Relief Valve Discharge Pipe Reduction? 1

[pmover]

all,

while i am investigating the asme code, i believe a knowledgeable individual can provide a quicker response and possibly direct me to the location within the code - hopefully that is!

equipment is boiler drain heat recovery unit.

relief valve size is 1-1/4" x ? with 4" vent piping to bldg roof and then reduced to 1-1/4" for roof penetration.

i know from experience that reduction of or restricting vent piping is not permitted as it defeats the purpose of the relief valve - a personnel and equipment hazard!

anyone with direction to specific section within the bpv code or international mechanical code is appreciated.
-pmover

 

[pmover]

for those whom are interested,

asme bpv code section 8, part ug-135, ug-136, and appendix m.
-pmover

 

[EGT01]

pmover,

As additional reference, you may also want to look at the American Petroleum Institue recommended practices for relief system design, API RP-520 Part I, Section 3.3 discussion on back pressure and API RP-520 Part II, Section 3 disccusion on relief device discharge piping.

API also lists some technical interpretations of these recommended practices on their website. Take a look at the following link which I think addresses your question specifically. The technical interpretations are given in Excel spreadsheet format, look at the discussion on row 11.

http://committees.api.org/standards/tech/docs/520bti.xls

 

[bdog]

Calculate the backpressure imposed by the downstream piping on the relief valve during a relief event. It may reduce the capacity of the valve. Though it is poor piping practice to have any piping reduction in relief discharge piping, you may be ok if the vessel is still protected from overpressure.

 

[unclesyd]

You will have a hard time convincing a jurisdictional inspector of the need or suitability of a reduction in discharge piping, especially of the magnitude in your case. As "bdog" said this a bad practice to get into. If you have to do this for some reason you had better have the relief valve capacity calcultions ready.

 

[pmover]

EGT01,

thanks for the link information.

unclesyd and bdog,
i wholeheartedly concur with your assessments, but i would not (ever!) recommend a piping reduction in relief valve discharge piping, even if the calcs support the reduction. the only justification i can see is cost-savings (not a technical decision!), which is a poor decision.
-pmover

 

[FRChem]

Supporting the above responses, I have come myself across a similar bad practice. That is connecting some other stream line piping to the Relief Valve discharge line. This would also bring about the same effect.