Pressure Seal Bonnet Leakage (Class 900 and Above)

Root Causes

Step-by-Step Repair Guide

1. 1

   Confirm the leakage location and pressure seal geometry

   First, confirm that the leak is at the body-bonnet pressure seal interface (not at the gland packing — which is a different issue). The pressure seal interface is typically 50–100 mm above the valve body flange, at the bonnet-body junction. Look for: weeping at the junction between the outer body wall and the bonnet, or at the bonnet retaining ring. Confirm the leak rate increases with pressure — this distinguishes a pressure seal failure (self-energised, so increases with pressure) from a bolted gasket failure (which is pressure-independent up to the bolt load).

   API 600 and ASME B16.34 require that pressure seal ring seating load is a fraction of the system pressure multiplied by the effective ring area — the ring is designed to be loaded harder by higher pressure. A leak that gets worse at higher pressure is therefore diagnostic of ring failure, not bonnet ring displacement.

2. 2

   Depressurise, drain, and cool the line — do not loosen the retaining ring under pressure

   Pressure seal valve disassembly under pressure is extremely dangerous. The bonnet is retained by a ring threaded into the body, or by a split retaining ring. If this is loosened under pressure, the bonnet will be ejected by the stored fluid energy. Full LOTO, verified zero pressure on both sides, and temperature below 60 degrees C are mandatory before any disassembly. Drain the body cavity through the body drain connection (if present) before loosening the retaining ring.

   Class 1500/2500 valves contain enormous stored energy at rated pressure. A 6-inch Class 2500 valve at 420 bar: the stored energy in 1 litre of fluid is approximately 42 kJ — sufficient to cause fatal injury if released suddenly.

3. 3

   Remove the bonnet and extract the pressure seal ring

   Unscrew or extract the bonnet retaining ring (follow the manufacturer's specific procedure — some require a special spanner, others use a yoke extractor). Lift the bonnet vertically from the body. Extract the old pressure seal ring. Inspect: (1) Ring deformation — measure ring height against the nominal. More than 1 mm compression indicates over-loading in service. (2) Ring surface — look for radial cracks, fragmentation, or material extrusion. (3) Body bore sealing surface — dye penetrant or bluing check for scratches, corrosion pitting, or out-of-round distortion.

4. 4

   Prepare the body bore and install new pressure seal ring

   Clean the body bore sealing surface with a lint-free cloth. Polish any light surface scratches with 400-grit emery paper followed by 600-grit — the seal surface must be smooth (Ra < 1.6 micrometres). Apply a thin film of high-temperature anti-seize (never general-purpose grease) to the ring seating surface. Install the new pressure seal ring with the correct orientation — the ring shoulder typically faces downward (toward the process side) so that process pressure acts on the back face. Confirm with the valve manufacturer's assembly drawing. Lower the bonnet carefully — it must enter the body bore concentrically to avoid cocking the ring.

   On valves with graphite pressure seal rings: the ring must be a new ring from the original valve manufacturer or an approved OEM source. Do not re-use the old ring even if it looks intact — a used ring will not achieve the initial interference fit and will leak at the first pressure test.

5. 5

   Reassemble retaining ring and hydrostatic test

   Engage the retaining ring fully (specified torque or turns per manufacturer's data sheet). On large valves (NPS 6 and above), a torque multiplier is typically needed to achieve full engagement load. After full engagement, perform a shell (body) hydrostatic test at 1.5x design pressure for 10 minutes (API 598 / ASME B16.34 test pressure for pressure seal valves). If there is any leakage at the pressure seal interface during the test, the ring is not correctly seated — full disassembly and reinstallation are required. Do not attempt to tighten the ring further to stop a leak during testing.

Applicable Standards

Related Products

Key Terms Explained

Unfamiliar with any terms used in this guide? Each links to a full engineering definition.