TECH TALK
Three Steps to Ensure Control Valve Reliability in PSA Applications
Although high-quality valves are available from a range of original equipment manufacturers ( OEMs ), valve performance issues will invariably occur . Over time , pressure swing adsorption ( PSA ) process pressures ranging from 300 to 700 psi contribute to equipment wear and result in leakage . This can impact process efficiency along with the volume and purity of the hydrogen end product . Leakage at the valve seat or stem also raises the risks of fugitive emissions and non-compliance with environmental regulations .
By Cris Sidwell , Global Product Leader , Control Valves - Flowserve
Pressure swing adsorption ( PSA ) applications are essential to operations in chemical plants , oil refineries , and other facilities that separate and purify a wide range of industrial gases , including hydrogen . Dozens of control valves , along with isolation and on-off valves , are installed with piping and instrumentation on large skids paired with adsorbent beds in the PSA process .
Less-than-optimal valve performance or equipment failure that disrupts production can create significant unplanned expenses for unit operators . However , companies can avoid unexpected downtime and delays — which in some cases , add up to millions of dollars — by taking the following three steps :
1 . Select the right valves for PSA application requirements with an emphasis on maximizing reliability .
2 . Ensure the valve supplier can provide expert , timely support , and services when urgently needed .
3 . Complete projects on agreed-upon schedules and within budget by partnering with an OEM experienced in PSA applications .
Major Challenges : High Cycle Rates and Pressures
PSA skids designed for hydrogen production incorporate dozens of control valves opening and closing in the right sequence at the right times . Most valves — along with the associated pneumatic actuators and positioners — cycle 120,000 to 150,000 times a year .
The high process pressures are necessary to separate carbon from methane in the feedstock to produce hydrogen . Cycling the pressure in the beds as many times a day as possible maximizes the amount of hydrogen produced . But the extreme conditions take their toll on valves ; it causes wear of valve seats , as well as the guides in globe valves , and the bearings in butterfly valves , which degrades shut-off capability and process control .
Another common failure mode is when valve stem leakage is caused by packing wear or failure , which can result in the loss of valuable gas feedstock and purified hydrogen . Stem leaks also release fugitive emissions , raising safety concerns and causing compliance issues .
The high number of cycles takes its toll on actuation as well . Piston or diaphragm seals can wear , causing pneumatic actuators to become less responsive . This leads to the loss of precise valve control . Actuator springs and connection points can also be affected . In such cases , actuators can lose throttling capability or the ability to move to a fail-safe position in the event of power loss , while valve-to-actuator and positioner-to-actuator connections can loosen over time and result in decreased throttling accuracy .
How to Address Common PSA Skid Valve Issues
When selecting butterfly or globe valves for PSA applications , it is important to consider how the valves are engineered to resist wear to deliver control accuracy and rangeability .
Here are three steps one can take , along with questions to ask OEMs , to enable them to confidently choose flow control equipment for PSA applications in operations :
1 . Select the right valves for PSA application requirements , with an emphasis on maximizing reliability .
Control valves on PSA skids must maintain tight shut-off . Unit operators should thoroughly evaluate valve design with respect to the most common wear points previously noted .
Underperforming valves can adversely affect process efficiency and lead to unnecessary maintenance and repair . However , wear-resistant globe and butterfly control valves are ideal for PSA processes if they are designed to meet stringent leakage requirements and can tolerate rapid cycling .
This is why some manufacturers subject their valves , actuators , and positioners to rigorous one million-cycle endurance tests . The test results are accepted as proof that the equipment can withstand the high cycling rates and temperatures typical of operating conditions in PSA applications .
Reliable high-cycle performance starts with thoughtful design . Seat design is a key differentiator among high-performance butterfly valves . The first recommended step is therefore that an individual specify a butterfly valve that has a seat design that adjusts to maintain contact with the disc , even after wear . This is less of an issue with globe valves , which have a tapered plug that can be driven farther into the seat to maintain proper contact .
• Material choices impact durability and wear as well . Specify a seat design that includes ultra-high molecular weight polyethylene ( UHMWPE ) components . This material possesses excellent wear resistance and can prolong seat performance .
• The operator should make sure to specify high-cycle , fluid power units when it comes to actuators . They should include wear-resistant seals , long-lifecycle springs , and very tight connection points with the valve and positioner . Splined shafts are best . It is best to avoid designs with a mechanical adapter between the shafts because of the high incidence of wear at the connection point .
22 Valve World Americas | September 2023 • www . valve-world-americas . net