At what temperature should an unfired waste heat steam generator have a low water cutout?

[SREisme]

I am trying to understand the differences between unfired waste heat steam generators and fired boilers as it applies to a low water cutout requirement. NFPA requires a low water trip on all fired boilers, presumably due to the number of boilers that have exploded when water was lost. I can not find a similar requirement for waste heat steam generators. I found one manufacturer of WHSGs that publishes a run dry capability up to 840F (450C), but no reason why.

Can anyone point me to an industry standard, research report, or study that identifies the maximum safe operating temperature of a dry boiler tube to avoid the potential for a steam explosion in the boiler on the reintroduction of water to a dry surface? It stands to reason that there is a limit somewhere between 536F (minimum homogenous nucleate boiling temp) and fired boilers (2000-3500F), but I have not found one in my search yet.


Thanks

SREisme

 

[davefitz]

If the unit has a steam drum, then the lo-lo water level is set by the mfr as the minimum level needed to prevent upsets in water circulaiton to the tube bundles. If the level drops below that level, then water circulation for some tube sections stops, and leads to rapid failure of the tubes or waterwall ( for a radiant furnace or fired tube boiler) . The design code requires automatic tripping for the burners in that situation. The code is meant both for safety of the operators and to protect the financial investment of the insurance compnay that covers the plant.

In the case of a waste heat recovery boielr with a steam drum, the tubes might not fail from overheat, but the thermal stresses from some tubes being more than 200F hotter than others will cause failure of the risers at the drum connection and damage to the steam drum- such riser failures can be a safety hazard to operators.

The waste heat recovery boilers permitted to run dry are "once thru steam generators" without a steam drum , and specifically designed to allow tube , riser, and feeder overheat and hight temperature variatiojs without generating excessive thermal stress.