OMB Valves s.p.a. con Socio Unico Via Europa, 7 24069 Cenate Sotto (BG) - Italy Ph +39 035 4256711 Fx +39 035 942638 942638 e-mail
[email protected]
TECHNICAL BULLETIN
REQUIREMENT FOR BLEED HOLE FOR SMALL SIZE CRYOGENIC SERVICE GATE VALVES (API602/ISO15761)
TB/001 - rev.1
Foreword This note identify the design criteria and dimension of the bleed hole in OMB gate valves for cryogenic service. Requirement Customers specification usually identify the needs of a bleed hole as a mean to discharge in line excessive pressure which could be trapped in the bonnet column of cryogenic valves when these bidirectional valves are in closed position. In case of gate valves the wedge seal both seats at the same time: with valve closed, heat expansion of gas in the column can produce an internal pressure greater than the valves design rating. Specifications BS 6364 States:
Shell 77/200 adds: Shell 77-200 BODY CAVITY RELIEF (PRESSURE EQUALISATION) PROVISIONS 3.3.1 Double seated valves used in liquefied service at operating temperatures below minus 50 degrees Celsius, that can see cavity over-pressurisation (i.e. exceeding 33% of the rated pressure at maximum design temperature as per appropriate class), shall have an internal c avity relief provision. A distinction distinction is made between between valve designs for: for: - double seated gate gate valves (flexible (flexible wedge type) type) with metal metal seats, - ball valves valves with seat supported (floating) (floating) ball and canter levered/soft seats, seats, - ball valves with seat supported (floating) ball and spring energised seat configuration, configuration, - b all all valves with trunnion supported ball and spring energised single or double relieving seat design’s. For: For: - flexible wedge type gate valves with metal seats, - seat supported (floating) ball valves with spring energised soft seats, - seat supported (floating) ball valves with canter levered (soft) seats and, body cavity over-pressurisation over-pressurisation shall be prevented by a pressure equalising hole, with a diameter of at least 3 mm, drilled in the closure member outside the seat facing area. Spring energised (single or (single or double) self relieving seat configurations (“soft, (“soft, metal and integral fire safe”), used in: in: - ball valves with seat supported (floating) ball and/or - ball valves with a trunnion supported ball are acceptable, provided the manufacturer shall proof the correct cavity relieving function, as specified in SPE 77/130 sections 3.10.3 3.10.3 and 3.13. For 3rd party witnessed functional acceptance/type acceptance/type test; refer to SPE 77/306 and T-2.253.730). T-2.253.730). Note: A double self relieving seat design prevents the risk of installing the valve incorrectly and eliminates the need for control and identification marking.
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08/01/2012 Revised template
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05/02/2007 First issue
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REV. Mod. 002/1
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OMB Valves s.p.a. con Socio Unico Via Europa, 7 24069 Cenate Sotto (BG) - Italy Ph +39 035 4256711 Fx +39 035 942638 e-mail
[email protected]
TECHNICAL BULLETIN
REQUIREMENT FOR BLEED HOLE FOR SMALL SIZE CRYOGENIC SERVICE GATE VALVES (API602/ISO15761)
TB/001 - rev.1
OMB standard
OMB has defined as a standard a vent hole drilled into the gate valves wedge with a hole dimension of 3mm and a configuration as per drawing table 3
table 3
Valve in closed position with backseat area not operating
OMB experience with the construction OMB fully guarantee that all the valves fitted with the vent hole as per above description are fit for purpose and passed prototype testing at temperature up to -196C.
1
08/01/2012 Revised template
SB
LG
LG
0
05/02/2007 First issue
SB
LG
LG
ISSUED
CHECKED
APPROVED
REV. Mod. 002/1
DATE
DESCRIPTION
Page 2/2
