Piping Standard

Which standard I should follow here?

You appear to have more than just "a spool." It sounds like a piping configuration with multiple elements.
You should follow the standard of "Good Sense" in all you do. You need to consider (a) operation, (b) maintenance and (c) fabrication.

- ball valves-
Ball valves are in a family of valves called "Quarter Turn" valves. To turn them on or to turn them off you need to be able to rotate the valve handle a quarter turn. This means that the arc of the handle must he clear of all obstructions. It also means that the valve must be reachable.

- pressure gauge-
A pressure gauge needs to be seen and read by the operator. From time to time it also needs to be removed and re-calibrated. To do this there must be a block valve between the pressure gauge and the line alone with a bleed valve to release the pressure in the gauge before removal. These valves also take room and if they are also ball valves then they also have quarter turn handles and need clearance.

- flow meter-
Flow meters come in all different types. most need some minimum up-stream and down-stream runs clear of any elbows or other piping elements. The also need to be removed and re-calibrated from time to time. Make sure you know and consider these requirements.

- globe valve-
This globe valve is most likely to adjust the flow based on what the flow meter is showing. Make sure that you can see the read-out on the flow meter when you stand at the globe valve.

- Y strainer-
A Y-strainer needs periodic cleaning. To clean it you need to remove the basket. To remove the basket you need to shut off the flow to the strainer and de-pressure or drain the strainer. This might mean that the whole line must be shut down or separate block valves a bypass valve must be provided for the strainer. If the valves to do this are also ball valves then the space to turn these quarter turn handles must also be considered. You must also provide a blow-off valve on the bottom and of the strainer to release the pressure before you open the strainer. You also need to provide space to remove the strainer basket.

I hope this helps.

Can u just tell me the normal distance that we keep between these items (roughly?)

No I cannot tell you how much distance to keep between these items. I do not know what kind of flow meter you have or what kind of strainer you have or what size these itens are. You said you had from 1/2" to 3" but that does not tell me which is which and I also do not know how large or small how clear or how congested the area available is.

Is any of the details related to the design of this system is mentioned some where in ASME B 31.3…? or any other standards….


Yes and No! If the project is a B31.3 Code project then some elements must be to code. This would include the material and the fabrication. The layout or arrangement is just good piping design.