Is there any rule of thumbs about the clearance of one vent pipe or drain pipe to another in liquid pipeline system? How far we must put one vent pipe or drain to another one?

Where are these pipes, in a building wall, in a building floor, buried underground, in contact with soil, above ground, in a pipe rack, what diameter, what are they carrying, what is the pipe material, are they insulated, what code?

B31.4 requires 12" clearance to foreign objects when piping is placed underground, or for pipe installed offshore.  There is no required clearance for onshore above ground pipe, but it is customary to design to the same 12" clearance, unless there is some very good reason that it cannot be done in specific cases.

The purpose are for hydrostatic test and  used during operation, cause i thnk when a lines or tanks cools the pressure drop and can cause syphoning or prevent drain beside that when the pressure rise in storage tank (in my case) due to an increase in temperature, it is necessary to release the excess pressure or air.  So i need to put vent at high point and drain at low point of pipeline. In this case let we assume i have 1,000 ft of pipeline (like Ludpeka1 said before) so what should be the horizontal spacing between vent and/or drain?

If its a diesel line, you won't have any air to worry about.  Hydrotest drains and vents are temporary and should be installed as required based on the location of the test segments and BY the test contractor.  Are you the test contractor?  Tanks will have their own psv installed ON THE TANK.  You do not need to worry about venting tank pressure in your distribution lines.  Product lines should be kept full and under slight positive pressure even when not in use.

I don't believe you need any vents or drains in the line, but, I have certainly not seen your PFDs.  I just highly doubt it.  Near metering or sampling or in various segments of the plant near pumps, for FREQUENT maintenance purposes you might want to install drains running to a closed hydrocarbon-oily water drain system.  You may also need small temperature expansion PSVs in segments of the line that can be blocked and exposed to fire or other high temperatures to run to those drains.  Those should be located only at low points.  Vents may be needed at pig launchers and some other locations where air must be removed from the launching barrel, or where segments of pipe must be removed frequently for operation purposes, not for infrequent maintenance.  

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