Valves are one of the main leakage sources in the pipeline system of the petrochemical industry, so it’s critical to the leakage of valves. Valve leakage rates are actually the valve sealing level, valve sealing performance is referred to as the valve sealing parts to prevent the ability of media leakage.
The main sealing parts of the valve including: the contact surface between the opening and closing parts and the seat, the fitting of packing and stem and packing box, the connection between the valve body and the bonnets. The former belongs to internal leakage, which directly affects the ability of the valve to cut off the medium and the normal operation of the equipment. The last two are external leakage, that is, media leakage from the inner valve. The loss and environmental pollution caused by external leakage are often more serious than that caused by internal leakage. Valve leakage is not allowed especially for high temperature and pressure condition, flammable, explosive, toxic or corrosive media, so the valve must provide reliable sealing performance to meet the requirements of its use conditions on the leakage. At present, there are five kinds of valve seal classification standards commonly used in the world.
International Organization for Standardization ISO 5208 specifies examinations and tests that a valve manufacturer needs to act upon in order to establish the integrity of the pressure boundary of an industrial metallic valve and to verify the degree of valve closure tightness and the structural adequacy of its closure mechanism.
There are 10 leakage rates specified in ISO 5208: A, AA, A, B, C, CC, D, E, EE, F, G and the rate A is the highest grades. There is a loosely defined correspondence between the leakage rate acceptance values of API 598 and leakage value rate A as applied to DN 50, rate CC-liquid for other than metal-seated check valves and for check valves rate EE-gas and rate G-liquid. Rate A, B, C, D, F and G correspond to values in EN 12266-1.
American petroleum institute standard API 598 is the most commonly used test standard for American standard valves. It is applicable to the following API standard valve sealing performance tests:
API 594 Flanged, lug, wafer and butt weld connection check valves
API 599 Flanged, threaded and butt-welded metal plug valves
API 602 Steel gate and check valves DN 00 and below for the oil and natural gas industry
API 603 Flanged and butt-welded corrosion-resistant bolted cover gate valves
API 608 Flanged, threaded and butt-welded metal ball valves
API 609 Double flanged, lug and wafer butterfly valves
American Association for standardization of manufacturers of valves and fittings MSS SP61 pressure test for metallic valves specifies the allowable leakage requirements are as follows:
(1) In the case that one of the sealing surfaces of the valve sealing seat is made of plastic or rubber, no leakage shall be observed during the duration of the sealing test.
(2) The maximum allowable leakage on each side when closed shall be: the liquid shall be the nominal size (DN) 0 per mm, 0 per hour.4 ml; Gas is the nominal size (DN) per millimeter, 120 ml per hour.
(3) The leakage allowed by the check valve can be increased by 4 times.
It should be noted that MS SSP 61 is often used for inspection of “fully open” and “fully closed” steel valves, but not for control valves. MSS SP61 is not usually used for testing American standard valves.
American national standards/American instrument association standards ANSI/FCI 70-2(ASME B16).104) is applicable to the control valve seal grade requirements. Metal-elastic seal or metal seal should be selected in engineering design according to the characteristics of the medium and the opening frequency of the valve. Metal seated valve seal grades should be is stipulated in the order contract, the rates I, Ⅱ, Ⅲ is used less due to request a lower level, generally choose Ⅳat least and V or Ⅵ for higher requirements.
EN 12266-1, tests on industrial valves part l specifies the pressure tests, test methods and acceptance criteria – mandatory requirements. EN 12266-1 meets the requirements of ISO 5208 for seal classification but lacks AA, CC and EE ratings. The new edition of ISO 5208 adds six levels of AA, CC, E, EE, F, and G and gives comparisons with several seal levels of API 598 and EN 12266.
It should be noted in the engineering design that API 600-2001(ISO 10434–1998) specifies that the sealing performance of the valve is tested in accordance with ISO 5208, but the leakage in tables 17 and 18 is equivalent to API 598–1996, not ISO 5208. Therefore, when API 600 and its sealing performance test API 598 standard are selected for the engineering design, the version of the standard must be clarified to ensure the uniformity of the standard content.
The relevant guidelines of API 6D(ISO 14313) for valve leakage are: “soft seated valves and oil-seal plug valves shall not exceed ISO 5208 A (no visible leakage), metal seat valves shall not exceed ISO 5208(1993) D unless otherwise specified.” Note in the standard: “special applications may require leakage less than ISO 5208(1993) class D. Therefore, leakage requirements higher than the standard shall be given in the order contract.