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They can be thought of as miniature check valves in which the disc is replaced with a needle. Standard sizes are 1/4 ", 3/8", 1/2 ", 3/4" and 1 ". Connections are typically threaded, with pipe threads. Used as an instrument valve in combination with a pressure transducer, or for sampling. Often used in hydraulic systems, usually made of stainless steel, steel or brass.
Another name is knife gate valve. It is used for systems for conveying liquids containing solids. They can "cut" any suspension. It is the first and only choice in the pulp and paper industry, in waste water treatment plants. Bilge water can contain sediment, so a knife valve is the right choice.
Structurally, the knife valve is similar to a gate valve, but its size from flange to flange is much smaller, the stainless steel gate valve itself is much thinner and equipped with a blade. The seat gaskets are made of Teflon.
The knife valve is usually used at pressures less than 10 bar.
The two main types of check valves are piston valves and rotary disc valves. Attention! These valves are not, and cannot be considered, a shut-off valve. Once contaminants have settled on the O-rings, these valves will flow in the opposite direction.
To solve this problem, a “twist” check valve was created. By design, this is an ordinary check valve, in which the disc is not attached to the spindle. In the open position, the disc can move freely up and down, and when the flow changes its direction, the disc will move down and close it. If necessary, the spindle can be screwed in and the valve will be hermetically sealed. A variation of this valve is the so-called "storm valve", which is specially designed to protect sanitary installations (you guessed it, these are toilets) from being flooded by sea water in a storm.
This also includes a more sophisticated check valve with an eccentric locking disc that does not completely close the seat when the direction of flow changes. It is used to prevent water hammer.
Another name is spring loaded safety valve. This is not a common-sense valve. It is a precise device designed to open at a given pressure, whatever the circumstances. This is often the last barrier between you and your Creator, and this is no joke. Safety relief valves protect equipment, boat and crew from dangerous overpressure. They are always on the alert!
Therefore, strict regulations govern the use, design and installation of safety valves. Their violation is a reason for insurance companies not to pay in case of accidents and other disasters. Therefore, double-check everything, check with the specialists from the manufacturer and with senior comrades if you are dealing with safety valves.Design features:
Inspection and repair of such valves can only be carried out by specially trained people who have completed courses at the factory and received a certificate. This procedure is usually performed annually, or more often as needed.
It should be understood that only the spring keeps the valve closed. When it opens, its resistance to flow will be minimal.
There are really dangerous devices that need special safety valves. These are devices that contain compressed gases and containers under pressure, where boiling liquids with a vapor phase are located, for example, a steam boiler. Valves for such devices have a small inlet and a large outlet. This makes it possible to expand the escaping steam or gas. Valves should open into an open space and not into a closed space. An exception is poisonous liquids and gases, which must enter a special neutralizer.
There are other varieties of spring loaded valves. The outlet valves, through which excess water after the pumps is discharged overboard, is triggered at 0.5 bar. Redundant valves, set at higher pressure, protect centrifugal pumps from overheating when fluid consumption changes. Pressure reducing valves must maintain pressure at any flow rate through the valve.
There are special valves that are designed to discharge large volumes of gases and vapors from tanks, as well as prevent negative pressure in them. In their design, another ingenious engineering solution is used: the outflow rate of the outgoing gases exceeds 20 m / s, at such speeds the risk of ignition or explosion of gas in tanks is minimal!