PG-71.3 The opening or connection between the boiler and the pressure relief valve shall have at least the area of the valve inlet. No valve of any description shall be placed between the required pressure relief valve or valves and the boiler, nor on the discharge pipe between the pressure relief valve and the atmosphere. When a discharge pipe is used, the cross-sectional area shall be not less than the full area of the valve outlet or of the total of the areas of the valve outlets, discharging thereinto. It shall be as short and straight as possible and so arranged as to avoid undue stresses on the valve or valves.
All pressure relief valve discharges shall be so located or piped as to be carried clear from running boards or platforms. Ample provision for gravity drain shall be made in the discharge pipe at or near each pressure relief valve, and where water of condensation may collect. Each valve shall have an open gravity drain through the casing below the level of the valve seat. For iron- and steel-bodied valves exceeding NPS 2 ½ (DN 65), the drain hole shall be tapped not less than NPS 3/8 (DN 10).
Discharge piping from pressure relief valves on high temperature water boilers shall be provided with adequate provisions for water drainage as well as the steam venting.
The installation of cast iron bodied pressure relief valves for high-temperature water boilers is prohibited.
PG-71.4 If a muffler is used on a pressure relief valve, it shall have sufficient outlet area to prevent back pressure from interfering with the proper operation and discharge capacity of the valve. The muffler plates or other devices shall be so constructed as to avoid a possibility of restriction of the steam passages due to deposit. Mufflers shall not be used on high-temperature water boiler pressure relief valves.
When a pressure relief valve is exposed to outdoor elements that may affect operation of the valve, it is permissible to shield the valve with a satisfactory cover. The shield or cover shall be properly vented and arranged to permit servicing and normal operation of the valve.
PG-71.5 When a boiler is fitted with two or more pressure relief valves on one connection, this connection to the boiler shall have a cross-sectional area not less than the combined areas of inlet connections of all the pressure relief valves with which it connects and shall also meet the requirements of PG-71.3.
PG-71.6 Pressure relief valves may be attached to drums or headers by welding provided the welding is done in accordance with Code requirements.
PG-71.7 Every boiler shall have proper outlet connections for the required pressure relief valve, or valves, independent of any other outside steam connection,. the area of opening to be at least equal to the aggregate areas of inlet connections of all of the pressure relief valves to be attached thereto. An internal collecting pipe, splash plate, or pan may be used, provided the total area for inlet of steam thereto is not less than twice the aggregate areas of the inlet connections of the attached pressure relief valves. The holes in such collecting pipes shall be at least ¼
in. (6 mm) in diameter and the least dimension in any other form of opening for inlet of steam shall be ¼ in. (6 mm).
Such dimensional limitations to operation for steam need not apply to steam scrubbers or driers provided the net free steam inlet area of the scrubber or drier is at least 10 times the total area of the boiler outlets for the pressure relief valves.
PG-71.8 If pressure relief valves are attached to a separate steam drum or dome, the opening between the boiler proper and the steam drum or dome shall be not less than required by PG-71.7.
PG-72 OPERATION OF PRESSURE RELIEF VALVES
PG-72.1 Pressure relief valves shall be designed and constructed to operate without chattering, with a minimum blowdown of 2 psi (15 kPa) or 2% of the set pressure, whichever is greater, and to attain full lift at a pressure not greater than 3% above their set pressure.
Pressure relief valves used on forced-flow steam generators with no fixed steam and waterline, and pressure relief valves used on high-temperature water boilers must be marked for these special services by the valve Manufacturer or Assembler.
PG-72.2 The set pressure tolerance plus or minus shall not exceed that specified in the following table:
-
See PDF for table 
PG-72.3 The spring in a pressure relief valve shall not be reset for any pressure more than 5% above or below that for which the valve is marked unless the new setting is within the spring design range established by the manufacturer or is determined to be acceptable to the manufacturer.
If the set pressure is to be adjusted within the limits specified above, the adjustment shall be performed by the manufacturer, his authorized representative, or an assembler. An additional valve data tag identifying the new set pressure, capacity, and date shall be furnished and installed, and the valve shall be resealed.
PG-72.4 If the set pressure of a valve is changed so as to require a new spring, the spring shall be acceptable to the manufacturer. The spring installation and valve adjustment shall be performed by the manufacturer, his authorized representative, or an assembler. A new nameplate as described in PG-110 shall be furnished and installed, and the valve shall be resealed.
PG-73 MINIMUM REQUIREMENTS FOR PRESSURE RELIEF VALVES
PG-73.1 Permissible Pressure Relief Valves
PG-73.1.1 Pressure relief valves shall be either direct spring-loaded safety valves, direct spring-loaded safety relief valves, or pilot-operated pressure relief valves.
PG-73.1.2 Power-actuated pressure relieving valves shall only be used for applications specified in PG-67.4.1.
PG-73.1.3 Deadweight or weighted lever safety valves or safety relief valves shall not be used.
PG-73.1.4 Unless otherwise defined, the definitions relating to pressure relief devices in ASME PTC 25 shall apply.
PG-73.2 Minimum Requirements
PG-73.2.1 All pressure relief valves shall be so constructed that the failure of any part cannot obstruct the free and full discharge of steam and water from the valve. Pressure relief valves shall have the seat inclined at any angle between 45 deg and 90 deg, inclusive, to the centerline of the disk.
PG-73.2.2 The design shall incorporate guiding arrangements necessary to insure consistent operation and tightness.
PG-73.2.3 The spring shall be designed so that the full lift spring compression shall be no greater than 80% of the nominal solid deflection. The permanent set of the spring (defined as the difference between the free height and height measured 10 min after the spring has been compressed solid three additional times after presetting at room temperature) shall not exceed 0.5% of the free height.
PG-73.2.4 To provide a means for verifying whether it is free, each safety valve or safety relief valve shall have a substantial lifting device, which when activated will release the seating force on the disk when the valve is subjected to pressure of at least 75% of the set pressure. The lifting device shall be such that it cannot lock or hold the valve disk in lifted position when the exterior lifting force is released. Disks of pressure relief valves used on high-temperature water boilers shall not be lifted while the temperature of the water exceeds 200°F (93
°C). If it is desired to lift the valve disk to assure that it is free, this shall be done when the valve is subjected to a pressure of at least 75% of the set pressure. Pilot-operated pressure relief valves shall be provided with either a lifting device as described above or means for connecting and applying pressure to the pilot adequate to verify that the moving parts critical to proper operations are free to move. For high-temperature water boilers, the lifting mechanism shall be sealed against leakage.
PG-73.2.5 The seat of a pressure relief valve shall be fastened to the body of the valve in such a way that there is no possibility of the seat lifting.
PG-73.2.6 A body drain below seat level shall be provided in the valve and this drain shall not be plugged during or after field installation. For valves exceeding NPS 2 ½ (DN 65), the drain hole or holes shall be tapped not less than NPS 3/8 (DN 10). For valves of NPS 2 ½ (DN 65) or smaller, the drain hole shall not be less than ¼ in. (6 mm) in diameter.
PG-73.2.7 In the design of the body of the valve, consideration shall be given to minimizing the effects of water deposits.
PG-73.2.8 Valves having screwed inlet or outlet connections shall be provided with wrenching surfaces to allow for normal installation without damaging operating parts.
PG-73.2.9 Means shall be provided in the design of all valves for use under this Section, for sealing all external adjustments. Seals shall be installed by the manufacturer, his authorized representative, or an assembler at the time of the initial adjustment. After spring replacement and/or subsequent adjustment, the valve shall be resealed. Seals shall be installed in such a manner as to prevent changing the adjustment without breaking the seal and, in addition, shall serve as a means of identifying the manufacturer, his authorized representative, or the assembler making the adjustment.
PG-73.2.10 Valve capacity may be restricted by restricting the lift of a valve provided the following requirements are met:
(a) The valve size shall be NPS ¾ (DN 20) or larger.
(b) No changes shall be made in the design of the valve except to change the valve lift by use of a lift restraining device described in (c) below.
(c) The restriction of valve capacity shall be permitted only by the use of a lift restraining device which shall limit valve lift and shall not otherwise interfere with flow through the valve. The design of the lift restraining device shall be subject to review by an ASME designee.
(d) The lift restraining device shall be designed so that, if adjustable, the adjustable feature can be sealed. Seals shall be installed by the valve Manufacturer or Assembler at the time of initial adjustment.
(e) Valves shall not have their lifts restricted to a value less than 30% of full rated lift, or 0.080 in. (2 mm).
(f) When sizing and selecting valves, the restricted lift nameplate capacity shall be determined by multiplying the capacity at full rated lift by the ratio of the restricted lift to the full rated lift.
PG-73.2.11 A pressure relief valve over NPS 3 (DN 80), used for pressure greater than 15 psig (100 kPa), shall have a flanged inlet connection or a welded inlet connection. The dimensions of the flanges subjected to boiler pressure shall conform to the applicable ASME standards as given in PG-42. The facing shall be similar to those illustrated in the standard.
PG-73.2.12 The pilot sensing line of pilot-operated pressure relief valves shall be adequately protected from freezing.
PG-73.3 Material Selections
PG-73.3.1 Cast iron seats and disks are not permitted.
PG-73.3.2 Adjacent sliding surfaces such as guides and disks or disk holders shall both be of corrosion-resistant material. Springs of corrosion-resistant material or having a corrosion-resistant coating are required. The seats and disks of pressure relief valves shall be of suitable material to resist corrosion by the lading fluid.
NOTES: The degree of corrosion resistance, appropriate to the intended service, shall be a matter of agreement between the manufacturer and the purchaser.
PG-73.3.3 Materials used in bodies and bonnets or yokes shall be listed in Section II, Parts A and B, and identified in Tables 1A and 1B of Section II Part D, as permitted for Section I construction. Materials used in body to bonnet or body to yoke bolting shall be listed in ASME B16.34. Materials used in all other parts required for the pressure relieving or retaining function shall be
(a) listed in ASME Section II
(b) listed in ASTM Specifications (see Note below) or
(c) controlled by the manufacturer of the pressure relief valve by a specification ensuring control of chemical and physical properties and quality at least equivalent to ASTM Standards (see Note below)
PG-73.3.4 Pressure relief valves may have bronze parts complying with either SB-61, SB-62 or SB-148, provided the maximum allowable stresses and temperatures do not exceed the values given in Table 1B of Section II, Part D, and shall be marked to indicate the class of material used. Such valves shall not be used on superheaters delivering steam at a temperature over 450°F (230°C) for SB-61 and SB-148, and 306°F (I50°C) for SB-62, and shall not be used for high-temperature water boilers.
NOTES: It shall be the manufacturer's responsibility to ensure that the allowable stresses at temperature meet the requirements of Section II, Part D, Appendix 1, Mandatory Basis for Establishing Stress Values in Tables 1A and 1B.
PG-73.4 Inspection of Manufacturing and/or Assembly
PG-73.4.1 A manufacturer shall demonstrate to the satisfaction of an ASME designee that his manufacturing, production, and test facilities and quality control procedures will ensure close agreement between the performance of random production samples and the performance of those valves submitted for capacity certification.
PG-73.4.2 Manufacturing, assembly, inspection, and test operations including capacity, are subject to inspections at any time by an ASME designee.
PG-73.4.3 A Manufacturer or Assembler may be granted permission to apply the V Code Symbol to production pressure relief valves capacity-certified in accordance with PG-69, provided the following tests are successfully completed. This permission shall expire on the sixth anniversary of the date it is initially granted. This permission may be extended for 6-yr periods if the following tests are successfully repeated within the 6-mo period before expiration.
(a) Two sample production pressure relief valves of a size and capacity within the capability of an ASME accepted laboratory shall be selected by an ASME designee. The maximum blowdown for these samples shall not exceed the value specified in the following table:
-
See PDF for table
The blow down for sample valves designed for use on forced flow steam generators with no fixed steam and waterline or high-temperature water boilers shall not exceed 10% of the set pressure.
(b) Operational and capacity tests shall be conducted in the presence of an ASME designee at an ASME-accepted laboratory. The valve manufacturer or assembler shall be notified of the time of the test and may have representatives present to witness the test.
(c) Should any valve fail to relieve at or above its certified capacity or should it fail to meet performance requirements in PG-72, the test shall be repeated at the rate of two replacement valves, selected in accordance with PG-73.4.3(a), for each valve that failed.
(d) Failure of any of the replacement valves to meet capacity or the performance requirements of this Section shall be cause for revocation within 60 days of the authorization to use the Code symbol on that particular t type of valve. During this period, the Manufacturer or assembler shall demonstrate the cause of such deficiency and the action taken toward against future occurrence.
PG-73.4.4 Use of the Code Symbol Stamp by an assembler indicates the use of original unmodified parts in strict accordance with the instructions of the manufacturer of the valve.
(a) An assembler may transfer original and unmodified pressure relief parts produced by the Manufacturer to other Assemblers, provided the following conditions are met:
(1) both Assemblers have been granted permission to apply the V or UV Code Symbol to the specific valve type in which the parts are to be used
(2) the Quality Control System of the Assembler receiving the pressure relief valve parts shall define the controls for the procurement and acceptance of those parts
(3) the pressure relief valve parts are appropriately packaged, marked, or sealed by the Manufacturer to ensure that the parts are
(a) produced, by the Manufacturer
(b) the parts are original and unmodified
(b) However, an assembler may convert original finished parts by either machining to another finished part or applying a corrosion-resistant coating to valve springs for a specific application under the following conditions:
(1) Conversions shall be specified by the Manufacturer. Drawings and/or written instructions used for part conversion shall be obtained from the Manufacturer and shall include a drawing or description of the converted part before and after the conversion.
(2) The Assembler's quality control system, as accepted by a representative from an ASME-designated organization, must describe in detail the conversion of original parts, provisions for inspection and acceptance, personnel training, and control of current Manufacturer's drawings and/or written instructions.
(3) The Assembler must document each use of a converted part.
(4) The Assembler must demonstrate to the Manufacturer the ability to perform each type of conversion. The Manufacturer shall document all authorizations granted to perform part conversions. The Manufacturer and Assembler shall maintain a file of such authorizations.
(5) For an Assembler to offer restricted lift valves, the Assembler must demonstrate to the satisfaction of the Manufacturer the ability to perform valve lift restrictions. The Manufacturer shall document all authorizations granted to restrict the lift of the valves, and shall maintain records of the lift restrictions made by the Assembler. The Assembler shall maintain a file of such authorizations.
(6) At least annually a review shall be performed by the Manufacturer of an Assembler's system and conversion capabilities. The Manufacturer shall document the results of these reviews. A copy of this documentation shall be kept on file by the Assembler. The review results shall be made available to a representative from an ASME designated organization.
NOTES: Within the requirements of PG-73.4 and PG-73.5, a
manufacturer is defined as a person or organization who is completely responsible for design, material selection, capacity certification, manufacture of all component parts, assembly, testing, sealing, and shipping of pressure relief valves certified under this Section.
An assembler is defined as a person or organization who purchases or receives from a manufacturer the necessary component parts or valves and assembles, adjusts, tests, seals, and ships pressure relief valves certified under this Section at a geographical location other than and using facilities other than those used by the manufacturer.
PG-73.5 Testing by Manufacturers or Assemblers
PG-73.5.1 Pressure Testing. Each pressure relief valve to which the Code Symbol Stamp is to be applied shall be subjected to the following tests by the Manufacturer or Assembler:
(a) The pressure-containing parts of each valve shall be hydrostatically tested at a pressure at least 1.5 times the design pressure of the parts. Parts meeting the following criteria shall be exempt from pressure testing:
(1) the applied stress under hydrostatic test conditions does not exceed 50% of the allowable stress; and
(2) the part is not cast or welded.
(b) Alternatively, testing may be performed pneumatically at a pressure 1.25 times the design pressure of the part. Pneumatic testing can be hazardous; it is therefore recommended that special precautions be taken when conducting a pneumatic test.
(c) Testing may be done in the component or assembled condition.
(d) When the valve is designed for discharging directly to atmosphere, the valve components downstream of the valve disk are exempt from pressure testing.
(e) Valve components downstream of the disk and fully contained within the body are exempt from pressure testing.
(f) These tests shall be conducted after all machining and welding operations on the parts have been completed.
(g) There shall be no sign of leakage.
