For opening and shutting a gate valve, the AWWA operating formula is 3 times the nominal valve size + 2 or 3 rotations of the working nut. A 6-inch gate valve, for example, would take 6 x 3 = 18 spins, plus 2 or 3 turns, for a total of 20 to 21 turns.
A turn of a gate valve consists of lifting the valve stem 1/8 inch and allowing it to drop back into place. For closing a gate valve, repeat this procedure in reverse order: lower the valve stem, release it, then lift it again. A complete cycle of opening and closing a gate valve takes two rounds through the operation.
A double-acting gate valve is identical to a single-acting one except that it has two sets of threads on its body, one set for each direction of movement. This allows the valve to function as a two-way valve as well as a one-way valve. For example, if there are no restrictions on the flow from A to B, then a 2-inch double-acting gate valve could be used instead of a 1-inch single-acting one. The same amount of fluid would flow in both directions through the double-acting valve.
The most common type of gate valve is the sliding gate valve. It includes an exterior handle or lever that's used to open and close the valve. When the handle is turned counterclockwise, for example, the valve opens.
Gate valves are full bore when fully open, which means there is nothing to block the flow because the gate and pipeline diameters are the same. The valve size is also determined by the bore diameter. For example, a 6-inch (152-mm) gate valve has a 6-inch (152-mm) bore with a thickness of about 1/4 inch (6 mm).
The advantage of using a full bore gate valve is that it allows fluid to flow through the valve quickly when opened. The pressure loss across the valve is low because there are no restrictions such as seats or liners inside the valve body. Also, full bore gate valves are usually equipped with straight stems instead of angled ones like other types of valves. This makes them easier to operate since there is no need to rotate the handle to overcome friction caused by the angle of the stem.
Disadvantages of using full bore gate valves are that they require more space than other types of valves and they are more expensive due to their larger size and heavier construction.
There are two main types of full bore gate valves: surface mounted and casing-mounted. Surface-mounted valves are installed directly onto the deck of a tank without needing special fixtures. They can be moved around the tank site as needed.
A gate valve is a manual linear-motion valve with a vertical rectangular or circular disc that slides over an opening to stop the flow and functions as a "gate." The name comes from the fact that the disc can be moved between open and closed positions like a gate. These valves are used for controlling the flow of water in pipes.
They consist of a casing with a horizontal base plate and a cover, which fits into the top of the casing. A hole is drilled through the center of the cover allowing it to be turned by a handle located on the side of the casing. The cover has teeth around its edge so that when it is turned it will not move unless acted upon by pressure from within the pipe being controlled. The disc itself is flat, with holes cut out of it so that it will not block the flow completely but allows some water to pass. When the cover is in the closed position, there is no flow because there are no openings in the cover for water to go through. However, when the cover is opened up, it creates a gap between itself and the casing large enough for water to flow through.
These valves are usually made out of cast iron or steel but plastic versions do exist as well.
This fullbore design has a minimal friction loss, which saves energy and lowers the total cost of ownership. A gate valve with no packing will leak slightly when closed, which is why most have some form of sealing material around the outside edge of the valve body.
When actuated, the gate arm lifts away from the seat creating a gap that allows fluid to pass through. Gate valves are typically driven by hydraulic pressure or an electric motor. In either case, when the pressure is removed the valve closes automatically. This prevents any hazardous conditions such as gas leaking into other parts of the facility.
Full bore design allows for smaller valves and shorter opening times which reduces energy consumption and costs. There's less metal moving against metal which reduces friction and heat generation. Also, since there's no packing to wear out, the life expectancy of a full-bore valve is longer than those with packing.
The term "full bore" was originally used in connection with oil wells. When a well was drilled to its intended depth, the drillers would sometimes find that it was not possible to complete the hole due to obstacles such as rock formations. In these cases, they would often install a full-bore casing pipe.