Valve Types and Functions Quiz

24 Questions

What is a disadvantage of using a gate valve for control applications?

Limited throttling capability

What is the primary function of an angle valve?

Changes flow direction (similar to an elbow with control capability)

A butterfly valve features a disc that rotates on a shaft to control flow. What is an advantage of this design?

Compact and lightweight

Why is a positioner used with a control valve?

To improve the accuracy and stability of the valve position

How does a positioner improve the performance of a control valve?

By compensating for pressure drops and friction in the actuator

What is cavitation in a control valve, and what effect does it have?

Rapid formation and collapse of vapor bubbles due to pressure drop, causing vibration and erosion

How can cavitation and flashing be prevented in control valves?

Reducing the pressure drop across the valve

Which of the following is NOT a component of a basic process control loop?

Final product

A temperature control system in a chemical reaction vessel is an example of a process loop for controlling which variable?

Temperature

The current flowing through a transmitter in a process loop can be considered a:

Process variable

In P&ID diagrams, a square box with a diagonal line represents:

Sensor

Instruments located in the physical process area (near the equipment) are represented by symbols in the:

Top half of the P&ID

The standard 4-20mA current loop is used in process control because it is:

Both a) and b)

In a pressure transmitter, a “live zero” calibration refers to adjusting the output signal when the:

Transmitter is powered on but no pressure applied

Which of the following statements about dead zero is INCORRECT?

It should be done frequently during operation

Why are transmitters used in process control systems?

To convert sensor signals into a standard format for easy transmission

Which of the following terms indicates the main difference between a two-wire and a four-wire transmitter?

Power supply

Which of the following is NOT a typical feature of a transmitter?

Flow control valve

An electronic capacitive differential pressure transmitter (DPT) can be used for level measurement. How does this work?

By measuring the pressure difference between the top and bottom of a tank

What is the drawback of using only proportional control?

Steady-state error (the process variable may not reach the setpoint)

Integral control (I) eliminates the steady-state error of a proportional controller by considering:

The accumulation of past errors

What is the potential issue with using only integral control?

Increase offset

Derivative control (D) anticipates future changes in the error by considering the:

Rate of change of the error

Which type of valve uses a disc that moves vertically to regulate flow?

Globe valve

Study Notes

Process Control Loops*
A process control loop typically consists of a sensor, controller, actuator, and final control element
The sensor measures the process variable (PV) and sends the signal to the controller
The controller compares the PV to the setpoint (SP) and calculates the error
The actuator receives the controller's output signal and adjusts the final control element to manipulate the process variable
Components of a Process Loop*
Sensor: measures the process variable (PV) and sends a signal to the controller
Controller: compares the PV to the setpoint (SP) and calculates the error
Actuator: receives the controller's output signal and adjusts the final control element
Final Control Element: manipulates the process variable (e.g., valve, pump, heater)
P&ID Diagrams*
Symbols in the top half of the P&ID represent instruments located in the physical process area (near the equipment)
Symbols in the bottom half of the P&ID represent instruments located in the control room or remote locations
Transmitters*
Used to convert sensor signals into a standard format for easy transmission
Can be two-wire or four-wire transmitters
Two-wire transmitters receive power from the signal loop, while four-wire transmitters have a separate power connection
Can have features like internal diagnostics, remote calibration capability, and linear output signals
Differential Pressure Transmitters (DPTs)*
Can be used for level measurement by measuring the pressure difference between the top and bottom of a tank
Can have features like zero elevation and suppression to adjust the span of the transmitter and compensate for static pressure
Smart Transmitters*
Offer digital communication capabilities
Can perform internal diagnostics and have remote calibration capability
Can be configured and calibrated using a handheld device or computer
Control Systems*
Error: the difference between the desired output (setpoint) and the actual output
Cycling: rapid on-off switching of the final control element
Direct Acting: the controller output increases with an increase in the controlled variable
Reverse Acting: the controller output decreases with an increase in the controlled variable
PID Controllers*
Proportional (P) control: adjusts the control signal based on the magnitude of the error
Integral (I) control: eliminates steady-state error by considering the accumulation of past errors
Derivative (D) control: anticipates future changes in the error by considering the rate of change of the error
PI controllers use a combination of P and I control, while PID controllers use a combination of P, I, and D control
PID Controller Tuning*
Ziegler-Nichols method: a common method for tuning PID controllers
Relay tuning method: another method for tuning PID controllers
Genetic algorithm method: a method for tuning PID controllers using optimization techniques
Final Control Elements*
Control valves: manipulate the process variable (e.g., flow rate, pressure, temperature)
Actuators: power the movement of the control valve (e.g., pneumatic, electric)
Valve trim: the internal parts of the control valve that come into contact with the process fluid

Test your knowledge of different types of valves, their functions and applications. Learn about gate valves, angle valves, and more.

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