Fire-Safe Ball Valve with Metal Secondary Seal for NGL Loading Racks

Description

In the storage, transport, and loading/unloading of natural gas liquids (NGL), liquefied petroleum gas (LPG), refined products, and chemical feedstocks, one of the gravest operational risks is an unexpected line leak that ignites. Once a fire starts, conventional soft‑seated ball valves with PTFE or nylon seats typically burn and char within minutes, causing total loss of sealing. This allows large volumes of flammable media to continue escaping, rapidly escalating the blaze. The fire‑safe ball valve offers an engineered safeguard through its dual‑stage sealing design: when the primary soft seal fails under fire exposure, a metal secondary seal built into the seat back‑up automatically engages, establishing metal‑to‑metal contact and limiting leakage to a safe level—buying critical time for personnel evacuation and firefighting efforts.

Construction and Post‑Fire Metal Sealing Mechanism

The defining feature of a fire‑safe ball valve lies in its seat’s two‑layer sealing configuration. Under normal operation, the forward PTFE (or reinforced PTFE) seat is pressed against the ball by spring preload and line pressure, achieving zero‑leakage shutoff. At the rear of the seat, a metallic lip—typically made of Inconel 625, 316L stainless steel, or Stellite—is embedded along with a flexible graphite ring. This secondary element remains inactive while the PTFE seal is intact. When an external fire raises the cavity temperature above approximately 250 °C, the PTFE begins to soften and decompose.

As the temperature climbs to 650–850 °C, the PTFE fully chars and shrinks in volume. At this point, the spring‑loaded metallic lip advances forward, contacting the ball surface directly. Simultaneously, the compressed graphite ring fills the clearance between the ball and seat, forming a composite metal‑plus‑graphite barrier. This post‑fire seal can withstand a hydrostatic test at no less than 80 % of the valve’s rated design pressure.

The stem incorporates a blowout‑proof thrust design, and an anti‑static grounding device (ball → stem → body) is fitted at the bottom to prevent static‑discharge ignition of any leaked medium. Fire‑safe ball valves must pass rigorous fire‑testing per API 607 or ISO 10497: after 30 minutes of exposure to an 800–1000 °C flame, the valve must still maintain an acceptable leakage rate (typically ≤ 50 mL/min, depending on size and pressure class) at 1.1 times the design pressure.

Key Technical Specifications

Typical Applications

• Main headers at NGL and LPG storage terminals, truck loading racks

• Heater inlet/outlet and reactor feed/effluent lines in refineries

• Compressor suction/discharge at gas‑compressor stations on long‑distance pipelines

• Loading arms and tank inlet/outlet at LNG receiving terminals

• Flammable‑media piping on offshore platforms and FPSO decks

• Process lines handling Class A and Class B flammable liquids and gases in chemical plants

Case Study – Enterprise Products Partners, Mont Belvieu, Texas

At Enterprise Products Partners’ NGL terminal in Mont Belvieu, Texas, all block valves on the propane loading rack (DN80, Class 300) are specified as fire‑safe. In June 2021, a static‑discharge flash fire occurred during a truck loading operation. The flame impinged on the fire‑safe ball valve at the loading arm connection for approximately three minutes before being extinguished by handheld fire extinguishers. Although the valve was positioned about 1.5 m directly above the ignition source, post‑incident inspection revealed that while the PTFE primary seat had lost resilience and become charred, the Inconel 625 metallic lip and flexible graphite ring had successfully engaged, forming a stable metal‑to‑metal secondary seal.

No external leakage was observed despite the valve cavity being filled with propane vapour. The valve was returned to the manufacturer for a repeat fire‑test, which confirmed that the secondary metal seal remained intact and functional. After replacing the primary seat, the valve was returned to service. This incident was featured in Enterprise’s internal safety bulletin as a prime example of how a fire‑safe valve effectively mitigated the consequences of a fire.

Selection Guidance

When specifying a fire‑safe ball valve, pay particular attention to these four points:

1. Fire‑test standard version – API 607 has been updated to the 7th Edition (2016), which imposes stricter requirements (including cycling under elevated temperatures). Always verify that the supplier’s fire‑test report is based on the current valid edition, not outdated or non‑standard tests.

2. Secondary seal material selection – Inconel 625 offers excellent high‑temperature strength and oxidation resistance, making it suitable for fire scenarios above 650 °C. 316L stainless steel is more economical and works for temperatures up to 500–600 °C. The purity of the graphite ring (≥ 98 %) directly affects the reliability of the secondary barrier.

3. Anti‑static grounding verification – After installation, measure the grounding resistance from the ball to the valve body’s earth terminal; it must not exceed 10 Ω per API 608. If necessary, add an external bonding jumper.

4. Operating torque considerations – After a fire event, engagement of the metal secondary seal may increase the valve’s operating torque. When selecting an actuator, reserve an extra 20–30 % torque margin to ensure the valve can still be operated post‑fire.