INTRODUCTIONThis tool utilizes HSG253 ("The safe isolation of plant and equipment") methodology for selecting ‘baseline’ process isolation standards. It is guidance on the general principles of safe process isolation. The tool can complement, but cannot replace, competent technical judgment and common sense. The reference has a free-to-download, web-friendly version through HSE website.
This is an overview of the use of the selection tool to select a final isolation method. For more information, please refer to HSG253 Appendix 6: Example of a selection tool to establish the ‘baseline standard’ for final isolation. (HSG253 reference is available below - Figure 11 page 59).
This is an overview of the final isolation methods. For more information, please refer to HSG253 Figure 4, Final isolation methods. (HSG253 reference is available below - Figure 4 page 26 and page 64 for the description of the methods).
R: Consider whether the associated risk is acceptable or whether there is a need to further reduce risk by eg risk reduction measures, extending the isolation
envelope, plant shutdown.
I: Positive isolation (Physical disconnection, eg spool removal - Double block, bleed and spade - Single block and bleed and spade).
II: Proved isolation (Double block and bleed, DBB - Double seals in a single valve body with a bleed in between - Single block and bleed, SBB).
III: Non-proved isolation (Double valve - Single valve)
This is an overview of the appropriate substance category, These are primarily (but not exclusively) based on the classifications given in the Chemicals (Hazard Information and Packaging for Supply) Regulations 2005 (CHIP), you may check this. For more information, please refer to HSG253 Table C, Substance category. (HSG253 reference is available below - Table C page 62).
Line size and pressure give a release factor (this reflects the potential rate of release). The options are high (H), medium (M), and low (L). (HSG253 reference is available below - Table D page 63).
Consideration of the location should include numbers at risk, escalation, and damage if a release occurs. Take into account the nature of the possible consequences if the isolation fails, eg vapor cloud explosion (VCE), toxic gas cloud, jet fire with potential for escalation, etc. The options are high (H), medium (M), and low (L). (HSG253 reference is available below - Table E page 63).
Release factor and location factor give an outcome factor, in the range A-C. (HSG253 reference is available below - Table F page 63).
BASELINE ISOLATION STANDARD
The substance category and outcome factor give the appropriate baseline standard for final isolation The options are R, I, II, and III. (HSG253 reference is available below - Table G page 64).