Code: 

A body of laws; as of a nation, state or industry group; arranged systematically for ease of use and reference. Examples include AWS D1.1; API 1104, ASME Section IX and MIL-STD-248.

Standard: 

A "rule" or basis of comparison in measuring quality, quantity, content, relative value, etc. ASTM standards for various products such as ASTM A36 for weldable structural steels, is an example.

Specification: 

A detailed description of parts of a whole; a statement or enumeration of particulars as to actual or required quality, size, etc. AWS Filler Metal Specifications A5.1 through A5.31 are examples.

Commercial Codes

Codes in the commercial world fall into one of two groups- pressure containment or structural. The American Society of Mechanical Engineers (ASME) dominates the pressure containment arena. The American Welding Society (AWS) controls the structural arena. Both organizations publish their documents to American National Standard Institute (ANSI) guidelines. All Codes are periodically updated and often the year of publication is stated after the Code’s title. For example AWS D1.1-98 would have been published in 1998.

Codes set the requirements for design, material selection, fabrication, assembly, testing and inspection. Some of the more popular Codes include:

ASME B31 .1 Power Piping: 

This Code covers boiler external piping for steam or vapor above 15 psig and high temperature water exceeding 160 psig and/or temperatures exceeding 250 F. Full penetration is required for all groove welds. Visual Testing (VT) is required for all welds. Radiographic Testing (RT) or Ultrasonic Testing (UT) is required if the temperatures exceed 750°F or if the temperatures are between 350F and750F with pressures over 1025 psig.

ASME B31 .2 Fuel Gas Piping

This Code applies to piping systems for fuel gases. These gases include manufactured gas, liquefied petroleum gas, natural gas and mixtures above the upper combustible limits. The piping systems that are covered by this Code maybe be in or between buildings from the meter set to the first pressure containment valve.

ASME B31 .3 Process Piping: 

This code covers piping in chemical plants.The quality of the weld is set by the owner's determination of the safety aspects of the fluid being contained.

ASME B31 .4 Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids: 

This Code covers piping for liquid petroleum products such as crude oil and gasoline.

ASME B31 .5 Refrigeration Piping and Heat Exchanger Components 

This Code applies to refrigerant and brine piping systems. It does not include self-contained or unit refrigeration systems that are subject to Underwriters Laboratories specifications.

MORE CODE INFO HERE..........