Valves, at a minimum, should conform to their relevant industrial or national standard for through or seat leakage, e.g., API 598 or ANSI B16.34 (Petrochemical), API 6D (Pipeline), and API 6A (Wellheads).
A seat leakage test is recommended for the fully assembled EBV at the FAT. For projects ordering many automated valves, test selected valves for seat leakage.
API 598 Valve Inspection and Testing defines many requirements for valves in the refining process. This specification is preferred for defining the seat-leakage requirements for quarter-turn valves with piston actuators or for gate and globe valves with electric or pneumatic motor actuators.
An overview of this specification shows:
• 60–100 psi minimum test pressure for these valves: ball, gate, plug, and metalseated butterfly.
• 110% of the design differential pressure for these valves: globe and resilientseated butterfly.
• No allowable leakage for resilient-seated valves.
• Leakage rates for metal-seated valves in drops per minute (16 drops per ml) for a liquid test and bubbles per minute for a gas test. These rates apply to valves grouped in four size ranges: 2-inch and under, 3- to 6-inches, 8- to 12-inches and over 14-inches.
ANSI/FCI 70-2-1991 Control Valve Leakage defines only the seat leakage for control valves. (ANSI B16.104-1976 Control Valve Seat Leakage, which is essentially the same as 70-2, is discontinued.) This specification is preferred for defining the seat-leakage requirements of quarter-turn type control valves. Sliding-stem control valves should not be considered for EBV service.
An overview of this specification shows:
• Six classes of leakage are defined, but only Class V (tight shut-off, liquids) and Class VI (tight shut-off, gases for resilient-seats) should be installed as EBVs.
• Class V seat leakage is 0.0005 ml per minute per inch of orifice diameter per psi of the maximum service drop across the valve plug.
• Class VI seat leakage is defined by the port diameter from one inch through eight inches in either ml or bubbles per minute. Test pressure is 50 psi.
A comparison of these specifications shows the allowable leakage for a four-inch, metal-seated valve at 100 psi differential would be 0.7 ml per minute, based on API 598; and 0.2 ml per minute, based on ANSI-70-2.
For applications tolerating no leakage (such as fuel gas line(s) to a furnace or boiler or a blowdown line from a reactor), install two valves in series (double block). When two valves are set in series, it is possible to locate an automated bleed valve between these two valves for fuel gas services, as shown in NFPA 85A. For other services, close the lead valve first and provide a pressure switch and alarm for leakage detection.
Another solution for situations which do not tolerate leakage is to provide an EBV with a minimum leakage class rating of Class VI or bubble-tight as defined by ANSI/FCI 70-2-1991, or a resilient-seated valve as defined by API-598. For leakage checking, the installation must include a downstream block valve and pressure gage.
When in hydrocarbon service, quarter-turn valves must be certified to the latest edition of API 607, Fire Test for Soft-Seated Quarter-Turn Valves, or to the Company-approved equivalent.
Recently, molded covers made of K-MASS fireproofing were fire tested successfully by following API- 607 for a quarter-turn valve with an elastomer liner. The results showed that, in fire areas, non-metal seated valves are successful as EBVs in hydrocarbon service.
Some special applications require different types of valves, such as:
• When pipeline-style, full-conduit, expanding-wedge-design, gate valves are involved in process applications above 350°F, the expanding wedge may bind in both the open and the closed positions.
• Y-pattern globe valves and Orbit valves in high-pressure refinery applications, such as a hydro-treater recycle compressor service, have proven reliable. For high-pressure and high-temperature applications requiring tight shutoff, select the Y-pattern valve.
• Maxon gate valves and actuators have been successful on low-pressure, cleanfuel services to fired units.