Pressure request for installation of double block and bleed valves? It is neccessary to install those valves on the low pressure lines or you can get away with just a block valve and a drain?
Whether you use DBB or not usually does not depend on the pressure of the system, but rather the degree of need to prevent leakage across the valve.  For example, you would not want an expensive product leaking across a valve that was supposedly closing off the sales meter, nor would you want a  gate valve from a gasoline line to a common header leaking and contaminating your diesel that was currently passing through the header, or a valve on a concentrated sulfuric acid line leaking across a common drain and backing into the drinking or cooling water supply, high or low pressure, although the leaked volume might be higher with higher pressure, the damage was probably already done no matter what the pressure or valve size. DB&B is a class of isolation called "positive energy isolation"  Positive energy isolation is required to protect workers during certain evolutions.  That is the only time that I know of that it is required by law.  There are many ways to accomplish positive energy isolation including:  (1) insert blinds; (2) removing or misaligning piping; or (3) double block and bleed.  The first two are appropriate for any work evolution.  The third has some restrictions where it can be used.

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