Considering NPSH When Making a Pump Selection

What are the things you look for when making a pump selection?

Which selection offers the highest efficiency at the rated condition?

Does the operating condition fall near the pump’s best efficiency point?

Which selection requires the smallest motor rating?

While those are all great factors to consider, NPSH is another factor that you must verify before settling on a pump selection.

What’s So Important About NPSH?

One common mistake many people make when new to the pump industry is to overlook Net Positive Suction Head (NPSH) when making a pump selection. When considering pump selections, the first thing we are taught is to look for a selection with high efficiency at the rated condition, and one where the rated condition falls close to the best efficiency point. When operating under an overwhelming mountain of new information, those who are new to the industry will sometimes forget to make sure the Net Positive Suction Head Required (NPSHr) characteristics of the pump are suitable for the application.

While it’s easy to understand how this happens, selecting a pump with excessive NPSH requirements can lead to large problems further down the road. While a pump may provide acceptable performance despite lower efficiencies, a pump that requires more NPSH than the system makes available will never operate as designed, and internal damage can be expected in relatively short order.

Ensuring the NPSH requirements of the pump are compatible with the NPSH made available by the system in which the pump is installed is a critical step in the pump selection process.

What Exactly is NPSH?

When talking about NPSH, there are three terms we need to get a handle one:

l Net Positive Suction Head Required (NPSHr)

l Net Positive Suction Head Available (NPSHa)

l Cavitation

We’ll take just a moment to briefly touch on each of these terms. However, if you want to learn a lot more about this topic please review this article by Jacques Chaurette.

Let’s start with cavitation.

Cavitation is the formation of small vapor bubbles in a liquid which then collapse almost as quickly as they form.

In a pump, cavitation occurs at the point of lowest pressure, which is at the inlet to the impeller vanes. When the vapor bubbles produced by cavitation collapse they generate a tremendous amount of pressure in a very small point. The result is a burst of tremendous pressure hitting the inlet portion of the impeller vanes. As a result, over time cavitation will cause significant internal damage to a pump impeller.

NPSHa is a measure of how close a liquid is too converting into a vapor and cavitating.

At sea level, the atmosphere exerts approximately 14.7 psia of pressure. The NPSHa present at the surface of a body of fresh water at sea level is approximately 14.7 psia minus the vapor pressure of the water, which varies based on temperature. If NPSHa is measured at a point above or below the surface of a body of fresh water, then the vertical distance between the surface and the point of measurement must be added (if positive) or subtracted (if negative) from the NPSHa.

NPSHr is the absolute pressure that must be present in a liquid for a pump to avoid cavitation while pumping the liquid.

NPSHr varies from one impeller and casing design to the next. NPSHr is also influenced by the pump’s materials of construction, wear within the pump, and the operating speed of the pump.

The bottom line is that NPSH available (NPSHa) in the pump system must be more than the NPSH required (NPSHr) of the pump at all times. If the situation is ever reversed, and the NPSHa of the pump system falls below the NPSHr of the pump, excessive cavitation will occur within the pump.

Once excessive cavitation occurs, pump performance will deteriorate immediately and significantly. If the situation is allowed to continue on a regular basis or for extended periods of time, internal damage to the pump will occur.

How to Select a Pump with Acceptable NPSHr Characteristics

They key to making a good pump selection with regard to NPSH is to make sure you have all of the information before making a pump selection. Do not attempt to make a pump selection without know what the suction conditions are and the NPSHa that will be available to the pump during all operating conditions.

A good rule of thumb is to require a 10% margin or 5 Ft of difference between NPSHa and NPSHr, whichever is greater.

For an example of how this might work in practice, take a look at the NPSHr curve below.

NPSHr Curve

For the sake of example, let’s say that this pump is going to operate a 1000 GPM most of the time and 1350 GPM some of the time. Using the rule of thumb above, we can come up with the following table.

As you can see, in this example the 5 Ft minimum margin is greater than the 10% minimum margin, so we go with the 5 Ft margin. Based on this information, we would recommend this pump only if NPSHa is at least 12.5 Ft with the pump operating at 1000 GPM, and at least 15 Ft while the pump is operating at 1350 GPM.

Practically speaking, NPSHa values of 12.5 Ft and 15 Ft are fairly low, and for the vast majority of cases a pump with these NPSHr characteristics would be suitable. However, there are two possible instances where this pump could run into trouble:

Suction lift: A suction lift arrangement is one where the suction source for the pump is below the pump. In that arrangement, it is possible for the water to be lifted far enough that NPSHa drops below 15 Ft.

High temperatures: As the temperature of a liquid increases, its vapor pressure increases and NPSHa reduces as a result. If this pump were applied in a high temperature application we would want to keep a careful eye on NPSHa.

The Consequences of Making a Bad Selection

What happens if we mess up and select a pump with NPSHr characteristics that aren’t a good fit for the system? Simply put: excessive cavitation.

If a pump is operating with inadequate NPSHa, then the combination of velocity head and friction head within the pump will cause small air bubbles to rapidly and repeatedly form and collapse at the lowest pressure point in the pump impeller.

If cavitation is allowed to continue for a prolonged period, the pump impeller will be damaged. Over time, cavitation will cause wear within the impeller, which will increase the NPSH required by the pump and increase the amount of cavitation occurring within the pump.

Thankfully, cavitation is generally a noisy phenomenon, and a pump experiencing cavitation is generally easy to identify. As a result, remedial steps can usually be taken before damage occurs as long as someone is monitoring performance of the pump.

In addition, a pump operating with inadequate NPSHa will produce diminished performance in the form of reduced head and flow production. Pumps operating with inadequate NPSHa are often said to be running “off the curve” meaning that the point the pump is operating at falls below, and not on, the published performance curve.

What Else Should a Pump Professional Know About NPSH?

One thing to keep in mind is that NPSHr and NPSHa are expressed in absolute pressure and not gauge pressure. In other words, NPSH includes atmospheric pressure. This is because the phenomenon we’re trying to avoid is vaporizing of liquid, and atmospheric pressure is what keeps water from vaporizing. However, keep in mind that in a closed-loop system (one that is not open to the atmosphere at any point), atmospheric pressure is not acting on the liquid, and NPSHa within these types of systems is sometimes very low.

Another topic that is related to NPSH, and that every pump professional needs to be at least somewhat familiar within is Suction Specific Speed (Nss).

Nss is an index of impeller design. It is a calculated unitless number that indexes impellers designs relative to each other. Basically, Nss takes the amount of head a pump generates, the speed at which it operates, and the amount of NPSH required for the pump to operate, and converts those values into a single value.

An aggressively designed impeller (high-head, high-speed, low NPSHr) will receive a high Nss value, while a more conservatively designed impeller will receive a lower Nss value.

High Nss values can be a predictor of a very high-energy pump, or of a pump with an oversized suction eye. These pumps are ones where care must be taken to maintain operation as close to BEP as possible as they are prone to experience vibration and cavitation as operation moves away from BEP, and some knowledgeable Engineers and pump users are wary of high Nss impeller designs.