Pipes and valves are critical in a compressed air system

There is more to an efficient compressed air system than the compressor itself. Piping, valves and fittings all play an important role in the overall health and efficiency of a compressed air system.

By Peter Modrow and Brian Mann April 4, 2024
Courtesy: Hitachi Global Air Power


Learning Objectives

  • Types of piping and the role piping plays in a compressed air system.
  • Understand how bypass valves help with maintenance needs to avoid downtime. Pressure relief valves keep the compressor, storage tanks and other components from being damaged should an error occur.
  • The impact air leaks have on the efficiency of a compressed air system (and the corresponding costs) and how to best remediate any leaks or issues.


Compressed air insights

  • Neglecting the maintenance of supporting components like pipes, valves and fittings in a compressed air system risks efficiency loss and potential shutdowns.
  • Small overlooked leaks in pipes and fittings significantly reduce efficiency, leading to increased power costs and unnecessary carbon emissions over time.

A lot of time is spent sourcing and properly installing the right air compressor for a facility, setting the maintenance schedule and working out the intervals for regular oil sampling per the original equipment manufacturer’s (OEM’s) recommended service intervals.

However, there are other often overlooked components in an optimized compressed air system that are critical to helping get the most out of the compressed air system: Pipes, valves and fittings. If one part of the compressor system’s supporting components isn’t installed, operating or designed correctly, there is a major risk of lowered efficiency or, in the worst-case scenario, a shutdown.

Piping is a compressed air conduit

Pipes are the conduits that carry the compressed air to the process once it leaves the compressor. All compressors come with a discharge pipe connection sized by the manufacturer to have the proper and minimal pressure drop for the best compressor operation. Added to this connection is a manual (or automatic) shut-off valve. This disconnects the compressor from the compressed air system in times of maintenance.

The system piping to the plant after the shut-off valve, needs to be at least the size of the discharge connection at the compressor. The size of the pipe at the discharge of the compressor should never be reduced. Increasing the pipe size, however, is often beneficial to a compressed air system from a control reliability standpoint. The piping should also have the least number of 90° elbows as possible. In a good installation, sweeping piping radius bends are the best solution. This helps reduce the pressure drop every 90° elbow affords. As the piping connects to the main header, it is always good practice to connect the air at the top of the main header. If this is not done, then the pipe coming from the compressor can act as a “drip leg” and allow condensation to drain into the compressor.

In terms of piping materials, there are several options. Each comes with their own advantages and disadvantages. A compressed air specialist can help determine which type works for the specific application.

Aluminum piping is often used because it is lightweight, non-corrosive and easy to work with. Galvanized piping is sometimes used but is heavy and challenging to fit. Copper piping is also used but can be expensive. Older systems often have iron pipes which should be replaced. Over time, the residual water from the compressed air system can react with the iron pipes, causing a buildup of corrosion and reducing air flow.

Properly installed piping plays an important role in the efficiency of a compressed air system. Courtesy: Hitachi Global Air Power

Properly installed piping plays an important role in the efficiency of a compressed air system. Courtesy: Hitachi Global Air Power

Valves main operation

In addition to the shut-off valve, a 3-valve bypass should be installed around each added and maintained piece of equipment, such as filters and dryers in the compressed air system. At some point, the in-line filters will need to be changed and maintenance will need to be performed. Another good practice to follow is when the piping contractor is onsite, have the installer add a pipe with a ball valve before the filters and dryer somewhere near the compressor room or penetrating an outside wall. At times, the compressor will need maintenance and to be shut down.

This additional connection allows the customer to hook up a rental compressor so maintenance can be completed without interrupting the air flow. This helps prevent unplanned downtime due to emergency maintenance situations.

Pressure relief valves improve safety, efficiency

Quality air compressors are designed with safety in mind. For the most part, users normally do not have to worry about a safety issue arising. In fact, to prevent a potential issue, air compressors come equipped with an air pressure relief valve (PRV), sometimes called a safety relief valve, which is designed to work if other safety features fail.

The pressure relief valve protects the compressor component it is attached to from being exposed to a pressure above its rated maximum operating pressure. This rating, called the maximum working pressure (MWP), is the pressure at which the sump has been certified to continuously operate safely. When a compressor is running at or below its maximum working pressure — when it’s running “normally” — the relief valve doesn’t do anything.

However, when the air pressure inside a compressor exceeds its MWP, the pressure relief valve will activate to “blow off” the excessive pressure within the compressor. Without a relief valve, the storage tank could rupture from the excessive pressure, damaging the compressor, other property near it, and maybe causing injuries (or worse) to anyone standing nearby.

Under normal circumstances, the air pressure in a compressor is controlled by a pressure switch in an electro/mechanical control system or, in the case of an electronic controller, a pressure transducer and controller settings. When the cut-out set pressure for the pressure switch is reached, the compressor will stop compressing air until the cut-in set pressure is reached at which time it will start compressing air again.

If the pressure switch fails, the compressor would not be able to start compressing air again, or potentially worse, not be able to stop. Most compressors also have a high-pressure safety switch that should stop the compressor if the pressure exceeds the unload set point.

A pressure relief valve is a straightforward safety backup to the pressure switch and high-pressure switch, or the controller set points, should any of these components fail with the compressor running. The safety relief valve is set above the high-pressure safety switch and at or below the vessel’s maximum operating pressure. Inside the valve is a spring, and the pressure created by compressing the spring keeps the valve closed under normal operating conditions. However, as the air pressure increases in pressure vessels (like the storage tank), it may exceed the rated pressure of the relief valve, causing the relief valve to open and the excess pressure to be blown off to the atmosphere.

Image 2: Pressure relief valves are an important safety feature added to prevent pressure from building up inside a compressor. Courtesy: Hitachi Global Air Power

Image 2: Pressure relief valves are an important safety feature added to prevent pressure from building up inside a compressor. Courtesy: Hitachi Global Air Power

Preparing for a pressure relief valve failure

What happens when the pressure relief valve fails depends on whether it fails with the valve open, or the valve closed? There are two key scenarios to consider.

1. Failing with the valve open

If the pressure relief valve fails to open, air will keep venting to the atmosphere, preventing the air stream from becoming fully pressurized. The compressor should be shut down and the relief valve replaced before the compressor is restarted. The open relief valve may cause a loss of production and possible danger to personnel as a result of the flow of high-pressure air potentially with flying debris. The open valve can also produce an unsafe sound level.

2. Failing with the valve closed

A pressure relief valve failing closed presents a potentially more dangerous situation. As noted earlier, the relief valve exists to allow excessive pressure to be “blown off” so the air pressure inside the compressor’s pressure vessels don’t exceed their rated specifications. If the valve fails closed, it is a malfunction. It is not a setting or normal mode of operation and this pressure venting can’t happen. Unless compressed air demand matches the compressed air supply, the pressure inside the compressor will continue to build. Eventually, the pressure increase would cause the storage tank to rupture, damaging the compressor and causing additional damage and injury to property and people nearby.

If the pressure relief valve is opening and whether that is because of excessive pressure in the compressor or because the valve is failing, users should have an air compressor service technician inspect it before running it again for two reasons:

  1. The certified technician can determine whether the valve opened due to a failing relief valve or excessive compressors pressure and perform any needed maintenance or service to get the compressor safely running again.

  2. Regardless of why the pressure relief valve opened, replacing it may be recommended to ensure safe compressor operation, depending on the valve manufacturer.

Running the compressor after the relief valve has opened can put property at risk and workers in danger.

Air leaks and the efficiency factor

If pipes are leaky, the system is losing air and efficiency. Checking for leaks in a compressed air system should be an on-going activity. One approach to managing leaks is to segment the plant into 10 to 12 relatively equal areas, based on the distribution of production equipment using the compressed air power. Assigning one segment per month for a leak audit allows the entire plant to be audited each year.

And the leak doesn’t have to be large to start dragging down efficiency levels. Even small leaks can become expensive. By some reports, up to a third of compressed air is lost due to leaks and the corresponding elevated system pressure to compensate for that loss.

If an industrial user has 200 HP online (approximately 1,000 acfm capacity), the leaks and inappropriate uses of compressed air consume a significant quantity of electrical power, which add to the cost of electrical power for the plant and unnecessarily contribute to carbon emissions.

Impact of leaks and inappropriate uses of compressed air for an industrial user with system specific power of 20kW/100 cfm, paying $0.0771/kW-hr operating 8,000 hours/year. Tons of CO2/year based on coal-fired utility.

Impact of leaks and inappropriate uses of compressed air for an industrial user with system specific power of 20kW/100 cfm, paying $0.0771/kW-hr operating 8,000 hours/year. Tons of CO2/year based on coal-fired utility. Courtesy: Hitachi Global Air Power

Piping leaks usually happen over time due to vibrations, hose and tubing degradation and failed compression fittings. To root out the sources of leaks, most OEMs recommend a leak audit. These audits often find inappropriate uses and loss of compressed air, so the exercise is absolutely worth the effort.

Most air auditors use an ultrasonic leak detector since most leaks are small and are not audible to the human ear, particularly in a noisy production environment.

A best practice is to document each leak when it is identified. Many ultrasonic leak detectors include cameras so the leak can be photographed allowing the user to include a description such as the location in the plant. And, along with documenting the leak information, attaching a two-part tag to the leak makes locating the leak for repair easy for the maintenance team.

Logging the intensity of the leak is also a good practice to follow. The leak intensity can be used to estimate the power cost of the leak and allows the maintenance team to determine if a leak is worth fixing. For example, the packing gland and seat in a valve are found to be leaking, but the leak cannot be repaired without replacing the valve. The cost of the valve, plus the labor associated with the repair, could cause the repair to be more expensive to fix than letting the leak continue.

Image 3: Compressed air system air leak auditing should be an on-going maintenance task. A good practice is to check the entire system annually. Courtesy: Hitachi Global Air Power

Image 3: Compressed air system air leak auditing should be an on-going maintenance task. A good practice is to check the entire system annually. Courtesy: Hitachi Global Air Power

While an air compressor is the largest and most important part of an efficient compressed air system, the piping, valves and fittings that carry the air from the compressor can have a huge impact on the cost to run the system and the operation’s safety.

Utilizing the right parts and connectors and remembering to regularly check these often-overlooked components can mean significant savings and a more optimized compressed air system.

Author Bio: Peter Modrow is Senior Product Manager at Hitachi Global Air Power. Modrow leads product management for the stationary division and brings 25+ years of experience developing and launching new products. He holds a Bachelor of Science degree in Engineering Technology from Texas Tech University in Lubbock, Texas. Modrow resides in Charlotte, North Carolina. Brian Mann is Product Manager, Service Plans & Warranty Products at Hitachi Global Air Power. He has been in the industrial compressed air industry for more than 14 years and is a CAGI Certified Compressed Air System Specialist. Brian holds a Master of Engineering degree in Mechanical Engineering from the University of Louisville and resides in Louisville, Kentucky.