Bleach plants: from rebuild programs to capital upgrades.

North American mills have shown no significant upturn in pulp mill and bleach plant capital spending over the past two years. There has been activity in areas of oxygen delignification systems and brownstock washer upgrades to meet environmental criteria, but not enough to indicate a trend. Many mills have settled into a strict capital spending mode, only venturing to spend beyond maintenance budgets if there is a fast return on the capital invested.

One area of maintenance/capital spending has been in upgrading and replacing the process equipment infrastructure of bleach plants to include medium consistency stock pumps, steam and chemical mixers, and tower equipment. A large percentage of this equipment, installed over the past 10-20 years, is considered obsolete. It is becoming a serious maintenance and production liability.

Compounding the issue of obsolete equipment, many of the original equipment manufacturers (OEM) no longer exist and dedicated product support for this equipment has disappeared with them. In most cases, the manufacturers have merged into other companies, which now only provide rebuild service--often through regional repair centers. Many local machine shops also provide rebuild service. Some have obtained patterns and castings for casing replacements. Regardless of where the rebuilds originate, high costs, delivery, quality and supplier support have become concerns with mill maintenance and production departments. Also of concern is the uncertainty over the future of rebuild centers and their commitment to continue supporting obsolete equipment.

Medium consistency positive displacement stock pumps currently present the most critical liabilities. The primary issues are rebuilds and reliability. The OEMs for positive displacement stock pumps have essentially disappeared. Costs estimates to rebuild these pumps are in the $25,000-$60,000 range, depending on the supplier, make and size of the pump. The rebuild costs are often carried as a mill maintenance budget item. The rebuild cycles occur from six months to three years, depending on age, make and mill service. The pump rarely returns to peak performance after a rebuild and production upsets are common towards the end of the rebuild wear cycle.

Early medium consistency centrifugal stock pumps were considered more reliable and developed higher head and better flow control than positive displacement pumps. Disadvantages included higher brake horsepower requirement, often leading to stock consistency below 10%. Lower consistencies increase heating and bleach chemical costs and can lead to bleach tower channeling.

The OEMs for the early centrifugal stock pumps still exist, although in most cases under different manufacturing names, offering various levels of customer support. These centrifugal stock pumps in operation today have a fair maintenance cycle, but still have penalties in areas of energy costs and the ability to maintain stock consistency above 10%. Upgrade packages to improve these deficiencies are offered by some suppliers.

The logical upgrade choice for positive displacement pumps is to replace them with the latest centrifugal stock pump design. These new pumps are capable of maintaining 12% consistency or higher at a pumping efficiency > 60%, with rebuild cycles of 3 years or higher, far superior to earlier designs discussed. There is also a continuing level of product support from the OEM.

The capital return period for the new centrifugal pump purchase and installation, based on positive displacement pump rebuild costs only, can be as long as 4-5 years. Lost production and higher process costs due to poor positive displacement pump performance towards the end of the rebuild cycle are usually not included in the capital justification. They should be. The replacement is now justified, despite the long return, by the concerns over the supplier and quality of the next positive displacement pump rebuild.

Earlier bleach plant steam and chemical mixers were designed with larger mixing volumes than the more recent models. These older models have much larger casings, rotors, etc., and require higher horsepower to operate. They have rebuild cycles of 2-5 years, depending on service. OEM issues on some mixer models are the same as discussed. Rebuilds are also often handled by regional or local repair shops. Due to the infrequent rebuild demand, many of the present rebuild repair centers may not have the patterns or castings for casings on some models. Higher metallurgy is also often required. This can lead to an expensive rebuild for an obsolete piece of equipment.

The capital return period for upgrading to current mixer designs, based on rebuild costs and power usage for older mixers, is 2-3 years. Chemical savings, based on increased mixing efficiencies, can significantly reduce the return period. The increasing concern over the supplier and quality of the rebuilds is also driving the final justification.

ABOUT THE AUTHOR

Bill Miller is a process consultant for Ilkka Seppa, Inc. (ISI), Marietta, Georgia, USA. Contact him by phone at [phone] or by e-mail at [e-mail]

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