2). The correct absolute pressure converted to head is 2.22 feet not 31.78 feet. Equation 2 ( Correct approach) 34 - 31.78 = 2.22 feet (Note: I have rounded off and assumed sea level for the example) At some point, you will be required to calculate the value for NPSH available. Why not be ready to do it the right way and avoid the unnecessary drama and expensive corrections?