With their direct influence on safe, economical plant performance, compressors are a vital component of the LNG liquefaction train. Protecting them from the ‘surge phenomenon’ helps avoid costly damage and unplanned downtime.

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.

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.

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.

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.

LNG project owners and engineering, procurement, and contractors (EPCs) need to consider the following factors when making decisions about surge control:

1. 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.

2. 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.

3. 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.

Severn has the solution to the enduring surge control challenge

Severn has engineered a sophisticated Anti-Surge Control Valve solution with advanced actuation capabilities to overcome this enduring challenge of LNG production.

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.

Contact us to find out how we can support your LNG development goals.