Rugged and Reliable Molecular Sieve Dryer Switching Valves

Natural gas and hydrocarbon liquids undergo many processing steps to final products. Dehydrating hydrocarbons is a key step before further processing, such as fractionation and liquefaction. Molecular sieve dryers, known for their efficiency and based on the adsorption principle, are the standard way to accomplish dehydration. Emerson’s Olivier Beyerman shared some background and considerations on this application.

Molecular sieve dryerA typical dryer has at least one tower in an adsorption state and one in a regeneration state at temperatures as high as 315°C. The process flow alternates between towers to enable continuous drying. The valves used to perform this switching operation are subjected to harsh conditions, including high cycle rates, high temperatures, thermal cycles, and the potential for erosive/abrasive dust.

The reliability of the molecular sieve dryers is directly linked to the performance of the switching valves. These valves, crucial in the dehydration process, must be reliable, remain fully operable across the thermal and mechanical cycles, be able to seal at process conditions to grant long-term efficient plant operations, and comply with fugitive emission requirements.

Molecular sieve switching valves operate as dividers between two streams: the process stream being dried and the hot gas regenerating the beds. Therefore, the switching valve must have a tight bidirectional shutoff to avoid intermingling these flows and negatively impacting dryer efficiency. Valves should be able to pass API 598 testing with no visible leakage at maximum shut-off pressure as specified in both directions.

Emissions are another important consideration. Just a few years ago, 10,000 ppm was allowable; today, it is 500 ppm, and enhanced leak detection and repair (LDAR) requirements of 100 ppm are being discussed. Polished stainless steel stems are mandatory for long-lasting low emissions. While steel stems may have been tolerable in years past, today, they are a path to constant violations of emission limits and frequent maintenance. Live-loaded packing is critical, as is certification to standards such as ISO 15848-1 and Shell SPE 77-300.

The Offshore Reliability Data Project (OREDA) confirms that generally, 99% of potential emissions can be eliminated simply by using quarter-turn valves instead of a rising stem.

AEV C-Ball Valves, a modern quarter-turn switching valve solution, offers significant advantages in molecular sieve dryer applications. These valves, featuring a highly polished stainless-steel stem protected from side load and live-loaded packing operators, can operate for extended periods without requiring a single packing adjustment. They effectively address challenges such as seat wear, leakage, and ongoing maintenance requirements, enabling compliance with stringent emissions regulations. The quarter-turn design, advanced high-temperature, and self-adjusting live-loaded packing ensure long-lived, effective emissions containment. The AEV 2XC severe service valve is designed to meet the specific needs of the dehydration processes.

It features a metal to metal seating with quarter turn rotation and very limited friction only across the last few degrees of rotation thanks to its double offset design; furthermore, the valve is torque seated, in this way, sealing is obtained by contact pressure between the rugged C-ball and the metal seat firmly assembled to the valve body.

In this way, the sealing is not dependent on pressure or springs, as typical of other ball valve designs; cavities under the seat, which can accumulate particle deposits compromising sealing performance, are completely eliminated, making it ideal for bidirectional shut-off and delivering a safer and more straightforward solution.

Furthermore, the non-rubbing rotation of the double-offset C-ball design reduces wear and extends maintenance intervals.

The material selection, including the hard coating of the ball surface and stellite seat overlay, provides high resistance to abrasion and erosion. At the same time, the robust trunnions and unique bearing design reduce the wear of rotating parts while preserving packing from lateral loads.

Visit the C-Ball Valves section on Emerson.com for more information on severe service zero leakage switching valves and their use in other challenging processes, including cryogenics, high temperatures, high pressures, emergency shutdown (ESD), and high integrity pressure protection (HIPPS) applications.

 

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