Sizing, Selection, and lnstallation of Pressure-relieving Devices (Installation) Part 3

8 PRD lsolation (Stop) Valves 8.1 General lsolation Ыосk valves may Ье used for maintenance purposes to isolate а PRD from the equipment it protects or from its downstream disposal system. Since improper use of an isolation valve may render а PRD inoperative, the design, installation, and administrative controls placed on these isolation Ыосk valves should Ье carefully evaluated to ensure that plant safety is not compromised. А PRD shall not Ье used as а Ыосk valve to provide positive isolation. 8.2 Application lf а PRD has а service history of leakage, plugging, or other severe proЫems that affect its perfonnance, isolation and sparing of the PRD may Ье provided. The use of isolation valves and/or sparing permits the PRD to Ье inspected, maintained, or repaired without shutting down the process unit. However, there аге potential hazards associated w�h m, the use of isolation valves. The ASME Boilerand Pressure Vessel Code, Section VIII Appendix М, Section М-5.6 discusses proper application of these valves and the administrative controls that shall Ье in place when isolation Ыосk valves аге used. Local jurisdidions may have other requirements. Additional examples of isolation valve installations эге given in 8.4. 8.З lsolation Valve Requirements 8.3.1 General lsolation (stop) valves аге allowed upstream and/or downstream of the pressure relieving device for the purpose of inspection. testing, and гераiг of the pressure relieving device ог discharge header isolation. 23 ТаЫе 1-Performance Differences with PRV Trim Characteristic Vapor Certified PRV Capacity is not cerlified bul сап Ье estimated using guidance in API 520 Part 1 (may need up to 25 % overpressure to achieve fuU lift) Liquid Relief Dual Certified PRV (Note1) Liquid Certified PRV Capacity is Certified Gapacity is Certified Capacity Is not certlfied, and is not addressed Ьу API 520 Part 1. Gapacity is Certified Up to 15 % fог liquid Up \о 1 О % for vapor, and Up to 20% fог vарог or liquid Up to 20 % for vapor or liquid Tendency to chatter in liquid relief rela�ve to liquid trim PRV lncreased Neutral Neutral Effect of setting medium on the opening characteristic PRV set on gas ог vapor but relieving liquid may open З % to 5 % higher PRV set on liquid but relieving vврог may ореп З %to 5 % lower Minor effect (i.e. within Code tolerances) Effect of required valve overpressure vs. set medium дпу shift up or down in the opening point may result in а similar shift in the point at which full lift is ac:hieved Capacity is Certifted Vapor Relief Range of Ыowdown availaЫe (see manufacturer fог PRV specific Ыowdown values, see Note2) See manufacturer for estimated capacity. NOTE1 The term Oual Certlfied covers PRVs that аге ЬOth vapor now certJned and liquld now certlЛeo where this duai cenificaUon 1s achleved without making trim changes when switching fluids during the flow testing. NOTE 2 These are typical va1ues obtained from valw manufadurers. User is cautioned to fulty understand the impact on operations when Ыowda.vn exceeds the operating margin. таые 2-Design Guidance for PRV Trim Options PRV lnlet Line Pressure Drop Hydraulic Calculation Basis Options Certified PRV Design Relief Media Vapor ✓ Vapor Dual ✓ ✓ Liquid ✓ Liquid Two phase Supercritical Uoe PRV Rated Capacity Uoe Required Relief Load ✓ ✓(Note 1) ✓ ✓ Vapor NOTE1 Liquid ✓(NoteЗ) ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Comments ✓(Note 1) CAUTION (Note 2) CAUTION (Note 3) ✓(Note 1) ✓(Note 1) ✓(Note 1) Мау Ье used if PRV exhiЫts modulating Ьehavior: see paragraph 7.3.7.3. NOTE 2 Application not recommended lf vapor is the oontrolling sizing case. Where vapor loads are not controlling, the c.apacity of the PRV relieving vapor wШ need to Ье estimated, see manufacturer for guidance. See ТаЫе 1 Jar Ыowdown characteristics. ��JvТ� 1iq��fi���� r� e�«��J;1� ��\��rJ� l�u� �,v��nr��:J:i�::C:�u7�Jn�r�r�l ��e ::s�J:c�%� :: ��� capacity with Кр = 1 (this is Ьecausethe valve is not very modulating at vэlve overpressures of less than 10%). See ТаЫе 1 for overpressure charaderistics. co u li iz 1 f 24 ln addition to prevlously noted lnlet and outlet pressure drop restrictions, isolation valves located in relief system piping shall meet the requirements specified below. The opening through all pipe and fittings (including stop valves) between а pressure vessel and its PRV shall have at least the агеа of the PRV inlet connection. See 5.2. For outlet isolation valves. to help minimize the built-up backpressure. the flow area in the outlet isolation valve should Ье equal to ог greater than the outlet агеа of the PRV. Butterfly valves and gloЬe valves аге not full area due to the presence of intemal elements and typically аге not designed for tight shut-off. ln addition, there is the potential for internal failure of the butterfly valve causing ап obstruction in the PRD inlet line. For these reasons, butterfly valves and globe valves аге typically not used as PRD isolation valves. While butterfy valves аге not recommended for usage as PRD isolation valves, high-performance butterfy valves сап Ье suitaЫe for isolating large relief headers. Butterfly valves used for these applications should Ье specified to а rigorous set of standards which assure that the disk will not fail in а position that will restrict flow and that provides а tight shut-off. Check valves shall not Ье installed in PRD inlet or outlet lines since these devices are normally closed and the check valve сап become stuck in the closed posilion ог fail in а manner causing ап obstruction in the PRD path. 8.3.2 lnlet and Outlet lsolation Valves For PRD inlet and ou�et isolation valves: а) Valves shall Ье suitaЫe for the llne service classification. Ь) Valves shall have the capaЫlity of being locked or car-sealed in the appropriate position. с) When gate valves are used, they shall Ье installed with stems oriented horizontally ог, if this is not feasiЫe, the stem shall Ье oriented downward to а maximum of 45" from the horizontal to keep the gate from falling off and Ыocking the flow. d) А Ыееd valve shall Ье installed between the isolation valve and the PRD to епаЫе the system !о Ье safely depressurized prior to performing maintenance. This Ыееd valve сап also Ье used to prevent pressure build-up between the PRD and the closed outlet isolation valve. е) The outlet isolation valve should never Ье closed while the vessel is in operation w�hout using ап inlet isolation valve that has first Ьееп closed with the space Ьetween the inlet isolation valve and the PRV adequately depressured. 11 the PRV inlet pressure is higher than the design pressure of the PRV Ьоdу ог bellows, then closure or the discharge valve could lead to PRV component overpressure. Procedures or work practices сап Ье used to facilitate proper sequencing, however consideration might also Ье given to using an intertocking system between the inlet and outlet isolation valves to assist with proper sequencing. Тhese work practices or procedures сап apply to relief device isolation valves in general. f) Consideration should Ье given !о painting the isolation valve а special color ог providing other identification. When placing the PRD into service, апу outlet valve should Ье opened fully before beginning !о ореп the inlet valve. lsolation valves should Ье opened gradually to help prevent unwanted opening of а PRD due to the momentum of the ftuid. The inlet and outlet isolation valves shall Ье ореп fully whenever the PRV is in service. А typical installation of inlet and outlet isolation valves for PRVs is shown in Figure 1 о. Typical installations of isolation valves mounted at the inlet of PRDs аге shown in Figure 11, Figure 12, and Figure 13. 25 lsolation valve with provision for car sealing or locking ореп (not required for atmospheric discharge, see Note 1 J Bonnet vent piping for bellows type pressure­ relief valves, if required (see Note 2) То closed system or atmospheric piping 'r--- Bleed valve installed оп valve � body (see Note Э) -1----1 Bleed valve lsolation valve with provision for car seaГ.ng ог locking open Flanged spool piece, lf requlred to elevate PRV pressure losses not more than З % of set pressure (see Note 4) Vessel NOTE 1 NOTE 2 NOTE 3 NOTE 4 See Section 8 for the use of isolation valves in pressure-relief system piping. See Section 1 О. Alternatively, а pipe spool with Ыееd may Ье provided. See 7.3 for detailed discussion оп inlet pressure drop. Figure 10-Typical PRD lnslallation with an lsolation Valve 8.3.3 lnstallation of Spare Relief Capacity ln corrosive and fouling services, ог other services which may require frequent PRD inspection and testing, consideration should Ье given to the installation of ап additional relief device, so that 100 % design relieving capacity is availaЫe while any PRD is out of service. Examples of this type of installation are shown in Figure 11, Figure 12, and Figure 1 З. Consideration should Ье given to storing the sраге PRVs until needed, to preserve their integrity and all1w bench testing just prior to installation. When spare relief devices аге provided, а mechanical inter1ock or administrative controls shall Ье provided which manages proper openlng and closing sequences of the isolation valves to ensure that overpressure protection of the vessel or equipment is not compromised. 26 Typically the inlet isolation valves for spare relief devices аге closed апd the outlet isolation valves are ореп. Тhе outlet isolation valve for spare relief devices сап Ье closed during operation if exposure to the fiuid is а сопсегn, however, the pressure tempeгature rating of the PRD outlet, the outlet isolation valve and intervening piping should Ье suitaЫe for the conditions upstream of the relief device iп case of leakage. Protection of the PRD from discharge system fiuids without closing the outlet isolation valve сап also Ье achieved either Ьу providing а purge or Ьу installing а discharge rupture disk. Three-way changeover valves are ассерtаЫе iп spare relief capacity applications provided the installation meets the size апd inlet pressure drop requirements (see 8.3.4). То closed (isolation valving required) or atmospheric discharge system NonrecoveraЫe pressure losses not more than З%ofset pressure (seeNote) Typical Ыееd valve Vessel NOTE See 7 .3 ror detailed discussion оп inlet pressure drop. Figure 11-Typical PRD lnstallation for 100 % Spare Relieving Capacity 27 То closed (isolation valving required) or atmospheric dlscharge system Typical isolation valve with provision for саг sealing or locking open (see Note 1) NonrecoveraЫe pressure losses not more than 3%ofset pressure (see Note2) Vessel NOTE 1 See Section 8. NOTE 2 See 7.3 for detailed discussion оп inlet pressure drop. Figure 12-Altemate PRD Arrangement for 100 % Spare Relieving Capacity 8.3.4 Three-Way Changeover Valves for Dual PRD lnstallatlons Three-way changeover valves are availaЫe, which are designed specifically for isolation valve service of dual PRD installations. Such installations provide 100 % of the design relieving capacity with one PRD while а relief device is out of service. Тhе second PRD may Ье permanently mounted on the three-way changeover valve or may Ье stored until needed to preserve its integrity and allow Ьench testing just prior to installation. Three types of changeover valves are availaЫe: а three-way ball valve comЬined w�h piping on the inlet and outlet, the shuttle type (see Figure 14) and the rotor type (see Figure 15). а) Three-way changeover valves, used for PRV installations, shall Ье sized to assure compliance with the inlet loss requirements of 7.3.4. Some three-way changeover valves are designed wilh minimum flow areas equal to or 28 То closed (isolation valving required) or atmospheric discharge system NonrecoveraЫe pressure losses not more than 3 % ofset pressure (see note) Typical Ыееd valve Positive lndication of active relieving device Vessel NOTE See 7 .З for detailed discussion оп inlet pressure dmp. Figure 13-Alternate PRD lnstallation Arrangement for 100 % Spare Relieving Capacity 29 Figure 14-Three-Way ChangeoverValve-Shuttle Туре Flgure 15-Three-Way Changeover Valve-Rotor Туре 30 greater than the inlet агеа of а PRV designed for that line size. Other designs, however, may have to Ье specified опе line size larger than the PRV to minimize inlet pressure losses. Ь) The three-way changeover valve should Ье designed to prevent both PRDs from being isolated at any time during its switchover operation. с) А positive indication of which PRD is active should Ье а required accessory for the three-way changeover valve. d) А Ыееd valve shall Ье installed between the inlet isolation valve and an isolated PRD to еnаЫе the inlet to the isolated PRV to Ье safely depressurized prior to performing maintenance. е) lndividual isolation valves may Ье used on the outlet side of PRDs that аге mounted on an inlet three-way changeover. When using individual outlet isolation valves the recornmendations of 8.3.2 should Ье followed. f) Three-way valves may also Ье used for outlet isolation. Desgns are availaЫe that will minimize the effects of built­ up backpressure when using the same pipe size as the outlet of the pressure relieving device. AII other recommendations of 8.3.2 should Ье followed. g) lsolation valves shall have the capabllity of being locked ог car-sealed in position. Only ап authorized person may break the seal and operate the valve (see 8.3.2 Ь ). Mechanical interlocks and/or management control pгocedures shall Ье provided which will assure the ргорег opening and closing sequences of the inlet and outlet isolation valves. 8.3.5 Use of Ball Valves as Three-Way Changeover Valve Ball valves аге availaЫe in а variety of configurations as shown in Figure 16. The two seat L-port configuration is the most commonly used oonfiguration for relief device selector service. Due to the variety of oonfigurations, caution should Ье taken so that the proper conf,guration is specified, the ports proper1y marked, and the valve proper1y installed. 8.4 Examples of lsolation Valve lnstallations An isolation valve downstream of а PRD may Ье installed at battery limits of process units (see Figure 17). The purpose of battery limit isolation valves is to allow process units to Ье removed from service for maintenance while other process units discharging into the main plant flare header remain in service. Similarly, relief system isolation valves may Ье used for equipment such as compressors, dryers, or coalescers, which аге spared and need to Ье shut down fог maintenance while sраге equipment remains online (see Figure 18). Number of seats 2 4 1 4 3-Way 'L" port 3-Way "L" port ���� Port arrangements �� 3-Way 'Т' port ���� Figure 16-Three-Way Changeover Valve-Ball Types 31 8.5 Administrative Controls Related to lsolation Valves Administrative controls sha/1 Ье in place that wi/1 prohiblt the inappropriate closing of isolation valves in pressure-relief system piping. These controls should require that the opening and closing of the isolation valves Ье done Ьу an authorized person. An updated list should Ье kept or all isolэtion valves located in pгessure-relier system piping which could isolate PRVs. Documentation or the required position and reason fог the lock ог seal should Ье provided. !! Periodic inspections of isolation valves located in pressure-relief system piping should Ье made which verify the position of isolation valves and the condition of the /ocking ог sealing device. Battery limit � � ) Process unit Паrе header Ыосk valve (see Note 1) �итf 1 lsolation Ыind polnt � Pressure-rellef valve installation (see Note 3) А Pressure vesselA Process unit relief header (self drэining) Pressure-relief valve installation (see Note 2) z ) z z �!J� � Pressure vessel В NOTE 1 See 8.4. NOTE 2 See Figure 11 эnd Figure 12. NOTE З See Figure з and Figure 1 О. Figure 17-Typical Flare Header Block Valves z � 32 lsolation Ыосk valves (see note) From spare compressor j �-�---ef г--- compressor nare headers lsolatюn Ыrnd pornts 11 � V"J N,,1 Process unitflare header Pulsation dampners First stage Second stage Third stage NOTE See 8.4. Figure 18-Typical lsolation Block Valves for Spare Compressor