Your air receiver tank works hard to keep your compressed air system running at optimal efficiency. For best results and safe operation, it’s important to make sure you have adequate storage capacity for your application. You also need to take proper care of your tank once it is installed. In this article we provide advice for air receiver tank sizing, safety and storage.
Properly sized and maintained air receiver tanks will contribute to years of reliable and efficient compressed air system performance.
Air Receiver Tank Sizing
The volume of compressed air storage capacity needed by a facility depends on several factors:
- The air compressor capacity in cubic feet per minute (cfm).
- Peak cfm requirements at moments of maximum demand.
- The consistency of airflow.
- The diameter of the piping.
A good rule of thumb for most applications is to have three to five gallons of air storage capacity per air compressor cfm output. So if your air compressor is rated for 100 cfm, you would want 300 to 500 gallons of compressed air storage. As we’ll explain in more detail below, 1/3 of the total storage capacity should be wet storage and 2/3 should be dry storage.
While the standard rule works well for many applications, you will also want to consider other variables in determining your compressed air storage needs. Flow consistency has a large impact on storage requirements.
- Facilities with very steady airflow, such as robotic facilities, typically don’t need as much stored air. That’s because they don’t have frequent high bursts of demand that rely on stored air. In this case, air storage can be reduced to two gallons per cfm of air compressor capacity. All storage should be wet storage in this case.
- Facilities with high variability in airflow and large peaks in demand may require larger volumes of stored air. This extra capacity will ensure that the system will be able to keep up with periods of high demand. Testing to determine cfm at peak demand will be needed to calculate air storage requirements.
The final consideration in determining compressed air storage requirements is the size of the pipework in the system. The pipes also store air for your compressed air system, and the larger the pipes, the more storage they provide. For systems with pipework of two inches or greater diameter, it may be worthwhile to consider that volume in the calculation.
Wet Versus Dry Compressed Air Storage
When shopping for an air receiver tank, you may be asked whether you want “wet” or “dry” compressed air storage. The difference is in the location of the air storage tank in your compressed air system; there is no difference in tank construction or design.
- “Wet” storage tanks are located before the air drying system. Air flows through the tank in this configuration, entering through the bottom port from the air compressor and exiting out the top to the dryer.
- “Dry” storage tanks are located after the air dryers to store compressed air that has already been dried and filtered. It is not necessary to flow the compressed air through the tank for dry storage.
Wet and dry storage have different benefits for your system.
- Wet storage increases the efficiency of your air dryer and prolongs the life of the pre-filter element by allowing excess water and lubricant to condense out of the system before it hits the filter and dryer. This also reduces pressure drop on the air dryer side of the system and provides a steadier pressure signal to the compressor controller.
- Dry compressed air is ready to use right out of the tank. This reduces the risk that the air dryer will become over-capacitated during high-demand events.
For most applications, it makes sense to have a combination of wet and dry storage.
The ideal ratio of compressed air storage for most applications is 1/3 wet to 2/3 dry capacity. For example, if you have a total of 1,200 gallons of compressed air storage, 800 gallons should be dry storage and 400 gallons should be wet. Dry air is ready to use on demand. The wet air tank increases the efficiency of the dryer and acts as a secondary reserve when dry air is exhausted. Dry air storage needs to be greater than wet storage to minimize the risk of over-capacitating the air dryer during periods of high demand.
An exception to this rule is for applications that have steady airflow without sharp peaks in demand. In this case, there is no need for a dry storage tank because air will simply flow through it without being stored up. This is often the case in robotic manufacturing facilities where airflow is consistent and predictable.
The Importance of ASME Certification and Safety
The American Society of Mechanical Engineers (ASME) has developed a set of codes and standards for pressure vessels, including air receiver tanks. The ASME Boiler and Pressure Vessel Certification Program sets rules governing the design, fabrication, assembly and inspection of pressure vessel components during construction. These rules include engineering standards for the thickness of the tank body, welds and joints, connections and other components of the tank. Tank manufacturers must conform to all of the rules to obtain ASME certification.
All air receiver tanks used in industrial applications must be certified by ASME for safety and performance. Non-code air receiver tanks should never be used, especially for industrial applications.
Some big box stores carry non-code air receiver tanks. While these may be cheaper, they have not undergone the rigorous manufacturing processes and quality testing needed to ensure that they are safe and reliable. Using a non-code air receiver tank could put your life and the lives of your coworkers at risk.
Air receiver tanks hold air under immense pressure. This creates safety hazards if the tank is not up to code or is not maintained properly.
Pressure vessels must be built to withstand high internal pressures over a long period of time. Over time, corrosion, stress and fatigue can make tank failure more likely. The most common causes of air receiver failure are:
- Faulty design/use of non-code tanks.
- Operation above maximum allowable working pressure (over-pressurization).
- Improper installation.
- Weld failure.
- Improper repair of cracks/leaks.
- Exposure to extreme environmental conditions (freezing or overheating).
- Safety valve failure.
The high internal pressures within an air receiver tank make failure extremely hazardous. Cracking or weld failure can cause the tank to burst with explosive force, projecting large pieces of metal or fragments of shrapnel at high speed. Air receiver tank failure may result in extensive damage to the facility and nearby equipment and severe injury or death for nearby workers.
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Air Receiver Tank Maintenance and Inspection
A well-maintained air receiver tank can last for many years. To get the most out of your investment, it is important to follow all operating guidelines, perform regular maintenance and inspection, and protect the tank from climate extremes.
Follow all operating guidelines, perform regular maintenance and inspection, and protect the tank from climate extremes to get the most out of your air receiver tank.
For safe operation, it is essential to follow all safety guidelines listed in the owner’s manual for your air receiver tank. To improve tank safety, be sure to:
- Only use ASME-certified air receiver tanks.
- Never over-pressurize the tank; follow operating guidelines for maximum pressure.
- Make sure the tank has a pressure gauge and it is functioning correctly.
- Periodically inspect the tank for corrosion, signs of weld seam stress, cracks, thinning of the vessel walls, and other defects. Any signs of corrosion should be addressed immediately to maintain the integrity of the tank.
- Make sure that the tank has an ASME-certified safety relief valve and the valve is working correctly.
- Drain the tank frequently to prevent liquids from accumulating inside the tank.
- Have all alterations or repairs completed by ASME-certified professionals to ensure that the repair meets quality standards.
- Provide safety training for air receiver tank operators.
Consult the OSHA guidelines for pressure vessel safety for more information.
If you are not sure whether or not your air receiver tank meets code requirements, you should have it inspected. Your local fire marshall may provide this service. They will stop in and test your tank with ultrasonic metal thickness testing technology. If your air receiver tank does not pass the inspection, it should be decommissioned and replaced immediately.
All air receiver tanks must also be inspected periodically once they are installed. OSHA does not mandate a specific testing interval, but it is recommended that all air receiver tanks be inspected at least annually. Your insurance company or local governing board may have different requirements. OSHA requires that formal inspections be performed by an inspector holding a valid National Board Commission and in accordance with the applicable chapters of the National Board Inspection Code. Manufacturers are required to keep records of formal inspections and make them available to OSHA representatives upon request.
The inspector will review the current operating certificate and perform an overall assessment of the air receiver, piping and other systems. This will include visually inspecting the vessel walls and all support and mounting bracket attachment welds for evidence of corrosion, cracking, denting, gauges, punctures or weld failures. Internal inspection of the vessel walls may be conducted using cameras or sensors. Where visual internal inspection is impractical, inspectors may instead perform thickness readings using an ultrasonic sensor to look for signs of vessel wall thinning. In addition, the inspector will check:
- Pressure relief system.
- Automatic condensate drain or manual drain valve.
- Connected piping.
- Proper securing of the tank by bolting it to a solid structure or concrete floor.
In between formal board inspections, companies should conduct frequent visual inspections of the air receiver tank to look for signs of corrosion, damage or weld failure. Check drains daily and pressure relief valves quarterly to make sure they are operating correctly. Contact your compressed air system manufacturer or installer immediately if you see any signs of problems with your air receiver tank.
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Outdoor or Indoor Air Receiver Tank?
Air receiver tanks can be installed either inside or out, depending on climate and space considerations.
Compressed air receiver tanks can be bulky, so many compressed air system owners would prefer to store them outside. Outdoor storage saves precious floorspace in the facility. It also helps to reduce strain on your HVAC system in warm weather. The compressed air storage tank radiates heat as hot air from the air compressor cools within the tank. Storing your tank outside avoids excess heat buildup in the air compressor room and also helps the storage tank perform its secondary job as a heat exchanger more efficiently.
The decision to store air receiver tanks inside, or outside of a facility are driven by multiple factors, including space considerations and the local climate.
On the other hand, outdoor storage leaves the air receiver tank vulnerable to temperature extremes and moisture damage. Make sure your climate is suitable for outdoor placement of your compressed air tank. Outdoor storage of the air receiver tank is only appropriate for environments that stay above freezing year-round. In freezing temperatures, outdoor tanks can ice up and even rupture — a costly and potentially dangerous outcome. If your area experiences freezing temperatures during part of the year, it is safest to keep your tank indoors.
If your area is subject to cooler temperatures with an occasional risk of icing, take special care of your tank in cooler weather. The tank will generate some heat on its own. However, if temperatures drop too far, the tank is still at risk of freezing. Insulating your tank and providing auxiliary heating during cold weather may be necessary to prevent damage.
Some other things to keep in mind when locating your air receiver tank:
- The tank should be located so that the entire outside surface can be easily inspected.
- The air receiver tank should not ever be buried.
- Make sure all drains, valves and gauges can be easily seen and accessed.
- Make sure that there is good airflow all around the tank to dissipate excess heat.
- Do not locate the tank near equipment, materials or products that could be damaged by heat radiating from the air compressor.
- If you are storing your air receiver tank outdoors, be sure to conduct frequent inspections to monitor for corrosion.
- Make sure the tank is not subjected to excessive vibration or at risk of impact by vehicles or manufacturing equipment.
Reliable and Safe Operation for Years to Come
Following safe operation, maintenance, inspection and storage guidelines will extend the life of your air receiver tank and ensure that people working with or near the tank remain safe. With proper care, your air receiver tank will continue to operate safely for many years to come.
About the Author
Derrick Taylor is co-founder and General Manager of PneuTech USA, email: Derrick.firstname.lastname@example.org, tel: 888-966-9007, Ext. 4000. He brings more than 20 years of direct experience in compressed air service, installation, sales and system design. Taylor is also co-owner of Fluid-Aire Dynamics, a leading distributor of industrial compressed air equipment in the Chicago, Milwaukee, Minneapolis and San Antonio markets.
PneuTech is a global manufacturer of air compressors, air dryers and compressed air equipment. For more information, visit https://pneutech.com/usa/.
All photos courtesy of PneuTech USA.
To read Part 1 of this article, visit www.airbestpractices.com/system-assessments/pipingstorage/how-your-air-receiver-tank-improves-system-efficiency.
To read similar articles on Compressed Air Storage System Assessments, visit www.airbestpractices.com/system-assessments/piping-storage.