Matt Byers, Baker Hughes, USA, outlines the engineering best practices used today to optimise the design of pressure relief valves operating under cryogenic conditions.
Pressure relief valves (PRVs) are a necessary requirement for overpressure protection within the LNG industry. However, not all PRVs are created equally when it comes to performance within cryogenic applications, and design temperatures as low as -320°F (-196°C). These applications require PRVs with enhanced sealing features to address the cryogenic conditions and perform their safety function.
PRVs are the last line of defence to protect equipment and personnel from an overpressure event. Construction materials, trim designs, and anti-galling measures are all critical components of a PRV design, as they help to address their challenging applications. Common PRV problems in these applications that need to be addressed include:
- Seat leakage: Thermal stress from low temperature causes material deflection. This deflection on a seating surface can result in leakage while the valve is closed, or immediately following a relief event.
- Galling of bearing/guiding surfaces: Anti-seize grease, commonly used in non-cryogenic applications to prevent galling, quickly deteriorates under cryogenic temperatures. This results in galling-induced wear between the metallic components, which leads to seat leakage, valve simmer, and ‘hang-up’ of guiding surfaces as the valve attempts to re-seat following a relief event.
- Process loss and fugitive emissions: Seat damage as a result of prolonged seat leakage, premature opening as a result of major seat leakage, or the PRV not fully closing after a relief event, due to excessive galling, can all lead to unwanted and costly release, or fugitive emission, of process fluid (Figure 1).
- End users have been implementing local fixes to reduce or delay the impact to their operation through reactive PRV maintenance. However, the severity of cryogenic requirements in the LNG industry extend further than the typical relief valve countermeasures that are commonly used today.
Reactive PRV maintenance
The traditional approach to managing PRV field issues has been a ‘good enough’ strategy of reactive maintenance. With reactive maintenance, most PRVs follow a regular service interval ranging between every six months to every three years, set by local site policies and best practices. While this approach may serve to maintain operations, it does not always properly analyse root cause failure modes and implement full corrective actions. Unfortunately, this approach has drawbacks due to the unpredictable nature of system operation and PRV performance status before and after a relief event.
This is an abridged version of an article that was originally published in the August 2020 issue of LNG Industry. The full version can be read here.
Read the article online at: https://www.lngindustry.com/liquid-natural-gas/19082020/dont-let-the-cold-catch-you-out/