Large LNG compressor problems require large Anti-Surge Valves which in turn present complex engineering challenges. Valves with shorter travel (the distance the plug moves) and suboptimal control compromise the efficiency and reliability of the compressor. But traditionally, it’s difficult to control valves with a long stroke involving more than 12” of travel.
With extensive experience developing Anti-Surge Valves, Severn Valves resolves these challenges. Our expert valve selection, integration, setup and calibration enable large valves with up to 24” of travel to meet stringent performance and safety specifications.
Severn’s Anti-Surge control valves act extremely fast – within a two-second window – when process upset is detected on an LNG liquefaction train.
To facilitate a rapid inflow of surge gas, these valves typically have an oversized capacity of 1.8-2.2 times the maximum volume they are likely to handle. However, this important safety feature has implications for valve size, weight, and travel.
It is harder for large Anti-Surge valves to achieve the balance of speed and precise control required for compressor protection and optimal flow management. To compensate for this, LNG surge management systems involving large valves are often configured so that valves are activated at a lower threshold. This approach ensures plant safety, but it can compromise plant efficiency.
The whole valve package: focusing on the trim, actuator, and positioner improves operability
Severn’s proven Anti-Surge Control Valve technology takes a complete, highly engineered approach to overcome performance challenges for optimally selected valves where travel exceeds 12”.
Our bespoke valve trims for precise pressure let down, are integrated with sophisticated, carefully selected actuators and positioners. The impressive power and controllability of the actuation package is an integral feature of our valves. Precision control is enabled using high-power piston actuators which ensure a rigid and robust instrumentation setup. This enables us to implement a longer stroke – giving better control of the surge management process – while meeting the application’s exacting requirements for speed and accuracy.
The entire valve assembly is expertly integrated and calibrated as a complete system to deliver optimum performance. Valves are tested using a Profiler tool which measures and validates performance according to the specific Anti-Surge specifications. The tool is independently calibrated, ensuring valve performance is verified against end user standards without concession.
Our experienced valve engineers leverage test outcomes to fine-tune the actuator, positioner and boosters. Treating the Anti-Surge solution as a system of carefully selected, finely orchestrated components facilitates optimised, compliant valve performance.
Control optimisation boosts LNG productivity and efficiency by reducing the LNG Compressor Problems
As the below graph indicates, when compressor Anti-Surge Valves are primed for speed, accuracy, and reliability, liquefaction trains can operate within the compressor’s maximum efficiency zone. The entire LNG plant benefits from this. Controlling surge quickly and safely without it escalating into a major event allows operators to maintain higher liquefaction flow/pressure rates which boost overall productivity.
Severn’s Anti-Surge Control Valve technology enables this balance of speed and accuracy to be achieved consistently, even with large valves. With our ongoing commitment to R&D, Severn carried out stringent Anti-Surge specification testing on a valve with a nominal bore of 30” and a linear stroke of 24”. It performed the full 24” stroke in less than two seconds and achieved incredibly precise positional control ensuring specifications can be met without concessions on all valve sizes.
With their direct influence on safe, economical plant performance, compressors are a vital component of the LNG liquefaction train, supporting broader LNG reliability and uptime. Protecting them from the ‘surge phenomenon’ helps avoid costly damage and unplanned downtime, reducing emergency shutdowns and safeguarding production targets.
Centrifugal compressors are critical to LNG production, raising the pressure of feed gas so it meets the thermodynamic requirements of liquefaction. Highly engineered and complex in nature, they are also expensive – typically costing millions of dollars, making protection of these high‑value assets essential to total cost of ownership.
Under normal process conditions, the impeller within a centrifugal compressor rotates at high speed accelerating the flow of the feed gas. It’s important that the inlet flow rate is sufficient to maintain this forward movement and keep the gas flowing along the liquefaction train. However, changing process demands across the wider system can cause fluctuations, bringing the need for extremely fast and accurate mitigation measures, including fast‑acting anti‑surge control and real‑time protection.
What is ‘compressor surge’ and why must it be controlled?
The surge phenomenon occurs when inlet flow stalls because it is insufficient to overcome the high pressure at the compressor’s discharge point. This results in a reversal of flow which can trigger a surge cycle. Compressed gas rushes backwards through the impeller, reversing the flow direction of the gas and thus reducing discharge pressure and potentially stalling the compressor. As discharge pressure lowers, forward flow resumes, only to be reversed again, creating instability near the surge line and repeated flow reversal.
Compressor stall and surge events happen within a 20 – to 50-millisecond window without warning and, if left unchecked, a surge cycle will continue indefinitely. The extent and immediacy of any consequences depends on the frequency and power of the surge force, the vibrations it generates, and the temperature of the surging gas. Associated risks range from short-term performance issues to catastrophic failure of the compressor and lengthy, costly unplanned downtime, which is why millisecond‑level response and asset integrity monitoring are critical.
LNG project owners and engineering, procurement, and contractors (EPCs) need to consider the following factors when making decisions about surge control, from specification guidance to lifecycle risk mitigation:
- Surge damages the compressor, compromising performance
A surge can damage compressor seals, with their replacement costing tens of thousands of dollars. Surges can also cause significant mechanical damage to bearings, the impellers or the shaft, and other critical components. As well as hindering the plant’s operational efficiency, a damaged compressor is more prone to leakage, posing a serious risk to safety and the environment, increasing maintenance costs and threatening safety and environmental compliance.
- Consequences of compressor surge can lead to LNG plant shutdown
Stable operation is vital to the safety and efficiency of an LNG liquefaction train. Flow reversal can lead to major process-related problems requiring emergency shutdown. The repair or replacement of damaged compressor components can also necessitate downtime. Unplanned LNG plant shutdown has a major impact on production. It can lead to significant financial losses, possibly up to tens of millions of dollars, further highlighting the critical importance of a fast, accurate solution, to protect production uptime and reduce lifecycle cost.
- Minimising the gap between ‘surge’ and ‘surge control’ maximises plant performance
Reducing the likelihood of compressor surge is critical. Yet, while a surge event can have serious consequences, an overly conservative approach to surge risk management constrains the compressor’s operating envelope, limiting overall plant efficiency. Effectively balancing risk requires a range of measures, from fast, accurate, dynamic control systems to predictive analytics and regular maintenance, so you can expand the operating envelope and optimise efficiency.
Severn has the solution to the enduring Surge Control for LNG Challenge
Severn has engineered a sophisticated Anti-Surge Control Valve solution with advanced actuation capabilities to overcome this enduring challenge of LNG production, combining high‑Cv trims with low‑deadband, fast‑stroke actuation for precision flow control.
Severn brings extensive experience in the design and manufacture of LNG valves for both the cryogenic and liquefaction phases of production. Previously, we’ve supplied severe and critical service valves for LNG liquefaction projects such as Gorgon and Icthys, demonstrating proven LNG references and cryogenic service expertise.
Contact us to find out how we can support your LNG development goals, request an engineering consultation with a Severn specialist today.