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Hydraulic Valve Applications in the Chemical Industry

For offshore oil and gas installations as well as refineries and petrochemical plants, the chemical industry uses hydraulic valves for a broad range of functions, flow material and environments. The most common valve functions in the chemical processing industry are:

¨      Shut Off (Gate Valves)

¨      Directional Flow (Check Valves)

¨      Flow Throttling (Globe Valves)

¨      Safety (Pressure Relief Valves)

¨      Control Valves (Electro-Hydraulic Valves)

About a quarter of all valves used in the chemical process industry are gate valves. They are well suited for on-or-off control, but inappropriate for throttling fluid flow because the gate valve’s seating can erode if used in a partially open position for extended periods. A gate valve is constructed with valve body and a gate, which can be a flat face, vertical disc or sliding type. Moving the gate down the valve body closes the valve. When fully open, gate valves do not restrict fluid flow, which is an important advantage. Gate valves can be designed to operate in very high temperature liquids/gases, generally up to 650 °C. For transferring acids, lyes, abrasives, liquids with suspended solids and corrosive slurries, gate valves are constructed with special alloys and anti-corrosion lining.

Globe valves are primarily used for flow throttling. They are classified as low capacity valves, which limits their application in the chemical process industry. In addition, after extended service, their seating can erode due to the effects of turbulent fluid flow. Globe valves are typically constructed with plug that has a flat or convex bottom. As the valve is closed, the plug lowers into the body until its mates with the horizontal seat for a decisive shut off. They are reliable where decreases in pressure are not crucial.

Electro-Hydraulic Control Valves

In general, all control valves regulate fluid flow in a chemical process. They are can be remotely opened or closed by electrical, pneumatic or hydraulic power. They are constructed with a valve body and actuator, which replaces the hand wheel of manual valves. Control valves can use water or oil as its power source. The control valve’s hydraulic system consists of a pump, an accumulator tank and a control unit.

Electro-hydraulic control valves are constructed with a spring return actuator and are connected to the reservoir tank, which contains an adequate volume of hydraulic fluid to fill the actuator. Completing the system is a solenoid valve and a small pump. The solenoid valve is used to energize to pump, which forces hydraulic fluid into the valve actuator. The actuator’s piston is driven against the spring, which moves the valve into the open or closed position, as required. To return the electro-hydraulic valve to the closed position, the solenoid is deenergized, which stops the pump. As a result, the spring-return of actuator forces hydraulic fluid back into the tank and the valve is moved into the closed position.

Electro-hydraulic control valve systems are an improvement over pneumatic systems, since the hydraulics eliminate the need for an air compressor, air filter, dryer and regulator. In addition, hydraulic systems can function normally at low temperatures, where air systems would freeze. They are an accurate positioning control that limits both the deadband and hysteresis, which are common to pneumatic power sources.