The case of the flooded boiler feed tank

By Ray Wohlfarth

 

One of the most overlooked issues inside a steam boiler room is water leaking from the steam system. At a recent visit to a brewery, water dripped steadily from the boiler feed tank overflow pipe. In my classes I refer to water leaks as a boiler killer. In addition to destroying the boiler, water leaks in a steam system can increase operating costs. The leaking water is water that the owner paid to heat and chemically treat. Each gallon or litre of water lost from the system is a gallon or litre that must be replaced. Fresh water contains oxygen and impurities that must be chemically treated and reheated. In addition, the freshwater will attack the metal surfaces and cause leaks or scale buildup. Looking at the tank gauge glass assured me what I thought, the tank was flooded. The normal water level in the tank should be about halfway up the gauge glass. There are several reasons for a flooded boiler feed tank.

 

Leaking check valve on boiler feed pump

Pressure always goes from high to low. This is one of the main tenets of steam boiler service. As shown in Figure 1, the feedwater pump discharge pipe should have a check valve on the outlet to stop water from leaving the boiler and backflowing to the boiler feed tank. Dirt from the system can accumulate inside the check valve, preventing it from closing tight.

This condition usually shows up when the boiler has been off for a while, and the system stabilizes. If the check valve on the boiler feed pump does not seat, water from the boiler could flow into the tank, flooding it. Water always tries to equalize itself.

Chances are the boiler water height is higher than the boiler feed tank. This will cause the boiler water to push back into the tank. The best way to diagnose this problem is to remove and inspect the check valve. Look for pitting or dirt accumulation inside the check valve. If there is, the check valve should be replaced. Additionally, using two spring-loaded check valves on the discharge piping helps prevent the boiler water from backing into the tank.

 

Restrictions in the return pipes

Restrictions in the return pipes usually show up about an hour or two after the boiler is started from a cold start. Most boiler feed systems are sized using a 20-to 30-minute cycle. When the steam leaves the boiler, the condensate should take about 15 to 20 minutes to return to the tank. If it takes longer the boiler feed tank will run out of water. The internal float valve will open and introduce fresh water to the tank. Once the condensate returns, there is too much water in the system and the tank floods.

This condition is more common on older steam systems, but it can occur on newer systems when the piping was not installed correctly. If it’s an old piping system, use an infrared temperature gun or thermal imaging camera to try to find the blockage. If it’s a new system, look for valves, rises, or restrictions in the return piping.

To diagnose this condition, begin with a cold system and the discharge valve closed on the boiler and the makeup water valve closed on the boiler feed tank. After starting the boiler, watch the pressure gauge. When steam pressure shows on the gauge, slowly open the boiler discharge valve and start a timer.

The hot condensate should return after 15 to 20 minutes. This can be verified by feeling the condensate pipe going into the boiler feed tank. Watch the water level in the boiler feed tank. If the tank runs out of water before the hot condensate returns, this indicates the condensate pipes are restricted, or the tank is undersized. If the system worked fine for a while and is now flooding, the tank is likely large enough, and the problem is with the condensate piping.

 

Leaking fill valve

The boiler feed tank has an internal float that monitors the water level in the tank. It looks like an old float inside a toilet. If the water level in the tank drops, the float valve will open and introduce fresh water into the tank to raise the water level to the set height. There are a couple of things to check on this.

To diagnose this condition, the first thing to do is to grab the makeup water pipe to the boiler feed tank. If the pipe feels cool, it could mean the fresh water is flowing into the tank. Look at the water level inside the tank. If the water is still feeding even though the water level is high enough, this usually means there is an issue with the float valve.

Check the incoming water pressure to the makeup water float valve. It should be around 20 psi or slightly lower. If the water pressure is higher than 20, the pressure could be high enough to force the float valve open and flood the tank. Try turning the incoming pressure below 20 psi. The makeup water pipe should have a pressure-reducing valve. If the pressure is below 20, the valve itself may be defective and should be replaced.

To check the float, unscrew the float from the rod and visually inspect it. It should be airtight with no accumulation on the float. Shake it to listen for any water inside the float. You could even place it in a sink filled with water to verify it floats. If the float is intact, the next step is to inspect the feed water valve. Look for scale or dirt inside the valve, which could stop it from closing tightly. If the valve, float, and incoming pressure are correct, look at the previous two causes.

Install a water meter on the makeup water pipe to monitor how much water is being introduced. This helps the water treatment technician when recommending the proper water treatment and also helps you diagnose a leak in the system.

 

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