I have to select check valves for high pressure pipes (70 bar = 1000 psi), for sizes between 2" and 6". Up to now we are using single check valves, but I suppose it may be better to use double check instead, since they should make a slighter water hammer, and probably less pressure drop also. Is this supposition right? Is there a better option?

Save your money.

Although two checks may be desireable for double protection from contamination during a flow reversal, checks do not help water hammer in the forward direction at all and do little to prevent a water hammer pressure wave from transversing in the reverse direction.  The main pressure wave is extremely fast moving and usually passes the check before it can close.  You may catch a reduced pressure secondary reflection, but by then any damage would have already happened.

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