Isolation & Permit-to-Work

Covers the isolation requirements that must be satisfied before any entry, inspection, or maintenance on a fired heater, and the operator's role within the permit-to-work (PTW) system. Safe isolation is the primary defence against energy release during maintenance.

Isolation Philosophy

Isolation for maintenance means physically preventing all hazardous energy sources from reaching the work area, and verifying that isolation is effective before work begins. The goal is not merely to close a valve — it is to create and confirm a safe working boundary.

For fired heaters, isolation philosophy is driven by three principles:

• Defence in depth: Multiple independent isolation points for each energy source. No single point of failure should expose personnel to hazardous energy.
• Positive isolation: Where possible, use physical disconnection (spade / spectacle blind) rather than reliance on valve seating integrity.
• Independent verification: The person installing isolation must not be the only person confirming it is effective. Physical checks are required — pressure tests, gas tests, visual confirmation.

Energy Sources to be Isolated

Fired heaters have multiple simultaneous hazardous energy sources. All must be addressed in the isolation plan before any maintenance work proceeds.

Double Block-and-Bleed on Fuel Gas

Single valve isolation is not acceptable for fuel gas systems where personnel will work in the vicinity or within the firebox. A single valve may pass (leak across its seat) due to wear, deposits, or mechanical damage — placing personnel at risk from invisible hydrocarbon accumulation.

The double-block-and-bleed arrangement uses two block valves with a bleed valve between them:

How it works:

• Block 1 (upstream): Primary isolation — closed against the supply pressure.
• Bleed valve: Open to a safe vent. Proves Block 1 is holding: if pressure builds in the cavity, Block 1 is passing and work must not proceed.
• Block 2 (downstream): Secondary isolation — prevents backflow from any adjacent pressurised source.

Permit-to-Work — Operator's Role

This page does not cover how to write a PTW. That is the responsibility of the issuing authority (typically a designated senior engineer or safety officer). The operator's role within the PTW system is specific and important.

Depressuring and Purging Before Maintenance

Before maintenance work begins on any pressurised component of the fuel or process system, the affected section must be depressured and purged clear of hydrocarbons.

Isolation Verification

Verification confirms that installed isolations are actually effective — not just that valves are in the correct position. Physical verification is required before any work begins.

• P
  Physical check: Walk the line and confirm each valve is fully closed and in the correct position. Check valve stem position and disc indicator where fitted.
• T
  Bleed cavity pressure test: Monitor bleed valve outlet for pressure accumulation over a defined hold period (typically 5–10 minutes ). Rising pressure indicates the upstream block valve is passing.
• P
  Spade confirmation: For positive isolations, physically confirm the spade is in the correct orientation (inserted, not in the bypass position) and the flange bolts are made up.
• T
  Gas test at work location: Authorised gas tester confirms LEL within limits at the actual point of work, not just at the isolation boundary.
• D
  Isolation register complete: All isolations listed with tag number, type, location, and confirming signature. No blanks permitted before the PTW is issued.
• P
  Electrical isolation test-dead: Maintenance electrician confirms all electrical supplies are dead using approved test instruments before any work on electrical equipment.

Re-commissioning After Isolation

Reinstatement of isolation must be planned and sequenced as carefully as installation. Incorrect sequence can pressurise pipework before all connections are confirmed made-up, or introduce fuel gas before purging is complete.

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