Isolation Selection Calculator
Follow the Appendix 6 methodology to determine the appropriate isolation standard
This is a guidance tool based on HSG253. It does not replace competent professional judgement, site-specific risk assessments, or company procedures. Always consult with qualified safety professionals.
Select the cell that matches your line size and pressure:
| Line Size / Pressure | >50 barg | 10-50 barg | <10 barg |
|---|---|---|---|
| ≥20 cm | H | H | M |
| 5-20 cm | H | M | L |
| ≤5 cm | M | L | L |
Isolation Methods Reference
Visual guide to acceptable isolation methods per HSG253 Figure 4
Physical Disconnection
Description: Complete physical separation of pipework or equipment.
Use: Highest level of isolation certainty. Required for Category 1 substances with high risk factors.
Double Block & Bleed + Spade
Description: Two valves with bleed point and spade between them.
Use: Positive isolation without complete disconnection.
Single Block & Bleed + Spade
Description: Single valve with bleed and spade fitted downstream.
Use: Positive isolation with verification via bleed.
Double Block & Bleed (DBB)
Description: Two isolation valves with bleed between them.
Use: Isolation can be proved by monitoring bleed point pressure.
Single Block & Bleed (SBB)
Description: Single isolation valve with bleed on protected side.
Use: Isolation proved by monitoring bleed for pressure/leakage.
Double Seals with Bleed
Description: Equipment with double sealing system and intermediate bleed.
Use: For rotating equipment (pumps, compressors) requiring isolation.
Double Valve Isolation
Description: Two isolation valves in series without bleed.
Use: Lower risk applications where proved isolation not required.
Single Valve Isolation
Description: Single isolation valve only.
Use: Low risk substances with low consequence of failure.
Compliance Monitoring Checklist
Based on Appendix 3 - Assess compliance with HSG253 requirements
Checklist Progress
Systems and Procedures
People
Worksite
Valve Selection Guide
Based on Table A - Sealing ability and security requirements
| Valve Type | Sealing Ability | Security Requirements | Notes |
|---|---|---|---|
| Ball Valve | Excellent | Lock in closed position | Best sealing performance. Suitable for critical isolation. |
| Plug Valve | Excellent | Lock in closed position | Good sealing. Requires regular maintenance of plug seals. |
| Butterfly Valve | Good | Lock in closed position | Moderate sealing. May leak with differential pressure. |
| Globe Valve | Good to Fair | Lock in closed position | Depends on seat condition. Regular maintenance critical. |
| Gate Valve | Fair | Lock in closed position | Can develop leaks over time. Not ideal for tight shut-off. |
| SDNR Valve | Poor | Lock in closed position + additional barrier | Self-draining non-return. Cannot be relied upon alone. |
| Needle Valve | Excellent | Lock in closed position | Small bore only. Excellent for precise isolation. |
Valve Security Methods
Securing Requirements
- Padlock with unique key
- Lock-out device on actuator
- Removable handle
- Car seal or tag
- Valve position indicator
Verification
- Visual inspection of valve position
- Check security device integrity
- Verify lock/tag matches isolation certificate
- Test bleed points where applicable
- Record valve serial numbers
Maintenance
- Regular seat inspection
- Leak testing program
- Stem packing maintenance
- Actuator servicing
- Isolation register updates
Key Stages Workflow
8-stage process for safe isolation based on Figure 3
Identify all hazardous substances, energy sources, and potential release scenarios. Review P&IDs, operating procedures, and previous incident reports.
Use Appendix 6 methodology to determine appropriate isolation standard. Consider substance category, release factor, and location factor.
Prepare isolation certificate, identify isolation points, arrange security devices, brief personnel, and ensure emergency procedures are in place.
Close and secure valves, fit spades/blinds as required, lock out electrical supplies, verify physical isolation is complete.
Remove residual hazardous materials. Drain liquids, vent/purge gases, flush pipework. Test atmosphere before entry.
Conduct atmospheric tests, pressure tests on bleed points, verify zero energy state. Establish continuous monitoring where required.
Execute planned maintenance/modification work. Maintain isolation integrity. Continue monitoring. Control access to worksite.
Remove temporary isolation equipment, restore plant configuration, pressure test as required, remove locks/tags, hand back to operations.
Quick Reference Cards
Essential guidance for field use
DVPF Sequence
- Drain all liquids to safe location
- Vent gases to safe atmosphere or flare
- Purge with inert gas or air (as appropriate)
- Flush with water or cleaning fluid
- Test atmosphere before entry
- Monitor continuously during work
Securing Methods Hierarchy
- Physical disconnection (best)
- Spading/blank flanging
- Locked valve + spade
- Double block & bleed
- Single block & bleed
- Double valve (lowest)
Positive Isolation Checklist
- All isolation points identified on P&ID
- Correct isolation category selected
- Spades/blinds correct specification
- All valves locked and tagged
- Bleed points monitored
- Isolation certificate signed
Variation Approval
- Proposed variation documented
- Alternative risk assessment completed
- Competent person approval obtained
- Additional controls identified
- All affected parties briefed
- Variation recorded in permit
Bleed Point Monitoring
- Open bleed before starting work
- Check for pressure indication
- Monitor for fluid/gas escape
- Record bleed point status hourly
- Stop work if leakage detected
- Close bleed during reinstatement
Emergency Actions
- Stop work immediately
- Evacuate to safe location
- Notify control room/supervisor
- Account for all personnel
- Do not attempt re-isolation
- Await emergency response team
Important Notes
Any variation from the calculated standard isolation must be formally approved by a competent person and documented on the isolation certificate with additional risk controls identified.
If assessment indicates "R" (may be intolerable), work should not proceed without senior management review. Consider redesign, different timing, or elimination of the activity.
Review isolation incidents and near-misses. Update procedures based on operational experience. Maintain an isolation equipment register showing inspection and test status.