Why do we use trevi test?
Trevi test is use for set pressure of safety valve with online while safety valve in service without adding pressure in whole system approaching the set pressure. Trevi test kit use the Hydraulic lift assist device to help pull spindle of safety valve to overcome spring force without increasing pressure in a system.
In early time in power plant we were not using trevi test for checking the pressure of safety valve instate of that we increase the pressure of system till the safety valve will not pop up than by comparing the control room valve pressure data we can set the pressure by tightening or loose the adjusting screw. But this method can be risky as well as time consuming and it can be reason of generation lose in the power plant.
trevi test also is not mentioned in the IBR books for set the pressure though we are using it for more easy and safety purpose.
Equipment used in trevi test
Trevi test kit consists of computer, Power pack, and Lift assist unit.
Principle
We applied pneumatic (Air) Supply to the power pack which converts pneumatic to hydraulic pressure by hydraulic pump. Oil Pressure line give the force to lift assist unit which force (Tensile) is given to load cell that convert signal into graph.
Calculation of applied force
Take differential pressure in running line and safety valve set pressure (∆P).
We already have valve disk area from which we can calculate the force which we have to apply to operate the safety valve.
F=∆P/A
Where, F = Pulled Force
∆P = Differential pressure
A= Valve Disk Area
Procedure
Ask Running line pressure to operation department and enter that pressure in software, Calculate the pulled force from above formula then enter it in software then operate pressure applying switch that plot the graph in software the first deflection in the graph indicates the operate value of safety valve if this value is more then set value than we open lock nut of safety valve then tight adjusting screw if the value is less then set value than we open lock nut of safety valve then lose adjusting screw then tight lock nut, then take second trial to set the setting pressure.
Advantages
- We can set safety valve pressure at low pressure also.
- There is no requirement of gaging another safety valve.
- More safer than another process.
Disadvantages
- As we are using software there can be error in checking pressure.
