Dehydration of gas in LNG liquefaction plants is one of the most important and critical processes, with major disruption possible in cases where the gas is not properly dried. This article examines the vital role of valves in the process, and offers 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.
By Fabrizio Delledonne, Marketing Manager, and Mike Semens-Flanagan, Global Engineering Director, IMI Critical Engineering
The dehydration of feed gas in LNG plants is a vital part of the liquefaction process, with valves playing 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 absorbs the present water content. In liquifying the gas, it is vital that it is completely dry because any water will freeze during the application and damage vital areas of the processing plant, such as the 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. There are several challenges that these valves face during the course of their operation, from the volume of operational cycles, through to the presence of solid particles in the fluid, as well as the thermal cycling itself.
This places the valves under constant strain and raises the threat of costly service disruptions due to 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 will need to be recalibrated before going back into production, and this is before factoring in the additional losses incurred due to 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 U.S., 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 dollars USD.
It is difficult to offer a definitive figure for the impact poor valves have on LNG production, but broader estimates from analysts highlight the need for better specification. McKinsey, for example, found that just 3.65 days of unplanned downtime a year costs the oil & gas industry USD $5.037 million. Even shorter periods can lead to noticeable losses, therefore 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.
Rising Stem Ball Valves
Rising stem ball valves (RSBVs) 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 three to four times a day). They need to be capable of withstanding thermal cycling, and must be resistant to wear related to the presence of solid catalyst particles in the media. The roto-translatory movement design concept of RSBVs 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 closure member (ball) and seats. If conventional ball valves are used in molecular sieve systems, the seats will tend to wear out very quickly and a tight shut-off cannot be provided.
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 unstable oil & gas prices, with an increased focus on taking a more holistic approach to operations that involve identifying and solving inefficiencies, and in order to reduce operational expenditure, the role of RSBVs are coming under ever closer scrutiny.
RSBVs are also heavy and have a large footprint, both of which generate several direct (material use) and indirect (installation) costs that engineers must account for during front-end 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 RSBVs might be a concern, with the required substitution of several trim components, including the stem, leading to high maintenance costs, which can even make it more cost-effective to purchase an entirely new valve.
Additional areas of concern include high friction in the shaft and a potential for rotating parts to get stuck, particularly in cases where abrasive fluid enters into 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 used in natural gas molecular sieves.
Key Considerations
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. It is also recommended to use bushing protectors to avoid entry of sieve particles between the stem and valve body.
The temperature of 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 to tolerance calculations between closure member and seats should also be taken into consideration.
The Search for Alternative Valve Types
Due to the challenges associated with RSBVs 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 the oil majors have already positively evaluated and accepted alternative solutions that collectively are pushing the industry towards making a change.
Unsurprisingly, this new generation of valves deliver a compact design and a reduction in weight, as well as improving maintenance access and lowering overall operational expenditure.
Double Eccentric Segmented Ball Valve
One solution is a double eccentric segmented ball valve, that offers a compact and simple design. It is easier to maintain and can deliver lower operating costs.
This type of valve is characterized 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 is typically non-rubbing, low running torque, and high-performance mechanical sealing. On this severe service, the ball and seats are hard faced, with chromium carbide on both inner and outer surfaces of the ball, providing the most durable hard surface.
Friction-free operation between the ball and seat makes this type of valve ideal for high cycle and high endurance processes like molecular sieve switching or high/low temperature operations. Most importantly, this type of valve is cavity-free to eliminate the possibility of an over-pressurized body cavity and has a quarter turn operation that allows for simple and effective automation along with emission-free performance.
Triple Eccentric Metal Seated Butterfly Valve
Another alternative is a triple offset valve. This valve type is being increasingly adopted for molecular sieve service for its non-rubbing metal seat design, excellent tightness performance, great compactness and full metal construction.
The valve typically has a full metal solid seal ring, which ensures excellent tightness performance over time, offering a long service life. Particularly, triple eccentricity makes it possible to achieve contact between the sealing surfaces only when the valve disc reaches its fully closed position. Special bushings can offer immunity to dirt and polymeric media that can enhance service life and intervals between maintenance checks.
The triple eccentric valve selection offers compactness, minimizing space and weight. For valve sizes exceeding 10”, a triple eccentric valve has these advantages over comparable valve designs.
Looking Ahead
The importance of the careful selection of valves for molecular sieving processes can never be overemphasised.
There have been cases where valves have generated unacceptably high annual operational and maintenance costs, in turn causing processing plants to suffer from 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.
