Published on 01 May 2019 by Tadd Pham
Single or double valves at a classification change should be suitable for the more severe line classification on either side of the valve(s) location. At a line classification break, a single block valve may be exposed to both service conditions; therefore, it must be adequate for the more severe of the two services. In the case of a double block valve installation at a line service classification change, it is possible for either valve to be closed against the more severe service condition; therefore, both valves must be designed for it.
Valves around control valves (upstream, downstream, and bypass) should be of the same Class rating as the most severe line classification.
Fugitive emissions usually account for about half of all VOC and toxic air emissions. Valves account for about half of fugitive emissions. Many operating locations require limits on the emissions and also on the allowable leak rate for valves. Purchasing low leak rate valves helps meet the required limits more cost effectively by reducing maintenance (e.g. packing replacement and repeated gland tightening).
Gate and globe valves should have flanged ends, bolted bonnet, and should be outside screw and yoke.
For Butterfly Valves
- Where butterfly valves are used adjacent to process equipment that requires depressurizing and removal for maintenance or other purposes, the valves should be of the flanged type.
- Butterfly valves should be limited to NPS 3 and larger.
For Soft Seated Valves
- When specified, pressure relief for body cavities should be provided for soft seated valves in liquid service. The liquid should be relieved to the upstream side, unless otherwise specified.
- Non-fire-tested type ball, plug, or butterfly valves should not be used in flammable material or other dangerous material services regardless of operating temperature, except for certain acid services. If a non fire-tested valve were to be employed in a flammable or dangerous material service, it would be of no value in performing its primary function in the event of a fire. Unacceptable internal and external leakage would result, increasing the hazardous condition.
- Fire-tested type ball, plug, and butterfly valves greater than NPS 4 should not be used as the first block valve for the holdup of large volumes of dangerous liquid materials (>10,000 gal (38,000 l) liquid hold up). For valves larger than 4 in., the leakage rates would be too high and would pose an unacceptable risk in releasing a large volume of liquid into a fire.
- Fire-tested butterfly valves per API STD 609 Category B should not be used in dangerous material services, and other process and general services unless the design is bi-directional, or meets the design temperatures and pressures specified for both the preferred and non-preferred flow directions. Fire-tested butterfly valves per API STD 609 Category B may be used in dangerous material services, and other process and general services if it is of equal strength in both directions.
- Soft seated ball, plug, and butterfly valves 2 in. and larger should not be used as block valves to the atmosphere.
For Soft-Sealed Butterfly Valves–Fire-Tested
- Unless otherwise specified, shaft material should be as follows:
- 13Cr, except for valves with austenitic stainless steel bodies.
- 18Cr 8Ni for valves with austenitic stainless steel bodies.
- Unless otherwise specified, disc material should be as follows:
- 18Cr 8Ni
- Carbon steel, overlaid with 18Cr 8Ni, 0.060 in. (1.5 mm) minimum finished thickness
- Cr plated discs are not acceptable.
- AWS A5.13 E/R CoCr-A hardfacing should be provided on seating surfaces when specified.
For Butterfly Valves for Water Service
Rubber lining of valve bodies should be as follows:
- The wetted surfaces of valve bodies should be fully lined, except that only a bonded rubber seat insert is required for non-throttling fresh water services.
- The lining should extend across the full flange face, and should have a minimum thickness of 1/4 in. (6 mm).
- The rubber lining should be 100 percent bonded to the body, flange, and the metal seat insert, if any. The lining-to-seat bond should be capable of withstanding a 160 psi (1100 kPa) tensile load.
For Compact and Extended Body Steel Valves
Threaded and socket welding steel gate valves per API STD 602 and steel globe valves conforming to the applicable requirements of API STD 602 can be used within the following limits:
- Valves should be outside screw and yoke.
- The specified line corrosion allowance is 1/8 in. (3 mm) or less; or, if the specified valves have a thickness in excess of the minimum API STD 602 specified thickness, the additional thickness may be considered as additional corrosion allowance.
- Valves should be bolted bonnet unless welded bonnet is specified. Bonnet welds should be maximum 225 Brinell hardness.
Reduced Port Valves
Reduced port or smaller than line size valves (with reducers) may be used instead of full port valves, provided:
- The increased pressure drop and reduced section modulus is considered in the design of the piping.
- Such valves are not used in horizontal lines sloped for continuous draining.
- Drains are installed at all additional low points caused by the installation of such valves.
- They are not used in erosive applications such as slurries, fluidized solids, or severe fouling or coking services.
- They are not to be used in pressure relief valve inlet or outlet piping, or in flare headers. In any pressure relieving system, the inlet and outlet piping systems are not to have any size reductions in that would cause flow restrictions. The safety valve orifice is to be the only flow restriction in the line.
- They are not used in lines specified to be pigged.
Wafer and Lug Type Valves
- Wafer type valves should not be used in hydrocarbons or dangerous materials service. The bolting of a flangeless valve (except for a lug-type) can be 30 percent or more cooler than the valve body itself. This temperature difference will cause differential thermal expansion between the valve body and the bolts, especially considering the extra long length of the bolts in this type of valve installation. This increases the likelihood of leakage.
- Wafer or lug type valves should not be used as the first block valve against storage tanks or vessels. Because of the increased likelihood of leakage at flangeless valves, it prohibits their use if they are the primary means of holding back larger volumes of dangerous materials. In this case, in addition to concerns of increased likelihood of operational leakage, there is a greater risk of the joints at a flangeless valve to open up in the event of a fire in the vicinity, or if the longer-than-normal exposed bolting are doused with water or steam during fire fighting in the area.
- Wafer (flangeless) and lug type valves should not be used in any service with a design temperature greater than 600°F (315°C). A flangeless valve is typically less expensive than a flanged valve; the initial impetus to accepting their use was based upon reduced cost. This cost saving potential was then weighed against the increased leakage potential. It was felt that 600°F would be a reasonable break point. It avoids using this valve type in high temperature hydrogen-rich-type services, while still opening the door in lower temperature (less severe) environments. This temperature limit had been 950°F for many years. However, recent thinking indicates a preference to be more conservative, and to limit the maximum temperature below the auto-ignition limit.
- For design temperature exceeding 400°F (200°C) the body material of wafer or lug type valves should have the same nominal coefficient of thermal expansion as the bolting material and adjacent flanges.
- Dual and single plate check valves per API STD 594 should not be used in reciprocating compressor or reciprocating pump service.
- Union bonnet ball check and lift check valves (straight-through type) use should be limited to portions of piping systems where pipe unions are permissible.
- Valves other than dual plate should be of the self-closing type, and should not depend on flow reversal or a spring to effect closure.
- Check valves should not be used in down-flow applications.
- Lift check valves should not be used in services subject to fouling, coking deposits, or erosion.
- The need for and design of external assist or damping devices for the specified check valve type in a specific compressor discharge service should be based on the full range of operating flow conditions.
- For design temperature >600ºF, threaded retainer plugs for hinge pins should be seal welded if valves are in flammable material service.
Knife Gate Valves
- Knife gate valves should not be used in dangerous material service.