A typical process plant has an installed base of valves that numbers in the thousands. Most of them operate flawlessly for years. Sooner or later, the random chaos floating through the universe will turn at least one of them into a leaker or otherwise render it useless for its intended purpose. Regardless of how well the plant’s predictive maintenance program foretells failures, there is either a valve repair or a replacement in the offing.
At a superficial level, valve repair would seem to be the maintenance response that would incur the least cost. In the long run, however, valve repair may be more expensive than simply scrapping the malfunctioning unit in favor of a new one. The choice is not necessarily an easy one.
In the final analysis, minimizing the valve’s life-cycle cost should drive the repair or replace decision. However, there is more to the cost minimization issue than simply the difference between the list price of a new and remanufactured valve. The idea behind remanufacturing a valve is bringing it back to a functionality level equal to that afforded by a new unit. In the absence of equivalent, or improved, functionality, the simple cost differential becomes irrelevant. Any reduction in quality compromises service life, safety and environmental compliance. There is a risk that the use of a remanufactured valve may introduce lesser quality and greater future process problems and repair cost.
In a valve, equivalent functionality is not the same as identical appearance. Reworking a valve always starts with a defective unit that has already endured the rigors of daily life in a plant. External corrosion, paint chipping and normal wear-and-tear take their toll. Even flanges slightly thinned by a refacing operation should not affect how the valve operates. These cosmetic issues are separate from functionality.
The importance of failure analysis
On some fundamental level, valves fail for only one reason—their characteristics and attributes are not suitable for the demands placed on them. Specified for an assumed service, perhaps the valve cannot withstand certain process upsets. Perhaps the party responsible for specifying them made a mistake. Perhaps the process has been modified over time.
In every case, there is a reason why the valve failed and it needs to be identified to avoid a recurrence. Each failure must be accompanied by a diligent, rigorous failure analysis to ferret out the root cause of the problem. Convert that information into a specification that explains to either the valve manufacturer or repair service what is needed to minimize your cost of ownership.
Consequences = cost
Many factors serve as an impetus for responding to a leaking valve. If the leak is external, the Environmental Protection Agency may have some opinion about the consequences of ignoring the leak and the resultant puddle of trimethyl flubdub dribbling into a sewer. The Occupational Safety and Health Administration may also weigh in if the leak endangers people working in the immediate area. On the other hand, an invisible, internal leak means the valve is providing neither positive closure nor reliable control. This introduces the possibility of producing unknown quantities of off-spec material. For one reason or another, you have no choice but to remove the defective valve from service. Besides, in the abstract, doing something about a defective valve is the right thing to do.
Whether the attention paid to the valve is a result of government coercion or concerns about marketing finished goods of diminished economic value, the question of buying or rebuilding involves a cost trade-off.
The cost of valve ownership far exceeds its purchase price. Except for the costs associated with installation, regular maintenance and eventual disposal, a valve represents a zero-cost investment up to the moment it fails. Then, the ownership cost rises instantly and dramatically. The actual time and expense involved in physically swapping a valve is swamped by the time and cost involved in the necessary supporting activities, such as:
- Shutting the process down.
- Flushing and purging the lines.
- Testing to confirm the replacement valve installation is leak-free.
- Bringing the process back online.
Taking a plant offline and bringing it back into production is a costly, labor-intensive activity that must be coordinated and scheduled. The argument that these workers are on-site anyway and the labor cost is irrelevant is specious, at best. Sure, they are already here, but they are normally otherwise gainfully employed. During a shutdown, normal work is not being completed. Either way, labor cost must be considered. It’s a rare organization that accounts for every minute of every labor-hour and corresponding payroll invested in the valve swap.
Since removing a valve from service most likely requires a process shutdown, the issue of lost revenue from product sales is important. Every shutdown has a scheduled duration associated with it. Every plant has a standard, expected production rate. And every finished product has a unit value in the marketplace. Determining the revenue lost during a shutdown is a straightforward exercise.
Another shutdown cost factor is the recycling or scraping of off-spec material produced while the plant is in transition from steady-state to zero and back again. Without a doubt, more cost elements need to be considered. The point is that the valve cost—either new or remanufactured—is insignificant, compared to the shutdown cost caused by a malfunctioning valve.
Large plants, with thousands of valves, probably purchase hundreds of them at a time to take advantage of quantity discounts. This may seem to make economic sense. If they’re not suitable for the service intended, however, there will be additional valve failures and a need to purchase even more. A string of failures makes the cheap valves look like consumable goods and the normal reaction is to start buying on a cost basis. This, of course, can lead to further quality reductions, more failures and the uncontrolled waste of money.