Monitoring dump valves (pneumatic safety)

Steve Kemp

Member
Join Date
Nov 2003
Posts
122
Any thoughts on this one? I'm designing the controls for an operator-loaded, light-guard protected hydro-pneumatic press (pneum approach stroke/intensified press stroke). It's a category 4 machine, so I'm using two pneumatically series dump valves, fed via the contacts of two series-connected positively guided relays, the normally-closed contacts of which form part of the monitor reset circuit. So far, so normal. I want to monitor each of the dump valves' outputs with a pressure switch. These switches come with a changeover microswitch as standard, so I figured on using the changeover aspect to "cross-monitor" their status and incorporate this as part of the monitor reset circuit. But I figured wrong: a single c/o isn't enough, but if I use "regular" relays to expand their function, these are in themselves a source of potantial failure. I know it's only a couple of mA and relays (the ones I know) go on for ever, but it's a safety thing. All opinions welcomed. Thanks, Steve.
 
How will you detect when the second dump valve fails? The pressure will be released by the first dump valve.
You may want to monitor the spool position of the dump valves
 
We had a very large hydraulic system for pasta extrusion, where we had cross monitoring ensuring pressure was equal across certain positions. Any pressure difference would initiate a dead stop.

The pressure difference was measured by a single special pressure switch, long time ago so can't remember details, I think it was square d, would have thought there would be modern equivalents.
 
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