Emergency Stop Pushbutton Design

I think he already bought the E-stop button, he just needs to know whether to hard-wire it or use software to control it. Hardwiring would give you the immediate stop you need, while using PLC Logic will not provide as fast a response, thus causing safety-related issues.
 
A hardwired "E-stop" still needs to tell PLC that it has been activated so the program can "park" the system at the safe place to be examined by the operator before a "safe to restart" command gets issued. The "safe-to-start" should never be the removal of the E-stop button activation.
 
Years back I worked in an automotive stamping plant, and all the presses had a corded pushbutton the operator held and it stopped the press cycle.

The second shift maintenance supervisor kept calling it a 'remote e-stop' and called it that when an OSHA inspector was there. Talk about problems.

To appease OSHA I had to write up him and give him a written instruction that it was a 'remote cycle stop'
Per the OSHA inspector an e-stop must shut off all power, motors, stop the flywheel within a reasonable time and stop all potential energy sources.
 
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The op mentioned that certain estop need to be done on a chemical process. What are those actions? In a process, unlike a machine, some things may need to be started, opened, etc during an estop. Just killing all power may not be correct in this situation. What things are needing to happen in the case of an estop on your chemical process?
 
The op mentioned that certain estop need to be done on a chemical process. What are those actions? In a process, unlike a machine, some things may need to be started, opened, etc during an estop. Just killing all power may not be correct in this situation. What things are needing to happen in the case of an estop on your chemical process?

By definition, an "Emergency Stop" is designated as a button that untrained people should use in the event of a life-threatening situation.

You have to stop moving objects as best you can.
You have to stops pumps in case chemicals, gasses, hot fluids etc. are spraying out of leaks.
You have to remove electrical power in case anyone is being electrocuted.

You do not have to think about what is the correct button to press, as an untrained person may not know the correct action to take.

Any button that puts the control system into a special "safe shut-down mode", is not an "Emergency Stop".

wikipedia said:
A kill switch, also known as an emergency stop (e-stop) and as an emergency power off (EPO), is a safety mechanism used to shut off machinery in an emergency, when it cannot be shut down in the usual manner. Unlike a normal shut-down switch or shut-down procedure, which shuts down all systems in order and turns off the machine without damage, a kill switch is designed and configured to abort the operation as quickly as possible (even if it damages the equipment) and to be operated simply and quickly (so that even a panicked operator with impaired executive functions or a by-stander can activate it). Kill switches are usually designed to be noticeable, even to an untrained operator or a bystander.
 
What about say a chemical process that is exothermic and requires cooling water pumps to keep temperature of a vessel from going out of control? I'm not being difficult, series question. In that case, would you still say have to shutoff cooling pumps?
 
What about say a chemical process that is exothermic and requires cooling water pumps to keep temperature of a vessel from going out of control? I'm not being difficult, series question. In that case, would you still say have to shutoff cooling pumps?

I think you miss the point.... an EMERGENCY STOP has to be uncontrolled, you have to remove motive power, electrical power, etc., as quickly as possible.

If the process requires a "controlled safety procedure", that is not an "emergency stop".

I understand where you are coming from, but if the chilled liquor, or even worse Glycol, pipework had burst, that is a definite "emergency", and no attempt at cooling the vessel can be made. In situations you are dreaming up, an emergency stop should kill everything and it is likely that an evacuation of the area would be on the cards. The designers of the system should have taken all this into account, call it "risk assessment" if you like.
 
An emergency stop is to protect the people - without any regard for the product and the equipment.

An E Stop is to put the system to a safe state. It is intended to be used even by the uninformed/untrained, yes, but that doesn't mean that it can't be designed to do complex/intelligent things.

For MACHINE safety, safe state usually means stop as fast as possible then cut power. Sometimes, though, you need a coast stop rather than a ramp stop.

For PROCESS safety, there are very different standards. Quite often the uncontrolled process is a larger hazard than whatever local thing they want to estop. You may need to prevent the building from exploding to protect the individuals in it. Often times this means there is redundancy to the control system, but sometimes it means a valve to either dump the contents of a tank or prevent anything else from flowing.
 
An E Stop is to put the system to a safe state. It is intended to be used even by the uninformed/untrained, yes, but that doesn't mean that it can't be designed to do complex/intelligent things.

For MACHINE safety, safe state usually means stop as fast as possible then cut power. Sometimes, though, you need a coast stop rather than a ramp stop.

For PROCESS safety, there are very different standards. Quite often the uncontrolled process is a larger hazard than whatever local thing they want to estop. You may need to prevent the building from exploding to protect the individuals in it. Often times this means there is redundancy to the control system, but sometimes it means a valve to either dump the contents of a tank or prevent anything else from flowing.

And what if the equipment needed to effect your version of an Emergency Stop is also compromised by the original need for the E-Stop. Now you need another E-Stop button to E-Stop the E-Stop sequence.

Read that wiki definition again, and be totally clear that what you are describing is not an "Emergency Stop".

You are describing a "Safe Stop", or "Process Stop", or "Shutdown Stop", or call it whatever you like, but don't call it an "Emergency Stop" if it doesn't stop everything and cut power, nor should it be the standard Red (Yellow Background) button intended as an Emergency Stop, or "Kill Switch".

Here's some further reading ....

A Rockwell White Paper
Wikipedia article
Machinery Safety
What designers need to know

... and thousands more.
 
Emergency stops can't be done on the screen. Have to be real E-stop buttons (not any kind of button). And it has to be hardwired but not just to regular inputs to the PLC. Either a dedicated safety relay or as safety inputs on a safety PLC or PLC with safety functions.

In the chemical industry it's common to have a safety system that is setup to avoid mixing the wrong chemicals and process related safety like that. It's a little bit different than the safety on machines and moving things.

The criteria for determining if you can use a regular stop function on a HMI or if it has to be an emergency stop has to do with safety. If there are no safety concerns (for humans) if the stop works or not when it is pushed, then it can be done as a stop button on a HMI. Can't be called emergency stop though but you can call it stop, quick stop, machine stop or something like that.
 
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I think he already bought the E-stop button, he just needs to know whether to hard-wire it or use software to control it. Hardwiring would give you the immediate stop you need, while using PLC Logic will not provide as fast a response, thus causing safety-related issues.

Response time of a PLC v a Safety PLC are not really any different. They are different, but not in response times.
 

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