Maintaining proper drum level in your industrial boiler is critical for the longevity of the pressure parts as well as the downstream equipment. Most modern boilers are fitted with automatic high and low drum level trip switches and drum level controls for this very reason. If your boiler does not have these vital safety components, you should seriously consider installing them before a mishap causes major damage to your boiler system or downstream equipment, as well as extended downtime for the processes that depend on them.
Drum level trips are needed because the fired fuels and flue gas temperatures in an industrial boiler can easily exceed the maximum allowable design temperature of the steel in the tubes and pressure parts. It is the water and steam on the opposite side of the tubes that keep the steel cool so it maintains its strength and abrasion resistance. Without cooling steam and water, the pressure parts would soften and degrade rapidly, dramatically shortening the expected life or causing a tube leak that requires a shutdown of the boiler system.
For the protection of the industrial boiler components themselves, a low water level trip is necessary. Level controls ensure that an adequate amount of water is present in the boiler at all times while a fire is present in the boiler. If the level drops below the minimum allowable level, the controls trip the fire and shut down the forced draft fan to prevent overheating of the tubes. In a gas-fired boiler, the cool down is rapid after the burners are tripped. In a solid-fuel-fired boiler, the cool down is much slower, since the residual fuel continues to produce heat. In solid-fuel boilers, an additional step, like dumping the grates, may be necessary to get the heat out of the furnace quickly to minimize damage.
For the protection of the components downstream of the industrial boiler, a high-level drum trip is required. A high drum level allows low quality (wet) steam to carry over into the superheater or downstream equipment. This wet steam can quickly damage components designed to see only gas (due to impingement or pitting), or at a minimum create scale in the tubes, reducing heat transfer and eventually causing them to overheat and fail. Wet steam is particularly troublesome to steam turbines, which have been known to wreck when low quality steam is introduced to the HP section. Like the low-level trip, a high drum level trip will shut off the fuel and shut down the FD fan; it will also trip the feedwater pump and open the rapid drain/blowdown valves, if the boiler is so equipped.
High- and low-level trip devices should be installed strategically in the drum or level gauge column and wired in series with the FD fan, burner valves and feedwater pump (high level only). These devices are inexpensive to purchase and install, and should be used for redundancy even if the boiler is equipped with a drum level transmitter.
To maintain the longevity of your industrial boiler, water control systems in the form of automatic high and low drum level trips are critical. They can prevent catastrophic failure of the pressure components due to overheating, and protect your expensive downstream equipment from damage or failure. If your boiler system is not equipped with these devices, take the steps to get them installed now.