Petrofac SECTION 9 VENTS, FLARES AND DRAINS. Vents, Flares and Drains. 1 Introduction

Transcription

1 SECTION 9 VENTS, FLARES AND DRAINS 1 Introduction 2 Vents and Flares 2.1 System Function 2.2 System Equipment 2.3 Sources of Released Fluids 2.4 HP Flare System 2.5 LP Flare System 2.6 LLP Flare System 2.7 Flare Tips 2.8 Maintenance/Atmospheric Vents 3 Drains System 3.1 System Function 3.2 System Equipment Figures 9.1 Flare System Schematic 9.2 Combined HP/LP Flare Coanda Type Flare Tip 9.3 Flare Tip Pilot and Ignition System 9.4 Open Drains System Overview 9.5 Closed Drains System Overview Training Services Page 1 of

2 9.1 INTRODUCTION On offshore installations there must be a provision for handling and dealing with hydrocarbons that are not recovered or contained during various scenarios. Such a provision is required both during normal operations and during upset or emergency conditions. During upset and emergency conditions the facility must be capable of accepting a full process plant depressurisation. A full process plant depressurisation may well involve dumping of hydrocarbon liquids to drains, as well as the flaring and venting of gases. Therefore such a facility will comprise two related systems: Vents and flares Drains 9.2 VENTS AND FLARES System Function Refer to Figure 9. 1 The vents and flares installed on offshore installations can comprise three separate systems operating at high pressure (HP), low pressure (LP) and low-low pressure (LLP). The purpose of these systems is to: Collect and safely dispose of hydrocarbons released at different pressures from process and utility equipment during platform normal operation, upset conditions or emergency situations Collect and safely dispose of water, probably containing hydrocarbons and released through pressure safety valves or rupture discs fitted on certain heat exchangers to cater for tube rupture Segregate, prior to liquid knockout, the collection of wet gases and dry gases, within each HP and LP system, to avoid the possibility of hydrate formation in pipework essential for controlled gas release Recover separated liquids and transfer them to the platform drains for recovery Gases from the three systems are burnt at HP, LP and LLP flare tips located adjacent to each other at the end of the flare boom. The flare system is designed such that in the event of maximum flaring conditions there is a minimal risk to personnel and equipment due to heat radiated from the flare tip. Training Services Page 2 of

3 9.2.2 System Equipment By referring to the system shown in Figure 9.1 it can be seen that three collection and disposal systems are provided: HP system comprising the HP flare drum, condensate recovery pumps and the HP flare tip located at the end of the flare boom LP system comprising the flare drum and the LP) flare tip LLP system comprising the flare drum and the LLP flare tip A combined high pressure and low pressure flare tip is shown in Figure Flare Ignition Refer to Figure 9.3 A means of igniting the flares is provided by a common system, which incorporates pilot lights, pilot igniters and a method of ignition. Each flare tip is equipped with three pilots which are fed by fuel gas. Each pilot has a dedicated ignition riser in which a mixture of instrument air and fuel gas is lit by a flame front generator located on the flare ignition panel. The ignited air/gas mixture or flame front is then diverted through the appropriate ignition riser to its particular pilot, which in turn ignites the associated flare tip. Failure of any of the flare pilots, low purge gas flow or high fuel gas filter differential pressure is indicated on the flare control panel. Normally cross-ignition takes place between the flare tips, once the first tip has been lit. However, the system must be checked to ensure that all the flares are burning Sources of Released Fluids Hydrocarbon gases, vapours and liquids are released to HP, LP and LLP flare systems from blowdown valves (BDVs), pressure safety valves (PSVs), bursting discs (pressure safety elements (PSEs)), pressure control valves (PCVs) and vents as follows: When equipment requires to be depressurised, as a result of an emergency, or prior to maintenance, BDVs are opened to vent hydrocarbons to either the HP or LP systems. BDVs are opened automatically by the emergency shutdown (ESD) system or manually by operator command Training Services Page 3 of

4 As a legal requirement pressure valves and other equipment items are protected against overpressure by PSVs and PSEs. Depending upon system pressure and nature of materials released they will relieve to the HP, LP or LLP system when the corresponding setpoint is exceeded Pressure control valves will pass hydrocarbon gas unable to be handled by system equipment to the flare systems to maintain normal operating pressure within the platform process and utility systems Vents pass fuel gas to the LLP flare after use by process and utility equipment for blanketing purposes, eg on drains, vessels and so on In general, hydrocarbons released from: Main process equipment items are routed to the HP system The LP/HP associated gas compressors depressurisation flows, necessary to ensure non-failure of oil seals are routed to the LP system All other atmospheric pressure discharges which are not discharged into a pressure flare system are routed to the LLP system HP Flare System Hydrocarbons that are vented to the HP flare system are routed to either one of two HP flare headers, the wet or dry header. The purpose of the two headers is to prevent the HP flare system becoming inoperative through possible blockage by hydrates. As hydrate formation is dependent upon temperature, pressure, free water and hydrocarbon gases being present under certain conditions, the system is designed to prevent this occurring by eliminating one or all of these conditions. Hydrocarbons vented to the wet header contain free water but, because of process conditions ie temperature and pressure, will not create low enough temperatures to form hydrates. Hydrocarbons vented to the dry header do not contain free water and may be disposed of safely from plant areas that could create low temperatures when vented, ie if the gas process were vented from a cold gas separator hydrates will form should water be present. For this reason the source and nature of the hydrocarbon is taken into account and routed accordingly. Purge gas (refer to Figure 9.1) is continually supplied to the HP dry flare header to blow any liquids that may be present through to the flare drum; prevent air ingress into the system and ensure a minimum flow at the flare tip. Likewise a minimum flow is introduced to the HP wet header. Training Services Page 4 of

5 Collections from the HP wet and dry headers enter the HP flare drum as separate streams. On entering the drum each stream hits a deflector causing liquids and gases to separate. Gas exits the drum from both ends, flows to a common header and then to the HP flare tip where the gas is disposed of by burning. The common header is manifolded to receive gas, from the gas export line, whenever the line is required to be depressurised. The liquid level in the drum is automatically controlled between upper and lower limits by the level indicating controller (LIC). At the upper limit the HP flare condensate pump A is signalled to start and transfer separated liquids to the pressure drain flash drum. If the level increases to a high level, the second pump is signalled to start. When the level drops to the lower limit both pumps are signalled to stop. The liquid in the drum is heated by electrical immersion heaters to ensure the liquid temperature is automatically controlled to between 8 C and 25 C eliminating the possibility of liquids freezing in the event of a cold release of hydrocarbons. They also reduce the flashpoint of the liquid to allow it to be handled in the drains system LP Flare System As with the HP system separate wet and dry LP flare headers segregate collections within the system. Hydrocarbons that are vented to the LP flare system are routed to either the wet or dry header. The purpose of the two headers is to prevent the LP flare system becoming inoperative through possible blockage by hydrates. Hydrocarbons vented to the wet header contain free water, but because of process conditions, ie temperature and pressure are not likely to form hydrates. Hydrocarbons vented to the dry header do not contain free water and may be disposed of safely from plant areas that could create low temperatures when vented. Certain process equipment can be permanently open to the LP flare wet and dry headers. Purge gas is continually supplied to the LP flare headers to flow liquids through to the flare drum, prevent air ingress into the system and present a minimum load at the flare tip. Liquid accumulating in the LP drum flows through a level valve (LV) before passing to the slops oil tank. Liquid level in the drum is automatically controlled by the LIC acting on the LV. The liquid in the drum is heated by a single electrical immersion heater to ensure the temperature is automatically controlled to between 8 C and 12 C. This is to eliminate the possibility of liquids freezing in the event of a cold release of hydrocarbons. Training Services Page 5 of

6 9.2.6 LLP Flare System The calculated maximum release of hydrocarbons to this system based on a fire arising provides for emergency depressurisation of the systems such as the glycol package and flotation units. Certain process equipment is also permanently open to the LLP flare system. The problem of freezing condensed or entrained water is not considered to exist in this system and a separate wet header is not required. Consequently, only one collection header is provided. Purge gas is continuously supplied to the extreme end of the header preventing air ingress into the system and providing a minimum flow of gas to the LLP flare tip. The single collection header routes released hydrocarbons to the LLP flare drum which separates out liquids by gravity. Gases leave the top of the drum and flow to the LLP flare tip for combustion. Separated liquids leaving the base of the drum flow through a loop seal to the slops oil tank. Elevation differences between the normal liquid level and the Slops oil tank allow liquids to be transferred by gravity. The transfer line maintains a minimum level by means of the liquid seal contained in one arm of the loop. The loop is continuously vented to the LLP flare header to prevent liquids being inadvertently syphoned out of the drum followed by gas breakthrough. Piping downstream of the loop is selfdraining and is normally free of liquid Flare Tips Hydrocarbon gases from each of the three flare drums are discharged to corresponding flare tips through dedicated flare headers. The flare tips are located at the extremity of a flare boom. The quantity of gas flared is measured and recorded by means of flowmeters installed in each header. Released hydrocarbon gases are burnt on discharge from the individual HP, LP and LLP flare tips. The continuous fuel gas purge to each HP, LP and LLP system maintains a continuous flame at the flare tips so that any additional release to the flare systems automatically ignites at the flare tip. As a further safeguard against possible extinguishment of the flare, continuously burning pilots are provided at each tip. A supply from the fuel gas system maintains the flame at each pilot. Should a pilot extinguish, a locally mounted temperature sensor relays a signal to the flare ignition panel which monitors all pilots. Re-ignition of any of the pilots is initiated manually. Typical flare tips have tulip type profiles (refer to Figure 9.3). Gas burns on the outside of the tulip and a protective layer of unburned gas between the flame and tulip bowl prevents the flame impinging on the bowl. The tips are designed for smokeless combustion and are capable of burning up to 25% weight/weight of crude oil or condensate without liquid fall-out and little smoke production. Training Services Page 6 of

7 The LLP flare tip is normally a conventional open pipe flame with the flame burning just outside the gas exit point. Combustion characteristics are dependent on gas flowrate and composition. The LLP flare is not designed to deal with appreciable quantities of liquid carryover, which if sufficient will cause burning liquids to fall away from the flare ( flaming rain ) presenting an extremely hazardous situation. 2.8 Maintenance/Atmospheric Vents The atmospheric vent system receives hydrocarbon releases from systems which have too low an operating pressure to allow connection into the flare system. These releases are manifolded together and piped to the atmosphere through a flame arrestor. The vent typically terminates at approximately 75% of the height of the flare stack or flare boom. In order to prevent the ingress of air into the pipework the system header is continually purged with fuel gas or nitrogen during startup and shutdown situations. Low points of the system headers are fitted with liquid knockout drain pots in order to remove any liquids formed in the system. Overpressurisation of the systems, for example as a result of partial blockage of the flame arrestor, is catered for by installing pressure alarms throughout the system, typically alarming at a setpoint of 0.1barg (1.5psig). As well as affording a facility for final depressurising of the various process vessels and tanks which are connected to the system, the vent header collects hydrocarbon and other gases from various sources such as: Hazardous drains tanks Compressor secondary seals Analyser vent Dehydration storage/vent systems Drilling mud sumps Produced water flotation unit vents Training Services Page 7 of

8 9.3 DRAINS SYSTEM System Function The drains system is installed in order to: Contain oil and hazardous chemical spillages, process equipment drainings and washdown water Recycle recovered oil back to the main process stream Safely dispose of any gases liberated in the separation process to the flare system or vent as appropriate Segregate hazardous and non-hazardous drainings Dump excess quantities of rain and deluge water directly overboard Allow large spillages of non-hazardous chemicals from storage bunds to be discharged safely to the sea System Equipment Refer to Figures 9.4 and 9.5 Non-hazardous and hazardous liquid wastes are segregated in order to prevent cross-contamination and hazardous situations arising. Innocuous non-hazardous liquids from non-hazardous area washdowns, drip trays, tundishes etc are collected in unclassified open drains and disposed of directly through the oily water disposal caisson to the sea. Potentially hazardous liquids from hazardous area washdowns, drip trays, tundishes etc are collected in classified open drains and directed to the classified open drains tank. Closed drains generally collect only hazardous liquids from process vessels or pipe manifolds and are hard piped from the drain point, ie the drain line is flanged or welded directly to the vessel or pipe. Once collected the fluids are directed to the pressure drains flash drum. Typically, a drains system installed for the segregation, collection, separation of hydrocarbons and disposal of liquid is divided into the following subsystems: (1) The open drains system (atmosphere skid draining, washdown, deluge and rain water draining). This is further subdivided into: (a) (b) The classified drains system (hazardous) The unclassified drains system (non-hazardous) Training Services Page 8 of

9 (2) The closed drains system comprising pressure drains. Closed drain manifolds are sometimes segregated into warm and cold headers to reduce the potential for hydrate formation within the system. Any flash gas from the liquids in the closed drains vessel is vented directly to the LP flare whilst recovered oil, including that recovered in the hazardous drains caisson, is pumped back to the separators for reprocessing. Training Services Page 9 of

10 Training Services 2010

11 SECTION 9 VENTS, FLARES AND DRAINS Figures 9.1 Flare System Schematic 9.2 Coanda Type Flare Tip 9.3 Flare Tip Pilot and Ignition System 9.4 Open Drains System Overview 9.5 Closed Drains System Overview