Boiler turndown

[tbedford]

We utilize high pressure, high temperature steam for our process. The steam is used to heat large volume cylnders over which we place sheet paper. The paper is warmed (170F)and will readily accept bleaching agents etc. in a short period of time.
We wish to conserve natural gas beyond that provided by our burner control turndown. Our idea is to turn our burners off at night and close the main steam line etc. This would save the energy within our boiler. We do realize our steam lines and process steam drums would require preheating to temperature again next morning.
Several questions I pose:

1. Are there any long term effects to the boiler with a temporary shut down of this nature?
2. Are there other cost-saving measures we could employ?
3. Is there an optimal set-back temperature/pressure to leave the system in stand-by mode overnight?


Thanks
Tom

 

[quark]

Not anything major that I foresee. I am not sure about water tube boilers but I have similar experience with fire tube boilers. The pressure was 10kgf/cm²g and we used to stop the boiler 3 days in a week. To prevent delays in startup next day morning, we used to close the stop valve. With ambient temperatures not below 15⁰C and occassional 6⁰C in the morning from 3 to 5 AM, the pressure drop used to be around 1.5kgf/cm²g.

1. There shouldn't be a problem with boiler shutoff but just take care of your piping. The condensing steam creates low pressure in the piping and corrosion due to air ingress may be a serious problem in future. My piping was relatively new so I couldn't get any data of pipe failure.

2. There are many conventional techniques which you may, already, be using. Condensate recovery, periodical maintenance of steam traps to avoid wastage of live steam, automatic blowdown control, desuperheating across pressure reducing valves etc. to name a few. What is the capacity and operating pressure of your boiler?

3. I don't have a solution nor a logic for this. We used to leave the boiler at the high pressure cut off point. Let us see what others say.

 

[goldnwhite]

You also have the option of installing a burner control system with timer controls. You could then program in your normal operational hours where it would modulate based on a parameter like pressure. During the off-hours it would execute warming cycles every so often to keep the boiler warm for the next timed period. This would reduce cycling and reduce the needed time for a warmup period.

The controls we just installed are from AutoFlame, although we do not use the time function as we operate 24/7. Similar systems are surely available for boilers used for building heat.

 

[willard3]

If you have water-tube boilers, you can turn them off. My client's water tubes run at 100 psig and are steaming in 5-10 minutes from cold shut-down.

There is a small fire-tube boiler with fully modulating gas train that maintains 2-5 psig on night cycle to keep the process piping up to temp.

 

[davefitz]

The term used for shutting down every night is " two shift cycling operation".

To minimize the rate of decay of steam drum pressure overnight, you need to limit the heat loss. During shutdonw, you would need to close all dampers, mainsteam stop valve, and use steam traps on the main steam line and superheater drains.

The main damage that could be caused is "fatigue damage", to thick walled presure parts ( steam drum , main steam outlet header), and to seals ,penetrations, supports, etc which would undergo large differntial growth during each startup/shutdown cycle.

The rate of increase in metal temperature of the steam drum shell and mean steam outlet header shell should be monitored if these thicknesses are over 4" thick, to limit the thermal stress to allowable fatigue limits. Refer to the old german boiler code TRD 301 annex 1 or the newer EU PED for the relationship between allowable rate of increase in metal temp vs wall thickness.

The boiler mfr may be able to recomend changes to the seals and supports to allow a longer fatigue life of those parts- they probably were not originally desinged to undergo more than 500 startup/ shutdown cycles.

Another issue if feedwater treatment, but this is not the forum for this issue.