Ball Valves

This is normally in LNG Services. When you close a ball valve a certain amount of liquid gets trapped in the ball itself. LNG. Over a period of time this LNG Trapped in the ball will expand causing tremendous pressure rise in the ball leading to subsequent leakages and more severe damages( LNG expansion ratio is 1:600 ). The cavity relief lets the LNG vaporising pressure getting reliieved in the pipe and keeps the ball of the valve within accpetable pressure limits. Since the valve cavity is on the upstream side this type of valves are normally used for unidirection services only.

Hope this answers ur query

Walter

Regular ball valves also create problems with Sodium Hypochlorite. When hypochlorite degrades, it creates vapors and the valve can literally explode.

Another problem, especially with PVC valves, is that when the ball is pressurized form the inside, the valve gets stuck, and operators tend to destroy the handle trying to open it.

The "vent hole" is provided by the manufacturer (be careful if you do it yourself because if the edges are not smooth you can damage the seals opening and closing the valve) and the valve has to be installed with the vent hole facing the pressurized side when the valve is closed. This is why the valve is mono-directional.

I started using a type of ball valve (produced by NilCor ) which has a system of seals and O-rings that allow the gas trapped in the ball to "burp" back in the pressurized side. The gas will always choose the pressurized side, therefore the valve can be used by-directionally.

These valves are very expensive, but of really good quality.

Also very commonly used for Bleach (NaClO) for the same reasons. Outgassing of the Chlorine causes over pressure within the valve body. The vent gives the gas some where to go before damaging the valve.

A venting ball valve is used in applications when the fluid (if trapped inside the port when the ball is closed) has the potential for expansion under certain conditions. For example, if a cryogenic fluid (liquid nitrogen) is flowing through a ball valve and the valve is closed, what happens to the liquid inside? As the liquid warms up, it expands to over 800x its volume. Your ball will explode, line will break, and you be in the plant managers office. The ball vent relief hole (pointed upstream) allows the trapped liquid/vapor to expand to a safe location. You would not want to vent downstream, since there's probably a reason you are closing the valve in the first place and that expanding fluid might affect you operations. This is the same reason why you don't use a a vented ball valve for bi-directional flow.

Next question: Using a reduce port ball valve for bi-directional flow is not advised because it will increase frictional pipe losses, phase change, and plugging. In general, for any bi-directional application you would not want to use any type of valve not designed for that purpose. Valves have different flow characteristics depending on which way they are installed. You need to check with the valve manufacturer.