Loop Checking and Field Instrument Testing Procedure

Everybody knows that loop checking and field instrument testing is the last piece of the puzzle in each project before commencing the commissioning of any system. It is time-consuming and depends upon the completion of other systems like piping, electrical equipment, control valves etc.

Each contractor or company has its own procedure and common practices for instrument loop checking and functional testing. I found this forum is good place to discuss this topic in more detail, for example:

  1. Can we have a common or guide line procedure for loop checking?
  2. Can we illustrate all the required drawings, specification and forms required for loop checking?
  3. Can we build a flow chart for loop checking which illustrates the rule of operation, maintenance, engineering and inspection for example?

What are your comments?

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  • This is a very good topic for discussion.

    We do the following in our organisation

    1.A detailed loop check format is prepared for each project 2.Above is based on latest I/O list , P&ID and process narrative. 3.For each I/O point, all connected details/elements like PLC/DCS I/O module,Panel Terminal Block, Marshalling panel Terminal Block, Junction Box Terminal Block,Instrument/Final Control element tag, Service description,calibration range,cable tag.... 4.We do this testing by involving owner's as well as prime EPC bidder's Engineer as witnesses. After successful completion, the document gets signed by them.

    We follow the same procedure for commissioning the logic as well as to demonstrate the same to end user!

    This is time consuming and we do it religiously to eliminate last minute surprises.


  • I fully agree with all 3 points. A guide is really needed. I takes much time to get agreed such procedure in every new project and a unified approach would be very effective here. To the second point I may add that a list of such documents would be very useful as well to avoid creating of a huge dossier and got all needed papers in place. Templates for certificates would be fine. As to the third point I'd a recommended list of personal of performing and witnessing parties. Mikhail Aravin, Senoir Instrument Commissioning Engineer.


  • The loop testing procedure can vary depending on the instrumentation and I/O infrastructure. A foundation fieldbus or Profibus PA based system would be different from a system using conventional or HART instrumentation on a system with HART I/O cards.

    While one can perform a loop test with a handheld communicator when HART I/O is not present on the automation system that process would require multiple people similar to performing loop tests with conventional instruments. One benefit HART instrumentation has over conventional is the non intrusive nature of the testing. One does not need to break the loop to insert a mA simulator. HART instruments have a Loop Test method that allows the tester to drive the mA output of the instrument from a hand held communicator or an Asset Management Software application, When using Asset Management software, testing can be done more efficiently because a single person can be commanding devices from one window and observing results on another. An additional benefit when using Asset Management software is that multiple instruments can be put into loop test simutaneously which allows one to incorporate interlock logic checks along with the loop tests which further streamlines the comissioning process.

    Since loop tests are often the final step before start-up, and projects schedules often slip, there is always pressure on the loop testing team to gain efficiencies to pull in the plant startup milestone.


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