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