VALVES - SELECTION OF VALVES AND ITS TYPES

 WHAT IS THE VALVE?

• A valve is a device that controls not only a flow, but also the rate, the volume and the pressure, direction of liquids, gases, toxic gases, slurries through a pipeline chute or passageway.

REASONS FOR FITTINGS?

1. Complete isolation
2. Throttle

TYPES OF VALVES:

Basically the valve types are classified into 8 types there are,

1. Screw down valve
2. Parallel slide valve
3. Gate valve
4. Check valve
5. Ball valve
6. Butterfly valve
7. Diaphragm valve
8. Bellow seal valve

SCREW DOWN VALVE:

• It uses threaded spindle to control valve position

There are four types of valve seat and geometry

1. Mitre valve
2. Flat valve
3. Globe valve
4. Hemispherical valve

PARELLEL SLIDE VALVE(PSV VALVES):

• It used a stream system where full flow and no control is required 

• Parallel disc forming valves lids are kept in the contact with valve seat by a spring

• Tight seal to flow is maintained by differential pressure acting on valves seat and lids.

GATE VALVE:

• It general service valve, it operates (open/close) by a disc or gate that slides down through the valve to block the flow.

RECOMMENDED USES:

• Fully open/close, non-throttling
• Infrequent operation
• Minimal fluid trapping in line
• These valves are often described in terms of sealing element. (Eg :solid wedge, parallel slide, parallel double disc and split wedge

CHECK VALVE:

• Check valves are described as reflux, non-return, back pressure, retaining valve.

• The check valves are designed to prevent back flow. Fluid flow in the desired direction opens the valve while backflow forces the valve closed.

• Most check valves are based on either swing concept or lifting concept. Some common types are,

1. Swing check valve
2. Split check
3. Dual plate check

BUTTERFLY VALVE:

• A flat plate is positioned in the center of pipe. The plate is connected to the Rotating handle which turns the plate either parallel or perpendicular to the flow.

• A tight seal obtained by a synthetic rubber fitted around the edge of the plate.

• Valve can be used with on actuator on remote.

BALL VALVES:

• Valve consists of a ball having a hole through its center.

• Ball fitted between the two seats of synthatic material.

• Quick Operation by a Rotating handle.

DIAPHRAGM VALVE:

• Diaphragm valve is a linear motion valve that is used to start, regulate and stop fluid flow.

• The name is derived from its flexible disc which mates with a seat located in the open area at top of the valve body to from seal.

BELLOW SEAL VALVE:

• This type of valve used on nuclear submarine.

• Used as stream stop / bypass valve

• Consists of welded valve body with removable bonnet assembly.

• Valves sealing is achieved by bellows.

SELECTION OF VALVES:

• The substance to be handled and required flow rate.

• The ability of valve to withstand the maximum working pressure and temperature.

• The ability of valve to resist attack by corrosion or erosion.

• Actuator requirements

• Maintenance and repair requirements