This document is intended for general information only. It is provided in good faith to interested parties. ENEFEN will not be
responsible for its suitability for any specific project or circumstance. The information provided represents ENEFEN’s opinion,
which may not necessarily be accepted by other Inspection Bodies (IBs) or Authorities Having Jurisdiction (AHJ). Reader is warned to use this information with caution.
VENTING OF INSTRUMENT GAS
The instrument venting requirements are described in the CSA B 149.1, CSA B149.2, and B149.3 Codes. Since the wording of these standards is somewhat convoluted we recommend for industrial installations a simpler practical guideline:
1. Vent connection size from any device shall not be reduced below the connection size of that device, but in no case shall be less than equivalent to 1/4" Sch 40 pipe ID = 0.364". (Note: We do accept 3/8" OD 0.035" wall instrument tubing although its ID is slightly smaller than that).
2. Vent lines longer than 50 ft shall be increased by one pipe or tube diameter for each additional 50 ft length, with the increase starting at the connection to the device (this means that the vent line has to be bigger from the start and not that it has to be gradually increased)
3. Vent lines shall be made of seamless steel, aluminum, or stainless steel tubing, or seamless carbon steel pipe (SCE 40 pipe is sufficient).
4. When possible each device shall be vented separately (on small skid packages this is usually simpler and less costly than building vent headers)
5. When multiple devices other than pressure relief devices (separate PSV or PSV internal to a PRV) are connected to a single vent line, this vent line shall have an area of not less than twice the total area of the connected vents. [Use formula => sqr((D1^2 + D2^2 + ....... + Dn^2)*2) ]
6. Multiple pressure relief devices (separate PSV or PSV internal to a PRV) may be connected to a single vent line if the inlet pressures do not very be more than 10% and this vent line has an area equal to the largest device opening plus 50% of the total area of other device openings.
7. The outdoor vent termination shall be equipped with a means to prevent the entry of water, insects and other foreign objects. (typically this means a "candy-cane" or 90EL termination pointing downwards and a screen at the outlet)
8. There shall be no valves, reductions, or other devices installed in the vent lines which could impede their operation, or in any way made them ineffective, isolated or bypassed.
9. All vents shall be terminated at a "safe location" including: a) not directed at operators face or any part of operator's body b) not located near walkways, driveways, stairs, access ladders, manual valves or other devices where operator's or public presence may be foreseen. c) not terminated under the buildings, soffits or any areas where vented gas may be trapped or accumulate d) not terminated in locations which may be covered by snow, ice, leaves, plants, or any other objects which may impede venting e) at least 1 meter away from building openings (windows, doors, vent louvers or cut-outs, etc), appliance vent outlets, or moisture exhaust ducts f) at least 3 meters away from mechanical air intakes (equipped with a fan), appliance air intake, or source of ignition. g) not located so that prevailing winds, downdrafts or gas buoyancy (negative or positive) could blow the gas back towards the above "unsafe locations" h) preferably directed "up and away" to allow any vented gas to be safely dispersed to the outside air. i) when vented gas contains poisonous compounds (such as H2S) additional precautions shall be taken to ensure its "safe" venting, including extending the above minimum distances and/or disposing of the vent gas to appropriately sized flare headers.