You do not state what kind of "hydro testing" you are engaged in, nor whether the valve is new or existing, but in any case I'm going to take a guess you are trying to field hydrostatically test against the closed, metal-seated valve discs. If that is the case, I believe there are a great many valves out there that are "200 OWG" (oil/water/gas) valves. I think what that basically means is that they have been originally tested at the factory and found to successfully hold 200 psi against the close valve discs/gate. This of course does not mean, however, that they would necessarily close tight at a higher say 250 psi level against the closed valve.
As the valve case is typically tested at the factory to much higher, e.g. twice the rating (you should be able to find this like the above permanently marked on the case) with the valve cracked or open , this would generally give confidence however that the pipeline containing the valve can be tested higher, but in doing so it would probably be advisable to test "through" the valve in a some open position e.g. with a new test boundary being a different bulkhead like a plug or blind flange, inserted "blank" or other etc.
While there are other reasons conventional metal-seated valves might leak e.g. even rather minor trash trapped between the gate and seat or scoring/damage previously caused by same, some modern compression resilient-seated gate valves may be more dependable for testing against closed gates bottle-tight, and also commonly carry a higher pressure rating of e.g. 250 psi. If you need more information, you may want to check the Mechanical/Valve forum.
The disc may have been closed on some construction debris. The corrective action is to create flow through the valve (open a nearby downstream hydrant) then exercise the valve to loosen/remove the debris.
Air may be trapped in the line. The corrective action is to flush the line to remove the trapped air. Add an Air release valve if necessary.
"Double disc" implies that each disc is being pressed against it's seal by an internal wedge. Are you saying that the fluid is getting past both discs or only the lower one and you are bleeding water from the gap? What type of seal do you have? Elastomer seals are better for tight shut off, but can pick up debris as bimr says. Also have you opened this valve under pressure? This could easily damage the seats such that they hold up to a certain pressure, but then leak. If it is leaking from the bleed just shut the bleed and see if it leaks past the second disc.
Can you just add a blind flange to the end of the valve and test against that? Testing against valves at close to their rated pressure causes difficulty as you have found out, which is why it is not recommended.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
I understand all the feedback and it is greatly appreciated.
These are Mech Joint DD Valves. Many times we have to make the
connection to the main service then install the balance of our service.
Most plumbing insp. - once they see adequate pressure it's fine
every so often we get an insp. that wants 250# thats when we run into trouble
What I am trying to find out or if there is documentation that if you exceed
the rating - this is what happens ie: the first gate lifts which causes the second etc. etc.
While all of us understand you are having problems passing some field line tests to the satisfaction of some inspectors at rather high pressure, there really isn't much other information to characterize exactly how/why these valves "fail". It does sound however like you are likely talking about the essentially double disk, metal seated valves per ANSI/AWWA C500 standard.
If you are not aware, leakage of 1 oz/hr/in of nominal pipe size has been/is actually allowed in even factory hydrostatic testing between the seats of this type of valve/per said specific standard (check specific manufacturer for any different/their expectations?) Depending on system configuration, filling, air evacuation and test requirements etc., it is possible loss of any water past the valve seats could cause the pressure to drop, at least some. Some variable designs of past and present double-disk valves may furthermore be better, or less subject to, regarding field leakage or field damage to seats etc. (that results in same in field tests) than others due to variable design features.
As has been mentioned in other posts, if you don't want any leakage by seats, other types of valves (e.g. resilient-seated gate valves) may be a better choice.
I don't have the standard here, and haven't seen "double-disk" valves tested for many years, but I believe the C500 standard has traditionally required the entire assembled valve case to be factory tested to twice the pressure rating on same, with the valve at least partway open (no leakage allowed through the valve structure in this test), then pressure is required to be applied between the disks to test both seats at the valve rating (the latter test only with allowable seat leakage as explained above). I believe the latter test is also accomplished with tap(s) in the valve case located in the middle body area between the disks, with these taps subsequently plugged for shipment.
