SIGTTO an introduction to the design and maintenance of cargo system pressure relief valves on board gas carriers 2nd edition september 1998ISBN 1 85609 163 5

Preventative and Planned Maintenance Procedures for Pressure Relief Valves 8 4.2 Pressure relief valve preventative maintenance philosophy 9 4.3 Pressure relief valve preventative mainte

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:- 1 85609 163 5 2nd Edition September 1998

3.2 Spring operated pressure relief valves 7

4 Preventative and Planned Maintenance Procedures for Pressure Relief Valves 8

4.2 Pressure relief valve preventative maintenance philosophy 9 4.3 Pressure relief valve preventative maintenance frequency 9 4.4 Pressure relief valve maintenance and inspection 9 4.5 Maintenance and care of diaphragms 11

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

2 REQUIREMENTS FOR PRESSURE RELIEF VALVES

_ 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

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

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

0 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 8 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

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.

3 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 : 1 to 3.0 : 1 This means that the area on the top side of the piston is larger than the seating area.

For example, with a 2 : 1 unbalance, the area of the top side is two times that of the seating area If the set pressure is 7 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 7 kg/cm2 = 84 kg Downward force = 2(12 cm2 x 7 kg/cm2) = 168 kg Therefore net force holding valve shut = 168 - 84 = 84 kgFor 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 1 bar = 1 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

SIGTTOThe operation of the valve and its maintenance is simpler than for the alternative pilot valve type, but it has the disadvantage that, in general, it cannot be set as accurately, particularly at low pressures.The different characteristics of the pilot operated and conventional spring loaded pressure relief valves are shown in Fig.4.

As the force holding the valve to its seat becomes zero at the pre-set opening pressure, the pilot operated valve lifts completely under the thrust exerted by the gas Whereas the spring loaded valve can be raised only by a further increase in pressure, since the compression of the spring requires an additional force to which must be added the static pressure due to the flow downstream of the valve The above diagrams illustrate this graphically

PRESSURE RELIEF VALVES

4.1 Scope

This section is applicable to both pilot operated pressure relief valves and direct spring operated pressure relief valves used for the protection of cargo tanks, hold spaces, insulation spaces, inter-barrier spaces and cargo piping Ship cargo system designs may be fully pressurised, semi-pressurised, or fully refrigerated Cargoes contained may be LNG, LPG, chemical gases, or Ammonia The design of

the tank, such as spherical, membrane, prismatic etc has no direct consequence to the

recommendations prescribed

I SIGTTO

I These guidelines do not specifically address equipment such as boilers, turbines, compressors,

f evaporators, pumps, heat exchangers etc The safety relief valves supplied as part of these systems

[ should be maintained in accordance with the manufacturer recommendations These valves are usually

I-t 4.2 Pressure Relief Valve PrevenI-taI-tive MainI-tenance Philosophy

Under normal operating conditions pressure relief valves should never need to operate They must not, however, be forgotten Regular routine maintenance is essential to ensure that they will function correctly if required LP valves, in particular, should be tested to ensure the pallet has not stuck to the seat

All personnel involved in the operation of the cargo systems protected by these valves should be familiar with their function, normal operation and maintenance requirements, as laid down by the manufacturers and system designers Operators should also be familiar with any means of closing them in an emergency

Table 1 provides a frequency matrix by valve type for various preventative type actions The combination of maintenance functions described requires actions by both the ship's crew and repair yard personnel They may also require input from the manufacturer or their designated service representative

Table 2 provides an action matrix by various valve types which can be applied during time at sea and also during shipyard overhauls These actions are specific in nature and are not intended to conflict with instructions of any particular manufacturer, who should be consulted in case of doubt or uncertainty

The following points should be taken into consideration when inspecting and overhauling pressure relief valves:-

1 The manufacturers instructions should be followed at all times

2 "Pirate" or home made spare parts should not be used This is particularly true of springs and diaphragms

3 Valves should be handled with care and always transported in the upright position This is

particularly relevant when being transported from the workshop to ship So many problems have been attributable to transit damage that many experienced ship operators now specify that, whenever possible, the larger pilot operated valves are overhauled in situ However, when making this decision consideration should be given to other work being carried out in the vicinity

of these valves and the amount of protection that can be afforded to them during the overhaul

If the decision is taken to remove valves to a shore workshop, it is imperative that each valve is tagged and a corresponding tag used to mark the position from which is was taken

4 Valves should normally be tested on dry air or nitrogen as this gives an indication of very small leaks through the bubble tester (figure 5) Should a water test be considered necessary the valve must be stripped and dried and then checked for seat tightness and set on dry gas This

is particularly important if it is intended for low temperature service

5 When removed for overhaul, a valve should be put on the test rig and popped before

dismantling and the results recorded

SIGTTO

6 Record sheets should be kept for all valves

7 Before removing valves, check cargo records and health and safety data sheets for any possible decontamination requirements

8 Never attempt to lap the valve nozzle to the disc.(see Appendix 3 for maintenance hints for

metal to metal seats)

9 Accuracy and repeatability of the set pressure will be impossible to achieve if the test rig does not have sufficient surge volume This is particularly true with spring operated valves, where

leakage may occur at 90% of set pressure and significant simmer at 95% set pressure A small test volume and feed rate may never be able to push the valve past the simmer point and

subsequently the valve will be set at a pressure higher than intended Therefore it can be seen

that it is not sufficient to connect an air hose to the inlet flange of the valve and test in this

manner

10 There is no accepted standard for surge tank capacity, but Figure 5 gives a graph of surge

tank volume related to valve orifice area It is published by the US National Board of Boiler and

Pressure Vessel Inspectors and is generally considered to be very conservative It can be seen

that a 150 mm NB valve, for example, with an orifice area of 113 cm2 (17.5 in2) and a design

blowdown of 7% would require a surge tank of (133ft3) 8 m3, a facility that few test shops

would have

An advantage of pilot valves is that they only require a small surge volume (in the order of 10

litres), due to the small capacity of the pilot

11 When the valve is on the test rig, it should be fitted with a bubble tester (figure 6) to enable the seat tightness and the exact point at which it starts to lift to be accurately determined Seat

tightness is generally quoted in bubbles per minute (bpm) For soft seat valves, 0 bpm is the

norm, whereas 30 bpm is common for metal seated valves These figures generally apply at

90% of set pressure API Standard 527 gives more information on this subject

12 Valves should always be overhauled and tested in a clean environment, using clean tools and

trained personnel

13 The valve discharge nozzle must be positioned to prevent exposure of personnel to a sudden

blast of air, water or other projectiles from the valve Ear protection may also be required when testing high pressure valves

4.5 Maintenance and Care of Diaphragms

Diaphragm failure, be it main or pilot, will result in the pressure relief valve opening and, at least initially,

an uncontrollable release of cargo Their cost bears no resemblance to the potential consequences

of

a failure For this reason alone these items should be examined and renewed at regular intervals When determining maintenance intervals consideration should be given to the cargo tank operating pressure and the aggressiveness of the products being carried Maintenance intervals may, of course, be modified in the light of operating experience

Spare diaphragms, manufactured from PTFE, Cryoflex™ or other synthetic materials, should be stored flat, in a cool dark place, such as an air-conditioned control room or office Kept in this manner they have a shelf life of up to 4 years, before being put into use

For valves fitted with metallic diaphragms manufacturers instructions on inspection and replacement should be observed Generally these will have longer shelf and service lives

Care should be taken to ensure that the spare diaphragm is fitted the right way up

A major manufacturer of relief valves for LNG duty, with non metallic diaphragms, suggests that: "all diaphragms are changed at the vessel's first refit after delivery; due to the wear and tear imposed during commissioning and testing Thenceforth, they should be inspected at the next refit and replaced at the refit following that This cycle, which assumes the first refit is 24 months after delivery and subsequent refits at 30 months intervals, should then continue throughout the vessel's life." This suggestion is the result of a number of years of trouble free operation

4.6 Spare Parts

It is strongly recommended that at least one complete set of manufacturers recommended spares is kept onboard the vessel for each type of pressure relief valve fitted

5 EMERGENCY CLOSURE OF PRESSURE RELIEF VALVES

If a boiler safety valve does not close after operation the result is reduced boiler pressure and subsequent loss or reduction in power The failure to close of a pressure relief valve on a gas tank can result in fire, explosion, or the formation of a toxic plume For this reason, all senior officers should be aware of the methods available for the emergency closure of pressure relief valves on their vessel and the location of tools and equipment required to carry out these operations It is also strongly recommended that owners consult with individual manufacturers of the relief valves fitted on their vessels and incorporate emergency closure procedures into the cargo operations manual

_SIGTTOListed below are some methods suggested by various manufacturers:

1 Auxiliary Setting Device 11 Connector 22 Diaphragm

2 Supply Tubing 12, Connector 23 Washer

3 Pilot Exhaust Tubing 13 Close Tee 24 Spring Pin

4 Pilot Bracket Bolt 15 Check Valve 25 Lower Body

5 Pilot 16 Connector 26 Body

6 Pipe Nipple 17 Connector 27 Spring

7 Field Test Valve 18 Pipe Plug 28 Ball

8 Field Test Plug 19 Body Bolt 29 0-ring

9 Connector 20 Lock Washer 30 Gasket

10 Check Valve 21 Upper Body

SIGTTO 5.2 Direct spring operated valves can generally be closed by fitting a test gag, which applies pressure to

the valve stem, forcing the disk onto the seat

5.3 In the event of a diaphragm failure on FMC low pressure valves, the temporary remedy depends on

whether the failure is the main or pilot diaphragm

If the pilot diaphragm fails, blocking off of the pilot valve vent will cause the valve to reseat

Should the main diaphragm fail, the tank pressure should be dropped to approximately 40% of set pressure, where the mass of the pallet assembly should cause the valve to close As a last resort the tank pressure should be dropped as low as possible and main outlet blocked off,

5.4 It is suggested that suitable spade blanks are kept, in the event that the inlet or outlet lines to

pressure relief valves reguire blanking in an emergency This is not an easy task and should only

be considered as a last resort, after careful planning and preparation.

If this action is undertaken, the position of the blank should be well marked and noted and it must be removed as soon as possible.

6 OPERATING PROBLEMS AND FAULTS

6.1 Operating Problems

Chattering: This is generally caused by the volume of the tank or vessel being too small in relation to

the capacity of the PRV As a result, the process fluid does not have sufficient kinetic energy to hold the valve open This is a design problem and would normally be rectified early in the ship's life However,

it can also be caused by incorrect setting of the blowdown ring (see fig.3) and excessive back-pressure

in the discharge pipe

This problem can also be encountered if valves are placed on a test rig where the volumetric capacity

of the rig is too small for the valve being tested Excessive chattering will cause serious seat damage (see 4,4,9.)

Continued Leakage after operation: This is normally the result of dirt being carried over with the

process fluid and becoming trapped between the seating surfaces The valve will have to be dismantled and cleaned It is also possible that the lifting device has been incorrectly fitted

Leakage: This may be the result of the set pressure being too close to the operating pressure It is

also possible that the spring is damaged, or the wrong spring is fitted and thus it is not possible to obtain repeatability of the set pressure Damaged or leaking seals and joints can also be a cause of seat leakage

6.2 Faults

A pilot operated relief valve can open and have reverse flow if the pressure in the vent line exceeds the pressure in the cargo tank This may occur if the discharged fluids vaporise in the mast A back-flow preventer is provided by some manufacturers to prevent this phenomena occurring

Vessel's carrying cargoes that require polymerisation inhibitors, such as Butadiene, may encounter problems with blockage of the sensing tube between the cargo space and the pilot valve

Bellows assisted relief valves commonly use stainless steel as the bellows material This is liable to suffer from chloride pitting, resulting in barely visible holes in the material and consequently leakage or early lifting of the valve These bellows should always be inspected very thoroughly at times of overhaul.Over-enthusiastic application of paint may result in the blockage of small orifices such as bonnet vents Upon removal, flakes of paint are likely to find their way into the pipe-work and thus become a potential cause of trouble at a later date, when the valve is back in service

SIGTTO

6.3 Chloride Stress Corrosion Cracking

Certain components in pilot operated safety relief valves have been known to suffer from chloride stress corrosion cracking These include; bolts, nuts, tube fittings and caps, It is generally discovered when the parts fail mechanically during dismantling The disturbing fact is that these components, in the normal course of events, would be expected to last the life of the valve

The cause of this problem is a high concentration of chlorides from sea water, relatively high metal temperatures (65 deg C ) due to solar radiation and the use of certain grades of austenitic stainless steel for the valve components

On one particular vessel all 16 caps on the check valves, used in the pilot valve pipe-work, were found

to have cracks through the whole thickness of the material, emanating from the bolt holes A seal failure

of this cap would have resulted in the inadvertent opening of the main valve

In another instance, corrosion was found at the lower end of the pilot valve auxiliary setter extension rods, the effect of which was to alter the set pressure of the main valve

It is recommended that these components are inspected during regular maintenance periods and replaced every 5 years Fasteners made of Hastelloy C are available for extended service life, but the cost of these fasteners is approximately 20 times that of 316 stainless steel fasteners

Table 1: Showing frequency of preventative maintenance actions for various types of

pressure relief valves