Servicing equipment during the Summer in Las Vegas gives one the opportunity to deal with heat-related failures to machinery in general, and boiler room machinery in particular, with the leading cause of boiler failure being the condensate return pump.
Vapor lock, or the more scientific name “pump cavitation,” is a condition in which bubbles of steam vapor form between the impeller (rotating part of the pump) and raceway (stationary part of the pump) of the return pump. This condition prevents the pump from pushing water under pressure into the boiler.
The most frequent cause of this condition is probably faulty check valves. Check valves allow fluids, water in this case, to flow only from the return pump into the boiler. Scale and debris in the piping and return tank often break off and prevent these valves from shutting completely. When this occurs, water under high pressure in the boiler (remember this water is above the normal boiling point because it is under pressure) is forced back through the piping into the pump. As the pressure on the super-heated water is reduced, the water flashes (boils) into steam inside the pump, preventing the pump from pushing more water into the boiler, causing vapor lock. You can often identify this problem by the “knocking” sound coming from the return tank and very hot piping from the boiler to the pump (if the check valves are faulty, spit will evaporate in a second or two on these pipes). In my experience, it doesn’t pay to attempt to repair these valves; just replace them.
YOU MUST SHUT OFF THE BOILER AND BLOW DOWN UNTIL THERE IS NO BOILER PRESSURE! DO NOT DO ANY REPAIRS ON A BOILER UNDER PRESSURE UNDER ANY CIRCUMSTANCES!
When the boiler has no pressure, replace the check valves (you might as well do them all). Be prepared for discovering a lot of damaged piping when you start repairs in this area. While you’re this far into the job, you might as well clean the Y strainer between the return tank and condensate pump.
Another type of component failure I frequently see is leaking ball float valves. This type of valve uses a ball-shaped float, similar in operation to the valve in your toilet tank, to control the addition of new untreated water to the condensate return tank. These valves nearly always leak and are often the root cause of rapidly developing piping failure. When these valves leak, untreated water continuously bleeds into the return tank, diluting boiler compound and wasting energy by cooling the condensate return tank (SEE THIS MONTH’S EZTIMERS AD ABOUT THE EZLEVEL DESIGNED TO END THIS BOILER DAMAGE).
Excessive temperature in the return tank is another source of vapor. Often, this is the result of steam traps sticking open (blowing through) and allowing a continuous flow of steam back into the return tank, raising the water temperature to a point where the pump cavitates becoming ineffective; the excess steam, mechanical action of the pump, and extra heat supplied by the pump motor are enough to heat the water inside the pump to the boiling point causing vapor lock. If several traps are blowing through, there will be a continuous stream of steam from the return tank vent on the roof. An easy way to troubleshoot this problem is to get a bag of ice and place it on the pump (not the pump motor, but the pump where the piping is). Alternately, if you have access to a means of adding cold water to the return tank itself, this often works. Using either method usually yields results within a minute or two, and the pump will begin to move water into the boiler. Obviously, if this is the problem, you need to find and then repair or replace the faulty steam traps (SEE OUR AD IN THIS MONTH’S ISSUE FOR A GREAT STEAM TRAP TESTER).
Boiler water inlet piping, as well as check valves and the boiler itself, are subject to both rapid pipe corrosion and accumulations of scale and large particles of debris. This type of deterioration is nearly always caused by improperly treated make-up (often referred to as city) water. Untreated water is high in dissolved oxygen content, and the oxygen in the water combines with carbon dioxide to form carbonic acid, which corrodes metal and leads to the clogging of piping. Additionally, elevated levels of dissolved solids are present in untreated water and are the cause of scale formation, which will also lead to clogging of pipes, valves, and boiler tubes. To control this situation, SOFT WATER AND BOILER COMPOUND ARE ESSENTIAL in most areas.
Contact me with any suggestions for future articles or service questions. I’m reachable at 702-376-6693.
