DGS-IU-012: Takreer Design General Specification (DGS)

TAKREER DESIGN GENERAL SPECIFICATION (DGS)

DGS-IU-012

PRESSURE RELIEF DEVICES

REV

DATE

0

MAR 2006

DESCRIPTION

REVIEWED ENDORSED

BY

APPROVED

Base Reference – Project 5601 ABU DHABI OIL REFINING COMPANY

DOCUMENT NUMBER: DGS-IU-012

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TABLE OF CONTENTS

1.0

GENERAL....... ........................................... ............................................ .............................................. ...3 ... 3 1.1 INTRODUCTION.........................................................................................................................3 1.2 PURPOSE......................................................................................................................................3 1.3 DEFINITIONS DEFINITIONS ..................................... ........................................... ........................................... ...3 ... 3 2.0 CODES, STANDARDS STANDARDS AND RECOMMENDED PRACTICES..... PRACTICES................................................ ........................................... ........... 4 3.0 REFERENCE DOCUMENTS.................................................................................................................5 4.0 DOCUMENT PRECEDENCE ......................................... ........................................... ............................ 5 5.0 SPECIFICATION SPECIFICATION DEVIATION DEVIATION / CONCESSION CONTROL ........................................... ................... 5 6.0 QUALITY ASSURANCE ASSURANCE AND QUALITY CONTROL................ CONTROL................ .......................................... .............. 6 7.0 DOCUMENTA DOCUMENTATION .......................................... ........................................... .......................................... 6 8.0 SUBCONTRACTORS/SUBVENDORS.................................................................................................7 9.0 HANDLING.............................................................................................................................................7 10.0 DESIGN...................................................................................................................................................8 10.1 PRESSURE RELIEF.....................................................................................................................8 10.2 VENDOR REQUIREMENTS.....................................................................................................18

APPENDIX 1 .......................................... ........................................... ............................................... ............... 20 APPENDIX 2 .......................................... ........................................... ............................................... ............... 22 APPENDIX 3 .......................................... ........................................... ............................................... ............... 28

ABU DHABI OIL REFINING COMPANY


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1.0

GENERAL

1.1

INTRODUCTION

This specification gives minimum requirements for design and engineering of pressure relief devices in gas, oil and chemical installations. 1.2

PURPOSE

This specification establishes minimum requirements for design and engineering of pressure relief and depressuring facilities in oil, gas and chemical installations, including cryogenic services. 1.3

DEFINITIONS

For the purposes of this specification, the following definitions shall apply: General Definitions:

CONCESSION REQUEST - A deviation requested by the SUBCONTRACTOR or VENDOR, usually after receiving the contract package or purchase order. Often, it refers to an authorization to use, repair, recondition, reclaim, or release materials, components or equipment already in progress or completely manufactured but which does not meet or comply with COMPANY requirements. A CONCESSION REQUEST is subject to the COMPANY approval. SHALL - Indicates a mandatory requirement. SHOULD - Indicates a strong recommendation. Specific Definitions:

BALANCED SAFETY/RELIEF VALVE - A safety/relief valve that incorporates means for minimizing the effect of back pressure on the performance characteristics - opening pressure, closing pressure, lift and relieving capacity. CONVENTIONAL SAFETY/RELIEF VALVE - A closed-bonnet pressure relief valve whose bonnet is vented to the discharge side of the valve. The Valve’s performance characteristics opening pressure, closing pressure, lift, and relieving capacity - are directly affected by changes on the back pressure on the valve. PILOT ASSISTED SAFETY/RELIEF VALVE - Operated in normal conditions by a pilot at maximum operating pressure within the 10 percent band of the set pressure. In case this pilot operated activation fails, the pilot assisted safety/valve will operated as a conventional safety/relief valve. PRESSURE RELIEF VALVE - A generic term applied to relief valves, safety valves, and safety/relief valves. RELIEF VALVE - An automatic pressure-relieving device actuated by the static pressure upstream of the valve, which opens in proportion to the increase in pressure over the opening pressure. A relief valve is used primarily for liquid service. SAFETY VALVE - Normally used in gas and vapor service or in steam and air service, is an automatic pressure-relieving device actuated by the static pressure upstream of the valve and characterized by rapid full opening or pop action.



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SAFETY/RELIEF VALVE - Normally used in gas and vapor service or in liquid service, is an automatic pressure-relieving device actuated by the static pressure upstream of the valve and characterized by rapid full opening or pop action. 2.0

CODES, STANDARDS AND RECOMMENDED PRACTICES

It shall be the VENDOR’S responsibility to be, or to become, knowledgeable of the requirementsof the referenced Codes and Standards. The following codes and standards shall, to the extent specified herein, form a part of this specification. The latest edition in force shall apply. American Petroleum Institute (API)

API RP 520

Recommended Practice for the Design and Installation of Pressure-relieving Systems in Refineries Part I “Design” Part II “Installation”

API RP 521

Guide for Pressure-Relieving Depressuring Systems

API RP 526

Flanged Steel Safety Relief Valves

API RP 527

Commercial Seal Tightness of Safety Relief Valves with Metal to Metal Seat

API RP 2000

Venting of Atmospheric and Low-Pressure Storage Tanks

American Society of Mechanical Engineers (ASME)

ASME

Boiler and Pressure Vessel Code Division I & II

ASME Section I

Steam Generators

ASME Section I

Power Boilers

ASME Section VIII

Pressure Vessels

International Organization for Standardization (ISO)

ISO 9001-2000

Quality Management System Requirements

ISO 9004-2000

Quality Management Guidelines for Performance Improvement System

ISO 9011

Quality Management Guidelines for Performance Improvement System

National Fire Protection Association (NFPA)

NFPA 58

Storage and Handling of Liquefied Petroleum Gases



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3.0

REFERENCE DOCUMENTS Project Specifications

DGS-IU-009

Instrumentation for Depressuring Systems

DGS-MU-002

Preservation and Export Packing

DGS-MU-003

Spare Parts

DGS-MU-009

Equipment Noise Control

DGS-MU-014

Minimum Shop Inspection and Certification Requirements

DGS-MW-002

Welding and NDE ( Nondestructive Examination) of Piping

DGS-MX-001

Painting

DGS-MW-006

PMI (Positive Material Identification) of Vessels and Piping

DGS-PU-003

Technical Specification for Piping Systems

MESC Specifications for Valves and Accessories

4.0

77/135

Safety/Relief Valves to API Standard 526

77/302

Material Acceptance Criteria, General Service

77/303

Material Acceptance Criteria, Special Service

77/304

Inspection, Testing and Certification, General Service

77/305

Inspection, Testing and Certification, Special Service

DOCUMENT PRECEDENCE

It shall be the VENDOR’S responsibility to be, or to become, knowledgeable of the requirements of the referenced Codes and Standards. The VENDOR shall notify the CONTRACTOR of any apparent conflict between this specification, the related data sheets, the Codes and Standards and any other specifications noted herein. Resolution and/or interpretation of precedence shall be obtained from the CONTRACTOR in writing before proceeding with the design/manufacture. In case of conflict, the order of precedence shall be stated in the AGREEMENT or other PROJECT documents as applicable. 5.0

SPECIFICATION DEVIATION / CONCESSION CONTROL

Any technical deviations to the Purchase Order and its attachments including, but not limited to, the Data Sheets and Narrative Specifications shall be sought by the VENDOR only through CONCESSION REQUEST format. CONCESSION REQUESTS require CONTRACTOR’S and COMPANY’S review/approval, prior to the proposed technical changes being implemented. Technical changes implemented prior to COMPANY approval are subject to rejection.



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6.0

QUALITY ASSURANCE AND QUALITY CONTROL

The VENDOR shall have in effect at all times, a QA/QC program which clearly establishes the authority and responsibility of those responsible for the quality system. Persons performing quality functions shall have sufficient and well defined authority to enforce quality requirements that initiate, identify, recommend and provide solutions to quality problems and verify the effectiveness of the corrective action. VENDOR’S proposed quality system shall fully satisfy all the elements of ISO 9001 - 2000 and ISO 9004 - 2000. The quality system shall provide for the planned and systematic control of all quality-related activities performed during design. The VENDOR shall identify in purchase documents to its SUBVENDORS all applicable QA/QC requirements imposed by the CONTRACTOR, and shall ensure compliance thereto. On request, VENDOR shall provide objective evidence of its QA/QC surveillance of its SUBVENDOR’S activities. The VENDOR shall submit certified reports of production tests as soon as the tests are completed satisfactorily. The COMPANY/CONTRACTOR reserves the right to inspect materials and workmanship at all stages of manufacture and to witness any or all tests. The VENDOR, 30 days after award but prior to the pre-inspection meeting, shall provide the CONTRACTOR with a copy of its Manufacturing and Inspection Plan for review and inclusion of any mandatory COMPANY/CONTRACTOR witness points. A Criticality Rating (CR) for each valve is listed on the data sheet or line list. The CR shall be used for various design and inspection functions by the CONTRACTOR. 7.0

DOCUMENTATION

VENDOR shall submit the type and quantity of drawings and documentation for CONTRACTOR’S authorization or information as listed in the Individual Material Requisitions and Purchase Orders. Mutual agreement on scheduled submittal of drawings and engineering data shall be an integral part of any formal Purchase Order. Comments made by CONTRACTOR on drawing submittal shall not relieve VENDOR or SUBVENDORS of any responsibility in meeting the requirements of this specifications. Such commends shall not be construed as permission to deviate from requirements of the Purchase Order unless specific and mutual agreement is reached and confirmed in writing. Each drawing shall be provided with a title block in the bottom right-hand corner incorporating the following information: a.

Official trade name of the VENDOR.

b.

VENDOR’S drawing number.

c.

Drawing title giving the description of contents whereby the drawing can be identified.



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d.

A symbol or letter indicating the l atest issue or revision.

e.

PO number and item tag numbers

f.

TAKREER Logo.

Revisions to drawing shall be identified with symbols adjacent to the alterations, a brief description in tabular form of each revision shall be given, and if applicable, the authority and date of the revision shall be listed. The term “Latest Revision” shall not be used. 8.0

SUBCONTRACTORS/SUBVENDORS

The VENDOR shall assume unit responsibility and overall guarantee for the equipment package and associated equipment. The VENDOR shall transmit all relevant purchase order documents including specifications to his SUBVENDORS and SUBCONTRACTORS. It is the VENDOR’S responsibility to enforce all Purchase Order and Specification requirements on his SUBVENDORS and SUBCONTRACTORS. The VENDOR shall submit all relevant SUBVENDOR and SUBCONTRACTOR drawings and engineering data to the CONTRACTOR. The VENDOR shall obtain and transmit all SUBVENDOR and SUBCONTRACTOR warranties to the CONTRACTORS/COMPANY, in addition to the system warranty. 9.0

HANDLING

Preparation for shipment shall be in accordance with the VENDOR’S standards and as noted herein. VENDOR shall be solely responsible for the adequacy of the preparation for shipment provisions with respect to materials and application, and to provide equipment at the destination in ex-works condition when handled by commercial carriers. Adequate protection shall be provided to prevent mechanical damage and atmospheric corrosion in transit and at the jobsite. Preparation for shipment and packing will be subject to inspection and rejection by COMPANY’S/CONTRACTOR’S inspectors. All costs occasioned by such rejection shall be to the account of the VENDOR. After inspection and test, equipment shall be completely free of water and dry before start of preparation for shipment. All references to instrument tubing in this document are assumed to be for 317L SS (not 316 SS). This specifically includes instrument air tubing. Where areas of high corrosion potential exist and are identified by COMPANY, the following alternate materials shall be considered for instrument tubing: 904 SS

254SMO SS 2205SS



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ENGINEER/CONTRACTOR shall confirm cost and availability of the alternate materials for COMPANY approval. All equipment and material shall be preserved and export packed in accordance with Project Preservation Specification, DGS-MU-002. 10.0

DESIGN

Relieving facilities for vessels in marketing installations of capacities up to and including 135 m3 shall be assessed in accordance with NFPA 58, storage and handling of liquefied petroleum gases. For pressure relief and venting of low-pressure storage tanks including refrigerated storage, refer to API RP 2000. For the rating and adjustment of safety valves on power boilers refer to the ASME Boiler and Pressure Vessel Code, Section I “Power Boilers.” Safety valves, relief valves, and rupture discs will be selected in accordance with governing Codes and Regulations. If the area in which the plant is located has no local codes or regulations, the ASME Power Boiler Code and the ASME Unfired Pressure Vessel Code are used as applicable. Relief valves will be applied in a ccordance with the following:

10.1

Pressure Vessels

ASME Section VIII

Power Boilers

ASME Section I

Steam Generators

ASME Section I

Tank Venting

API RP-2000

PRESSURE RELIEF

Set pressures (SP) and maximum relief pressures (MRP) of safety/relief valves, expressed in relation to the design pressure (DP) of the protected equipment, all expressed in gauge pressures, shall not exceed the values given in the table below. Note:

For the pressure setting of the safety/relief valve instead of the design pressure (DP), the maximum allowable working pressure (MAWP) is also used. MAWP is determined by the pressure vessel fabricator and is equal to or greater than DP. Set Pressure (SP) Non-fire conditions

Fire Conditions

Maximum relieving pressure (MRP) Non-fire Conditions

Fire Conditions

Single Valve

100% of DP

100% of DP

110% of DP

121% of DP

Multiple Valves

one valve 100% of DP the others at 105% of DP

one valve 100% of DP the others at 105% of DP** 110% of DP*

116% of DP

121% of DP



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*

Relief valves for fire protection may only be set at 110% of DP if they are installed in addition to adequate relief protection of the process equipment against non-fire situations.

** For set pressures below 10 kg/cm2, staggering of set pressure becomes impracticable because of the difference between the set pressure tolerance of 3% (according to ASME VIII UG 134) and the value of 5% of the DP, becomes too small. The above shall also apply to safety /relief valves discharging liquid and flashing liquid. 10.1.2

Evaluation of Relief Requirements

10.1.2.1

For each safety/relief valve the applicable causes for relief and the resulting relief requirements shall be determined. The possible causes are listed in API 520, Part 1, Table 2. A more detailed description of the causes for relief is given in API 521, Chapters 2 and 3. The following remarks about the listed causes in API 520/521 can be made: a.

For new designs no correction shall be made for the difference between maximum working pressure and relief pressure with respect to suppression of vapor production, except and subject to COMPANY approval, where an appreciable difference exists between maximum working pressure and set pressure (SP), and subsequent reduction in the quantity of gas/liquid to be relieved.

b.

For new designs the cooling effect of an air-cooled heat exchanger resulting from natural draught in case of fan failure shall not be taken into account.

c.

For the case of excess flow from a high pressure source (high pressure - low pressure interface) the largest contribution of either the fully open control valve or the fully open bypass valve shall be taken into account. The calculation shall be based on vapor breakthrough (no liquid) and shall be made for the actually installed valves. Additionally it shall be ensured that in the highly improbably event of both control valve and bypass being fully open the downstream pressure will not exceed 150% of MAWP.

10.1.2.2

10.1.2.3

In addition to the above, the quantities to be discharged in the event of general emergencies, shall be determined for each safety/relief valve. The following emergencies shall be considered separately, unless one emergency will precipitate the other: a.

Total cooling water failure

b.

Total electricity failure - Partial electricity failure, that is failure of one cable or transformer

c.

Steam failure, total or partial

d.

Instrument power supply failure (gas, air, electrical etc.)

e.

External fire, for each probable fire area.

For each safety/relief valve, the relevant relief conditions will be established. The data of all pertinent relief conditions shall be stated on the safety/relief valve data sheet.



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The size of the safety/relief valve and the inlet and outlet piping shall be determined using the largest relief load. For reporting the calculations of safety/relief valve size, a calculation sheet shall be used. See the example as shown in Appendix 1. Or, the approved safety/relief valve vendor’s calculation program and printout, shall be used. The formulas used are derived from API RP520. In the formula for vapors, the compressibility factor (Z) for vapor will generally be equal to one. For high pressure gas systems the real value of Z should be used in order to avoid oversizing of safety/relief valves. 10.1.2.4

Equipment shall be protected against high pressure due to fire under the following conditions: a.

If the equipment is located in an area where a sizable fire might occur, and

b.

If it is conceivable that the equipment is blocked in without having been emptied first when such a fire occurs.

During a fire, all feed and output streams to and from the fire affected equipment and all internal heat sources within the process are assumed to have ceased. Potential fire areas shall be established and clearly shown on a plot plan. The fire areas shall be numbered and the number shall be indicated on the safety/relief valve data sheet. 2

For process units, a fire area of typically 300 m , should be developed on the basis of the drainage design of the plot. Height of the flame to be considered shall be 8 meters from grade or platform on which liquid can accumulate (concrete platform), except for air coolers where no height limitation shall be assumed. If protection of equipment against operational emergencies by a safety/relief valve is already foreseen, the same valve may serve for the fire/emergency case, provided it is of adequate size for either emergency. Vapor relief requirements for fire relief shall be calculated in accordance with the procedures given in Appendix 2. 10.1.2.5

In the rare instances where a gas-filled vessel (no liquid level) can be engulfed by fire, it is possible that the pressure vessel cannot be adequately protected by a typical safety relief valve only. Without the presence of liquid boiling off inside the vessel to keep metal temperature down while rapidly approaching relief pressure, a vessel can experience severe reductions in strength of materials due to high metal temperature before the interior pressure reaches relief pressure. The only driving force for internal pressure increase in such a situation is gas expansion. ASME in the later editions of BPV-VIII confirms this situation. Please note that a standard safety relief valve may still be necessary to satisfy non-fire relief contingencies.

10.1.3

Selection of Relieving Devices

10.1.3.1

General As a rule spring loaded relief valves should be used as pressure relief devices. For special applications pilot operated safety/relief valves may be applied.



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Rupture discs and explosion covers which are non-reclosing safety devices shall not be used as sole protection against over pressure without the COMPANY’S agreement. For further applications of bursting discs reference is made to paragraph 10.1.4.1. 10.1.3.2

Safety/Relief Valves

10.1.3.2.1

When selecting the type and size of relief valves only those which have established data and are guaranteed by the MANUFACTURER(s) for the service conditions shall be used. For reasons of standardization, the following range of standard sizes in accordance with API Standard 526 “Flanged Steel Safety Relief Valves” shall be used for valves not exceeding 8T10 size: Inlet/Orifice/Outlet

Orifice Area (sq. in.)

(sq. cm.)

1D2

0.110

0.71

1E2

0.196

1.26

1 1/2F2

0.307

1.98

2H3

0.785

5.06

3K4

1.838

11.86

4L6

2.853

18.41

4P6

6.380

41.16

6Q8

11.050

71.23

6R10

16.000

103.23

8T10

26.000

167.75

(A different standard may prevail for existing refineries or plants.) The risk of unstable valve operation, e.g., in liquid filled systems, where oversized valves can cause chattering of the valve, makes it sometimes necessary to make use of sizes not included in the standard range. The orifices 4M6 and 4N6 are also allowed in addition to the above. For required orifices larger than 8T10, pilot-operated valves may be used; the process medium should then be dry and clean. 10.1.3.2.2

Safety/relief valves shall have as a minimum flanged inlet connection of 150 lbs ANSI RF, or equal to the vessel or pipe rating, whichever is greater, and outlet connection of 150 lbs ANSI RF. The flanges shall form an integral part of the body. The set pressure limit according to API 526 shall not be taken less than 21 kg/cm2g (300 psig.) at 232ºC (450ºF) except for the sizes R and T for which the set pressure limit shall be not less than 16 kg/cm2g (230 psig) at 232ºC (450ºF). Flange face finish shall be 125-250 Ra in accordance with the piping class of the connected piping, refer to DGS-PU-003 Technical Specifications for Piping Sy stems.



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Flange shall be integrally cast type; slip-on or weld-on flanges shall not be allowed, except as approved by the COMPANY. 10.1.3.2.3

Valve materials shall be suitable for inlet and outlet temperatures that result from the extremes of operating and emergency conditions. The possibility that the valve may leak under otherwise normal operating conditions must be considered. For the choice of valve spring materials, only the extremes of non-fire conditions shall be considered. Material selection according to API RP 526 is recommended. The standard materials for the temperature range 15ºC to 232ºC (59ºF to 450ºF) are for body and bonnet ASTM A 216 Gr. WCB or WCC and for the valve trim AISI 316, with the additional requirements given in 10.2.1. The springs of safety/relief valves shall be protected with a suitable coating to prevent the occurrence of general corrosion and/or hydrogen sulfide stress corrosion cracking. Coatings of cadmium or zinc are not allowed, owing to the risk of liquid metal embrittlement during service and/or hydrogen embrittlement during the galvanizing process. Suitable aluminum coatings are permitted. Coatings shall be approved by COMPANY. For services not indicated in API RP 526, the material shall be selected in consultation with the COMPANY.

10.1.3.2.4

Non-balanced Safety/Relief Valves A non-balanced safety/relief valve shall be used in t he following cases: a.

When the variable back pressure is not larger than the allowed difference between the maximum relief pressure (MRP) and valve SP. This is usually 10% of SP in most instances, it may be 21% of SP for fire relief when the valve is set at equipment DP (design pressure).

b.

When the constant back pressure is less than 50% of the set pressure and the valve is set for the difference between upstream DP and constant back pressure.

Local regulations may not accept non-balanced valves set at equipment DP if the actual opening pressure could at times be higher as a result of the back pressure generated by other relief valves blowing at the same time. In that case, the use of balanced valves, or a slightly lower setting of conventional valves, should be considered for those valves which may relieve as a result of a common emergency. Conventional safety/relief valves connected to a closed discharge system shall always have a closed bonnet. 10.1.3.2.5

Balanced Type of Safety/Relief Valve A balanced type of safety/relief valve shall be provided when the variable back pressure (gauge) is higher than allowed for a conventional v alve. The back pressure (gauge) shall not exceed 50% of the set pressure (gauge) for relief of vapors or flashing liquids.

10.1.3.2.6

Balanced Bellows Safety/Relief Valves Safety/relief valves of the balanced type shall have a vented bonnet which shall be vented to atmosphere in such a manner as to prevent the ingress of rain and dirt. Hazards to personnel should be avoided. However, any flow from the vent should be visible.



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Balanced bellows valves, with a bonnet vent open to atmosphere, shall not be used on streams which cause the valve to freeze up when it leaks. If this happens, the bonnet and balancing bellows will fill up with ice due to condensation and subsequent freezing of the moisture in the air, making the valve inoperative. Spring-loaded, non-balanced valves and pilot-operated valves, the pilot of which stays relatively warm, are less sensitive in this respect. 10.1.3.2.7

Pilot Operated Safety/Relief Valves Pilot operated safety/relief valves use the process pressure acting on the top side of the disk as the closing force. The pilot valve, which is a spring-loaded valve, connects the cavity above the disk either to the process or to the atmosphere, or a low pressure system. The pilot operated safety/relief valve can operate at process pressures close to the set pressure because of the high positive closing force which increases with the process pressure. Normally pilot operated valves are provided with soft seals which give a better valve tightness. The design features of a pilot operated valve allows for larger orifice sizes than the maximum 8T10 size for spring-loaded valves. For smaller and medium sizes these safety/relief valves are more expensive. A field connection for checking the set pressure shall be provided. Due to corrosive attack from several sources, a special material of construction n ote is required: All control/sensing tubing and compression fittings used with pilot-operated Safety/Relief valves shall be 316L SS PVC coated minimum.

10.1.3.2.8

Thermal Expansion Relief Valves (TERV) Thermal expansion relief valves are required in liquid filled systems when the system can be blocked in and submitted to heat input from the atmosphere or process. The theoretical pressure rise for most liquids lies in the range 5 to 15 kg/cm2 for each degree Centigrade of temperature increase. In practice, the theoretical pressure rise is not attained because systems are rarely totally liquid full and usually have small leakages through valve seats or gaskets. Calculations of the pressure rise are thus of little use in formulating realistic guidelines for application of thermal relief valves. “Refinery practice for many years dictated the use of TERVs on the cold side of heat exchangers that could be blocked in. In the late 1970s and early 1980s, ASME modified BPV-VIII so that all relieving devices used on ASME Code-Stamped Pressure Vessels must be either Code-Stamped Safety Valves, Safety Relief Valves or Relief valves as defined in BPV-VIII. This specifically included valves used to protect the pressure vessel in a thermal expansion case. A few years later, BPV-VIII was further modified to state that all ASME Code-Stamped Valves must have a calculated relieving quantity included with the mandatory information concerning the valve. The following factors should be taken into account when deciding whether to fit thermal expansion relief: a.

Is the linework or equipment in continuous operation and thus not routinely isolated without being depressured and drained. Most continuous process equipment and linework is in this category?



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b.

Is the liquid highly toxic, corrosive or an LPG?

c.

Is there a weak point in the system such as a flange where expansion could relieve without serious consequences?

d.

Is it likely that the system will be totally liquid filled (i.e., to more than 95%)?

As a general guide, thermal relief valves are not n eeded for: a.

Process plant piping.

b.

Storage or transport piping sections which are not normally shut in for operational or emergency purposes.

c.

Lines in which there is normally two-phase flow.

d.

Heat traced lines which are not blocked i n as part of normal operations.

Thermal relief valves are normally fitted to: a.

Sections of piping containing more than 500 liters of LPG or toxic material which could be normally blocked in.

b.

Piping in storage areas or transport lines which will be regularly blocked in during normal operation.

In all cases, operating procedures should be such that overpressure due to thermal expansion does not occur even when a relief is fitted. It is recommended that for each major project, Operating Company, or site, a more detailed practice on application of thermal reliefs is developed, taking local climate and operating conditions into account. The TERV standard size for piping applications is 3/4 x 1 inch flanged with a minimum orifice area of 0.71 cm2. Bronze valves, which are allowed for water only, may have screwed connections. 10.1.3.2.9

Safety/Relief Valves for Steam Service Safety/relief valves for steam service discharging to atmosphere shall be of the exposed-spring type if operating above 250ºC. Test rods shall not be provided unless required by statutory regulations. (Test rods which prevent the stem moving are sometimes used for blocking the safety/relief valve during hydrotesting of the equipment). Lifting devices shall be provided for steam, hot water and air service only.

10.1.4 10.1.4.1

Arrangement of Safety/Relief Valves Safety/relief devices shall be arranged so that their proper functioning is not hindered by the nature of the vessel’s contents. The use of protecting devices such as rupture discs, goose-neck seals or purge arrangements may be necessary but these shall not interfere with the proper functioning of the safety/relief device.



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Rupture discs may be used upstream of safety/relief valves to protect them from continuous contact with a corrosive or solidifying process fluid, or used on equipment normally operating under vacuum if the ingress of oxygen via the leaking valve could cause problems. If a rupture disc is used upstream of a safety/relief valve, a pressure indicator alarm shall be provided between disc and valve. This arrangement permits detection of disc rupture or leakage. If an indicator or alarm is applied, the requirements set by the various design codes should be taken into account. For liquids with a high pour point, insulation and heat tracing of piping upstream and downstream of the safety/relief valves (or bursting discs) should be applied. Flushing with a low viscosity hot process flow upstream, and sometimes also downstream, of the safety/relief valve may be applied. In the case of flushing of the downstream lines, a local knock out facility will be required. This knock out drum will also serve to prevent introduction of high pour point fluids into the main flare system. 10.1.4.2

Safety/relief valves on cold process streams shall have an uninsulated inlet line of sufficient length to prevent icing of (non-leaking) valves. Special attention shall be paid in this respect to valves which discharge into the atmosphere, i.e. in those having open outlet which could clog with ice. Safety/relief valves on hot process streams may be installed on an uninsulated length of inlet line, which will keep the valve cold if the process stream is liquid or vapor with enough incondensables to maintain a cold gas cap and if there is not any danger for deposit formation and solidification on the safety/relief valve inlet.

10.1.4.3

Where the possibility of a safety/relief valve becoming plugged by hydrates exists, one or a combination of the following measures could be considered: a.

Spring-loaded or pilot-operated safety/relief valves should be fitted with heat-tracing. This will prevent hydrate formation in case of small leaks across the valve seat.

b.

Spring-loaded safety/relief valves can be provided with a bursting disc fitted parallel to the valve.

c.

Passing pilot valves should be provided with small capacity bursting discs mounted in the pilot pick up line or pilot to dome line. These discs shall be fitted with an outlet pipe, which shall discharge to atmosphere at a safe height.

d.

Alternatively, a complete back-up system to the required number of safety/relief valves can be provided. The back-up system shall consist of an equal number of safety/relief valves with bursting discs mounted upstream of each safety/relief valve.

e.

Glycol or methanol injection in relief system piping.

The bursting discs mentioned in the foregoing alternatives shall have a nominal bursting pressure which is above the safety/relief valve setting but below the hydrostatic test pressure, i.e. 130% of the design pressure.



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It should be realized that this arrangement of backing up the relief valves with bursting discs has no basis in the pertinent API Recommended Practices or ASME Codes. It should be regarded as an additional measure to enhance safety if the measures that have been taken to keep the statutory safety/relief valves operational become inadequate. 10.1.4.4

Safety/relief valves shall be connected to the protected equipment in the vapor space above any contained liquid or to piping connected to the vapor space. Safety valves installed in ASME BPV-1 piping are a notable exception to this requirement. Below is the following exception: For vessels fitted with a demister mat, a relief outlet downstream of the mat is only allowed if the velocity head of the largest relief flow is not greater than 3 times that of the maximum design operating flow. The relief connection shall be upstream of the mat for larger relief flows.

10.1.4.5

The flow area through all pipe and fittings between a pressure vessel and its safety/relief valve shall at least have the same flow area as the inlet of the valve. The pressure drop of the inlet piping for a gas, vapor or flashing liquid flow, based on the actual safety/relief valve capacity, shall not exceed 3% of the valve set pressure. Exceptions to this requirement are only allowed in the case of a pilot-operated valve with a suitably arranged remote pilot connection, or when it can be shown on the basis of measured safety/relief valve characteristics that a higher inlet p ressure drop can be tolerated. When two or more safety/relief valves (spares not counted) are pl aced on one connection, the crosssectional area of this connection shall be at least equal to the combined inlet areas of the valves, and the above pressure drop requirement shall apply for the combined flow of the valves.

10.1.4.6

Spring-loaded and pilot-operated or assisted safety/relief valves, as well as thermal expansion valves, shall always be installed in the upright position. Liquid shall be drained from the valve, both at the valve inlet and outlet.

10.1.4.7

Safety/relief valves discharging to atmosphere shall be located at the maximum practical elevation to economize on discharging piping. Also, safety/relief valves connected to a closed relief system shall be located above the relief header, to prevent back flow of liquid or condensate. For instance, valves protecting tall columns shall be positioned on the overhead vapor line.

10.1.4.8

If the valves cannot be put above the header, they shall be lined up to discharge into a local drain vessel. Alternatively, if the problem of elevation is confined to a few valves, and if the COMPANY agrees, outlet lines to the header shall be heat-traced from the safety/relief valve to the highest point of the line. Such an arrangement is not permitted for valves which discharge a medium which can leave a deposit. The heat-tracing can be omitted if the safety/relief valve, as well as the connecting header, handle only products which vaporize completely at the lowest ambient temperature.

10.1.4.9

Safety/relief valve outlet lines should be connected to the top of the header, or at least so that the latter cannot drain back into them, not even with the header full of liquid.



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10.1.4.10

The provision of block valves should be avoided wherever possible because of the attendant maintenance and procedural requirements which safe interlock systems demand. However, where the need to replace a safety/relief valve can be expected, thereby causing unacceptable production loss, operational upset or air pollution, an additional (spare) safety/relief valve shall be provided. Block valves with suitable interlocks paragraph (10.1.4.11) or, alternatively, coupled change-over valves, shall be provided upstream and downstream of safety/relief valve(s). Safety/relief valves which discharge via individual outlets to atmosphere need no outlet block valves. Care must be taken that the maximum inlet pressure drop is not exceeded, paragraph (10.4.5), particularly when change-over valves are used. A vent connection shall be provided between upstream block valve and safety/relief valve. Spare safety/relief valves are not required: a.

For thermal expansion relief valves.

b.

If the safety/relief valve protects equipment which can readily be isolated without interrupting the process.

10.1.4.11

The block valves used in the inlet and outlet of safety/relief valves and their spares and 1 inch and 1-1/2 inch valves on the inlet and outlet of thermal expansion relief valves may be sealed rather than locked, in a manner to be agreed by the COMPANY.

10.1.4.12

Rupture Discs Rupture discs shall be used: a.

Where relief loads caused by unexpected sources cannot be handled fast enough by relief valves. Tube rupture venting high pressure gas/vapor into the liquid-filled shell side of a heat exchanger can conceivably produce a shock wave (> Mach 1) phenomena that can damage the pressure vessel regardless of the safety relief valve of rupture disc configuration. This is not considered to be a likely governing condition except where the tubeside gas is corrosive.

b.

On the inlet side of a relief valve to protect the valve if the process fluid is extremely corrosive, or polymerizes, or to insure against valve leak age of hazardous and toxic fluids.

10.1.5

Construction

10.1.5.1

Valve Body Valve bodies shall be minimum cast steel for all services except low pressure air and water. Parts contacting the process fluid are determined by line classification. Minimum inlet connections for flanged valves are 1-inch. All steel valves with inlet flange size 2-inch or smaller, shall be 150 lb. minimum. Use 3/4 x 1-inch screwed valves for thermal relief on water service.



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10.1.5.2

Nozzle Disc and Spring All flanged valves shall have full nozzles with one piece disc construction. Screwed valves shall have insert type nozzles with MANUFACTURER’S standard disc. The disc is a piston type held against process pressure by a spring. Disc, nozzle, guide, and ring are MANUFACTURER’S standard stainless steel except where the line classification dictates a better material. Spring material is carbon steel up to 230ºC and tungsten steel for higher temperatures. The spring is adjustable for popping pressures within Code allowances above a nd below initial set pressure.

10.1.5.3

Accessories

10.1.5.3.1

Bonnet and Cap Bolted pressure tight bonnets for all conventional valves except where the code requires open bonnets shall be used. Balanced valves shall have vented bonnets. Bonnet and cap material is carbon steel, unless process fluid entering bonnet on balanced safety valves requires alloy construction. The cap and bonnet will be provided with lugs for wire seal after setting the valve. Gags Gags are not to be used.

10.1.5.3.2

Lifting Levers Lifting levers are provided on all safety valves used on steam, air service, and hot water above 60ºC.

10.2

VENDOR REQUIREMENTS

10.2.1

Materials Materials selection shall also comply with the Technical Specification for Piping Systems, DGSPU-003 for carbon steel pressure containing parts: a.

Specific approval of the COMPANY is required for use of types of steel with a specified 2 minimum tensile strength exceeding 480 N/mm .

b.

Carbon content shall not exceed 0.23%, except forgings and castings which shall be limited to 0.25% maximum.

c.

Carbon equivalent Ceq = C +

Mn 6

+

C r + M o + V Cu+Ni + 5 15

≤

0.42%

The material selection of the body (including bonnet and/or bottom flange), external bolts, studs and nuts, etc. shall be in accordance with the piping class. Valve bodies specified as 316 shall be 316L stainless steel minimum. Carbon, low alloy and stainless steel bodies shall be externally painted per Section 10.2.5. External, uninsulated bolts and nuts shall be shop coated with TAKECOAT-1000 from Takenada Seisakusho Co., Osaka, Japan or authorised equivalent in Painting Specification DGS-MX 001. Insulated bolts shall also be coated if the service temperature is less than 200°C. All bolting installation requirements from the coating MANUFACTURER shall be followed in addition to installation requirements from other Project specifications.



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Screws/rivets for fixing the valve identification plate to the safety/relief valve, by VENDOR, shall be corrosion resistant. 10.2.2

Material Acceptance Material acceptance criteria shall be in accordance with the requirements as specified in DGS-MU-014 and Type “B” certificate (En 10204, 3.1B); the original or Red stamped copies, verified by COMPANY, approved by i nspectors.

10.2.3

Welding Welding certificates and NDE (nondestructive examination) shall be in accordance with the requirements as specified in DGS-MW-002, Welding and NDE of Piping.

10.2.4

Marking The VENDOR supplied valve identification plate shall also include the CONTRACTOR’S valve tag number.

10.2.5

Painting Surfaces to be painted or coated shall be dry and free from burrs, weld spatters, flux, dust, grease, oil and other foreign matter before any paint is applied. See Painting Specification, DGS-MX-001. All carbon steel, low/intermediate alloy and stainless steel parts of the safety/relief valve, in contact with the atmosphere shall be blast cleaned per DGS-MX-001, Painting Specification. A two-component, chemical-resistant paint system shall then be applied. The finish color of the final finish shall be according to the Color Code given in DGS-MX-001, Painting Specification, unless otherwise specified in the requisition. The valve stem or spindle and gasket-contact surface of flanges shall not be painted but shall be protected against corrosion with a suitable protective fluid.

10.2.6

Documentation VENDOR shall provide a table showing the valve tag number, orifice type, orifice diameter, spring type, spring size, spring range, spring coding, spring dimensions and number of coils, valve set pressure, valve size and rating and valve type.

10.2.7

Inspection and Testing Inspection, testing and certification of the valves shall be in accordance with the requirements as specified in MESC Specification 77/304 and 77/305, and DGS-MU-014, Minimum Shop Inspection and Certification Requirements. PMI-Positive material identification for all alloy pressure-containing components, shall be in accordance with the requirements as specified in DGS-MW-006, PMI-Vessels and Piping.



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APPENDIX 1 SAFETY RELIEF VALVE CALCULATION UNIT

TAG NUMBER

SERVICE

JOB NUMBER

FLUID

1 2

Operating Pressure (P F)

kg/cm g 2

Max. Operating Pressure (P O)

kg/cm g

Liquid Specific Gravity (G F)

kg/liter

Molecular Weight Vapors (MW) Temp. At Acc. Pressure (T A)

ºC

Specific Heat Ratio (k) Constant, based on k (C) 2

Max. Allowable working pres. (P)

kg/cm g

Flow coefficient (K)

2 3

kg/cm2 g

Design Pressure (Pd ) Set Pressure (PP)

12

2

13

2

kg/cm g

14

kg/cm g

Back Pressure (var.) (Pb)

11

2

kg/cm g

4

Net Spring Setting (P S) = PP - PC

5

Accumulation

%

15

6

Accum. Absolute Press. (PA)

2

16

7

Super Heat Correction (K SH)

17

8

Accumulation Factor (C a)

18

9

Viscosity Correction (K V)

10

HAZARD

- RV -

kg/cm g

See note

19 2

Back Pressure (Const.) (P C)

kg/cm g

20

DESIGN BASE

V (VOLUME)

W (WEIGHT)

21

REMARKS

3

kg/h

22

m /h

º Total Electricity Failure

23

º Total Cooling Water Failure

24

º Total Instrument Air Failure

25

º External Fire

26

º Non - Fire, other Failure

27

º Thermal Expansion

(Take nominal size)*

28

º Steam Failure

29

DESIGN BASE

30 EVALUATION OF ORIFICE SIZE

Vapors

A=

31

Steam

W x TA . z 4 . 9 x C x K x PA x MW

A=

Liquids

W 340 x K x PA x K SH

V x GF A=

23.5 x PS x C a x K v

Units W in kg/h

Ca = 0.6

TA in º kg/liter

CF in

V in m3/h PA in kg/cm2

A=

Type: Remarks:

SQ. IN.

Conventional, balanced (Piston, Bellows-Piston, Bellows) If C is unknown take C = 315 If K is unknown take K 0.9 (to be checked after selection of vendor, but not higher than 0.9) If viscosity < 20 cS take K V = 1.0 z = compressibility factor

For liquid gas flows the required orifice areas for the liquid and gas parts are calculated using the formula for liquid and gas respectively and sum the orifice areas Formulae based on API RP 520, ASME Boiler and Press. Vessel Code Sect. VIII (DIV I) Manufacturers may apply other formulas, if based on certified data.



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APPENDIX 1 (Continued)

* Nominal size = 1º x 1º, 300 x 150 ANS, A = 0.11 SQ. IN.

Model Description Type Inlet size & rating Outlet size & rating Nozzle area / letter Manuf. Model Nr.

Made By

:

Date :

Title :

Issue

Checked By

:

Date :

CALCULATION OF

Appr. By

:

Date :

RELIEF VALVE ORIFICE

LOCATION

:

Eng. By

:

Sheet No.

TAKREER

:

Drawing No.

PLANT

:

Date By Project & Group No. :



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APPENDIX 2 CALCULATION OF VAPOR RELIEF REQUIREMENTS FOR FIRE EXPOSURE

For fire conditions the following calculation procedure shall be used: Note:

Units of measurement shown are an example, for reference only. Use project units of measure for all project calculations.

1)

The boiling temperature of the liquid at the relief pressure is determined and subsequently the latent heat of vaporization at these conditions.

2)

The internally wetted surface of the vessel is calculated. For the wetted surface take the wetted surface up to a height of 8 m above grade or in the case of spheres or spheroids at least the elevation of the maximum horizontal diameter or a height of 8 m, whichever is the greater. The term “grade” usually refers to ground level, but may be at any level at which a sizable fire could be sustained (e.g., concrete platform). In case of a variable liquid level within the above specified height the calculation of the wetted surface shall be based on the following:

3)

•

Storage vessels are assumed to contain their maximum working volume.

•

Process vessels are assumed to contain their normal working volume.

•

Columns (packed and tray) are assumed to contain a liquid volume corresponding with the height from the bottom to the highest (LC) level controller connection and the liquid on the trays which can be calculated on the basis of normal pressure drop over the columns.

•

Cases not dealt with in the above should be considered individually.

If all floors are self draining and any liquid is led away from the equipment which has to be protected, the heat input shall be calculated as follows: Q

=

43.2 FA0.82 (based on formulae of API RP 520. Q - 21 000 FA0.82 Btu/hr)

= Q

=

Total heat input in kW

A

=

Wetted surface in m2

F

=

Environmental factor; depends on presence of insulation jacket or a protective layer.

Following valves for F shall be used: F No insulation

1.0

Insulation thickness 25 mm and over, Note 1

0.3

Heating or cooling jacket

0.6

Earth covered storage above grade

0.03

Underground storage

0.0



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Refer to the graph of this Appendix: Heat input in case of fire for equipment. If drainage cannot be provided in the area under the vessel, the heat input shall be calculated as follows: Q = 63 A, (kW) (Reference is made to API 520 table A1). The rate of discharge in kg/s in case of fire is: Total heat input rate in kW Latent heat of vaporization in (kJ/kg) NOTE 1: Following insulation requirements and alternatives apply.: a)

The sheeting shall be made of galvanized steel or stainless steel and be fastened with stainless steel or galvanized self-tapping screws, not with aluminum blind rivets. The combination of sheeting and insulation shall be resistant against prolonged fire exposure. (Ref. API 521: 3.12.2.1).

b)

Suitable insulation, in descending order of excellence, is: 1) Shaped foam glass panels. These are heat-resistant, do not soak up oil and will not burn as a wick, they are, however, expensive. 2) Ceramic fibre blanket at least 20 mm thick (e.g. Fibrefax or Kaowool) with outer sheeting over the regular insulation. This system will soak up oil but will not burn as a wick as long as the sheeting is intact. 3) Mineral wool insulation throughout, at least 100 mm thick with outer sheeting. The wool, or at least the outer layers, should have a heat-resistant binder. Because the mineral wool cannot withstand the higher temperatures that a fire can attain (+1000ºC) it will gradually deteriorate to the 25 mm required by the code, if the outer sheeting stays intact. This system will soak up oil but will not burn as a wick as long as the sheeting is intact. This system is the most common form of “fire-resistant” insulation for vessels. 4) Example: A storage vessel without insulation on a self-draining floor. •

Total volume

170 m3

•

Maximum operating volume

160 m3

Normal operating temperature

35ºC

•

Maximum operating temperature

40ºC

•

Maximum operating pressure

4.2 bar ga (vapor pressure of liquid content at maximum working temperature of 40ºC)

•

Composition of liquid

75.0% butane-1

content (% mol.)

21.0% butadiene-1,3

•

4.0% benzene ABU DHABI OIL REFINING COMPANY


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•

•

Feed stream to the vessel

30 000 kg/h at 35ºC,

(supplied by pump)

s.g. = 0.608

Offtake from the vessel

30 000 kg/h at 35ºC

(by a pump)

s.g. = 0.608

The pressure inside the vessel is assumed to exceed the atmospheric pressure in all circumstances. Calculation: Design pressure is 4.2 + 1.7 = 5.9 bar (g) = maximum allowable working pressure. Non-fire case maximum relief pressure is 1.1 x 5.9 bar (g). Fire condition: maximum relief pressure is 1.2 x 5.9 = 7.1 bar (g). Fire Case:

Wetted Surface: Lengths between tangent lines

12 m

Internal diameter

4m

The vessel has dished ends and is vertically arranged on a support so that the lower tangent line is 2.0 meters above grade. Therefore, when calculating the wetted surface, a height of (8.0 2.0)m = 6.0m shall be taken into account. Thus the wetted surface area is: 2

(π x 4) + (π x 4 x 6.0) = 88 m . Boiling temperature: The bulk of the vapors is relieved when the vapor pressure of the mixture has the value of 7.1 bar (g). The boiling temperature of the mixture at this pressure is determined with standard calculation methods and programs using the following formula: “Vapor pressure of pure component multiplied by the mole fraction in the liquid equals the partial pressure of component, and sum of partial pressure equals total pressure (mixture follows Raoult’s law).” At this temperature the: Vapor pressure of butene-1

= 8.43 bar (a)

Vapor pressure of butadiene-1,3

= 8.10 bar (a)

Vapor pressure of benzene

= 0.57 bar (a)

Partial pressure of butene-1

= 6.33 bar (a)

0.75 x 8.43 Partial pressure of butadiene-1,3

=1.70 bar (a)

0.21 x 8.1



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Partial pressure of benzene

= 0.22 bar (a)

0.4 x 0.57 Total Pressure

= 8.05 bar (a)

This checks with the value of 7.05 bar ga. The latent heat of vaporization of the mixture at 62.5ºC is found by adding the products of weight fraction in the vapor phase and latent heat of vaporization of pure component (kJ/kg) for all components. Mole fraction in vapor phase =

partial pressure total pressure

According to Dalton’s law. Thus, Vapor mole fraction butene-1 Vapor mole fraction butadiene-1,3 Vapor mole fraction benzene Molecular weight butene-1 Molecular weight butadiene-1,3 Molecular weight benzene •

= 0.788 = 0.211 = 0.003 1.002 = 56.10 = 54.10 = 78.11

Converting mole fractions to weight fractions results in: Vapor weight fraction butene-1 Vapor weight fraction butadiene-1,3 Vapor weight fraction benzene

= 0.792 = 0.204 = 0.004

Latent heat of vaporization of butene-1 at 62.5ºC Latent heat of vaporization of butadiene-1,3 at 62.5ºC Latent heat of vaporization of benzene at 62.5ºC

= 309.35 kJ/kg = 334.88 kJ/kg = 405.62 kJ/kg

Hence, the average latent heat of vaporization of the mixture is: 0.792 x 309.35 + 0.204 x 334.88 + 0.004 x 405.62

= 314.95 kJ/kg

From this result it may be judged that simply taking the latent heat of the major component (78.8% mol. Butene-1) gives a good approximation: 309.35 kJ/kg. •

Average molecular weight of vapor 0.211 x 54.10 + 0.788 x 56.10 + 0.003 x 78.11 = 55.9

•

Heat input to the vessel 0.82

Q = 43.20 x 1 x 88º

= 1698 kW (see Page 6)



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•

Rate of discharge 1698 314.95

•

=

5.39kg / s

Effect of insulation When applying 25 mm suitable insulation with galvanized steel sheeting, the rate of discharge becomes: 43.20 x 0.3 x 880.82 314.95

= 1.61 kg

/s



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HEAT INPUT IN CASE OF FIRE FOR VESSELS, COLUMNS, ETC.



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APPENDIX 3 LINE-UP OF THERMAL EXPANSION RELIEF VALVES

Thermal expansion relief valves for equipment which can be shut-in and isolated from the process. Examples: parallel filters, heat exchanger with bypass. The thermal expansion relief valves (TERVs) can in nearly all cases be lined-up to discharge in the downstream direction, over the downstream isolation valve. The spring setting (differential test pressure) of the TERVs must not exceed the design pressure of the protected equipment minus the maximum operating pressure in the point of discharge for the TERVs. The spring setting of a TERV so lined-up can generally be quite low (1 kg/cm2) without causing troublesome inadvertent relief, because there is also an upstream isolation valve which is closed when the equipment is isolated. The spring setting of the TERV must also be higher than the pressure drop between its inlet and outlet tiein points with the isolation valves open. This pressure drop should normally be quite low, unless the downstream valve is a control valve.



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DGS-IU-012: Takreer Design General Specification (DGS)

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