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Avoid Instrument Issues During Revamps

Reusing existing devices can save money but requires a thorough assessment.

By Girish Sathyanarayana, KBR Hydrocarbons

May 09, 2011

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• Check material compatibility of existing instruments against the new service and conditions.
• Use "risk registers" to record and escalate potential risks to project management and the end user.
• Involve all stakeholders when making the decision to reuse or replace existing instruments.
• Identify the requirement to perform PHA and SIL assessments if an existing instrument is replaced with one of a different size, type or operating principle.

Control valves. These need the most attention and time. It's prudent to use sizing software from the manufacturer of the installed valves, to ensure the adequacy check is accurate. Figure 1 shows a decision tree for checking control valves.

Most valve manufacturers keep records of the serial number and valve details in their archives for a considerable length of time. It's important to have adequacy checks carried out or reviewed by the original manufacturer and to carefully analyze its recommendations. Adequacy check reports should record the recommendation for each valve, clearly indicating if the valve is suitable for reuse as is, requires modifications or needs replacement.

The process designer and the end user also should review the adequacy reports and confirm that the process requirements are clearly understood and the decisions made are accurate.

If you're replacing valve internals (trim), the trim material must suit the new process conditions. The method (at vendor's workshop or in-situ) and the timing of the trim replacement need careful planning with the vendor, the owner and the construction team. Inform piping and process disciplines if the new valve has a bigger or a smaller body size valve than the current one.

Existing on-off valves typically are assessed for the actuator sizing for the new shut-off pressure as well as for the revamp pressure and temperature conditions. You must check the mechanical compatibility of the new actuator with the existing valve stem and space if the decision is to retain the current valve body and only replace the actuator.

Flow instruments. Inline devices like vortex, mass, magnetic and other types of flow meters also require careful adequacy checks. Generally for inline instruments, their respective vendors carry out the sizing calculations and provide the recommendation.

Typically, flow instruments are checked for the new flow range, rangability, accuracy, pressure, temperature and pressure drop. The process designer must confirm that the revamp pressure drops across the existing flow meters are acceptable.

Existing orifice plates also should undergo adequacy checks -- although replacing them doesn't cost a fortune. Calculate new pressure drops across each orifice plate with the existing orifice beta value. Unlike transmitters available today, earlier generation ones didn't have wide differential pressure (DP) ranges. Hence, a review of the existing DP transmitters for the new DP range is a must. Recalibrate existing DP transmitters for the new DP, if they can handle the range. Keep flow turndown in mind when making the reuse or replace decisions.

Process designers also may want to know the permanent pressure loss across the existing orifice plates for the revamp case.

Safety relief valves. Adequacy checks of safety valves pose similar complexity as those for control valves. Usually you must review the set pressure, spring range, material compatibility, pressure and temperature. One of the most common issues that crops up is the close gap between the operating pressure and the safety valve set pressure. In cases where the difference is less than 10%, avoid reusing conventional safety or balanced bellows valves because there's a possibility of spurious popping that can cause operational disturbances. Carefully assess such applications and consider alternatives like pilot-operated or air-assisted safety valves, depending on the type of service.

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