Introduction: Why is This Important?
The molecular sieve dehydration unit is an important process in any plant that uses natural gas as a feedstock. Whether the plant is processing natural gas to make LNG, ammonia, or is processing gas to extract NGLs, it is imperative the gas is properly dried. Water in the gas passed along into subsequent processes can cause the formation of hydrates or destroy valuable catalyst.
Molecular sieve dehydration is currently the process by which almost all water is removed from gas. The switching valves are critical components in this process. If these valves do not perform as expected, the drying process will be compromised and the molecular sieve drying unit will not dry the gas to the required specifications.
Selection of the proper valve type for use as a molecular sieve switching valve is the first step to success in an optimally operating system. Several valve manufacturers claim the valve type they offer is perfect for molecular sieve switching valve applications, but few have a proven track record in actual service. Many valve types have been tried in this critical service; few have performed well. Of all of the valve types utilized for switching valves in molecular sieve dehydration service, the rising stem ball valve has a superior and proven track record.
First and foremost, in a dryer, the valve must seal tightly. If it is not possible to obtain tight shutoff, the leaky valve allows wet gas to enter the drying tower during the regeneration cycle. This leakage lengthens the regeneration cycle, wastes energy, and will not allow the desiccant to be fully regenerated, resulting in increased operating costs. The valve also must withstand high regeneration temperatures. Taking into consideration temperatures typically found in regeneration cycles and considering temporary excursions above typical regeneration temperatures, the switching valve should be designed for a maximum of 800° F (426° C).
The valves must be capable of withstanding the frequent cycling that is characteristic of dehydration cycles. For example, if a system is on eight-hour cycles, a valve could cycle three times per day, seven days per week, 365 days per year. If planned maintenance of the system is every five years, and this maintenance includes rebuilding of the beds and repair of the switching valves, the valve could see 5500 cycles between repairs. Not many valve types are capable of withstanding this many cycles in a hot, dry and, sometimes, hostile environment.
The rising stem ball valve provides tight shutoff, withstands frequent cycling, and handles high temperatures better than other valve types in this service. Other valve types do not have an equal track record in molecular sieve dehydration service, because no other valve provides the tight seal and friction-free operation in the same manner as a rising stem ball valve.
Taking shortcuts, or trying to lower costs of a unit by selecting unproven valve designs, typically proves to be a false economy. Everyone desires lower costs and a premium product for a discounted price, but in the case of dehydration switching valves, buying cheaper products usually proves to be most expensive course of action. John Ruskin (8 February 1819 – 20 January 1900), English art critic and social thinker, said it best:
It’s unwise to pay too much, but it’s worse to pay too little. When you pay too much you lose a little money, that’s all. When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing it was bought to do. The common law of business balance prohibits paying a little and getting a lot. It can’t be done. If you deal with the lowest bidder, it is well to add something for the risk you run. And if you do that, you will have enough to pay for something better.i
The most cost effective way to save money on dryer valves is to purchase proven technology during the planning and construction of the plant. This “buy it once” mentality almost always results in a process that performs within the required specifications, with significant reductions in downtime or unexpected shutdowns.
Construction, Startup, and Commissioning
The construction, start-up, and plant commissioning phases are critical in bringing a new plant or system online. It is possible to avoid many common problems seen in dehydration switching valves by implementing proper techniques and procedures during the construction and start-up phases of the project. The most common valve issue seen during the construction/startup is foreign matter in the valves. This foreign matter typically comes from the construction of the piping into which the valves are installed.
Welding operations, by their nature, are dirty. It is recommended to clean welding residue from the lines before installing the valves, which is best accomplished by flushing the entire system. Only when the lines are clean should installation and operation of the valves commence, since damage to the valves can occur if this critical cleaning operation is not performed. The most common debris found in piping and valves following construction includes weld slag and miscellaneous debris from the construction process.
Hard particles in weld slag can damage coatings, platings, and overlays. If a valve closes on the particulate, the base material can yield, compromising the integrity of the coatings. Once this occurs, the coatings, especially hard coatings such as tungsten carbide and Stellite®ii, may crack and chip, exacerbating the problem. This damages the valve sealing surfaces significantly.
Damage Caused by Weld Slag During Construction that was Found at Startup
It is common for other types of debris to find their way into piping systems during the construction phase. These can be anything from bits and pieces of wood, juice cans, safety helmets, hand tools, or other debris left behind by construction crews. Anything left in the pipe can be a source of damage to valve components. A thorough cleaning of the entire system is critical for a successful startup.
Most molecular sieve dehydration switching valves are automated, and actuator operation directly affects valve operation and performance. For pneumatic actuators, which are the most common type used in this service, it is important that supply lines be adequately sized to deliver the appropriate volume of instrument air to smoothly open and close the valves without jumpingiii.
Saving a few dollars by using smaller instrument lines at the time of construction can adversely affect the smooth, efficient operation of critical service valves once the plant is in operation. When the valves do not work properly, the unit does not work properly.
In cases where electric actuators are preferred, it is imperative that the actuator settings are consistent with the valve on which it is installed. Some valves are torque seated while other valve types are position seated. It is critical that the electric actuator be properly set, or the valve cannot close or open fully, adversely affecting performance. Improper torque settings and/or position settings are commonplace when field personnel, unfamiliar with the operation of the valve or actuator, adjust these settings; typically resulting in poor valve performance or damage.
It is normal for dust or fine powder to escape the beds, especially following new construction or the reworking of a drying tower that involved the change out of desiccant. Valves designed for this service, especially the rising stem ball valves mentioned earlier, will handle normal dust and carry over without issue.
Once a plant is past the construction and start-up phases and has been in operation for a period of time, the most common cause of damage to valve sealing surfaces is molecular sieve desiccant escaping the screens and finding their way into the valves. If the desiccant escapes the tower, it can find its way between the valve sealing surfaces. This can result in damage to the closure members when the valve closes on this material, causing yielding in the base material supporting the hard facing. No valve trim is designed to adequately handle this foreign material. The solution is proper installation of the molecular sieve desiccant and ceramic balls that make up the components in the drying tower, and proper installation of the screens.
Once seat damage has occurred and seat leakage begins to cause concerns in the efficiency of the unit, a common practice for well-intentioned operators is to attempt to get the valves to “seal tighter.” This attempt usually means increasing the closing air pressure on the instrument regulator. This results in more closing force being applied to the valve and can result in a temporary improvement in sealing. In the long run, however, increasing air pressure above the manufacturer’s recommendations results in accelerated wear (best case) or complete failure of a key component (worst case). Simply keeping the desiccant in the tower enables the plant to avoid problems.
Typical Component Damage Caused by Excessive Closing Force
Another common problem in the operation of switching valves is cycling the valves too fast, which can cause accelerated wear on critical valve components and premature valve failure. The most effective speed controls are adjustable valves placed in the exhaust ports of the three-way or four-way solenoid valves on the instrument panel. If the speed controls are placed in the supply line, the actuator could “starve” for air, resulting in jumping or erratic operation. Placing the speed control in the exhaust port eliminates this problem and will allow precise regulation of the valve operating speed. This simple, but often overlooked, detail can greatly extend valve operating life.
Re-pressuring and de-pressuring of the towers is dependent on many factors, such as tower size, pressure, flow restrictors, etc., and most experts agree that de-pressuring of the tower should be gradual, no faster than a 50 psi change per minuteiv. Some form of flow control is necessary for the tower to gradually de-pressure and if this flow rate is not taken into consideration and the appropriate flow limiter installed at the time of plant design, high fluid velocities can occur when the de-pressuring valve opens. If this high flow rate is not taken into consideration at the design stage, the de-pressuring valve is likely to be damaged. The solution is to consider the potential flow rates at the de-pressuring line and install the appropriate flow restricting devices. Changing trim materials in the valve does not solve this problem.
Turnarounds, Shutdowns, Repair, and Maintenance
Once the plant is operational and all of the construction and start-up bugs have been worked out, it is common for a dryer to operate continuously for an extended period of time, perhaps five years or more. Eventually the beds will require attention and a turnaround is scheduled. During this turnaround, it makes economic sense to inspect and repair all equipment in the system so that the next run cycle can be long and trouble free.
To ensure proper operation of the unit, rebuilding of the beds must receive the same care and attention that was given at the time of construction. It is necessary to remove and replace the ceramic balls and desiccant and to inspect and replace the screens along with all of the packing. It is common for an operator to experience problems on next startup similar to those during initial plant startup. Loading of the ceramic balls and desiccant is critical, and the same care and attention to detail is critical, to keep the ceramic balls and desiccant in the tower and out of the valves. While this can seem elementary, many operators are forced to re-learn these start-up lessons immediately following a turnaround.
It is a common practice, and highly recommended, to refurbish the switching valves during a turnaround.
There are two schools of thought as to the extent of repairs performed during a turnaround:
- One is to inspect and replace only those components exhibiting damage or wear. This approach can make sense in cases where the service support and spare parts are easily accessible. Additional parts, if necessary, could be expedited and additional service personnel called in when needed. If this approach is chosen, care must be taken to ensure parts are available and can be acquired within the time frame. If the plant is isolated in a location where importation of parts is difficult, visas for service personnel are problematic, or the valve components are special (special material, size, pressure class, etc.), this approach is less viable.
- The alternate approach is to prepare to replace all components in the valve and have these parts on hand prior to shutdown. While this approach seems the most expensive alternative, it can save money in the long run by avoiding costly future delays. Both approaches have their merits; each operator must decide which is best for their operation.
In addition to the valves, consideration should also be given to the actuators. Whether electric or pneumatic, the actuator is the key component in the successful operation of the dryer. It is easy to focus on the repair of the tower and the valves and completely overlook the actuators. Remember, the actuator can stop the operation of your plant just as quickly as any other critical component.
It is also highly recommended that personnel chosen to conduct the repairs on valves and actuators be qualified to perform the task. Any company can claim to have the expertise to repair valves and actuators to factory specifications; few, however, have the factory training or necessary knowledge. Saving a few dollars here can cost many dollars later in the form of unplanned shutdowns or poor performance from the valves/actuators that were badly repaired.
Proper routine or preventative maintenance is another way to extend valve life and/or eliminate that “call in the middle of the night.” Valve and actuator manufacturers will have a recommended preventative maintenance schedule for their products, and these schedules are based on the experience they have accumulated over the years. Following these recommendations can save much more than they cost and prove to be a valuable investment.
Extending the operating life on switching valves used in molecular sieve dehydration service is not rocket science; it is simply paying attention to some simple details.
- Install the valves into a clean system.
- Keep the molecular sieve desiccant and ceramic balls in the drying tower.
- Maintain recommended air pressure on the actuator so the appropriate closing force is applied (and not exceeded).
- Control the speed of operation to conform to manufacturer recommendations.
- Ensure properly sized instrument piping is installed.
- Follow the manufacturer’s recommended preventative maintenance program.
- Properly repair valves and actuators according to the manufacturer’s recommended procedures.
- Consult the valve and actuator manufacturer for the appropriate repair parts.
If these simple steps are followed, the life of your molecular sieve dehydration switching valve will be greatly extended and will improve performance.
Written by Mike Wood
i Ruskin, John. Common Law of Business
ii Stellite is a trademarked name of the Deloro Stellite Company
iii Starving the actuator for air pressure
iv Gas Processors Suppliers Association. (2004) Engineering Data Book 12th Edition. (Volume 1 Section 1-15). Tulsa, OK. Gas Processors Suppliers Association