TIPS & TOOLS:  Ammonia Detection

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Current fire and building codes, as well as IIAR-2, require that engine rooms and refrigerated spaces be protected by automatic ammonia detectors. This article discusses the application and settings of ammonia detectors, and recommendations for their application.

Ammonia Levels

Ammonia Concentrations and Responses

400 ppm......... Immediate throat irritation
1,700 ppm...... Cough
2,400 ppm...... Threat to life after 30 minutes
5,000+ ppm.... High likelihood of mortality after brief exposure

Where should Ammonia Levels be Set?

The Uniform Mechanical Code calls for the emergency exhaust to be activated when the concentration reaches 50% of the IDLH, which would be 150 ppm. It also calls for the engine room to be shut down when the concentration reaches 25% of the LFL, which would be 37,500 ppm (UMC 1977). IIAR-2 calls for the emergency exhaust to be activated at a level not exceeding 1,000 ppm.

Ammonia Concentration Action Levels

The following table shows ammonia levels and what actions should be taken to enter the affected area.

0-25 ppm................................................................................. No Protection required
25-300 ppm................................................................ Air Purifying Respirator required
300-37,500 ppm........................................ SCBA Required, Level A Suit recommended
Above 37,500 ppm.................................................................................. Exit the area

Ammonia Detector Levels Recommendation in Engine Rooms

25 ppm........... warning – beacon and audible
< 1,000 ppm... activate emergency exhaust

In Cold Rooms

25 ppm........... warning – beacon and audible
35 ppm........... alarm level – shut off flow of ammonia to evaporator

Types of Ammonia Detectors

There are three types of ammonia detectors currently being used:

Solid State

Lowest in cost – both from a first cost standpoint and a maintenance standpoint. However, many of them are also easily fooled by other gases – paint fumes, exhaust gas, perfumes, etc. leading to false alarms. Depending on manufacturer, the sensors require replacement every 8-10 years.

Solid State is sensitive to low concentrations of gas.

Electrochemical

These sensors are more specific to ammonia gas – and are more costly. They use a chemical sensor that has to be replaced periodically. The warmer the sensor environment or the more ammonia leaks it experiences the faster the decline of the sensor. They are also adversely affected by moisture.

All ammonia sensors need to be checked periodically – but since electrochemical sensors become less and less sensitive with time – they eventually fade and will not sense a release. For this reason, Manning recommends testing every three months. Sensors last from 6 months to 5 years, depending on manufacturer and application.

Infrared

These are very specific to ammonia. They are as reliable as solid state. Their cost is similar to electrochemical. However, they cannot sense low concentrations (35 ppm) of ammonia.

Where to locate sensors

Ammonia is lighter than air – especially when the surrounding air is cold. Thus the most sensitive place to put a detector is up in the ceiling area – but this is also the least accessible for testing. In any case detectors mounted at a high level in the room should be at least 1-2 feet below the ceiling to minimize humidity change problems. In engine rooms, IIAR-2 requires that the detectors be located in an area where refrigerant from a leak is likely to concentrate. In blast freezers, sensors should be located below the level of the air unit, to protect against humidity problems. The argument can be made that if product protection is the primary consideration then put the sensor at ceiling level, and if personnel protection is the primary concern put it at eye level.

Maintenance of Sensors

Ammonia sensors should be tested according to manufacturer’s recommendations. There are two kinds of services to perform – calibration and testing. Testing simply involves running some ammonia past the detector and making sure it goes off. Calibration involves taking a sample bottle of ammonia at the proper concentration and making sure that the detector is going off at the right level. If a release occurs, and the EPA or OSHA get involved – the ammonia detectors may well be tested to see if they work.

Conclusions

There are no perfect ammonia detectors out there. Each has its advantages and disadvantages. The electrochemical are the best choice for avoiding false alarms – but are a bad choice if the customer does not do regular maintenance.