Warm-Up Rates for Steam Systems 1

[scotty36]

Does anyone know of a guide for the warm-up rates for steam systems? I cannot find anything in ASME B31.1 or Spirax publications.

I am purposefully not giving details because I am after guidelines not specifics. However, it should be noted that the systems in question are in industrial power generation - over 400degC with 240t/h of steam.

Not being a steam guru, I expect you should limit the flowrate from the boiler during warm-up to allow a slow and steady expansion of the piping steel and steady condensate removal?

A guideline of deg/hr based on diameter-length of line (or mass of steel) would be great.

 

[quark]

http://www.armstrong.be/pdf/N_101.pdf

Besides giving excellent data on steam system and steam trapping, this document deals with startup conditions(page 17).

Regards,

 

[davefitz]

I think the main limits on rate of warmup for steam systems are:

a) limit thermal stress in thick pressure vessels, such as HP steam drum , turbine casing, thick valves. The thermal stress developed during warmup is proportional to the {rate of change of increase in temperature ) times ( the square of the wall thickness), per the TRD 301 annex 1 ( german boiler code) . If you do not want to perfrom the calculation for a specifc case, a conservative estimate for carbon steel components is 200F /hr if the wall thickness is 6" thk, for a 3000 cycle life.

b) some valves have problem with differential heating between the thick valve body and the thin cage that guides the plug, so sticking of the plug can occur if the valve is heated up too fast.

c)drain capacity of the steam line steam traps- a lot of condensate can buildup during inital warmup

 

[gmorin]

Generally your warmup rate is limited by your traps, orifices, and/or vent valves. If the line is properly designed and you have traps or orifices, I can't see any way that you could get into trouble by heating your line too fast. Large vent valves would be more likely to get you into trouble as they would heat the line much faster. Even then, I'd be more worried about water accumulation than thermal effects.

Lots of things can happen during warmup - pipe walking off supports, water hammers, inadequate drainage causing unexpected expansion, valves seizing up due to pressure on one side and not the other, etc.

Most of these are best addressed by ensuring drainage and proper procedures, venting, trapping, etc. - not by limiting warming rates.

I would guess if you were to do a calculation you would find that you'd hit some physical limit - trap/orifice flow rate, boiler warmup rate, end user warmup rate (turbine casing), etc. before you came anywhere near the point where thermal differentials in pipe during heating would be a concern. (As mentioned above, thick valve bodies could be a concern) The example above assumed 6" thk steel and still used a 200F/hr rule of thumb - I don't know your pressure, but I assume from your other condiitons that your pipe would be significantly thinner than that.