Update on Ammonia Diffusion Tanks
Since 1994, jurisdictions which adopted the Uniform Mechanical Code (UMC) have required ammonia refrigeration systems to be equipped with a diffusion tank to which all pressure relief valve discharge pipes must terminate. As a state, California has adhered to the California Mechanical Code (CMC) which is a derivative of the UMC.
The most recent version of the UMC (2015) and CMC (2016) have removed all references to diffusion tanks. Since the 2016 CMC became effective on January 1, 2017, this means that the “era of diffusion tanks” has ended (1994-2016). It is important to clarify that 2015 UMC §1114.1 requires that “Systems containing other than Group A1 or B1 refrigerants shall discharge to atmosphere through an approved flaring device.” Since ammonia is classified as B2L, this requirement would apply to ammonia refrigeration systems unless exempted by the AHJ after a review of “rational engineering analysis” that unacceptable hazards would not result from an atmospheric release. IIAR has made clear in a letter dated April 25, 2017 that it does not advocate the use of flaring devices on ammonia refrigeration systems.
It is also worth noting that the International Fire Code (2015 IFC) and California Fire Code (2016 CFC) includes an “ammonia diffusion system” as one of five available options for relief valve termination (see 2015 IFC §606.12.5), but does not mandate a diffusion tank in all installations. Similarly, ANSI/IIAR 2-2014 §15.5.1 recommends discharging relief valves directly to atmosphere, but lists “water diffusion systems” as an approved exception if approved by the authority having jurisdiction (AHJ). In summary, there will still be circumstances in which a local AHJ, owner, or designer may recommend/require the installation of a diffusion tank, but that should be a design consideration on a case-by-case basis.
For additional information check out the blog post at Safety Engineering Network which also addresses this topic.
Historical Diffusion Code References
1. 1994 – First UMC Version Requiring a Diffusion Tank
1994 UMC §1119 Ammonia Discharge
Ammonia systems shall be provided with an emergency discharge into a tank of water provided exclusively for ammonia absorption. At least 1 gallon (379 mL) of fresh water shall be provided for each pound (454 g) of ammonia in the system. The water used shall be prevented from freezing without the use of salt or chemicals. The tank shall be substantially constructed of not less than 1/8-inch or No. 10 M.S.G. (2.51 mm) steel. The horizontal dimensions of the tank shall be equal to or less than one half of the height. The tank shall have a hinged cover or, if of the enclosed type, shall have a vent hole at the top. Pipe connections shall be through the top of the tank. The discharge pipe from the pressure-relief valves shall discharge ammonia in the center of the tank near the bottom but not more than 30 feet (9104 mm) below the surface of the water.
EXCEPTION: An emergency discharge is not required for ammonia-water absorption unit systems installed outdoors serving a dwelling unit provided that the discharge is shielded and dispersed.
2. 2009 – UMC Requirement for the Size of the Diffusion Tank was Reduced
2009 UMC §1120.0 Ammonia Discharge.
Ammonia shall discharge into a tank of water that shall be used for no purpose except ammonia absorption. At least one (1) gallon (3.785 L) of fresh water shall be provided for each pound (454 g) of ammonia that will be released in one (1) hour from the largest relief device connected to the discharge pipe. The water used shall be prevented from freezing without the use of salt or chemicals. The tank shall be substantially constructed of not less than ⅛” (3.2 mm) or No. 10 MSG steel. The horizontal dimensions of the tank shall be equal to or less than one-half the height. The tank shall have a hinged cover or, if of the enclosed type, shall have a vent hole at the top. Pipe connections shall be through the top of the tank. The discharge pipe from the pressure-relief valves shall discharge ammonia in the center of the tank near the bottom but not more than thirty (30) feet (9,144 mm) below the surface of the water.
Exception: An ammonia-water absorption unit system installed outdoors serving a dwelling unit, provided the discharge is shielded and dispersed.
3. 2015 – Removal of Diffusion Tank Requirement in UMC
2015 UMC §1112.10.2 Discharging Location Exterior to Building.
Pressure-relief devices designed to discharge external to the refrigeration system shall be arranged to discharge outside of a building and shall be in accordance with the following:
(1) The point of vent discharge shall be located not less than 15 feet (4572 mm) above the adjoining ground level.
Exception: Outdoor systems containing Group A1 refrigerant shall be permitted to discharge at any elevation where the point of discharge is located in an access-controlled area accessible to authorized personnel only.
(2) The point of vent discharge shall be located not less than 20 feet (6096 mm) from windows, building ventilation openings, pedestrian walkways, or building exits.
(3) For heavier-than-air refrigerants, the point of vent discharge shall be located not less than 20 feet (6096 mm) horizontally from below-grade walkways, entrances, pits or ramps where a release of the entire system charge into such a space would yield a concentration of refrigerant in excess of the RCL. The direct discharge of a relief vent into enclosed outdoor spaces, such as a courtyard with walls on all sides, shall not be permitted where a release of the entire system charge into such a space would yield a concentration of refrigerant in excess of the RCL. The volume for the refrigerant concentration calculation shall be determined using the gross area of the space and a height of 8.2 feet (2499 mm), regardless of the actual height of the enclosed space.
(4) The termination point of a vent discharge line shall be made in a manner that prevents discharged refrigerant from spraying directly onto personnel that are capable of being in the vicinity.
(5) The termination point of vent discharge line shall be made in a manner that prevents foreign material or debris from entering the discharge piping.
(6) Relief vent lines that terminate vertically upward and are subject to moisture entry shall be provided with a drip pocket having a length of not less than 24 inches (610 mm) and having the size of the vent discharge pipe. The drip pocket shall be installed to extend below the first change in vent pipe direction and shall be fitted with a valve or drain plug to permit removal of accumulated moisture. [ASHRAE 15:18.104.22.168]
2015 UMC §1114.0 Special Discharge Requirements.
2015 UMC §1114.1 General.
Systems containing other than Group A1 or B1 refrigerants shall discharge to atmosphere through an approved flaring device.
(1) Ammonia absorption systems serving a single dwelling unit.
(2) Where the Authority Having Jurisdiction determines upon review of a rational engineering analysis that fire, health, or environmental hazards will not result from the proposed atmospheric release.
(3) Lithium bromide absorption system using water as the refrigerant.
4. 2015 – IFC Option for Diffusion Systems
2015 IFC §606.12.5 Ammonia refrigerant.
Systems containing more than 6.6 pounds (3 kg) of ammonia refrigerant shall discharge vapor to the atmosphere in accordance with one of the following methods:
- Directly to atmosphere where the fire code official determines, on review of an engineering analysis prepared in accordance with Section 104.7.2, that a fire, health or environmental hazard would not result from atmospheric discharge of ammonia.
- Through an approved treatment system in accordance with Section 606.12.6.
- Through a flaring system in accordance with Section 606.12.7.
- Through an approved ammonia diffusion system in accordance with Section 606.12.8.
- By other approved means.
Exception: Ammonia/water absorption systems containing less than 22 pounds (10 kg) of ammonia and for which the ammonia circuit is located entirely outdoors.
5. 2014 – IIAR 2 Option for Diffusion Systems
ANSI/IIAR 2-2014 §15.5.1 Atmospheric Discharge.
Pressure relief devices shall discharge vapor directly to the atmosphere outdoors in accordance with this section.
EXCEPTION: In lieu of relieving directly to atmosphere, the following methods of discharging ammonia from pressure relief devices shall be permitted where approved by the AHJ:
- Discharge through a treatment system.
harge through a flaring system in accordance with Section 15.5.2.
- Discharge through a water diffusion system in accordance with Section 15.5.3.
- Discharge using other approved means.
6. 2016 – Removal of Diffusion Tank Requirement in ASHRAE 15
ANSI/ASHRAE 15-2016 §22.214.171.124.2 Ammonia (R-717).
Pressure relief valves serving systems using ammonia as a refrigerant shall discharge in accordance with one of the following:
a. To atmosphere in accordance with Section 126.96.36.199
b. Internally in accordance with Section 188.8.131.52
c. To a treatment system approved by the authority having jurisdiction