Boiler Drum safety valve

[Nivrah]

Hello fellow engineers,
I am sizing 2 boiler drum safety valves set at 7412 kPag(drum design pressure) and 7620 kPa (1.028% of 7412 kpag). The required discharge capacities and minimum discharge area of the valves have been calculated and checked.
However, I have also been told to calculate the required discharge pipe diameter to ensure that the discharge velocity does not achieve sonic speed. Why is that the case? or does it matter is the discharge is sonic? Implications if discharge is sonic?

many thanks!

 

[pmover]

to ensure that the safety valve capacity is not limited by high back-pressure.

i suggest further reading in ASME B31.1 122.6.2 Discharge Piping From Pressure-Relieving
Safety Devices. remember, the safety valve outlet flow pressure is much less than the safety valve setting, provided the outlet is directly to atmosphere.

hope this helps.
-pmover

 

[Nivrah]

Pmover,
the output is atmospheric but given the high upstream pressure, would we not expect sonic ("choked velocity") flow immediately after the discharge?
the only scenario i can think of is that the velocity will be sonic immediately after the discharge orifice but the velocity along the discharge pipe will then decrease below sonic speed since pressure losses and wall resistance is expected.

 

[Fizza453]

please also see non-mandatory appendix II of ASME B31.1.

Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN

 

[Fizza453]

PG-71 of ASME Section I gives some installation details/guidelines of discharge pipe also.

Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN

 

[Nivrah]

Thanx Fizza453.I will look into it