Positive Isolation
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7/27/2019 Positive Isolation http://slidepdf.com/reader/full/positive-isolation 1/2 POSITIVE ISOLATION torusinsurance.com Torus Risk Engineering BEST PRACTICE ADVICE Best Practice Centre Piper Alpha Incident. (6 July 1988, North Sea) During maintenance work, a pump discharge PSV was removed. The open pipe rom the pump was not properly secured with a blank fange and when the pump was started the release o hydrocarbons caused an explosion and re. The resulting cascade o events caused the death o 167 people and total loss o the platorm. This is believed to be the largest single insurance loss rom re and explosion at around US$1.3Billion. The most reliable orm o positive isolation or pipework is to close the open end with a blank fange o the appropriate material, size and rating, which is properly installed with the correct gaskets and securing bolts so that it could withstand the system design pressure. For positive isolation between two systems, the most reliable method is to remove a spool piece and install blank fanges on the open ends. Where the piping conguration does not allow or the use o blank fanges, as described above, then a spade or spectacle plate may be inserted between fanges. All spades and blinds should be clearly marked with material, size and rating. For small bore piping it may be preerred to use a cap or a plug, in place o a blank fange. For short duration isolation a double block and bleed may be used. The block valves should be secured closed and the bleed valve used to ensure that there is no pressure build up between the block valves. This type o isolation is a “proved” isolation, rather than a “positive” isolation, as the integrity o the isolation requires re-proving at intervals. It is not suitable or critical isolations, such as conned space entry. For highly hazardous materials the bleed should be vented to a sae location. I the bleed is connected to other equipment, such as a fare system, then the possibility o a back pressure should be considered, as this could aect the integrity o the double block isolation. Background A leak review o the UK North Sea oshore installations, carried out by the HSE, revealed that 21% were due to an open pathway being provided by the lack o positive isolation. All installations, whether onshore or oshore, which handle hazardous materials, should have procedures to ensure that positive isolations are installed and that monitoring systems are in place to ensure compliance.
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Transcript of Positive Isolation
7/27/2019 Positive Isolation
http://slidepdf.com/reader/full/positive-isolation 1/2
POSITIVE ISOLATION
torusinsurance.com
Torus Risk Engineering
BEST PRACTICE ADVICE
Best Practice Centre
Piper Alpha Incident.(6 July 1988, North Sea)
During maintenance work, a pump dischargePSV was removed. The open pipe rom the pumpwas not properly secured with a blank fangeand when the pump was started the release ohydrocarbons caused an explosion and re. Theresulting cascade o events caused the death o167 people and total loss o the platorm. This isbelieved to be the largest single insurance lossrom re and explosion at around US$1.3Billion.
The most reliable orm o positive isolation or pipework isto close the open end with a blank fange o the appropriatematerial, size and rating, which is properly installed withthe correct gaskets and securing bolts so that it couldwithstand the system design pressure.
For positive isolation between two systems, the mostreliable method is to remove a spool piece and installblank fanges on the open ends.
Where the piping conguration does not allow or the useo blank fanges, as described above, then a spade or
spectacle plate may be inserted between fanges.All spades and blinds should be clearly marked withmaterial, size and rating.
For small bore piping it may be preerred to use a cap or aplug, in place o a blank fange.
For short duration isolation a double block and bleed maybe used. The block valves should be secured closed andthe bleed valve used to ensure that there is no pressurebuild up between the block valves. This type o isolationis a “proved” isolation, rather than a “positive” isolation,as the integrity o the isolation requires re-proving atintervals. It is not suitable or critical isolations, suchas conned space entry. For highly hazardous materialsthe bleed should be vented to a sae location. I thebleed is connected to other equipment, such as a faresystem, then the possibility o a back pressure should beconsidered, as this could aect the integrity o the doubleblock isolation.
BackgroundA leak review o the UK North Sea oshore installations,carried out by the HSE, revealed that 21% were due toan open pathway being provided by the lack o positiveisolation. All installations, whether onshore or oshore,which handle hazardous materials, should haveprocedures to ensure that positive isolations are installed andthat monitoring systems are in place to ensure compliance.
7/27/2019 Positive Isolation
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POSITIVE ISOLATION
This publication is intended as a general overview and discussion o topics that might be relevant to insureds and it is notintended to be, and should not be used as, advice in relation to any specic situation nor a representation o Torus’ view onany particular risk, circumstance or claim. Torus Insurance Company (either Torus Insurance (UK) Limited, Torus SpecialtyInsurance Company Torus National Insurance Company, Torus Insurance (Europe) AG or Torus Insurance (Bermuda)Limited, collectively “Torus”) will accept no responsibility or any actions taken or not taken on the basis o this publication.
I you would like urther advice, please contact [email protected]
Copyright © 2010 Torus Insurance Company. All rights reserved.
torusinsurance.com
BEST PRACTICE ADVICE Continued
A less secure orm o isolation is to use valves whichprovide a double seal in a single-valve body with a bleed inbetween (e.g. double-wedge gate, parallel expanding gateor double-seal ball valves) or isolation, as it is not alwayspossible to test the downstream seal. There is an additionalconcern in that the security o the isolation depends upon theimmobilisation o a single valve operating stem.
Single block valve isolation is NOT regarded as positiveisolation. It is generally used as the primary isolation priorto installing the positive isolation, such as a blank fange orspade. Where single block valve isolation is used the worksite should not be let unattended and generally the periodo use should not extend beyond one shit.
Spectacle Blind Drain with Cap
For small bore piping involving requent activities, suchas sampling or draining level instruments, then a doubleblock valve arrangement may be acceptable, i the systemis on the operator’s visit routine at least once every shit.The upstream valve should always be a screw down type,such as a gate valve, as this is more secure than a 90oaction valve, like a ball valve, which is susceptible to beingaccidentally knocked open.
Summary
Positive isolation means the secure closing o pathwaysrom the containment envelope, by physical blocks, suchas blank fanges, spades, spectacle blinds, or caps andplugs or small bore piping. The use o valves or isolation
should be or short durations only and be closely controlled.
References
“Reducing Offshore Hydrocarbon Leaks – a UKRegulatory Initiative” Loss Prevention Bulletin 168,IChemE 2002.
HSG253 “The Safe Isolation of Plant and Equipment,”Health and Safety Executive, U.K., 2006.
Contact us
For further information please contact ourEngineering Practice Leader for Onshore Oil & Gas
Peter Wong
+1 (832) 314 [email protected]
