Sophisticated Anti-Surge valve technology from Severn Compressor Surge Control leverages advanced actuation capabilities to deliver precision control, maximising LNG plant efficiency, safety, and productivity.
Controlling centrifugal compressor surge is a fundamental aspect of efficient LNG production, and Anti-Surge Control Valves play a vital role.
Surge events can damage critical rotating equipment, harm plant performance, and result in costly unplanned downtime. Yet a conservative approach to surge risk management compromises plant efficiency and productivity by constraining the compressor’s operating envelope. Identifying an acceptable trade-off between these two extremes is increasingly difficult as trends in LNG production evolve.
The escalating challenge of LNG Compressor Surge Control
Challenges surrounding compressor surge control have become significantly more difficult in the quest for optimised efficiency. With global demand for LNG growing, the capacity of existing and future ‘mega trains’ is increasing. This requires larger valves sizes, which makes it harder to achieve fast, accurate control in anti-surge applications.
Surge events can occur within milliseconds and without warning, meaning Anti-Surge Control Valves must operate with exceptional speed and precision. However, fundamental principles of physics and engineering make it harder to achieve critical performance parameters as valve size increases. The mass of larger valves introduces greater mechanical inertia, so more force is required to control the travel of the plug (the main moving part). Handling larger volumes of gas also introduces significant dynamic inertia and pressure delays, further impeding valve response times.
On LNG plants where Anti-Surge Control Valve travel exceeds 18”, operators traditionally had to tolerate conservative surge control measures, compromising overall plant efficiency. Until now…
Raise the surge control line, boost efficiency
Severn’s latest innovation addresses issues associated with larger Anti-Surge Control Valves head on. Our engineers have applied their technical expertise to the development of a robust, highly engineered solution.
Benefits include fast operability, high controllability, excellent reliability, and extremely accurate valve control. This means the valves respond very quickly (in less than two seconds, including any deadband), without compromising controllability when surge is imminent. As the below diagram shows, these superior performance characteristics enable centrifugal compressors to operate closer to the surge control line, optimising performance and efficiency.
Severn recently completed testing on a proof-of-concept Anti-Surge Control Valve with a nominal bore of 30” and a linear stroke (the distance the actuator travels) of 24”. The valve specimen’s pressure rating was ANSI 150, and it was designed with a flow coefficient (Cv) exceeding 7700, in line with the process conditions of a typical Anti-Surge application.
Testing showed that the valve meets all required parameters of the world’s most stringent Anti-Surge specifications. A significant highlight was its ability to perform the full 24” stroke in less than two seconds while simultaneously achieving incredibly precise positional control. The exceptional performance enables accurate and reliable control close to the compressor surge control line, striking an effective balance between safety and efficiency without concession or compromise.
There is a great variation in centrifugal compressor process conditions, in terms of feed gas properties and operating parameters such as flow rate, pressure, and temperature. It follows that Anti-Surge Control Valves must be individually specified and designed to optimise compressor performance and, in turn, maximise LNG plant performance and efficiency. Future articles in this series will explain how technical aspects of our Anti-Surge Control Valve technology, such as control optimisation and trim design, are customised to meet specific application needs and requirements.
Meanwhile, if you’re looking for expert technical input on centrifugal compressor surge control or other severe or critical service LNG valve applications, contact us at sales@severnvalve.com
Severn has a strong track record in the design and manufacture of valves that overcome the most challenging technical issues. LNG projects we’ve supplied in the past include Ichthys and Gorgon.
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.