I have tried to understand the basis for the International Institute of Ammonia Refrigeration’s (IIAR) set points they recommend in one of their Technical Bulletins and one of the ANSI/IIAR codes. Some background data to help understand this analysis:
- 1% = 10,000 ppm
- Normal Anhydrous Ammonia (99.97% or greater) has an LEL of 16% (or 160,000 ppm)
- Studies have shown that NH3 can have a lower LEL of 10% (or 100,000 ppm)
- IIAR/ANSI 2-2008 calls for the emergency ventilation to kick in at 1,000 ppm.
IIAR Bulletin 111 – Guidelines for Ammonia Machinery Room Ventilation calls for emergency ventilation to kick in at no more than 25% of the LEL or 40,000 ppm (i.e. 25% of 160,000). The 25% of the LEL is from NFPA 70 and is meant to provide a safety factor when dealing with the LEL of flammable materials.
In our experience, most facilities use the 40,000 ppm setpoint from Bulletin 111. If they all used the 1,000 ppm set point found in ANSI/IIAR 2-2008, all would be OK; however, when a facility uses the IIAR Bulletin 111 set point of 40,000 ppm, there may be a potential gap. Everything appears to be OK, as the facility would still be safe. Still, the safety margin in the design parameters set in IIAR Bulletin 111 is reduced by 1/2 and, therefore, would not be following NFPA codes.
The LEL of NH3 is lowered via the entrainment of oil in the ammonia. Oil is used in the compressors and seals and leaks into the ammonia system. It is nearly impossible to obtain oil removal efficiencies from the compressor manufacturers. A few have told me they do not have the data and that regular draining of the oil pots will be sufficient to ensure that not enough oil gets entrained in the ammonia and that the LEL would drop to 10%. But we know we can get down to 10%, and a refrigeration system can still operate, although not as efficiently, but it is still running, and a leak into the engine/compressor room could develop a flammable atmosphere well ahead of the time the ventilation fan kicks on. Here is how this would happen:
- there is enough oil entrained in the Ammonia to lower the LEL to 10%. What does this mean? The engine room has 100% air volume. If we replaced 10% of the volume of air with the NH3 vapor that has the oil entrained in it, that is a FLAMMABLE ATMOSPHERE (e.g., 100% LEL)
- Using IIAR Bulletin 111, our design basis, Section “3.2.1 Emergency Ventilation Rate” states: The lower flammability limit for ammonia/air mixtures is approximately 16% by volume (160,000 ppm). The National Fire Protection Association (NFPA 30 — Flammable and Combustible Liquids) defines “adequate ventilation” as that which is required to maintain concentrations below 25% of the lower flammability limit (i.e., 4% or 40,000 ppm by volume for anhydrous ammonia). A goal of the ventilation system design and operation under emergency conditions is to purge ammonia vapors in an attempt to maintain ammonia concentrations below 4% by volume (40,000 ppm) in the event of an ammonia leak.
And IIAR Bulletin 111 Section 3.5.2 Emergency Shutdown states:
Machinery rooms should be designed with emergency electrical power shutdown capability. The electrical power shutdown should disconnect power from the entire machinery room with the exception of ventilation exhaust fans (with non-sparking type motors) and any equipment that is Class I, Division 2, Group D compliant. The recommended machinery room concentration to initiate machinery room electrical shutdown is 15,000 ppm. Lower threshold concentrations to initiate electrical shutdown can also be considered. The electrical shutdown concentration should not exceed 25% of the LFL (i.e. 40,000 ppm).
Here are the numbers:
Normal NH3
LEL = 16% (160,000 ppm) – 25% of this LEL is 40,000 ppm
Ammonia with Oil
LEL = 10% (100,000 ppm) – 25% of this LEL is 25,000 ppm
Using the set point of 40,000 ppm, our engine room will be well past the NFPA Code requirement to maintain the atmosphere to at least 25% of the LEL. WE ARE STILL BELOW THE LEL of 100,000, but we are NOT maintaining the safety factor of 1/4 in regards to the LEL. This could be an issue with your compliance with Electrical Hazardous Locations Code.
Facilities have two options in regard to the installation of electrical equipment in their ammonia areas, primarily in their engine rooms:
1) install all electrical as a Class I Division 2 locations or
2) use ventilation to ensure the room will not exceed at least 25% of the materials LEL.
Using the IIAR Bulletin 111 set point of 40,000 ppm for our fans to kick on is NOT maintaining the safety factor of 25% of the materials LEL when we have oil entrained in our ammonia vapor. In order to maintain this safety factor, facilities should NOT use the IIAR 111 set point, but rather the ANSI/IIAR 2 set point of 1,000 ppm or lower their set point to 25,000 ppm in order to comply with the Electrical Code for Hazardous Locations.