A Coverage on Safety Valve
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Transcript of A Coverage on Safety Valve
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A coverage on safety valve
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Purpose of safety valve
• To safeguard the equipment against exceeding design pressure.
• A boiler without Superheater will have two safety valves in steam drum / Shell.
• A boiler with superheater should have one safety valve on superheater & two safety valves on Steam drum.
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A safety valve versus relief valve
• A relief valve is meant for releasing the liquid in a proportional manner in order to maintain some system pressure on a continual basis.
• A safety valve should instantly lift and release the medium by a fixed quantity and bring down the pressure.
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Main parameters for safety valve
• Relieving capacity in kg/h• Steam temperature • Set pressure• % accumulation• % blow down• Downstream pressure
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Relieving capacity of safety valves for a boiler
• All the safety valves put together should relieve the entire steam that is generated by the boiler without casing an increase in pressure by 5%.
• The drum safety valves together should release 75% Boiler steam generation capacity.
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Electromatic safety valve
• An electromatic safety valve is a pressure switch initiated safety valve which is set to lift on particular pressure.
• Whereas a regular safety valve can lift only after an accumulation of pressure
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Definitions by code regulations
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Definitions by code regulations
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Terminologies used in safety valve
• LIFT OR POP - REFERS TO THE OPENING OF THE SAFETY VALVE
• BLOWDOWN - THE PRESSURE DROP BETWEEN LIFT AND RESEATING OF THE SAFETY VALVE
• RESEAT - CLOSING OF THE SAFETY VALVE WITHOUT CHATTERING
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• SIMMERING - STEAM THAT LEAKS BY THE SEAT AND DISK PRIOR TO THE LIFTING OF THE SAFETY VALVE. ACCEPTABLE CONDITION WITHIN 1% OF LIFTING PRESSURE.
• CHATTERING - THE RAPID POPPING AND RESEATING OF THE SAFETY VALVE
Terminologies used in safety valve
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• GAG - A MECHANICAL DEVICE PLACED ON THE SAFETY TO PREVENT THE SAFETY FROM LIFTING DURING TESTING AND BOILER HYDROSTATIC TESTS
Terminologies used in safety valve
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• MAWP• Design pressure• Operating
pressure• Set pressure• Overpressure• Accumulation• Blowdown
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Flow area of safety valve
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Accumulation & blow down
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Weather hood provides complete protection for yoke, spring and valve steam for outdoor installations
Test gag is used for preventing the valve from lift during hydro test / safety valve testing. Test gad should be turned down only finger tight. It must be removed after testing is completed
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Safety valve sizing as per IBR-293
Where,A = reqd nozzle orifice / bore area, in mm2.E = reqd sat steam relieving capacity, kg/hC = IBR constant for the manufacturerP = set pressure, bar (a)Es = reqd relieving capacity of SH steam, kg/hTs = deg of superheat ( deg C)
PCTE
A ss
*)1000/*7.2(1+
≈
Saturated steam – kg/h
PCEA*
≈
Superheated steam – kg/h
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ASME sec I –clause – PG-67
• Safety valves must discharge all the steam generated without allowing the boiler pressure to increase beyond 6% of design pressure.
• Any economiser that may be shut off from boiler shall have one or more safety relief valve with relieving capacity based on its heat duty.
• One or more safety valve shall be set at or below MAWP of boiler.
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ASME sec I –clause – PG-67
• The highest safety valve set pressure shall not be more than 3% of MAWP.
• The complete range of all saturated steam safety valve shall not exceed 10% of highest pressure to which any valve is set.
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ASME sec I –clause – PG-67 on safety valve construction
• Safety valve shall be so constructed that any failure of a part of safety valve should not obstruct the free & full discharge of steam.
• Safety valve shall be direct spring loaded pop type, with seat inclined at 45 -90 deg to spindle axis.
• The saturated steam safety valve should have a total relieving capacity of at least 75%.
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ASME sec I –clause – PG-67 on safety valve construction
• When two different size valves are used at drum, the smaller safety valve shall at least equal to 50% of relieving capacity of bigger valve.
• Safety valve shall be designed to vent the steam without chattering with a minimum blow down of 2 psi or 2% of set pressure whichever is greater.
• Safety valves shall lift fully at a pressure not exceeding 3% of set pressure.
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ASME sec I –clause – PG-71 on safety valve mounting
• Safety valve shall be erected with its spindle upright.
• There shall not be intervening valve or obstruction between valve & the boiler.
• The safety valve stub should not be longer than a corresponding Tee fitting.
• The stub inner flow area should not be less than the safety valve inlet flow area.
• There should not be any obstruction to steam flow path in the vicinity of safety valve.
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ASME sec I –clause – PG-71 on safety valve mounting
• When a discharge pipe is used, the flow area should not be less than that of the safety valve discharge nozzle flow area.
• The discharge pipe should be short and straight and shall be so arranged to avoid undue stresses.
• Each valve shall have the body drain below the seat and shall be given a open drain pipe connection.
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ASME sec I –clause – PG-71 on safety valve mounting
• If a muffler is used, it shall have sufficient outlet area to prevent back pressure.
• The tolerance on set pressure is as per the table. 1% set press>1000 psi
10 psi> 300 ≤ 1000 psi
3% of set press> 70 ≤ 300 psi
2 psi≤ 70 psi
toleranceSet press
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ASME sec I –clause – PG-71 on safety valve mounting
• The spring in a safety valve shall bot be reset for any pressure more than 5% above or below that for which it marked.
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The points related to installation
• Minimum length stub
• Rounded inlet nozzle
• Higher tube inlet area
• Body drain
• Drip pan arrangement
• Drip pan drain.
• No load on safety by escape piping.
• Anchoring of escape piping
• Minimum bend length for escape piping.
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Boiler Hydro test after
erection of safety valve
Hydrotest at higher pressure than operating pressure
• In case of flanged connection, the safety valve shall be removed. Blind flange shall be used.
• In case of welded safety valve hydrostatic plug to be used.
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Boiler Hydro test after erection of safety valve
Hydro test at operating pressure
• In case of flanged / welded safety valve, the safety valve shall be in place and gag can be used for hydro test at rated pressure.
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Typical safety valve data sheet from a vendor
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Typical safety valve data sheet from a vendor
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Safety valve-a vendor drawing
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Safety valve under floated condition.
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Installation recommendations
A safety valve should not be
installed on on s stub with smaller
inlet area that that of safety valve
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Installation recommendations
A safety valve should be erected with the
spindle upright
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Installation recommendations
A safety valve should be mounted directly on the nozzle independent of any other connection and attached as close as possible having a well rounded approach that provides smooth, unobstructed flow from the vessel or line.
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Installation recommendations
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Installation recommendations
The cross sectional area of the discharge pipe shall not be less than the full area of the valve outlet.
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Installation recommendations
Discharge pipe should be small and straight & arranged to avoid undue stresses on the valve. Discharge pipe diameter above drip pan should be adequately to avoid blow back of steam.The distance between the centre line of valve & discharge line should not exceed 24 inches.
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Installation recommendations
The cross sectional area of the discharge pipe shall not be less than the full area of the valve outlet.
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Installation recommendations
When the length of discharge pipe is long, it may be necessary to increase the diameter in the upper part of the stack.
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Installation recommendations
Recommendations on drip pan sizing
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Installation recommendations
When the length of discharge pipe is long, it may be necessary to increase the diameter in the upper part of the stack.
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PRESSURE %PRESSURE %
103103
100100100
959595
939393
MAWPMAWPMAWP
ALLOWABLEOVERPRESSUREALLOWABLEALLOWABLEOVERPRESSUREOVERPRESSURE
TYPICAL SETPRESSURETYPICAL SETPRESSURE
TYPICAL OPERATINGPRESSURE(“THE MONEY MAKER”)
TYPICAL OPERATINGPRESSURE(“THE MONEY MAKER”)
SET PRESSURETOLERANCE
SET PRESSURETOLERANCE
TYPICAL RESEAT
PRESSURE
TYPICAL RESEAT
PRESSURE
BLOWDOWN,% OF SET
BLOWDOWN,% OF SET
ASME Section 1 Valves- pressure relations