Loop check is the process of confirming the components of the control loop is working correctly. Loop checking is done only after the completion of cable laying and all installations and before commissioning.
The loop checking includes verifying the transmitter’s process variable (PV) for display to the operator, for use in the control strategy, and for historical trending.
Before loop check starts, all the instruments in the loop should be calibrated, Two groups of technicians should be ready, one at the field and other at the HMI. Alarms and trip actions of PLCs shall be checked at loop check.
Loop Check procedure:
Check the entire loop to ensure all wires and tubing are connected. Check for proper voltages and set instrument air pressure to the recommended manufacturer’s requirements.
According to the specification, check air/electrical supply to all instruments are correct otherwise set, especially air supply to valves and pneumatic actuators.
For electronic loops, check polarities and measure loop impedance in advance, where necessary.
Each transmitter and switch should not be recalibrated during the loop test since all transmitter and switches are installed after finished the workshop calibration.
If a transmitter exists in a loop, the output signals equivalent to 0%, 50% and 100% of the range of the instrument will be generated manually (dummy signals) to verify the function of the loop, such as the variable zero setting of the electronics transmitters or connectors of standard DC currents or nozzles of retention and fins in the pilot of pneumatic transmitters.
Connect a known source to the field transmitter, apply 0%, 50%, 100% signal to the transmitter through it. Check the PLC/DCS controller, HMI system and alarm, trip system for the correct response.
If a local mounted controller exists in a loop, the signal for checking the valve operation to be generated manually using the manual operation device on the controller prior to the automatic operation check. Automatic operation checks to be conducted by changing the controller’s set point. The scan time for different final control elements are shown:
For those control loop, set it to manual mode and check indications for PV signal points; 0%, 25%, 50%, 75%, 100% observe and correct it if needed. Return process variable to 0%, put the controller in the the automatic mode, place P.B. at 100%, reset time and derivative to the minimum setting with set point at 50% and ensure that controller is in direct action. Simulate process variable signal 0%, 25%, 50%, 75% and 100%, observe and record all findings.manually make switch and level switch loops, their contact to be manually make and/or break for ensuring the corresponding instrument’s responses.
For thermocouple loops, verify the shutdown characteristics by disconnecting the cables and confirm that the polarity of the thermocouple extension cables is maintained when they are reconnected. To help identify the thermocouple extension cables, the following guidance is provided:
- Negative extension wires are always coloured as red
- For type K thermocouple, the negative wire is slightly magnetic. It has a lower resistance in ohms/foot for the same size wire.
- For type R thermocouple, the negative wires softer than the positive wire.
For resistor temperature detector (RTD) loops, check the function of the instrument and the control valves in the loop by connecting the resistor approximately 120 ohms at the terminal heads, and visually verify the correct connections when closing the terminal heads.
During loop testing, the following items shall generally be confirmed or checked :
(1) In control room or at the local panel:
-Indication and output signal level on the DCS console or board mounted instruments
-Indication of trend recorders associated with DCS
-Function of flow counters
-Function of annunciators
-Action of controllers of DCS or conventional type instruments
(2) In the Field:
-Action of control valves including associated accessories, such as solenoid valves, limit switches, etc.
-Range of transmitter
-Indication of remote mounted output gauge