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P&id Symbols - Is It A Switch Or Controller?


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#1 JamieG

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Posted 07 April 2015 - 07:00 AM

Hello - I have a very basic question that is escaping myself and my colleagues. There seems to be a difference of opinion on how to correctly identify the following on a P&ID:
 

  • A pressure transmitter is installed on a distillation tower.
  • The transmitter is wired to a safety PLC and then to an on/off valve where it will close the valve on high pressure.
  • Sidenote - the PLC also transmits this pressure signal to the DCS via ethernet so as to take advantage of the operators seeing an additional pressure reading.

My question is, does the symbol for the PLC contain the identifier "PC" as in pressure controller or "PSH" as in pressure switch - high?

 

In my mind, a switch is reserved for a sensing element with only discrete measurement capabilities. The switch then triggers a relay which manipulates a final element. In this case, we are using an analog instrument that triggers the final element similar to a switch but we are also using the full range of measurement of the instrument.

 

I can honestly see both ways. Any thoughts?

 

Thanks,

Jamie

 

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#2 Zauberberg

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Posted 07 April 2015 - 08:08 AM

What does the P&ID legend say? We cannot know what these symbols and rhomboids represent.

 

Advantage of using a transmitter over a switch in safety loops, is that you can change the trip set point - which is impossible for switches. Perhaps the "PC" designation is used to emphasize this fact.



#3 JamieG

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Posted 07 April 2015 - 12:37 PM

I apologize. I may not have been very clear with my picture. Please see attached. I will be using a transmitter and not a switch. The argument is that the logic solver will be a PLC and if that should be labeled as a pressure controller or a pressure switch. In my mind, there is no switch present so labeling it as a switch would be incorrect. I could be wrong though.

Thanks again!

 

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#4 curious_cat

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Posted 07 April 2015 - 01:01 PM

Advantage of using a transmitter over a switch in safety loops, is that you can change the trip set point - which is impossible for switches. Perhaps the "PC" designation is used to emphasize this fact.

 

Is there a corresponding downside?



#5 MTumack

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Posted 07 April 2015 - 03:57 PM

 

Advantage of using a transmitter over a switch in safety loops, is that you can change the trip set point - which is impossible for switches. Perhaps the "PC" designation is used to emphasize this fact.

 

Is there a corresponding downside?

 

 

Higher Pricing, more complicated programming based on potential for multiple outputs from Pressure controller, and more complicated trouble shooting (moving from binary system to a more analog system).

 

A good high quality pneumatic pressure switch (no need for electric even) can run you a quarter of the cost of a good high quality electric Pressure controller without even factoring in the higher cost of installation / maintenance.

 

Basically the difference between a single speed bike with no brakes and a 10-speed bike with brakes. Obviously you can do more with the 10-speed, but do you want to pay the extra money? Do you HAVE to spend the extra money? If you need a Ford, is there a point in you buying a Mercedes?



#6 curious_cat

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Posted 08 April 2015 - 05:57 AM

 

 

Advantage of using a transmitter over a switch in safety loops, is that you can change the trip set point - which is impossible for switches. Perhaps the "PC" designation is used to emphasize this fact.

 

Is there a corresponding downside?

 

 

Higher Pricing, more complicated programming based on potential for multiple outputs from Pressure controller, and more complicated trouble shooting (moving from binary system to a more analog system).

 

Nice points. In addition, what I was thinking was that in a safety loop sometimes I might like something simpler. A good high quality pneumatic pressure switch has those upsides. 

 

The ability to change the trip set point may not always be a desirable feature in a safety loop. 

 

I think, these considerations will have to be weighed in individual cases. 



#7 JamieG

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Posted 08 April 2015 - 08:44 AM

While I think that switches have been reliable and used often, they dont necessarily meet the requirements of a Safety Instrumented System. I believe that they lack the online diagnostics that an analoag transmitter can provide, thus increasing their rate of dangerous, undetected failures.

 

This is the reason that I am using a transmitter and not a switch. Although I'm certain that switch manufacturers will step up to the game if they want to sell to ISA 84 compliant companies.






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