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17 February 2026 13 min read Engine Room, Operations, Safety
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Standfirst: Isolations that hold at zero pressure but pass as soon as you load the system are a recurring cause of burns, flooding, unintended starts and near-misses. This article sets out how to plan, prove and police isolations under real operating conditions, and what to do when they fail.

Contents

1. Why it matters

Permits to Work (PTW) and Lock Out Tag Out (LOTO) stop people being exposed to energy. The weak point is often the isolation that looks fine on a drawing and even passes a zero-pressure check, but leaks the moment you introduce differential pressure or a backfeed. That is when a flange warms and flashes condensate, a pump windsmill-starts, or a main engine moves under a sniff of starting air. Most serious incidents I have seen had a tag in place and a permit signed; the energy just found a path we didn’t anticipate.

Under ISM you must identify hazards and establish safeguards. Charterers and PSC look for robust isolation practice, not just paperwork. If your isolations don’t survive pressure, your PTW system is cosmetic, and you are one valve seat away from an injury, pollution or unplanned off-hire.

2. Isolation failure mechanisms under pressure

Valves that pass under load

Gate, globe and butterfly valves can pass when seats are cut, stems are twisted, or the pressure reverses. A single valve may hold at atmospheric test but seep with 6–10 bar across it. Non-return valves often pass on fouled seats or low closing force.

Reverse and cross-connection backfeed

Crossovers and common returns allow pressure to arrive from the “wrong” side. A tank high in the ship can create static head into a supposedly isolated low line. Instruments, sample lines and drains can bypass your main isolation.

Stored and induced energy

Hydraulic accumulators, air receivers, VFD DC link capacitors and spring-charged breakers hold energy regardless of valve or switch position. Thermal expansion in trapped fluids will generate pressure even if you started at zero.

Dynamic effects

Sea motion, water hammer and pump cycling can force marginal seats to lift. Turning gear interlocks can be defeated by a passing starting valve; the crank moves a few degrees and your feeler gauge is now a projectile.

3. Planning isolations that withstand load

Start with the energy: mechanical movement, pressure, temperature and electricity. Identify every path the energy can take using the latest P&IDs and a physical walk-down. For high-energy or hazardous media, a single valve is not a barrier. Use a hierarchy:

1) Remove the source (drain/depressurise/de-energise). 2) Positive isolation (spade/spectacle blind or spool removal). 3) Double block and bleed (DBB). 4) Single valve only where risk is genuinely low and demonstrably tight.

Mark isolations on the drawing, nominate valve numbers, and write the proving method into the permit. Identify all potential backfeeds: UPS, shaft generators, cross-connection lines, returns, instrument take-offs.

4. Proving isolation: tests that mimic operating pressure

Double block and bleed, proven

Shut two valves in series with an open bleed between to a safe drain. Apply or allow full upstream pressure. Prove zero pressure at the bleed with a calibrated gauge and confirm no flow at the bleed for a stable period. For engine room liquids and air, five minutes minimum without rise is a sane baseline; extend for hot systems where thermal creep is likely. Record the gauge reading and time on the permit.

Electrical: lock, tag and try

Isolate, lock and tag. Prove your tester on a known live source, test all conductors to all others and to earth, then re-prove your tester. Expect 0 V AC and 0 V DC; treat anything above 50 V as live. Discharge capacitors on VFDs per OEM and confirm DC link below OEM-stated safe voltage before touch. Consider backfeeds from UPS, space heaters and control circuits; isolate neutrals where required by shipboard practice and confirm no induced or backfed voltage.

Hydraulic/pneumatic

Bleed accumulators to 0 bar at the manifold and confirm stable zero for at least five minutes with upstream pressure applied. Where decay testing is used, agree an acceptable decay rate in writing; as a guide, anything other than a flat zero in an isolated, bled leg is a fail until explained and risk-assessed.

5. Systems that often fool you

Sea water and cooling

Sea chests and overboard valves pass more than we admit. With 6–8 m head, expect 0.6–0.8 bar at the chest. If your downstream gauge creeps from 0 bar with the bleed open, the seat is passing. For sea chest work, fit a spade or a blind spool where practicable; lash and pin valves; keep the bleed open to a safe sump while the job is live.

Fuel and lube oil

Heated fuel expands. A deadleg between two tight valves will pressurise as it warms. Crack the bleed and keep it open to a recovery tank. Purifier inlet stop valves are notorious for seeping; DBB and verify cold and hot. Expect 6–8 bar on booster lines; any steady bleed suggests passing.

Steam and condensate

Steam stop valves that “feel” shut can pass enough to flash at an opened flange. Use DBB with a cracked drain; prove cold line and zero pressure at the drain before breaking. Watch for reverse flow via traps and bypasses.

Starting air

A passing starting valve or leaking non-return can move the engine on the turning gear. Isolate at the receiver, shut the engine branch, open and monitor the manifold drain for zero flow, and physically confirm the indicator cocks are open. Treat any hiss at the drain as a fail and investigate; don’t rely solely on the interlock.

Hydraulics and pneumatics

Pitch systems, stabilisers and ramps have accumulators and pilot-operated checks. Isolate, de-energise the pilot circuits, and bleed at the manifold until gauges are at 0 bar and stable with upstream pressure present. Movement from gravity or trapped loads can generate pressure; mechanically secure the load.

Electrical distribution

Backfeed via bus-ties, shaft generators, UPS and auto-changeover logic catches engineers out. Rack out and lock breakers, fit shutters, and test for dead on both sides. Be wary of space heater feeds and control supplies arriving via small fuses. Don’t trust neon pens; use an approved two-pole tester and the prove–test–prove method.

Fixed firefighting

Deluge and CO₂ systems must only be isolated under management control. Pilot lines and release circuits can hold energy. Where testing is authorised, use dummy loads and blanks per OEM; never rely on a single valve to prevent discharge.

6. Field techniques to detect passing early

Before you break a joint, look, listen and measure. Feel for temperature difference across a valve body. Listen with a stethoscope or ultrasonic probe for hissing past a seat. Use soap solution on air fittings. Fit temporary gauges upstream and downstream. Keep the bleed cracked and draining to a safe point while you watch for creep. If it creeps now, it will surge when you start work.

7. What ‘good’ looks like

A marked-up P&ID in the permit pack, isolation points verified by walk-down and labelled in situ. DBB with the bleed locked open to a safe drain. Zero pressure or zero voltage proven with calibrated instruments, recorded with time and signature. Remote starts defeated and tagged, local controls tried and found inoperative. Keys held in a controlled box; no master keys in the job area. A toolbox talk that explicitly covers how the isolation could fail and what the stop-work trigger is. Independent verification on high-energy jobs.

8. When an isolation fails: escalation pathway

If a bleed shows flow, a gauge creeps, or a circuit proves live, stop. Make the system safe: close the job area, reinstate guards, and withdraw people. Hold the permit. Notify the duty engineer and Chief Engineer. Add barriers: a third valve, a spade, or a breaker racked out at a higher level. Re-test from scratch. Update the risk assessment and permit, brief the team, and only restart when two competent people agree the isolation is proven. Record a near-miss and raise a maintenance defect for the passing component. Inform the Master for any critical system or delay to operations.

9. Case patterns and lessons learned

Steam branch flange burn: A fitter cracked a 7 bar steam flange after “closing” a single globe valve. No DBB, no drain open. The line had warmed by seepage and flashed condensate on opening. Lesson: DBB with an open, proven cold drain, and time to stabilise.

Bunker overflow: A crossover stop between port and starboard settling tanks passed during transfer. The “isolated” tank filled and overflowed to deck. Lesson: Positively isolate with a spade or remove a spool for transfers; verify levels and alarms, and monitor the “isolated” side.

Engine kick on turning gear: Starting air branch isolating cock wept. With indicator cocks open, the main started to move. The job stopped with no injury. Lesson: Drain and monitor the branch; if any hiss persists, strip and lap or fit a blank. Never rely on interlocks alone.

10. Auditing and continuous improvement

Sample permits weekly. Check that isolations are described, located on drawings, and proven with recorded values and times. Verify that DBB was used where required. Trend the number of failed isolations caught at proving stage; this is a healthy metric when it rises initially with vigilance. Calibrate testers and gauges and keep certificates accessible. Close the loop with toolbox talk feedback and post-job reviews.

11. Practical checks and expected values

Use OEM and class rules first. Where ship procedures allow, the following are pragmatic yardsticks:

  • DBB bleed: 0 bar at the bleed, no drip for at least 5 minutes under normal upstream pressure. Any rise or wetness is a fail.
  • Sea suction: with 6–8 m head, expect 0.6–0.8 bar at the chest; downstream gauge should remain at 0 bar with bleed open. No creep allowed.
  • Fuel booster lines: typical 6–8 bar; downstream 0 bar stable at bleed. Watch for thermal gain; maintain an open bleed to a safe tank.
  • Hydraulic circuits: 0 bar at the manifold and actuator ports, stable for 5 minutes. No rebound after cycling controls de-energised.
  • Starting air: 0 bar at engine branch drain with receiver charged; silence at the drain. Any hiss is a fail.
  • Electrical LV/MV: 0 V AC and 0 V DC between phases and to earth on the isolated side; for insulation checks, ≥1 MΩ to earth is a common minimum for shipboard 440 V circuits, but follow IEC/class and OEM limits. VFD DC link below OEM safe value before touch.
  • Steam: 0 bar and ambient temperature at the drain; drain stays dry with upstream live. Line remains cool to touch (with due caution) before breaking.

12. Regulatory and company requirements

ISM Code requires identification of hazards and establishment of safeguards; PTW and energy isolation are central to that. SOLAS demands operability and maintenance of valves and closures; poor isolations can lead to PSC deficiencies under ISM functional elements. OCIMF TMSA and SIRE focus on isolation verification and LOTO discipline; expect observations if DBB is missing on high-energy work. Electrical practices should align with IEC 60092 and flag requirements; the prove–test–prove principle is widely recognised by regulators (mirrored in UK HSE Electricity at Work guidance). Company procedures normally stipulate DBB or positive isolation for hot work, confined space entry and invasive maintenance.

13. Training the team

Drill prove–test–prove until it is habit. Walk junior engineers through P&IDs and the plant so they can spot backfeeds. Practise fitting spades and blanks. Coach the team to be suspicious of “quiet” systems and to keep bleeds open and visible. Reinforce stop-work authority when a gauge creeps or a hiss is heard.

14. Records and evidence

Attach marked drawings, photos of locks and blanks, and gauge readings to the permit. Note times and values when isolation was proven and re-proven after breaks. File defective component reports for passing valves. Capture lessons learned in the job close-out and feed them into your next toolbox talk.

15. Final reminders

  • Single valves are not barriers for high-energy work.
  • Always prove under expected differential pressure, not just at zero.
  • Backfeeds arrive quietly via small lines and control circuits.
  • Keep bleeds open and observable while work is live.
  • If anything creeps, you have not isolated. Stop and escalate.

Review questions

  1. Why can a valve that seems tight at zero pressure pass when the system is live?
  2. List three potential backfeed paths that can defeat an isolation plan.
  3. Describe the steps of a proven double block and bleed isolation.
  4. What is the prove–test–prove method and why is it necessary?
  5. How would you verify that a sea chest isolation is holding under static head?
  6. What additional control is required for VFD-fed motors after isolating the breaker?
  7. When working on a steam line, what indicates the isolation is effective before breaking the flange?
  8. What is an acceptable reading at a DBB bleed point and for how long?
  9. How can thermal expansion create pressure in an apparently isolated section?
  10. What escalation steps do you take if a bleed shows a steady drip?
  11. Give two reasons why a starting air system can move a main engine despite isolation.
  12. Which systems on your ship must never be isolated without management approval, and why?
  13. What minimum insulation resistance would you typically accept on a 440 V shipboard circuit, and what standard informs this?
  14. How do you ensure that a hydraulic actuator cannot move once isolated?
  15. What evidence should accompany a permit to demonstrate isolation quality?
  16. How would you adjust your proving time for hot systems and why?
  17. What common SIRE or PSC observations relate to isolation and LOTO?
  18. How do you train new engineers to recognise isolation failure modes?
  19. When is a single valve isolation acceptable, and what must you document?
  20. What is your ship’s process for reporting and rectifying a passing valve discovered during PTW?

Glossary

PTW (Permit to Work): Formal written authority to perform a specific task under defined controls.

LOTO (Lock Out Tag Out): Physical locking and tagging of energy-isolating devices to prevent operation.

DBB (Double Block and Bleed): Two isolation devices in series with a vented bleed between to prove tightness.

Backfeed: Energy arriving from an unintended route, often via returns, cross-connections or auxiliary supplies.

Accumulator: Device storing fluid energy, commonly nitrogen-charged in hydraulic systems.

VFD (Variable Frequency Drive): Electronic drive that can retain charge in DC link capacitors after isolation.

UPS (Uninterruptible Power Supply): Backup power source that can maintain voltage on circuits after main isolation.

Spectacle blind/Spade: Positive isolation device inserted between flanges to physically block flow.

Deadleg: Section of piping with trapped fluid and no flow path, prone to thermal pressurisation.

Prove–test–prove: Electrical safety method: verify tester on known live, test circuit, re-verify on known live.

Isolating cock: Small valve used to shut off a branch, e.g. starting air line to an engine.