Separator Letdown Control Valve Solutions
Ensure separator letdown valves can withstand the harsh
conditions and provide uninterrupted control.
Ensure separator letdown valves can withstand the harsh
conditions and provide uninterrupted control.
High-pressure separator letdown valves are among the most severe control valve applications in a refinery and are found in the Hydrocracker and Hydrotreater units. The hot high-pressure separator (HHPS) and cold high-pressure separator (CHPS) each have two letdown valves that maintain separator level to ensure proper separation of liquid and gas products. Because of the multiple constituents involved along with the entrained gases and catalyst, these valves are exposed to a number of challenges:
•Outgassing
•Flashing
•Cavitation
•Vibration
•Entrained catalyst
•Corrosive H2S
The negative consequences of these phenomena, including erosion, excessive vibration, and noise, makes sizing and selecting valves especially difficult.
The hot high-pressure separator (HHPS) cuts the heavy and light reaction products and recovers the hydrogen flashed off during the letdown from the reactor. The heavy product liquid is sent to the fractionator, stripper, or a combination of both. The light product liquid is cooled and injected with wash water to absorb ammonia (NH3) and hydrogen sulfide (H2S). The mixture is further cooled before entering the cold high-pressure separator by the recycle gas exchanger and fin fan (HHP separator cooler).
The cold high-pressure separator (CHPS) is designed to allow all overhead gas to flow through a high-pressure scrubber before entering the recycle compressor. Redundant separator depressurizing valves are located in this line for depressurizing the unit in emergency situations or during shutdowns of the unit. These valves will see high pressure drops with associated noise and vibration for short periods of time. A multi-stage trim valve is often necessary due to the high pressure drops, entrained catalyst, and two-phase fluid phenomena. A multi-step, low-noise valve should be applied under those conditions. These valves are typically specified with a fire-safe requirement for the actuation and instrumentation.
Emerson has specific Fisher™ sizing and engineered solutions for all separator letdown applications. These solutions are designed to address outgassing and flashing effects that can occur in more HHPS letdown valves and CHPS letdown valves. They offer protection against cavitation, erosion, plugging, and leakage
Fisher DST-G trim is specially designed for clean or dirty services where the fluid has dissolved gases that are released from a solution due to a reduction in pressure (outgassing). In separator letdown valves, outgassing typically causes two types of damage: cavitation and erosion by the fluid stream. DST-G trim stages the pressure drop, similar to the DST trim, to prevent cavitation. Different than the DST trim, erosion from the entrained gas is controlled by the special slotted lower cage. These slots further separate the flow stream into smaller, lower energy jets that are less likely to cause damage.
Fisher NotchFlo DST control valves are designed for applications prone to cavitation, like separator letdown. The trim in this valve utilizes multi-stage, axial flow process to control pressure drops, prevent cavitation, and pass entrained particulate. It also features a protected seat design that helps avoid clearance flow erosion for long-term shutoff integrity.
To verify proper valve operation after installation, a Fisher FIELDVUE digital valve controller can be used to monitor valve performance. The DVC6200 digital valve controller provides diagnostic reviews without interrupting the process in order to identify potential performance issues. This helps ensure proper operation and tight shutoff over the service life of the valve.
