OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.

June 13, 1990

                        REGIONAL ADMINISTRATOR

FROM:                   PATRICIA K. CLARK
                        DIRECTOR DESIGNATE

SUBJECT:                Dilution Ventilation and Intrinsic Safety
                        in the Mechanical Control Room for an 
                        Anhydrous Ammonia Refrigeration System

This is in response to your memorandum of April 9, regarding the above subject. As you know, the applicable industry consensus standards for refrigeration systems, including the system that uses ammonia as a refrigerant, are contained in ANSI/ASHRAE 15-1989. Section 10.14(j) of the ANSI/ASHRAE standard, however, precludes the ammonia refrigeration system control room from being classified as a Class I, Division 2 location, if the provisions contained in Sections 10.14(h) and 10.14(i) of the same standard are met.

The company at time of the inspection had not installed a ventilation system meeting the specifications of the ANSI/ASHRAE standard and, consequently, was in apparent violation of 1910.307(b), for exposing employees to explosion hazards. We advise that a 1910.307(b) citation be issued.

Because the company, [company name withheld], is proposing to install a ventilating system to meet the design requirements specified in the ANSI/ASHRAE standard, the company, will have abated 1910.307(b) violation when the system is installed. They would then not be required to install blower fans that are intrinsically safe.

It should be noted that the installation of a vapor detector system as described in your memorandum may potentially eliminate the explosion hazard but would not eliminate the inhalation hazards likely to cause death or serious physical harm.

The health hazards associated with ammonia are recognized by ANSI, as referenced in Table 2 of ANSI K61.1 - 1989 (copy attached), and a release of ammonia to contaminate the control room area in concentrations greater than 2,500 ppm in air, could be fatal if the necessary precautions are not taken. Though the referenced ANSI standard is not applicable to the situation you described, the inhalation hazard is recognized and must also be addressed by the company.

In order to protect against both safety and health hazards, the vapor detector system to be installed shall be capable of providing an audible alarm at an ammonia vapor concentration of 2,500 ppm in the air in the control room, and shall be capable of activating the exhaust blowers when the ammonia vapor concentration in the air in the control room reaches 40,000 ppm. Employees who may have to enter the control room during emergencies shall be fully equipped with the appropriate personal protective equipment, and shall be trained to meet OSHA standards at 1910.120, 1910.134, and 1910.1200.

If your staff members have any questions, they may contact Sanji Kanth at FTS 523-8031.