Don't forget to take vapor pressure into consideration.
by Jim Elsey
September 11, 2018
This is the third article in a five-part series. Read the first part of the series here and the second part of the series here.

In the first part of this five-part series, we defined net positive suction head available (NPSHa) and walked through a simple calculation based on ambient temperature water for a flooded suction condition. In part two, we worked through an almost identical situation except the water source was at a level below the pump centerline, which is a suction lift situation. In both cases, the systems were open to atmospheric pressure, located at the elevation of sea level. The liquid temperature was at ambient 68 F.

Flooded Suction Pumping Hot Water

In this third part of the series, we will investigate what happens when the liquid temperature is above ambient. Before we get to the formula and the calculation, I recommend a review of the definition for vapor pressure. I covered vapor pressure in my April 2018 Pumps & Systems column should you wish to revisit. A brief summary is as follows.

A liquid in an open container will eventually evaporate to a vapor unless some other force is present to prevent the change from occurring. In most examples, that force is simply atmospheric pressure. Even a bowl of water left on the kitchen counter will evaporate away over time, but it will happen at a significantly faster rate with each incremental increase in temperature. The amount of energy associated with the vapor pressure is subtracted from the total energy level available for the NPSHa. The energy associated with the vapor pressure is always a negative quantity. In the context of NPSHa, vapor pressure is not a friend to the pump.

For a given temperature, a liquid exerts a certain pressure to the atmosphere, and the atmosphere exerts a counter pressure in return. For water temperatures below 212 F, the atmospheric pressure is greater than the vapor pressure. Those two pressures (vapor and atmospheric) would be in equilibrium if the water was at 212 F at sea level, where the water would begin to boil. If the pump example was at higher elevations (lower ambient pressure and consequently a lower equilibrium temperature) the water would boil at a lower temperature.

Vapor pressures for water at various temperatures are easy to calculate or obtain in a reference source, but it is often very difficult to find reliable information on vapor pressures for other liquids. It is not uncommon for the end user/operator to be using the incorrect vapor pressure in their calculations.

The Formula for NPSHa

Remember that we are calculating NPSHa, so we do not need to include velocity head. Velocity head would be included if we were measuring NPSHa. Please refer to Image 1.

flooded suction hot waterImage 1. Flooded suction with hot water (Image courtesy of the author)

The water level is 10 feet (hst) above the pump centerline, and let us assume the water level will remain at that height for this example. We will assume the supply rate is the same as the demand rate. In a real world situation, you must calculate NPSHa based on the expected worst-case scenario. For example, the application could be a batch process where the tank will be nearly empty at some point.

NPSHa formula

For ease in working the example, I have calculated the total friction losses (hf) as 3.2 feet. Note that friction losses are technically lower for hot water than cold, but we will ignore the small difference for the examples in this series.

The tank is open to atmospheric pressure, and the system is located at an elevation of sea level. The absolute pressure in feet of head (ha) is 35.4 feet as a result. Remember from the formal NPSHa definition for absolute pressure (ha)… “is the absolute pressure as measured in feet of head of the liquid being pumped at the surface of the liquid.” Absolute atmospheric pressure is 14.7 pounds per square inch absolute (psia). To convert to feet, multiply by 2.31 and divide by the specific gravity of the liquid being pumped.

Equation 2

In real life examples, the actual atmospheric pressure will be slightly lower and vary with weather/barometric pressure and elevation above or below sea level.

Standard pressure at sea level may also be listed as 14.696 psia in lieu of 14.7. Either way, it rounds off to 14.7 psia. The more important fact is to know the actual elevation for the pump location and adjust your calculations accordingly.