Subject: Article highlights the challenges of picking the correct valves for high cycling adsorption processes.
By Keith Nehring, Emerson Automation Solutions
Keith Nehring, Chemical Industry Manager for Emerson Automation Solutions’ flow control products, recently published an article in the March 2020 issue of Chemical Processing describing the challenges when selecting valves for high cycling adsorption processes. His article is titled Select the Right Valves for Adsorption Processes and is summarized below.
What Is Adsorption?
Adsorption is the adhesion of atoms, ions or molecules to the walls of a porous material. The component held within the pores is then purged from the adsorbent bed during a regeneration process.
A very common adsorption process is used to remove water from instrument air. Air containing moisture is pumped into a cylinder filled with adsorbent beads at pressure. After adsorption of the moisture, dry air leaves the vessel. Once the beads are saturated with moisture, the bed is taken offline for regeneration. Here, heated purge gas raises the temperature of the loaded bed, driving off the adsorbed moisture.
The beds constantly cycle from one mode to another. Depending upon the bed type and process, the cycling may occur every in a range from once every minute to once every eight hours.
The Adsorption Valve Challenge
Selecting the right valves for high cycling, critical service adsorption process is not a simple matter. Keith describes the issues:
Many plants rely on adsorption to remove or separate specific components of a liquid or gaseous mixture. This mass transfer process typically involves multiple valves (Figure 1). Stroking frequency of those valves can exceed 60,000 cycles per year and maintaining industrial gas purity and efficiency requires tight shutoff to Class VI.
Figure 1. Reliable operation requires multiple specialty valves designed for high cycles.
Many problems occur from using the wrong valves. Poorly performing valves can lead to increased maintenance, unit trips and decreased efficiency. For example, when surveying ethanol producers from around the world, some of the most commonly reported maintenance problems related to adsorption include:
Improper selection of control valve assemblies for adsorption processes can cause unscheduled downtime. During this downtime, a typical 50-million-gal/yr ethanol facility can suffer more than $10,000/hr in lost revenue.
Key Design Decisions
Selecting the correct valves for an adsorption process is critical for long term service and equipment efficiency. Keith explains:
Proper selection of all adsorption valves requires adequate attention to two major concerns: tight shutoff and robust design.
Tight shutoff:
If the valves don’t shut tightly, the beds will leak and, therefore, decrease the process efficiency and increase costs. A recommended practice is to specify Class VI shutoff for all valves in the adsorption process to ensure the highest process efficiency.
Figure 2 (Figure 5 in article). A PTFE seal suits most applications and can ensure tight shutoff.
Typical process temperatures allow for the use of soft seals. Opting for a form of polytetrafluoroethylene (PTFE) for soft seals (Figure 2) will give the durability needed for the multitude of cycles while also providing a tight seal and Class VI shutoff. PTFE is a fairly low friction material, thus reducing the force needed to seat and unseat the valve. However, PTFE is limited to 450°F. So, if the adsorption units run above that temperature, the best option is to select a metal seal despite its drawbacks such as higher leakage rate, torque and wear. It’s critical to choose a valve with at least Class IV shutoff to ensure the smallest leakage possible as well as a hard-faced seat material to lengthen the life of seats.
Robust design:
This is crucial because valves must withstand a high cycle count. Not selecting a robust-enough design may lead to premature failure, which will incur increased cost through process downtime and additional equipment expenses. Choose a valve that has been tested to one million or more cycles to ensure reliability. Verify the testing took place at pressures equal to or greater than those expected in your process.
Also, consider bearings when selecting valves. PTFE-lined polyetheretherketone (PEEK) bearings are a great choice for high-cycle butterfly valves in adsorption units. However, PTFE-lined PEEK bearings also have a 450°F limit. Therefore, in some higher temperature units, you may need to opt for metal bearings. Insist upon metal bearings that use a hard material or have been hardened. Two possibilities are solid R30006 bearings or nitriding a softer material such as S31600A valve with metal bearings requires increased torque to operate, so you must select an actuator with suitably higher output torque.
When using sliding stem valves, opt for stainless steel trim with hard-facing. Hard-facing critical trim components such as the valve plug will keep wear to the minimum. Using a hard-faced plug also will minimize galling and increase valve life.
The last thing to consider when selecting both butterfly and globe valves is the packing system. It helps align the stem of a globe valve and the shaft of a rotary valve to operate and shut off properly, and, of course, also serves to prevent leaks. Many packing systems experience increased wear over time that can lead to greater atmospheric leaking. Using a live-loaded PTFE packing will ensure emissions are limited.
Summary
Keith summarized the topic saying:
Adsorption is a high-cycle process that creates wear on valves. Proper selection of packing and seals, along with ensuring the valve design is robust enough to withstand multiple cycles, will lead to a longer life and trouble-free operation.