Elements of breathing-air compressor systems
Industrial plants often require operators to work in environments where ambient air is not suitable for long-term exposure. This requires installing an airline respiratory program or breathing-air system (Fig. 1). Plant breathing air must meet the Compressed Gas Association, Grade-D requirements. OSHA CFR 29-1910.134 states that breathing air must comply with the Compressed Gas Association’s G7.1 standard for breathing air.
What information is needed to select and implement a total plant breathing-air system? The following guidelines provide the information and resources needed to make these critical safety and health decisions.
Hazard assessment results should be reviewed in detail with the plant’s safety and health department. The assessment should detail physical hazards in the plant, areas where a material or substance is off-gasing or releasing toxic/noxious vapors into the ambient air. A fork lift burning diesel fuel or gas inside a building, a smoke stack in close proximity to an air compressor intake, or a manufacturing process that releases particulates into the air are examples of individual hazards.
Determining where each hazard is located within the plant as well as what concentration levels are present at each location is essential. If in doubt, consult the American Conference of Government Industrial Hygienists (ACGIH) Guide for specific threshold limit values for chemical substances and physical agents, and biological exposure indices. The values listed are adopted values and may change from year to year.
This evaluation is a critical phase in the decision process and is vital to selecting the right type of respirator(s) and the design of the compressor system itself.
Is breathing air needed on the production lines as well as for mobile personnel performing routine maintenance around the plant? This decision is essential in designing where the plumbing for the breathing system is located and where the connection points for the worker’s respirators will be placed.
In a fixed system, the plumbing conveniences must be modified to an inverted-T, so the air piping does not act as a conduit for residual moisture and condensation.
Mobile or portable systems can be mounted to a cart or forklift for flexibility. Depending on the type and location of the hazard, the compressed air system may require special materials such as HEPA intake filters or custom, skid-mounted units for systems located in washdown areas.
Once the concentration level of the hazard is determined, the appropriate airline respirator can be selected. This selection will require the aid of a trained safety equipment professional who can make recommendations as to the best airline respirator to be used, based on the hazard assessment.
Type C airline respirators are available in two functionally different designs, positive pressure/ pressure demand and constant flow. Constant flow respirators are available in three basic facepiece styles: hood, half mask, and full-face mask.
More than one style of airline respirator may be necessary, based on the extent of the hazards at each location in the plant. Since all respirator equipment manufacturers are required to submit their equipment with all components and fittings for testing and review, each manufacturer should provide equipment that includes a Test Certificate Approval Number. Manufacturers put their equipment through this laborious and expensive certification process to ensure that all breathing-air delivery equipment conforms to NIOSH regulation 87-116.
Federal regulations require all airline respirators (positive pressure/pressure demand) to deliver a minimum of 4 cfm to each worker. If hood-style respirators (constant flow) are selected, 6 cfm per worker is required.
The number and style of airline respirators is essential to calculating the size of the compressor required for the plant air system. Multiplying the total number of workers using the plant air system times the required flow rate to yields the approximate output required from the compressed air system. If additional workers may be added to the system in the future, the additional number should be added to the final count of workers for calculating flow requirements.
A decentralized system with dedicated compressors is recommended. It should be a redundant system or with bottled air as a backup. It is possible to tap into the plant air system, but be aware that piping and lubricant affect the breathing-air system.
If the compressor is used for other air needs in the plant, such as running pneumatic tools or plant machinery, each air tool’s consumption will need to be totaled and added to the overall size of the compressor. It is always prudent to add another 10%-15% output capacity to the overall system to compensate for any unforeseen air consumption needs or unplanned equipment upgrades.
Rotary screw compressors supplied with USDA certified food grade lubricant are ideally suited for a plant breathing-air system. In addition to low maintenance requirements, rotary screw compressors operate at much cooler temperatures than reciprocating compressors and are designed for continuous operation, which ensures a continuous supply of quality compressed air.
Rotary screw compressors also produce minimal oil carryover, usually in the 2_4 ppm range. Using a food-grade lubricant helps ensure the safest possible breathing air. USDA Certified, FDA recognized, food-grade lubricants are required because they are biodegradable with little or no human toxicology issues, and the flash points are significantly higher than normal petroleum based lubricants.
A low operating temperature removes the risk of residual burns and CO production. Avoiding CO production is another benefit of using rotary screw compressors. Reciprocating compressors operate at much higher temperatures and the residual burn may result in too much CO production.
All compressors take ambient air and compress it internally. While the lubricant injected into the airend actually removes most contaminants, it is critical that the compressor used for breathing air be located in a clean-air environment away from harmful dusts and gases. A separate compressor room with ample fresh air ventilation is recommended in certain dusty or contaminate-prone environments.
A compressor output pressure of 120_150 psig is adequate to operate any respirator on the market. Delivering air throughout the plant can be done with stainless steel, carbon steel, or certain approved plastic pipes that do not give off any harmful vapors.
The next component required in a plant breathing air system is a refrigerated air dryer. This component removes moisture from the compressed air and delivers the air to the receiver tank at approximately 70 F, ideal for breathing-air requirements. A refrigerated dryer is better than desiccant dryers, because the air delivered from desiccant dryers is too dry and will irritate the worker’s eyes and throat.
The refrigerated system is also less expensive and more compact in size. Air processed in the dryer is stored for use in an air receiver tank. This stored air ensures a smooth delivery to the users; it is not intended to act as an emergency air supply.
The cooled and stored air must be filtered to remove all remaining particulates, vapors, and tastes to achieve Grade-D air quality. The first stage of filtration is a basic particulate filter. The second stage includes a coalescing oil filter (Fig. 2). A high-capacity charcoal filter must be used to remove organic vapors, odors, and tastes.
Specific features to be considered for the filtration system start with automatic drains on the first and second-stage filters and tanks. Differential pressure gauges are recommended on all three filters. All three filters, in series, must produce 99.9998% efficiency at 0.01-micron particle size with a maximum pressure drop of 10 psi.
After the air passes through Grade-D filtration, a CO airline monitor must be installed to continuously monitor the level of CO in the breathing air. High-CO warning lights and alarms can be installed close to the workers or can be placed in a central engineering control room (Fig. 3).
The last stage of the breathing air system is the proper distribution of air to the worker’s respirators. The point where the respirator hose is attached to the breathing-air system is called the point-of-attachment (POA). All POA’s are also regulated by NIOSH, and must have an air pressure regulator, pressure gauge, 125-psig safety relief valve, and a quick-connect coupling approved for use by the respirator manufacturer.
Respirator fittings must be recognizably different than tool fittings to reduce the risk of plugging into a connection with point-of-use lubrication installed. The work environment must also be considered when selecting a POA device so its components can be protected to ensure proper functioning.
The process outlined here shows that many variables must be identified to have a safe and functioning plant breathing-air system. Supplying dependable breathing air is a serious matter and should be left only to professionals. Selecting the right breathing-air system supplier and a reputable manufacturer will ensure the proper setup, training, and testing of the complete system.
Questions about breathing air compressor systems can be directed to Ray Ellis at 800-866-8100. Portions of this material were contributed by Kaeser Compressors, Inc. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, email@example.com .
Typical compressed air usage*
|CGA grade||Typical uses|
|A||Industrial compressed air|
|I||Self-contained breathing apparatus (SCBA) air|
|D||OSHA breathing air|
|E||Self-contained underwater breathing apparatus (SCUBA) air|
|J||Specialty grade air, analytical applications|
|*Typical uses defined in this table are not all-inclusive Compressed Gas Association, Inc.|