PREPARED BY CHECKED BY
NA
PTT PUBLIC COMPANY LIMITED ONSHORE COMPRESSOR STATION 4 PROJECT
APPROVED BY CERTIFIED (PTT) REV. DATE NO. D1 14-Feb-08 D2 12-Jun-08
NA REVISED BY
APPROVED BY NA NA
DESCRIPTION ISSUED FOR ITB REVISION ISSUED FOR ITB
EMERGENCY DEPRESSURIZING PHILOSOPHY PHC 0804.02-10.06 REV.D2 TOTAL 7 PAGES
AREA CODE OF SITE LOCATION GENERAL AREA: 010 PTT PLC. CONTRACT NO.
.
PTT PLC. PROJECT NO. 0804.02
EMERGENCY DEPRESSURIZING PHILOSOPHY
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.06 PAGE 2 OF 7 REV: D2
CONTENTS SECTION 1.0
INTRODUCTION ........................................... ................................................................. ............................................ ........................ 3
2.0
SCOPE .......................................... ................................................................ ............................................. ..................................... .............. 3
3.0
GENERAL REQUIREMENTS ........................................... .................................................................. .............................. ....... 3
4.0
DEPRESSURIZING REQUIREMENTS................... REQUIREMENTS ......................................... ........................................... ..................... 4
5.0
CALCULATION OF DEPRESSURIZING LOADS.................... LOADS......................................... ............................ ....... 5
6.0
INSTRUMENTATION .............................................. ..................................................................... .................................... ............. 6
7.0
INSTALLATION .............................................. ..................................................................... ........................................... .................... 6
8.0
REFERENCE GUIDELINES ............................................ ................................................................... ................................ ......... 6
EMERGENCY DEPRESSURIZING PHILOSOPHY 1.0
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.06 PAGE 3 OF 7 REV: D2
INTRODUCTION This document sets out the philosophy for the design of the emergency depressurizing (EDP) system for the ONSHORE COMPRESSOR STATION 4. The philosophy is based primarily on PTT standard practice and the recommendations of API. It shall be incorporated into the FEED and EPC phases of the project. The document does not attempt to define the overall methodology to be followed in the design of the emergency depressurizing system but it does identify certain design criteria, which shall be adopted in s pecific areas. General design guidance will be found in the publications referenced in section 8.
2.0
SCOPE The emergency depressurizing system is one of the safeguarding systems, which protect personnel and equipment from potential hazards, created by unexpected operating excursions. This document addresses the requirements for emergency depressurizing facilities for the project. Related issues are covered in the following documents: •
PH-0804.02-10-02 Overpressure Protection Philosophy
•
PH-0804.02-10-05 Flaring and Venting Philosophy
•
PH-0804.02-10-03 Emergency Shutdown Philosophy
Where an issue is not specifically addressed in this document, the guidance contained in the related PTT project engineering specification and API recommended practice shall be followed (see section 8).
3.0
GENERAL REQUIREMENTS
3.1
Overpressure Protection All pressure equipments shall be adequately protected against overpressure from all identified pressure sources as set out in document PH-0804.02-1002, Overpressure Protection Philosophy. Unless the process design for the system is inherently safe, overpressure protection shall be provided using, in most instances, a pressure-relieving device to remove excess fluid from the system under pressure. Relief disposal systems shall be provided to safely dispose of this fluid without creating additional hazards.
EMERGENCY DEPRESSURIZING PHILOSOPHY 3.2
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
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Functionality Systems containing large inventories of gas and liquefied gas at high pressure have the potential to create a major hazard during the process upset. In the event of fire, vessels containing gas at high pressure cannot always be adequately protected by a pressure-relieving device. There may be insufficient wet area available to absorb the heat from the fire without raising the vessel wall temperature and thus significantly reducing the integrity of the vessel. As a result, the vessel may fail at a pressure, which is considerably lower than the relief valve set pressure. To minimise the potential for the creation of a major hazard, provision shall be made to depressure equipment containing large inventories of high pressure gas/liquid in the event of a fire, a serious operating excursion or utility failure.
4.0
DEPRESSURIZING REQUIREMENTS
4.1
Pressure Reduction The requirement to depressurize shall be considered for all process equipments in according to the recommendations of API RP 521. It requires that any equipment with an operating pressure in excess of 17.24 barg (250 psig) shall be provided with a depressurizing facility. It is envisaged that all high pressure systems within the the OCS 4 shall be provided provided with depressurizing facilities. Discharge shall be to the flare/vent/vent via the blowdown header. Locations and requirements shall be defined during FEED. The depressurizing capacity for each system shall take into account the specific requirements for each location but, unless otherwise indicated, the recommendations of API RP 521 shall be followed. Each depressurizing facility shall be capable of depressurizing protected equipment, having a wall thickness not less than 25 mm, from design pressure to 50% or 6.90 barg, whichever is lower, within 15 minutes. Thinner vessels may require a different depressurizing rate and this rate shall be specified by the vessel designer. The depressurizing rate calculated shall be regarded as preliminary. Provided the integrity of the protected equipment would not be compromised, this rate may be reduced if this would be beneficial to the design of the flare/vent/vent system. However, any deviation from the API recommended practice shall be endorsed by the vessel designer. Depressurizing requirements for fire and each process and utility upset shall be determined. The fire case case will normally set the size of the depressurizing valves due to the generation of additional vapour by the heat input from the fire. Equipment shall wherever possible be depressured as part of a system. In selecting appropriate systems, account shall be taken for the normal response
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of any control valves in the system and the potential for items of equipment to be automatically isolated from the depressurizing valve.
4.2
Impact on Flare/vent/Vent Header Sizing The depressurizing rate shall not govern the capacity of the flare/vent/vent header. Should the calculated depressurizing load require a larger header than is necessary for the maximum identified emergency flare/vent load, consideration shall be given to reducing the depressurizing rate or by phasing the depressurizing of sections of the process. Regardless of phasing, consideration shall be given to the possibility for and the consequences of a common mode failure. Subject to capacity constraints, the depressurizing rates selected shall, wherever possible, minimise the potential for low temperature discharges, which would either impact on materials selection or create disposal problems. Depending on the flare/vent loads identified for various overpressure scenarios and the calculated depressurizing loads, it may be cost-effective to provide a separate depressurizing header. The back pressure generated at the depressurizing valve shall be limited such that critical flow is maintained through the valve. For the calculation of flare/vent system back pressures for a fire case, the depressurizing rate for a system shall not be considered to be co-incident with the fire loads determined for the equipment being depressured. Where appropriate, the depressurizing loads from other systems in the same fire area and the possible phasing of these systems s ystems shall be taken into account. The calculated depressurizing rate is an initial maximum rate which will immediately begin to decay. If depressurizing loads are phased, the dynamics dynamics of a depressurizing scenario shall be taken into account if this approach would result in a significant reduction in the size s ize of the flare/vent header. Since the magnitude and possible phasing of depressurizing loads may have a significant impact on the required capacity of the flare/vent system, definition of depressurizing requirements shall be established as early as possible during FEED.
5.0
CALCULATION OF DEPRESSURIZING LOADS The required rate of depressurizing for each system shall be calculated using the approach set out in API RP 521 section 3.19.
EMERGENCY DEPRESSURIZING PHILOSOPHY
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
6.0
INSTRUMENTATION
6.1
Configuration
PHC-0804.02-10.06 PAGE 6 OF 7 REV: D2
The depressurizing system is part of the emergency shutdown (ESD) system and shall meet the requirements set out in document PH-0804.02-10-03, Emergency Shutdown Philosophy. Specific functional requirements shall be shown in the Cause and Effect Charts and Safety Requirement Specification (SRS) The ESD system logic shall ensure that all necessary shutdown actions are completed before the depressurizing of a system s ystem may be initiated. Appropriate indication shall be provided in the control room. Depressurizing systems shall be actuated manually from the control room.
6.2
Depressurizing Valves Depressurizing valves shall be low noise, tight shut-off globe valves which fail open. The need for secure air supplies shall be considered to eliminate a common mode failure in which all valves fail open and discharge simultaneously to the flare/vent. One or more valves may be provided on any item of equipment or system. The opening of the valves in a multi-valve installation may be staggered in order to meet any limitations on maximum depressurizing rate set by available flare/vent header capacity. The response time of the depressurizing valves shall be taken into account in assessing depressurizing requirements.
7.0
INSTALLATION Depressurizing valves shall be located as near as possible to the hydrocarbon inventory to be depressured. When selecting a location, the following factors shall be considered: •
•
8.0
the anticipated response of control valves in the s ystem the possible impact on vessel temperatures (e.g. due to the depressurizing of a ‘cold’ section of plant through a ‘warm’ one)
•
damage to trays and other internals in vessels
•
damage to solids contained in vessels (e.g. molecular sieves)
REFERENCE GUIDELINES Emergency depressurizing systems for the OCS 4 shall be designed primarily in accordance with the requirements of this project specification. Unless
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otherwise stated, they shall also meet the requirements of API 521 Guide for Pressure-Relieving and Depressurizing Systems (Fourth Edition, March 19 97).
