What is loop check and How to perform it in Analog loops?

What is loop check?

Loop checking is defined as a pre-commissioning operation that starts after the equipment is installed on-site and loop checking of equipment is performed between the installed field equipment and its associated control systems, such as PLC or DCS.

Various methods to do loop check in an Analog Input loop

A 4-20 mA/HART transmitter current loop can be loop checked in two ways:

  • Inject physical sensor input
  • Force output current

Inject Physical Sensor Input

Apply a real physical input to the 4-20mA/HART transmitter. For example, applying physical pressure to a pressure transmitter or inputting a simulated temperature sensor signal from a calibrator into a temperature transmitter or placing a temperature sensor in a temperature bath.

The engineering unit reading at the control system end is checked in five points: 0 percent, 25 percent, 50 percent, 75 percent, and 100 percent in each phase of the test. Because the sensor must be detached or separated and the signal injection must be connected, this takes a long time.

Need one person in the injecting field to calibrate the input and one person in the control room for verifying the reading shown in the control system screen. To ensure reliable NAMUR NE43 failure alarm indication in the control system, simulate a sensor failure by disconnecting the sensor cable (not the transmitter wires).

The advantage of this loop check method is that the transmitter works, the transmitter signal is routed to the correct channel on the input board and the 4-20 mA range setting of the transmitter matches the control system range setting, which is very important.

As an extra check, a multi-meter can be introduced into the loop.

This is the sole option if the 4-20 mA transmitter or system does not support HART.

Force Output Current

Another way to check Analog input loop is by forcing the transmitter output current. For this method, we need to force 4-20mA/HART transmitter’s output current irrespective of applied input. This technique is called the loop test.

A field person must use a handheld field communicator to first increase the current to 4 mA and then to 8 mA, 12 mA, 16 mA, and 20 mA during each test step.

It may be feasible to force the output current from the local operator interface without utilizing a handheld field communicator if the 4-20 mA/HART transmitter has a local display. It is checked that the transmitter signal is marshalled to the correct input card channel by loop checking from the field.

If the plant’s Asset Management System (AMS) includes Intelligent Device Management (IDM) software, the technician can force the output from an inside workstation instead of going to the field. A second technician double-checks the control system’s reading.

Note that the IDM software cannot verify the reading since it looks at the reading from the transmitter end, not the 4-20 mA current received in the controller, which is what matters in a current loop check.

When checking for loops in the IDM software, the transmitter signal destined for the wrong input channel on the card may not be detected. Therefore, it is necessary to separately check the correct type of device.

Various methods to do loop check in an Analog Output loop

Control Valves Loop Checking (4-20 mA/HART)

A 4-20 mA/HART-based control valve positioner current loop can be loop-checked in three ways. They are

  • Observe Valve Movement
  • Read position feedback
  • Read-back loop current

Observe Valve Movement

A second person in the field monitors the actual valve movement while the operator at the control system switches the control loop to manual and sets the control system output to 0%, 25%, 50%, 75%, and 100% for each stage of the test.

This loop check method has the advantage of ensuring that the valve moves freely over its entire range of motion. As an extra check, a multi-meter can be introduced into the loop. This is the sole option if the 4-20 mA positioner or system does not support HART.

Read Position Feedback

The operator at the control system sets the control loop to manual mode, first sets the control system output to 0%, then sets it to 25%, 50%, 75% and 100% for each test step, and a second person runs the IDM software. Use to read the position feedback valve from the valve positioner.

The advantage of this periodic testing method is that it does not require a person to go to the site.

Read-back Loop Current

For each phase of the test, the control system operator places the control loop in manual mode and sets the control system output to 0%, 25%, 50%, 75%, and 100%. Meanwhile, a second person uses the IDM software to read the loop current as received by the 4-20 mA/HART valve positioner.

This loop check method also does not necessitate a field visit. There is no sign, however, that the valve is truly moving.

In all the above methods, the control loop is placed in manual mode and the control output is first set to 0% at each test step and then to 25%, 50%, 75%, and 100%. The difference is how the equipment technician verifies that the correct signal is being received from the 4-20 mA/HART valve locator.

The fourth method is to measure the current in the field with a multi-meter rather than a 4-20mA / HART valve positioner to verify if it matches the transmission from the control side. However, the multimeter’s voltage drop is much lower than that of a 4-20mA/HART valve positioner, so the results may be erroneous.

Also Read: Loop checking & Procedure

Once this loop check is done, the technician enters the values observed during the loop check in loop test checklist.

Why do we need to do loop checks?

The loop check is used to find and locate various errors and deficiencies in the various instruments such as filed, controller, HMI interfaces, etc. We can also find the issues in wiring, cabling, engineering issues, and mechanical issues.

Loop check can only locate and identify errors, but loop check can’t correct the errors or any deficiencies found. They inform all the details to the concerned authority about the deficiencies to correct.

Basic troubleshooting steps involved while doing loop check

Various troubleshooting steps are

  • Tracking control loop problems
  • Loop Malfunctions
  • Field Checks & Plant Commissioning
  • Verification of DCS and PLC Cards
  • Examine a Valve Positioner
  • Loop Isolators and VFD check

1. Tracking control loop problems

The operator may report a bad valve or a loop that isn’t responding as it should be the first sign of a control loop problem. Any of these occurrences should prompt the technician to start troubleshooting. The first step is to verify the loop current value by measuring the 4-20 mA signal.

Broken, disconnected or shorted wires; a bad loop power supply; or faulty instrumentation are all possibilities when the loop current measured is not as expected. Check the loop power supply if there is no problem with the wires. If there is no output from that supply, a meter with a 24-V loop power function can be used to fill in the gap. If the loop functions properly, the power supply is clearly the source of the problem.

Once both the wiring and the power supply have been checked, it’s time to test the generator. Use mA simulation on a loop calibrator, process calibrator, or Multimeter clamp to replace the transmitter. If the loop works as it should, the problem is with the transmitter.

Finally, if a final control element (i.e., valve positioner) is suspected, the mA source/simulation mode can send a signal to the element and provide feedback.

2. Loop Malfunctions

An incorrect loop could be a faulty I/O (I/O) card in the PLC, a Distributed Process Control System (DCS), or a bad final control (eg I/P in the valve positioner). It is usually best to start with field tests of the transmitter, local or remote indicator, or final control.

Measure the loop current using a clamp meter as an end control and compare the value to a local position indicator on a valve or other end control. Send this information to the operator to check the result.

For the loop current, use a clamp meter to measure the loop current, then ask the operator how well the value displayed on the control panel matches the actual loop current. This allows you to quickly determine which PLC or DCS I/O board is handling that particular cycle. You can also use the mA source/simulate mode to send a known signal to the control room and compare the operator reading to the actual loop current.

Some loops show random fluctuations or intermittent errors that need to be tracked over time. You can use the scaled mA output clamp meter to measure the loop current without breaking the circuit and produce an equal and isolated mA output. This output can be fed to a Digital Multimeter with a logging function that allows recording over time.

3. Field Checks & Plant Commissioning

A loop current meter is used to check each and every loop for current in a matter of seconds while doing either field check or plant commissioning. While doing this, no need to disconnect any devices in the plant. A multi-function clamp meter is designed to find and rectify the error if the loop is not working properly. Sometimes we need to use the classic troubleshooting technique when the power supply is not available on some loops.

Classic troubleshooting consists of wire checking, supply, I/O cards in the control system. In this method, we use a special type meter to pass the signal to the I/Os and connect the operator or concerned authority to check the result. If the operator agrees to what is being sent, there may be a problem with the transmitter itself. There may be a wiring problem with the transmitter itself or with a new installation.

4. Verification of DCS and PLC Cards

We can use an mA clamp meter to examine the PLCs and DCSs I/O board operation by using faultless signal sources. Disconnect the process loop and use the meter’s mA source mode to apply the known signal value and compare it to the value shown on the operator display for the 4-20mA input card. Voltage input cards can be tested in a similar way using the Meter’s voltage source function.

5. Examine a Valve Positioner

Operators could use a milliamp clamp meter for the electronic valve positioner. It is referred to as a preventive maintenance program. This method is done periodically. It is relatively easy to perform a quick operation check using the meter as a signal source while observing the valve stem position, mechanical position indicator, or flow indicator when the input is changed according to the manufacturer’s instructions.

6. Loop Isolators and VFD check

Apply an mA input signal to a loop isolator and use the clamp-on current measuring function to measure the device’s 4-20 mA output. For valves that report their position using 4-20 mA, a two-channel simultaneous source/measure function can be used. An mA process clamp meter can also be used to feed a signal into a variable frequency drive (VFD) to simulate a normal input while the technician watches.

Because they can replace a number of different instruments and perform a variety of functions, today’s mA process clamp meters can save instrumentation and automation technicians a lot of time. Check the specifications and feature/function sets of a multi-function tool before purchasing it to ensure that it will meet all of your troubleshooting requirements.

Advantages of doing loop check

  • Can find and locate the various errors in a plant
  • Reduce maintenance and shut downtime.