Issue: 8/05

Editor’s Note: “Back to Basics” is a column that will run periodically in PME reviewing the basic principles of plumbing engineering.

Call me strange, but I always enjoy standing in a mechanical room where a circulator is running so quietly that I can barely hear it. To me this epitomizes one of the benefits of a quality hydronic system—the silent conveyance of heat from where it’s produced to where it’s needed.

It’s unfortunate that not every hydronic circulator operates in such virtual silence. Beyond the fact that larger circulators produce higher sound levels, there are small circulators that sound like a cross between an espresso machine and an empty can of shaving cream. They have the mechanical equivalent of acute indigestion. Left unchecked, such an operating condition is both annoying and destructive.

Although some churning sounds from circulators are inevitable as the system is initially deaerated, sustained cracking sounds coming from the circulator’s volute are often indicative of vapor cavitation. It’s a condition that can be elusive at times, coming and going as the system operates in different modes and temperatures.

Fortunately, cavitation can usually be avoided through proper design and installation. This article discusses the cause of cavitation and how to avoid it through proper design and analysis.

What’s Happening?

Figure 1. Vapor pressure of water versus its temperature.

Vaporous cavitation occurs as water entering the eye of a circulator’s impeller flashes into vapor. You could say that the water “boils” as it enters the eye of the impeller.

Contrary to what some people think, water will boil over a wide range of temperatures both above and below 212˚F. Boiling occurs whenever and wherever the pressure of water drops below its vapor pressure (e.g., the minimum pressure that must be maintained on liquid water to prevent it from boiling).

The vapor pressure of water depends strongly on its temperature. The higher its temperature, the more pressure has to be maintained on water to prevent it from flashing into vapor. Figure 1 shows the relationship between water temperature and vapor pressure for water between 50˚F and 250˚F.

The vertical axis on the right side of the graph shows gauge pressure (e.g., the pressure on a standard pressure gauge at sea level). The vertical axis on the left side of the graph shows absolute pressure (e.g., pressure relative to a perfect vacuum).

The most familiar boiling point of water is 212˚F when the water is at atmospheric pressure (e.g., 0 gauge pressure). If the water’s temperature increases above 212˚F, the pressure that must be maintained on it to prevent boiling increases. For example, if the water is heated to 250˚F, it must be maintained under at least 15 psi gauge pressure to prevent it from boiling. At temperatures less than 212˚F, the pressure at which boiling occurs is lower than standard (sea level) atmospheric pressure (14.7 psia). Figure 1 shows that water at 170˚F boils at a gauge pressure of about -8.6 psi. This explains why water boils at less than 212˚F at higher altitudes (where the atmosphere exerts less pressure).

Pressures that allow water to boil at substantially less than 212˚F can be created in hydronic systems, especially near the eye of the circulator’s impeller. The result is instantaneous formation of vapor pockets. The mixture of liquid and vapor entering the impeller is now a compressible fluid. This substantially increases noise and decreases the circulator’s ability to maintain design flow rate.

Implosive Results

Cavitation begins when the water entering the eye of the impeller flashes into millions of tiny vapor pockets. The density of this vapor is about 1,500 less than that of liquid water. In other words, the water molecules take up about 1,500 times more space as vapor compared to liquid. This is comparable to a single kernel of popcorn expanding to the size of a baseball.

What happens next is arguably the most interesting but also the most destructive aspect of vaporous cavitation. As the mixture of liquid and vapor flows outward between the impeller disks, its pressure increases because of the head energy it is gaining. When the mixture reaches its vapor pressure, the vapor pockets instantly collapse like tiny punctured balloons. This is what causes the “crackling” sounds characteristic of vaporous cavitation.

As the vapor pockets collapse, tiny amounts of liquid water are accelerated into the voids, in some cases reaching velocities in excess of twice the speed of sound! These tiny bullets of water are called “microjets.” Although tiny in size and duration, they are potent missiles capable of continually ripping away tiny amounts of metal from the surfaces around them. Over time, their effect can be devastating. After seeing an impeller severely eroded by cavitation, you might conclude it was used to convey wet gravel rather than water.

Don’t Aid the Enemy

Figure 2. Piping details that both discourage and encourage cavitation.

Unfortunately, some hydronic systems are designed, installed, or operated such that cavitation is unknowingly encouraged.

Any design or installation detail that creates a drop in pressure near the inlet of a circulator is a potential cause of vaporous cavitation. If such details can collectively pull the water pressure down below its vapor pressure, cavitation instantly occurs. The lower the pressure at the eye of the impeller, the more severe the symptoms.

The following design/installation/operating details should be followed to avoid cavitation:

• Don’t locate the circulator upstream (pumping toward) the expansion tank. The differential pressure created by a circulator pumping toward the expansion tank is subtracted from the static water pressure at the circulator’s inlet. If the resulting pressure drops to or below the vapor pressure, cavitation will occur.

• Don’t put throttling valves, flow-checks, or other components with high flow resistance near the circulator’s inlet. Anything that creates significant turbulence or pressure drop at the circulator’s inlet should be avoided. As a rule, install at least 12 pipe-diameters of straight pipe upstream of all in-line circulators.

• Don’t operate the system at low water pressure. The lower the static pressure at the circulator inlet, the smaller the pressure drop required to initiate cavitation.

• Don’t design for high water temperature operation. The higher the water temperature, the higher the system pressure must be to suppress cavitation. You may even come across a circulator that operates quietly when cool, then starts crackling away when the system reaches a higher temperature.

• Do install a good air separator. Dissolved gases in water can create “gaseous” cavitation as they are driven out of solution during the first few heating cycles. Although not as destructive as vaporous cavitation, gases going in and out of solution as they pass through the circulator will create noise.

Figure 2 shows some examples of these design/installation considerations.

Predicting Cavitation

Figure 3. Density of water versus its temperature.

Over the years, engineers have established a standardized method of predicting when a pump will cavitate within a given piping system. This method is based on a quantity called Net Positive Suction Head (NPSH). It combines the effect of fluid density, pressure and flow velocity relative to the formation of vaporous cavitation.

Since the piping through which a fluid travels affects its temperature, pressure and velocity, the value of NPSH will be different at different locations in the system. When trying to avoid cavitation, the location of interest is usually the circulator’s inlet port. To be specific, we can say that a given piping configuration and its operating conditions make a certain NPSH Available (NPSHA) to the circulator at its inlet.

Part of the head energy a fluid possesses as it enters a circulator depends on its pressure. If a pressure gauge were mounted near the circulator’s inlet, its reading could be converted to a head value by multiplying psi by 144 and dividing by the fluid’s density (in lb/cubic-foot).

Table 1

The same fluid also contains head energy due to its velocity. The faster the flow moves, the more “velocity head” it possesses. To calculate this, you need to find the fluid’s velocity (in feet per second), square it, and divide by 64.4. The equations in Table 1 can be used to calculate flow velocity in various sizes of copper tubing.

The fluid’s vapor pressure (the pressure at which vapor pockets begin forming) also gets factored into NPSHA. It sets the “threshold head” above which the fluid must be kept if cavitation is to be avoided.

Equation 1 pulls all these head contributions together mathematically.

Equation 1

Equation 1

Where:

v = velocity of the fluid entering circulator (ft/sec)

p_i = pressure at circulator inlet (psi gauge)

p_v = absolute vapor pressure of fluid entering circulator (psia)

D = density of the fluid entering the circulator, (lb/cu.-ft.) (see Figure 3)

NPSHA could be described as the total head of the fluid above the head value at which cavitation would occur; hence, the word net. Like any value for head, NPSH is expressed in feet.

Here’s an Example

Equation 1 with figures filled in.

Assume water enters a circulator through a 1-inch copper tube at a flow rate of 12 gallons per minute. The water’s temperature is 160˚F, and a pressure gauge at the circulator inlet reads 10 psi. What NPSHA does this piping system make available to the circulator?

Solution: Start by getting the flow velocity for the water using Table 1.

v = 0.367(12) = 4.4 ft/sec.

Now look up the vapor pressure and density of water at 160˚F from Figures 1 and 3 respectively:

Vapor pressure = 4.7 psia.

Density = 61.0 lb/ft³

Finally, put these numbers in Equation 1.

So, what does this number tell us? By itself, not much. To make it relevant, we need something to compare it to.

That’s where the pump manufacturer comes in. With the circulator mounted in a test stand and operating at a specific flow rate, technicians reduce the NPSH available at the inlet of a circulator until cavitation occurs. The resulting number is called the Net Positive Suction Head Required (NPSHR) at that flow rate.

The test procedure is repeated at different flow rates because higher flows increase the pressure drop through the inlet of the volute, and thus, increase the NPSHR of the circulator. Many manufacturers provide a NPSHR vs. flow rate curve for a given circulator on the same graph as the pump curve.

NPSHR values may not be published for smaller wet rotor circulators. In such cases, use a (conservative) value of five feet of head.

The Bottom Line

To avoid cavitation, make sure the NPSHA provided by the piping system and its operating conditions is greater than the NPSHR of the circulator. The greater the NPSHA is relative to the NPSHR, the greater the safety margin against cavitation. When making the comparison, use the NPSHR value at approximately the same flow rate at which the circulator will operate.

Cavitation, like corrosion, is best avoided through good design rather than corrected when it suddenly appears. Use the design concepts we’ve discussed and the mathematical checks to keep the circulators in your systems cavitation free. Then, relish in the virtually silent conveyance of comfort only hydronic systems can provide.

PME Congratulates John Siegenthaler for 2005 APEX Award!

PME would like to congratulate John Siegenthaler for winning a 2005 APEX Award of Excellence for technical writing for his article, “Hydronic Fundamentals, Part 1,” from the January 2004 issue. This is his third consecutive year of winning an APEX award.