-
1
- #1
BryceBunting
Industrial
- Aug 24, 2015
- 4
I read with interest a post which asked, "What is the difference in a Position Seated and a Torque Seated Valve"? In reading the answers, I was disappointed that none came close to facts. As such lets start:
There are 4 Basic Valve Sealing Mechanisms:
1) INTERFERENCE FIT
2) PRESSURE ASSISTED SEAL
3) POSITION SEATED
4) TORQUE SEATED
INTERFERENCE FIT:
A Resilient (rubber) seated butterfly valve is an example of an Interference Fit Sealing Mechanism. This is a very technical way of saying that the OD of the Disk is larger than the ID of the liner. Hence, when the disk moves to the closed position, interference is created which seals the valve. Other examples of Interference Fit valves are:
a) Gate Valves (think Wedge Gate)
b) Plug Valves
c) Crusher Seated Ball Valves
Typically, Interference Fit designed valves:
1) Are Bi-Directional
2) Will seal independent of line pressure
PRESSURE ASSISTED SEAL:
A free floating ball valve is an example of a Pressure Assisted Sealing Mechanism. This design uses the line pressure to force the ball, gate or plug into the seat. Theoretically, the higher the pressure (up to the full rated pressure), the better a seal is created. As such, these valves are typically uni-directional, but not always. As in the case of the ball valve, which has 2 seats, and seals bi-directional. One must be careful with application of Pressure Assisted Seal valves for 2 reasons:
1) In low Delta P application, you can see significant leakage!
2) In Flow Up applications, some valves, like Knife Gate Valves, need to be installed BACKWARDS in order to properly isolate.
Other examples of Pressure Assisted Seal Valves are:
1) Knife Gate Valves (single seated, where the seat is on the P2 side of the valve)
2) Single Offset High Performance Butterfly Valves
POSITION SEATED VALVES:
These are valves which simply must be in a specific position in order to seal. Two good examples would be:
1) High Performance (double offset) Butterfly Valves
2) Parallel Disk Gate Valves (high pressure valves used for steam isolation)
In both the above mentioned valves (one being rotary, the other being linear), if they are not in the correct position, they will not seal.
TORQUE SEATED VALVES:
A Torque Seated Valve is a version of a Position Seated Valve which not only must be in the correct position, but also have a specific torque to achieve sealing. The primary example of this valve is a Triple Offset Valve (Butterfly design). The Triple Offset Valve or TOV has a 3rd offset which is the sealing cone. Like a double offset butterfly, both create a cam effect. Unlike the double offset though, the triple can be in the proper position and still leak considerably.
Understanding these various SEALING MECHANISMS is crucial to properly applying any valve. Feel free to send questions as I am willing to help!
I teach a very detailed 8 hour VALVE APPLICATION FUNDAMENTALS Course and this is just one of the topics covered.
Valve APPLICATION FUNDAMENTALS Link:
There are 4 Basic Valve Sealing Mechanisms:
1) INTERFERENCE FIT
2) PRESSURE ASSISTED SEAL
3) POSITION SEATED
4) TORQUE SEATED
INTERFERENCE FIT:
A Resilient (rubber) seated butterfly valve is an example of an Interference Fit Sealing Mechanism. This is a very technical way of saying that the OD of the Disk is larger than the ID of the liner. Hence, when the disk moves to the closed position, interference is created which seals the valve. Other examples of Interference Fit valves are:
a) Gate Valves (think Wedge Gate)
b) Plug Valves
c) Crusher Seated Ball Valves
Typically, Interference Fit designed valves:
1) Are Bi-Directional
2) Will seal independent of line pressure
PRESSURE ASSISTED SEAL:
A free floating ball valve is an example of a Pressure Assisted Sealing Mechanism. This design uses the line pressure to force the ball, gate or plug into the seat. Theoretically, the higher the pressure (up to the full rated pressure), the better a seal is created. As such, these valves are typically uni-directional, but not always. As in the case of the ball valve, which has 2 seats, and seals bi-directional. One must be careful with application of Pressure Assisted Seal valves for 2 reasons:
1) In low Delta P application, you can see significant leakage!
2) In Flow Up applications, some valves, like Knife Gate Valves, need to be installed BACKWARDS in order to properly isolate.
Other examples of Pressure Assisted Seal Valves are:
1) Knife Gate Valves (single seated, where the seat is on the P2 side of the valve)
2) Single Offset High Performance Butterfly Valves
POSITION SEATED VALVES:
These are valves which simply must be in a specific position in order to seal. Two good examples would be:
1) High Performance (double offset) Butterfly Valves
2) Parallel Disk Gate Valves (high pressure valves used for steam isolation)
In both the above mentioned valves (one being rotary, the other being linear), if they are not in the correct position, they will not seal.
TORQUE SEATED VALVES:
A Torque Seated Valve is a version of a Position Seated Valve which not only must be in the correct position, but also have a specific torque to achieve sealing. The primary example of this valve is a Triple Offset Valve (Butterfly design). The Triple Offset Valve or TOV has a 3rd offset which is the sealing cone. Like a double offset butterfly, both create a cam effect. Unlike the double offset though, the triple can be in the proper position and still leak considerably.
Understanding these various SEALING MECHANISMS is crucial to properly applying any valve. Feel free to send questions as I am willing to help!
I teach a very detailed 8 hour VALVE APPLICATION FUNDAMENTALS Course and this is just one of the topics covered.
Valve APPLICATION FUNDAMENTALS Link:
