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msalaheldin
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Engineering Fields - Control Engineering - Automation Engineer Egypt - Member - Mohamed Salaheldin

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Solenoid Connections Standard

03/17/2008 2:49 AM

Is there any standard that specify whether to connect a solenoid directly to the input signal or to connect the solenoid to the main power and have an open contact from DCS to energize the solenoid. Hereunder, I will describe the situation: There is an O2 analyzer to measure max limit of O2 in the sample about 0.3 ppm. In some condition the process uses O2 to kill a certain reaction so that the O2 suddenly increases in the sample to a very high limit in mole percent. to avoid this , I need to put two solenoids one to close the way of the sample line and the other to open a way of Nitrogen ( to avoid the vaccum in the O2 measuring cell). I have a suggestion to connect the solenoids to 220Vac while put a normally open contact in the Line. However, I prefer to connect the solenoids directly to DCS ( digital output) without need for power cable. Please advise.
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PWSlack
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#1

Re: Solenoid Connections Standard

03/17/2008 5:35 AM

So, the analog signal from the analyser goes into some form of comparator, and when the signal goes above a certain level a switch changes position to power a solenoid valve that alters the supply of O2 to quench the reaction? A trip-amplifier will do the job.

http://www.manufacturingtalk.com/news/sro/sro100.html

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Vulcan
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#2
In reply to #1

Re: Solenoid Connections Standard

03/17/2008 6:20 AM

I think he's asking about this:

He wants to know if he should use one or the other.

I'm not aware of a standard but I've seen installations that use both types of connections.

I think that if the DCS output can handle the current load of the solenoid, it should be okay to connect it directly.

Depending on the type of output, you might have to put a diode across the solenoid coil to kill any transients that will be generated when you switch the coil on and off.

If your system uses two, different types of power, such as in the first case, you may have to use a relay to drive the coil. Further, if all the other outputs already use relays, I'd advise that you stick with the existing practice. Nothing confuses people more than an installation that employed multiple techniques or methods.

Speaking from experience...

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PWSlack
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#3
In reply to #2

Re: Solenoid Connections Standard

03/17/2008 7:17 AM

Quite.

Any standards will therefore be local to the poster, and not global.

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caramba
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#4
In reply to #2

Re: Solenoid Connections Standard

03/17/2008 9:46 AM

Great draft, Vulcan!

As for me I used to apply first variant even if DCS output can't handle the coil load (I add a transistor this case). Of course diode is needed to be added for prolonging contacts' life. I prefer first variant as it protects from casaulties more sensetive part.
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msalaheldin
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#6
In reply to #4

Re: Solenoid Connections Standard

03/18/2008 2:24 AM

Dear Caranba,

Please clarify more the first phrase " I used to apply first variant even if DCS output can't handle the coil load ".
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caramba
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#8
In reply to #6

Re: Solenoid Connections Standard

03/18/2008 2:56 PM

Dear msalaheldin. I would ask to accept my apology.

Therewas just my comment in reply to Vulcan and I'd marked it myself as off-topic. But ok, I try to clarify.

If DCS output hadn't been designed to handle coil current loading itself, that is it would be enough to put between Relay and DCS output transistor as amplifier. And think that work done. Yes? But I prefer to put the transistor then interim relay and then one more relay which switches terminal load. I do find this approach more safer for DCS/PLC maintenance. It's just my preference only. I do not pretend to recommend it for everyone.

Regards,

CaraMba.
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caramba
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#9
In reply to #6

Re: Solenoid Connections Standard

03/18/2008 3:10 PM

One more thing to add. I'm prefering first variant shown on Vulcan's draft as it allows to separate distinctly control and power circuits therefore it makes operation of maintenance personnel more comfortable and safe.

Regards.
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design-engineer
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#10
In reply to #6

Re: Solenoid Connections Standard

03/19/2008 5:29 AM

Back EMF will be generated when the coil is de-energised and this could damage the DCS. Diodes are installed across the coil to prevent this

Deseng

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Solrac
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#5
In reply to #2

Re: Solenoid Connections Standard

03/18/2008 12:04 AM

Totally agree with putting a diode, specially for some DCS systems (some brands have built in supressors, others do not).!
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NestorLeal
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#7

Re: Solenoid Connections Standard

03/18/2008 11:02 AM

First, you must find out whether a low voltage solenoid (24 VDC) can handle the delta-Ps across the valves and drive the valve actuator open or closed.

If so, I would prefer to use the low voltage solenoids driven by a 24 VDC DCS output, like the right-hand drawing in Vulcan's post.

If not, then you will need a high voltage solenoid (220VAC), and an interposing relay to drive the solenoids, like the left hand drawing in Vulcan's post .

Even in the first case, it might be convenient to use an interposing relay, so you can use the same unique DCS output to drive both solenoids. Use the NO and NC contacts from the relay to drive the O2 and the N2 solenoids, so that when one solenoid is energized, the other is de energized, and viceversa. This is a design decision that will depend on details of your system: Do you have extra DCS outputs available?, are you already using relays?, are your standard solenoids 24 VDC or 220 VAC?, etc.

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caramba (3); design-engineer (1); msalaheldin (1); NestorLeal (1); PWSlack (2); Solrac (1); Vulcan (1)

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