Steam Relief Valve Discharge Piping Design

[RRE]

I have a steam relief valve that failed when the inlet piping to the valve cracked. The mechanism is being investigated. Can you provide me a reference (API like code) where I can find how the discharge piping of a steam relief valve needs to be designed? The vent piping was supported.

 

[gerhardl]

The main question for all steam piping when components fail is to check if the system have a sufficient condensate drainage with proper designed steam-trap points and correctly selected type and dimension of steamtraps, with correct installation and operation (all working?)

Your failure could be caused by one ore several of following.

a) Water hammer(condensate)caused by inadequate condensate draining
b) Mechanical, caused by weak, wrong materials and design for distribution of forces, wrong anchoring and support. (Plus point a and c?)
c) Thermic forces (plus point a and/or b?)
d) Corrosion caused/helped by a,b,c and age.
e) Inproper operation

You seem to lean on b. In my experience this is more seldom alone the cause of any failure. More than 80% of steam system problems are probably caused by inadeqate condensate handling within the system.

For design support, check for instance

http://www.gestra.com

or

http://www.spiraxsarco.com

 

[davefitz]

There are 2 other causes of failures of the inlet pipe not disussed by gerhardahl.

a) locating teh relief valve too close to the outlet of an elbow on the main pipe can lead to fluid-eleastic vibrations at the relief valve inlet nozzle, and a hi cycle fatigue failure is possible.

b) the support of the relief ehaust pipe may be interfering with the thermal expansion of the relief valve and its inlet nozzle, which must lose out when compared to the pipeline forces generated in the main pipe .

 

[JimCasey]

(1) There is a Safety-relief valve topic in the Chemical Engineers forum.

(2) Lots of stuff could still have gone wrong.
a. The outlet piping is usually not attached to the valve. Usually there is an elbow with a condensate pan on it, draining to a floor drain. The actual stack is suspended from the building structure. This allows for thermal expansion and ensures that there will be no condensate accumulated in the outlet side. Safety-relief valves also usually have a drain hole in the body, altho it is usually plugged.
b. Could have been any Murphy's law fluke, such as a casting flaw or a bad weld.
c. The reaction force on a SRV can be VERY high. Main steam safety valves, for example, are supplied with dual outlets to cancel reaction force, or at least installed alternately on the main steam header so the reaction vectors cancel. There have been cases involving tragic fatalities where Safety valves lifted and the reaction force ripped the header loose.