For some forms of valves, the trim is not hard to identify and understand, as you move the trim for other forms of valves is usually confusing. Although the term “valve trim” is much more closely regarding linear and non-return valves, it's also important in quarter-turn valves like stainless steel sight glass also.
The control valve community normally takes the definition of trim a lttle bit further with the help of guides, bushing and cages to globe-type valve trim descriptions. Additionally, packing system components including lantern rings (if present) and packing followers are added too.
The exterior from the stem is threaded, even though the portion on the stem from the valve is smooth. The stem threads are isolated in the flow medium because of the stem packing. Two different styles these designs are offered; one using the handwheel coupled to the stem, for them to rise together, plus the other having a threaded sleeve which causes the stem to elevate through the handwheel. This type of valve is indicated by "O. S. and Y." is a kind of design for NPS 2 and larger valves.
The threaded the main stem is from the valve body, along with the stem packing down the smooth section that is certainly exposed to the atmosphere outside. In this case, the stem threads will be in contact with all the flow medium. When rotated, the stem plus the handwheel to elevate together to start the valve.
When a gate valve is fully opened, the gate disc is very withdrawn through the flow path to the bonnet. For this reason, pressure to succeed drop through gate valves is minimal. When closed, the gate disc seals against seats inside valve body which is often integrally cast or, more probable, welded or threaded into place. Metal seats are most typical and seats are usually lapped after installation. In certain applications, the seats can be hardened. Seats on both sides on the gate let the valve to be played with bi-directionally. Where some leakage is acceptable, the gate seals with no extra sealing ring.
Gate valves must not be used in besides fully opened or fully closed positions as partially opened gates can expose the valve internals to accelerated and uneven wear and vibration. Another problem with employing a gate valve in regulating service is the gate will not open the valve passage uniformly, turning it into difficult to create a consistent rise in flow for just a given turn with the stem.