Gate Valve and an E-Stop 1

[elfman]

I have a system that I am using a gate valve to restrict flow leaving a tank - details of what are going on in the tank are not important though it does involve a motor. I have an E-Stop on the control panel that will kill power to the motor(s). However, if I kill power to the gate valve actuator I could, potentially have an overflow condition that the E-Stop should prevent, not make worse. I have been told that E-Stops should always be hard-wired. Does my gate valve require special circumstances where, when an E-Stop signal is received, the PLC will power the contactors to open the gate valve actuator? What are your thoughts or are mine too confusing?

Thanks in advance.

 

[jraef]

I doubt you will get an easy answer because there is no absolute consensus on the issue of E-Stops. But I think you will find it boils down into two base philosophies:

1) E-Stop means every put into a safe state, powered off when appropriate. This is what I subscribe to. Don't just kill power to everything, because of the exact scenario you describe. Some things are BETTER OFF being powered into a safe condition. Think of a Nuclear reactor. If an E-Stop killed power to everything, including the motors for the control rods, the reactor goes critical! In that industry, they use the term SCRAM to denote a rapid shutdown to a safe condition (SCRAM stood for Safety Control Rod Axe Man; based on a guy who had the job of holing a fireman's axe to cut the manila rope holding the control rods if everything else failed). That to me sounds more like your situation.

2) E-Stop means immediately remove all potential sources of energy from everything. The Automotive industry seems to like this a lot but my argument against it, aside from the above, is that it also denotes a false sense of security. I have seen people hit an E-Stop button and then go into electrical equipment assuming it was "safe". The only true method of removing power is a visible blade disconnect device in my opinion (I'm even a little distrustful of circuit breakers).

Bottom line, you know your system best. There is no valid argument in favor of leaving something in an unsafe condition.


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[FreddyNurk]

In addition to jraef's great consensus regarding the operation of E-Stop, its worth considering that the function of the E-Stop, particularly as an electrical circuit, is probably not the be all and end all in terms of safety.

What I mean in terms of this, is that you're looking at the application of the E-Stop in terms of its electrical control, whereas from the mechanical perspective, if you had a gate valve with a spring (i.e. requires power and actuator to close), then you can safely disconnect the E-Stop circuit (i.e. hard wired closed loop circuit, break in the circuit means E-Stop function activated) and get your functionality that way.

I'm aware I'm probably oversimplifying it, and its not possible in all cases (as per jraef's example of the nuclear reactor), but looking at other aspects of how to operate the system may be of benefit. A wholistic HAZOP or risk assessment in terms of the control of the whole system should highlight such things, though it can depend on how rigorously its applied.

Please excuse my ignorance in terms of valves, a spring loaded gate valve may not exist.

 

[elfman]

Thank you jraef! That is exactly what I wanted to understand, but hadn't thought it through well enough. Also, FreddyNurk, your comments are appreciated. And just so you know, the valve we are using will only move if the actuator is either working electrically, or there is a manual hand wheel that can raise it, unfortunately, no spring exists to rely upon.

Thanks again.

 

[xnuke]

jraef,

I agree with your post. I'm really sorry to nitpick about one tiny detail, but it's a misconception I hear so often that I feel the need to correct it whenever possible.

If an E-Stop killed power to everything, including the motors for the control rods, the reactor goes critical!

Nuclear reactors operate critical, and even go supercritical when raising the power level during startup or when any plant transient causes power level to rise. There is nothing dangerous inherently indicated by the words critical and supercritical. An uncontrolled supercritical reactor can be dangerous if the protection system fails to shut it down, and that is cause for worry.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
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[jraef]

Thanks for that. Having never worked in a nuke plant, I humbly accept your semantics correction.

I'll store that tidbit into what my wife calls my "fountain of useless information" so that I can pull it out and dazzle someone at a cocktail party some day.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[controlsdude]

my 2 cents is if you buy a valve and you push estop, have the valve go to the closed position when power is turned off. If this is a safe condition. So in other words buy the valve that is normally closed to flow with no power applied. To me that is fail safe condition. If the safe condition was open then buy it as a normaly open valve.

 

[elfman]

The problem with a normally closed valve is it would swamp/overflow our equipment as the valve is controlling flow from our equipment not flow into our equipment. Also, we are using a gate valve with an electric actuator - basically there is no 'normally' on it when the actuator motor stops turning, the valve remains in that position. I feel like jraef hit the solution on the nose or so to speak.

 

[jraef]

Big spring on the valve, manila rope holding it open, guy with an axe. Problem solved.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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