Bonaire,

I can't say I know the full history behind the liquid valves certified to 25% overpressure but at one time you could get liquid relief valves either certified at 25% or 10% overpressure.  Those certified to 25% overpressure could be used in ASME code equipment provided their capacities were adjusted based on the actual overpressure experienced.  Most valve manufacturers provided the necessary information to properly size a valve rated at 25% overpressure when used to meet code requirements of 10% overpressure.  

You may want to look at API RP-520 Part I, 7th edition, January 2000, Section 2.2.1.2.6 which has also has a discussion about this.

As I understand, liquid relief valves offered by most valve manufacturers today are certified to 10% so that should not be an issue now but you need to confirm that with your valve supplier.

Bonaire,

Sometimes in liquid service on PD pumps, hydraulic systems, etc you have a relief requirement to protect the piping from a blocked in condition where the popping characteristic of a conventional relief valve is undesirable (chattering). A hydraulic proportional opening relief valve is needed in these situations. These valves start to open at the set pressure and become more open as the pressure increases, and then close the same way. This gives a smooth action in an cyclic overpressure situation where as a conventional valve would beat itself to death after a relatively small number of opening cycles.

These are non-certified valves which are applicable to certain liquid situations beyond the ASME code requirements.

Is this the type of valve you are discussing?

best wishes,
sshep

ASME Sec VII was revised in Summer '88 (if memory serves) to require codestamped liquid-service valves to:
(1) Have rated capacity at 10% overpressure (110% of set pressure)
(2) reclose after 10% Blowdown (90% of set pressure)

Everybody redesigned their liquid-service code-valves for "pop" action.

Before that, typically there was something like a .6 capacity derate factor at 10% accumulation and rated flow was at 25% OP.  Blowdown could go as much as 40%.  If you had a pressure excursion you could lose a LOT of product.  

Please note that there is no blowdown requirement for liquid service valves in the ASME Code