Boiler blowdown requirements are outlined in the various sections of the ASME Boiler & Pressure Vessel Code, and possibly in specific regulations issued by the local jurisdiction (i.e. state, province, county, or city) where the boiler is installed.  

The section of ASME Code to use in answering this question depends on the type of boiler in question.  ASME Section I deals with power boilers, those boilers that generate steam for electrical power generation (except nuclear) or for use in process systems (oil refining, chemical manufacture, pulp & paper, pharmaceuticals, food processing, etc.), or both.  ASME Section III covers nuclear power plants.  ASME Section IV is for hot water heating boilers used in hotels, hospitals, schools, etc.  

One way to tell what type of boiler you have (and thus determine the applicable Code section) is to look at the manufacturer's nameplate on the boiler and determine what Code stamp has been applied.  The following stamps are associated with Section I:  A, E, M, S; the following stamps are associated with Section III:  N, NA, NPT; the following stamps are associated with Section IV:  H, HLW.  The stamp is usually applied to the upper left corner of the nameplate and is contained within what looks like a stylized 4-leaf clover.  

Another way to tell what type of boiler you have is to look on the original Manufacturer's Data Report (i.e. the "Code papers" supplied by the boiler manufacturer) if you have it.  This is the official Code document signed by the fabricator and the Authorized Inspector certifying that the boiler was designed, built, inspected, tested and stamped in accordance with the applicable section of the Code.  The particular section of the Code the boiler is built to will be prominantly identified.  

All Code sections are available from ASME.  Most engineering organizations that work with boilers, pressure vessels, piping, etc. will have current and past editions of the Code for reference.  

The ASME B31.1 Power Piping Code has a section specifically on "Near Boiler Piping". The boiler codes cover everything including the actual flange or connection right on the boiler. Things like feedwater, steam leads, blow down, etc are covered by B31.1.The information you require is likely in there.

In the ASME Section 1 "Power Boiler", PG-58.3.6 stated that the blow off piping for all boilers, except forced-flow steam generator with no fixed steam and water line, high-temperature water boilers, and those used for traction and/ or portable purposes, when the maximum allowable working pressure exceeds 100 psi shall extend through and including the second valve. The blowoff piping for all traction and/ or portable boilers and for forced circulation and electric boilers having a normal water content not exceeding 100 gal are required to extend through only one valve.
Requirements for Blowoff you can find on the PG-59.3.