ASME SEC VIII UG-140 Overpressure protection by System Design

[Sawsan311]

Dear All,

I would like to enquire your interpretation of ASME SEC VIII UG-140 clauses about the role of system design for overpressure protection, do you agree that:

a) if no PSV is relief on and the system design by ISD or SIF SIL-3 function is to eliminate the overpressure scenario, then we would accept their role only if the pressure resulting from the overpressure scenario would not exceed MAWP ? or would not exceed the corrected hydrotest pressure? or code allowable accumulation?
b) if an only if PSV is present while role of system design is reducing its relief requirement , then the code allowable accumulation to be considered shall be 116% including fire scenario.

I am bit confused about UG-140 stating the role of system design when the system pressure is not self limiting and referring to MAWP not being exceeded while at the same time in item (5) stating that the overpressure limit shall not exceed 116% of MAWP and then stating test pressure. Is there a rational behind the 116% being correlated to 130% multiplier for the corrected hydrotest pressure.

Thanks

Regards,

 

[davefitz]

In my experience at thermal power plants the 2 cases that make use of that paragraph would be feed pump discharge piping and atmospheric flash tanks.

In the case of the pump discharge piping, there is a factory curve of the pumps max discharge pressure with the original impellor, and if that is less than 110% piping MAWP ( as permitted by the power piping code) then a safety releif valve is not required, but a small thermal relief valve ( 3/4" ) must be applied for thermal expansion issues during outages. This implies several assumptions that need to be confirmed by the designer inlcuding that the impellor will not be replaced by a higher discharge pressure capacity impellor.

In the case of the atmospheric flash tank, the vent sizing must ensure that the diametedr is large enough to prevent the backpressure from exceeding the tank's MAWP with all valves that feed the tank are failed wide open. This is a tricky proposition, as this involves choked flow through many valves, choked flow thru the vent pipe ( and silencer) , and is complicated further when the scope of the supply of the vent is not by the same contractor who supplies the tank and valving. As per the code, the end user must accept this configuration.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick