MORE TOPICS | Aftermarket
Maintenance Considerations for Pumps Used in Amine Treating Applications
Amine units are critical in transforming raw natural gas.
Todd Keefe | Sundyne
| IMAGE 1: Amine treating units are typically located near the front end of a gas treating plant. (Image courtesy of Sundyne)
Heavy duty pumps used in amine treating and other hydrocarbon processing applications are designed to run for decades. The initial cost of a pump deployed in these applications is approximately 15% of the total life cycle cost. Operating/energy expenses can consume another 40% of life cycle costs, leaving almost half of the total costs to maintenance and optimization. For this reason, it is critical to streamline maintenance activities that keep pumps operating at optimal conditions.
When raw gas exits a well, it contains methane (CH4), with smaller amounts of water (H2O), ethane (C2H6), propane (C3H8), butane (C4H10), carbon dioxide (CO2), nitrogen (N2) and hydrogen sulfide (H2S). Removing impurities and heavier hydrocarbons (which can be sold as byproducts) produces pipeline quality, methane-rich natural gas, which can be liquefied or sent to power plants.
Amine scrubbing is an important gas treating application. It removes impurities from a gas stream by pumping aqueous alkylamine solutions onto the raw gas to absorb H2S and CO2.
“Lean” amines chemically react with raw gas, causing impurities to transfer onto the amine solution. Removing H2S and CO2 creates “sweet gas” (mostly methane), which exits the “absorber” while the rich amines bring impurities to a stripper/regenerator. Intense heat breaks the chemical bonds between the rich amines and the absorbed acid gases. Once regenerated, the amines (now returned to a lean state) are cooled and reused.
This process is repeated multiple times. In simple terms, amine treating is like a reusable chemical sponge that picks up impurities from natural gas, gets heated to release the impurities and then goes back around to pick up more.
Primary Maintenance Activities for Pumps in Amine Scrubbing
Amine solutions, such as monoethanolamine (MEA), diethanolamine (DEA) or methyldiethanolamin (MDEA), are hot, abrasive and chemically active substances. They can cause a variety of issues for pumps, including corrosion, cavitation and seal leakage. To prevent this, maintenance personnel should focus on the following.
Seal inspection and maintenance
Amine pumps handle aggressive chemicals and operate at highly elevated temperatures. Seal arrangements are an important design consideration that impacts safety and maintenance. Maintenance personnel should actively monitor seal temperatures, seal flush/cooling, elastomer conditions and shaft/sleeve wear. It is important to have replacement seals readily available.
Some amine treating units use sealless magnetic drive pumps, which replace mechanical seals with a static containment shell or shroud that forms a completely sealed liquid end or pressure boundary that removes any potential leak paths. Removing the seals removes a common point of failure. It also simplifies maintenance, because sealless magnetic drive pumps have fewer working parts and do not require seal support systems.
Bearings and vibration
Routine vibration monitoring is another important activity. Maintenance teams should look for increased vibration or for any imbalance or misalignment. Tracking vibration trends is more useful than single or occasional vibration measurements.
Cavitation
Amine pumps can be particularly sensitive to suction conditions. It is important to select pumps with lower net positive suction head required (NPSHr) than the system’s net positive suction head available (NPSHa). NPSHa is the fluid pressure energy available at the suction port of a pump, minus the fluid’s vapor pressure. If the pressure at the pump suction gets too low, vapor bubbles can form. If vapor bubbles move inside the pump, they collapse and cause cavitation.
In an amine treating unit, NPSHa is a function of the piping and the entire amine system, while NPSHr is a characteristic of the pump itself. In amine units, suction conditions can decline when absorber/stripper liquid levels drop or when amine temperature rises. Any resulting cavitation in the pump can damage the bearings or the impeller, which could eventually lead to a pump failure. To avoid this, suction pressure and temperature should be monitored as part of preventive maintenance.
Impeller inspection
The condition of the impeller has a direct effect on head and flow. During turnarounds, maintenance technicians should look for corrosion or any loss of material on the impeller from chemical attack. Accumulated deposits should be removed, and if signs of wear are noticeable, the impeller should be replaced, as unintended changes in clearances on the impeller will reduce hydraulic efficiency.
Coupling
Alignment between the pump and the motor should be checked during routine maintenance. Any misalignment issues can cause unwanted vibration, bearing wear and mechanical seal problems. Laser alignment is helpful because even small alignment errors can create substantial forces at high operating speeds.
Oil changes
For a centrifugal pump used in amine treating, oil-change intervals are generally based on bearing temperature, oil condition, contamination and a pump manufacturer’s recommendations. A variety of factors can warrant more frequent oil changes, such as water or amines entering the bearing housing, high bearing temperature, particulate contamination or any mechanical seal or shaft seal leakage. If any of these conditions are detected, the pump’s oil should be changed, and the source of contamination should be investigated.
Typical Maintenance Guidelines
Amine service pumps are good candidates for condition monitoring and predictive maintenance programs that feature a combination of continuous and periodic activities. Continuous monitoring should be conducted for vibration, bearing temperature, seal leakage, suction/discharge pressure and flow rates. Periodic/preventive maintenance activities can include coupling inspection, alignment verification, seal inspection, bearing condition assessment, etc.
Aftermarket Services Site surveys
The pump condition and maintenance activities referenced above can be complemented with site surveys, as field-service engineers often bring unique insights to troubleshooting “bad actor” pumps. Site surveys provide startup assistance for new and repaired units.
Rerates
If process conditions change and operators identify units not running near best efficiency point (BEP), hydraulic rerates can help to get things back on track. Efficiency improvements are achievable without requiring a new motor, footprint or piping.
Why Amine Treating Matters
Natural gas is one of the world’s most relied upon energy sources. It is widely used for electricity generation and heating and as a feedstock for petrochemicals. New data centers—as well as surging demand for liquefied natural gas (LNG) in Asia and around the globe—continue to increase demand for natural gas, and amine treating is required to clean natural gas before it can be transported, sold and used.
Removing H2S and CO2 from gas streams via amine treating prevents corrosion in pipelines, valves, vessels and heat exchangers. Pipeline operators typically impose strict limits on H2S, CO2 and water content. CO2 does not have any useful heating value, but it takes up space, so removing it improves the quality and value of the gas.
Todd Keefe is a marketing communications executive who has worked in the pump and compressor industry for decades. He may be reached at todd@firpr.com. For more information, visit sundyne.com.
In This Issue
