The wedge gate valve is one of the first choices of isolating valve, with good sealing capabilities and low fluid friction losses. Leakage rates should be of the order of 10 ml/hr/25 mm seat bore for metal-to-metal seals. The wedge is effectively clamped between two full-bore seats and seals on both sides.
The taper of the wedge can compensate for wear on both the gate and the seats. Low pressure valves up to DN350 will be full-bore but some larger sizes may have a slight bore reduction. Check the design detail carefully if "pigging" is required.
The wedge gate must be lifted clear of the seat bores to open the valve fully. With manual valves, this can take some time. Water hammer is not usually a problem. Wedge gate valves can be suitable for liquid-solid mixtures. Problems can be experienced when closing the valve if sediment accumulates in the bottom between the seats.
The wedge gate is available in three different styles each with slightly different characteristics. The styles are:
· Solid Wedge
· Flexible Wedge
· Double Disc Wedge
Water applications. Cast steel valves are available in sizes from DN50. Forged steel valves can be as small as 1/4". When closed the solid wedge can become jammed between the seats if there
is a significant drop in temperature or the valve body is distorted by pipe forces and moments. Figure 3.6 shows a high pressure solid wedge valve with hand wheel.
The flexible wedge is designed to overcome these problems. The wedge is made in one piece but the sealing faces are sprung on cantilevers from the centre. When the valve is closed.
Figure 3.6 Section through a high pressure solid wedge gate valve
It is the spring load which is applied to the seat faces, not the wedge force directly. Thermal contraction or body distortion increases the spring load without jamming the gate. Some flexible wedge gates can be fitted with PTFE inserts to enhance sealing but operating temperatures are limited. Valves with solid wedges may have flexible wedge gates as an option. The double-disc design or expanding wedge gate valve has a gate made up of two separate, but connected discs. The discs are aligned but allowed some angular freedom. When the discs are in position over the seats, an inter-spaced central spherically-mounted wedge attached to the stem forces the discs apart on to the seats. The spherical mounting allows the discs to comply with the seat faces and clearances assure balanced seating forces. This valve design seals on both the upstream and downstream seats, see Figure 3.7, which shows the internal construction and method of operation. The central valve body cavity can be fitted with a drain to verify sealing integrity. For some applications the discs can be coated with an elastomer to improve sealing and reduce corrosion. Seat faces can be hard faced to reduce wear, corrosion, galling and friction. For applications with significant entrained solids, purge connections can be fitted to the seats for cleaning prior to opening or closing.
The double-disc design cures two other problems associated with solid wedge designs. If unexpected forces and moments are applied to the valve body, attempting distortion, the gate
can still be freed because the seat sealing loads are removed before gate withdrawal is attempted. The independent discs can follow their respective seats if the valve body distorts
slightly. Temperature changes, while the valve is closed, do not cause the gate to jam solid. This valve design is available in sizes from DN50 to DN600, 2" to 24" nb, with pressure ratings up to 100 barg on 600 Ib flanges. Body materials include steel, stainless steel, duplex stainless steel and nickel alloys to cope with a temperature range of-200 to 816~ The bonnet can be extended to permit near ambient operating conditions for the stem seal and actuator, which can be powered.
Figure 3.7 Construction and operation of expanding wedge gate valves
All styles of wedge gate valves are prone to "chattering" when used for flow regulating or throttling. The wedge is loosely guided by cast passages in the body when not in contact with the seats. Both the wedge and seats can wear or be damaged. The diameter and unsupported length of the stem, plus the design of the wedge attachment to the stem, determine the valve's susceptibility to vibration. Wedge gate valves are recommended for isolation only. The wedge gate lifts out of the seat bores to open the valve. The upper portion of the valve body must be wide enough to accommodate the gate. This means the bonnet attachment can be almost elliptical. Some designs use circular bonnets which makes the valve body longer than strictly necessary but greatly improves the access to the internals. Bolted bonnets are standard on steel valves rated for 16 barg and over. Small brass and bronze valves, for 10 barg applications, will have screwed bonnets and integral seats. Carbon steel valves generally have screwed 11/13 Cr seats and some designs include "O" ring seals. Integral seats may be optional for valves in applications where crevice corrosion can cause problems. Suppliers should be consulted about facilities to maintain the seat faces.
Seat designs can be modified for fire-safe applications. Valve bodies can be partially lined by fitting long seat inserts through the connections. The location of the bonnet on the body is critical for the gate to be centrally placed between the seats. If the gate is offset axially, the stem will be bent when the valve is closed, possibly leading to fatigue. A good, positive bonnet location is essential for long trouble-free operation; a spigot is probably best for low pressure valves, a ring-type joint performing a double function on high pressure valves. A spiral-wound gasket under the spigot is a good method of sealing low pressure bonnets. Large valves may have a clamped bonnet and so alignment is assured. The bonnet can be extended for cold and hot applications.
The bottom of the packing box should be fitted with a backseat bush of reasonable length. This provides stem guidance and alignment. If it is too short, rapid wear can occur if the valve is
operated often. The bottom of the stem should form a good seal with the backseat bush to allow repacking with the valve on-line. Both components may be described as "stainless steel" when they are made of 11/13 Cr, AISI 410. Ensure that corrosion will not be a problem or else the back seal will be ineffective when required. Small brass and bronze valves will have screwed glands. Steel valves are generally bolted. A form of graphite packing is the popular standard. Standard valves can have the packing box extended for vacuum applications. Most valves are of the outside screw, rising stem type. Depending upon the pressure rating, valves over DN200 may have geared hand wheels; valves over DN350 will always have geared hand wheels.
Brass/bronze is popular for small low pressure water applications. Cast and forged carbon steel are popular for larger installations. Valves with closed die-forged bodies may have reduced ports. Some forged body valves have flanges attached by butt welding or friction welding; check the quality assurance on the joint. Forged body valves can have welded bonnets for increased integrity. The weld can be difficult to inspect so integrity may not be verified.
A few manufacturers make cast iron valves for PN16 ratings. These valves feature an inside screw/non-rising stem with gunmetal seats and gunmetal inserts in the cast iron wedge. Aimed at waterworks applications, the valves can operate between -10 to 220~ Table 3.2 indicates the sizes and ratings of some popular wedge gate valves.
1) Bellows sealed packing box with butt weld connections option
Table 3.2 Popular wedge gate valves
Because the wedge gate valve seals effectively on both seats it can be used for process "double block and bleed" applications; the soft-seated versions proving very efficient. The body cavity can be bled at the bottom, like a drain, or through the bonnet. The bleeding can be automatic, triggered by the valve closure, or manual. Standard valves, from ANSI 1501b to 1 5001b, can be adapted.
A special version of the wedge gate valve is available in small sizes for use in instrument impulse lines and similar applications. Sometimes called a plug valve it is better described as a circular wedge valve. It is a linear valve and looks similar to a needle valve but the ports are in-line allowing the valve to be cleaned with rods if necessary. The circular wedge is attached to the stem and does not rotate as it is pressed into the seat.
The rising stem is sealed by a screwed gland with PTFE packing. The seat may be metallic or non-metallic and replaceable. Barstock valves, with reduced ports, in stainless steel with Delrin TM seats for pressures up to 414 barg at temperatures between -29 to 121~ are made in screwed pipe sizes from ¼” to ½” female and ¼” to ¾” male. Socket and butt weld ends are optional.