Stringent Anti-Surge Valves specifications can pose challenges for larger valves on LNG liquefaction trains. But Severn’s bespoke Anti-Surge Control Valves ensure efficiency and compliance.

LNG compressor manufactures are placing increasingly stringent performance requirements on Anti-Surge valves, and specifications are often written without considering valve size. Yet while the standards are easily achievable for smaller valves, they become significantly more difficult as valve travel (the distance the plug moves) increases. Since specifications are primarily based on safety and efficiency requirements, any deviations introduce risk; concessions are not generally accepted.

Technical challenges for large Anti-Surge valves

Fundamental principles of physics make it harder for larger valves to achieve the optimised, highly efficient control required in Anti-Surge applications. The weight of the valve plug and the long linear stroke of the actuator introduce mechanical inertia and lag, which can result in a slower or more erratic valve response. Consequently, to ensure safety, end users have traditionally had to tolerate a conservative approach to surge control when using larger valves. This compromises overall system efficiency.

Severn’s Anti-Surge Control Valves are engineered to overcome these challenges, and we recently put a 30” test specimen through its paces with dynamic testing. Results prove that even large valves commonly found on LNG mega trains can fully comply with rigorous compressor manufacturer specifications.

Dynamic testing explained

Dynamic testing for Anti-Surge valves determines how a valve performs when subjected to operational parameters that will be encountered on the liquefaction train. Speed, precision and stability are the critical features evaluated.

Assessing speed of response

Anti-Surge valves must react quickly via automatic control when an imminent surge event is detected, and the control signal is activated.

Compressor manufacturers’ dynamic testing parameters specify a maximum time from cut off (i.e. when the valve is fully closed) to fully open. This is typically around 2 seconds. Another specified parameter is the time taken for the valve to move from cut off to 80% open. The target timeframe for this is generally 1.5 seconds.

Opening time specifications are inclusive of dead time (the delay between the control signal being sent to the valve and the beginning of the valve response). It’s common for dead time to be allocated its own required value. For instance, the preferred time might be 0.3 seconds and the maximum allowable time 0.5 seconds.

Validating precision control

When a valve’s ability to quickly achieve the ‘fully open’ position is critical for safety, it is only necessary in emergency surge prevention situations. Much of the time a surge event can be avoided via carefully modulated control of the Anti-Surge Valve. In fact, opening too far may result in an excessive amount of compressed gas being recycled, which is wasteful and harms efficiency.

That’s why dynamic testing also assesses a valve’s ability to achieve precise movements without too much overshoot (when the valve moves more than required) or stiction (when plug travel is jerky due to friction). Proving that any overshoot or stiction are minimal provides assurance that the valve can achieve accurate, precision control.

Take the Baker Hughes dynamic testing specification. This compressor manufacturer describes one test for valve moment involving continuous changes in the control signal from ‘98% opening’ to ‘2% opening’ over a five-minute period. Stiction of less than 1% is preferred, and the maximum allowable stiction is 1.5%. In other words, the test looks to identify whether friction causes any significant deviation between input signal and the valve’s position.

Demonstrating Stability

As well as maximising efficiency, smooth operation of Anti-Surge Control Valves is vital to avoid compressor surge being caused inadvertently.

Oscillation (repeated overshoot and correction), or the use of extra force to overcome stiction, can destabilise the compressor, pushing it into surge rather than avoiding it. What’s more, a valve that responds too aggressively to small, high-frequency signal variations may be overly sensitive to noise and mechanical vibrations in nearby equipment, including the compressor. If the valve moves unexpectedly and unintentionally due to environmental factors, resultant pressure fluctuations could be hugely detrimental to compressor performance and the wider liquefaction process.

Frequency response testing plays a critical role here. It shows how quickly the valve opens and closes in response to control signals at different frequencies, identifying whether it overshoots or oscillates. Tests demonstrate the sensitivity and linearity of the valve and actuator assembly, showing whether they respond accurately to small control inputs.

Bigger valves, better control

The 30” Anti-Surge Control Valve demonstrated exceptional speed, precision, and stability in dynamic testing. Results show that our bespoke valve design coupled with sophisticated actuation allows large valves to operate reliably and safely within a centrifugal compressor’s maximum efficiency zone.

Severn’s experience supplying technically challenging control valves for critical equipment enables us to meet customers’ safety and efficiency requirements without compromise. Check out our previous articles which look specifically at the customisation of valve trims and control optimisation to facilitate this high level of performance. Or contact us to find out how valve engineering expertise can benefit your LNG project.