Cause & Consequence
- Uncontrolled release of a flammable gas or liquid
Description of Process
The 3rd stage compression system comprises of 2x 100% trains designed to handle 85.3 MMSCFD of gas.
Description of Incident
During routine plant maintenance routine function testing of SDV, central control room informed the instrument technician that valve had not met its function stroke limits. Valve has been in service since 1992.
Upon investigation, an instrument technician heard audible noise coming from the grease injector fitting on the valve stem. The system was then shut down and isolated for further examination.
The valve was removed from service and sent to vendor to simulate the exact site process test conditions. During the test the “official leakage rate” was recorded from the grease injector fitting, which indicated failure of lower stem seal, allowing gas to pressurise behind the grease injector fitting.
The valve was then stripped down and the valve components were found to be in good condition. However, closer inspection indicated that the lower stem seal had ‘collapsed’ and the grease injector NRV check-valve exhibited corrosion on the check-valve seating faces.
A full engineering report was compiled by the vendor, which indicated the root cause as being “injection of grease in a reverse flow direction, causing the lower seal lip seal to collapse”. Secondary component failure of the grease injector fitting found “Corrosion on the check valve seating faces, probably due to the length of time in service, and the environment location of the valve”.
Post Incident – ALL valves in hydrocarbon service were inspected (visual & FLIR Camera) and checked for signs of leakage through injector fittings.
Good Practice Guidance
The valve had historically been lubricated with grease through the stem injector fitting to aid functioning performance of the valve. This fitting had been wrongly assumed to be a grease injector fitting. In fact its purpose is a stem sealant injector fitting (where the valve is injected with sealant in the event of a lower stem seal failure). Therefore, greasing the cavity between upper and lower stem seal, has caused the collapse of the lower stem lip seal, causing gas to migrate up to the injector fitting.
Stop all greasing in valve stem seal cavities and plug injector ports. (Ensuring plugs are compatible material with valves).
Determine an appropriate course of action to complete a visual survey of gas hydrocarbon system valves greater than 2” this inspection should determine the following:
a. Confirm the existence sealant injector fittings.
b. Confirm presence of leaks at these fittings via FLIR camera inspection.
c. Report presence of leaks to allow each site to assess the suitability for continued use. (As per the requirements of companies weeps, seeps & leaks procedure). Maintain valve register for injector fitting installed .This register should then uploaded into your maintenance management system.