Rethinking Valve Specification for LNG Liquefaction: A smarter alternative to rising stem ball valves

In this article, we examine the vital role of valves in the process and offer key criteria for their selection, as well as looking at three alternative valve designs: rising stem ball valves, double eccentric segmented ball valves, and triple eccentric butterfly valves.

Dehydration of feed gas in LNG plants is a vital part of the liquefaction process, and valves play a major role. The efficiency of a molecular sieve system is highly dependent on valve performance, specifically tightness and reliability.

In operation, the wet gas flows through tanks, where a solid-particle catalyst adsorbs the water. When liquifying the gas, it is vital that it be completely dry, as any water will freeze during the process and damage vital areas of the processing plant, such as piping systems.

To enhance the reaction and regenerate the catalyst, there is continuous thermal cycling facilitated by valves at the top and bottom of the tank, which let the gas come in and out. These valves face several challenges during operation, from the volume of operational cycles to the presence of solid particles in the fluid, as well as thermal cycling itself.

This places the valves under constant strain and increases the risk of costly service disruptions from critical component damage. The financial impact of this downtime is serious, not least in an industry where unexpected interruptions can quickly compound. Maintenance is expensive and often lengthy, as the system must be recalibrated before returning to production, before factoring in additional losses from lost or contaminated product.

Recent events have brought issues of this kind into sharper focus. In August 2022, Reuters reported that unplanned disruptions at several LNG plants in the US, Nigeria and Australia had caught major traders off guard, forcing them to pay much higher costs for alternative supplies. The final figure is still unknown, but losses from this event easily ran into the hundreds of millions of US dollars. This story followed on from another major train outage at Qatar’s Ras Laffan in February 2022, where a week-long delay put the brakes on one of its 7.8 million tonne annual output.

It's difficult to offer a definitive figure for the impact poor valves have on LNG production, but broader analyst estimates highlight the need for better specifications. McKinsey, for example, found that just 3.65 days of unplanned downtime a year costs the oil and gas industry $5.037 million. Even shorter periods can lead to noticeable losses, so it makes sense for engineers to consider where improvements can be made, even if the components themselves are only a small part of a much larger system.

The rising stem ball valve (RSBV) type is the most frequent choice for valves operating in molecular sieve systems in the LNG sector.

These valves are required to cycle frequently (up to 3-4 times a day). They need to withstand thermal cycling and be resistant to wear caused by the presence of solid catalyst particles in the media. The roto-translatory movement design concept of RSBV addresses these concerns and makes them suitable for this service.

This explains why conventional ball valves and gate valves are not used for molecular sieve systems. Conventional ball valves have continuous line contact between the closure member (ball) and the seats. If conventional ball valves are used in molecular sieve systems, the seats will wear out quickly, and a tight shut-off cannot be achieved.

Also, due to continuous contact, frictional resistance is high, meaning the actuator's operational force requirements will be high as well. The LNG industry has therefore relied on RSBV. Nevertheless, this valve type also has its downsides, and currently, there are attractive alternative valve designs on the market.

First of all, the complex design results in a corresponding increase in downtime, operational and maintenance costs, that are increasingly perceived as intolerable for the industry. In fact, in an era of volatile oil and gas prices, with an increased focus on a more holistic approach to operations that involves identifying and solving inefficiencies to reduce operational expenditure, the role of RSBV is under ever closer scrutiny.

Secondly, RSBV are heavy and have a large footprint, both of which generate several direct (material use) and indirect (installation) costs that engineers must account for during frontend engineering design project phases.

The importance of valve weight cannot be overstated, with industry placing a great emphasis on reducing the weight of processing plant components in order to reduce shipping and installation costs.

Maintenance on RSBV might be a concern, with the need to replace several trim components, including the stem, leading to high maintenance costs that may even make it more cost-effective to purchase an entirely new valve.

Additional areas of concern include high friction in the shaft and the potential for rotating parts to get stuck, particularly when abrasive fluid enters the guiding slots. Also, the wear and tear related to the linear movement of the stem may lead to higher emissions compared to quarter-turn valves.

Moreover, the valves have to undergo frequent thermal cycles. During regeneration mode, the molecular sieve is flushed with hot gas, typically at 350°C. During purification mode, it is brought down to ambient temperature.

Gas purification residuals (crushed adsorbents) are often present in outlet gases and can pass through screens and flow through the valves towards downstream lines, causing the abrasion of sealing components and jeopardising valve integrity.

Conversely, whenever membranes are used for CO2 removal without pre-treatment, corrosive gases (sour/acid) are present, especially in offshore installations. Historically, non-rubbing, rising-stem (tilting) ball valves have been the standard for natural gas molecular sieves.

There are multiple considerations when specifying valves for the dehydration of feed gas in LNG liquefaction plants, but the key points are as follows.

Firstly, it is necessary that valves should be cavity free. If the valve being used in the process has a cavity, then there is a high possibility that the sieve particles will be trapped inside the cavity rendering the valve inoperable after some time.

Secondly, the valves should have metallic hard-faced seats. It is impossible to eliminate the contact between the closure member and seats completely during the shut off time, and therefore it is vital that metallic hard-faced seats are used to avoid any abrasion between closure member and seats.

Thirdly, hard-faced bushings should be used to avoid any abrasion by sieve particles. Also, it is recommended to use bushing protectors to avoid the entry of sieve particles between the stem and valve body.

The temperature of the gas used for regeneration ranges from 200°C to 350°C. This means that valves undergo significant numbers of thermal cycles. As such, consideration of valve material and gap/tolerance calculations between closure member and seats should also be taken into consideration.

Due to the challenges associated with RSBV there is an increasing trend for industry to seek out alternative valve designs that can help lower operational and maintenance costs while simultaneously increasing productivity.

While this move away from RSBV is still in its early stages, a number of oil majors have already evaluated and accepted alternative solutions that are collectively pushing the industry towards change.

Unsurprisingly, this new generation of valves delivers a compact design and a reduction in weight, as well as improving maintenance access and lowering overall operational expenditure.

One solution is the double-eccentric segmented ball valve, which offers a compact, simple design. It is easier to maintain and can deliver lower operating costs.

This type of valve, typified by the C-REX™ manufactured by IMI, is characterised by an eccentric C-shape ball; the cam action provides non-rubbing rotation, which is a key prerequisite for valves operating in molecular sieves services.

In operation, the valve offers non-rubbing, low running torque and high-performance mechanical sealing. For severe service, the ball and seats are hard-faced with chromium carbide on both the inner and outer surfaces of the ball, providing the most durable hard surface.

Furthermore, the C-REX™ has a compact, lightweight design, weighing up to 50% less than the alternative RSBV. Its metal seated torque sealing mechanism ensures excellent tightness in both directions, with reliable performance over the valve’s lifetime.

Friction-free operation between the ball and seat makes the C-REX™ valve ideal for high-cycle, high-endurance processes such as molecular sieve switching or high- and low-temperature operations. The double eccentric segmented ball valve also offers minimal rubbing between seating elements during travel, in turn reducing torque, friction and wear to extend service life. Crucially, this type of valve is a top entry design that offers easy access to the valve internals for inline maintenance.

Most importantly, this type of valve is cavity-free to eliminate the possibility of an over-pressurised body cavity and has a quarter turn operation that allows for simple and effective automation along with emission-free performance.

With its simple design, the C-REX™ maintenance operation is recognised to be fast and effective, minimising operational costs related to maintenance aspects.

Another alternative is the triple offset valve, such as IMI’s MV series. This valve type is increasingly adopted for molecular sieve service for its non-rubbing metal seat design, excellent sealing performance, compact design, and full-metal construction.

The MV series typically features a full-metal solid seal ring, which ensures excellent tightness over time and offers a long service life. In particular, triple eccentricity allows contact between the sealing surfaces only when the valve disc reaches its fully closed position. The floating resilient seal ring enables uniform contact pressure across the sealing circumference, ensuring tightness in both directions.

Special bushings offer immunity to dirt and polymeric media, enhancing service life and increasing maintenance intervals.

Like the C-REX™, the MV series is a simple design with a one-piece integrated body and a one-piece shaft and disc arrangement.

The triple eccentric valve selection offers compactness, minimising space and weight in situ. For valve sizes exceeding 10", the triple eccentric valve has these advantages over comparable valve designs.

The importance of carefully selecting valves for molecular sieving processes cannot be overemphasised.

There have been cases where valves have generated unacceptably high annual operational and maintenance costs, resulting in processing plants suffering product losses far exceeding the purchase price of the valves.

More recently, several plants have turned to alternatives for molecular sieves, choosing quarter-turn valves such as triple eccentric butterfly valves and double eccentric segmented ball valves, improving their uptime and significantly reducing maintenance costs.

Learn more about how IMI’s technologies and expertise optimise LNG processes in the midstream sector.

- Fabrizio Delledonne, Marketing Manager, Process Automation at IMI .

A version of this article originally appeared in the March 2023 edition of LNG Industry magazine