EXP-MN-SM085-En-R0 - Safety Valves and Burst Discs
Short Description
Safety Valves and Burst Discs...
Description
MECHANICAL MAINTENANCE SAFETY VALVES AND BURST DISCS
TRAINING MANUAL COURSE EXP-MN-SM085 Revision 0
Field Operations Training Mechanical Maintenance Safety Valves and Burst Discs
EQUIPMENT SAFETY VALVES AND BURST DISCS CONTENTS 1. OBJECTIVES ..................................................................................................................4 2. THE FUNCTIONS OF SAFETY VALVES ........................................................................5 2.1. INTRODUCTION.......................................................................................................5 2.2. OPERATION .............................................................................................................6 2.3. STANDARDS ............................................................................................................9 3. THE PRINCIPAL CATEGORIES ...................................................................................10 3.1. SPRING VALVE/ HIGH PRESSURE SAFETY VALVE ...........................................10 3.1.1. Different types .................................................................................................12 3.1.1.1. Screw plug valve ........................................................................................12 3.1.1.2. Thermal expansion valve ...........................................................................13 3.1.1.3. Regulator valve ..........................................................................................14 3.1.1.4. Safety valve with operating lever................................................................15 3.2. PILOT OPERATED VALVE.....................................................................................16 3.3. TANK VENT VALVE................................................................................................17 3.4. PRESSURE REDUCING VALVE ............................................................................18 3.5. BURST DISC ..........................................................................................................19 4. APPLICATIONS OF VALVES........................................................................................20 4.1. SPRING VALVE ......................................................................................................20 4.1.1. Pressure Safety Valve (PSV) ..........................................................................20 4.1.2. Pressure Relief Valve (PRV) ...........................................................................20 4.2. TANK VENT VALVE................................................................................................21 4.3. BURST DISC ..........................................................................................................22 4.3.1. Arrangement of burst discs..............................................................................22 4.3.1.1. Conventional burst disc ..............................................................................22 4.3.1.2. Back pressure or anti-vacuum mountings ..................................................23 4.3.1.3. Reversed burst disc ...................................................................................24 4.4. ADVANTAGES AND DISADVANTAGES................................................................25 5. PRINCIPLE OF OPERATION........................................................................................26 5.1. TERMINOLOGY USED...........................................................................................26 5.2. SPRING VALVE ......................................................................................................26 5.2.1. Valve installed on gases or steam. ..................................................................27 5.2.2. Valve installed on liquids .................................................................................28 5.3. VALVE WITH ADJUSTING RINGS .........................................................................29 5.4. PILOT-OPERATED VALVE ....................................................................................31 5.5. TANK VENT VALVE................................................................................................32 5.6. BURST DISC ..........................................................................................................33 5.6.1. Conventional burst disc ...................................................................................33 5.6.2. Reversed burst disc.........................................................................................33 5.6.3. Warning note ...................................................................................................33 6. REPRESENTATION AND DATA ...................................................................................35 Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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6.1. REPRESENTATION ON PID ..................................................................................35 6.2. DIAGRAMMATIC REPRESENTATION OF VALVES..............................................35 6.3. DATA SHEET..........................................................................................................36 7. OPERATION OF VALVES.............................................................................................37 7.1. PRESSURE SETTING ............................................................................................37 7.1.1. Sequence of preparation for pressure setting and pressure setting ................39 7.2. LEVEL 1 MAINTENANCE .......................................................................................40 7.2.1. Valves in general.............................................................................................40 7.2.2. Burst disc.........................................................................................................40 7.2.3. Disposal...........................................................................................................40 7.3. ASSEMBLY AND INSTALLATION ..........................................................................40 7.3.1. Assembly.........................................................................................................40 7.3.2. Installation .......................................................................................................41 7.3.3. Isolation of safety valves .................................................................................41 7.4. EXAMPLE OF MONITORING PROCEDURE .........................................................42 7.4.1. Subject ............................................................................................................42 7.4.2. Scope ..............................................................................................................42 7.4.3. Standards applicable .......................................................................................42 7.4.4. Identification and marking................................................................................43 7.4.5. Operations performed......................................................................................44 7.4.5.1. In-service valve inspections .......................................................................44 7.4.5.2. Workshop overhaul of valves .....................................................................44 7.4.5.3. Bench testing BEFORE dismantling...........................................................45 7.4.6. Inspection after triggering in service ................................................................45 7.4.6.1. Role of the Operator...................................................................................45 7.4.6.2. Role of the zone inspector..........................................................................46 7.4.7. Inspection after an overpressure without triggering.........................................46 7.4.7.1. Role of the Operator...................................................................................46 7.4.7.2. Role of the zone inspector..........................................................................46 7.4.8. Burst discs.......................................................................................................46 7.5. EXAMPLES OF LOGICAL DIAGRAMS ..................................................................47 8. EXERCISES ..................................................................................................................50 9. LIST OF FIGURES ........................................................................................................53 10. LIST OF TABLES ........................................................................................................54 11. ANSWERS TO EXERCISES .......................................................................................55
Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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Field Operations Training Mechanical Maintenance Safety Valves and Burst Discs
1. OBJECTIVES
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2. THE FUNCTIONS OF SAFETY VALVES 2.1. INTRODUCTION A safety valve is a safety device designed to protect capacities against overpressures or negative pressures. Its operation must be exceptional and its position must remain normally closed. Industrial capacities are characterised by a maximum operating pressure which must not be exceeded. This is the pressure used as a basis for calculating the strength of the component materials used in the capacity.
Figure 1: Examples of valves Adjustment: Spring Counterweight
Discharge: To the open air Channelled Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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2.2. OPERATION Safety valves evacuate any excess energy from the system to be protected in order to limit the maximum pressure in the protected device to a level which is acceptable for it. A safety valve is a safety component whose operation is exceptional. Its normal position is the closed position. The valve is designed to evacuate a gas flow because, for an equal volume, more (pneumatic) energy is evacuated in the gas phase than in the liquid phase.
Figure 2: Operation of safety (relief) valves
Valve closed = rest position When the internal pressure of the installation to be protected is less than the calibration pressure of the valve (i.e. of the spring), the spring presses the clapper against its seat.
Valve open = work position When the internal pressure of the installation to be protected exceeds the calibration pressure of the valve, it lifts the clapper and allows the excess energy to be discharged. The spring is compressed and remains compressed for as long as the internal pressure exceeds the spring force. Figure 3: Spring-type safety valve By discharging the excess pressure, the internal pressure is reduced. As soon as the internal pressure drops below the calibration pressure of the safety valve, the spring presses the valve against its seat. The discharge orifice is extended by a vertical vent which favours dilution and which, in case of ignition, Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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keeps the flames away from the wall of the tank protected by the valve. The top end of this vent is also protected by a shutter, which prevents water or foreign matter from entering the valve / vent tube assembly. When two valves are used, they are usually installed on a twinning valve. This enables either a single valve to be put in service (with the other valve isolated) or both valves at the same time (index in the intermediate position). The inlet orifice of the twinning valve is connected to the installation, on the tapping provided for this purpose. The valves are installed on the tapping provided for this purpose. The safety valves are installed on the outlet orifices of the twinning valve. The operating principle of the twinning valve prevents both safety valves from being isolated at the same time.
Figure 4: Example of safety valves on a twinning valve
Figure 5: Diagram of a twinning valve
It is necessary for the position of the isolated valve to be clearly indicated. Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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When the two valves are operated simultaneously (in which case neither valve provides 100 % of the required flow-rate), the position of the twinning valve (intermediate position) must be guaranteed by a safety chain or a lead seal.
Figure 6: Safety valves on a twinning valve
The safety valves can also be installed on a manifold, in cases where more than two safety valves are required for each tank to be protected. When necessary, these devices enable a valve to be removed for a check, an inspection or replacement, without completely emptying the tank.
Figure 7: 4-valve manifold
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2.3. STANDARDS Standards concerning safety valves, applicable both in France and at our subsidiaries, must be taken into account before any operation or before carrying out any work on these devices. The main standards concerned are listed below. Decree dated 4 December 1998 (Arrêté du 04 décembre 1998): Concerning the operational monitoring of safety valves and steam or gas pressure devices. Decree No. 99-1046 dated 13 December 1999 (Décret no 99-1046 du 13 décembre 1999): Concerning equipment under pressure Decree dated 15 March 2000 (Arrêté du 15 mars 2000): Concerning the operation of equipment under pressure EC Directive 97-23 (Directive CE 97-23) from the European Parliament and the Council Meeting held on 29 May 1997: Concerning the alignment of legislation between member states for equipment under pressure. NF EN 14382 (October 2003): Safety devices for gas pressure regulation stations and installations - Safety valves for service pressures up to 100 bars NF EN ISO 4126-1 (July 2004): Safety devices for protection against excessive pressure - Part 1: safety valves NF EN ISO 4126-4 (July 2004): Safety devices for protection against excessive pressures - Part 4: controlled safety valves NF EN ISO 4126-7 (July 2004): Safety devices for protection against excessive pressures - Part 7: common data
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3. THE PRINCIPAL CATEGORIES 3.1. SPRING VALVE/ HIGH PRESSURE SAFETY VALVE
Figure 8: Examples of spring valves A spring valve is composed of: A body in cast steel connected to the capacity to be protected and to the exhaust system. A seat, screwed into the body, of which the upper section is carefully machined in order to provide sealing. An adjusting ring which is screwed to the seat and can be operated from the exterior by a worm screw. A lift ring which is screwed to the adjusting ring and can be operated from the exterior by a worm screw. A valve guide which rests on the upper part of the body through a seal. A disc valve which slides in the valve guide and is held against the seat by the spring, thus providing sealing. A stop maintained against the upper side of the valve guide and against which the disc valve stops when it opens. A pin, the spring and its washer constituting the system which forces the plug against the seat. An adjusting screw and its nut, allowing the spring tension to be adjusted Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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A jacket, resting on the top of the valve body through a seal and fixed by bolts, which protects the spring A bonnet screwed to the top of the jacket and connected to it by a sealing which prevents any modification of the valve setting. BONNET ADJUSTING NUT CONVENTIONAL SAFETY VALVE
ADJUSTING SCREW SEAL S
SEALING
SPRING WASHER SPRING PIN JACKET VALVE GUIDE STOP DISC VALVE
TO EXHAUST SYSTEM
LIFT RING
ADJUSTING RING SEAT
BODY
Figure 9: Composition of a spring valve
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3.1.1. Different types 3.1.1.1. Screw plug valve It is possible to place an interlock pin on these valves which presses down the disc valve and therefore inhibits the safety function of the valve. After lifting, the valve may leak. In such cases we need to try to restore sealing by using the interlock pin to press down on the disc valve. The interlock pin must not be left on a valve during normal operation.
1 3 4 5 6 7 8 9 10 11 15 16 17 18 19 20 21 22 23 37
Bonnet Plug Baseplate Disc valve Valve carrier ring Valve carrier Guide Stop screw Retaining ring Rod Adjusting screw Adjusting screw nut Plug screw Plug screw seal Plug seal Bonnet seal Stop screw seal Spring washer Spring Nameplate
Figure 10: Screw plug valve
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3.1.1.2. Thermal expansion valve Gradual lift type valve fitted in product lines, particular on offsites, to ensure line expansion on isolation, and also installed in cooling water circuits.
S
Figure 11: Thermal expansion valve
Figure 12: Example of thermal expansion valve
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3.1.1.3. Regulator valve Placed on the discharge line of a pump, it regulates the pressure to a given value and modulates the flow rate, which can vary between the maximum and zero. Overpressures and pressure surges are thus eliminated if rapid closure occurs. Anti-surge accumulators are also necessary in order to avoid pressure surges.
Figure 13: Regulator valve
Figure 14: Example of a regulator valve
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3.1.1.4. Safety valve with operating lever Fitted on the steam systems which they protect, these valves can be recognised by a lever which enables the disc valve to be unstuck from its seat from time to time. This operation, specified in the 1926 regulations, is not performed today because it is too dangerous and has been replaced by tests (Trévitest) on the site.
Figure 15: Valve with operating lever
Figure 16: Examples of valve with operating lever
Two safety valves per tank are generally fitted either to a change-over valve or to a manifold, which allows one of the valves to be isolated while ensuring that the other operates This arrangement allows valve maintenance to be performed without shutting down the equipment
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3.2. PILOT OPERATED VALVE In certain applications where conventional valves do not perform satisfactorily, it may be necessary to fit pilot-operated valves (steam plant – LPG spillage from D5)
Figure 17: Example of pilot-operated valve
Figure 18: Pilot-operated valve Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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3.3. TANK VENT VALVE The double valve is particularly recommended in cases where an inerting system or pressurisation system is used in the tanks. It prevents damage to the tank if there is a lack of inert gas or a failure in a system component, and thus complements pressurisation systems. It also allows atmospheric air to be admitted when the tank is emptied or when a drop in temperature occurs.
Figure 19: Tank vent valve
ATMOSPHER
FROM GAS SYSTEM
CAPACITY SIDE
Figure 20: Diagram of a tank vent valve
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3.4. PRESSURE REDUCING VALVE A pressure reducing valve is a self-contained valve device (requiring no external energy supply), which enables the fluid obtained at its output to be at a pressure that is lower than the input pressure, and to be maintained more or less at a constant value, with or without circulation. Figure 21: Pressure reducing valve
Note: The designation "pressure reducing valve" refers to two types of equipment: Pressure reducing valve reserved for compressed air; steam and gases, which are compressible, Pressure reducing valve used for liquids, which are incompressible.
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3.5. BURST DISC This device protects against overpressures or underpressures. Component which does not re-close, designed to burst at a predetermined pressure in order to protect systems or isolated facilitiess against unacceptable pressure or vacuum conditions.
. Figure 22: Burst disc
Connection: Between flanges
Advantages: Fluid-tight Good resistance to temperature Withstands most acids, alkalis and organic solvents
Disadvantage: Systematic replacement (mandatory stock)
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4. APPLICATIONS OF VALVES 4.1. SPRING VALVE 4.1.1. Pressure Safety Valve (PSV) PSV's are mainly installed on compressible gases, including steam They are also used on process and utility fluids to protect equipment against overpressures. The valve opens without requiring energy other than that of the fluid Figure 23: Examples of PSV's
4.1.2. Pressure Relief Valve (PRV) A PRV opens under the pressure of the internal fluid, usually in proportion to the increase in pressure PRV's are used in liquid systems to protect the capacity and prevent thermal expansion. They can also be used in the pumping system to protect against overpressures.
Figure 24: Examples of PRV's
PRV's are characterised by rapid opening, or opening which is proportional to the pressure rise, depending on the application A PRV will in general act as a PSV when it is used in a compressible gas system, but opens proportionally when used in a liquid system
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4.2. TANK VENT VALVE Mainly used on the fixed roofs of storage tanks in the oil sector, vent valves avoid overpressures or underpressures in a tank
Figure 25: Tank vent valve
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4.3. BURST DISC These are generally installed either upstream of valves in order to isolate them from corrosive products or as a relief device
Figure 26: Burst disc (1) They must be carefully fitted on special flanges and the metal used must be determined according to the upstream fluid.
Figure 27: Burst disc (2)
4.3.1. Arrangement of burst discs 4.3.1.1. Conventional burst disc Capsule type mounting supports have two rings assembled by screws and fitted between two pipe flanges which do not provide sealing between the disc and the rings. Sealing requires the assembly to be tightened between pipe flanges. Capsule type supports exist in two types: The semi-nozzle type which is fitted inside the drilling circumference of standard flanges. The flange of a burst disc is sufficiently ductile to seal the assembly and act as a gasket.
Figure 28:Semi-nozzle type arrangement
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The full-nozzle type whose diameter is the same as that of the flanges and is drilled for the insertion of bolts. The Figure shows a full-nozzle type arrangement for a conventional burst disc installed between 2 pipe flanges.
Figure 29: Full-nozzle type arrangement
4.3.1.2. Back pressure or anti-vacuum mountings A conventional burst disc is arranged with its concave side towards the vacuum or back pressure, which may result in reversal of the dome and premature bursting. This problem is overcome by using a perforated mounting, which exactly adapts to the shape of the disc that it supports and allows the disc to withstand an effective operating pressure of 1 bar, preventing it from reversing.
Figure 30: Burst disc - back pressure and anti-vacuum support
Some discs are thick enough to tolerate a total vacuum without support. The vacuum support, shown by the points, is always placed under the disc.
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4.3.1.3. Reversed burst disc Whereas a conventional disc is mounted with its concave side towards the pressure, a reversed disc has its convex side oriented towards the pressure. A reversed or inverted disc is always under compression. The reversed disc can withstand a vacuum and therefore does not need a back pressure mounting.
Figure 31: Arrangement of a semi-nozzle reversed disc
The disc is mounted in a support which has, on its downstream side, a diagonal arrangement of sharp blades. If an overpressure occurs, the disc reverses and is cut or torn by the blades to ensure relief.
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4.4. ADVANTAGES AND DISADVANTAGES
ADVANTAGES
Can be re-used after use if readjusted Spring valve
Pilot valve
Simple fitting to equipment
More flexible to use
DISADVANTAGES
Needs to be readjusted regularly Pressure setting must be performed by specific personnel Weight of the valve for large diameters Requires somewhat more maintenance Usable on clean fluid
Vent valve
Total safety for instantaneous relief
Burst disc
Absence of leaks by virtue of construction Good resistance to corrosion if valve metallurgy is correctly chosen
Can only be used once, since activation destroys it Replacement means shut-down of the unit or isolation of the device
Table 1: Advantages and disadvantages of the different types
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5. PRINCIPLE OF OPERATION It is the result of 2 dynamic forces caused by the velocity of the fluid and the rise in static pressure. In the lifting of a valve, 2 cases must be considered: Valve installed on gases or steam. Valve installed on liquids
5.1. TERMINOLOGY USED Maximum Allowable Operating Pressure (MAOP): Equipment design pressure. Operating Pressure: Normal equipment operating pressure . Set Pressure: Spring adjustment pressure set on a test bench at ambient temperature Exhaust to Atmosphere. The Set Pressure is normally equal to the Opening Pressure. . Opening Pressure : Pressure at which valve starts to rise. This pressure must not be greater than the pressure of the equipment. Closing Pressure : Pressure at which valve is entirely closed.
5.2. SPRING VALVE Valve closed: idle position When the internal pressure of the tank is less than the set pressure of the valve (the spring) the spring keeps the disc valve against its seat. Valve open: working position If the internal pressure of the tank exceeds the set pressure of the valve, the disc valve rises and allows the excess energy to escape. The spring compresses and remains compressed as long as the internal tank pressure is higher. Figure 32: Spring valve The escape of the fluid reduces the internal pressure. Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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As soon as it returns to below the set pressure of the valve, the spring keeps the disc valve against its seat. The outlet orifice is extended by a vertical vent which helps dilution and in the case of ignition keeps the flames away from the wall of the tank that the valve protects. The upper part of the vent is itself protected by a cover which prevents water or foreign objects from entering the valve/vent pipe assembly. The weight of this cover is calibrated so that it lifts under the pressure. The valve opens when:
P1 ×
πD ² 4
> P2 ×
πD ² 4
+ kx
kx: action of spring (adjustable) P1 and P2: pressures acting on each side of the valve. Weight of parts ignored The effect of the spring can be replaced by that of a counterweight but this is no longer used.
5.2.1. Valve installed on gases or steam. Normal Operation: Valve closed: Process pressure: P2 Spring pressure: P1 P2 is less then P1: P2 < P1 When P2 rises to equilibrium with P1 (P2 = P1): The specific thrust of the disc valve becomes zero. The gases succeed in entering the valve faces for Pressures < 1 to 2 % of the set pressure. There is a hiss and the valve is said to "fizz". If the Pressure P2 continues to rise, the valve opens to the fully open position: Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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The disc valve comes against the stop and the overpressure escapes to the exhaust system
5.2.2. Valve installed on liquids Normal Operation: Valve closed: Process pressure: P2 Spring pressure: P1 P2 is less then P1: P2 < P1 When P2 rises to equilibrium with P1 (P2 = P1) The valve remains closed The valve only opens when Pressure of Liquid P2 > P1 Spring For the disc valve to lift completely: The pressure P2 must be higher by 25%
Figure 33: Operation of a simple valve
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5.3. VALVE WITH ADJUSTING RINGS The principle is identical to the previous case as far as the "hiss" stage The role of this type of valve is to allow rapid opening in order to avoid this phenomenon rapid closure in order to avoid loss of pressure in the capacity and bouncing of the disc valve on the seat.
Figure 34: Valve with adjusting ring
Role of the adjusting ring The disc valve having started to lift, the gases enter the chamber 1 and can only escape through the annular space formed by the outer edge of the adjusting ring and the edge of the disc valve. This space is adjustable by moving the ring. If the space is tightened, the speed of the gas is reduced, while the pressure increases. The bearing area is also increased and the pressure on it creates a force which is able to overcome the action of the spring, promoting a sudden lifting of the disc valve.
Examples of ring adjustment The valve set to the MAOP "fizzes". The adjusting ring is too low. It rises distinctly but the closing is slow and results in a loss of pressure: The adjusting ring is too high. Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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Role of the baffle If we raise the baffle, its outer edge comes closer to that of the Valve Carrier and we have the same phenomenon of pressure increase in chamber 2 as previously. If the baffle and the ring are set "HIGH", the spring will be unable to overcome the total of the forces exercised by P2 + chambers 1 & 2. To create a rapid drop and return to P2 < P pressure setting, the baffle must be set lower.
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5.4. PILOT-OPERATED VALVE
PHASE 1 The pressure PS < pressure PR (adjustment pressure), the pilot sensor is open and the valve piston is under pressure. P
THE VALVE IS CLOSED
PHASE 2 The pressure PS rises and reaches 95% of PR, the 3-way system hydraulically isolates the volume 5. The valve head remains under pressure. THE VALVE STAYS CLOSED
Volum
PHASE 3 The pressure PS reaches the value of PR, the pilot sensor trips and the fluid contained at the surface of the head piston is purged. THE VALVE OPENS Volum
PHASE 4 The opening of the valve lowers the pressure PS to a value< PR. The pilot sensor hydraulically isolates the purging of the valve head. A further lowering of the pressure causes the pilot to flip over and: RETURN TO PHASE 1
Volum
Figure 35: Operation of a pilot-operated valve Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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5.5. TANK VENT VALVE If the capacity pressure falls: the disc valve on the Gas System side rises the disc valve to Atmosphere remains pressed against its seat gas is admitted into the capacity
If the capacity pressure rises: the disc valve on the Gas System side is pressed against its seat the disc valve to Atmosphere rises the excess pressure is sent to atmosphere
1
2 ATMOSPHER
CAPACIT
Figure 36: Tank vent valve DISC VALVE 1: PROTECTION AGAINST UNDERPRESSURE DISC VALVE 2: PROTECTION AGAINST OVERPRESSURE
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5.6. BURST DISC This device protects against overpressures or underpressures.
5.6.1. Conventional burst disc A conventional burst disc is a component which does not re-close. It is designed to burst at a predetermined pressure in order to protect systems or isolated facilities against unacceptable conditions of pressure or vacuum.
5.6.2. Reversed burst disc A reversed disc is always under compression. The disc is mounted in a support which has, on its downstream side, a diagonal arrangement of sharp blades. If an overpressure occurs, the disc reverses and is cut by the blades to ensure relief.
Figure 37: Reversed burst disc
5.6.3. Warning note For safety reasons, it is important to pay attention to the following recommendations: Discharge from the burst disc continues until the pressure reaches equilibrium. The discharge may include parts of the burst disc. The products discharged must be removed to a safe place. The sound levels caused are sometimes extremely high. The intensity of burst can be very high and impose reaction forces. Incorrect connection can cause overpressure in the system. The useful life of a burst disc cannot be guaranteed.
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Certain types of disc are not suitable for hydraulic applications (reversed dome disc). Burst disc devices are sometimes significantly heavy. Burst discs can be fragile. Burst discs may have sharp edges. Alignment errors or damage can cause leaks. The re-use of discs is not recommended after they have been removed.
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6. REPRESENTATION AND DATA 6.1. REPRESENTATION ON PID
Figure 38: Representation of valves on a PID
6.2. DIAGRAMMATIC REPRESENTATION OF VALVES
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6.3. DATA SHEET
Figure 39: ROSA PSV data sheet sample
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7. OPERATION OF VALVES 7.1. PRESSURE SETTING They are fitted with a non-aging metal plate fixed permanently to the device bodies. The plate specifies (at least) the nominal pressure of the body or of the flange drilling template, the nominal diameter of the device and its pressure setting. Figure 40: Pressure setting bench Safety valves are factory-adjusted to within 0.1 bar of the pressure specified in the official engineering documents. They are designed to permit easy adjustment of the setting. Valve pressure setting follows very strict rules because the valves are SAFETY devices. TOTAL Group Specifications require readjustment once per year (more or less). We must differentiate however between readjustment and commissioning of a valve. When equipment is commissioned, the valves are usually set by the supplier, but the pressure setting certificates are only valid for 60 to 90 days depending on the country.
Figure 41: Adjusting a valve
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In all cases, an equipment must only be commissioned with these valves if the certificate dates from less than 90 days, after which the maintenance rules take effect. Each valve is fitted with a nameplate showing these characteristics, and an additional plate showing the date of the last pressure setting. These plates are INDISPENSABLE. All valves have data sheets and certificates, which must ABSOLUTELY be kept for inspection. The pressure setting of a valve, i.e. the pressure at which it starts to open must not be higher than the maximum operating pressure. When the internal pressure of the tank is less than the set pressure of the valve, the spring keeps the disc valve against its seat. If the pressure exceeds the set pressure of the valve, the disc valve rises and allows the excess energy to escape.
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7.1.1. Sequence of preparation for pressure setting and pressure setting
Dismounting the unit
VERIFICATION on the bench
If pressure > 110% open pressure Stop the verification
WORKSHOP overhaul
Calibration on bench
Verify Clean Remove Refit
Pressure gauge with maximum EM calibration certificate
Pressure = 90 % open pressure Leak test on bench
If one bubble in a time of < 45 s (no leaks) Otherwise (leak): needs grinding in
Certifying body test on bench
Refitting to unit
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7.2. LEVEL 1 MAINTENANCE 7.2.1. Valves in general Check the external state of corrosion, particularly for offshore sites, and lubrication of nuts and bolts for easier dismantling for inspection.
7.2.2. Burst disc Burst discs do not generally require maintenance because they have no moving parts.
7.2.3. Disposal Used components must be disposed of in accordance with local and national regulations. It is essential to be aware of these instructions before installing a burst disc.
7.3. ASSEMBLY AND INSTALLATION 7.3.1. Assembly Place the discharge side of the support on a bench with the throat upwards. Check that all surfaces are clean and smooth and that the radius is not damaged. Check the information shown on the disc label (identification no., burst pressure, operating temperature). Fit the pressure side part of the support into the discharge side part. Assembly o Standard assembly Insert and tighten the assembly screws which fix the two parts of the support. These screws must not be overtightened. o Assembly to a predefined torque Carefully turn the disc assembly over, line up the holes and insert the assembly screws. Tighten the screws evenly and gradually according to the specified torque. The disc assembly can now be installed in the system. Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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Note: No sealing ring must be fitted between the disc and the discharge side of the support. Sealing rings may be fitted between the disc assembly and the pressure side of the support. If these are required, they must be supplied by MARSTON.
7.3.2. Installation Check that the information shown on the disc and support labels is correct for the position in question. When installing the assembly between flanges, check the position and direction of the disc Sealing rings must be fitted between the support and the system flanges. Tighten the flange bolts gradually in order to ensure that the system does not leak. The tightening torque of the flange bolts does not affect the performance of the disc.
7.3.3. Isolation of safety valves All valves connected to the flare systems can be closed, separately or in groups, by means of an isolation valve. An isolation valve which is always: Open in operation. Closed during maintenance. This valve is equipped with a locking system (chain and padlock) which protects the safety valve against operation. The safety system is operated in accordance with the CGS 05-05 Instruction entitled: "Isolation of safety valves".
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7.4. EXAMPLE OF MONITORING PROCEDURE 7.4.1. Subject The purpose of this procedure is to define the actions to be undertaken in order to check the condition and correct operation of safety devices installed on equipment under pressure which are monitored by the Corrosion-Inspection department. It also deals with the intervention frequency and the management of monitoring. It concerns in-service on-site inspections as well as workshop inspections during unit shutdowns or special interventions.
7.4.2. Scope This procedure applies to safety devices which are intended to protect equipment under pressure against overpressures or underpressures: spring valves, pilot-operated valves, tank vent valves, burst discs recirculation of treatment product injection pumps The following valves are not monitored: compressor lubricating oil system discharge.
7.4.3. Standards applicable Ministerial Order of 4 December 1998: operational monitoring of safety valves on equipment under steam or gas pressure. French Standards NFE 29410 to 29425 CTG Rule RG TUY-F3 Ministerial Order of 15 March 2000 concerning the operation of equipment under pressure Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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Maintenance Procedure TECH 307-1 GS EXP 212
7.4.4. Identification and marking Before being installed on site, all new valves must be identified by the Central File Manager of the inspection department. He enters the manufacturing characteristics in the CMMS. The valve is then sent to an approved repairer who performs a triggering test and fits a sealed metal nameplate bearing the following engraved information: station identifier equipment identifier pressure setting date of pressure setting
Figure 42: Example of identification Burst discs are catalogued in the Central File lists.
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7.4.5. Operations performed 7.4.5.1. In-service valve inspections Nature It is carried out by the Zone Inspector. It includes the following inspections: check of the general condition of the valve and associated equipment (changeover valves, pipework) verification of sealing verification of indications on the nameplate verification of consistency of required pressure settings with pressures in the equipment protected by means of the computer application Frequency For valves installed on equipment under the normal regulatory system, the inspection takes place when the unit is being prepared for shutdown, during the 12 months prior to the shutdown. For valves installed on equipment covered by a P.C.S., the inspection takes place during the external examinations specified in the procedure. For valves installed on common systems, inspection takes place at the same time as the systems inspection.
7.4.5.2. Workshop overhaul of valves valve which has been triggered in service, in accordance with 5.4.2 valve which has been subjected to an overpressure according to 5.5.2 valve delivered individually to the overhaul workshop (not during a Scheduled Shutdown) 100% of the valve population is delivered to the workshop per batch (Scheduled Shutdown) Nature Overhaul is carried out by an approved repairer of TOTAL GABON, in accordance with a repairer procedure which meets TOTAL GABON specifications Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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The overhaul includes the following at least: a preliminary bench test in accordance with 5.3.1 dismantling check of components return to compliance if necessary final bench test Overhaul report Frequency A workshop inspection takes place at each re-qualification and/or major intrusive examination (RBI analysis) of the equipment protected, whether under regulations or not.
7.4.5.3. Bench testing BEFORE dismantling Circumstances The specifications for workshop valve overhauls includes a systematic bench test BEFORE any other verification In all cases, an invitation must be sent to the inspector responsible. The test must be monitored by an inspector or its designated representative.
7.4.6. Inspection after triggering in service In-service triggering is taken into account in all cases except when the pressure recorded was not more than 10% higher than the pressure setting of the valve, in which case the incident is closed
7.4.6.1. Role of the Operator The Operator records all cases of valve triggering: significant facts" of the control room and issues a failure notice in SAP.
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When such an incident occurs, it also notifies the inspector responsible for the site via email. If the valve has closed again correctly, it archives the pressure recording (if the pressure was recorded) corresponding to the incident
7.4.6.2. Role of the zone inspector The zone inspector validates the failure notice in the SAP computer application and follows up the work order until its closure. If this is not the case, the inspector asks for the valve to be removed for workshop overhaul. If the valve continues to leak, the inspector asks for it to be removed for workshop overhaul.
7.4.7. Inspection after an overpressure without triggering 7.4.7.1. Role of the Operator This case is characterised by the ESP monitoring pressure transmitter threshold being exceeded without triggering the valve. The Operator is informed by an alarm on its management system. The Operator records the event in the "daily log: significant events" of the control room. When such an incident occurs, it also notifies the inspector responsible for the site via email and creates a failure notice in Unisup.
7.4.7.2. Role of the zone inspector The zone inspector validates the failure notice in the SAP computer application and follows up the work order until its closure.
7.4.8. Burst discs Burst discs are replaced at each regulatory shutdown.
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7.5. EXAMPLES OF LOGICAL DIAGRAMS
Figure 43: Logical diagram for removal/refitting 1) 2) 3) 4) 5)
SIC issues a malfunction form to search for the causes and induced effects during the daily meetings the inspector attends the test and countersigns the malfunction form after on-site verification that refitting was compliant either in the technical dossier for the equipment, or in the file provided for that purpose for piping
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Figure 44: Logical diagram for issue from store (1) full list of valves to be overhauled plus list of valves to be tested before removal (2) the inspector attends testing for 10% of the designated valves (3) by countersigning the original list (4) either in the technical dossier for the equipment, or in the file provided for that purpose for piping
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Figure 45: Logical diagram for removal in store (1) full list of valves (2) the inspector attends testing (3) by countersigning the original list (4) either in the technical dossier for the equipment, or in the file provided for that purpose for piping
As shown in the procedure and the logical diagrams, monitoring follows precise rules with which compliance is mandatory The valve is a SAFETY device
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8. EXERCISES 1. What does PSV mean?
2. Is the safety valve a Safety device? Regulation device?
3. With respect to the tank which it protects, a valve is set to a value Higher than the design pressure of the tank? Lower than the tank pressure? At least equal to the tank pressure?
4. What is the opening pressure?
5. When do we say that a valve fizzes?
6. If P2 > P1, what happens? The disc valve opens The disc valve closes The disc valve does not move
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7. What is the minimum number of valves on a tank? 1 2 3
8. Draw the representation of a valve on a PID
9. What information is found on a valve?
10. A check shows that the pressure is at 110% of the normal pressure setting: what should I do? Stop Continue to 120%
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11. If I have 10 bubbles in 45 seconds, my valve is considered to be good Yes No
12. What are the advantages of burst discs?
13. What are the disadvantages of burst discs?
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9. LIST OF FIGURES Figure 1: Examples of valves...............................................................................................5 Figure 2: Operation of safety (relief) valves .........................................................................6 Figure 3: Spring-type safety valve .......................................................................................6 Figure 4: Example of safety valves on a twinning valve.......................................................7 Figure 5: Diagram of a twinning valve..................................................................................7 Figure 6: Safety valves on a twinning valve .........................................................................8 Figure 7: 4-valve manifold....................................................................................................8 Figure 8: Examples of spring valves ..................................................................................10 Figure 9: Composition of a spring valve.............................................................................11 Figure 10: Screw plug valve...............................................................................................12 Figure 11: Thermal expansion valve..................................................................................13 Figure 12: Example of thermal expansion valve ................................................................13 Figure 13: Regulator valve.................................................................................................14 Figure 14: Example of a regulator valve ............................................................................14 Figure 15: Valve with operating lever.................................................................................15 Figure 16: Examples of valve with operating lever.............................................................15 Figure 17: Example of pilot-operated valve........................................................................16 Figure 18: Pilot-operated valve ..........................................................................................16 Figure 19: Tank vent valve.................................................................................................17 Figure 20: Diagram of a tank vent valve ............................................................................17 Figure 21: Pressure reducing valve ...................................................................................18 Figure 22: Burst disc ..........................................................................................................19 Figure 23: Examples of PSV's ...........................................................................................20 Figure 24: Examples of PRV's ...........................................................................................20 Figure 25: Tank vent valve.................................................................................................21 Figure 26: Burst disc (1).....................................................................................................22 Figure 27: Burst disc (2).....................................................................................................22 Figure 28:Semi-nozzle type arrangement ..........................................................................22 Figure 29: Full-nozzle type arrangement ...........................................................................23 Figure 30: Burst disc - back pressure and anti-vacuum support ........................................23 Figure 31: Arrangement of a semi-nozzle reversed disc....................................................24 Figure 32: Spring valve ......................................................................................................26 Figure 33: Operation of a simple valve ..............................................................................28 Figure 34: Valve with adjusting ring ...................................................................................29 Figure 35: Operation of a pilot-operated valve...................................................................31 Figure 36: Tank vent valve.................................................................................................32 Figure 37: Reversed burst disc ..........................................................................................33 Figure 38: Representation of valves on a PID ...................................................................35 Figure 39: ROSA PSV data sheet sample .........................................................................36 Figure 40: Pressure setting bench .....................................................................................37 Figure 41: Adjusting a valve...............................................................................................37 Figure 42: Example of identification...................................................................................43 Figure 43: Logical diagram for removal/refitting.................................................................47 Figure 44: Logical diagram for issue from store.................................................................48 Figure 45: Logical diagram for removal in store.................................................................49 Training Manual : EXP-MN-SM85-EN Last Revision: 20/03/2008
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10. LIST OF TABLES Table 1: Advantages and disadvantages of the different types..........................................25
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11. ANSWERS TO EXERCISES
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