Boiler turndown
Boiler turndown
(OP)
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
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





RE: Boiler turndown
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.
RE: Boiler turndown
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.
RE: Boiler turndown
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.
RE: Boiler turndown
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.