Author Topic: When Three-Element Drum Level Control Goes Out of Control  (Read 956 times)

Offline Jason R

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Three drum element control works by keeping the feed water flow rate into the boiler equal with the amount of steam leaving the boiler. The third variable is the measurement of the drum level, which has the minor role of correcting for small drum level variances (like trim controls in an airplane). But what happens when you lose either flow transmitter?

Losing either flow transmitter

Three-element drum level control compares feed water flow and steam flow as a ratio. The controller is trying to keep this ratio one-to-one and will either decrease or increase feed water flow accordingly.

Losing the signal from the feed water flow transmitter, in the most extreme circumstances, will result in a feed-water flow of zero units. If the steam flow transmitter is indicating, for example, a flow of 15 units, the drum level controller will try to increase feed water flow to match the steam flow from the drum. This results in the drum level rising to dangerously high levels but this is prevented, luckily, by the signal from the drum level transmitter. However, the drum level transmitter is calibrated to only correct for small variances in boiler drum level and its rate-and-reset tuning will not stabilize large level swings. In fact, the drum level will be rising and falling like a yo-yo.

Whereas losing the signal from the feed water flow transmitter results in a high drum level, losing the signal from the steam flow transmitter results in a low drum level. If the steam flow transmitter is indicating, for example, a flow of 0 units of steam, the drum level controller will try to decrease feed water flow to match the steam flow. This results in the drum level decreasing to dangerously low levels. Again this is prevent by the signal from the drum level transmitter but results in the rising and falling drum level.

If a transmitter failure compromises the drum-level control strategy, an operator should take manual control.

Causes of flow transmitter failure

There are a number of causes of transmitter failures with each cause ranging from simple to complex. Most of the time, the problems are simple to correct.

The flow transmitter has a specific flow range; it hasn?t failed

This is a problem when a boiler is at bar- minimum load or just starting up. The steam and feed water flows are so low that they don?t even create a signal by the flow transmitter. At this point, the drum level should be controlled manually.

The transmitter is isolated from the flow sensing element

During maintenance, or other reasons, the isolating valves to the transmitter are closed. Also, the equalizing valve on the transmitter may be open.

Dirt and air in the transmitter sensing lines

Dirt and air in the sensing transmitter sensing lines could be causing a bad pressure differential across the transmitter?s diaphragm. If this is the case, the sensing lines should be vented and blown down as per procedure.

In the case of a steam flow transmitter, it could take hours for the sensing lines to fill back up with condensate. This can be done faster by applying wet rags to the condensate pots connected to the primary flow element to help condense the steam faster. Another option is to carefully fill each condensate leg with condensate.

A bad transmitter or wiring

The transmitter may be faulty. It should be properly assessed by someone familiar with the transmitter (like an instrumentation mechanic). For humility, be sure that the problem is not being caused by a simpler reason as outlined above.
« Last Edit: Nov 09, 2014, 16:40 by JasonR »
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