An Introduction to the Design and Maintenance of Cargo System Pressure Relief Valves on Board Gas Carriers First Published in 1994 by: © Society of International Gas Tanker and Terminal Operators Ltd Fully Revised and Updated in February 1998 Printed and Distributed by WITHERBY& CO LTD 32-36 Ay/esbury Street, London EC1R OET Telephone:- 44 (0)171 253 5413 Fax:- 44 (0)171 251 1296 ISBN:- 85609 163 2nd Edition September 1998 Contents Page No, Introduction Requirements for Pressure Relief Valves 2.1 2.2 2.3 General Valve design requirements Design and installation Types of Pressure Relief Valves 3.1 Pilot operated valves 3.2 Spring operated pressure relief valves Preventative and Planned Maintenance Procedures for Pressure Relief Valves 4.1 Scope 4.2 4.3 4.4 4.5 4.6 Pressure relief valve preventative maintenance philosophy Pressure relief valve preventative maintenance frequency Pressure relief valve maintenance and inspection Maintenance and care of diaphragms Spare parts 9 11 11 Emergency Closure of Pressure Relief Valves 5.1 Emergency closure Operating Problems and Faults 11 12 13 6.1 6.2 Operating problems Faults 13 13 6.3 Chloride stress corrosion cracking 14 In-Service Testing 19 Appendix 1: IMO IGC Code - Extracts from Chapter Cargo Tank Vent Systems 21 Appendix 2: Definitions and Terminology Used in Regard to Pressure Relief Valves 29 Appendix 3: Metal to Metal Seat Valves - Precautions and Hints for Lapping Seats 29 Appendix 4: Pressure Levels 31 Appendix 5: Fire Activated Pressure Relief Systems 33 References September 1998 34 Hi ; SIGTTO INTRODUCTION In 1992, an incident involving relief valve failure occurred during the loading of a semi-pressurised LPG carrier The cargo being loaded was propane and the pressure relief valves (PRVs) were set to operate at 11 bar gauge During the final stages of loading and at a pressure of only bar gauge, one of the two pressure relief valves fitted to a cargo tank lifted and failed to re-seat There were no guidelines available on board, or at the terminal, to describe, or suggest, emergency methods of closing the valve Eventually, the valve was physically blanked off with the tank pressure reduced to 0.02 bar gauge; by which time a large quantity of product was lost to the atmosphere Subsequent investigation of the valve indicated that there had been failures of the pilot valve diaphragms These failures were believed to have occurred because the diaphragms had been in service for too long Luckily, the product did not ignite and the only loss was commercial Had the cargo been ammonia or VCM and the wind direction blown this towards a densely populated area, the consequences could have been fatal Application of gas or nitrogen, at cargo tank pressure, to the top of the main valve dome would have closed the valve and considerably limited the amount of propane released to the atmosphere Unfortunately the crew were not aware of this simple procedure A similar incident occurred the same year, in the Far East, when a fully pressurised LPG carrier had a relief valve failure This also resulted in a large vapour cloud enveloping the jetty area Investigation showed that the cause of the problem was lack of maintenance As a result of these and some other similar incidents, SIGTTO produced "Guidelines on the Maintenance of Pressure Relief on board Gas Carriers" In 1998 this booklet was revised and expanded and the title changed to: "An Introduction to the Design and Maintenance of Cargo System Pressure Relief Valves on Board Gas Carriers" It must be stressed that this publication is a general guide to ship's staff and not intended to replace manufacturers instruction manuals Furthermore it is recommended that ship's staff responsible for the maintenance of these valves attend a manufacturers training course It is also hoped that the book may be of use to officers studying for certificates of competency For further information on the inspection and maintenance of pressure relief valves, API Recommended Practice 576 - Inspection of Pressure Relieving Devices, is thoroughly recommended The SIGTTO Secretariat would like to acknowledge the personal assistance given by members of the Society and the Safety Relief Valve Industry in the production of this book REQUIREMENTS FOR PRESSURE RELIEF VALVES 2.1 General The International Maritime Organization (IMO) Codes for Gas Carriers (see Appendix 1) requires at least two pressure relief valves of equal capacity to be fitted to any cargo tank with a volume greater than 20 m3 Below 20 m3 one pressure relief valve is sufficient The types of valves normally fitted are either spring-loaded or pilot-operated relief valves Pilot-operated relief valves may be found on Types A, B and C tanks while spring-loaded relief valves are usually only used on Type C tanks and pipe-work The maximum allowable pressure in the vapour space of type 'A' tanks and prismatic Type B tanks is 0.7 barg The Kvaerner Moss spherical Type B tanks can operate at the slightly higher pressure of 1.9 barg However, the normal operating pressure for both Type A and Type B tanks is generally 0.25 barg Type C tanks are pressure vessels and are generally designed to operate at pressures up to 18 barg September 1998 _ SIGTTO The use of pilot-operated relief valves on Type A and B tanks ensures accurate operation at the low pressure conditions prevailing; while their use on Type C tanks, for example, allows variable relief settings to be achieved using the same valve This may be done by changing the pilot spring or, more usually, by fitting one or more auxiliary(or complimentary) setters Figures 1a and 1b show typical pilot operated relief valves Other types of pilot valve are available for adjustment of "set pressure" and "blow-down pressure" Pilot operated relief valves with adjustable settings may be provided for two reasons Firstly, they may be used to provide a higher set pressure than normal, but not exceeding the Maximum Allowable Relief Valve Setting (MARVS), during cargo handling (sometimes referred to as "harbour" setting) Secondly, they can improve the loading limits of type 'C' tanks By causing relief valves to lift at pressures below those required to avoid over-stressing of the tank structure, viz below MARVS, the reference temperature, used to determine the tank filling limit, can be reduced This in turn reduces the difference between reference temperature and loading temperature and consequently reduces the cargo "shut out" volume Such adjustment, however, is not necessary on vessels having an "adequate vent system" under the amendments to the IGC and GC cases (8.2.18) Further information on this subject may be obtained from the SIGTTO/IACS publication; "Application of Amendments to Gas Carrier Codes Concerning Type C Tank Loading Limits" A further use of adjustable relief valves is if a Type B spherical tank has to be discharged, in an emergency, by pressurisation Auxiliary setters are fitted to the pilot valve to ensure tank pressure does not exceed the design pressure Whenever such valves are used for more than one pressure setting, a proper record must be kept of any changes in the pilot valve springs with the pilot assembly cap always being resealed after such changes A record must also be kept of the use of auxiliary setters Ideally a dedicated cupboard, should be mounted in the Cargo Control Room When pressure settings are changed the tank high pressure alarms should be adjusted accordingly Cargo tank pressure relief valves relieve into one or more vent stacks Vent stack drains are provided and should be checked regularly, to ensure no accumulation of rain water etc., in the stack Accumulation of liquid can have the effect of altering the pressure relief valve setting due to the resulting increased back pressure from the vaporising liquid, or frozen water This may prevent the valve lifting at its set pressure, or in the case of a pilot operated relief valve the valve may open and have reverse flow if the pressure in the vent line exceeds the cargo tank pressure The use of a back-flow preventer will prevent a pilot operated relief valve from opening due to back-pressure in the vapour column However, should a head of liquid accumulate in the vent riser of a diaphragm operated valve it may provide sufficient pressure to the underside of the diaphragm to overcome tank pressure on the top of the diaphragm and allow the valve to open The IMO Codes require all pipelines, or components which may be isolated when full of liquid, to be provided with relief valves to allow for thermal expansion of the liquid These valves, often referred to as "thermal relief valves", can relieve either into the cargo tanks themselves or, alternatively, they may be taken to a vent stack via liquid collecting pots with, in some cases, level switch alarm and a liquid vaporising source 2.2 Basic Requirements Pressure relief valve design must take account of two basic requirements These are accidental overpressurisation and accidental over-heating of the tank contents Over-pressurisation must be prevented, while keeping the amount of gas discharged through the pressure relief valve to a minimum; for reasons of environmental protection and economy The valve must therefore be designed to shut off at a pressure just below its opening pressure, once the overpressurisation has been relieved Accidental over-heating will result in vaporisation of the liquid and increased pressure which can only be safely reduced by discharge of gas through the pressure relief valve The Classification Societies September 1998 SIGTTQ have defined tank design rules which fixes the amount of gas to be discharged as a function of the liquid volume in the tank, the dimensions of the tank and the thermal insulation of the tank The rules create a requirement for high gas flow rates Pressure relief valves are therefore principally designed to take account of the following requirements: a) To provide an effective seal until the pre-set opening pressure (set pressure) is reached b) A precise and clean release of gas is achieved irrespective of cargo temperature c) Complete opening of the valve to give full flow d) Complete closing of the valves at a pressure slightly below the opening pressure This is normally 3%-7% of opening pressure, but in the case of diaphragm valves operating at pressures of less than 100 mbar this range is extended to more practical limits, in the order of 10 to 15% e) Operation to be free from the effects of frosting which may occur within the valve Operation to be unaffected by acceleration due to movement of the ship in a seaway, or list or trim g) The valve must repeatedly open at the prescribed set pressure, (see App I - 8.2.5.) h) Back pressure in the vent pipe system does not impede full flow of the valve The capacity of a given pressure relief valve is governed by various factors, including pressure, temperature and the fluid being handled A general formula for the capacity of a valve is given below and shows how this is affected by these factors 2.3 Design and Installation The sizing of relief valves is stipulated by the various Administrations, based on Chapter of the IGC Codes and, as such, is beyond the scope of this publication However, problems in service can often be alleviated by good pipe-work design and installation practices Most manufacturers recommend that each valve has a separate expansion bellows fitted in the vent line, before joining a common header Failure to observe this requirement can result in unacceptable loads being transmitted via the vent lines, due to thermal contraction and expansion This may impose undue loads on, or cause malfunction of, the second valve Pipe-work should always be adequately supported Depending on the grades of stainless steel used in construction, painting, for corrosion prevention, may or may not be required Should it be deemed necessary to paint the valves the coatings should be applied judiciously, as numerous malfunctions of relief valves have been attributable to the blockage of small orifices by paint and paint flakes September 1998 SIGTTO _ Supervision during construction of the vessel and re-installation (if removed for overhaul), should ensure that there is no undue stress imposed on the valves due to poorly fitting pipe-work Pipe-work should also be sighted internally for any debris, prior to the fitting of valves TYPES OF PRESSURE RELIEF VALVES 3.1 Pilot Operated Pressure Relief Valves (see Figure 2a 2b and 2c) A pilot operated pressure relief valve consists of a main valve and a pilot valve The main valve has an unbalanced piston or diaphragm (unbalanced member) Tank pressure is applied to the top of the piston or diaphragm via the pilot As the area at the top of the piston or diaphragm is larger than the bottom, the valve remains closed When the set pressure is reached, the pilot valve opens venting the space above the piston to atmosphere, or the vent stack An example of the forces involved in the operation of a pilot operated valve is given in the description of figure 2c : SIGTTO _ The unbalance of the piston (moving member) usually ranges from 1.2 : to 3.0 : This means that the area on the top side of the piston is larger than the seating area For example, with a : unbalance, the area of the top side is two times that of the seating area If the set pressure is bar and the seat area is 12 cm2, then the net forces holding the seat closed, immediately prior to opening, is 84 kg Upward force = 12 cm2 x kg/cm2 = 84 kg Downward force = 2(12 cm2 x kg/cm2) = 168 kg Therefore net force holding valve shut = 168 - 84 = 84 kg For the valve to open, the pilot valve must de-pressurise the cavity on the top side of the piston to a pressure equal to 50% of the inlet pressure When that occurs, the forces are in balance and the valve is on the threshold of opening The piston will then move upwards and the pressure will remain constant during this period When the pilot closes the top cavity is re-pressurised and the piston closes For simplicity in the above example it has been assumed that bar = kg/cm2 3.2 Spring Operated Pressure Relief Valves (See Figure 3) The piston or valve seating is accomplished through the pressure exerted by a coil spring onto the top of the piston or valve As the tank pressure increases, so the valve-seating contact force is reduced, making the setting of an exact set pressure more difficult to accomplish This is particularly true at low pressures The opening of the valve and the amount of opening is, therefore, dependant on the compression of the spring Any pressure, due to flow downstream of the pressure relief valve, will also tend to close the valve 18 September 1998 SIGTTO IN-SERVICE TESTING Ship's staff are often reluctant to undertake this operation and when they it is generally with some trepidation, particularly if the test results in the main valve actually opening If the test will result in the main valve opening, it should only be undertaken after consideration has been given to:- the tank contents, previous cargo, ship's position and weather conditions The operation should be properly planned and carried out under a Permit to Work At least two people should be in attendance, appropriate protective clothing i.e gloves, goggles, boiler-suits, should be worn and communications with the Officer of the Watch, via hand held VHP radios, established Arguably, it is better to discover that a pressure relief valve will not re-seat when at sea, with the wind across the deck, than in port, in close proximity to a crowded industrial area There are generally two types of test that may be carried out:1 Ensuring the freedom of operation of the main valve A simple check is carried out by removing the plug and opening the valve on the upper housing, thus releasing the pressure above the diaphragm and allowing the valve to open The opening of the valve will be audible, at which point the vent valve is shut causing the valve to re-seat, the plug can then be replaced This operation should be carried out every three months, particularly on fully refrigerated tanks, operating at low pressures, where there is a greater chance of valve seats sticking, due to the low tank pressure Figure shows the position of the vent valve on an FMC Safety Relief Valve Checking the set pressure and operation of the pilot valve This test will indicate the pressure at which the pilot valve operates This requires a manufacturer's field test kit, such as that shown in fig This procedure only tests the operation and set pressure of the pilot valve, the main valve does not open Typically it would be used before and after adjustment of the pilot valve A brief outline of this operation is given below, but in practice the manufacturer's procedure should be followed i The pilot discharge tube is removed and replaced with a 1,5mm orifice and the test kit connected to the field test valve ii With the metering and vent valve closed, the gas bottle is opened and the regulator set to about 14 barg iii The metering valve is then slowly opened whilst observing the test gauge The pressure is raised until there is a sudden and rapid increase of gas flow from the orifice plug fitted to the pilot discharge This is the set pressure of the valve September 1998 19 SIGTTO Appendix IMO IGC CODE - EXTRACTS FROM CHAPTER - CARGO TANK VENT SYSTEMS 8.1 General All cargo tanks should be provided with a pressure relief system appropriate to the design of the cargo containment system and the cargo being carried Hold spaces, inter-barrier spaces and cargo piping which may be subject to pressures beyond their design capabilities should also be provided with a suitable pressure relief system The pressure relief system should be connected to a vent piping system so designed as to minimise the possibility of cargo vapour accumulating on the decks, or entering accommodation spaces, service spaces, control stations and machinery spaces, or other spaces where it may create a dangerous condition Pressure control systems specified by chapter should be independent of the pressure relief systems 8.2 Pressure Relief Systems 8.2.1 Each cargo tank with a volume exceeding 20 m3 should be fitted with at least two pressure relief valves of approximately equal capacity, suitably designed and constructed for the prescribed service For cargo tanks with a volume not exceeding 20 m3, a single relief valve may be fitted 8.2.2 Interbarrier spaces should be provided with pressure relief devices complying with recognised standards 8.2.3 In general, the setting of the pressure relief valves should not be higher than the vapour pressure which has been used in the design of the tank However, where two or more pressure relief valves are fitted, valves comprising not more than 50% of the total relieving capacity may be set at a pressure up to 5% above MARVS 8.2.4 Pressure relief valves should be connected to the highest part of the cargo tank above deck level Pressure relief valves on cargo tanks with a design temperature below 0°C should be arranged to prevent their becoming inoperative due to ice formation when they are closed Due consideration should be given to the construction and arrangement of pressure relief valves on cargo tanks subject to low ambient temperatures "Valves should be constructed of materials with a melting point above 925°C Consideration should be given to lower melting point materials for internal parts and seals if their use will yield a significant improvement in the general operation of the valve" 8.2.5 Pressure relief valves should be prototype tested to ensure that the valves have the capacity required Each valve should be tested to ensure that it opens at the prescribed pressure setting with an allowance not exceeding ± 10% for to 1.5 bar, ± 6% for 1.5 to 3.0 bar, ± 3% for 3.0 bar and above Pressure relief valves should be set and sealed by a competent authority acceptable to the Administration and a record of this action, including the values of set pressure, should be retained aboard the ship 8.2.6 In the case of cargo tanks permitted to have more than one relief valve setting this may be accomplished by: installing two or more properly set and sealed valves and providing means as necessary for isolating the valves not in use from the cargo tank; or installing relief valves whose settings may be changed by the insertion of previously approved spacer pieces or alternative springs or by other similar means not requiring pressure testing to verify the new set pressure All other valve adjustments should be sealed September 1998 21 SIGTTO 8.2.7 The changing of the set pressure under the provisions of 8.2.6 should be carried out under the supervision of the master in accordance with procedures approved by Administration and specified in the ship's operating manual Changes in set pressures should be recorded in the ship's log and a sign posted in the cargo control room, if provided, and at each relief valve, stating the set pressure 8.2.8 Stop valves or other means of blanking off pipes between tanks and pressure relief valves to facilitate maintenance should not be fitted unless all the following arrangements are provided: suitable arrangements to prevent more than one pressure relief valve being out of service at the same time; a device which automatically and in a clearly visible way indicates which one of the pressure relief valves is out of service; and pressure relief valve capacities such that if one valve is out of service the remaining valves have the combined relieving capacity required by 8.5 However, this capacity may be provided by the combined capacity of all valves, if a suitably maintained spare valve is carried on board 8.2.9 Each pressure relief valve installed on a cargo tank should be connected to a venting system, which should be so constructed that the discharge of gas will be unimpeded and directed vertically upwards at the exit; and so arranged as to minimize the possibility of water or snow entering the vent system The height of vent exits should be not less than B/3 or m, whichever is the greater, above the weather deck and m above the working area, the fore and aft gangway, deck storage tanks and cargo liquid lines (Where B is the vessel's beam) 8.2.10 Cargo tank pressure relief valve vent exits should be arranged at a distance at least equal to or 25 m, whichever is less, from the nearest air intake or opening to accommodation spaces, service spaces and control stations, or other gas-safe spaces For ships less than 90 m in length, smaller distances may be permitted by the Administration All other vent exits connected to the cargo containment system should be arranged at a distance of at least 10m from the nearest air intake or opening to accommodation spaces, service spaces and control stations, or other gas-safe spaces 8.2.11 All other cargo vent exits not dealt with in other chapters should be arranged in accordance with 8.2.9 and 8.2.10 8.2.12 If cargoes which react in a hazardous manner with each other are carried simultaneously, a separate pressure relief system should be fitted for each cargo carried 8.2.13 In the vent piping system, means for draining liquid from places where it may accumulate should be provided The pressure relief valves and piping should be so arranged that liquid can under no circumstances accumulate in or near the pressure relief valves 8.2.14 Suitable protection screens should be fitted in vent outlets to prevent the ingress of foreign objects 8.2.15 All vent piping should be so designed and arranged that it will not be damaged by temperature variations to which it may be exposed, or by the ship's motions 8.2.16 The back pressure in the vent lines from the pressure relief valves should be taken into account in determining the flow capacity required by 8.5." The pressure drop in the vent line from the tank to the pressure relief valve inlet should not exceed 3% of the valve set pressure For unbalanced pressure relief valves the back pressure in the discharge line should not exceed 10% of the gauge pressure at the relief valve inlet with the vent lines under fire exposure as referred to in 8.5.2." 22 September 1998 SIGTTO _ 8.2.17 Pressure relief valves should be positioned on the cargo tank so that they will remain in the vapour phase under conditions of 15° list and 0.015L trim, where L is defined in 1.3.23 8.2.18 The adequacy of the vent system fitted on tanks loaded in accordance with 15.1.15 is to be demonstrated using the guidelines developed by the Organisation A relevant certificate should be permanently kept on board the ship For the purposes of this paragraph, vent system means: the tank outlet and the piping to the pressure relief valve; the pressure relief valve; the piping from the pressure relief valve to the location of discharge to the atmosphere and including any interconnections and piping which joins other tanks This paragraph may apply to all ships regardless of the date of construction 8.3 Additional Pressure Relieving System for Liquid Level Control 8.3.1 Where required by 15.1.4.2, an additional pressure relieving system to prevent the tank from becoming liquid full at any time during relief under the fire explosion conditions referred to in 8.5 should be fitted to each tank This pressure relieving system should consist of; 8.4 one or more relief valves set at a pressure corresponding to the gauge vapour pressure of the cargo at the reference temperature defined in 15.1.4.2; and an override arrangement, whenever necessary, to prevent its normal operation This arrangement should include fusible elements designed to melt at temperatures between 98°C and 104°C and to cause relief valves specified in 8.3.1.1 to become operable The fusible elements should be located, in particular, in the vicinity of relief valves The system should become operable upon loss of system power if provided The override arrangement should not be dependent on any source of ship's power Vacuum Protection Systems 8.4.1 Cargo tanks designed to withstand a maximum external pressure differential exceeding 0.25 bar and capable of withstanding the maximum external pressure differential which can be attained at maximum discharge rates with no vapour return into the cargo tanks, or by operation of a cargo refrigeration system, need no vacuum relief protection 8.4.2 Cargo tanks designed to withstand a maximum external pressure differential not exceeding 0.25 bar, or tanks which cannot withstand the maximum external pressure differential that can be attained at maximum discharge rates with no vapour return into the cargo tanks, or by operation of a cargo refrigeration system, or by sending boil-off vapour to the machinery spaces, should be fitted with: two independent pressure switches to sequentially alarm and subsequently stop all suction of cargo liquid or vapour from the cargo tank, and refrigeration equipment if fitted, by suitable means at a pressure sufficiently below the maximum external designed pressure differential of the cargo tank; or vacuum relief valves with a gas flow capacity at least equal to the maximum cargo discharge rate per cargo tank, set to open at a pressure sufficiently below the external design differential pressure of the cargo tank; or other vacuum relief systems acceptable to the Administration September 1998 23 _ SIGTTO 8.4.3 Subject to the requirements of chapter 17, the vacuum relief valves should admit an inert gas, cargo vapour or air to the cargo tank and should be arranged to minimize the possibility of the entrance of water or snow If cargo vapour is admitted, it should be from a source other than the cargo vapour lines 8.4.4 The vacuum protection system should be capable of being tested to ensure that it operates at the prescribed pressure 8.5 Size of Valves Pressure relief valves should have a combined relieving capacity for each cargo tank to discharge the greater of the following with not more than a 20% rise in cargo tank pressure above the MARVS: 24 the maximum capacity of the cargo tank inerting system if the maximum attainable working pressure of the cargo tank inerting system exceeds the MARVS of the cargo tanks; or vapours generated under fire exposure September 1998 SIGTTO Appendix DEFINITIONS AND TERMINOLOGY USED IN REGARD TO PRESSURE RELIEF VALVES Auxiliary Setter: The auxiliary, or complementary, setter is a device that enables the set pressure of a valve to be temporarily increased without the need for changing its initial setting It is normally screwed onto the pilot valve and the force of its spring is added to that of the pilot valve When removed the valve will operate at its original setting Auxiliary setters are machined to fit a particular valve and supplied with unique serial numbers, corresponding to the valve, which should be checked before fitting Backflow Preventer: This is fitted to pilot operated relief valves to prevent the valve from opening due to back-pressure in the vent line It consists of a check valve fitted in the supply tube to the pilot valve Back Pressure: Back pressure is the pressure existing at the outlet of a pressure relief device due to pressure in the discharge system Slowdown Pressure: Slowdown pressure is the value of decreasing inlet pressure at which no further discharge is detected at the outlet of a safety relief valve, after the valve has been subjected to a pressure equal to or above the popping pressure Blowdown: Slowdown is the difference between actual popping pressure of a pressure relief valve and actual reseating pressure expressed as a percentage of set pressure or in pressure units Bore Area: Bore area is the minimum cross-sectional flow area of a nozzle Built-up Pressure: Built-up back pressure is the pressure existing at the outlet of a pressure relief device occasioned by the flow through that particular device into a discharge system Chatter: Chatter is the abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in which the disk contacts the seat Closing Pressure: Closing pressure is the value of decreasing inlet pressure at which the valve disk re-establishes contact with the seat or at which lift becomes zero Coefficient of Discharge: Coefficient of discharge is the ratio of the measured relieving capacity to the theoretical relieving capacity Cold Differential Test Pressure: Cold differential test pressure is the inlet pressure at which a pressure relief valve is adjusted to open on the test stand This test pressure includes corrections for service conditions of back pressure and/or temperature It is sometimes abbreviated to "cold set" Disk: A disk is the pressure retaining movable element of a pressure relief valve which effects closure In larger valves it is referred to as the pallet Flutter: Flutter is abnormal, rapid reciprocating motion of the movable parts of a pressure relief valve in which the disk does not contact the seat Inlet Size: Inlet size is the nominal pipe size of the inlet of a pressure relief valve, unless otherwise designated Lift: Lift is the actual travel of the disk away from closed position when a valve is relieving September 1998 25 SIGTTQ Lifting Device: A lifting device is a device for manually opening a pressure relief valve by the application of external force to lessen the spring loading which holds the valve closed Sometimes referred to as easing gear Nozzle Area, Nozzle Throat Area: See Bore Area Nozzle: A nozzle is a pressure containing element which constitutes the inlet flow passage and includes the fixed portion of the seat closure Opening Pressure: Opening pressure is the value of increasing inlet pressure of a pressure relief valve at which there is a measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling or hearing Outlet Size: Outlet size is the nominal pipe size of the outlet of a pressure relief valve, unless otherwise designated Over-pressure: Over-pressure is a pressure increase over the set pressure of a pressure relief valve, usually expressed as a percentage of set pressure Pilot Valve: A pilot valve is an auxiliary valve which actuates a major relieving device Popping Pressure: Popping pressure is the value of increasing inlet pressure at which the disk moves in the opening direction at a faster rate as compared with corresponding movement at higher or lower pressures It applies only to safety or safety relief valves on compressible fluid service Pressure Relief Valve: This is a generic term applying to relief valves, safety valves or safety relief valves They are all devices that under excessive upstream static pressure automatically open and allow the process fluid to flow until normal pressure has been restored, but each has its own uses and limitations Relief Valve: An automatic pressure relieving device actuated by the static pressure upstream of the valve, which opens in proportion to the increase in pressure over the opening pressure These valves have closed bonnets to prevent the release of the process fluid and may be provided with easing levers They are primarily used for liquid service, such as for a hydrostatic or thermal relief on liquefied gas systems Relieving Pressure: Relieving pressure is set pressure plus overpressure Reseating Pressure: Resealing pressure is the value of decreasing inlet static pressure at which no further leakage is detected after closing The method of detection may be a specified water seal on the outlet or other means appropriate for the application Reseating Pressure: See Closing Pressure Safety Relief Valve: An automatic pressure actuated relieving device suitable for use as a safety valve when in gas or vapour service and as a relief valve in liquid service Safety relief valves are either "conventional", where the spring is covered with a pressure-tight housing and the opening pressure may be affected by changes in back pressure Or "balanced" where the effects of variable back pressure must be minimised Balancing is achieved by the use of a bellows and or balance piston Safety Valve: This is an automatic pressure relieving device actuated by the pressure upstream of the valve and without the aid of any external energy source It is designed to discharge a certified quantity of fluid to prevent a predetermined pressure from being exceeded and normally characterised by rapid full opening or pop action A safety valve is usually characterised by an exposed spring, in which case it should not be used where escape of the process fluid in the region of the valves is undesirable An 26 September 1998 SIGTTO easing lever is often provided for manual lifting to check the operation of the valve and a flexible wire may be provided to enable emergency de-pressurisation of the system They are typically used for steam boilers Seat: A seat is the pressure containing contact between the fixed and moving portions of the pressure containing elements of a valve Security Seal: A lead or plastic seal, used to prevent unauthorised adjustment of relief valve or auxiliary setter Set Pressure: Set pressure is the value of increasing inlet pressure at which a pressure relief valve displays one of the operational characteristics as defined under "opening pressure" or "popping pressure" Simmer: Simmer is the audible or visible escape of fluid between the seat and disk at an inlet static pressure below the popping pressure and at no measurable capacity It applies to safety or safety relief valves on compressible fluid service Thermal Relief Valve: Any cargo pipe-line that may be isolated in a liquid full condition should be provided with a relief valve to prevent excessive pressure build up if the liquid vaporises This is also known as a hydrostatic, or liquid line relief valve September 1998 27 SIGTTO Appendix METAL TO METAL SEAT VALVES PRECAUTIONS AND HINTS FOR LAPPING METAL TO METAL VALVE SEATS Reconditioning of the seat surface may be accomplished by lapping with a flat cast iron ring lap coated with Grade No 1000 Silicone Carbide compound or its equivalent The following precautions and hints will enable maintenance personnel to a "professional" job of lapping seats : a Keep work materials clean b Always use a fresh lap If signs of wearing (out of flatness) are evident, recondition the lap Reconditioning of laps is accomplished by lapping them on a flat lapping plate The lapping should be done with a figure-eight motion as indicated in the Figure To assure the best results when lapping seats, the laps should be reconditioned after each usage and checked with an optical flat c Apply a very thin layer of compound to the lap This will prevent rounding off the edges of the seat d Keep the lap squarely on the flat surface, and avoid any tendency to rock the lap which causes rounding of the seat e When lapping, keep a firm grip on the part to prevent the possibility of dropping it and damaging the seat SIGTT O f Lap, using an eccentric, or figure-eight motion, in all directions, while at the same time, applying uniform pressure and rotating the lap slowly (Again, see Figure, above) g Replace the compound frequently after wiping off the old compound, and apply more pressure to speed the cutting action of the compound h, To check the seating surfaces, remove all compound from both the seat and the lap Then, shine the seat with the same lap using the lapping motion described above Low sections on the seating surface will show up as a shadow in contrast to the shiny portion If shadows are present, further lapping is necessary and only laps known to be flat should now be used Only a few minutes will be required to remove the shadows i When the lapping is complete, any lines appearing as cross scratches can be removed by rotating the lap (which has been wiped clean of compound) on the seat about its own axis j The seat should now be thoroughly cleaned using a lint-free cloth and a cleansing fluid 30 September 1998 September 1998 31 SIGTTO Appendix FIRE ACTIVATED PRESSURE RELIEF SYSTEMS Chapter 15 (15.1.1) of the IGC Code states that "No cargo tanks should be more than 98% liquid full at the reference temperature" Prior to the amendments to the Gas Carrier Code (IMO MSC Circular letter 604) for Type C Tank Loading Limits this could result in a loss of loading volume up to 12% A possibility to reduce this loss of cargo capacity is given in Chapter of the IGC Code (see appendix 1), by the provision of an additional pressure relieving system for liquid level control, that is activated in the event of a fire, lifting at a pressure lower than the normal set pressure The Cryogas system was one method of taking advantage of the allowances in 8.3 of the IGC Code.fCryogas is no longer in existence, but some of these systems are believed to be still in service.) It makes use of a second pilot valve that is connected in parallel with the first pilot, but its operation is blocked by a bellows sealed shut off valve, kept closed by a fusible plug melting between 98 and 104 degC The second pilot has a variable micrometer adjustment for altering the set pressure, which is set slightly above tank pressure; thus permitting full loading The first pilot is set at a pressure up to MARVS A fire will cause the fusible plug to break, opening the shut off valve and any rise in tank pressure due to radiant heat from the fire will cause the tank pressure to rise above the set point of the second pilot and the main valve will open Provided that the vessel's vent system is deemed to be adequate under the terms of the IMO guidelines in the amendments to the IGC Code, this system now serves little purpose and many owners have opted to remove it September 1998 33 SIGTTO References API Recommended Practice 576 - Inspection of Pressure Relieving Devices, Published by the American Petroleum Institute 1220 L Street Northwest Washington DC 20005 IMO IGC Code Published by International Maritime Organization IMO 140E "Application of Amendments to Gas Carrier Codes Concerning Type C Tank Loading Limits" SIGTTO/IACS "Chloride Corrosion Cracking of Safety Valve Components" Anderson Greenwood & Co SIGTTO Panel Meeting March 1990 34 September 1998 .. .An Introduction to the Design and Maintenance of Cargo System Pressure Relief Valves on Board Gas Carriers First Published in 19 94 by: © Society of International Gas Tanker and Terminal... other similar incidents, SIGTTO produced "Guidelines on the Maintenance of Pressure Relief on board Gas Carriers" In 19 98 this booklet was revised and expanded and the title changed to: "An Introduction. .. cavity on the top side of the piston to a pressure equal to 50 % of the inlet pressure When that occurs, the forces are in balance and the valve is on the threshold of opening The piston will then