A pressure Relief Valve is a safety device designed to protect a pressurized vessel or system during an overpressure event. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure
This totally depends on the fail safe position of the valve, if valve is fail close valve then it is normally closed valve and if valve is fail open valve then it is normally open valve
Leakage class is a prescribed standard. Here the categorization of valves is done on the basis of allowed levels of leakage. Generally used standards are Class 4, Class 5 and Class 6
Class 6 is considered to be highest with bubble tight shutoff.
Decision on which type of valve should be used totally depends on frequency of its operation. If a valve is required to be operated at frequent intervals then it should be considered for operating automatically.
But in cases where frequency of operation is less, then manual operation can be used.
At times location of valve is also very important, if Valve has very low frequency of operation but it is located at remote place then also automation may be essential
The body flange and bonnet flange are joined by studs and nuts, with a gasket of suitable design/material inserted between the flange faces to facilitate proper sealing.
In pressure seal type, Bonnet takes the help of its bolts pull itself up. This in turn creates a seal between the gasket and the inner diameter of the valve body, facilitating tight sealing of bonnet against seal gasket.
In other words pressure of fluid itself is used to facilitate the sealing.