PID block and out of range process parameter

Hi,

I have a basic query about how PLC handles out of range process parameter.

I have a control valve which is modulated to maintain its downstream pressure at some setpoint, say 30 bar. It receives the process variable from a pressure transmitter located downstream.

If the pressure at the downstream rises above some value, say 34 bar, I want the valve to close to ensure safe condition. Is it possible to implement this requirement in the PLC software? If yes, how?

Thank you.

Compare instruction and a latch?

Hi,

The valve is modulating type and it needs 4-20mA analog signal for operation. In order to close the valve, the current output should be either 4mA or 20mA, depending on the type of valve.

How will the scheme you suggested- comparator + latch, work in the above scenario? Do you mean the latched output can be used to change the value of current? If yes, please explain how.

The other method I'm thinking of is to use a solenoid valve in the instrument air supply line to the valve. When the pressure goes high, the latched output from PLC can be used to energize/ de-energize the solenoid valve and cut-off the air supply, causing the valve to go to its air failure position (which is CLOSE in this case).

The problem with this method is that it requires the use of additional hardware- the solenoid valve and additional wiring. Is there a way to achieve the same result using software?

Do it with logic. No extra hardware. The beauty of PLC's.

If process variable is lower then 34 bars move output from PID instruction to the analog output card. If process variable is equal to or greater then 34 bars move 4ma or 20ma to the analog output card ( the correct raw value for your card).
You may need a small deadband to prevent cycling of your valve.

See rdrast post #2 add (s) to instruction(s).

It may be better to put the PID instruction in manual and manipulate the output. With other programming to have a bumpless transfer.

Your PLC will dictate this.

You can put a high Kp Ki gain to make it close at 34 bar, but it will be very sensitive to any process changes.

Another way is, you can add a comparison logic at the output of the PID:

If PV > 34 bar, then the analog output is overwrite with 0% (you can use a multiplexer to overwrite it).