Automatic Block Valves - On/Off Guidelines

I'm not sure there is any one standard for this. In some cases, all valves are drawn the same size. This may be functionally okay, but the absence of scale makes it harder to match the P&ID with the appearance of the installation. On the other hand, scaling P&IDs to the actual sizes of valves/pipes tends to make the small valves unreadable in the drawing. Once upon a time I tried showing valves/pipes in proportion the square root of their physical sizes. This looked somewhat better, but was unwieldy. Another method would be to adopt a default valve/pipe size of say half the largest size involved, and then add to it the actual size of each valve/pipe. Then the smaller ones would still be large enough to see, and the larger ones would be noticeably larger.

Sorry if i framed the question incorrectly.. what i am trying to ask is guidelines for VALVE sizing based on the line size... eg- for a 2" line should the block valve be 2" or one size smaller or two size smaller..

On a 2" line you use a 2" valve and on a 6" line you use a 6" valve and on a 1" pipe a 1" valve and so on. However, there are numerous valve types on the market and it is advisable that each application should use a valve type best suitable for the intended purpose.

Generally speaking, the nominal size of on/off valve should be as same as the nominal size of the pipe.

For On/OFF valves generally butterfly valves are used which are of same size as that of line size. For Control valves, size of valve may differ than line size as we have to control the flow between 10% to 90% of max flow in order to achieve better controllability. Generally in P&ID Control Valve/On-Off valve size is not shown as size of valve is calculated on the basis of process conditions and line size which are known only after P&ID. So we can represent all valves by a same size on a P &ID.