What Instruments Do in a Process Plant
The four jobs of instruments — measure, show, control and protect — and how they join together in a control loop.
8 min
Activities: 0 of 2
The senses of the plant
A process plant moves and changes liquids and gases. Operators cannot see inside pipes and vessels, so they depend on instruments. Instruments are the senses of the plant. They tell people, and computers, what is happening inside.
Instruments do four jobs. They measure a value, such as pressure. They show it, on a gauge or a screen. They help control the process, by sending the value to a controller that moves a valve. And they protect people and equipment, by stopping the process when a value becomes dangerous.
A simple control loop
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1
Measure
A level transmitter measures the liquid level in a tank.
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2
Send
It sends the level as an electrical signal to the control system.
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3
Compare
A controller compares the level with the wanted value, called the set point.
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4
Act
The controller moves a control valve to let more or less liquid out.
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5
Repeat
The loop repeats all the time, so the level stays close to the set point.
Put the control loop in order
- 1The controller compares the level with the set point
- 2The controller moves the valve
- 3The signal goes to the control system
- 4The transmitter measures the level
What is the set point?