The first part of this article appeared in the December 2005 issue, page 36, and discussed loop assessment strategies and the automation hierarchy.
An effective predictive maintenance policy uses process diagnostics and performance analysis tools to focus maintenance resources on problematic loops and components that offer the greatest return on investment. Surveys have shown that adopting an effective loop maintenance policy provides considerable improvement in plant operations. One multi-industry loop audit survey reported that only 32% of loops have acceptable or excellent performance.
Many industries now are adopting some form of condition based maintenance (CBM) as a predictive maintenance policy, according to another survey. The objective of predictive maintenance is twofold. First, it aims to address under-performing and problematic loops before they become intolerably faulty, when chances for costly unscheduled shutdowns are high. The second objective is to improve the cost effectiveness of the maintenance effort, which necessitates prioritizing loop maintenance.
Prioritizing loop maintenance on a plant-wide scale is based on various policies, four of which are:
• Loop static priority (LSP) criteria
• Loop fault severity (LFS) criteria
• Contribution to unit fluctuations cost model (UFCM)
• Contribution to pseudo fluctuations cost model (PFCM)
Loop static priorities (LSP)
In prioritizing maintenance based on LSP criteria, each loop in the plant is assigned a static priority weight. A loop with a higher LSP is given precedence when there are multiple loops in need of maintenance. The determination of a loop in need of maintenance is based on separate criteria -- excessive values of the average or standard deviation of closed loop setpoint error. The loop static priority is usually assigned by process engineers based on loop criticality. The static priority of a loop is time invariant, but it could be recipe-dependent. LSP also is independent of the loop fault modes. Static loop priorities are usually assigned before attempting any plant-wide maintenance activity. Figure 1 shows the ordering among a number of loops based on LSP, in a report that shows the general tuning status of a number of PID loops.
|Figure 1. Proritizing loop maintenance based on loop static priority. Click on image for larger version.|
Loop fault severity (LFS)
When prioritizing plant-wide loop maintenance based on the severity of fault modes, a loop is viewed as a cut in an automation hierarchy (Figure 2) having the following levels:
|Figure 2. Automation hierarchy|
• Main process
• Device: Process devices like sensors, actuators, and communication networks
• Control: Regulatory control algorithms such as PID, IMC and MPC
• System: System level automation programs in PLC, DCS and PC
• Supervisory: Supervisory control subsystems such as HMI, recipe management, alarm level, production operators and engineers
These levels are organized according to the logical order of system layers that fall between an operator -- the highest level -- and the underlying process of interest -- the lowest level -- when the operator initiates an action that affects the process.
A number of fault modes have been defined for various levels of the automation hierarchy. By defining appropriate loop assessment indices at each level, we can determine the need for loop maintenance when there’s a considerable deterioration of the loop assessment index.
Many loop problems occur at lower levels in the automation hierarchy. The problems first manifest themselves as a drift and deterioration in performance and assessment indices for the corresponding levels. As problems worsen, symptoms cross borders and diffuse to higher levels, thereby affecting performance indices at the higher levels. Thus, deterioration in an appropriately defined assessment index for a higher level –- for example, the percentage of time a loop is operating in manual mode -– might indicate a severe problem at the lower levels. Evidently, more severe problems provide better opportunity for improvement if the problem is solved. Thus, higher-level assessment indices could be a guide to identifying the problematic loops that might provide a better return on maintenance effort.
An effective strategy that identifies faulty loops needing maintenance provides better economic return on maintenance effort and is based on monitoring the higher-level performance indices before moving down to lower-level performance indices. In other words, among the hundreds of loops a site might have, the maintenance effort should be focused on loops with assessment indices in higher levels of the automation hierarchy that exhibit problems and deficiencies because the faults in these loops are likely to be more severe.
When basing loop maintenance on the LFS method, loop assessment indices are used for both determining whether a loop needs maintenance as well as prioritizing that maintenance.
Unit fluctuation cost model (UFCM)
Measuring maintenance activity effectiveness by means of key indicators has long been standard industry practice. A commonly used indicator is the ratio of unscheduled process shutdowns to the sum of scheduled and unscheduled process shutdowns. An effective maintenance policy should drive this ratio to lower values over time.