Everything You Need to Know About How Control Valves Operate
Water, oil, gas, and more drive industry. Without a network of pipes to flow through, everything would be at a standstill. The city of Los Angeles alone delivers some 167 billion gallons of clean water a year to customers.
What do industry giants Kraft Velveeta, Coca Cola, or Pacific Oil and Gas have in common with the city of Los Angles?
They all use control valves to regulate the motion of liquids or gases through their systems. Although liquid cheese, sugar syrup, and natural gas are very different, the mechanisms that control their flow are similar.
Read on to learn about different types of valves and how they work.
What’s Your Process Loop?
If you are moving a product, there may be hundreds of points where there is some important variable that needs control. Pressure, flow, temperature, level, etc. all have effects on your end result.
One little disturbance in a variable can change other points, which in turn affects the next loop. Valve controls manipulate fluid flows to limit changes. They limit process variability and increase efficiency.
Rotating Types of Control Valves
Valves can be automated or operated manually. There are several different types, each suitable for certain uses. control valve body types. We can divide them into two groups: linear and rotating.
Rotating control valves have an element that turns within the flow to modulate it.
Full or Segmented Ball Valves
A rotating control valve suitable for on/off service, wide pressure and flow variability. Segmented ball valves are appropriate for slurry.
Ball valves can be used to modulate the flow of fluids. The ball acts as a plug, which stops or allows full or partial flow through the valve.
The cost is between globe valves and butterfly valves.
A lower cost and less precise alternative to other types of rotating valves. They use less space and are available in large diameters. It is a ring-shaped, inline valve with a disc in the middle, that works as the plug.
Another lower-cost alternative to ball valves. Used at low-pressure drops under high flow rates.
Linear Control Valves
Linear control valves use the straight-line motion of a sliding stem to adjust flow.
Globe valves are a linear control valve. They come in three basic types: single-seat, double-seat or three-way.
Good for a variety of flow characteristics and provide precise control of flows. Best for clean fluids. Tends to be the most expensive choice.
A type of linear control valve also known as a knife or guillotine valve. Used for isolation or cutoff. Not suited to modulate or throttle flow. Usually, they block low-pressure gas and airflow. They come in single and double seat versions.
These valves are suitable for abrasive or corrosive fluids. They are used for on/off service and modulation. Air pressure provides the action to collapse the sleeve closing the valve.
Pinch valve construction prevents leaks and forms an excellent seal. Selection of the right valve for your control loop is only part of the task.
Valve Size Selection
Many valves are oversized for the process they control. Under normal operating conditions, they are at a low percentage of their rated flow capacity. Valves that are oversized deliver less precise adjustments than correctly sized valves.
This may be to adjust for expected failures in previous process loops or engineering inexperience and error. There are more than 30 pieces of information to collect to calculate the correct type, size, and characteristics of your control valve. For example:
- Type of fluid
- Temperature of fluid
- Viscosity of fluid
- Specific gravity of the fluid
- Flow capacity required
- Inlet pressure
- Outlet pressure
- Pressure drop during normal flowing conditions
- Pressure drop at shutoff
- Permissible noise level
- Degrees of superheat or existence of flashing
- Inlet and outlet pipeline size and schedule
- Special tagging information required
- Body material
- End connections and valve rating
- Action desired on air supply failure
- Instrument air supply available
- Instrument signal
- Valve type number
- Valve size
- Valve body construction
- Valve plug guiding
- Valve plug action
- Port size
- Valve trim materials required
- Flow action
- Actuator size required
- Bonnet style
- Packing material
- Accessories required
This list is taken from the Emerson Control Valve Handbook, 4th Edition.
A control valve’s response speed is how fast it changes position after it receives a signal. The faster a valve moves into position, in response to small changes, the more precise the control of the process variable.
The dead time or lag time between a signal and the position of the valve plug is lost time in production. These moments of destabilization control steal your productivity.
You need the appropriate actuator to deliver a speedy and effective response.
Control Valve Actuators
The body part of the control valve stays in contact with the product. The actuator opens and closes the valve in response to the need to modulate the flow. Actuators match the valve body type: linear or rotating.
There are three different systems that provide movement. They are pneumatic, hydraulic, and electrical.
Pneumatic actuators use air pressure to create a linear or rotating movement. A hydraulic actuator uses liquid instead of air pressure. Electrical actuators use motors.
All systems require a power source. Electrical actuators use power to drive the motors, while hydraulic and pneumatic actuators draw power to a compressor pressurize the liquid or air. Pneumatic actuators operate in the widest range of environmental temperatures.
Pneumatic actuators are used under hazardous or harsh conditions because they do not spark or create additional heat.
Choose the Right Control Valves
The selection of the correct control valves for your project loop is a vital decision. Cost-effectiveness, quality, and safety are on the line. Keep the flow of liquids and/or gases running smoothly.
Industrial transport of liquids and/or gases is part of many products, from energy to foodstuffs. Depending on the nature of the items moved, you may need rotary or linear valves.
As important as the choice of valves is the choice of actuators. Actuators provide the speedy response and automation required for accurate process control.
Let our experts help you select the right control valve from our large range. Contact us today!