Proactive Preventative Maintenance Programs Pay Off in Maximizing Power Production

The specific fall-outs from deregulation within the electric-power industry are legion; many of the historical facility-management performance “givens” no longer exist. Long-held attitudes toward past maintenance practices must be revisited. The alternatives to these changes are totally unacceptable – non-competitiveness and an inability to survive in today’s competitive electricity marketplace are just two consequences. The bottom line now is to maximize power production and to minimize heat rate.

As a result, all past plant-maintenance practices and their close relationship to plant betterment must be reviewed. The term “plant betterment” has now been broadened to mean many things to many people – reverse heat-rate and/or MW-production deterioration that may have occurred over the years, uprate power production through reducing excessive original design margins; alter component configuration to accommodate wide load swings and frequent on/off cycling; and increase operating stability, reliability, and availability.

Older maintenance philosophies such as, “if it ain’t broke, don’t fix it” just do not work any more.

Component classification – critical and non-critical – will go a long way toward accomplishing these new goals. Operators must determine which components directly affect heat rate and/or MW output on a continuing basis. In which components will catastrophic failure immediately shut the plant down? Where will gradual component deterioration adversely affect heat rate and/or MW production over a period of time? Where are safety considerations a critical concern?

Control valves are an excellent case in point in this new power-plant-betterment world. Many times, poor control-valve operation can easily account for up to a two- to five-percent loss in power production. In any power plant, the sheer number of valve scan vastly exceed the number of other items in other component classifications.

Basis of Valve Classification

One good basis for reviewing power-plant valves as discrete components for maintenance-review purposes is to classify them into two categories – critical severe services and non-critical services.

Critical, severe-service control valves are those valves that through flow modulation and/or dead shutoff capability directly affect power production or heat rate. Inherent in these characteristics is controllability and zero leakage. When these critical functions are combined with high pressures across valve shutoff sealing faces (plug and seat) and high differential pressure control within their modulating functions (trim), they constitute the “short list” of valves that must be tagged for thorough, periodic attention. (Special care must be taken here to include any general-purpose valves that may have been erroneously installed in critical severe-service applications.)

Valve maintenance neglect ultimately results in damaged trim.

This will define the attractive economics of a proactive, preventive-maintenance program, rather than merely a reactive, corrective, (after-the-fact-failure) maintenance program to ensure continuous maximum power production at the lowest possible heat rate. After all, it can be assumed that when originally purchased, the valve application/selection was carefully evaluated. Unless some misapplication occurred at the time, why not take steps to preserve the benefits of this initial selection judgment?

In reviewing all the valves present against these criteria, usually, 12 to 15 such critical, severe-service valves exist in a typical fossil-fueled power plant unit and somewhat more in a typical nuclear power-plant unit.

Most of these critical, severe-service control valves can be readily identified by their function, such as:

• Main feedpump (MFP) or boiler feed pump (BFP) minimum flow control valves
• Feedwater control valves
• HP, IP, and LP turbine bypass valves
• Spraywater control valves

In nuclear plants, there are, in addition, some of these specialized critical, severe-service control valves such as:

• Residual heat removal (RHR valves)
• High-pressure core injection (HPCI)valves
• Reactor coolant system (RCS) valves
• Chemical Volume Control System (CVCS) valves

Proactive Preventive Maintenance Program

In the long run, a proactive preventive maintenance program saves money. Not only can it anticipate potential catastrophic valve failures due to possible misapplication or improper use, it can preclude consequential effects such as unexpected unit trips, unscheduled outages, and possibly severe damage to other plant components.

What really constitutes proactive preventive maintenance for severe-service critical control valves is that, periodically, all valves in this classification should be inspected. The question is: How is this best done – effectively and economically?

The extreme possibilities here both have their downsides. Certainly all power-plant maintenance people cannot afford to be severe-service, critical valve experts. At the other end of the spectrum, the perceived high cost of always using OEM service technicians to perform these periodic disassembly, inspection, and reassembly functions can appear to be intimidating, although in the long run, it can be optimally effective.

But there is a middle ground here that can be very effective when properly used. A close plant maintenance/OEM-service relationship eliminates the actual and perceived negatives of the two extreme approaches. Here, plant maintenance is the “monitor;” the OEM service is a combined “consultant/expert functionary.”

At several periods after initial valve operation(scheduled outages, for example) the OEM service technician should be called upon to conduct the disassembly/inspection/reassembly function. During these inspections, physical observation and critical measurement are recorded to immediately reveal any adverse trends. This established pattern can be monitored by plant maintenance and the OEM service technician. The important thing here is that this pattern be firmly established upon recorded physical observation and measurement, not merely inferred from changes in related operating parameters. Operational parameter irregularities can frequently provide suggestions on how to correct any anticipated problem areas in a timely fashion.

In addition, potential problems resulting from misapplication such as oversizing (which may cause modulating operation with the plug too close to the seat) will be revealed early in the game. Such a condition can cause premature plug/seat wear due to unnecessarily high fluid velocities in this area.

Other evidence of equipment “strain” or reduced controllability that may result from power uprating can be identified and corrected before costly damage occurs.

As a general rule, performing this procedure should highlight such items as gaskets and seals; packing spacers, followers, and the packing itself; plug, seat, and velocity-control elements; and actuator bushings and o-rings.

Figure 1 qualitatively illustrates the optimum situation. This plot of the total number of valve maintenance tasks vs. elapsed time shows where the proactive preventive-maintenance tasks line crosses the reactive corrective-maintenance tasks line. This cross-over indicates the break-even point where the plant is operating at its peak performance in terms of both efficiency and safety. In order to achieve optimum results, it is necessary to consistently consider valve-maintenance tasks across all critical valves involved. In other words, a “task” is defined as the disassembly, inspection, and replacement of all failed or damaged valve parts.

Figure 1

The Proper Role of Predictive Maintenance

No two power plants are the same, due to differing so-called “equal-to” components that may well have been originally provided in a duplicate plant design. Also the possibly adverse effects of differing operating characteristics during plant life cannot be readily taken into account.

Therefore the application of predictive-maintenance philosophies can be tricky. One unit’s experience simply cannot be tied to its “duplicate” sister unit. This is not to say that there isn’t an important role for predictive-maintenance practices in power-plant operations. Carefully kept, detailed records based upon physical inspection and measurements for individual units that are kept up to date over a period of time, in many cases, can be used in conjunction with a proactive preventive-maintenance program to effect significant economies.

These targeted severe-service control valves often constitute a final control element in power production. Therefore they not only deserve the special attention afforded by proactive preventive maintenance, but its strict and periodic application ultimately can pay off not only in terms of improved power production and heat rate, but also in reduced maintenance costs and unit reliability.

Published in SOLUTIONS Fall 2000