What will happen if the superheater safety valve lift first in the boiler?

Answer 1

If the superheater safety valve lifts first the excess pressure will be relieved and no damage will be done due to overheating. This is a good thing and the way it should work. If the boiler safety valve were to open first there would not be enough of a steam flow through the superheater to "cool" the tubes of the superheater which could result in severe damage. +++ ??? That does not read right at all. For a start, steam only flows through the superheater when the engine cylinder or turbine is operating. Locomotive boilers' superheaters don't have safety-valves on them, and the regulator is upstream of the superheater, but they don't overheat when the regulator is closed. The boiler safety-valve would not lessen the flow of steam through the superheater anyway - its role is to vent excess steam hence prevent over-pressure in the boiler. A safety-valve on a superheater would similarly only protect the pipes from over-pressure, not from overheating.In fact it's difficult to think of a situation in which a superheater can be overheated to the point of damage.

OK Let us look at your rebuttal to my answer and I will explain why you are wrong. First you say that steam only flows through the superheater when the engine cylinder or turbine is operating. This is true but it is the first indication that you don't really understand thermodynamics. The steam is not flowing but the temperature continues to rise. Just because the safety valve lifts you don't automatically shut off the fire and this heat continues to act on the superheater raising the temperature to a level where it could damage the metal of the superheater tubes. In other words, the superheater tubes get hot from the fire and the steam gets superheated by carrying this heat away. If this heat is not removed the tube overheats and becomes damaged.

But hey don't take my word for it lets see what the experts have to say. In the 40th edition of Steam, Its Generation and Use by Babcock & Wilcox Chapter 23 Page 6 under Safety and relief Valves paragraph 6 and I quote "For drum boilers with superheaters, Babcock & Wilcox (B&W) prefers to follow the Code allowed procedure of setting the safety valves so that the superheater valve(s) lift first at all loads, thereby maintaining a flow of steam through the superheater(s) to provide a measure of over-heat protection."