Safety Valves for power plant

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    Safety ValvesErection &

    Commisioning-Athiannavi. ESM/Engg-Valves, BHEL-richy

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    Valve is a mechanicaltool used for controllingthe fluid flowparameters in a

    pressurised system.These valves areoperateddaily/continuously in asystem.

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    But Safety valve is safety device. It isnot operated daily or continuously.

    Safety valves are meant for safety only.

    Safety valve is a over pressure protection

    equipment.

    In a best system, Safety valves are to bemaintained in working condition but not

    to be operated.

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    Reasons for raising pressure in a systemReasons for raising pressure in a systemReasons for raising pressure in a systemReasons for raising pressure in a system1.1.1.1. AccumulationAccumulationAccumulationAccumulation ofofofof fluid within afluid within afluid within afluid within a

    boundary.boundary.boundary.boundary.2.2.2.2. ChangeChangeChangeChange in specific volume due toin specific volume due toin specific volume due toin specific volume due to

    heatheatheatheat absorption.absorption.absorption.absorption.3.3.3.3. Change in fluid velocityChange in fluid velocityChange in fluid velocityChange in fluid velocity withinwithinwithinwithin

    shorter duration of time.shorter duration of time.shorter duration of time.shorter duration of time.

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    SAFETY VALVE 1700 SERIES

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    1) Do not go near discharge side of a safety valve.

    2) Body drain and cover plate vent must be piped

    to a safe area. If left open, steam will escape and

    present a burn hazard to personnel near the valve.) !lways gag a safety valve before ma"ing ring

    ad#ustments.

    $) %hen pulling hand lever for inspection purpose,a rope should be attached to the handle of sufficient

    length &minimum ' to 1( m long) to protect the

    operator from escaping steam, dust, fly ash etc.,

    SAFETY

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    Exercise caution when examining asafety valve for audible leaage. S!"E#$E%TE& STE%' is ()T VISIB*E.

    Safety valves should be mounted toprovide ade+uate access, - aroundthe valve plus overhead, for disassembly

    and maintenance.

    !se ear0muffs during the ad1ustment ofvalves.

    !se hand gloves during ad1ustment ofvalve in the hot condition.

    SAFETY

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    Ensure the disc is available. Identifythe disc and store it safely with &!se+uence re+uired for assembly.2hec for disc identification beforeassembly.

    2hec for tag no. or &! no. as per

    pacing slip. (on0availability ordiscrepancy must be informed to themanufacturing unit. 2hec for test

    certificates for each safety valve. 2hec whether the inlet si3e, orifice,

    material grade, set pressure etc. are

    as per the boiler Setting &iagram.

    STORAGE

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    2hec that the seals are intact on ring pin,spring ad1uster and overlap collar. 2hec for

    transit damages, if any.

    Ensure that the end covers are ept inposition and removed only 1ust before

    erecting in place. 'eticulous care should betaen to ensure dirt or foreign particles donot enter into the inlet or outlet of the valve.

    Ensure the valves are stored in crates onsleepers in a covered area with the inlet sidedownwards. Valves should never be laid onthe side to avoid damage to internals.

    STORAGE

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    Identify and hand over the $ydro test plugs tothe user after the $ydraulic test is completed.

    Ensure the valves are never sub1ected to

    impact by bumping or dropping during loadingand unloading and disassembly and assemblyof valve.

    &o not lift valve with lever or spring. Slingvalve as per )4'.

    Valves should be erected with spindle verticalwithin a tolerance of 5 from the vertical

    plane.

    Ensure proper pre0heating and post heating,stress relieving and (&T procedures arefollowed when welding to the stub.

    ERECTION

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    Ensure that the vent pipe is separately supportedand does not touch the drip pan unit and that thereis sufficient allowance for expansion in hori3ontaland vertical directions.

    Ensure the cover plate vent and drain areconnected with pipe of ade+uate si3e to safelocations.

    Extend the vent pipe over roof to avoid rain watercollection into drip pan. Insulate the inlet and bodyup to the cover plate.

    %fter hydrostatic test of boiler, remove hydro testplug and ensure disc is replaced and valvereassembled without affecting setting as per theprocedure given in )4' manual.

    ERECTION

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    !se cali"rate# $ress%re ga%ges for safetyvalve floating. loat the lo'est set safety valvefirst, gag the valve after setting is over an#

    float the ne(t higher set valve an# so on till allvalves are floate# an# chec)e# for the set$ress%re 'ithin *+ tolerance level.

    ry to achieve the setting 'ithin a minim%m

    no. of $o$s a"o%t $o$s.

    A$$ly gag-loa# finger tight in hot con#ition toavoi# s$in#le "en#ing.

    emove gags after floating of safety valves.

    Chec) an# ens%re s%fficient a$$roach an#maintenance s$ace all aro%n# the valve.

    Commissioning

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    1) Ambient temperature

    2) Discharge Stack Binding against drip pan elbow (Cold & Hot conditions).

    3) Vibration of the header from upstream bends.

    4) Lesser operating Gap.

    5) Very Short Blow down setting.

    6) Improper Locking of Ring pins, Spring Adjuster & Release Collar.

    Valve operation

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    Excessive pressure loss at the inlet of the safety valve will cause extremely rapid opening and

    closing of the valve, which is known as chattering .Chattering may result in lowered capacity as

    well as damage to the seating surface of the valve. Severe Chattering can also cause damage to otherparts of the valve.

    To reduce chatter, the area of the inlet nozzle should he at least equivalent to the inlet area of the

    safety valve and that nozzle should be short as possible.

    The inlet nozzle corners must be rounded to a radius of minimum 25% of the diameter opening.

    Ensure the pressure drop to the inlet of the valve should not be greater than 50% of the expected

    blow down of the safety valve.

    Safety valves should be installed at least eight to ten pipe diameters downstream from any bend in a

    streamline. This distance should be increased if the direction of the change of the steam flow is fromvertical upwards to horizontal in such a manner as to increase density of the flow in the area directly

    beneath the safety valve nozzles.

    Safety valves should not be installed closer than eight to ten pipe diameters either upstream or

    downstream from diverging or converging Y fittings In cases where piping configuration renders

    the two above recommendation impractical or impossible, the downstream corners of the nozzleinlets should be rounded, leaving the upstream corners square. The nozzle entrance should be

    rounded so that the radius at the downstream corner will be equal to one-fourth of the nozzle

    diameter. The radius should be reduced gradually leaving only a small portion of the upstream

    corner sharp.

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    The safety valve nozzle should never be installed in a steam line in a position directly opposite a

    branch line of equivalent size.

    Excessive steam line vibrations are known to produce shifts in safety valve set pressures. Results

    of sizable vibrations will be to introduce possible chatter with ultimate damage to the valve.

    Exhaust, drain and cover plate vent piping must be installed so that they will not impose any

    stresses on the safety valve. These stresses can produce distortion and leakage at pressures below

    the set point.

    Discharge piping should not be supported by the valve. The maximum weight on the outlet of thevalve should not exceed the weight of a discharge flange, elbow and drip pan unit.

    Clearances between the valve exhaust piping and the discharge stack should be sufficient to

    prevent contact when considering thermal expansion of the boiler, valve, discharge stack and

    increased stack movement due to vibration and temperature when the valve is relieving.

    Steam flowing vertically out of the discharge elbow produces a downward reaction on the elbow,

    in proportion on the quantity of steam flowing and its velocity. In large, high capacity valves this

    force can equal several thousand kilograms and can produce severe stresses in the valve necks.

    The bending stresses are determined by the amount of the reactive forces, combined with the

    moment arm, or horizontal distance between the vertical centreline of the valve and the vertical

    centre line of the outlet elbow.

    In no case should discharge piping be of a smaller size than the valve outlet .The length of thedischarge stack will govern its size and with increased lengths, stack diameters will increase. This

    will prevent blow back from the discharge stack. Excessive water being pulled through the valve

    will also produce a blow back under certain conditions and should be investigated prior to faulting

    the discharge stack.

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    For optimum performance, safety valves must be serviced regularly and maintained. So that

    servicing can be properly handled safety valve should be located for easy access. Sufficient

    working space should be provided around the safety valve to permit access to adjusting rings. Iftwo or more safety valves are located close together, the outlets should be parallel, to offer as

    much protection as possible to personnel repairing or working close to the safety valve. Cross

    beams may be provide above valve centre line for easy disassembly of safety valves.

    For valves being installed that are tested for set pressure with the use of a hydro set, it should be

    realized that only the set pressure is being verified. Other factors such as blow down, lift, reactionforce, proper inlet welding, proper discharge stack sizes and effects of thermal expansion cannot

    be determined.

    Because foreign material passing into and through a safety valve may damage it, the system on

    which the valve is tested and finally installed must also be inspected and cleaned. New systems,

    especially, are prone to contain welding beads, pipe scale, and other foreign materials which areinadvertently trapped during construction and destroy the sealing surfaces the first few times the

    valve opens. Wherever possible, the system should be purged thoroughly before the safety valve is

    installed.

    It is recommended that the safety valve be isolated during hydrostatic testing of the boiler either

    by blanking off the valve, gagging the valve, or installation of hydrostatic test plugs. i.e. For weld-end inlet valves, completely assembled valves may be installed without necessity for disassembly.

    Vertical alignment should be within 1 from a vertical plane. During welding, where possible, the

    valve neck should be insulated to reduce thermal effects. When stress relieving, the same will

    apply. In all cases the valve neck should be insulated at least up to the inlet neck-valve body bowl

    juncture.

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    1) To change the popping pressure of the valve removethe cap and lever assembly, loosen the locknut and turn

    the spring adjuster clockwise to increase pressure.

    2) After each adjustment of the spring adjuster the

    locknut should be tightened.

    3) The arm on top spring washer should always be free

    from bearing against the yoke rod. This can be

    accomplished by holding a screwdriver between the arm

    and the rod to prevent any movement of the top springwasher while adjusting the spring adjuster.

    Popping Point Adjustment

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    1) Always gag the Safety Valve for protection, in case boiler

    pressure rises while making ring adjustments. Adjusting ring

    lock pins are threaded into the valve body and engage notches

    which are cut into the rings. To adjust either ring thecorresponding ring pin must be removed. A screw driver can be

    used to turn the rings.

    2) Spare ring pins when supplied will be of excess length and

    have to be individually ground to match each valve. The tip of

    the pin should not touch the bottom of the ring notches.

    Ring Adjustments

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    1) Using a reference plane is established where the adjusting ring is level with thebushing seat. From this setting, factory adjustments are made by adjusting one notch

    downward for each 42.2 kg/sq.cm of set pressure. Ring position should be checked prior

    to testing. Slight deviations from these settings are possible because of tests conducted

    in the plant. NOTE: This provides a starting point only.

    2) The lower ring is used to obtain a clean popping action and to cushion the closing

    action of the valve. DO NOT ATTEMPT TO ADJUST BLOWDOWN WITH THE

    LOWER RING.

    3) The ideal position must then be found for the set of operating conditions present. If

    simmer is present or valve fails to lift, the lower ring should be moved upward slowly,one notch at a time to remove the simmer. The most ideal position for the lower ring is

    the lowest position that does not introduce simmer or a buzzing sound. In this

    connection, it is imperative that extreme care be used in conditioning the seat surfaces.

    4) Ensuring correct alignment, and establishing the proper clearance, so that

    mechanical causes of simmer will be reduced to a minimum.

    Lower Ring Adjustment

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    1) When shipped from the factory, upper adjusting rings are set level with the bushingseat.

    2) This should be checked prior to field setting and should be regarded as a starting

    position only.

    3) It is possible that this position is not strictly in seat level, because of testing at the

    plant. Starting position for blow down check should place bottom surface of upper ringlevel with bushing seat. Since the bushing seat is not visible through the service ports. It is

    necessary to use a cross-reference relationship. Raise the upper ring until its bottom

    surface in level with the disc holder, which is visible through the ports. Then, referring to

    O&M manual, lower the upper ring the required number of notches. This will place the

    upper ring on a plane with the bushing seat. This is the starting point for blow down tests.

    4) Further adjustments may be necessary depending on test results. When further

    adjustments are required to obtain final blow down setting, the upper adjusting ring should

    be moved 5-10 notches at a time

    5) The upper ring is used to obtain full lift at the popping pressure. However, its position

    also determines the point at which the valve begins to drop out of full lift and starts the

    closing portion of its cycle.

    Upper Ring Adjustment

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    1) It will further be noted that there is a distinct difference between the actual

    overlap setting on the valve and the point at which the overlap begins to take

    effect.

    2) This can be understood since the area in the overlap vent begins to reduce

    considerably ahead of the point where upper corner of the overlap level actually

    enters the floating washer.

    3) Excessively high overlap settings are liable to cause seat damage when the

    valve closes. It is therefore desirable to set the upper ring in such a position as

    to cause the valve to stay in full lift for as short time as possible most desirous

    complete.

    Overlap Setting

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    1) When steam safety valves are subjected to an excessively high water level,

    the valve can be expected to have a long blow down which the upper ring

    adjusting ring position will be unable to correct.

    2) It is recommended that the cause of high water level be corrected so valves

    may function correctly at the intended conditions. If a super heater valve is set

    with low temperature steam, it is advisable to for increase the blow down to

    compensate the change in the density and other thermal effects taking place when

    the steam is brought up to working temperature.

    3) An approximate rule is to add 0.5% of set pressure to the blow down foreach 55.6C of steam temperature below the final temperature.

    High water level

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    After testing the valve for proper set point and blow

    down, the ring pins, and top lever pin will be sealedfor all external adjustments. Seals shall be installed by

    the manufacturer at the time of initial shipment and

    after field adjustment or repair of the valves either bythe manufacturer, his authorized representative

    repairer, or the user

    Sealing Valves after Test

    Bl d

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    1) The lower and upper adjusting ring pins of any particular valve is not

    interchangeable.

    2) Spare ring pins when supplied will be of excess length. These pins are to be

    ground at site so as to match the valve under maintenance. The tip of pin shouldNOT touch the bottom of the ring notch. Also the pins should not bear against the

    rings.

    3) DO NOT attempt to adjust blow down with the lower ring.

    4) The upper adjusting ring should not be set below the seat level and the lower

    adjusting ring should not be set above the seat level. Lock both the ring pin

    settings using a seal wire to avoid disturbance during system/valve operation and

    line vibrations.

    5) It is possible to raise the upper ring too far and prohibit attainment of full

    lift.

    Blow down

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    1) Excessive line vibrations, presence of foreign particles,

    discharge piping stress, back pressure, reaction force etc., can lead

    to pin failure over a prolonged period of exposure. The ring pins

    being the most critical spare part, it is desirable that one pin pervalve (for drum, SH and RH SVs) and one ring per four valves is

    stocked as essential spares by customer.

    2) To reduce blow down MOVE RING UP (TURN COUNTERCLOCKWISE).

    3) To increase blow down MOVE RING DOWN (TURN

    CLOCKWISE).

    Pin failure

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    1) Safety Valve can be easily disassembled for inspection, reconditioning

    seats, or replacing internal parts, without altering the spring load and hence the

    set pressure.

    2) Be sure that the there is no steam pressure in the drum or header thenproceed.

    3) To retain the spring setting of the valve when and only when, a disc is to

    be replaced. Remove the top lever pin and top lever. Loosen cap set screw andlift off cap and drop lever assembly. Remove the cotter pin and turn the

    Release collar down until it is tight against the top of the spring adjuster, then

    back off turn. Install cotter pin in lower hole of spindle. Remove the two top

    yoke rod nuts and the four cover plate stud nuts, and lift the spindle, spring and

    spring washers, cover plate, disc and disc holder assembly out of the valve.

    DISASSEMBLY

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    Users scope

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    1) Quantity of steam being discharged through the valve, i.e. the actual

    installation capacity exceeding that of the test boiler, thus permitting the valve

    to flow its full rated capacity.

    2) Quality of steam being discharged.3) Discharge piping stresses

    4) Back pressure.

    5) Ambient temperature.

    6) Shipping or storage damage.7) Improper gagging.

    8) Improper bolting of flanges

    9) Damage due to foreign material in the steam.

    10) Damage, during disassembly and assembly in the field

    Users scope

    CHATTER

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    Causes RemedyPRV Oversized for the

    Installation - Actual flow is

    less than 25% of Rated

    Capacity

    Check the Required Capacity & Replace PRV if necessary

    Starvation (insufficient

    flow)

    Valve lift may be restricted to reduce the valve rated capacity if possible. Contact Valve

    Engg.Short Blow down setting Increase the Blow down setting.

    Wrong Adjusting Ring

    Setting - Upper Adjusting

    Ring is too High

    Lower Upper Adjusting Ring to Even With or Slightly Below Seat Level

    Inlet Piping Undersized for

    PRV - "Starving" the PRV

    Increase Inlet Piping to at least Inlet Size of PRV inlet. A valve should never be installed

    on a fitting having a smaller inside diameter than the inlet connection of the valve

    Outlet Piping Excessive

    Length - More Built-Up

    Back Pressure

    Redesign Outlet Piping so that Back Pressure does not Develop.

    Outlet Piping Undersized for

    PRV - More Built-Up Back

    Pressure

    Increase Outlet Piping to at least Outlet Size of PRV

    Pressure Drop in Inlet

    connecting Pipe is more

    Redesign Inlet Piping so that Pressure Drop is less than 3% of Set Pressure. Check and

    confirming the proper welding at valve inlet.

    CHATTER

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    Causes Remedy

    Provided offset for

    thermal expansion is not

    correct

    Modify the Discharge Piping arrangement if required, for providing a

    minimum Clearance of 25 mm in HOT condition to take care of Thermal

    Expansion of exhaust pipe as well as any changes by site during erection of

    Exhaust pipe or drain connectios.Chattering Reduce the chattering by proper adjustment

    More pressure drop in

    the exhaust piping

    Modify the exhaust piping routing to reduce the pressure drop if required.

    The gap betweenexhaust pipe and Drip-

    Pan is not sufficient

    Ensure proper clearance of 25 mm minimum, such that exhaust piping do notbind on the drip pan (valve outlet) so that loads are not transferring to valve.

    More back flow from

    exhaust piping.

    Ensure the proper erection of exhaust pipe and silencer.

    Downward force is notinline with axis

    Modify the Discharge Piping arrangement if required, for providing aminimum Clearance of 25 mm in HOT condition to take care of Thermal

    Expansion of exhaust pipe as well as any changes by site during erection of

    Exhaust pipe or drain connectios.

    Material failure Ensure the proper material/ heat treatment

    DRIP PAN / DISC DAMAGE

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    Causes Remedy

    Not properly installed Gags should be applied such that its legs are

    seating properly to the Yoke bottom

    machined surface and the gag stem is

    seating on the Spindle Centre. You are

    requested to take personal Care during this

    activity, especially for the SHSVs.

    During floating of the Drum Safety valves, it

    has been observed that the pressure at the SH

    SV inlet is also almost same as that of the

    drum pressure. This is due to absence of flow

    through the SH coils, the pressure drop is

    almost zero and the pressures at SH and

    drum is almost same. In this the SH SV (in

    gagged condition) is experiencing a high

    pressure and coupled with vibration may

    result in gag slippage if not properly

    installed.

    If the Drum SVs are floated first, then the

    SH SV set pr. can be temporarily increased

    by spring adjustor tightening and then

    properly Gagging the SHSVs to prevent gag

    slippage. Then the SH SVs setting can be

    relaxed and then floated to avoid the

    problem referred by you.

    GAG SLIPPAGE

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    Causes Remedy

    One major reason for high blow

    down in Drum Safety Valves is

    the Drum water level available at

    the time of floating. If water

    levels are kept above normal

    then the chances of water lifting

    during valve floating is there

    which would result in high blowdowns.

    Try to maintain drum water levels at nearest to

    lower limit during floating.

    Upper adjusting ring would have

    been disturbed

    Check with the position of the Upper adjusting

    ring with ref. to other Safety Valves (set with

    normal blow down) available on the boilerDrum. In case of any difference, we will get to

    know the required adjustment. Normally on a

    drum safety valve we need not have to adjust

    the OLC for getting the required blow down.

    High Blow down in Drum Safety Valve

    Leaking/Passing

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    Causes Remedy

    Setting disturbed Adjust the Spring Adjuster and lock the position by the Locknut.

    Seat or disc damage Lap/Change the disc/seat. Seat cutting to be done if required.

    Seat damaged by Improper

    Gagging

    Rectify the seat damage by lapping

    Long simmer Adjust the Lower Adjusting Ring position.

    Physical obstruction in seat Blowout the obstruction by opening the valve with the provided manual lifting lever.

    Spindle is bent by cold

    gagging or over gagging

    Rectify/Change the spindle.

    Improper tightening of Lock

    Nut.

    Ensure proper tightening so that proper locking of the spring adjuster is ensured.

    High ambient temperature Ambient temperature is to be reduced by identifying the source of heat. The ambient

    temperature near by the safety valve should be 40C maximum.

    Impurity in steam/Foreign

    particles damages the seat.

    Change/lap the disc/seat. Seat cutting to be done if required. Maintain the steam condition

    without impurities. For servicing/seat-cutting please contact BHEL/Valves Services

    Department.

    Wrong working pressure

    which is closer than 6% of

    set pressure difference

    Increase the set pressure or reduce the working pressure to have "6% of set pressure"

    operational gap.

    The gap between exhaust

    pipe and Drip-Pan is not

    sufficient

    Ensure proper clearance of 25 mm minimum, such that exhaust piping do not bind on the

    drip pan (valve outlet) so that loads are not transferring to valve.

    Leaking/Passing

    N t ti t l

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    Causes RemedyRelease collar is

    left unlocked.

    The release collar if left unlocked could rotate and come

    down. If system vibration is more or during valve lifting this

    can happen and the release collar would sit on top of the

    lever/spring adjustor and prevent the valve from reseatingeven at very low pressures of the system. Spare spindles,

    supplied to sites as replacement / spares, this hole is to be

    drilled on spindle in site only after setting of the release collar

    or by measurement.

    More External

    Load on Safety

    Valve Body/Outlet

    by DischargePiping or Drain

    connections.

    Modify the Discharge Piping arrangement if required, for

    providing a minimum Clearance of 25 mm in HOT condition

    to take care of Thermal Expansion of exhaust pipe as well as

    any changes by site during erection of Exhaust pipe or drainconnections.

    Not reseating even at very low pressures

    Ring Pin Broken

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    Causes Remedy

    The Lock Pin should

    bear against the Ring.

    Spare ring pins when supplied will be of excess length. These pins

    are to be ground at site so as to match the valve under maintenance.

    The tip of pin should NOT touch the bottom of the ring notch. Also

    the pins should not bear against the rings.

    Material failure Ensure the proper material/ heat treatment

    Excessive Line

    Vibration

    If vibration exists due to turbulent flow or improper piping support,

    correct the pipe supports to arrest the line vibration.

    More Back Pressure Redesign Outlet Piping so that Back Pressure does not Develop, if

    required.

    More External Load

    on Safety Valve

    Body/Outlet by

    Discharge Piping or

    Drain connections.

    Modify the Discharge Piping arrangement if required, for providing a

    minimum Clearance of 25 mm in HOT condition to take care of

    Thermal Expansion of exhaust pipe as well as any changes by site

    during erection of Exhaust pipe or drain connections.

    Exhaust pipe

    expansion clearances

    is not sufficient

    Ensure proper clearance such that exhaust piping do not bind on the

    drip pan so that loads are not transferring to valve.

    g

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    $ave a nice day6

    ThankY

    $ave a nice day6

    ThankY