Have you had a boiler that can’t maintain its water level and keeps dropping out on low water? Having serviced many different boiler rooms and systems, this is one of the more common problems that happens when a steam system is not maintained properly. It’s an issue that develops over time and is not readily apparent. So, what is the issue? First we have to talk about feedwater tanks.
What is a Feedwater Tank?
A feedwater tank is a non-pressurized vented vessel. The tank does not build pressure internally, as all flash steam is immediately vented to the atmosphere. The typical temperature of a feedwater tank is 180–190°F. This will vary system to system, but it’s very important to pay attention to this temperature when servicing a steam system.
Let’s say the feedwater tank has a normal operating temperature of 180°F. The pump is an on/off pump that only comes on when the boiler calls for water. Recently, maintenance personnel have noticed that the pumps run longer than usual. Normal run time used to be about 20 seconds, but now the pump runs for over a minute very loudly, and the boiler starts to intermittently go down on low water throughout the day. The sound is like marbles or gravel being sucked through the pump. You check the feedwater temperature and it now says 212°F and there is a plume of steam coming out of the feedwater vent. What’s the cause? The feedwater pumps are cavitating when trying to fill the boiler with water.
Cavitation is the rapid formation of vapor in water that occurs with rapid changes of pressure. This phenomenon of small rapid explosions inside the pump wears down pump impellors and housings. On properly designed and installed feedwater pumps, cavitation occurs when the inlet water to the pump is too hot and boils inside the pump. Since the feedwater tank is not pressurized, the pump relies on the suction head pressure from the tank to maintain good pumping characteristics inside the pump. (Look up pump NPSH for more details on this). Since the water at the suction of the pump is so close to boiling temperature, the rapid pressure change inside the pump will cause mini boiling of the water, which is where the marble or gravel sound comes from. The cavitation wears the pump out and eventually cannot supply the boiler with water, and you end up having to replace the pump.
So What Caused the Cavitation?
The increase in water temperature is generally the main cause of cavitation in a feedwater pump. It can be due to an improperly designed feedwater system, but for this example we are assuming the system had been working well. The increase in water temperature usually comes from steam blowing by steam traps. If steam traps start to fail, the steam will leak past and end up at the feedwater tank, artificially heating the tank beyond its design temperature and eventually rising out the vent. The only way to fix that steam plume is to fix the steam traps.
Checking Pump Operation
Check all the traps and make sure they are operating properly. You will probably find some that are leaking through and once they are replaced, the feedwater temperature should go back to normal inside the tank. When the temperature is around 180°F, the pump will be less likely to cavitate and therefore will be able to keep up with the boiler demand. Listening to the pumps operate is a great tool for troubleshooting and checking pump operation. The pump should have a consistent low drone that is even in tone. If you also hear a high-pitched squealing or the dreaded marbles through the pipes, this is an indicator that the pump may be cavitating and needs service.