Bespoke trim designs optimise compressor Anti-Surge valves for the specific application demands of the LNG plant where they’re installed.
Internal components of a valve that come into contract with process media is collectively known as the trim. They are critical to its reliable operation. This is especially true of Control Valves that protect centrifugal compressors from surge events on LNG liquefaction trains.
Compressor Anti-Surge Control Valves are often exposed to high flow velocities. Severn’s Repair Intelligence data (based on our engineers’ analysis of historic OEM valve products) shows this can lead to severe operational issues including high noise levels, vibration, erosion, and premature failure. These problems bring the risk of safety issues, production loss, and unscheduled shutdowns. Spiralling costs can be incurred too, due to frequent maintenance and replacement of damaged parts.
Since process conditions vary considerably between LNG projects, tailoring compressor Anti-Surge Valve trims to handle specific gas flow characteristics is highly advantageous. As well as facilitating precise and reliable control, this enhances valves’ ability to endure the operating conditions, improving longevity and reducing potential maintenance and safety concerns.
Customising Compressor Anti – Surge Valves for Endurance and Performance
To enable the release of process gas, Anti-Surge Valves are sized (as per specification) to have 1.8-2.2 times the required capacity for the process conditions. This ensures enough gas is supplied to the compressor to prevent surge and avoid a trip situation. Due to the high velocities created by the process, the gas must be carefully controlled, and the trim fulfils two main duties. The first is to enable the controlled release of the process gas that can cause the compressor to go into surge. The second is to control the velocity of the gas passing through the valve.
As well as having potential to cause erosion, the velocity of the gas can result in high aerodynamic noise levels which may exceed acceptable plant limits. High noise levels are also indicative of vibration within the trim which may affect the valve and downstream equipment, potentially inducing various modes of failure. Lagging the pipework is often seen as a solution, but this does not address the root cause. It is more effective to optimise the valve trim for the process conditions.
Severn’s bespoke trims optimise surge gas control
Severn’s Anti-Surge Control Valves benefit from engineered solutions to address the high capacity and velocity control requirements of the application. Trim components that benefit most from engineer-led customisation are the plug and the cage.
Precision engineering of cage geometry, informed by Severn’s expert analysis of flow conditions and Repair Intelligence insights, improves valve responsiveness and overall control. For instance, the 3D flow path of the cage can be engineered to reduce deadband and improve controllability. Our Multi Labyrinth Trim (MLT) can handle up to 48 stages of velocity reduction and our 1CC trim offers a high-capacity option which maximises throughput. To balance the need for high capacity and precise velocity control, we have also developed the Vari-Stage Trim which can be configured to meet end users’ exact process requirements.
A valve’s cage helps to balance pressure and reduce turbulence as the plug lifts and surge gas enters. Even so, there can be a delay in the equalisation of pressure in the area above the plug with the changes below it. This can lead to severe vibration and instability if the system does not respond in adequate time.
Severn’s Repair Intelligence has identified that process forces acting on a valve plug can severely detract from valve performance in operation. To combat this, we have developed a ‘Waggon Wheel’ plug design, in addition to our standard options, which can be implemented where conditions dictate. The design ensures process forces are equalised across the plug allowing for rapid movement and accurate control.
All these solutions are designed and engineered using Severn’s robust sizing and selection process, technical expertise, and years of Repair Intelligence.
Valve engineering expertise
Severn’s Anti-Surge Control Valve Technology is rooted in an adaptive design process led by experienced engineers. Applying our valve engineering expertise to compressor Anti-Surge challenges on an application-by-application basis results in durable, highly functional valves which ensure reliability, efficiency and safety.
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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.