Boiler Safety valve Terminology
1.) Gagging of safety valve:-
A gagging tool is used on the water drum safety valves for the pressure setting. The safety valves are gagged one by one and the blow off pressure is set for the other. Gagging is nothing but a locking of safety valve to avoid any blow off of safety valve during steam test of any boiler.
2.)Full lift safety valve
A safety valve which, after commencement of lift, opens rapidly within a 5% pressure rise up to the full lift as limited by the design. The amount of lift up to the rapid opening (proportional range) shall not be more than 20%. A full lift safety valve is one in which the disc lifts sufficiently, so that the curtain area no longer influences the discharge area. The discharge area, and therefore the capacity of the valve are subsequently determined by the bore area. This occurs when the disc lifts a distance of at least a quarter of the bore diameter. A full lift conventional safety valve is often the best choice for general steam applications.
3.) over pressure & blow-down of a safety valve
Pressure at which safety valve has to achieve its full discharge capacity normally set pressure +10%.
Difference between the pressure at which the valve pops and that at which it closes is the blow-down. The valve remains open until the pressure falls a few kg/cm2 below the original popping point because when it does open, a greater area of disk surface is presented to the upward rush of escaping steam and further upward pressure is exerted on the valve lip. The amount of blow-down may be varied by raising or lowering the adjusting ring on the outside of the valve seat, thus decreasing and increasing the area of the small ports through which the steam escapes from the “huddling chamber”.
4.) Lifting pressure and reset pressure of spring loaded safety valve :
Lifting pressure = This is a pressure at which safety valve will lift/puff. Lift pressure can be set by increasing/decreasing the tension of spring.
Reset pressure=This is a pressure at which safety valve will reset/settle-down. Reset pressure can be set by movement of compression screw in clockwise/anti-clock wise direction.
Blowdown : Blowdown is the pressure difference between set pressure and pressure at which the valve reseats expressed as a % of the set pressure.
Blow-down% =[Set pressure-Reset Pressure]x100/Set pressure
Usual range of blowdown % setting of safety valve is 2.5 to 4%.
Set pressure : It is the pressure at which the safety valves starts to open measured at valve inlet normally 1.1 x working pressure. Higher set pressure lead to invite accident & lower setting is lead to frequent puffing of safety valve which may hamper process.
Reset Pressure: It is pressure at which safety valve is fully closed. Setting limit within blowdown range 2.5 to4% is the best safety
Chattering is the rapid opening and closing of a pressure relief safety valve. The resulting vibration may cause misalignment valve seat damage and if prolonged can cause mechanical failure of valve internal and associated piping. Some cause of chattering include excessive inlet pressure drop excessive back pressure and over sized relief valve and a relief valve which must handle widely carrying flow rates. We will explain the under normal process conditions the vessel pressure is below the set pressure of the relief valve/safety valve and the pressure at the safety valve is same as vessel pressure, the pressure at the safety valve increases by the same amount and if the pressure exceeds the relief valve set the pressure valve open.
6.) Hang up of safety valve:
when the valve does not reset back due to some setting defect. This is called hang up of safety valve.