Control Loops - Control Valves Quiz

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Questions and Answers

What is a primary characteristic of a single acting spring opposed actuator?

Requires a constant air supply to maintain position

Operates solely on mechanical friction without air pressure

Uses a piston to drive the valve directly

Utilizes a spring to return the actuator to its default position (correct)

Which type of actuator uses pressure on both sides of the piston to control its position?

Air cushion actuator

Single acting actuator

Double acting actuator (correct)

Diaphragm actuator

What is the main function of a valve positioner?

To match the valve position with the controller output signal (correct)

To stabilize pressure changes in the system

To amplify the input signal for better actuator performance

To manually adjust valve positioning during maintenance

How can a valve positioner modify the relationship between input and output air signals?

By providing split-range control among other methods Signup and view all the answers

What does the output pressure gauge on a valve actuator indicate?

The pressure applied to the actuator for valve positioning Signup and view all the answers

In pneumatic actuation, which component is typically a diaphragm connected to a stem for mechanical advantage?

Diaphragm actuator Signup and view all the answers

What occurs when the signal pressure is reduced in an air cushion type piston actuator?

Compressed air pushes the piston back up Signup and view all the answers

What is the function of instrument input pressure gauge during troubleshooting?

To show signal pressure from the controller Signup and view all the answers

What is the primary function of a control valve in a control loop?

To regulate the flow of materials by producing a differential pressure drop Signup and view all the answers

Which component connects the actuator to the valve body in a sliding stem valve system?

Bonnet Signup and view all the answers

What role does the actuator play in a control valve system?

It provides motion to the valve, changing instrument signals into mechanical movement Signup and view all the answers

What does the valve positioner respond to in a control valve system?

The comparison between the position of the stem and the value of the instrument signal Signup and view all the answers

In a valve system, what is the purpose of the diaphragm?

To increase force to move the stem Signup and view all the answers

What component is typically used to override the actuator's function manually?

Handwheel Signup and view all the answers

What is the function of a regulator in a control valve system?

To control the pressure of the process fluid upstream of the device location Signup and view all the answers

What action is induced by the spring in a control valve system?

To provide energy that moves the valve in the opposite direction to the diaphragm loading Signup and view all the answers

What is the primary purpose of a globe control valve?

To change the direction of flow Signup and view all the answers

What feature distinguishes a three-way control valve from other types?

It can divert or blend flows between two output ports Signup and view all the answers

What is the role of the diaphragm in an air regulator?

To sense the pressure on the outlet side Signup and view all the answers

How can the output of a controller be reversed?

By configuring the current to the pneumatic transducer Signup and view all the answers

What aspect of butterfly valves contributes to their lower manufacturing costs?

Simpler design with fewer components Signup and view all the answers

What is a primary advantage of ball or segmented ball control valves?

They allow for tight shutoff and modulating applications Signup and view all the answers

What occurs to a control valve in a fail-safe situation?

It is forced into a specific fail-safe position Signup and view all the answers

Which actuator type is commonly paired with butterfly control valves?

Spring and diaphragm actuators Signup and view all the answers

What happens to an air to close valve when it loses its instrument air supply?

It fails open. Signup and view all the answers

What is the purpose of a lockup relay in a control valve system?

To seal in the existing signal during power loss. Signup and view all the answers

Which actuator response is considered direct acting?

Increasing pressure causes the stem to extend. Signup and view all the answers

Which statement accurately describes a fail in place condition for control valves?

They remain in the last position prior to failure. Signup and view all the answers

How can the combination of actuator and valve actions be determined?

By examining the identification plate for ATC or ATO. Signup and view all the answers

What occurs during a power failure in an air to open valve?

The valve closes. Signup and view all the answers

What is one characteristic of electric motor actuators regarding power loss?

They naturally fail in place. Signup and view all the answers

If a valve is moving material into a tank and experiences a power failure, what is the potential risk?

The tank could overfill due to uncontrolled flow. Signup and view all the answers

What is the first step in the instrument air regulator procedure?

Handwheel is turned Signup and view all the answers

What happens to the control valve if both the spring and diaphragm actuator are direct acting and air is lost?

The valve fails open Signup and view all the answers

Which component is essential for regulating upstream pressure in a back pressure regulator?

Pilot valve assembly Signup and view all the answers

Which of the following valves allows for the addition of material to a tank to stop the flow during a power failure?

Fail-closed valve Signup and view all the answers

What type of motion does a butterfly valve utilize to open?

Rotary motion Signup and view all the answers

Which pressure regulators are utilized to control downstream fluid pressure?

Pressure reducing regulators Signup and view all the answers

What is one functionality a valve positioner can provide?

Position the valve stem in reference to the instrument signal Signup and view all the answers

What is commonly the most used control valve body style in processing?

Globe valve Signup and view all the answers

Study Notes

Control Loops - Control Valves and Regulators

Objectives:
- Identify components of a control valve system from a drawing or device.
- Define terms related to valves and final control elements, and understand operating scenarios where they are beneficial.

Reading Summary - Introduction

Final Control Element: The last active device in a control loop, often a control valve.
Common Control Valve Components:
- Instrument air actuates the valve.
- Regulates material flow by creating a pressure drop across the valve.
- Actuator converts signals (linear or rotary motion) to control flow mechanisms within the valve body.
- Common valve components include: bodies, bonnets, valve plugs, stems, actuators(spring diaphragm, spring piston, double acting piston).
- Valve plugs move to open or close flow, impacting the flow rate of valves as they move towards/away from the valve seat.

Reading Summary - Control Valve Failure Conditions

Failure Conditions: Control valve failures due to air loss or other issues.
- Actuators without provisions for returning to a predetermined position in power failure cases can fail to a predetermined intermediate position.
- Engineers choose appropriate valve failure response based on safety criteria.
- Fail Open - Air to close loses, valve opens due to the return spring being more powerful than the diminished supply air (example pressure vessel relief).
- Fail Closed - Air to open fails, valve closes (example tank overrun protection).
- Fail in Place - Natural spring action unless the valve has a locked-up relay.

Reading Summary - Control Valve Actuators

Actuator Types:
- Actuators convert instrument signals to linear or rotational motion.
  - Direct acting: Pressure increase extends the stem
  - Reverse acting: Pressure increase retracts the stem
- Combinations: Direct/direct, Direct/reverse, Reverse/direct, Reverse/reverse
Categories of Pneumatic Actuators:
- Spring and diaphragm, piston type.
- Common spring/diaphragm actuator: single diaphragm supported by a diaphragm plate, stem, often preferred for low cost and high mechanical advantage.
Single Acting Spring Opposed Actuator: Diaphragm type piston opposed by a spring; example is an air cushion type piston actuator where pressure is trapped and then the air compresses as the piston is pushed.

Reading Summary - Valve Positioners

Valve Positioner Function:
- Match controller output signals with valve position.
- Locate moving parts related to the instrument signals received from the loop controller
- Optionally modify the input/output instrument air signal relationship
Valve Positioner Operation with Pressure Gauges
- Instrument input pressure gauges show the signal pressure from the controller (or an I/P transducer).
- Output pressure gauges show the actuator pressure.
- Supply pressure gauges denote the instrument supply pressure.

Reading Summary - Control Valve Configurations

Globe Valves: Most common; flow can change direction
Three-way Valves: Special globe valves with 3 ports, for mixing, diverting, or blending two streams.
Butterfly Valves: Lower manufacturing costs and higher flow capacity, often with rotary motor actuators (vs linear piston style).
Ball/Segmented Ball Valves: Used for on/off shutoff or modulating flow of fluids.

Reading Summary - Instrument Air Regulators

Control Devices: Self-actuating devices operating from the process stream.
Regulator Parts: Inlet and outlet for pressure control and regulation.

Reading Summary - Checking for Understanding

Globe Valve Flow Increase: Flow increases as the valve plug moves away from the seat.
Spring & Diaphragm Actuator Fail Open: If air is lost when the valve needs to close, the spring will always open the valve.
Fail Closed Valve Application: Valves to prevent material addition to a tank in case of instrument failure.

Reading Summary - Additional Questions

Pneumatic Valve Positioner Gauges: Instrument, output and supply
Common Control Valve in Processing: Globe style
Butterfly Valve Operation: Rotary motion
Three-way Valve Uses: Divert, mix, replace
Recommended Air Supply for Control Valves: Information not included in text.

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Description

Test your knowledge on control valves and regulators in control loops. This quiz covers key components, definitions, and scenarios where control valves are crucial. Identify parts and understand their roles in regulating flow within a control system.