TCG valves have two common types of design, slab gate (Fig. 4.9), and expanding.
Fig. 4.9. Slab gate valve.
Courtesy: Orion valve.
Expanding gate valves can be produced as either single expanding or double expanding (Fig. 4.10). A double expanding valve has two half disks and it is used for isolation of the piping system or a component when it is closed.
Fig. 4.10. Double expanding gate valve.
Courtesy: Orion valve.
What is the difference between single expanding and double expanding gate valves? A double expanding wedge is expanded in both open and closed positions, and the two seats are sealed in both open and closed conditions. A single expanding wedge is expanded only in the closed position and the two seats are sealed. Thus, fluid enters the cavity in the open position in a single expanding gate valve, but the cavity is not filled in either open or closed positions for a double expanding gate valve. Double expanding gate valves can guarantee zero leaks to the cavity in both closing and opening conditions. One advantage of a double expanding valve is that this valve contains two half wedges with more disk flexibility in high temperatures due to thermal expansion. Double expanding is the preferred choice of valve for very dirty services in high temperature ranges above 200°C.
TCG valves are not recommended for fluid control (throttling) since prolonged usage under throttling will result in quick damage to the seats of the valves as well as internal valve components. The main parts of the double expanding and slab gate valves are body, bonnet, wedge (gate) halves, seats, and stem. The body and bonnet are the main pressure-containing parts. The disk(s) are positioned by the stem to block the flow or open the fluid path. The wedge assembly in a double expanding gate valve includes two halves, one male and the other female, which are coupled together, usually with springs.
A slab gate valve, also called a single disk, is designed to provide isolation of the piping system or a component when it is in a closed position. This type of valve is also not suitable for fluid control (throttling). Just as the double expanding gate valves, prolonged usage of a slab gate valve under a throttling operation will result in wearing and damage of the seats and internal components. The main parts of a double expanding and slab gate valve are body, bonnet, wedge (gate), seats, and stem. The body and bonnet are the main pressure-containing parts. The seats are usually floating-type seats with the springs at the back to ensure proper tightness between the gate and seats.
A double expanding valve is two wedge gate valves in series with one bleed gate valve between. A double expanding valve can be as much as 50% more expensive on average than a slab gate valve in small sizes and 25%–30% more expensive in larger sizes. Single expanding valves are usually less expensive than double expanding. Single expansion is a good choice if a more robust option than a slab gate valve is required, and the valve is most often in the closed position. Comparing the metal seat ball valve with a TCG valve, both slab and expanding are more expensive in large sizes such as 30″ and 36″. In some cases, a ball valve in smaller or medium sizes could be more expensive than a TCG valve. For example, a 12″ Class 1500 ball valve with API 6D design could be more expensive than a slab gate valve in the same size and pressure class and API 6D design. One way to estimate the relative cost of the valves is to compare their weights. As an example, a 3″ slab gate valve in class 300 was compared to a ball valve with the same size, pressure class, and material. The slab gate valve was almost twice as heavy as the slab gate valve. Of course, the weight of a valve is different from one supplier to another one.
A double expanding gate valve is not necessarily a better valve than a slab gate type. But double expanding is a more robust valve since less friction and wearing happens between the seat and the disk during disk traveling. In addition, less fluid is accumulated and pressurized into the cavity so double expanding has a longer packing life compared to the slab gate valve. However, the actuation of a double expanding valve is challenging because this valve is a torque-seated type and over-torquing the valve could damage the disk. Therefore, extra attention should be paid to sizing and selecting the actuator for a double expanding gate valve to avoid over-torquing.
The wedge is expanded at the top (open) and bottom (closed) points in a double expanding gate valve. Expansion of the wedge toward the seats by the mechanical stem force and sealing pressure between two halves provides tight sealing with two seats in both directions. Tight contact of the wedge with the seats at open and closed positions increases the breakaway torque in both closed and open situations, which increases the actuator sizes. For this reason, an end user may avoid selecting a double expanding valve to prevent having a larger actuator. As mentioned earlier, over-torquing an actuated double expanding gate valve can damage the seat, wedge, and stem. Slab gate valves are not torque-seated so there is no risk of damage to valve components due to over-torquing.