Essentially, cold loop checking refers to the testing of equipment for the purpose of being able to make a precise action though a PLC or ICSS. It’s done without powering up so that loop resistance and continuity checks can be performed to test the wiring integrity.
But there is much more to it than that and this article will do a deeper dive on the fundamentals of Cold Loop Testing and Loop Checking in general. And the best place to begin is in understanding the basics of the Control Loop.
The Control Loop
The control loop contains all the essential pieces to move the control elements. Each control loop has a specific purpose. The controller passes information to the actuator dependent on the feedback signal to regulate the process and is manipulated from a control action.
What is Loop Checking
A loop is a term used to describe the instruments and software that work together to make a measurement or control process variables. During the equipment life, every loop is checked continually to ensure its integrity. The control loop is checked to make sure that it works as designed and that it is monitored.
After installation is complete, the system should work as intended. This means that the transmitter’s process variable is being monitored and verified to ensure accuracy, and is being used in the control strategy along with historical trend analysis.
Cold Loop Checking
The control panel’s terminal block needs to be tested to make sure it works correctly. Field cabling begins from the field instrument and ends at the control panel.
Continuity is tested for each of the branch cables to verify the continuity of the cables between the controllers.
Hot Loop Checking
Once the cold loop checking if fully complete, the loop is connected to a controller and power is applied.
The input of the controller system can be adjusted based on the feedback system. The measured process output is used by the controller to compare against the setpoint and determine how much corrective action is needed. To match the process output, the controller’s output should be adjusted manually.
Each I/O is subjected to hot loop check. During testing, the entire loop functionality is checked by simulation of the signal with power up from DCS or PLC.
The following are tested for –
- Calibration range
- Activation of –
- Inputs
- Outputs
- Field switches
- Solenoids
- System updates –
- PLC
- DCS
- SCADA
Difference Between Hot and Cold Loop Checking
Analog Loop Check
The phrase “covert failure” means that the error can go undetected and thus it’s important to test the 4-20 mA loop’s integrity.
To completely verify linearity, the analog loop check is usually done in 54 mA, 8 mA, 12 mA, 16 mA, and 20 mA and it can also be done in reverse order.
It’s a good idea to check the integrity of the 4-20 mA loop periodically to make sure it doesn’t degrade over time as a result of device replacement.
An analogue loop check verifies multiple aspects of the 4-20 mA current loop</strong >:
- The current signal isn’t constrained.
- The current signal is not biased.
- The control system range matches or mismatches the transmitter range.
- Non-linearity and signals characteristics.
Digital Loop Check
Digital signals can be sent and received without errors and they can be replicated infinitely. Digital communication does not need to be checked to make sure the message is received correctly, unlike the 4-20 mA loop.
However, if the device address is mapped in a wrong way, some readings may be swapped and rejected which is referred to as “Overt Failure”. An overt failure should immediately notify if the signal is sent correctly.
When a transmitter is solely relying on digital communication, the same number is received in the control system end if the transmitter measures 123.45 degrees C. Thus the digital loop check is not required.
