Application Discussion
AD124
June 15, 2003

The purpose of this process is to remove Acid Gases (H₂S, CO_2, COS) from a raw natural gas stream to make the gas composition meet Sales Gas requirements. The Amine Treating Unit removes CO2 and H2S from sour gas and hydrocarbon streams in the Amine Contactor. Counter flowing the amine solution hydrocarbon stream absorbs the acid contaminants. The stripped gas is removed overhead, and the amine is sent to the regenerator. In the regenerator, the acidic components are stripped by heat and reboiling action and disposed of, and the amine is recycled.

The sour gas streams enter the bottom of the Amine Contactor. The cooled lean amine enters the top of the contactor column. The sour gas flows upward counter-current to the lean amine solution. An acid-gas-rich-amine solution leaves the bottom of the column at an elevated temperature, due to the exothermic absorption reaction. The sweet gas, after absorption of H2S by the amine solution, flows overhead from the Amine Contactor.

There are two severe service valves associated with the Amine Treatment process. The level valve controls the level in the tower as to not flood the overhead section or allow the level to drop too low which causes inefficient absorption as the gas stream flows from bottom to top of the tower. The other valve is the liquid pump bypass valve, which is used as a minimum flow bypass valve to protect the pump. This valve may not exist due to a variable speed pump driver.

The fluid is a water/amine solution that should not have sour components remaining in it but some specifications require this valve to meet NACE guidelines. For the purposes of fluid flow the valve acts like water so expect cavitation and related problems.

Typically, inlet pressure range from 500 PSI to 1000 PSI with outlet pressures around 100 PSI. Temperature is usually 150 °F. Because of variance in plant or changes in composition from plant to plant, valve size can range from 1 to 6 inches. The pressure drop ratio is in the recommended range for Cavitrol III 2 stage trim. The standard Cavitrol 400 Series SST trim is typically used. The trim requires good turndown, as it is a minimum bypass application. This valve will try to open as soon as the flow controller detects insufficient suction flow to pump.

For the installations that require these valves to meet the NACE guidelines use 17-4PH SST DHT cage, 316SST/Stellite plug and Stellite seat ring.

This is a closed system. For this reason some installations may experience solids buildup in the fluid stream that do not pass through small passage trim typically used for cavitating services. If this situation arises applying Dirty Service Trim or NotchFlo DST is the recommended solution. The DST trims can be designed to pass particles up to ¾” in some cases.