Boiler controls - low flow

[remp]

Hi

I am retrofitting the controls on a couple of high temperature hot water boilers approx 4000Kw each and generate water to 140degC.. They operate in duty/standby.

The plant operator wants to be able to go from one boiler to another boiler on a weekly basis but does not want the water in the standby boiler ot go cold. I can understand this becasue it will take time to heat up and thermal shock cannot be good for the metal in the boiler. I have a 2 port modulating valve on each boiler outlet so is it ok to have a trickle of water going thro the standby boiler with the valve say 95% closed to keep the temp topped up? The boiler burner comes on to maintain 140deg C and goes off again through the boilers owns controls.

Also in operating mode can the flow in the boiler reduce as the first stage of control during times of load load?
Is there any damage that can be done to the boiler if the flow is excessively low....

 

[ChrisConley]

Minimum flow requirements would determine what flow you could put through the boiler. Some boilers come on 100% fire to start, and with a very low flow you can imagine this will cause serious problems. Boiler shop drawings should include minimum flow requirements.

Your two problems: time to heat up and thermal shock cannot be good for the metal in the boiler, are opposite problems.

If you allow sufficient time for the boiler to heat up, thermal shock shouldn't be an issue. How critical is it that the 140C temperature not drop at all? A large system wouldn't have a huge drop as you alternate between boilers.