BS 6755 Valve Testing Standard: Explained for Engineers and Procurement Teams

BS 6755 (Testing of Valves) is a British Standard that defines mandatory production testing requirements for industrial valves before they leave the factory. It was one of the earliest valve testing standards and remains referenced on many UK, European, and international project specifications — particularly in oil & gas, power, water, and chemical plant construction. Understanding BS 6755 is essential for procurement engineers, inspection teams, and piping engineers specifying valves for British and international projects.

BS 6755 Structure: Two Parts

BS 6755 is divided into two parts, each covering a different aspect of valve testing:

• BS 6755 Part 1 (BS 6755-1): Specification for production pressure testing requirements — covers the shell (hydrostatic body) test and the backseat test
• BS 6755 Part 2 (BS 6755-2): Specification for fire type-testing requirements — covers fire resistance testing to ensure the valve maintains pressure containment after a fire

Part 1 is the most commonly referenced section in valve purchase orders and inspection test plans (ITPs). Part 2 applies specifically when fire-safe valves are specified (fire-safe ball valves, fire-safe butterfly valves, etc.) and is equivalent in intent to API 607.

BS 6755 Part 1: Shell Test (Hydrostatic Body Test)

The shell test (also called the body test or pressure-containing parts test) verifies the pressure integrity of the valve body, bonnet, and all pressure-retaining components. The test procedure per BS 6755-1 is:

• Test Medium: Water (or other liquid specified by the purchaser) at ambient temperature; pneumatic test with gas (compressed air or nitrogen) is permitted for specific valve types but requires additional safety precautions
• Test Pressure: 1.5 × the maximum allowable pressure (MAP) at 20°C — this is identical to ASME B16.34 Class rating × 1.5 at room temperature. For a Class 150 carbon steel valve (MAP = 19.6 bar at 38°C), the shell test pressure is approximately 29.4 bar
• Valve Position: Valve in partially open position (half-open) during shell test so fluid pressurises the full internal cavity
• Test Duration: Minimum 1 minute for valves DN ≤ 50 mm; minimum 2 minutes for DN 65 to DN 150; minimum 5 minutes for DN 200 and above
• Acceptance Criteria: No visible leakage through the body wall, welds, or pressure-retaining bolted connections during the test period

BS 6755 Part 1: Seat Leakage Test

The seat leakage test (also called the closure test or disc seat test) verifies the sealing performance of the valve closure element against the valve seats. The test parameters per BS 6755-1 are:

• Test Medium: Water (liquid seat test) at ambient temperature; low-pressure pneumatic seat test (air or nitrogen at 5–7 bar) may be used as an alternative or additional test
• Test Pressure: Liquid seat test — 1.1 × the maximum allowable differential pressure across the valve at the specified temperature; for most valves this equates to approximately 1.1 × ASME Class rating maximum pressure
• Valve Position: Valve in fully closed position; torque applied to the stem per manufacturer's specified operating torque
• Test Duration: Same as shell test — minimum 1 minute (DN ≤ 50), 2 minutes (DN 65–150), 5 minutes (DN ≥ 200)
• Acceptance Criteria: Leakage must not exceed the allowable rate specified in the standard — Table 1 of BS 6755-1 defines allowable drops per minute for liquid testing, based on valve size and seat type

BS 6755 Leakage Rates vs API 598 Leakage Classes

The most common question from procurement engineers is: how does BS 6755 seat leakage acceptance compare to API 598 leakage classes? The relationship is:

In practice, BS 6755 and API 598 have similar acceptance criteria for production testing — both require zero leakage for soft (elastomeric or PTFE) seats and permit a small allowable leakage for metal seats that increases with valve size. For soft-seat ball valves and butterfly valves, both standards effectively require bubble-tight closure. For metal-seat gate valves, globe valves, and check valves, the allowable leakage is size-dependent — always verify the applicable table in the project-specified standard.

BS 6755 Part 2: Fire Type-Testing Requirements

BS 6755-2 specifies the fire resistance requirements for quarter-turn and multi-turn valves. Fire-type testing involves subjecting a valve to a controlled fire exposure followed by pressure testing to verify the valve still maintains acceptable pressure containment and closure performance after fire damage to soft components (seats, seals, packing). The test sequence in BS 6755-2 is:

1. 1Pre-fire functional test — verify valve operates normally and seats meet leakage requirements
2. 2Fire exposure — valve exposed to flame at 650–1000°C for a specified duration (typically 30 minutes per BS 6755-2; API 607 similarly specifies 30-minute burn)
3. 3Post-fire pressure test — valve subjected to shell pressure test while still hot, and seat leakage test after cooling
4. 4Acceptance — no catastrophic failure; post-fire leakage must not exceed specified limits (typically higher than the pre-fire seat test to allow for fire damage to soft seats, relying on metal-to-metal secondary seat)

BS 6755-2 is essentially equivalent to API 607 in its requirements — most manufacturers who qualify to one standard will also test to the other. When a project specification says 'fire-safe to BS 6755-2 and API 607', both fire test certificates should be requested from the valve manufacturer.

Comparison: BS 6755 vs API 598 vs ISO 5208

What to Check When Ordering Valves to BS 6755

• Specify which part: 'Tested to BS 6755-1' (production test) or 'Fire-safe per BS 6755-2' (fire test) — don't simply say 'BS 6755'
• Confirm test medium: water hydrostat is standard; if your process handles gas only and you cannot tolerate water contamination, request a pneumatic alternative test and confirm the manufacturer can perform it safely
• Request test certificates: BS 6755-1 shell test certificate, seat leakage test certificate, backseat test certificate (if applicable for gate and globe valves) — all should bear the inspection agency stamp if TPI (Third Party Inspection) is required
• Fire-safe valves: request BS 6755-2 type test certificate plus test report from a recognised fire testing laboratory (e.g. Intertek, Bureau Veritas, TÜV) — not just a self-certified document
• Combined specifications: Many projects specify 'API 598 and BS 6755-1' — both are generally compatible and a valve manufacturer can issue certificates for both if the valve passes both standards' criteria

Vajra Industrial Solutions: Valve Testing and Certification

All valves supplied by Vajra Industrial Solutions are production-tested at the manufacturing facility and shipped with test certificates. Standard test documentation includes: hydrostatic shell test certificate (to BS 6755-1, API 598, or as specified), seat leakage test certificate, material test reports (MTRs / Mill Certificates), and dimensional inspection certificate. For critical service valves, Third Party Inspection (TPI) by Bureau Veritas, Lloyd's Register, DNV GL, TÜV, or client-approved agencies can be arranged. Fire-safe type test certificates (BS 6755-2, API 607) are available for applicable valve types.