Diagnose faults in process control systems from 4 to 20 mA without opening the loop

Field test control design

Anyone who works in a manufacturing plant knows that there's nothing more devastating than an unplanned shutdown. Lost production time means lower yields, which can lead to huge price differences. In a process plant, a breakpoint at a critical point in the process can lead to the scrapping of an entire batch of product, further amplifying the loss of productivity.

The pliers section of the Fluke 773 is detachable and is connected to the body of the test tool via an external cable.

Keeping your process up and running requires efficient and accurate troubleshooting tools. As instrumentation technicians who have worked in oil, gas and chemical plants for the past 15 years, this is all too clear. He was responsible for the maintenance and troubleshooting of pressure, flow, temperature and pH transmitter processes in a chemical plant. As you know, powering off your device is one of the last resorts.

He's been using Fluke tools since his first foray into the field. "When you work in process control, you need to have equipment you can trust; None of the instrumentation technicians I know choose to use Fluke tools in all of their test equipment. ”

When he saw the Fluke 773 milliampere process clamp meter at a trade show in early 2011, he saw a huge possibility of using it in his factory. The 773 measures milliampere (mA) signals without breaking the loop, making it an ideal tool for troubleshooting transmitters, valves, and programmable logic controllers (PLCs) commonly found in process plants. The meter can also output and measure DC voltage, so it can be used to diagnose voltage input and output devices. The jaw head section of the meter is detachable and can be used as a remote jaw that connects to the main unit via an extension cable, making it ideal for measurements in tight spaces.

Uncover intermittent issues without disconnecting loops

"I've found that the biggest benefit of the 773 is that it can diagnose the failure of a live unit without having to lose power, without missing any problems in the process. If you're dealing with a chemical process, you'll have very quick and intermittent problems that can eventually lead to major accidents. I can take the 773 next to the process instrument, connect it, look at the output, and compare it to the instructions. With the mA simulation function, it prevents the transmitter from becoming a point of failure in the loop, and the clamp meter itself provides 24V loop power, making it suitable for performing alternative and isolation tests. ”

Before using the 773, he had to go to the power supply, power off the circuit, go back to the instrument, connect the meter in series, and then power the circuit back on for calibration checks. "I don't have to shut down the instruments every time, and I save three or four minutes per instrument, which adds up to a lot because every hour of downtime in the process costs thousands of dollars."

Not limited to stone core testing

The most common scenario is to use 773 for pH transmitters. "Our process was very difficult with pH measurement equipment, so I had to calibrate the pH transmitter on site. With the 773, I never have to turn off the instrument to test pH probes and transmitters. ”

When an operator contacts him with a pH instrument, he asks the operator to take a sample from the process and measure the pH using a core test strip. "The core test strips don't lie, but they aren't accurate enough. So, if they tell me that the stone core test shows a pH error, I'll go over and troubleshoot the pH probe and the instrument. ”

The process environment is very harsh and can damage the instrument. According to his estimates, probably only 20% of the time the problem is related to the process and not to the instrument. "Using the 773 helps me eliminate process issues quickly, so I don't have to remove and replace parts unnecessarily."

He carries with him a pH-range sample paper that indicates the correct mA value for the product in the process. He attaches the clamp meter to the transmitter and places the pH probe into the standard buffer to verify that it is working properly. "If the milliamp readings don't match, I know there's something wrong with the probe." If the probe is damaged, simply replace it and use the 773 for quick calibration. "Since it's a fault diagnosis, I can use the clamp meter for on-site calibration. In addition to using the milliamp reading function, I sometimes also use the milliamp output function to send a signal back to the control system to make sure there are no problems. ”

A 2-in-1 is far better than a single function

He found the voltage readout feature on the 773 to be particularly useful. "In the measuring instrument, the voltage of all the devices is 24V, so you have to make sure that the voltage of the loop is correct. If the same instrument can measure both voltage and current, troubleshooting can be very quick to determine if there is a power supply problem with the instrument. ”

For example, if a cable is scratched during installation, it can cause intermittent power issues. "In this case, it would be very useful if the instrument that measures the current can also measure the voltage. You can first read the voltage at the instrument to determine if the power supply is correct or consistent. If the power supply is good, you know that the circuit is in good condition. ”

Isolate the process from the instrument

The 773 is also very useful for reinstalling the instrument after maintenance. Not long ago, his team maintained the entire process, removing all the instruments and calibrating them with the Fluke 744 Documented Process Verifier. They reassembled some of the valves and then put all the units into service. Soon after, plant operations noticed a difference in flow rates – slightly on the lower side. In addition, at a particular set point, the control valve did not open.

"With the 773 clamp meter, we quickly proved to our operations manager that our process instruments were working as well as they were before maintenance. We calibrated it with the 744 and proved that the instrument was good when it was in the instrument room; After reinstallation, we were able to demonstrate that the milliampere output was consistent with the display on the control system. Because we don't have to shut down anything, we can show it 6 or 7 parameters in the loop very quickly.

In the end, it turned out that there was a process problem, and they quickly found and fixed the problem. ”

Diagnose faults in process control systems from 4 to 20 mA without opening the loop

Diagnose faults in process control systems from 4 to 20 mA without opening the loop

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