When Sizing Met Safety: A Storage Tale

By: Neil Mehltretter

Compressed air storage receivers are easily forgotten. They have no moving parts (except for the drains) and usually do their jobs quietly and reliably (as long as that drain is looked after). They provide a buffer to limit compressor cycles and store air for short periods of high demand. Size and locate them properly and they will allow your compressors to respond efficiently and reliably to system events. Receiver tanks also provide a vessel to collect and remove moisture. This enhances dryer operations and protects pneumatic equipment and production. The flip side is that condensate can affect the tank’s physical integrity (though this may take many years to happen). In addition, there are basic but very important safety requirements for tanks. So it pays to consider both the performance effects and safety when applying storage receivers.

The most important requirements when designing a compressed air station are flow, pressure, and air quality. These three parameters will determine how large or small your system components (filters, dryers, piping, etc.) should be, including the receiver tank(s). In addition to sizing these components for system flow, each has a maximum rated pressure and they must be higher than the compressors’ operating pressure.

For example, if your compressor is rated for 125 psig it will be critical to ensure that your dryers and filters can handle that particular pressure. While these items are usually paired with a compressor, the rated pressure for your storage receivers and piping also needs to be considered.

Safety First!

nameplate Storage receivers can be manufactured to any specific pressure requirement, with typical off-the-shelf units rated between 150 psig and 200 psig maximum allowable working pressure (MAWP). In our example — with a 125 psig compressor — we should choose a 150 psig MAWP rated storage receiver. Note that receivers must be manufactured in accordance with ASME Boiler Pressure Vessel Code VIII, Div. 1.

receiver-tankCompressed air storage receivers are also governed by OSHA 1910.169 and require a pressure gauge, a safety relief valve (AKA “pressure relief valve”), and a drain valve. The gauge must measure pressure at least up to the MAWP.  Further, it should be large enough and accurate enough so an accurate reading can be made at a glance. It should be mounted in an easily accessible viewing location. We also recommend an isolation valve between the pressure gauge and the storage receiver. If  the gauge ever needs to be replaced, you won’t have to shut the system down and bleed off the tank.  

The safety relief valve (SRV) must be sized for the full compressed air flow of the system, and it should be set to open at a pressure less than or equal to the MAWP of the storage receiver. In some cases, e.g., large systems, you may need to install multiple safety relief valves in parallel to handle the full potential flow of the system. SRVs should be placed where they can be easily inspected, and since a safety relief valve might theoretically open at any time, it must be placed so that no person could be injured by the airflow. For this reason, many safety relief valves are installed with deflector covers to direct the compressed air downwards rather than directly out from the tank. Keep this mind when selecting the location of the safety relief valve.

The cross-sectional area of the receiver opening should be equal to or greater than the cross-sectional area of the relief valve(s) opening(s) so as not to impede flow. For this same reason, do not install an isolation valve between the storage receiver and the safety relief valve.

Drains

This may seem obvious, but connect any drain you use at the bottom fitting of the tank. As to the type of drain, there are plenty of choices. The worst one is a manual valve. Cheap to buy and easy to install, it will either be closed and not do its job, or be left open and waste air. A 1/4″ drain valve will consume 95 cfm per minute at 90 psig. If that drain was open all the time it would cost the plant $15,000 per year (based on $0.10/kWh) and possibly impact system pressure. Fortunately, there are several affordable no-loss demand drain options. Some require 115V, 1ph power and some don’t require any power. These only open when condensate is present. Select models with a test button and check them routinely. When drains fail, there may be no outward sign. Meanwhile, oily water builds up in the tank, preventing the tank from doing its primary jobs, and eventually gets downstream.

Where oh Where…and Why?

Storage sizing varies by application and compressor control type. There’s a strong case for storage in multiple locations: wet storage prior to air treatment (1), dry storage after air treatment but still in or very close to the compressor room (2) , and point-of-use storage at the end users (3).

Size Matters

Storage is about volume, so tank size matters. Calculating storage requirements is not usually hard, but if it gets complicated, slightly oversizing will likely have no negative impacts. Undersizing is a different story.

Check out our previous blog post that talks in general about storage placement and design parameters.

For base-load rotary screw compressors, we recommend sizing storage receivers in the compressor room such that you can maintain the system supply for a minimum of 30 seconds. This ensures that you have enough response time to start and load a compressor to meet demand without significant pressure drop that could affect production.

Storage volume recommendations do vary based on the control type (i.e., fixed speed, variable capacity, variable frequency drive, and modulation). However, if your base-load compressor will load and unload frequently, 5 gallons of storage per cfm of base-load compressor is a good general rule. You can break this up into 1-3 gallons on the wet side, and 3-5 gallons on the dry side.

For more details on storage sizing, check this calculator on our website.

Point of Use Storage

Many plants have rapid air demands, also known as High Intermittent Demand Events, that will drop system pressure significantly and starve other points of use for air. Point of use storage can really be helpful in this scenario and they are easily sized and installed. Many of these point-of-use receivers are already included with end-use equipment. However, if your system does not include point-of-use storage, it can be added later on using simple calculations. A metered valve to the point-of-use receiver is also highly recommended so as not to draw down the entire production area for this one application during a high-demand event, and also to slowly re-pressurize that receiver after the event has passed. 

How much does that thing weigh?

receiver-on-mezzanineWe often get the question of whether it is OK to mount a receiver on a mezzanine, a roof, or suspended from the ceiling.  Like I mentioned above, storage receivers can fill with water if the drain valve is not working, so you must consider the maximum receiver weight when it’s full of water in your load calculations. In this installation, the user opted to save floor space by hanging the 1060 gallon tank from the ceiling.  Empty, the tank weighs 2700 lbs.  Full of water, that tank weighs over 11,500 lbs.  So here are two good ideas:  1) install a bulk liquid separator between the compressor and the tank to remove liquid before it hits the tank and 2) select a high-capacity automatic drain on the tank  that can be wired to alert you if the drain is not working. 

receiver-mountingAll storage receivers come with mounting feet for good reason: to mount the receivers securely.  Storage receivers have an enormous amount of potential energy, as we saw in the picture above. When you compress air you store energy. As pressure drops and the air flow moves through to the point of use, we release that energy. Insufficiently supporting your receivers can result in catastrophic failures.  Not to mention the simple fact that tanks are often tall…and made of steel.  Better not to let them tip over. 

Periodic Inspection

Receivers should be evaluated regularly to ensure that they are in safe operating condition. Many insurance companies, states, and local authorities require receivers and other pressure vessels to be inspected regularly (some yearly) by an outside party to ensure that they are still fit for service. We’ve seen receivers made in 1965 still in service fifty years later. Bottom line, make sure your receiver is fit for use. Otherwise not only could you have rust and scale in your compressed air system, but you could also have a rocket.

receiver-failure2Pictured here is a 25-year old 400-gallon vertical tank used as a wet tank between the compressor and dryer. Someone noted that the tank had a leak in the top bell. Rather than isolate, depressurize, and lock it out of the system, they took no action. Five hours later, the tank failed from a combination of rust and pressure. Thankfully no one was in the building at the time of the failure. 

Some receivers have manhole covers for inspection purposes. Therefore it is critical to ensure that ample service space is available around the receiver. In some municipalities, a specific amount of space is required by law in front of the manhole cover as well as the entirety of the receiver. Check locally to make sure you’re complying with necessary codes.

The Last Word

Tanks quietly do a lot of heavy lifting in your system. They reduce cycling and wear on the compressors, remove moisture to protect pneumatic tools and production equipment, store air for short periods of high demand, etc. Compared to other system components, they need little attention. It really pays to properly install and protect them.