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It is a common misconception, within the valve industry, to assume that safety relief valves must meet the same seat tightness criteria as defined for control valves in ANSI-FCI 70-2 (Class I to Class VI Leakage Class). However, they do not. Seat tightness for all Section VIII (“UV” Stamped valves) must be in accordance to API-527 (standard) unless otherwise specified by the manufacturer (See ASME Section VIII, Div. 1, UG-136[d][5]).

API-527 methods of determining the seat tightness of metal- and soft-seated pressure relief valves, including those of conventional, bellows, and pilot-operated designs. The maximum acceptable leakage rates are defined for pressure relief valves with set pressures from 103 kPa gauge (15 psig) to 41,379 kPa gauge (6000 psig). If greater seat tightness is required, the purchaser shall specify it in the purchase order. The test medium for determining the seat tightness—air, steam, or water—shall be the same as that used for determining the set pressure of the valve. For dual-service valves, the test medium—air, steam, or water—shall be the same as the primary relieving medium.To ensure safety, the procedures outlined in this standard shall be performed by persons experienced in the use and functions of pressure relief valves.

No problem, it's pretty simple actually.
There are six different seat leakage classifications as defined by ANSI FCI 70-2. The most commonly used by Gemco Valve are CLASS I, CLASS IV and CLASS Vl. CLASS I is also known as dust tight and can refer to metal or resilient seated valves. CLASS IV is also known as metal to metal. It is the kind of leakage rate you can expect from a valve with a metal shut-off disc and metal seat. CLASS Vl is known as a soft seat classification. Soft Seat Valves are those where the seat or shut-off disc or both are made from some kind of resilient material such as Teflon.

It is a common misconception, within the valve industry, to assume that safety relief valves must meet the same seat tightness criteria as defined for control valves in ANSI-FCI 70-2 (Class I to Class VI Leakage Class). However, they do not. Seat tightness for all Section VIII (“UV” Stamped valves) must be in accordance to API-527 (standard) unless otherwise specified by the manufacturer (See ASME Section VIII, Div. 1, UG-136[d][5]).

It is a common misconception, within the valve industry, to assume that safety relief valves must meet the same seat tightness criteria as defined for control valves in ANSI-FCI 70-2 (Class I to Class VI Leakage Class). However, they do not. Seat tightness for all Section VIII (“UV” Stamped valves) must be in accordance to API-527 (standard) unless otherwise specified by the manufacturer (See ASME Section VIII, Div. 1, UG-136[d][5]).