It is permissible to isolate high pressure from lower pressure by means of a valve or valves. How many valves, how they are configured etc., depends on the magnitude of the hazard which may result.
If through leakage of valves is cause for concern, a block and bleed or double block and bleed may be required for isolation. In some other cases, a single check valve may suffice. It depends on the hazard.
If, however, a piece of instrumentation such as a pressure gauge may be damaged, or worse still, potentially rupture with injury, merely as a result of the misoperation of a single hand valve (a highly probable event), most designers would find the resulting risk to be unacceptable and would require a means to mitigate that hazard.
Let's say, for instance, that a line may be at high pressure during some operations, but may require accurate measurement of a lower pressure during a separate operation or step. There are a number of ways to do this safely. One is to specify a pressure transmitter rather than a pressure gauge, which has sufficient overpressure handling capacity to survive misoperation of its block valve. Another would be the removal of the instrument during high pressure operation. Another would be the installation of a protection device such as a relief valve or rupture disc. Yet another might be physical, electrical or pneumatic interlocking which prevents the valve from being misoperated. Which of these measures is best for the particular situation requires a basic hazard identification exercise based on the specifics of the application.
