Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

Numerous Set-points For One Instrument


This topic has been archived. This means that you cannot reply to this topic.
3 replies to this topic
Share this topic:
| More

#1 blooma

blooma

    Junior Member

  • Members
  • 12 posts

Posted 15 October 2008 - 04:58 AM

Hello all,

I'm working on a control problem. I have a chlorine gas line and I need to control the pressure.
I have one PIS instrument.
my question is: can I use 4 different set-points to the same instrument? (LL,L,H,HH) what should be written in the DATA-SHEET ? (min ,max and oper pressures)

thanks
Blooma

faculty of chem. eng
Technion
Israel

#2 djack77494

djack77494

    Gold Member

  • ChE Plus Subscriber
  • 1,282 posts

Posted 15 October 2008 - 09:09 AM

QUOTE (blooma @ Oct 15 2008, 01:58 AM) <{POST_SNAPBACK}>
I'm working on a control problem. I have a chlorine gas line and I need to control the pressure.
I have one PIS instrument.
my question is: can I use 4 different set-points to the same instrument? (LL,L,H,HH) what should be written in the DATA-SHEET ? (min ,max and oper pressures)


Blooma,
I don't fully understand your question or its context. Most process facilities would have a Distributed Control System (DCS) or, for smaller facilities, a Programmable Logic Controller (PLC) as the "brains" behind control of the facility. Both are complex capable industrial control systems. Field measurements like your chlorine pressure are typically done using a suitable transmitter which converts your process variable (i.e. pressure) into an electronic (or pneumatic) signal. The signal usually goes to a control room and is an input to your DCS (or PLC). The DCS can be programmed to perform various actions depending on the value fo the process variable. The actions can be continuous, in which case we'd tend to call it a "controller", or they can be discrete. I'm not sure of your situation, though your reference to LL,L,H,HH suggest four discrete "events" are to be detected. Probably 4 actions would then be desired based on those events. So, I guess the answer to your question is "yes, one instrument with 4 setpoints can be used". Alternately 4 less expensive and less reliable "switches" could be purchased and installed in the field. Each would have a setpoint and would change its state (i.e. on/off) as your process variable reached the setpoint. I do not know what you mean by a PIS; it's not an abbreviation I've seen before.

#3 blooma

blooma

    Junior Member

  • Members
  • 12 posts

Posted 16 October 2008 - 02:10 AM


Thank you very much, djack
Your answer helped a lot.

By saying PIS - i mean Pressure Indicator Switch.
I think that what i`m missing is the way a controller works or the terminology.
how does it works? does the instrument gets a continues signal and operate the switch when it reaches HH/H/L/LL according to the 4 set-points i want?
because i thought that a set-point is the point that the controller "keeps trying to get back to" - like in control valves when i define a set-point and the valve keeps a constant pressure downstream.
i think i mixed up a few professional terms and i would love a clarification.

blooma

#4 djack77494

djack77494

    Gold Member

  • ChE Plus Subscriber
  • 1,282 posts

Posted 16 October 2008 - 05:32 PM

blooma,
You ask a lot. Traditionally, the field instrument that is in contact with your process is either a transmitter (which provides a continuous measurement) or a switch (which is like an on/off device and can only detect a single value). {Let me correct myself already and say that more than one switch can be combined into a single device.} A switch has a setpoint and will change its status when the setpoint is reached. That status change may be manifest by setting off a visual and/or audible alarm, by initiating other control actions (e.g. automatic shutdown, open a valve, close a valve, start a motor, etc.), initiating a recordable event in a data logging system, etc.

Transmitters send their output to a remote instrument or a DCS or a PLC. Depending on system capability and setup, you can use the transmitter's signal for indication, recording, control, and as inputs to other hardware or software switches. Assuming you have a control function, then the transmitter's signal would be compared against the controller's setpoint to compute an "error function". An output, usually proportional to the error, would be generated in the controller. This output would be sent to a control valve or other device that could act to reduce the error (hence control the process). Much complexity is possible, and it's very difficult to produce a generalized answer to such a broad question.




Similar Topics