Boiler Drums and Level Indicators

Generally speaking a drum will have a differential pressure transmitter giving signal to drum level electronic controller in control room, a Truescale type of indicator that visually indicated and has contact alarms for control room, a level switch device that will cause instant shutdown on extra Hi or extra low water level, and one or two sight glasses, usually one at either end of the drum. Good practice is for every device mentioned to have its own pair of tappings and isolation valves on the drum, in case of malfunction/blockage/human error in opening after a shut.

Rok

I believe that code requires a gauge glass on the boiler drum, in spite of their frequent issues. The plant I worked at had 3 level transmitters for a 2 out of 3 shut down on lolo water level and a level gauge on each side of the drum. One was a magnetic type gauge (I believe a K-Tek (sp?)) and the other was a bullet glass sight glass. I believe the boiler code will spell out most of the requirements for you.

Hope that this helps, good luck.

ASME section I still mandates at least one "direct visual water indicator" ie. gauge glass for HP steam boilers. Anything over a certain pressure requires 2 gauge glass indicators, one of which can be replaced by a continuously displayed electronic representation of the level in the control room.

Betomachine describes a very robust system that should be the standard for any high pressure boiler. 2 out of 3 voting is becoming more common for very good insurance against running the boiler dry or carrying over to a turbine, while reducing nuiscance trips due to a single failed device. This system allows for online maintnenace and calibration of all devices.

In summary, 3 DP transmitters for control purposes, 1 other remote level indicator (magnetostrictive, capacitance probe, ...) to drive hard alarms and a single gauge glass (c/w viewing hood) to satisfy code. Gauge glass and "alarm" gauge can share a set of drum taps and all 3 transmitters can share a set of drum taps, as long as there are no intervening valves to isolate both at the same time.