Tailor to your needs

An article by Colin Findlay, Severn Glocon Group, UK, outlining how customised cryogenic valves can enhance productivity and provide a significant commercial advantage. (Published in the April 2020 issue of LNG Industry).

Recent mega-project developments in the US, Russia and Qatar herald what many consider will be a golden age for the LNG industry. A 2019 McKinsey report, ‘Global Gas and LNG Outlook to 2035’, envisages that global demand will increase at 3.6% per year. According to the report’s authors, “liquefaction projects will require more than US$250 billion of investment in the next 15 years, with most of the investment expected in Africa, the Middle East and North America.”

Meeting this demand will require both new projects and the extension or expansion of existing liquefaction facilities to increase capacity. The current industry trend to ‘go mega’ offers a solution, but brings with it a host of complex technical challenges as facilities adjust to handling larger volumes of cryogenic fluid.

When tier one assets such as compressors operate at a higher capacity, there are significant repercussions for associated equipment in the loop, including control valves. As the volume of cryogenic fluid increases, so does pipeline diameter, meaning valves need to be bigger. All of this has an impact on factors related to the pressure and velocity of the process medium. Ultimately, this results in arduous production demands, which require expert technical input to ensure the safe and efficient handling of LNG.

Figure 1. Cryogenic valves need to handle arduous production demands reliably.

Simply reusing blueprint valve specifications from earlier or smaller projects can result in outdated or inappropriate selection of valve types, technologies and materials. Valves are a vulnerable part of any system and, especially in an LNG context, internal or atmospheric leakage represents a major hazard in terms of the environment and safety. When an incident occurs, it can escalate into a major event very quickly, potentially resulting in lengthy, unplanned downtime. 

Conversely, customised cryogenic valves, highly engineered to handle the specific demands of severe and critical service applications, can provide a significant commercial advantage. 

Cryogenic valve requirements

Cryogenic valves must provide repeatable, bubble-tight shut-off during various phases of the -160°C liquefaction process, and for all downstream applications until the point of regasification. They need to achieve this in a controlled manner, in the face of significant differential thermal dynamics. Valve design must accommodate minimum heat leakage, minimum cooldown mass and cold impact strength materials, with ambient air conditions considered, as well as the temperature of the LNG itself. Cryogenic valves are designed to operate at a temperature range spanning from 80°C to -196°C, in order to ensure they can comfortably handle extremes at either end of the spectrum.

Typical cryogenic valve applications

Some of the most demanding cryogenic valve applications in LNG production are those associated with liquefaction phase compressors. Anti-surge, hot gas bypass and Joule-Thomson let-down processes involve extreme operating parameters, which can be exacerbated in mega-liquefaction scenarios. 

For instance, the Joule-Thomson effect plays a vital role in cooling the feed gas during liquefaction. Control valves are used to achieve the effect, handling mixed phase and gaseous flows at a high differential pressure to accomplish the desired cooling. Fast and responsive valve operation is essential. However, this demanding severe service application brings a host of technical challenges, such as increased potential for high noise and erosion. What is more, large flowrates give rise to high power conversion rates, so the valves must be capable of precise throttling at cryogenic temperatures. 

Likewise, anti-surge valves play an important role protecting LNG compressors. The performance and reliability of the compressors has a direct impact on overall production and profitability. Unplanned downtime for these – or any aspect of the refrigerant loop – reduces production and can result in contractual penalties. Furthermore, compressors are big-ticket assets, so any damage results in costly repairs. 

Compressors are most at risk during plant startup and commissioning. Therefore, anti-surge valves provide throttling control, recycling a portion of the discharge flow as the compressor reaches capacity. During normal day-to-day operation, anti-surge valves remain closed, but they must be ready to open quickly (in less than two seconds) in the event of a surge, to protect the compressor impellers from the sudden reverse flow of a surge event. 

Get the specification right

In the absence of dedicated international standards for LNG cryogenic valves, most specifications are rooted in the British Standard BS 6364:1984 ‘valves for cryogenic service’, combined with additional end-user requirements or other relevant international standards. 

To meet the exacting severe service demands of cryogenic valve applications, such as Joule-Thomson let-down and compressor anti-surge, it is good practice for individual valves to receive a robust, intelligence-led specification. This is generally based on the valve’s specific requirements (such as speed of operation, pressure differential, etc.) and the operating conditions it will contend with (from velocity of the process medium to ambient temperature). 

Getting the specification right unlocks the potential for advanced valve engineering input, which can deliver significant benefits in terms of efficiency, performance, reliability and safety during startup, commissioning and ongoing operation. Creating time and space for engineering contractors, operators and valve suppliers to collaborate during the front-end engineering and design (FEED) stage can result in breakthrough, cost-effective solutions to help mitigate risk in the face of escalating demands. 

Advanced trim design

Trim modification or customisation can have a considerable impact on a valve’s ability to perform well in cryogenic LNG applications. The trim refers to the operating parts exposed to the process medium, and its design has a major bearing on factors such as velocity control, noise and erosion. An effective trim design can represent the difference between reliable, efficient performance and serious problems, such as increased risk of leakage and poor process control. 

Any high-velocity process medium can cause valve cavitation, erosion and abrasion of internal components, resulting in poor control and a higher likelihood of premature failure. Simply using harder materials does not go far enough to address the problem and can introduce unnecessary expense. It is more effective to address the problem at source with applied engineering skill.

Severn primarily uses specially designed trims, such as its own multi-labyrinth trim (MLT), to address some of the most arduous challenges surrounding velocity and pressure in cryogenic valves. With an MLT, the incoming flow medium is segmented into smaller streams, each of which follows a tortuous path of multiple flow turns which achieve reductions in pressure and velocity. This technique removes kinetic energy and lowers pressure in a controlled manner. The number and nature of the turns is precisely calculated, based on the expected fluid velocity at the inlet and the optimum velocity at the outlet. In this way, flow speed can be carefully controlled in each flow passage within the valve, ensuring operation is managed purposefully across its entire service range. 

MLTs are especially well suited to compressor anti-surge valves where performance is greatly enhanced through well-controlled velocity of the cryogenic fluid. This solution can reduce the vibration and noise typically associated with LNG compressors, as well as improve reliability and extend valve life. 

Retrofitting cryogenic valves

It is not just new LNG projects that can benefit from customised trims for cryogenic valves. When production volumes increase at existing plants, the cryogenic valves originally installed may no longer be fit for purpose. Replacing them with new products is costly and often takes too long, but strategic retrofit solutions offer an effective alternative. This approach enables operators to meet shifting production demands and avoid lengthy downtime in a cost-effective manner, while extending the life of the valves. 

Figure 2. Thorough cryogenic testing reduces the risk of problems during startup.

Far from being a second-rate solution, when a retrofit is handled well, it can enable a cryogenic valve to perform better than ever. With the addition of an innovative, customised trim, existing valves can handle larger volumes with ease. What is more, valve engineers have the benefit of insight into real production challenges that may not have been apparent at the original point of specification. In addition to this, if the valves have been in service for a long time, modern technologies and manufacturing techniques could offer new ways to enhance their performance.

Testing and ongoing management

While there is no dedicated industry standard for valves destined for cryogenic LNG applications, it is necessary to test any control valve handling hazardous substances for seat leakage. This is generally done in accordance with industry standards BS 6364, ASME B16.34 and ANSI FCI 70-2, under cryogenic conditions specified by the end-user. It is regular practice for cryogenic valves to undergo testing at temperatures of -196°C to verify that they can perform reliably at supercool temperatures beyond what they are expected to encounter. 

Typically, at least one cryogenic valve of each type per LNG train or system undergoes cryogenic testing. In some cases, all severe and critical service cryogenic valves are tested individually. 

Once a test specimen has passed standard hydrostatic and seat leakage assessments at ambient temperature, it is purged of previous test fluids before submersion in a tank of liquid nitrogen. When it reaches the test temperature, it is filled with helium gas (which does not freeze or liquefy at -196°C) and carefully assessed for any seat or pressure shell leakage. 

Thorough cryogenic testing reduces the likelihood of having to troubleshoot during plant startup and provides a high level of quality assurance for the end-user. Ideally, in-factory tests should go one step further, in order to facilitate proactive lifetime management of cryogenic valves. In addition to the regulatory testing, establishing each valve’s performance footprint provides a valuable benchmark for future performance data throughout its life. This enables plant managers to quickly ascertain whether set criteria are being met, and ensures the rapid identification of any deviation from standard performance, so that intervention can take place in a timely and effective manner. 

The hidden strength of customisation

Cryogenic valves can be an LNG plant’s Achilles heel or hidden strength. As production volumes increase to meet rising global demand, there is an acute need to focus on maximising their safe and reliable performance. Cryogenic valves with a customised design may result in higher upfront CAPEX than off-the-shelf products, but the OPEX benefits more than compensate. 

Applying specialist engineering skill to valve design mitigates risk and maximises reliability. Combining this with robust cryogenic testing and performance benchmarks ensures cryogenic valves are primed for enduring good performance. In the context of an LNG facility, cryogenic valves are a relatively small piece of equipment, but they can make a big difference to productivity. Customising them to deliver optimum performance offers a route to commercial advantage.