SELECTING A VALVE TYPE

When speaking of valves, it's easy to get lost in the terminology. Valve types are used to describe the mechanical characteristics and geometry (Ex/ gate, ball, globe valves). We'll usevalve controlto refer to how the valve travel or stroke (openness) relates to the flow:

1. Equal Percentage: equal increments of valve travel produce an equal percentage in flow change

2. Linear: valve travel is directly proportional to the valve stoke

3. Quick opening: large increase in flow with a small change in valve stroke

So how do you decide which valve control to use? Here are some rules of thumb for each one:

1. Equal Percentage (most commonly used valve control)

a. Used in processes where large changes in pressure drop are expected

b. Used in processes where a small percentage of the total pressure drop is permitted by the valve

c. Used in temperature and pressure control loops

2. Linear

a. Used in liquid level or flow loops

b. Used in systems where the pressure drop across the valve is expected to remain fairly constant (ie. steady state systems)

3. Quick Opening

a. Used for frequent on-off service

b. Used for processes where "instantly" large flow is needed (ie. safety systems or cooling water systems)

Now that we've covered the various types of valve control, we'll take a look at the most common valve types.

Gate Valves

Best Suited Control: Quick Opening

Recommended Uses:

1. Fully open/closed, non-throttling

2. Infrequent operation

3. Minimal fluid trapping in line

Applications: Oil, gas, air, slurries, heavy liquids, steam, noncondensing gases, and corrosive liquids

Advantages: Disadvantages:

1. High capacity 1. Poor control

2. Tight shutoff 2. Cavitate at low pressure drops

3. Low cost 3. Cannot be used for throttling

4. Little resistance to flow

Globe Valves

Best Suited Control: Linear and Equal percentage

Recommended Uses:

1. Throttling service/flow regulation

2. Frequent operation

Applications: Liquids, vapors, gases, corrosive substances, slurries

Advantages: Disadvantages:

1. Efficient throttling 1. High pressure drop

2. Accurate flow control 2. More expensive than other valves

3. Available in multiple ports

Ball Valves

Recommended Uses:

1. Fully open/closed, limited-throttling

2. Higher temperature fluids

Applications: Most liquids, high temperatures, slurries

Advantages: Disadvantages:

1. Low cost 1. Poor throttling characteristics

2. High capacity 2. Prone to cavitation

3. Low leakage and maint.

4. Tight sealing with low torque

Butterfly Valves

Best Suited Control: Linear, Equal percentage

Recommended Uses:

1. Fully open/closed or throttling services

2. Frequent operation

3. Minimal fluid trapping in line

Applications: Liquids, gases, slurries, liquids with suspended solids

Advantages: Disadvantages:

1. Low cost and maint. 1. High torque required for control

2. High capacity 2. Prone to cavitation at lower flows

3. Good flow control

4. Low pressure drop

Other Valves

Another type of valve commonly used in conjunction with other valves is called a check valve. Check valves are designed to restrict the flow to one direction. If the flow reverses direction, the check valve closes. Relief valves are used to regulate the operating pressure of incompressible flow. Safety valves are used to release excess pressure in gases or compressible fluids.