We have tested 3/4" 800 globe valve upto 225 kg/cm2. (The purpose was to make testing assembly that can test upto 140 kg/cm2. As we have no availibility of 1500 rating valves). There was no leakage through glands and bonnet flange. The valve was tested in open and close conditions. The question is how much pressure the valve can sustain with no leakage. is there any formula? or how much hydraulic pressure at atmospheric temperature  3/4" 800 rating valve can sustain without leakage?

800 is not an ANSI class, it is a frequent desination for CWP(cold working pressure)or WOG (Water-Oil-Gas) ratings. It is the manufacturer's self-established rating for maximum working pressure.  

Do not operate at greater than 800 psi.(56 kg/cm2)

225 kg/cm2 is 3200 psi.  You went 4 times the rating of the valve.  Although I have searched and not been able to find the procedure for rating a WOG valve, it would seem reasonable to assume the same 50% overpressure for testing used with an ANSI valve.  Shell test to no greater than 1200 psi, 85 kg/cm2.

I thought that a class 800 valve referenced an API #800.   These threaded or socket welded valves have an allowable working pressure of 2000 psi.

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