Pneumatics are versatile and inexpensive compared to electromechanical solutions and we are often called in to troubleshoot applications that are constrained by cost, time, or complexity. Or, in this case, constrained by explosion-proof requirements and a loss of power. Problem solving is a fluid process, so we'll break down the final solution.
One of our customers requested a system design that used a pneumatic cylinder to extend and retract a conveyor section through a doorway. Normally, the conveyor section would remain extended, but in case of an emergency where the door needed to close, the cylinder would have to retract the conveyor section one final time.
Using a reserve pneumatic source without power
In this emergency situation, the compressed air source might be on. It might have just been turned off because of the emergency, or it might have been off for an extended period of time due to maintenance or a shut-down. In any case, there needed to be a reserve of compressed air available to retract the cylinder and conveyor one more time. The catch was that only pneumatic controls could be used because of the hazardous environment and the prohibitive cost of electrical components that would meet the required explosion-proof safety standards.
We designed a system which used an air tank and two pilot operated check valves, one at the tank inlet and one at the tank outlet. These pilot operated check valves from NGT are "bubble tight" with an extremely low leak rate. The pilot operated check valve on the inlet side of the tank is there to allow air into the tank and also to keep it from escaping from that side of the tank if the air to the system is ever off.
The pilot operated check valve on the outlet side of the tank also keeps air from escaping from that side of the tank. When air is off for an extended period of time, the valves and other components downstream of the tank could potentially leak air and the resulting drop in reserve air pressure might be enough to keep the cylinder from retracting when necessary.
A pilot operated two-position, four-way valve controls the cylinder itself. In its default position, it is keeping air pressure on the cylinder to keep the conveyor extended. There is a second two-position, four-way valve that is activated by the door. This valve taps into the air supply at the outlet of the air tank, so it should always have pressurized air available. It is also a poppet style valve with a very good seal to minimize leaks at this valve. When the door is up, this valve is not passing air and the cylinder will be extended.
When the door drops, it shifts this valve so that pilot air is simultaneously sent to the valve which controls the cylinder and the pilot operated check valve at the outlet port of the air tank to release air there and make it available to move the cylinder.
Throughout the process, the project's scope evolved. There were several different valve types and valve actuators that we could have selected and we settled on this solution to satisfy the application parameters, the customer's schedule, and their budget. The in-house engineers are tremendous conveyor designers, but they look to us for help with pneumatic solutions to unique problems like this and we appreciate the opportunity to work with them. Together, all of their criteria was met while making a reliable and safe retractable conveyor that operated in emergency situations.
Rusty Richardson is a pneumatic account manager at Cross Company Motion Solutions. This article originally appeared on Cross Company's blog. Cross Company Integrated Systems Group is a CFE Media content partner. Edited by Chris Vavra, production editor, CFE Media, cvavra(at)cfemedia.com.
Cross Company is a CSIA member as of 12/15/2016.
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