Selection Of Valves Criteria and Basics For Piping Network

Valve is a component used for controlling and regulating flow of liquids or gases to maintain the process parameters such as flow, pressure, level etc. Valve as a final control element plays an important role in any process plant. To slash cost of production and to increase productivity, majority of the valves are automated in modern plants. The important aspects to be considered while selecting a valve are Performance, Reliability, Compactness & Energy consumption. Thus the user needs to have high degree of awareness in the field of valvology, automation & control accessories.

Valve:

Valves are broadly classified as:

• By design

• By function

Design:

This classification is based on the motion of the valve stem.

The available designs are:

1) Liner

2) Rotary

The types of valves covered in the Linear category are Gate, Globe,

Diaphragm & Pinch type where as Ball, Butterfly & Plug valves belong to Rotary group. 

Function:

This classification is based on the application of the valve.

1) On/Off or Isolation

2) Control or Modulating.

Normally Gate, Ball & Plug valves are preferred for Isolation i.e. On/Off application, whereas Globe, Diaphragm & Butterfly valves are preferred for control application. Ball & Plug valves with characterized trims are used for control application.

The third category is of Non Return valves where swing check & lift check Valves are available.

In all the types of valves number of variations in construction, material of construction, seat configuration, end connections etc. are available.

Gate valves:

They get their name from Gate like closure member. The varieties available in Gate valves are Wedge Gate valve (Solid & Flexible wedge), Split wedge gate valves, Parallel slide gate valve, Knife edge etc.

Globe valves:

In this type the seating is parallel to the line of flow. These valves are normally used for modulating applications. Possible variations in this type are valve with parabolic plug, Double seated globe valve, Angle type globe valve etc.

Diaphragm valves:

Normally recommended for Corrosive, Abrasive, Sticky, Viscous & Aseptic services. The two designs of Diaphragm valves that are available are Weir type & Straight through type.

Ball valves:

The variation available in the design & construction of ball valves makes it the most popular valve.

• Design: Floating & Trunnion mounted.
• Construction: Full bore & Regular bore.

Seat configurations: Soft seat, Metal seat, Primary metal & Secondary soft seat.

Butterfly valves:

Widely used for On/Off & control applications. They are available in centric design, single/Double/Triple offset design. Variations in the seat materials available are soft seated, metal seated, lined etc.

Plug valves:

They are available in Lubricated & Non lubricated design. Pressure balanced & non-Pressure balanced type are available in the Lubricated design & Lined & sleeved type are available in the Non Lubricated design.

Few of the critical applications in modern industry where valves are required to perform are

• Valve for Cryogenic application for temperature as low as – 196 deg cel. 
• Valve for hot reformer gas for temperature over 1100 deg cel. 
• Valve for vacuum system to 0.02 torr. 
• Valve for high pressure Synthesis gas at 400 bar. 
• Valve for piggable line. 
• Cavity free construction food grade valve 
• Valve for high-pressure drop with anti-cavitation trim. 

From above it is obvious that basic valve selection is a complex phenomenon & is dependent upon following key factors:

• Process fluid 
• Temperature 
• Pressure 
• Concentration 
• Permissible pressure drop 
• End connections 
• Safety regulations 
• Frequency of operation. 

Automation:

Automation or actuation of valves is perhaps the most critical & important aspect in the valve automation system.

Automation is commonly used for:

• Inaccessibility (Remote operation) 
• Continuous valve operation 
• Sequence control 
• Ease of operation 
• Operation in Hazardous environment 
• Reduction in operating costs 
• Fail safe requirement, etc. 

Methods of automation could be:

1) Electro - Pneumatic

2) Electric

3) Electro Hydraulic

4) Self actuated

The key factors involved in selection of actuators are: 

• Energy source available 
• Hazardous area classification 
• Control requirement 
• Process response 
• Valve characteristics including torque 
• Operating cycle 
• Valve orientation in pipe line 

Over 90% of the automated valves in the modern chemical process industry are with Pneumatic actuators. The types of Pneumatic actuators available are Piston type & Diaphragm type. Pneumatic actuators have the significant advantage of achieving Fail position by just introducing springs.

Electrical actuators are normally preferred where Pneumatic lines are not available & where fail safe condition is not desired.

Electro Hydraulic actuators are used for higher torque requirement, can give fail safe conditions but are expensive & bulky.

Special application in Refineries & Petrochemicals MOV demand Fire protection for Actuators & control accessories in addition to the valve with built in fire safe features. There are many proven & certified fire protection systems available today. They include fireproof enclosures, Fire Jackets; Special epoxy based fireproof compounds, which are moulded with the equipment.

Control accessories:

Based on the control desired & the area classification, selection of control accessories has to be done. Control accessories include Solenoid valve, Limit switches, Proximity switches, Filter regulator, Manual Override, Speed controller, Quick Exhaust valve, Thermal fuse, Position transmitter, Positioner etc.

Thus from above it is clear that the selection of valve is a complex process which requires expertise in Valve, Actuator & control accessories. A valve manufacturer, an actuator manufacturer or Control accessories manufacturer in isolation may not be able to supply the appropriate system. Today there are many organizations worldwide who can design & supply a tailor made valve automation system as they have acquired the necessary expertise in field of valve, Actuator & accessories & accept single point responsibility for the entire system. Hope you liked this selection of valves basic guide. 

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