Minimum Pressure rating for boiler feedwater, blowoff valve and flange

[stonehappy]

Hello,

I have question about ASME B31.1 Power Piping.

In case of feedwater and blowoff, when designing the piping, the presssure shall exceed MAWP by either 25% or 225 psi whichever less.

Acording to ASME B31.1 "Poer Piping" 122.1.7

It says " The minimum pressure and temperature rating for all valves and fittings in steam, feed water, blowoff, and miscellaneous piping shall be equal to the pressure and temperature specified for the connected piping on the side that has higher pressure, ..."

When designing the feedwater, blowoff valve and connecting flange, does "the pressure and temperature specified" in 122.1.7(ASME B31.1) mean MAWP or "25% or 225 psi exceeded MAWP"?

Thank you!!

 

[davefitz]

For feedwater, the higher pressure side is the feed side from the feed pump. The pump dead head pressure would be the max overpressure, and the feed piping and design pressure may equal that dead head pressure minus a permitted 10% overpressure ( see B31.1 overpressure clause) minus any credit for gravity head from pump center el. Please note that pumps may be supplied with an actual developed pressure about 5% over the value specified in the purchase spec. After this design P is determined, you then must confirm that this value is either 25% over MAWP or 225 psi higher.

For superheater, economizer, blowdown ,etc the drain valve and piping design pressure would be MAWP plus gravity head. MAWP is the steam drum design pressure, and the steam drum is usually higher than the drains, unless they built the boiler upside down