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Writer's pictureTaliya Mammadhasanzada
Aug 23, 20205 min read

Self-actuated Valves (part 3)šŸ§©

Warm greetings to all šŸ¤©in the 20th post on #Day20 ! Today, we will cover the 3rd topic of Valves which is Self-Actuated Valves. Here we gošŸ¢šŸ›,



Self-actuated valves include any valve that regulates itself. These include check, and pressure relief valves.

CheckšŸ¢

There are two main types of check valves, lift and swing. The picture below shows a lift check valve.



  • General Information

Check valves, such as the one shown below (1)-(2), prevent fluid from flowing through until pressure builds up and forces the valve to open. Check valves allow fluid flow in one preferred direction and prevent back flow, or flow in the opposite direction. These kinds of valves begin to close as the pressure in a pipeline drops and the fluid momentum slows down. When the flow direction reverses, the check valve closes completely.

In the schematic below (1)-(2), the fluid travels from left to right. The gate prevents any backflow from occurring.






  • Equipment Design

The pictures below show a cutaway view of a swing check valve. The fluid enters from the left, forcing the valve open. When the fluid stops flowing or when there is backflow, the valve will swing closed.



Lift check valves are typically used in vertical flowlines. Either a spring or the force of gravity keeps the gate in the closed position. If the fluid pressure is great enough, the gate will open and the fluid will flow up through the pipe.

  • Usage Examples

Check valves are used in applications that can't allow any backflow. The picture below shows a check valve that is in place to prevent any backflow into the feed pump.



Advantages

  • Unlike most valves, swing check valves provide tight sealing in low pressure applications.

Disadvantages

  • Lift check valves should not be used for fluids with suspended solids because settling will occur.

  • Swing check valves leak at low pressures, whereas most valves provide tight shutoff at low pressures.


Pressure Relief ValvesšŸ§©

Pressure relief valves, commonly known as PRVs, prevent system breakdown by releasing pressure.

  • General Information

Pressure relief valves open when the system pressure becomes too high or too low, and reclose once the system pressure has been restored to a safe level. Opening the valve decreases pressure in the system by allowing fluid to discharge. All of these allow the system to stay below a set pressure limit. The pressure that the valves will open to if exceeded can be set or calibrated. Pressure relief valves can be grouped into four categories: direct-loaded and pressure-actuated, pilot-operated and pressure-actuated, temperature-actuated, and power-actuated. The most common PRVs are direct-loaded and pilot-operated.

  • Equipment Design

Direct-loaded pressure relief valves involve a spring-loaded disc. Valves of this type are meant to lower system pressure by discharging fluid from the process. This valve is incorporated into the process such that the fluid exerts a force on the disc. The disc is held in place by a force provided by a spring in the opposite direction. At a specific fluid pressure the force the fluid exerts on the disc is greater than that of the spring holding the disc in place. The disc then rises and some fluid is allowed to discharge from the system. The pressure at which the fluid overcomes the spring can be set by adjusting the spring.


Ā 

In high back-pressure situations, a balanced-bellows design is preferred over the conventional design described above. In this design, bellows present allow for compression and expansion before pressure reaches the spring. Also, there is a vent in the bonnet to ensure exposure to the atmosphere, which guarantees that the relief will open regardless of the back-pressure applied. Below is a picture of a direct-loaded pressure valve where the arrows show the direction the fluid would flow when the disc is raised.


In pilot-operated pressure relief valves, such as the one shown below, the system fluid is responsible for causing the valve to open and close. This is unlike the direct-loaded valve type where the spring was responsible for valve closing.


Ā 

The valve consists of a piston, a dome, and a pilot. The piston is the movable part of the valve that will open if the pressure limit is exceeded. The dome is the open space above the piston. The pilot acts as a connecting piece between the valve inlet and the dome. When fluid is flowing at a safe pressure, the fluid pushes against the piston. šŸ§ŖHowever the fluid also flows through the pilot and into the dome, pushing the piston from the opposite direction with the same pressure. The piston is cleverly designed to have a larger surface area on the dome side so that even though the same pressure is exerted on both sides of the piston, there is a larger force on the dome side due to the larger area. This produces a net force that seals the piston, preventing flow past the piston.


šŸ§ŖšŸ§ŖThe pilot itself is a three-way connecting valve that is self-actuated. Under safe pressure the pilot connects the inlet to the dome as described above. If pressure exceeds a certain limit the pilot will switch and connect the inlet to a discharge stream. Now the system fluid is only pushing against the piston from one side and the pilot opens. The valve outlet discharges into the same stream as the pilot discharge allowing for very fast pressure relief. Note these valves may also be spring operated.


Ā 

The picture to below, (1), is that of a direct-operated PRV. (2) the direct operated PRV is a pilot-operated PRV.



  • Usage Examples

The main function of a pressure relief valve is for safety considerations.


Advantages

  • Protects equipment from rupturing and other mechanical failures.

  • Does not need replacement often.

  • Allows for a system to continuously run at safe pressures.


Disadvantages

  • Can be expensive to install, especially at small sizes.

  • Does not perform well at low temperatures.

  • Does not perform well with viscous fluids, or if there are solids suspended in the fluid.

  • Requires a minimum system pressure for use. If pressure drops below the minimum value the actuator becomes confused and does not know whether to open or close the valve. These valves are often coupled with a bypass system if the pressure drops too low.

ReferencesšŸ“–

  1. http://encyclopedia.che.engin.umich.edu/Pages/TransportStorage/Valves/Valves.html

  2. Gas Conditioning and Processing Volume 1, 7th edition

Additional Resources šŸ§ŖšŸ”‘

You can get deep insight about Process/Chemical Engineering from these sourcesšŸ˜‰:

  1. https://t.me/ebookstorage/210-Introduction to Process Engineering and Design (2015)

  2. https://t.me/ebookstorage/211-Elementary Principles of Chemical Processes

  3. https://t.me/OilAndGas/18122- Valve Sizing Sheet

  4. https://t.me/ebookgate/1127- Engineering Fluid Mechanics Book

  5. https://coursemania.xyz/course.html?id=433291- Free course on Fundamentals of Fluid Mechanics

  6. https://t.me/ebookstorage/178- Engineering Heat Transfer

  7. https://t.me/ebookstorage/159- Fundamentals of Engineering Thermodynamics (9th Edition) (2018)

  8. https://t.me/ebookstorage/183-Heat Transfer applications and principles

  9. https://t.me/ebookstorage/171-Design and Operation of Heat Exchangers and their Networks (2020)

  10. https://t.me/ebookstorage/215- Industrial Separation Processes (book)

  11. https://t.me/ebookstorage/214- Advanced Process Engineering Control (book)

  12. https://t.me/OilAndGas- Information about Oil&Gas (mainly arabic lang)

  13. https://t.me/chemical_worlds- Chemical Engineering Books, Quizzes and GATE Study Group

  14. https://t.me/chemical_environmental- Discussion group related to Chemical Engineering Problems

  15. https://t.me/chemicalengineeringworld_cew- Everything related to Chemical Engineering

  16. https://t.me/ebookgate- Chemical Engineering E-books (Telegram Channel)

  17. https://www.youtube.com/channel/UCqioh32NOJc8P7cPo3jHrbg- Piping Analysis

  18. https://www.youtube.com/channel/UCQfMyugsjrVUWU0v_ZxQs2Q -Mechanics of engineered devices

  19. http://chemicalengineeringguy.com/- suggests a wide range of courses in Chemical engineering (you can find free courses on topic of Aspen HYSYS, Aspen Plus)

  20. https://www.youtube.com/user/LearnEngineeringTeam- suggests working principles of every engineered devices, equipment and etch.

  21. https://www.youtube.com/channel/UCR0EfsRZIwA5TVDaQbTqwEQ- suggests great information about pumps, compressors with animation.


Today we have covered learning about Valves. Now, time to say goodbyešŸ‘‹šŸ» until tomorrow and Stay tuned for more content šŸ˜‰šŸŒāœØ!



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