PREPARED BY CHECKED BY APPROVED BY CERTIFIED (PTT) REV. DATE NO. D1 14-Feb-08 D2 11-Jun-08
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PTT PUBLIC COMPANY LIMITED ONSHORE COMPRESSOR STATION 4 PROJECT
NA REVISED BY
APPROVED BY NA NA
DESCRIPTION ISSUED FOR ITB REVISION ISSUED FOR ITB
FLARING AND VENTING PHILOSOPHY PHC-0804.02-10.05 REV D2 TOTAL 10 PAGES
AREA CODE OF SITE LOCATION GENERAL AREA: 010 PTT PLC. CONTRACT NO.
.
PTT PLC. PROJECT NO. 0804.02
FLARING AND VENTING PHILOSOPHY
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 2 OF 10 REV: D2
CONTENTS
SECTION 1.0
INTRODUCTION ........................................................................................ 3
2.0
SCOPE ..................................................................................................... 3
3.0
OVERPRESSURE PROTECTION .................................................................... 3
4.0
DISPOSAL SYSTEMS ................................................................................. 4
5.0
FLARE SYSTEM......................................................................................... 5
6.0
FLARE SYSTEM BACK PRESSURES ............................................................. 7
7.0
FLARE COMPONENTS................................................................................ 8
8.0
CONTINUOUS VENTING ............................................................................10
9.0
REFERENCE GUIDELINES ...........................................................................10
FLARING AND VENTING PHILOSOPHY 1.0
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 3 OF 10 REV: D2
INTRODUCTION This document sets out the project philosophy for the design of flaring and venting systems for the ONSHORE COMPRESSOR STATION 4 (OCS 4). The philosophy is based primarily on PTT standard practice and the recommendations of API. It shall be incorporated into the EPC phases of the project. The document does not attempt to define the overall methodology to be followed in the design of flaring and venting systems but it does identify certain design criteria which shall be adopted in specific areas. The design shall comply with the general design guidance given in the publications referenced in section 9.
2.0
SCOPE As part of the project, a new flare header system will be installed to serve the OCS 4 and FTP facility. This document addresses the design of the flare system and venting systems. Related issues are covered in the following documents: •
PHC 0804.02-10.02 Overpressure Protection Philosophy
•
PHC 0804.02-10.03 Emergency Shutdown Philosophy
•
PHC 0804.02-10.06 Emergency Depressurizing Philosophy
Where an issue is not specifically addressed in this document, the guidance contained in the API recommended practice shall be followed (see section 9).
3.0
OVERPRESSURE PROTECTION All pressure equipment shall be adequately protected against overpressure from all identified pressure sources as set out in PHC 0804.02-10.02, Overpressure Protection Philosophy. Unless the process design for the system is inherently safe, overpressure protection will 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. The integrity of the relieving device and the equipment it protects is dependent upon the ability of the disposal system to dispose of the discharged fluid without limiting the discharge flow.
FLARING AND VENTING PHILOSOPHY 4.0
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 4 OF 10 REV: D2
DISPOSAL SYSTEMS Fluid discharged from pressure relieving devices shall be disposed to one of the following locations: •
another part of the process
•
atmosphere
•
flare
•
blowdown header
•
oily water drainage system
The discharge location shall be selected on the basis of the phase and hazardous nature of the fluid.
4.1
Discharge to the Process This is the preferred disposal route, since excess material is contained within the process unit. However, opportunities which do not create the potential for overpressure elsewhere in the unit will be limited.
4.2
Discharge to Atmosphere To safeguard the environment, atmospheric discharges shall be limited to nonflammable and non-toxic fluids. Any requirement to discharge hydrocarbons to atmosphere shall be highlighted at the earliest opportunity. The discharge point shall be at a safe location and the flow directed away from areas where personnel may be present. The discharge line shall be sized to limit the backpressure at the relief valve to the following: •
conventional valve
: 10 percent of set pressure
•
balanced valve
: 30 percent of set pressure
The calculated pressure drop shall include the tail pipe exit loss and be based on the rated flow through the valve and a pipe roughness of 0.46 mm for carbon steel pipe. This value reflects the potentially corroded nature of relief valve tail pipes. The size of the line shall be sufficient to limit the fluid velocity to 0.5 Mach. No isolating valve shall be installed in the tail pipe.
FLARING AND VENTING PHILOSOPHY
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 5 OF 10 REV: D2
Tail pipes shall be provided with a low point drain to allow condensate to be removed together with a suitable cap to prevent ingress of rain and foreign bodies. Discharge lines shall not be combined to form a common vent. CONTRACTOR to undertake an assessment of the noise generated when the relief valve is discharging at the maximum anticipated rate. If applicable, a silencer shall be provided to reduce noise to an acceptable level. Account shall be taken of any additional pressure drop introduced by the silencer.
4.3
Discharge to Flare All relief devices capable of relieving hydrocarbons during any overpressure scenario shall discharge to the flare system. The flare system is the principal disposal system for relief valve discharges and is discussed further in section 5.
4.4
Discharge to Blowdown Header Thermal relief valves shall discharge hydrocarbon to the blowdown header. In addition, blowdown valves for depressuring equipment shall also discharge hydrocarbon in to the blowdown headers.
4.5
Discharge to Oily Water Drainage System Non-flammable and non-toxic (non-hydrocarbon) discharges from thermal relief valves shall be sent to the oily water drainage system.
5.0
FLARE SYSTEM
5.1
Flare Configuration The flare system shall meet the relief/blowdown requirement for the GDF. By design, the flare system will also include provision to handle the relief/blowdown from a future GSP within PTT Site (to be part of a separate Project).
5.2
Flare Headers Hydrocarbon discharges from relief devices shall be segregated according to temperature and routed to one of two flare headers.
5.2.1 Warm Flare Header The warm flare header shall collect all relief device hydrocarbon discharges which are at -10°C or above. The header shall be fabricated from carbon steel.
FLARING AND VENTING PHILOSOPHY
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 6 OF 10 REV: D2
Final material selection and temperature breakpoint are to be confirmed by CONTRACTOR.
02
5.3
Warm Blowdown Drum Warm wet discharges to the warm flare header and the warm blowdown header shall be routed to a warm blowdown drum. The vapour shall then be routed to the flare header.
5.4
Liquid Relief The presence of any liquid in a flare header system can cause hazard and create the potential for mal-operation of the system.
02
The potential to discharge liquid to the flare header shall be eliminated by design, for example by the suitable selection of design pressures and the adequate sizing of vessels. If appropriate, consideration shall be given to the use of SIS to eliminate large potential liquid flows.
02
5.5
Discharge Pipework The cross-sectional area for flow in the discharge line between the relief device and the flare header shall not be less than that of the relief valve discharge flange. The cross-sectional area for flow in each section of the flare header system shall not be less than that in the preceding section. Discharge pipework shall be self-draining from the relief valve discharge flange to the blowdown drum with no low points. The piping between the drum and the flare shall also slope to the blowdown drum. The minimum slope shall be 1 in 400. If a low point in the flare header is unavoidable, provision shall be made for the detection of liquid accumulation and the draining of that liquid to a suitable location before it can impact on the safe operation of the flare system. Appropriate operating procedures shall be instituted to guard against build-up of liquid. Where hydrocarbon liquid may flash across a relief valve and there is the potential for hydrate formation to occur in the header and in the relief valve, CONTRACTOR shall design system to minimise the potential for blockage Where two-phase flow is possible, piping supports shall be designed to resist the forces generated by slug flow. The design temperatures for the flare headers shall take into account the most extreme actual release conditions associated with any failure contingency and any dependable change in temperature which takes place in the system.
FLARING AND VENTING PHILOSOPHY 5.6
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 7 OF 10 REV: D2
Purging Requirements In order to protect against air ingress, a small, continuous purge gas shall be introduced at the upstream end of each main flare header to maintain a slightly positive pressure. The purge rate shall be set to maintain a minimum velocity of 0.03 metres per second. It may be appropriate to combine this with the purge of the flare riser (see section 7.3).
5.7
Design Conditions Equipment in the flare system shall have a minimum design pressure of 4.0 barg.
6.0
FLARE SYSTEM BACK PRESSURES
6.1
Header Sizing When assessing back pressures for the fire scenarios, consideration shall be given to the simultaneous discharge of material from plant depressuring systems. The hydraulic model of the flare system shall be produced by using proprietary software such as Flarenet. This model shall be used to calculate relief valve backpressures for each overpressure scenario for use in relief valve sizing. As part of this model, the back pressures for utility failure scenarios, each fire area and single operational/equipment failure cases shall be calculated. Double jeopardy scenarios shall not be considered but any dependence of one utility supply on another shall be taken into account. When considering the maximum flare load which might be realistically generated in a major failure scenario, credit shall be taken for the operation of the Safety Instrumented System (SIS) to isolate material and energy sources and prevent or limit the generation of some relief loads. In calculating the worst flare load, it shall be assumed that the SIS function, whose failure would give rise to the maximum flare load, has failed to operate. Any continuous flow of gas to the flare shall be taken into account in deriving backpressures. Flare headers and relief valve discharge pipework shall be sized to limit the backpressures at relief valves to 10 percent of set pressure for a conventional valve and 30 percent for a balanced valve. Since most plant systems operate at high pressure, it is unlikely that backpressure will have a major impact on relief valve sizing. Pressure drop calculations for carbon steel headers and pipework shall be based on a pipe roughness of 0.046 mm. Calculations shall assume the rated flow for the relief valve in individual discharge lines but the calculated flow in flare headers and sub-headers.
FLARING AND VENTING PHILOSOPHY
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 8 OF 10 REV: D2
Sonic conditions at piping discontinuities such as at branch connections and reducers can result in potentially damaging acoustically induced vibration. Where relief valve back pressure does not govern the sizing of any piping segment, the selected line size should limit the maximum velocity in the line to 0.65 Mach.
6.2
Fire Areas The fire area considered for each item of equipment shall be a circle of diameter 56 feet with the item of equipment at its centre. It shall be assumed that any item of equipment within this circle is affected by the fire and contributing a co-incident relief load. Backpressures shall be determined for two scenarios: •
relief valve discharge alone
•
co-incident system depressuring.
7.0
FLARE COMPONENTS
7.1
Type of Flare Flare shall be Elevated type.
7.2
Flare Tip The flare tip shall be sized to pass the maximum anticipated emergency flare load with a pressure drop of not more than 0.5 bar in order to minimise the impact on relief valve back pressures. The gas exit velocity from the tip shall be a maximum of 0.5 Mach.
7.3
Riser Purge To minimise the potential for air to enter the flare system via the tip, each riser shall be purged continuously with gas while in service to limit the extent to which a flammable mixture is allowed to exist in the riser. The required purge rate shall be calculated to give a minimum velocity in the flare risers of 0.1m/s When determining the purge rate, consideration shall be given to the potential for burn-back at the tip. At a sufficiently low purge rate it is possible for the flame to burn back and become established inside the tip. This would likely result in damage to the tip and a reduced service life. CONTRACTOR shall ensure that the purge rate provides an exit velocity sufficient to minimise damage from burn back. As the system cools after a hot release, there is the potential to create a partial vacuum. In line with the design philosophy for the flare systems for the
FLARING AND VENTING PHILOSOPHY
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 9 OF 10 REV: D2
existing PTT system, there shall be no water seal drum to protect against air ingress at the flare tip. Provision to inject supplementary gas shall therefore be provided to prevent air ingress. The system shall be initiated by high temperature, low pressure or a combination of both. The required gas injection will be high but of relatively short term duration.
7.4
Gas Seal A gas seal shall be provided within each riser upstream of the flare tip to help prevent the ingress of air via the tip. No credit shall be taken for the presence of the seal when setting the required riser purge rate unless vendor recommendations are supported by test data.
7.5
Thermal Radiation The height and precise location of the n flare shall be set by thermal radiation considerations. The height of the flare shall not be less than 30 metres. Radiation intensities shall be calculated for the maximum emergency flare load using a segmental method which adequately addresses the presence of two adjacent flames from the two in-service flare risers. CONTRACTOR shall develop contours for the radiation intensities specified in section 7.5.2 for key wind speeds and directions.
7.5.1 Flame Emissivity Radiation intensity is directly proportional to flame emissivity. This can vary considerably, depending upon the gas composition, the combustion efficiency and the extent to which the flame is shielded by soot. Flare vendors often claim and utilise low emissivities, which allow them to be more competitive in specifying a reduced height for a flare, but they do not make available any supporting test data. For many applications, the API recommended value of 0.3 will be safe, but this may not necessarily be conservative for streams containing appreciable quantities of unsaturated fluids. PTT has specified the following emissivities for use in this project: Hydrogen, methane
0.15
Natural gas
0.23
Ethane, propane
0.25
Other hydrocarbons
0.30
7.5.2 Maximum Exposure Times The acceptability of radiation intensities in a particular location will be influenced by the nature and frequency of tasks routinely carried out in the area and the ease with which a task could be terminated and the system left in a safe condition in the event of an emergency.
FLARING AND VENTING PHILOSOPHY
PTT PUBLIC COMPANY LIMITED PHILOSOPHY
PHC-0804.02-10.05 PAGE 10 OF 10 REV: D2
Maximum allowable thermal radiation intensity has been set by PTT as 6.31kW/m2 the perimeter of the sterile zone. This figure shall include solar radiation.
7.6
Smokeless Flaring The requirement for smokeless flaring shall be established taking into account the quantity of gas continuously flared . A minimum of 10 percent smokeless capacity shall be provided based on the maximum anticipated emergency flaring rate.
7.7
Emissions CONTRACTOR shall undertake dispersion analyses to verify that flare emissions under all anticipated operating modes comply with Thai regulations as identified in ES 91.04, HSE Legislation.
7.8
Oxygen Analyser An oxygen analyser shall be provided in each flare riser to monitor the effectiveness of the purge and to optimise the rate to minimise cost. The reliability of analysers in this service to date has been limited and therefore it is not appropriate to use their output for direct control of the purge rate.
8.0
CONTINUOUS VENTING Non-hazardous streams such as steam and air may be intermittently or continuously vented to atmosphere to a safe location. Storage tanks containing non-hazardous fluids may be provided with an open vent. .
9.0
REFERENCE GUIDELINES Relief disposal systems for OCS 4 shall be designed primarily in accordance with the requirements of this project specification. Unless otherwise stated, they shall also meet the requirements of API 521 Guide for Pressure-Relieving and Depressuring Systems (Fourth Edition, March 1997).
