A Technical Overview of Design, Safety, and Corrosion Considerations In Fire Water Systems

Firewater systems are critical safety infrastructures on offshore oil rigs, onshore coastal facilities and floating production, storage, and offloading (FPSO) vessels. Unlike inland-based systems that rely on municipal water supplies, offshore, vessels & coastal facility fire systems draw seawater directly from the surrounding environment. This requires specialised equipment and robust corrosion control strategies to ensure long-term reliability (and minimise potential critical safety risks). These safety systems are designed with multiple layers of defence to address a range of fire scenarios—from minor hydrocarbon spills to large-scale fires that can potentially threaten life and cause large scale environmental impact. The integrity of each component is vital to the system’s performance during emergencies.

Key Components of Firewater Systems

• Firewater Pumps: The heart of the system, these pumps draw in large volumes of seawater and pressurise it into the firewater ring main. Offshore facilities typically use two or more pumps for redundancy, powered by independent diesel engines or hydraulic units to ensure operation during power loss.
• Jockey Pump: A smaller pump that maintains system pressure during standby conditions, preventing unnecessary cycling of the main pumps.
• Pump Configurations: Options include submerged caisson-type pumps and dry-mounted diesel-hydraulic pumps, depending on platform design and operational requirements.
• Firewater Ring Main: A looped network of large-diameter piping that distributes pressurised seawater across the facility. Constructed from corrosion-resistant materials such as copper-nickel (Cu-Ni), glass-reinforced plastic (GRP), or titanium, the ring main ensures water delivery even if part of the system is compromised.
• Deluge Systems: Automatically activated by fire and gas (F&G) detection systems, these systems release water or foam through open nozzles to suppress fires in high-risk zones.

• Foam Systems: Used for flammable liquid fires, foam is mixed with water in a proportioning device and delivered via deluge systems or foam monitors. Fluorine-free foams (FFF) are increasingly adopted for environmental compliance.
• Monitors and Hydrants: High-capacity water cannons and strategically placed hydrants enable manual firefighting and localised response.

Emergency Operation of Firewater Systems

In the event of a fire, the firewater system is automatically activated through a sequence of coordinated actions:

• Fire Detection: Sensors within the integrated Fire and Gas (F&G) detection system identify heat, smoke, or flame signatures.
• Alarm and Response: Upon detection, the system triggers an alarm and initiates the firewater pumps, drawing seawater and pressurising the ring main.
• Deluge Activation: Deluge valves open rapidly, often before the pumps reach full pressure. The system is designed to accommodate this pressure surge to prevent pipework damage.
• Fire Suppression: Water, or a water-foam mixture, is discharged through nozzles or monitors to suppress the fire, cool surrounding equipment, and prevent escalation.
• Emergency Shutdown (ESD): In severe incidents, the ESD system isolates hydrocarbon sources and halts processing operations to contain the hazard.

The Importance of Fire-Safe Certified Isolation Valves

While pumps and piping are vital, the reliability of Isolation valves is equally critical. These valves must maintain integrity under extreme heat to ensure uninterrupted water delivery during a fire.

The reality is that some fire water systems were (are) built and specified with either rubber lined valves or non-fire safe certified valves meaning that either costs have been prioritised over safety, or criticalness of the application hasn’t been appreciated, and although the chances of an incident are small, the consequences could be fatal. Rubber lined valves are often specified as they can significantly reduce costs, cheaper body materials such as cast iron and carbon steel can be used as the rubber prevents the seawater contacting this, these valves will function correctly until an emergency occurs and the rubber melts or burns away. The valves are then rendered useless and will not perform the function they are needed to perform during this crucial time. This can lead to loss of system pressure, or leaks, meaning the relevant sections of pipe cannot be isolated, the water pressure isn’t high enough or the water cannot be directed to where is needed to extinguish the fire. This is not just for rubber lined valves, but it is also the same for other non-firesafe designed and certified Butterfly valves. 

Operators must ensure that the highest levels of safety are maintained and critical to this, it means ensuring the correct valves are specified for safety critical applications. These choices can stem from cost-saving measures or a lack of understanding of the application’s criticality. However, the consequences of valve failure during a fire can be catastrophic.

Fire Risks and Valve Failure Modes

Standard Isolation valves often use polymer-based components (e.g., PTFE seats and seals) that degrade at fire temperatures (750°C–1000°C). This can lead to:

• External Leakage: Loss of sealing allows pressurised water to escape, reducing firefighting effectiveness.
• Internal Leakage: Through-seat leakage compromises isolation, potentially flooding unintended areas.

Loss of Operability: Heat distortion can jam the valve, preventing manual or remote operation.

Fire-Safe Certification Standards

To mitigate these risks, valves must be tested and certified to recognise fire-safe standards:

• API 607: For quarter-turn valves with non-metallic seats.
• API 6FA: Covers metal-seated valves under fire conditions.
• ISO 10497: International fire type-testing standard.

Typical Fire Test Procedure:

1. Preparation: Valve is pressurised to 75% of its rated pressure and closed.
2. Fire Exposure: Valve is engulfed in flames for 30+ minutes at 750–1000°C.
3. Cooling and Inspection: Leakage is measured post-burn.
4. Operability Test: Valve must be cycled open and closed to confirm functionality.

Fire-Safe Valve Design Options

Several valve designs are available for firewater systems, each with distinct advantages and limitations:

Double Offset Valves

Firesafe certified Double offset valves, these are generally designed with a primary polymer seal and metallic backup metal seal, this means that in the event of a fire the PTFE seal can burn away or melt, and the metal seal will then come into contact sealing face; the risks are that if the polymer doesn’t fully carbonise or flow away from the secondary metal seal it risks becoming jammed between the two sealing faces and can stop the valve from closing fully, causing an internal leak. These valves have graphite packings and gaskets to prevent external leakage in the event of a fire.

Triple Offset Valves

This torque seated valve is the preferred option and provides metal to metal torque seating. This design uses metallic and graphite to provide leak tight sealing in the event of a fire. The negatives of this are that, when used in seawater applications, galvanic corrosion can occur causing premature failure of the valves, further details of what galvanic corrosion is and the issues are explained later in this article. There is however another option:

The OCT-SW Valve: A Graphite-Isolated Solution

Developed by Severn Glocon, Oblique Cone Technology is a patented triple offset valve design, enhanced further with graphite isolated parts for Sea Water service the OCT-SW valve addresses both fire safety and corrosion resistance:

By using the same principals as a standard Triple Offset valve, but enhancing this further with patented design technology and understanding the headaches galvanic corrosion can cause for operators. With the OCT-SW, all graphite components are isolated from contacting the line media during normal operation, eliminating the risk of galvanic corrosion. But as they remain within the valve design, they provide reliable sealing in the event of a fire.

The OCT-SW design uses a primary metal seal, and polymer back-up seal. By using the metal as a primary seal and torque seating the disc closed, it means that the metal seal does 99% of the valve sealing, meaning that the Polymer seal is only there to provide the last 1% and ensure full zero leakage isolation. What this means is that in the event of a fire, if the polymer, deforms, melts or carburises, the metal-to-metal seal still remains and provides a tight dependable seal.

Adopting a metal-to-metal torque seated design, means that even when exposed to the extreme heat of a fire the polymer seal cannot flow between the two metal sealing surfaces, therefore eliminating the risks that can be seen with Double Offset valves.

For Severn, safety is paramount which is why this valve includes enhanced safety which include dual anti blowout protection and fugitive emission certified packing as standard. The OCT-SW design combines the best aspects of Double and Triple Offset valves while eliminating their respective weaknesses.

Industry implications and conclusion

For industries with high fire risks, specifying fire-safe certified Isolation valves for firewater systems should be a non-negotiable best practice.

• Safety and reliability: These valves ensure the firewater system remains operational and can perform its critical function of extinguishing or controlling a fire.
• Regulatory compliance: Many regulatory bodies and industry standards, particularly in the oil and gas sector, mandate the use of certified valves for fire-prone areas.
• Asset protection and business continuity: Protecting the firewater system’s integrity directly protects personnel, high-value assets, and ensures business continuity.

By investing in certified, fire-safe Isolation valves, facility operators can be confident that their firewater system are a resilient and reliable defence against the worst-case fire scenario.