Relief/Safety Valves & Burst Discs

3 January 2026 13 min read Aux Machinery, Engine Room, Mechanical

Design, selection, installation, testing, failure modes, and real-world accident lessons (merchant, cruise, fishing, mega yacht).

Introduction

Relief devices are the last engineered barrier between a pressure system and a rupture. On ships they protect:

People (steam / hot oil / refrigerant / CO₂ discharge injuries, blast effects)
Plant (boilers, compressors, receivers, hydraulic systems, heat exchangers, piping)
The ship (fires, explosions, flooding from ruptured cooling lines, loss of propulsion/power)
Class/flag compliance (survey items; missing/isolated devices are common detentions)

But they’re also one of the most misunderstood pieces of machinery on board: wrong set pressure, gagged/isolated valves, undersized discharge, “temporary” blanks, poor drain/condensate arrangements, corroded springs, and burst discs installed backwards or with the wrong rating.

Definitions and what each device actually does
Where ships use relief devices (system-by-system map)
Relief valve types (spring, pilot, thermal, P/V valves)
Burst discs (types, selection, and where they belong)
Set pressure, accumulation, blowdown, and backpressure (the math you must respect)
Sizing philosophy (steam, liquid, gas, two-phase)
Discharge piping design (reaction forces, noise, drainage, icing, safe location)
Marine-specific realities (motion, corrosion, vibration, fouling, salt, crew practice)
Vessel-specific guidance
- Mega yachts / superyachts
- Cruise ships
- Fishing vessels (incl. RSW / ammonia / freezing plant)
- Machinery space ventilation & crankcase / scavenge considerations
Testing and maintenance (what class expects vs what actually works)
Failure modes & troubleshooting (symptoms → root cause → action)
Accident abstracts (relief devices involved or safety function defeated)
Practical checklists (watchkeeper / chief / refit yard)
Glossary
Tags + SEO pack
Placeholders for diagrams, photos, animations

1) Definitions

1.1 Safety valve vs relief valve (practical shipboard distinction)

Safety valve: typically for compressible fluids (steam/air/gas). Characterised by rapid pop action to full lift.
Relief valve: typically for liquids (fuel oil, lube oil, hydraulics). Often modulating (opens proportionally).
In standards/spec sheets you’ll see “SRV” used loosely for both; on board, call them by service and function.

1.2 Pressure relief valve (PRV) vs pressure/vacuum valve (P/V valve)

PRV/SRV protects closed pressure systems (boiler, air receiver, hydraulic manifold).
P/V valve protects cargo tanks from overpressure and vacuum (loading/unloading, thermal breathing, inert gas operations). It is a tank safety item and interacts with vapour return systems, flame screens, and inert gas.

1.3 Burst disc (rupture disc)

A non-reclosing pressure relief device that ruptures at a defined differential pressure.

Very fast response, very tight seal (no simmer), no moving parts
But single-use, sensitive to installation errors, and requires disciplined spares/maintenance.

2) Where ships use relief devices (system map)

2.1 Steam & hot water systems

Boiler drum safety valves (main + auxiliary)
Superheater outlet safety valves (where fitted)
Economiser safety valves / relief arrangements
Steam reducing stations (downstream protection)
Calorifiers / domestic hot water (thermal relief)

2.2 Compressed air systems

Main / emergency air receivers
Control air systems
Compressor discharge (interstage reliefs; aftercooler protection)
Starting air lines (local reliefs depending on arrangement)

2.3 Refrigeration & HVAC plant

Condensers/receivers (HFC/HFO, CO₂, ammonia systems)
Chillers (titanium condenser circuits, seawater-cooled)
RSW/plate freezer plants on fishing vessels (often ammonia)
Relief discharge to safe location (very specific on ammonia)

2.4 Fuel oil, lube oil, hydraulic and thermal oil

Positive displacement pump discharge relief
Hydraulic power packs (servo systems, stabilisers, steering gear)
Thermal expansion relief (blocked-in sections)

2.5 Heat exchangers and coolers

Tube rupture and overpressure scenarios (especially when high-pressure side can pressurise low-pressure side)
“Blocked outlet” thermal expansion scenarios

2.6 Firefighting / CO₂ & gas systems (relief / burst discs may exist)

Cylinder safety devices, manifold protection, temperature-driven venting provisions (system dependent)

3) Relief valve types (and what actually fails at sea)

3.1 Spring-loaded (direct acting)

Most common onboard.

Pros: simple, robust, self-contained
Cons: sensitive to backpressure, corrosion, deposits, and poor discharge line design
Typical failures: set drift, seat damage → simmer/leak, spring corrosion, guide sticking

3.2 Pilot-operated

Used where tight sealing is needed at operating pressure, or large capacity.

Pros: less seat leakage, stable near setpoint
Cons: pilot contamination = no lift or nuisance lift
Marine risk: small pilot orifices + dirty systems = unreliability unless filtration is excellent

3.3 Thermal relief valves (TRVs)

Small valves protecting blocked-in liquid volumes from thermal expansion.
Common on fuel/hydraulic lines with isolation valves.

3.4 Pressure/Vacuum valves for tanks

Must match tank design pressure and vapour handling arrangement
Flame screens can foul; wrong maintenance causes restriction
Vacuum side is commonly neglected until tanks “pant” or deform

4) Burst discs in marine service

4.1 Disc types you’ll actually see

Forward acting (tension): dome bursts in tension
Reverse buckling: dome buckles then ruptures (often better cycle life)
Composite / scored: more accurate burst pressure, predictable opening area
Graphite / metal depending on corrosion and temperature

4.2 Where burst discs make sense onboard

Refrigeration: as secondary protection or where tight sealing is critical
Instrumentation: protecting pressure sensors from spikes
Hazardous/contaminating fluids: where a leaking PRV is unacceptable
As PRV isolation: disc upstream of PRV to keep valve clean (only if engineered correctly—see below)

4.3 Burst disc + PRV in series (common trap)

If you install a disc upstream of a PRV:

You must manage the space between disc and PRV (tell-tale gauge/vent)
Any pressure build-up there changes the effective setpoint and can prevent correct relief.
This configuration can be excellent or a silent defeat of the protection if done wrong.

5) Set pressure, accumulation, blowdown, backpressure

5.1 Set pressure

The pressure at which the device begins to open (or disc bursts). Must be:

≤ MAWP (maximum allowable working pressure) of the protected equipment
Set considering static head, pressure drops, and normal operating margin

5.2 Accumulation (overpressure during relieving)

When the system is in a credible “worst case” scenario, pressure may rise above set while flow stabilises. That permissible rise is defined by applicable rules/standards and equipment design.

5.3 Blowdown

For spring safety valves: difference between opening pressure and reseat pressure.

Too small → chatter
Too large → long pressure decay, process upset

5.4 Backpressure (the killer onboard)

Superimposed backpressure: already present in discharge system
Built-up backpressure: created by flow through discharge piping
Too much backpressure causes:
reduced capacity
unstable opening
delayed closing or continuous simmer
setpoint shift

Ship reality: long discharge runs “to a safe place” + small pipe + poor supports = unreliable valves.

6) Sizing philosophy (what engineers get wrong most often)

Relief sizing is scenario-driven. You size for credible worst-case events, not average operation.

6.1 Typical shipboard credible scenarios

Fire exposure (external heating of vessel)
Blocked outlet / downstream valve closure
Control valve failure open
Tube rupture (HP → LP side)
Thermal expansion of blocked-in liquid
Pump deadhead (PD pumps especially)
Runaway heating (steam coil stuck open)

6.2 Steam vs liquid vs gas vs two-phase

Steam/gas: compressible, choked flow possible, requires discharge design attention
Liquid: incompressible, big forces, risk of water hammer
Two-phase: can be brutal; if you guess wrong you grossly under-size

Diagram placeholder: “Relief sizing decision tree (steam/gas/liquid/two-phase)”
[INSERT DIAGRAM: mh_relief_sizing_tree.svg]

7) Discharge piping design (where installations become dangerous)

7.1 Reaction forces

A lifting safety valve can generate huge thrust loads. Discharge piping must be:

adequately supported
expansion accommodated
not used as a “handle” that loads the valve body

7.2 Drainage and condensate

Steam relief discharges can fill with condensate. On lift:

slugging → violent forces
delayed lift behaviour
severe vibration

7.3 Safe discharge location

Away from walkways and workstations
Consider wind/stack effects; don’t route into intakes/AC inlets
Hot discharge must not impinge on insulation, cables, or personnel

7.4 Noise and fatigue

Discharges can exceed safe noise levels quickly; fatigue cracking of brackets is common.

Photo placeholder: typical poor discharge support with cracked bracket
[INSERT PHOTO: srvs_discharge_support_failure.jpg]

8) Marine-specific realities (why “textbook PRV” isn’t enough)

Salt + humidity: external corrosion, seized adjusters, spring degradation
Vibration: set drift, cap loosening, pilot line failures
Crew practice: “temporarily gagged,” isolation valves left shut, missing seals
Fouling: soot, oily aerosols, scale
Space constraints: long discharge lines, sharp bends, shared headers
Class survey cycles: valves can be “maintained on paper” yet fail in real duty

9) Vessel-specific coverage

9.1 Mega yachts / superyachts (comfort + silence + access reality)

Typical architecture: chilled water plant + FCUs/AHUs, high hotel loads, tight noise limits.

Relief/burst disc hotspots:

Chiller/receiver relief arrangements (often in crowded machinery spaces)
Seawater condenser fouling → high head pressure → nuisance trips/relief events
Quieting measures leading to undersized discharge lines (don’t do this)
Tight joinery means access panels must be planned or maintenance is skipped

Operational guidance:

Treat HVAC/chiller relief devices as charter-critical: nuisance trips lead to “adjusting” setpoints—this is a serious governance risk.
Verify discharge doesn’t recirculate into ventilation intakes (common on yachts due to compact routing).

Animation placeholder: chiller head pressure rising with fouled condenser → relief path
[INSERT ANIMATION: yacht_chiller_overpressure_path.json]

9.2 Cruise ships (scale + redundancy + public safety)

Cruise ships run massive “hotel plants” with:

multiple chillers (often staged)
extensive air handling
huge domestic hot water systems
complex steam/hot water networks depending on design

Relief priorities:

Redundancy philosophy: relief devices are not redundancy; they’re last resort. Don’t let “we have 3 chillers” justify poor protection.
Crowded technical spaces: discharge routing must consider public/crew areas and air intakes.
Testing discipline: large fleets trend toward condition monitoring; relief devices still require physical verification and correct certification.

9.3 Fishing vessels (RSW, freezing plants, ammonia/CO₂ trends)

Fishing vessels are special because refrigeration can be the business.

RSW systems and factory plants introduce:

large refrigeration capacities
long operating hours in harsh conditions
heavy vibration (trawling)
rapid loading/freezing cycles → pressure transients

Relief/burst disc hotspots:

Receiver relief sizing vs rapid heat loads
Ammonia systems: relief discharge handling is a major safety design item
CO₂ systems (emerging): high pressures demand impeccable disc/valve discipline

Chief’s practical note:
On fishing vessels, relief devices often fail indirectly: not because the valve is “bad,” but because the system is operated in transient modes (sequential freezer starts, rapid pull-down) that were never matched to the installed relief scenario basis.

9.4 Machinery spaces (including ventilation interaction)

Overpressure protection isn’t isolated from ventilation realities:

A relief discharge can heat/contaminate adjacent intake air paths
A high-rate gas discharge into a confined machinery space can create oxygen displacement risks (depends on medium)

Engine-room specific applications:

Starting air receivers and compressor stages
Fuel oil service systems (PD pumps)
Hydraulic packs (steering gear, CPP, stabilisers)
Steam/hot water lines for heating and services

Diagram placeholder: “Machinery space pressure systems map and relief locations”
[INSERT DIAGRAM: eng_room_relief_map.svg]

10) Testing and maintenance (what actually works at sea)

10.1 Bench testing vs in-situ reality

Bench test confirms set pressure and blowdown in clean conditions. Onboard reliability needs:

correct discharge design
correct installation orientation
no isolation defeat
clean process medium reaching the valve

10.2 What to inspect routinely (crew-level)

Evidence of leakage/simmer (salt tracks, staining, unusual noise)
Missing seals, tamper marks, caps loosened
Isolation valves: position, locking, signage
Discharge line: supports intact, drains clear, no water pockets
For burst discs: correct tag/rating, installed direction, no gasket creep, no corrosion

10.3 Class/flag readiness pack

Certificates and set pressures (as-installed list)
Cause-and-effect (for any monitored plant)
Discharge drawings (especially if modified)
Spare discs (correct rating + material) and gaskets

11) Failure modes & troubleshooting (fast diagnostic table)

Symptom: PRV lifting at normal load

Likely causes: wrong setpoint, backpressure, thermal expansion event, control failure, fouled condenser (HVAC), stuck valve
Actions: verify pressures with calibrated gauge, check discharge restrictions, confirm recent changes, inspect seat

Symptom: PRV never lifts but system overpressures / trips

Causes: gagged/isolated, seized spindle, pilot blockage, disc installed wrong rating, discharge blocked creating backpressure-induced nonperformance
Actions: stop and make safe, verify physical configuration, check isolation valves, consult certificates, function test under controlled conditions

Symptom: chatter (hammering) on lift

Causes: excessive backpressure, undersized valve, two-phase flow, liquid relief in gas-rated valve, poor inlet piping
Actions: don’t “live with it”—chatter destroys seating; revisit installation and scenario basis

Table placeholder: full troubleshooting matrix (20+ faults)
[INSERT TABLE: relief_devices_fault_matrix.csv]

12) Accident abstracts (relief devices involved or protection function defeated)

Case A — Tank explosion sequence where tank protection and vapour/inerting management were critical

Abstract (engineering take-away): A tanker explosion sequence illustrated how rapidly tank conditions can transition from “normal operations” to catastrophic when vapour control and safety barriers fail or are overwhelmed. For engineers, the key lesson is that tank pressure protection (P/V arrangements), vapour management, and inerting integrity must be treated as an integrated safety function rather than separate systems. Post-event learning across industry emphasised verification of protective devices, correct operational configurations, and the dangers of latent unsafe states developing unnoticed. dieselduck.info

Case B — Offshore incident where a burst disc / safety device did not perform as expected

Abstract (engineering take-away): An offshore incident report described a scenario where a burst disc within a pressure protection arrangement did not function as intended, underlining a classic vulnerability: burst discs are reliable only when correctly selected, correctly installed, and correctly monitored (including management of any trapped volume between devices, and strict control of spares/rating). The incident reinforces that a “sealed” protection device can quietly become the weak link if installation orientation, rating, or inspection regime is wrong. imcaweb.blob.core.windows.net

Case C — Accidental release event highlighting the hazard of high-rate gas discharge into enclosed spaces

Abstract (engineering take-away): A casualty investigation into an accidental fixed-gas release highlighted the lethal nature of rapid gas discharge into ship spaces, especially where access/escape is limited and crew are unprepared. Although not a classic “overpressure relief” event, the engineering relevance is direct: any relief or safety device that can discharge large volumes (CO₂, refrigerant, compressed gases) must be routed, interlocked, and procedurally controlled so the protective action does not create a secondary fatal hazard. dieselduck.info

13) Practical checklists

13.1 Watchkeeper “daily eye” (5 minutes)

Any valve weeping/simmering?
Any discharge line vibrating, cracked brackets, missing supports?
Any relief discharge pointing into a walkway / intake?
Any isolation valve found shut (or not locked as per procedure)?
Any abnormal compressor head pressure trends (HVAC/fishing plants)?

13.2 Chief Engineer “monthly discipline”

Cross-check certificates vs actual installed tags and set pressures
Verify locked open isolation valves (where fitted)
Confirm drains on discharge legs are clear
Spot-check calibration of local gauges used to “judge” relief behaviour
Review any nuisance lifts: treat as defect investigation, not annoyance

13.3 Yard/refit acceptance (relief devices)

Relief scenario basis documented (what events were sized for)
Discharge routing signed off for: forces, drainage, safe location, intakes, noise
Access panels provided (or maintenance will not happen)
Burst disc spares list delivered (correct rating/material/orientation notes)

14) Glossary (shipboard-focused)

MAWP: Maximum allowable working pressure
Set pressure: Pressure where device begins to open / disc bursts
Blowdown: Reseat differential for a safety valve
Backpressure: Pressure opposing valve discharge (superimposed + built-up)
Chatter: Rapid opening/closing that destroys seats and stability
Thermal expansion relief: Protection for blocked-in liquids heating up
P/V valve: Tank pressure/vacuum valve (cargo tank protection)

15) Tags + SEO pack

Title (SEO): Relief / Safety Valves & Burst Discs on Ships: Design, Testing, Failure Modes, and Accident Lessons
Slug: relief-safety-valves-burst-discs-shipboard
Meta description: Chief-engineer level guide to shipboard relief valves and burst discs: types, sizing philosophy, discharge design, testing regimes, common failure modes, and accident lessons for yachts, cruise ships, fishing vessels, and engine rooms.
Tags: #HVAC #Refrigeration #Boilers #CompressedAir #Hydraulics #SafetyValves #ReliefValves #BurstDiscs #MarineEngineering #ClassSurvey #SOLAS #PipingSystems #FishingVessels #Superyachts #CruiseShips

16) Media placeholders (copy/paste)

[INSERT ANIMATION: burst_disc_reverse_buckling.json]

[INSERT DIAGRAM: relief_valve_types_overview.svg]

[INSERT DIAGRAM: backpressure_effect_on_safety_valve.svg]

[INSERT DIAGRAM: burst_disc_installation_orientation.svg]

[INSERT PHOTO: corroded_safety_valve_spring.jpg]

[INSERT PHOTO: discharge_line_poor_support.jpg]

[INSERT ANIMATION: safety_valve_pop_action_lift.json]