Safety -- Control Reliable

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Thread Starter

Adrian

Safety Circuits on machinery need to be 'Control Reliable which requires redundancy and monitoring (Self Checking). But how much redundancy is required? I have seen many designs and ideas demonstrating electrical redundancy but not too much regarding pneumatic redundancy. The system I am working on is a small pneumatic press (1400 lbs force, ine inch stroke) controlled by a solenoid valve with the parts loaded/onloaded manually. A light curtain is between the operator and press.

First, is a dual valve (i.e Ross Controls or Herion) needed to control the press? A regular 5/2 solenoid valve does not have redundancy.
What about a pilot operated check valve to prevent the press from moving/drifting down?
Second, I have seen many machines that incorporate a dump valve that exhausts all air pressure when the E-stop is pressed. To make the system 'Control Reliable', shouldn't the dump valve have redundancy built-in or two dump valves being used?

And how does the E-stop safety relay monitor the dump valve for proper operation. Auxillary contactors are monitored using NC contacts.
Please respond with any ideas and comments.

Adrian
 
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Edward Oskam

Be very carefull here! Pneumatics don't need redundancy, but need to be fail-safe! Whenever electric power is disconnected e.q. by em-stop, all dangerous situations need to be solved automatically. i.o.w. everything needs to open! Here is the largest problem, since opening a cylinder may also cause a dangerous situation. When in doubt, make sure no personnel is in the area by closing the area with a gate.
This is a lot cheaper then a pair of operator-hands!
 
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Bob Peterson

This would appear to be basically a punch press. There are special regulations which apply to such things that you should read up on.

I am not overjoyed with using a check valve on a pneumatically operated ram and thinking that is safe. I am not an expert in such things but I would guess that you would want to exhaust both sides of the ram, and lock it in place mechanically. I don't htink a check valve would be adequate.

Bob Peterson
 
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Curt Wuollet

Easy to monitor the dump valve(s), just put a pressure switch (or two) inline to ensure that pressure was released. You now have the world's
most expensive air press. In a case like this, you might want to survey industry practice to find out when enough is enough. Or talk to a factory insurer. High force presses are special, but there has to be some point where you've met your obligation. I'm sure Factory Mutual or OSHA have
some guidelines on the subject. Many I've seen use finger switches with an approved safety control and the rest is normal practice. But, who knows if that's legal. OSHA would be the authority here, I'd bind to that rock. I think you could even get an inspection and blessing if warrented.

Regards

cww

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Richard Dahlgren, BSEE

Adrian,

A small pneumatic press is covered by CFR1910.212. Mechanical power presses are covered by CFR1910.217 (OSHA). The requirements under 217 are considerably more stringent than the requirements under 212.

You do not need a redundant valve, or a monitored valve in the US or Europe, as far as I know. You can look at the OSHA standards at the OSHA website.
 
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