Optimized Joule-Thomson Control Valves are Critical for LNG Operations

Liquified gas applications typically require high pressure and cryogenic temperatures to achieve gas separation and liquification. These processes invariably require significant amounts of energy, so the efficiency of these unit operations drives plant economics. Therefore, proper equipment specification and high performance are a prerequisite for profitable operation.

The main cryogenic heat exchanger (MCHE) is considered the heart of the LNG facility, making the closely-coupled Joule-Thomson (J-T) control valves feeding the MCHE the most critical of control valves. When these valves are specified and engineered correctly, licensed process controllers can optimize operations, resulting in increased LNG throughput with less energy use.

Conversely, the negative ramifications are significant when these high-pressure-drop cryogenic control valve assemblies throttle poorly. This article will focus on J-T control valves and discuss the key design features required for efficient LNG throughput and long service life.

LNG temperature controls. The liquification of natural gas requires processing temperatures in the range of -250°F or lower. This is normally accomplished using a combination of turbo-expanders and J-T valves, with the process using the J-T effect to cool the gas by reducing the process gas pressure very quickly. The turbo-expanders in the process are more efficient than J-T valves when running at steady state, but the J-T valves are installed in parallel to replace the turbo-expander when it is out of service, and to handle the increased cooling demands found during startup and transition conditions.

The J-T valve/turbo-expander combination (marked 6 in FIG. 1) is often found closely-coupled to the MCHE, but it can also be used in several locations where further cooling is required.

FIG. 1. This figure shows the more critical control valves in a typical LNG plant. The valves marked “6” are the J-T control valves, with an emphasis on the critical “warm” and “cold” J-T control valves feeding the main cryogenic heat exchanger.

In normal operation, the turbo-expander lets down process pressure to cool vapors, and it uses that energy to perform other work as required by the process. However, the flow through the turbo-expander is limited by the capacity of the unit, and it may be subject to a variety of process and equipment constraints. During system transitions, such as startup and grade changes, the J-T valve is brought into service to provide additional cooling. Some LNG facilities have no turbo-expanders at all, relying solely on the throttling performance of the J-T valves.

The J-T valve must be controlled very carefully to avoid process excursions, upsets and inadvertent shutdowns, or to achieve increased LNG throughput. It is not uncommon for the J-T control valves to receive and respond to setpoint changes as small as .05%. If the valve can be positioned precisely and consistently, advanced process controls can be utilized to optimize the system, and to minimize the startup and cooldown times required with off spec grade changes.

Such advanced controls can provide dramatic economic benefits, but they are only possible if the J-T control valves have been specified correctly to provide very tight flow control on a reliable repeatable basis.

Optimized J-T control valve key considerations. J-T valves are subjected to punishing conditions, including cryogenic temperatures, large pressure drop-induced vibration and multi-phase fluid conditions, yet they must accurately throttle to meet extremely small setpoint changes. Such process conditions pose several design challenges:

• The severe service trim configuration and materials of construction must be chosen to withstand and manage the large pressure differentials (~700 psid) caused by handling multi-phase cryogenic fluids without incurring valve damage. Custom cage characterizations are also required to ensure the installed gain of the assembly is ideal for accommodating advanced loop tuning efforts.
• Consistent and accurate valve position response is critical for tight flow control, which can be challenging with these large cryogenic valves. Low friction cryogenic balanced plug seals provide low deadband, and tight throttling accuracy when executing extremely small step changes, with minimal to no overshoot.
• Insufficient extension bonnet lengths can lead to mechanical integrity concerns such as valve stem, packing and valve positioner feedback freezing. A sufficiently long extension bonnet is key to keeping the packing box operating per design.
• High performance pneumatic actuation, paired with reliable linkage-less feedback from a smart digital positioner, provides insights into control valve health via diagnostics monitoring valve friction, supply air pressure, travel deviations, instrument tubing leaks and other critical parameters.
• To best ensure the as-shipped control valve performance matches the required installed control valve throttling performance, a dynamic factory acceptance test performed in a -320°F cryogenic pit is highly recommended.

FIG. 2 depicts a high performing J-T valve that meets all the criteria outlined above.

FIG. 2. This NPS 16-in. optimized proprietary J-T control valvea capable of controlling down to .05% step changes at -320°F.

 

A well-designed J-T valve will provide consistently accurate positioning regardless of process changes over a long service life. Poorly designed J-T control valves are susceptible to excessive friction, deadtime and deadband, along with ice formation and deteriorating control response. Eventually the valve will degrade to a point where automated control is impossible and manual control is the only option (FIG. 3).

FIG. 3. A poorly designed J-T valve (left) will allow significant external icing, respond poorly and require continual operator intervention. A high-performance J-T valve, such as the proprietary J-T control valvea on the right, enables fully automated control on a continuous basis.

 

Manual J-T valve operation requires diligent operator attention and intervention to avoid process trips and upsets, and it dramatically slows process startups and shutdowns, reducing the overall LNG throughput of a facility. This can have significant negative consequences for plant operations.

Design considerations. A common practice in the LNG industry is specifying cryogenic control valves, including J-T valves, to comply with on/off isolation valve seat leakage standards. It is possible to procure control valves meeting these standards, but these types of valves may not be able to provide the level of throttling precision required in critical applications.

This implication primarily arises with large, high-pressure, balanced cryogenic globe-type control valves specified to meet on/off isolation valve seat leakage standards, such as BS6364.  Balanced plug seals exist to provide the required minimal seat leakage, but these components can add a significant amount of friction to the assembly, which if not properly accounted for can greatly inhibit the tight throttling performance of a control valve.

MCHE J-T control valves are inherently susceptible to this concern. The control valves can meet the project specified isolation cryogenic seat leakage requirements via factory acceptance testing (FIG. 4) but installed dynamic performance of the valve may not be ideal for optimized licensed process controller tuning. These valves can be continuously modulating when the LNG facility is running, thus is important to ensure valves throughout the facility are being specified correctly for their respective duties.

FIG. 4 This optimized J-T control valve is shown as it undergoes testing by undergoing high-performance throttling step tests within a cryogenic pit at -320°F.

Economic impact in action. Few valves can affect the plant’s bottom line more than a J-T control valve because energy optimization is closely tied to MCHE efficiency. Therefore, a reliable and precise J-T control valve directly enables higher-tier automated control and corresponding benefits.

Recently, an LNG facility was operating poorly and suffering significant economic penalties. A brief review of operations found the MCHE J-T control valves installed in the process were the primary problem as they required manual control and nearly constant vigilance to keep the plant running. Because of numerous trips and upsets, the operators were forced to run the J-T control valves very conservatively, severely reducing production and efficiency.

The MCHE J-T control valves were replaced with the author’s company’s optimized J-T control valves, providing immediate and accurate response. This allowed the higher-order controls provided by the process licensor to be returned to automatic, optimizing startup, shutdowns and grade changes. The net economic impact was production equal to one additional LNG tanker for export sale, a value upwards of $200 MM. Efficiency also improved since the process could quickly and automatically respond to ambient weather conditions safely, allowing the plant to run at design capacity without risking trips.

If a LNG or air separation plant is struggling to achieve tight process control, a good place to start the troubleshooting process is by reviewing the performance of the J-T control valves. If they are not running in a fully automated mode or if they are providing inconsistent response, a valve upgrade will easily justify the cost and generate very quick payback. GP

NOTE

^a CL600 Fisher™ Large EWT-C J-T valve

Reid Youngdahl is a Valve Technical Specialist at Emerson for their flow controls products in Marshalltown, Iowa. He began working for Emerson in 2013 as an applications engineer, providing global control valve technical support for various industrial markets, such as oil and gas, power generation, and original equipment manufacturers. Youngdahl currently provides technical consultation for severe service control valve applications to optimize customer solutions. He earned a BS degree in mechanical engineering from Iowa State University.