Understanding Loop Diagrams and Process Loop Sheets

Because the design engineer has likely not looked at these signals together (i.e., there was no corresponding loop diagram to be subjected to QA/QC), often there will be minor I/O errors in the design. The electrician will need to resolve the errors on-site or issue a request for information (RFI) to the design team to obtain the necessary data. All of this is very time-consuming during the construction phase, and the contractors are under tremendous pressure to get the plant built.

Likewise, suppose an installed valve actuator requires troubleshooting. In an ideal situation, an as-built loop diagram or process loop sheet for the actuator will include all the information the maintenance electrician will need to check the power and signals to/from the actuator. (When a construction project is completed, the design engineer will prepare an updated set of the contract drawings called “as-built” or “for record” that captures the details of what was actually built verses the initial plans.) However, suppose there is no loop diagram for the actuator. The electrician will have to search through PLC panel drawings, other drawings and equipment manuals to determine how the actuator is wired and powered.

Often, if there are no loop diagrams, no drawing will exist that shows how the terminal screws of the valve actuator have been wired and what the terminals and configuration do. Thus, without a loop diagram, a simple 10-minute troubleshooting task can turn into hours of difficult trial-and-error troubleshooting to find the issue.

• Loop name. This is a brief description of the process being monitored or controlled.

• Loop number. This is a unique, four-digit number identifying the instrument loop.

• Item. This is a unique, four-digit alpha identifier for each piece of field equipment in the loop. These identifiers, called instrument codes, are defined in the ISA-5.1 standard for instrumentation. Identifiers for other devices will be typically defined in an end-user-specific standard. It is important to note that there are many of the same types of instruments. That is why each device and piece of equipment should be assigned a unique ID that is only used once at the plant. For example, ISA-5.1 provides a method of doing this using loop numbers, but each plant may have its own internal convention. The important takeaway is that the identifier must be unique.

• Description. This area explains the identifiers in the Item block. This block is valuable to anyone unfamiliar with ISA standard terminology and/or the plant’s instrument/ device tagging system.

• Type. This block provides details about each device. For example, it may identify a flow element (FE) as either a magnetic flow meter, a Venturi flow tube, or an ultrasonic flow meter.

• Criteria. This block contains requirements that are unique to the device or piece of equipment, such as the color of a pilot light lens, calibrated pressure, temperature, low-flow range requirements, elevations of level switches and analysis analyzer ranges. This block or the Type block may also contain information indicating which contractor is responsible for providing the device or piece of equipment. If there are special installation or commissioning requirements that are unique to a particular instrument, these will sometimes be also included on the process loop sheet.

• Tag/service. This block is used to identify the item and what it does in the process. This is useful for an individual who does not understand the process or plant requirements, as well as someone assembling a control panel who must identify each piece of equipment on the panel by the process that is either being controlled or monitored.

• Quantity. This is the number of items.For example, if multiple items of the same type are needed a quantity is listed rather than having multiple row entries. Likewise if similar, but slightly different, items are needed it is best to list them separately in separate rows.