NPSH: Not Pumping So Hot
Wouldn’t that be a profound discovery if it were in fact true? Especially if ol’ Hydronicus wrote it after fixing a shredded paddle wheel in one of his lead-piped heating systems.
Last month we looked at what causes cavitation in circulators. We talked about several installation details that actually “invite” cavitation. Obviously the idea is to avoid such details. Good installation practices, like pumping away from the expansion tank; keeping the water pressure at a reasonable level; and not installing valves upstream of the circulator all help prevent cavitation. Occasionally however, you may have to lay out a system that puts you outside your comfort zone regarding circulator cavitation. What you need is a way to verify that cavitation definitely won’t occur.
Long Name, But Nice Concept: Over the years pump engineers have established a standardized method for predicting cavitation thresholds in many types of piping systems. Of course, being engineers they were unable to communicate their ideas without the assistance of numbers and formulas. And once they had their numbers and formulas, they had to come up with a name that sounded sophisticated. (We engineers do this for job security reasons you know.) Hence was born the techno-acronym Net Positive Suction Head, or NPSH.
Now that I’ve picked on engineers a bit, let me say that NPSH is really a pretty simple concept. In essence it’s just a way to predict how far removed the fluid entering a pump is from its boiling point. It’s kind of a numerical safety margin. Stay within the safety margin and you don’t get cavitation.
NPSH is a concise way of describing the overall condition of a fluid as it enters a circulator. It combines the effect of temperature, pressure and fluid velocity into a single number. Obviously, the piping system through which the fluid travels affects the temperature, pressure and velocity it has entering the circulator. Because of this we say that the piping system makes a certain value of NPSH “available” to the circulator. And because five letters are even more impressive than four, the term NPSHA was adopted. The A stands for “available.”
Part of the head a fluid has going into the circulator depends on its pressure. If a pressure gauge was mounted near the circulator’s inlet, its (psi) reading could be converted to a head value by multiplying by 144 and then dividing by the fluid’s density (in lb/cubic foot).
The same fluid also has head because of its velocity. The faster it’s moving, the more “velocity head” it has. To get a number, for this you need to find the fluid’s velocity (in ft. per second), multiply it by itself (e.g. “square it”) and then divide the result by 64.4. Use the graph in Figure 2 to get fluid velocities in copper tubes at different flow rates.
The fluid’s vapor pressure (the pressure at which vapor pockets begin to form) must also get factored into NPSHA. In effect it sets the “reference pressure” above which the fluid must be kept if cavitation is to be avoided.
The NPSHA concept puts all these things together mathematically. It could be described as the total head of the fluid above the head value at which cavitation will occur. Hence the word net. Like any other value for head, NPSH is expressed in feet. If you put all this together mathematically and clean up the algebra, you get the following formula:
v = the velocity of the fluid entering the circulator (in ft. per second, see Figure 1) pi = the pressure gauge reading at the circulator inlet (in psi gauge) pv = the absolute vapor pressure of the fluid going into the circulator (from red curve in Figure 3) D = the density of the fluid entering the circulator, (in lb/cubic ft., read from blue curve in Figure 3)
Example: What is the NPSH available to the circulator shown in Figure 1? Water enters the circulator at 12 gpm through a 1–inch copper tube. Its temperature is 160 degrees F. The inlet pressure gauge reads 10 psi.
Solution: First look up the velocity of water flowing at 12 gpm through the 1–inch tube. From Figure 2 you get 4.4 ft. per second. Next look up the density and vapor pressure of water at 160 degrees F from Figure 3: Density equals 61 lb/cubic ft., and vapor pressure equals 4.7 psia. Finally, plug these numbers into the formula:
So what does this number tell us? Well, by itself, not much. To make it useful we need something to compare it to. That is where the pump manufacturer comes in.
Yet Another Acronym: Manufacturers test their circulators to find where cavitation begins. In a test stand, manufacturers reduce the NPSH available at the inlet of a circulator until it cavitates. They add a safety factor, and call the resulting number the NPSHR. No, that last letter is not a misprint. The R stands for “required.” NPSHR is the manufacturer’s specified minimum value of NPSH that must be provided by the piping system to prevent the circulator from cavitating.
The test performed finds the circulator’s NPSHR at several different flow rates. As flow rate through a circulator increases, so does its NPSHR.
That is because the faster the fluid goes into the circulator, the greater the head loss due to fluid friction inside its volute. This head loss pulls the fluid closer to cavitation upstream of the impeller and must be accounted for. Many manufacturers show the NPSHR values for a circulator right on the same graph as its pump curve. Check it out in some of the pump catalogs in your office. NPSHR values may not be published for smaller wet rotor circulators. In such cases use a conservative value of 20 ft. of head. Assume this value applies over the entire flow range of the small circulator.
The Comparison: Now that we’ve gotten the acronyms described, the rest is a piece of cake. Plain and simple: To avoid cavitation make sure the NPSHA provided by the piping system is equal to, or preferably greater than, the NPSHR of the circulator. The greater the NPSHA value is compared to the NPSHR, the greater the safety margin against cavitation. When making the comparison, remember to use the NPSHR value at about the same flow rate the pump will operate at.
Undoubtedly Hydronicus Unprofitus would have been pleased to think that someone, someday, would apply his axiom regarding wet heat. You’ve now got a way to check for cavitation as you design, and take corrective actions, if necessary, to prevent it. Perhaps, we should update Hydronicus’s axiom to: “If in doubt … get your calculator out.”
The future of hydronic heating is bright and bountiful for those who avail themselves of the materials and design/installation methods currently offered, as well as those soon to appear. I’ll do my best to keep you posted on the technical issues of hydronic in 1998, while many other PM columnists help you manage your business affairs wisely. Stay tuned.