Once this hurdle has been cleared, more challenges arise. In this article, we’ll take a look at one of the most common obstacles that companies face during the retrofit process: that of understanding how the existing safety equipment ties into the controls system and identifying which parts must be modified to ensure compliance.
Deciphering the relationship between the safety and controls systems
Sometimes the challenges of dealing with older equipment can make engineers feel like they’re stuck in a real-life version of an adventure thriller movie – think “Indiana Jones and the Temple of the Lost Controls Cabinet” – minus the heroic soundtrack and exotic locales. The safety system’s design may be sound, but the ways in which it relates to the controls system is a riddle that muddles the minds of all who dare to tackle it.
Ultimately, a working safety system is supposed to send a stop signal to any machinery capable of performing hazardous motion in the event that it detects an unsafe condition. The safety control will be monitoring the safety devices, but safety engineers also need to identify locations for monitoring key equipment parameters that may affect operator safety (such as pneumatic or hydraulic pressure on a press) and determine where to send the stop signal.
The stop signal needs to halt all hazardous motion in a way that doesn’t create an additional safety risk or damage the equipment. You can compare this to the stopping options you have when approaching a red light in a car at 30 mph – you could either gently tap the brakes (a safe and sound option), pull the parking brake (which might cause a tailspin), or put the car into “Park” and probably destroy the transmission. If at all possible, you’d choose the first option.
Dealing with patchy documentation and incomplete schematics
With mature equipment, documentation is often unavailable, incomplete or incorrect. Employees change over, records get thrown out, and equipment suppliers go out of business. Over the years, maintenance and engineering teams may have modified a product’s wiring or controls without recording these changes, causing inconsistencies with the documentation.
In these situations, there are things you know you don’t know (known unknowns) as well as things you don’t know that you don’t know (unknown unknowns). To cope with this, you can contact OEMs for documentation, scour the internet, or speak with experienced operators and plant maintenance personnel.
Have a plan and keep things as simple as possible
It’s important to have a plan. Map out your control systems and identify your points for the safety ties before you start installing the safety components. System design and system installation should be two distinct activities. Although this may require an engineer to spend several days with a DVM crawling over the tool, keeping these activities separate will minimize surprises and make the process more efficient in the long run.
You should also keep things simple. The safety control must be “separate and isolated” from the machine control (there is a reason you see yellow boxes hanging off the side of machines). Build it as a separate subassembly, and don’t try to fit the safety solution into an existing controls cabinet. Focus on making the system functional and avoid the temptation to make “improvements” unless absolutely necessary.
Also, know when to cut your losses. At some point, it may be easier and cheaper to install a new control system than work with the existing system if the latter can’t support all the required safety functions. Don’t be afraid to call in outside expertise, since a functional safety specialist may have a workable solution ready.
Finally, be sure to validate and test every safety component on the new solution. The equipment needs to come to a safe, controlled stop in the time specified. Once you’ve verified that everything interacts correctly with the machine control, you can congratulate yourself for ensuring a safe working environment for your team.
originally appeared on Smart Industry Forum on August 21, 2019 athttps://www.smartindustry.com/articles/2019/new-strategies-for-functional-retrofits-of-old-safety-systems/