'Look Mom, No Cavitation!'
Call me weird, but one thing I always enjoy is standing in a mechanical room where a circulator is running so quietly that I’d have to put my ear on it to hear any sound. To me this epitomizes one of the benefits of a quality hydronic system — the silent conveyance of heat to where it’s needed. I could stand in that room for quite a while, serenely appreciating all those Btu’s carried along in near silence. Unfortunately not every circulator operates with such soothing silence. Besides the obvious fact that larger circulators inherently produce more sound, there’s the occasional small circulator that sounds like a cross between an espresso machine, and a can of shaving cream that’s almost empty. It has the mechanical equivalent of acute indigestion. Left unchecked its symptoms are not only annoying but often destructive. Although some churning is inevitable during initial start-up and deareating of the system, sustained cracking sounds are usually indicative of vapor cavitation. It’s a condition that can seem elusive — coming and going as the system operates in different modes and temperatures. Fortunately, cavitation can usually be corrected — and perhaps even more importantly — predicted. The key is to understand what causes it, and follow through by not creating conditions that “invite” it. “Boilony:” Cavitation begins as water entering a circulator’s impeller starts to boil. Contrary to what most people think water can boil over a wide range of temperatures both above and below 212 degrees F. Boiling occurs whenever and wherever the pressure of water drops below its vapor pressure. A practical definition of vapor pressure is the minimum pressure that must be maintained on water to prevent it from boiling. If you’ve ever opened a warm bottle of soda you’ve probably experienced the effect of lowering the pressure of a liquid below its vapor pressure. When the cap is on the bottle the liquid inside appears essentially free of bubbles. But pop the top and the pressure under which the soda was bottled is instantly released. The pressure on the liquid drops below its vapor pressure, and pockets of previously dissolved carbon dioxide come out of solution almost instantly. 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. For any water temperature between 50–250 degrees F, the blue curve gives the corresponding pressure at which water (at that temperature) begins to boil (e.g., its vapor pressure). When using the graph in Figure 1 keep in mind that the main vertical axis shows absolute pressure. This is the pressure on the water relative to a total vacuum. The outer pressure scale shows “gauge” pressure. As its name implies gauge pressure is what’s read off a standard pressure gauge (one that reads zero before it’s installed). The most familiar boiling point of water is when the absolute pressure on it is 14.7 psia, or 0 psi gauge pressure. It’s the pressure the atmosphere exerts at sea level. As the water’s temperature increases above 212 degrees F, the pressure that must be maintained on it to prevent boiling increases. For example, if the water is heated to 250 degrees F, it must be under a minimum of 15 psi (gauge) pressure to prevent it from boiling. At temperatures below 212 degrees F the pressure at which boiling begins is less than standard (sea level) atmospheric pressure (14.7 psia). Figure 1 indicates that water at 170 degrees F will boil at an absolute pressure of about 6 psia. This corresponds to a gauge pressure of –8.7 psi (e.g., 6 – 14.7). This explains why water boils at less than 212 degrees F at higher altitudes where the atmosphere exerts less pressure. Pressures that allow water to boil at substantially less than 212 degrees F can be created with relative ease in many hydronic systems, especially near the eye of the circulator’s impeller. The result is instantaneous formation of vapor pockets. The mixing of liquid and vapor entering the impeller is now a compressible fluid. In addition to noisy operation, the ability of the circulator to sustain its design flow rate is significantly reduced. Want to hear it? When the opportunity presents itself, slowly (and momentarily), turn the inlet isolation valve on an operating wet rotor circulator toward its closed position. At some point you’ll hear the water inside transform into a foamy mix. The experiment works best with hot water and relatively low system pressure. Another convincing demo of cavitation is to fold over and almost pinch off a garden hose carrying a fast stream of water. The Rice Krispies guys would really be jealous of the sound you’ll hear. Inward Reflection: Cavitation begins when part of the water entering the eye of the impeller flashes into millions of tiny vapor pockets. The density of the water vapor is about 1,500 less than that of liquid water. In other words, some of the water molecules take up about 1,500 times more space as vapor compared to as liquid. It’s comparable to a single kernel of popcorn expanding to the size of a baseball. Imagine what that would do to your microwave! What happens next is perhaps the most interesting if not the most destructive aspect of cavitation. As the mix of liquid and vapor flows out between the impeller disks its pressure increases (e.g., the impeller adds head to it). When the mix reaches its vapor pressure the vapor pockets instantly collapse like tiny punctured balloons, an effect called implosion. To get the proportions right think again of the baseball, only this time instantly collapsing back into a kernel of popcorn. The collapsing vapor pockets create the “crackling” sound associated with cavitation. Vapor pocket implosion can, under the right conditions create a phenomena that’s truly hard to imagine but nonetheless real. As the vapor pockets collapse tiny amounts of liquid water are accelerated into the voids, in some cases reaching velocities faster than the speed of sound! These tiny bullets of water are called “microjets.” Although tiny in size and duration they are none the less potent, in some cases impacting metal surfaces with such force as to literally rip away minute amounts of metal. Over time, however, their effect can be devastating. After seeing an impeller severely eroded by cavitation you might think it was used to pump liquid sand paper rather than clean water. Courting The Enemy: Believe it or not many hydronic systems have attributes that actually invite cavitation. Anything that encourages a drop in water pressure near the inlet of the circulator is a potential culprit. Whether acting alone or in concert, if these conditions can pull the water pressure down to its “trigger value,” (its vapor pressure) cavitation begins. The lower the pressure at the eye of the impeller, the more severe the symptoms. Here are some things to avoid: Don’t locate the circulator upstream (pumping toward) the expansion tank. If you do, the differential pressure created by the circulator is subtracted from the static water pressure at the pump inlet. Reach the magic number and you’ve got cavitation. Don’t put throttling valves, flow checks or other components with high flow resistance near the circulator’s inlet. Anything that creates a significant pressure drop coaxes the water closer to cavitation at the worst possible spot. As a general rule install at least 10 pipe diameters of straight pipe upstream of inline circulators. Use only isolation flanges or full-port ball valves for isolation. Don’t operate the system at low water pressure. The lower static pressure at the circulator inlet, the smaller the pressure drop required to bring on cavitation. Closing the make-up water valve will eventually cause the pressure to drop in any hydronic system. Maintaining a decent static pressure is especially important in low-rise buildings that don’t have the piping height to inherently generate the pressure. Don’t design for high water temperature operation. The higher the water temperature, the higher the system pressure must be to prevent boiling. Sometimes you may even come across a circulator that operates quietly at low water temperatures only to fizzle and pop when the system gets up to higher temperature. Do be especially careful in designing systems with high head circulators, or multiple circulators in series. The higher the pressure differential across the circulator(s), the more the pressure at the circulator’s inlet will drop if, for example, the circulator pumps toward the expansion tank, or the static pressure gets too low. Do install a good air separator to quickly purge the system of dissolved gases that can create “gaseous” as opposed to vapor cavitation. Although not as destructive as the latter, gases going in and out of solution as they pass through the circulator will still get your ear’s attention. Figure 2 shows a system that no enlightened Wet Head would ever install. Look it over and describe eight things that are wrong. (Look for the answers at the end of the column.) Decavitating: No, this doesn’t refer to some new dental procedure. It’s about what to do if you’re called in to fix a “noisy pump” complaint. After ruling out mechanical problems like motor bearings, or couplings, cavitation is almost surely the problem. Check the static pressure and water temperature near the circulator inlet. Remember that low pressures and high temperatures encourage cavitation. Also look for piping components that could cause a significant pressure drop just upstream of the circulator. Always check the location of the expansion tank relative to the circulator. One or more of these things is almost always responsible for creating the problem. Circulators pumping toward expansion tanks are common in older systems, as are high water temperatures. If, for example, the original circulator was replaced with a higher head model, or the original one-zone system was subsequently divided up with zone valves, there could well be a higher ³P across the circulator. Perhaps just enough higher to cross over the cavitation threshold under certain conditions. It’s also wise to check any strainers near the circulator inlet. Once you’ve confirmed cavitation, isolate the circulator, remove the impeller and inspect for damage. Look for abrasion of the impeller and volute. If anything but extremely minor abrasion is present, the affected parts should probably be replaced. Two adjustments that can sometimes correct mild cavitation are boosting the system’s static pressure and/or lowering the system’s operating water temperature. Obviously you have to be judicious in adjusting water pressure upward to avoid creating a new problem with the relief valve. Likewise, lowering the water temperature too much might correct cavitation but spawn the new complaint of insufficient heat delivery. Remember, it’s a system; changing anything can affect everything. If you’ve tried adjustments to static pressure and water temperature to no avail, and you’re convinced the circulator model is appropriate for the flow/head requirements of the circuit, it’s probably time for surgery. If valves, elbows or other head-robbing/turbulence-inducing components are located near the circulator’s inlet, they’ll probably have to be removed, or at least relocated. While you’re at it, be sure to relocate any expansion tank that’s installed downstream of the circulator. Next month we’ll look at ways of predicting the occurrence of cavitation on paper, rather than waiting to see if the circulator runs as smoothly as you hope it will. In the meantime, keep up the pressure to eliminate cavitation — literally. Answers to Figure 2: Flow-check and tight turns near circulator inlet creates significant pressure drop and turbulence. Pumping toward expansion tank lowers circulator inlet pressure. When only one zone is open circulator operates at high pressure differential. High water temperature and low pressure reduce safety margin against cavitation. Use adifferential bypass valve. No air separator is used. Misapplied high head circulator. Use circulators with “flat” pump curves for zone valve systems. Low-rise distribution system cannot inherently maintain system pressure. Closed make-up water valve will allow system water pressure to drop over time.
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