Loop checking basic Procedure

what is mean by loop checking?

Loop Checking means, First we want to check the continuity of cable. Then whether  24 V DC or  – 20 mA DC is coming from DCS or JB want to check. That is called loop Checking.
After installation of instruments we must check full loop(instrument-junction box-control room) to ensure that whether the instruments are working properly or not, and whether is it passing the signal or not,whether is it receiving the signal or not.before we start the plant we need to check loop. during the loop check we give some known input and check the corresponding output at other side(control room).if the output is corresponding to input, the loop is ok.if not means, something fault in between the loop. for example.if you want to check ON/OFF control valve,during the loop check,when you give 24 v dc from control room ,the valve must open fully, and when you give v dc the valve must close.

Defining the Loop

Basic to any discussion of control loops is “feedback” control. In this control, the loop starts by measuring the process variable (PV). It then compares the PV to the desired value, that is, the set point (SP), and acts on the difference between SP and PV (error) using a control algorithm (typically PID). The loop then outputs to the final control element. The diagrams below indicate that the main elements of the loop are:
• transmitter/sensor (for measuring the PV)
• process controller (with an operator-entered SP and control algorithm)
• final control element (valve/actuator and accessories)

 

Ensure these steps before loop checking

1. Before carrying out the loop test or the sequence test, all related instruments must be calibrated, this can be seen from the instrument label. The final loop test will be performed to ensure that the instruments installed are functionally correct. Each transmitter and switch must not be re calibrated during the loop test, since all transmitters and switches are installed after the workshop calibration is completed.
2. When performing tests, two groups will be required, one in the field and other in the control room. The contractor is required to provide proper communication for all involved in test verification, i.e. field radios or walkie talkies
3.Associated alarms and trip actions of DCS/PLC and/or board-mounted instruments shall be checked during the loop test. When particular loop ties into DCS/PLC system, a technician may be required in the rack room for troubleshooting
4.When the interlocking inputs and / or outputs are associated with electrical equipment such as pump motors, compressors and valves, this sequence test shall be carried out together with the electrical and mechanical discipline to ensure the protection of the equipment.

Loop Test Procedure

1. Before performing the loop check, check the entire circuit to make sure all cables are connected. Check the appropriate voltages and instrument air pressure according to the manufacturer’s recommended requirements.
2. To test the circuit, configure the air and / or electrical supply, as applicable, the air supply pressure for the control valves and other pneumatic actuators must be verified and configured correctly according to the specifications.
3. For electronic loops, check the polarities and measure the impedance of the loop before it is necessary. Before performing the loop test, the DCS must be de-energized and confirm that all field cables are connected correctly. Then the DCS.
4 must be energized again. The final circuit test must be performed to ensure that the installed instrumentation is functionally correct. Each transmitter and switch must not be recalibrated during the loop test, since all transmitters and switches are installed after workshop calibration

5. 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 in manual mode (dummysignals) to verify the function of the loop, such as variable zero variation of the transmitters or by connecting the standard DC current generator or the retaining nozzle and the fin in the pilot of the pneumatic transducers.

6. Connect a known precision source to the primary field transmitter. The assimilation of the measured variable is applied to the transmitter for reading at zero, half scale and full scale, according to the instructions recommended by the manufacturer. Note in the PLC system that the indication is shown as simulated. Also note on the MMI screens (ManMachine Interface) on the PC. You must indicate the appropriate actions in Screen and Alarm Screens

7. If there is a local controller mounted in a loop, the signal to verify the operation of the valve will be generated manually using the manual operation device in the controller before the automatic operation check. An automatic operation check will be performed by changing the setpoint of the controller.

8. Control loop: Place the controller in manual mode and simulate the signal of the process variable from 0%, 25%, 50%, 75% and 100% of the scale and note on the monitor that the process variable indication shows signals assimilated. Return the process variable to 0%, place the controller in automatic mode, set P.B. 100%, restart the time and derived to Minimum adjustment with 50% set point and make sure the controller is an indirect action. Simulate the process variable signal 0%, 25%, 50%, 75% and 100%, observe and record all the findings

9. For pressure switches and level interrupters, their contact must be made manually and / or interrupted to guarantee the corresponding responses of the instrument.

For thermocouple loops, check the burn characteristics by disconnecting the cables, and confirm that the polarity of the thermocouple extension leads is maintained when they are reconnected. To assist in the identification of the thermocouple extension cables, the following guidance is provided.

(1) The negative extension cables are always colored in red as in the PER ANSI C96.1.
(2) For the K-type thermocouple (Chromel / alumel), the negative cable is slightly magnetic. It has a lower resistance in ohms / feet for the same size cable.

(3) For R type thermocouple (copper alloy / nickel copper), the negative wires are softer than the positive wire.11.For the loops of the resistance temperature detector (RTD), check the function of all the instruments and control valves in the circuit by connecting the resistor approximately 120 ohm in the terminal heads, and visually verify the correct connections when closing the terminal heads.

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