Follow a general pneumatic specification

March 31, 2020
Check out this guideline for some points to include in a pneumatic requirement specification, or your design may suck under pressure

When designing anything, it’s important to follow a general requirements specification. Machine configuration, mechanical design drawings, control systems, schematics and control programs all benefit from careful development and use of specifications.

For some reason a machine system that is often missed, or one that doesn’t get a lot of “press” in specifications is the Pneumatic Requirement Specification (PRS). The scope of this specification starts at the plant main air supply and ends at mechanical motion where the actual work is performed on a device being manufactured. The PRS should cover that range, so a machine that includes pneumatic devices should meet or exceed all these general requirements. The following defines some of what is required, but be sure to add what is needed for a complete PRS.

To start, the machine must be capable of operating at a pressure of 115 psi. This is a pressure easily achieved by most plant main air compressors. This plant air must be connected via a quick-disconnect connection or, better yet, through a manual, lockable shutoff valve, which provides lockout/tagout (LOTO) capability, that exhausts downstream machine air when turned off.

Downstream of the main air shutoff, the plant air must be filtered and regulated to appropriate operating pressure, typically 80 psi. Although the plant air may be filtered and dried before reaching the machine, it must be filtered and moisture removed before entering the machine pneumatic circuits to ensure a long, trouble-free life for the pneumatic components. Depending on the application, a 10-micron down to as small as a 0.01-micron filter may be needed in a clean room, for example.

The regulated and filtered main air supply must also include a two-way solenoid actuated valve to disconnect and vent through a silencer the machine side of the valve to atmosphere when an emergency stop is pressed. This removes motion causing pneumatic hazards similar to how electrical hazards are removed by opening appropriate safety contacts.

A pressure gauge should also be included to clearly provide visual indication of the main supply pressure, and a digital pressure switch input to the controller is best practice. This pressure switch must be monitored in the PLC program to stop the machine and indicate the fault if the pressure drops below an adjustable setpoint.

Pressure gauges and switches are also a best practice for critical process-related functions such as pressing, crimping and forming. In these critical processes, the pressure gauges must be calibrated when installed and re-calibrated yearly, for example, during the machine lifecycle.

For operator safety, any air-venting and exhaust functions must include a muffler. The type of noise suppression depends on plant requirements, but maximum noise levels near a machine and over an extended period are in the 75-to-80-decibel (dB) range. Additionally, these venting functions must not release oil vapor into the plant environment, and some applications, such as clean rooms, may require venting of air outside of the room.

Downstream of proper machine air preparation are the control valve, cylinders, actuators, hoses, fittings and flow controls, among other devices. A common valve type is a three-way, center-exhaust that is used to advance and retract or raise and lower a cylinder. When both sides of the valve are de-energized, the air to the cylinder exhausts, which is a safe state due to the release of motion causing air from the cylinder.

On anything but the smallest, simplest machines, the control valves should be manifold-mounted with a common air supply and exhaust as it simplifies installation and control.

In the past, discrete wiring to a valve bank was used. That control method should be limited to systems with four or fewer actuators. Industrial Ethernet protocols such as EtherNet/IP, Profinet and similar should be specified in the Industrial Internet of Things (IIoT). A single Ethernet cable and power cable can easily control and monitor dozens of solenoid valves in several banks/manifolds. Pneumatic requirements should carefully document these time-saving diagnostics improving valve bank architectures.

A requirement to use different hose colors for the return or home circuit (blue) and the extend or advance circuit (orange) helps a technician to understand the circuit better when troubleshooting. Each hose must also be labeled much like a wire in an electrical schematic. The pneumatic schematic must include hose numbers similar to wire numbers and a unique device designator for each pneumatic device so it can be labeled with an engraved tag on the equipment.

The pneumatic hoses must include flow controls for all motion causing cylinders and actuators. Typically, best practice is to flow control the air out of a cylinder not into a cylinder, but careful design is required. Additional control at the cylinder, in the form of a check valve may be necessary to keep the cylinder from dropping when the air is removed.

Many more requirements are needed to design a pneumatic system, so be sure to define what is required before starting the design that will then detail exactly how it will be done.

About the author: Dave Perkon
About the Author

Dave Perkon | Technical Editor

Dave Perkon is contributing editor for Control Design. He has engineered and managed automation projects for Fortune 500 companies in the medical, automotive, semiconductor, defense and solar industries.

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