Boiler Water Level Control

My understanding is that there must be 3 different types of level devices used together on a drum level for safety

I suppose it depends which country you are in and its rules. In Australia we only have to have two types. One for level control and the other for a independant check.

Here in Tasmania we use 3 types. One electrical transmitter that gives the level controller a PV, one non-electrical indicator that gives the operators a chance to see the level in a power outage or controlroom catastrophy, and one that is purely a selfcontained crash shutdown device that hopefully never comes into actuation that stops pumps/postitons vent valves ect. Of course there should be a sight glass at either end of the drum.

Rok

By using the Hydrastep which measures resistance between a probe and the wall of the enclosure, I think that the readings are more accurate than the displacer type because of of the density changes in the water due to the temp. Am I correct in this line of reasoning. I intend on using differential pressure type transmitter for my water level control and the Hydratstep for my safety shutdowns. Any comments or suggestions as I am not a Boiler specialist.

It sounds like you've got a solid plan. The Hydrastep will be most accurate because its sensing is insensitive to temperature/pressure changes. The manufacturer's data sheet for the Hydrastep gives some impressive numbers:

"An independent report by Factory Mutual Research concluded that the probability of Hydrastep missing a trip condition is less than 1 in 300 million and that nuisance trips will be less than 1 in 10 million."

I'd bet on those odds.

The differential instrument should work well to give an analog input to the DCS. However, both the displacer and the differential can give erroneous readings when boiler conditions are outside of assumed values. It may be necessary to place level control in manual during startup, shutdown & large transients. That will have to be part of operator training.

If your installation is in the US, remember that the ASME Boiler Code requires both direct (gauge-glass) and indirect (instrumentation) level indication.

Can ultrasonic level controls be usedon boilers? I have never seen it done, thought I would pose the question.

Theres is a system that uses Radar for level control and it is developed and Sold By Gestra If you would like more details please contact me

Cheers