What is a gate valve?
Gate valves are used to restrict the flow of a fluid that is flowing through a pipe in a straight line. This is an effective and reliable way to stop the flow of a fluid. Gate valves are meant to operate either fully open or fully closed. The slow opening and closing of the valve helps to avoid fluid hammer, which can harm the pipes attached to the valve. Gate valves are not intended to use for throttling flow, so don’t leave them half-open. There are 3 important parts of a gate valve. They are the gate, the stem, and the bonnet.
The gate in a gate valve can come with several variations. You may see an occasional knife gate used, or if you work with crude oils, you may see a slab gate. However, in the marine industry the most common gate is the wedge. This style of valve uses a wedge-shaped piece of metal that can be moved into the valve body to stop the flow of a fluid. This uses metal-on-metal contact to create a positive pressure seal. However, under very low pressure (5 psi or less), gate valves may have some seepage.
The stem is the part of the valve that is used to move the gate into the open or closed position. Normally, this is a screw-styled piece with a handle that is twisted by hand to raise or lower the gate. There are 2 types of stem in gate valves called rising or non-rising stems. A rising stem is one in which the handle moves away from or toward the valve when it is turned. This provides users the ability to quickly tell if the valve is open or not just by looking at it. A non-rising stem stays in place when turned, which is useful when space is limited.
A bonnet is the part of the valve that the gate rests in when it is in the open/ raised position. It is normally on the top of the valve. There are several variations of bonnets that you may see on a gate valve. The differences are normally comprised of how the bonnet is attached to the valve. A common variation of the bonnet is the union bonnet. Union bonnets are secured to the valve directly by screwing onto the valve and can be easily removed for maintenance or inspection of the valve’s internal parts. Bolted bonnets are similar, but they are used for higher pressure-valves. These are likely to be a little more sturdy than a union bonnet. If you are dealing with pressures of 2250 psi or higher, you may need to use a pressure seal bonnet. This style of bonnet actually becomes less likely to leak as pressure increases, so a minimum pressure that stays above 500 psi needs to be maintained.
Pigging is a common term used when discussing gate valves. It is the process of sending a device known as a pig through a line of pipe to clear debris. The pig is a close fit with the diameter of the pipe it is going into. A gate valve is meant to be full bore, or piggable, which means that when it is fully open a pig that fits in the pipe can also fit through the valve without problems. The result of a piggable valve is that it will have a lower resistance when open and will not effect pressure of fluids that are passing through it. Some other valves may be piggable, but it’s not common.