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1、 Reference number ISO 23251:2006/Amd.1:2008(E) ISO 2008 INTERNATIONAL STANDARD ISO 23251 First edition 2006-08-15 AMENDMENT 1 2008-05-01 Petroleum, petrochemical and natural gas industries Pressure-relieving and depressuring systems AMENDMENT 1 Industries du ptrole, de la ptrochimie et du gaz nature
2、l Systmes de dpressurisation et de protection contre les surpressions AMENDEMENT 1 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:20:39 MDTNo reproduction or networking permitted without lice
3、nse from IHS -,-,- ISO 23251:2006/Amd.1:2008(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer pe
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6、l Secretariat at the address given below. COPYRIGHT PROTECTED DOCUMENT ISO 2008 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission i
7、n writing from either ISO at the address below or ISOs member body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2008 All rights reserved Cop
8、yright International Organization for Standardization Provided by IHS under license with ISO Licensee=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:20:39 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 23251:2006/Amd.1:2008(E) ISO 2008 All rights reserved iii Forew
9、ord ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a techni
10、cal committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of e
11、lectrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to
12、the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for i
13、dentifying any or all such patent rights. Amendment 1 to ISO 23251:2006 was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore structures for petroleum, petrochemical and natural gas industries, Subcommittee SC 6, Processing equipment and systems. Copyright International Or
14、ganization for Standardization Provided by IHS under license with ISO Licensee=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:20:39 MDTNo reproduction or networking permitted without license from IHS -,-,- Copyright International Organization for Standardization Provided by IHS under license wit
15、h ISO Licensee=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:20:39 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 23251:2006/Amd.1:2008(E) ISO 2008 All rights reserved 1 Petroleum, petrochemical and natural gas industries Pressure-relieving and depressuring system
16、s AMENDMENT 1 Page 50, 5.15.7.4 Replace the last sentence at the end of subclause, “In calculating the heat absorption.wetted area term.” with the following: In calculating the heat absorption due to fire exposure, use Equations (6) and (7), applying an exponent of 1,0 to the wetted area term along
17、with the following values of C1 and C2: In SI units: C1 = 66 300 C2 = 108 900 In USC units: C1 = 21 000 C2 = 34 500 Page 65, Clause 5 Add a new subclause, 5.23, after 5.22: 5.23 Overfilling process or surge vessel 5.23.1 General Many process or surge vessels, including columns and towers, have a liq
18、uid level present during normal, start-up, or shutdown conditions. Experience has shown that this equipment can be overfilled under certain conditions. If the source pressure of a liquid feed or supply line can exceed the relief device set pressure and/or the design pressure of the equipment, then o
19、verfilling shall be included in the system design. System design options to deal with liquid overfill include but are not limited to a) increasing the system design pressure and/or pressure relief device set pressure within pressure design code allowances, b) designing a pressure-relief system that
20、can safely accommodate the overfill (including the effects of operator intervention response as discussed in 5.4), c) installing a safety instrumented system (SIS) to prevent the liquid overfill. Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee
21、=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:20:39 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 23251:2006/Amd.1:2008(E) 2 ISO 2008 All rights reserved For all three options, all phases of operation shall be evaluated. Particular attention shall be given to st
22、art-up and other non-routine operations where process conditions (e.g., flow rates, temperature and density) can be different from normal and where conditions that lead to overfilling can be more likely to occur when compared with normal operations. If option a) is chosen, the user shall validate th
23、e design (foundation, piping, etc.) consistent with the liquid overfill scenario. If option b) is chosen, the user shall select the pressure-relief devices, design their inlet and outlet piping, validate the foundation design, etc. consistent with the liquid overfill scenario. If operator interventi
24、on is used as part of the system design, then the risk of failure of the operator to properly intervene shall be addressed. The pressure-relief device may discharge back into a disposal system, such as a lower-pressure section of the process, a flare or to other disposal system. If it is a basis for
25、 the disposal-system design, then the user shall consider liquid overfill when designing the knockout drum, collection headers, etc. The design of the disposal system shall prevent the discharge of liquids above their flash point directly to atmosphere (into the air or on the ground) if there is an
26、unacceptable risk as a result of a vapour-cloud explosion or other hazardous condition. See 6.3.2.4 for cautions regarding the atmospheric discharge of liquids. See 6.6 regarding options for the disposal of liquids. 5.23.2 Mitigation measures When designing the system to mitigate liquid overfill, th
27、e following can affect the design and shall be evaluated: a) risk of failure of the operator to respond properly; b) operator training and operating procedures that include instrumentation behaviour; EXAMPLE If a differential-pressure or displacer-level measurement reads low compared to actual level
28、 when the fluid specific gravity is less than the design gravity. This can mean that the indicated level cannot reach 100 % even if the actual level is well above the measured range. c) availability of instrumentation that is required for adequate operator intervention; d) availability of instrument
29、ation that is required for safety instrumented system (SIS) response; e) potential consequence associated with relief-stream disposition (e.g., discharge back into the process, discharge to atmosphere, discharge to a treatment system, such as a flare, etc.); f) pressure design code requirements. If
30、operator intervention and/or SIS options are selected, then a risk analysis method shall determine the adequacy of the protection. 5.23.3 Level instruments Some criteria to consider when evaluating level instrumentation and alarms to demonstrate availability and independence from the basic process c
31、ontrol system include the following: a) whether the level instruments used for safeguards against overfilling are on separate process taps from the process control system; b) whether level instruments used for safeguards against overfilling are susceptible to the same common mode device failures as
32、those used for the basic process control system. Diverse instrumentation can minimize the potential for common-mode device failures (e.g., differential pressure and radar, displacer and float, etc.); c) whether the instruments are proven in use for the specific process applications; Copyright Intern
33、ational Organization for Standardization Provided by IHS under license with ISO Licensee=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:20:39 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 23251:2006/Amd.1:2008(E) ISO 2008 All rights reserved 3 d) whether the range
34、 of at least one of the level measurement(s) can indicate a valid level reading over the full range between the high critical alarm point and any shutdown or interlock point; e) whether operating characteristics of the level measurement during off-design, start-up and shutdown operations are conside
35、red in the display of level, setting alarms, trip points, operator training and operating procedures. Page 96, 6.6.2.3 Add the following paragraph at the end of 6.6.2.3 a): Sizing of the quench drum is discussed in 7.3.2.3.2. In addition to the quenching of vapours, the drum is typically required to
36、 provide vapour-liquid separation, liquid retention and disposal. If the quench drum is part of a header network consisting of multiple relief devices or tie-ins, then see 6.7 and 7.3.4 for additional design guidance. Page 97, Clause 6 Add a new subclause, 6.7, after 6.6.5: 6.7 Disposal through comm
37、on vent stack The general principles in 6.3 also apply to designs involving multiple relief devices discharging into a common header manifold that is either vented directly to atmosphere through a common vent stack or through a separator, quench, blowdown, scrubber or other drum where liquid is coll
38、ected and the vapour is discharged through a common vent stack. In particular, the considerations that it is necessary to evaluate for these systems shall include the following: a) guidance on determining design loads is given in 7.1; b) relief manifold system design is discussed in 7.2.3 and 7.3.1.
39、3; c) guidance on liquid disposal options is given in 5.23 and 6.6. Because these systems can encounter a wide range of relief loads, consequence assessment shall evaluate both the scenarios representing the design loads but also smaller releases that result in a lower discharge velocity and, conseq
40、uently, a greater tendency for the plume to drop to grade level in a hazardous concentration. Guidance on the specification and location of the vent stack is given in 7.3.4. Note that the guidance in 6.3.2 applies only to individual relief device tail pipes that discharge to atmosphere. CAUTION If t
41、here is a vapour-cloud explosion hazard associated with one or more relief cases or discharges, then one of the following shall be used: disposal by a flare (see 6.4); discharging into a lower pressure system (see 6.5); application of HIPS (see Annex E); eliminating the relevant relief cases (redesi
42、gn of equipment, etc.). The user shall assess hazards other than a vapour-cloud explosion associated with the release and determine appropriate mitigation measures. Page 115, 7.3.2.1 Change title from “Knockout drums” to “Flare knockout drums” Copyright International Organization for Standardization
43、 Provided by IHS under license with ISO Licensee=Boeing Co/5910770001 Not for Resale, 07/25/2008 03:20:39 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO 23251:2006/Amd.1:2008(E) 4 ISO 2008 All rights reserved Page 117, 7.3.2.1.2, below Equation (39) Replace the units f
44、or , “expressed in megapascal-seconds (centipoise)”; with “expressed in millipascal- seconds (centipoise);” Page 118, 7.3.2.1.2 Replace the paragraph following 7.3.2.1.2 b), “The volume occupiedvapour disengagement.”, with the following: The volume occupied by the liquid should be based on a release
45、 that lasts 20 min to 30 min. A larger hold-up volume can be required if it takes longer to stop the flow. Any accumulation of liquid retained from a prior release (from pressure-relief devices or other sources) should be added to the liquid indicated in items 6.7 a) and 6.7 b) to determine the avai
46、lable vapour-disengaging space. The risk of overfilling the flare knockout drum shall be assessed. Most flares are not designed to effectively combust liquid. These evaluations should consider the effect of the amount of liquid in the knockout drum prior to the release including the removal time, th
47、e liquid weight on the flare header and the flare stack mechanical integrity, the discharge of liquid from the flare (i.e., potential for flame-out, excessive smoke and unburned hydrocarbon emissions, discharge of “burning rain”, pool fires around the flare stack, etc.); the flare siting/location in
48、 proximity to areas where people can be exposed, to property fence-lines, units, etc. Page 127, 7.3.2.3 Replace the title and text of 7.3.2.3, “Sizing a quench drum” with the following: 7.3.2.3 Knockout drums venting to atmosphere 7.3.2.3.1 General Atmospheric blowdown systems are knockout drums with stacks open to atmosphere. CAUTION If there is a vapour-cloud explosion hazard associated with one or more relief cases or discharges, then one of the following shall be used: disposal by a flare (see 6.4); discha