Ammonia compressors in boil-off gas systems present several safety concerns that need to be addressed to ensure a safe working environment. Here's a breakdown of key areas:

1. Equipment and Design: Compressors must be specifically recommended by the manufacturer for ammonia service [1]. They should be designed for a working pressure of at least 250 p.s.i.g. [1]. Adequate measures, like drainable liquid traps, should be in place to minimize liquid entry into the compressor [1].

2. Pressure Relief: Compressors must have a relief valve capable of discharging the full capacity of the compressor connected to the discharge before any shutoff valve [1]. A spring-loaded safety relief valve, set at a pressure not exceeding 300 p.s.i.g., should be connected between the compressor discharge and the discharge shutoff valve [2].

3. Shutoff Valves and Pressure Gauges: Manually operated shutoff valves should be installed on both the suction and discharge connections of the compressors [2]. Pressure gauges of the bourdon-tube type must be installed on the suction and discharge of the compressor before the shutoff valves, and the compressor should not be operated if either gauge is removed or inoperative [2]. Pressure gauges should be graduated to at least one and one-half times the maximum pressure that can be developed [1].

4. Piping and Fittings: Plant piping should contain shutoff valves located as close as practical to compressor connections [1]. All piping, tubing, and fittings must be made of material suitable for anhydrous ammonia service and designed for a pressure not less than the maximum pressure to which they may be subjected in service [3].

5. Electrical Safety: Electrical equipment and wiring for use in ammonia installations should be general purpose or weather resistant as appropriate and installed and maintained properly [4].

6. Emergency Preparedness: Stationary storage installations must have at least two suitable gas masks in readily accessible locations [5]. Full-face masks with ammonia canisters approved by NIOSH are suitable for emergency action involving most anhydrous ammonia leaks, especially outdoors [5]. For concentrated ammonia atmospheres, a self-contained breathing apparatus is required [5]. Additionally, these installations should have an easily accessible shower or a 50-gallon drum of water [5].

7. Operational Considerations: The refrigeration system should have controls to govern compressor operation based on the load, as evidenced by the pressure in the container(s) [6]. An emergency alarm system should be installed to function if the pressure in the container(s) rises to the maximum allowable operating pressure [6].

8. Compressor Separators: An entrainment separator of suitable size and design pressure should be installed in the compressor suction line of lubricated compression, equipped with a drain and gaging device [6].

9. Maintenance and Records: Each equipment modification, repair, test, calibration, or maintenance service should be recorded with the date, nature of work, and the person performing the work [7].

10. General Safety Measures: Storage areas should be kept free of readily ignitable materials [8]. Precautions should be taken against damage from vehicles 10, 12.

Addressing these concerns through proper design, maintenance, and emergency preparedness can significantly reduce the risks associated with ammonia compressors in boil-off gas systems.