Flexible wedge gate valve

Details Flexible wedge gate valve Flexible wedge gate valve

1999-11-04

2002-01-15

Fox, John (Department: 3753)

Valves and valve actuation

Reciprocating valve

Gate

C251S327000

Reexamination Certificate

active

06338469

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to wedge gate valves in fluid flow systems and more specifically concerns flexible wedge gate valves having features to prevent pinching, thermal binding, and overloading due to over-travel, and to prevent damage to valve seats and disks which typically occurs when a wedge gate valve is opened or closed under conditions of high pressure and high fluid flow.
BACKGROUND OF THE INVENTION
The flexible wedge gate valve shown in
FIG. 1A
(prior art) is the most commonly used isolation valve in critical service applications in nuclear and fossil fuel electric power generation plants and in petrochemical plants where reliable sealing under high temperature is an important requirement. In safety-related systems, these valves are operated by an electric motor actuator, and the valve and actuator assembly is commonly referred to as “motor-operated valve” (MOV).
The closing function of the MOV is achieved by moving the wedge into a seated position that blocks the fluid flow. Motion imparted to the stem by the actuator imparts the necessary opening and closing movement to the wedge. During the opening and closing strokes the motion of the wedge is guided by rails attached to the body which cooperate with slots incorporated in the wedge to constrain the wedge to a linear path and support the load imparted to the wedge by the pressure and flow of the process fluid.
The flexible wedge gate valve provides a metal-to-metal seal between the faces of the seats (which are typically welded to the body in critical service applications) and the seating faces on the wedge, which is also commonly called a “disk” or “gate”. The total wedge angle between the two disk faces is relatively small and typically ranges from 6° to 15° in the industry. This relatively small wedge angle provides mechanical advantage and converts the externally applied stem force to a normal force at the seating faces which is much larger than the applied stem force, providing a tight seal across the disk under both high and low differential pressure applications. Compared to solid wedge gate valve designs in which the disk is too stiff to accommodate seat face distortion without adversely affecting sealing ability, the conventional flexible wedge design provides improved sealing capabilities by better accommodating seat face distortion resulting from changes in operating pressures, temperatures, and externally applied forces/moments to the valve ends. Because of its ability to provide a better seal than the solid wedge gates, the flexible wedge gate design has become the most common choice in critical service applications.
In spite of the advantage of better sealing capabilities compared to solid wedge designs, there are several problems with flexible wedge gate valves that have not been overcome by present-day designs:
1. Recent full-scale blowdown tests on several flexible wedge gate valves show that opening or closing operation under high energy flow conditions can inflict severe damage to the disk guide ear elements, body guide rails, and the downstream seat and disk faces, causing a substantial and unpredictable increase in the stem thrust required to operate the valve, which in extreme cases can prevent the valve actuator from fully opening or closing the valve.
2. Closing the valve hot and allowing it cool down can result in a condition commonly referred to as “thermal binding” which can cause even the so-called “flexible” wedge to get stuck in the closed position due to differential temperature between the wedge and valve body at the time of closing and the resulting differential thermal contraction while cooling. The present day flexible wedge gate design evolved from the solid wedge design with the goal of overcoming thermal binding problems, which are most pronounced in solid wedge gate valves. The flexible wedge gate valve designs have not been totally successful in achieving the goal, as evidenced by a number of reports by the United States Nuclear Regulatory Commission (NRC) including NUREG—1275 Vol. 9 and other industry reports. It has been found that the opening thrust can increase substantially and unpredictably in current flexible wedge gate designs when subjected to unfavorable temperature changes. In extreme cases, motor burn-out, motor stall, and damage to the seat faces and stem to disk connection can occur when the electric motor actuator attempts to open a valve that is stuck in the closed position due to thermal binding.
3. External forces and moments applied to the valve ends by the attached piping can cause the faces of the seats (which are welded to the body) to move closer together, distort, and become angularly misaligned with respect to each other. With the disk in the closed position, seat displacements resulting from external forces and moments can lead to “pinching” of the disk and create significant variations (increase or decrease) in the sealing contact force between the disk and seat faces. The current flexible wedge gate valve designs, especially those for high pressure service, do not have sufficient disk flexibility to accommodate such seat face displacements/distortions without a degradation of sealing ability and/or an unpredictable increase in thrust required to open the valve.
4. It is the firm seating of the wedge against the inclined seat surfaces that provides the seat contact stress necessary to initiate sealing. Disk seating is controlled by torque sensitive switches of the valve operator mechanism, rather than by thrust or by actual disk position. Since the stem and disk friction realized in service can be significantly less than the conservative values used for determining the torque switch trip setting, the actual stem thrust can be significantly higher than actually required to wedge the valve and achieve a seal, therefore the gate is subject to being wedged deeper than desired, which can overload the disk and seats. Failure or improper setting of the torque switch can also result in overload of the disk and seats.
The above mentioned problems are discussed in more detail here below to provide an insight into the responsible mechanisms.
During valve operation under high energy flow conditions, conventional flexible wedge gate valve designs can suffer severe damage to the disk, disk guides, and the seats, as shown in the tests conducted by the NRC and reported in NUREG/CR-5406 and NUREG/CR-5558. With the disk in the intermediate travel position, the high flow velocities and high differential pressure across the disk causes high internal reactions at the sliding interfaces of the internal valve components. As the valve is opened or closed under these conditions, severe damage can occur at two distinct locations: (1) at the lower edge of the guide rail to disk guide slot interface, and (2) the disk and downstream seat interface at approximately four o'clock and eight o'clock positions.
The disk-to-guide rail clearances in flexible wedge gate valve designs offered by different manufacturers span a wide range, from as low as 0.020 to 0.040 inch (typically referred to as “tight guide clearance” designs) to as high as 0.20 to 0.30 inch (typically referred to as “large guide clearance” designs). In designs having tight guide clearances and/or long guides, the disk rides on the guides during almost the entire stroke and makes contact with the downstream seat face only in the last 5 to 10 percent of the stroke near the fully closed position. The high differential forces across the disk in the intermediate travel position are transmitted to the guides, resulting in very high peak contact stress at the lower edge of the disk guide slot. The contact stress is high enough to cause severe damage to the guide surfaces, causing a significant and unpredictable increase in the stem thrust required to operate the valve, which may prevent the valve from achieving its fully open or closed position. This type of damage has been found to occur in valves with a short guide length, which allows the disk to tip during travel, as well as

Affiliated with

Also associated with

Rating

Flexible wedge gate valve does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Flexible wedge gate valve, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Flexible wedge gate valve will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2828983