Chemical, petrochemical and refining applications such as pipeline transfer services, waxy oil charge, polyethylene terephthalic acid (PTA) reactor feed or water injection require American Petroleum Institute (API)-compliant pumps that can deliver typical flow rates of 300 to 1,500 gallons per minute (gpm) (68 cubic meters per hour [m3/h] to 340 m3/h) and heads up to 15,000 feet (4,572 meters).
The pumps commonly used for these applications include API multistage, integrally geared pumps and BB3- or BB5-style pumps. They achieve higher pressures by connecting multiple liquid chambers in a series, sometimes at higher speeds. Increased final discharge pressure is accomplished through increased rotational speed, impeller trim size, number of stages (impellers) and diffusion pressure recovery.
The plants using these pumps run operations around the clock, so unplanned downtime should be avoided at all costs, as the value of a lost day of production vastly exceeds the cost of the pump. How do plants ensure pumps are optimized to run at peak efficiency, in a manner that extends the mean time between maintenance and reduces maintenance costs?
Proper Selection & Design
It starts with proper selection and process design. Two stage pumps that use the addition of a speed increasing gearbox can replace up to 12 stages, which offers a significant reduction in material costs. It minimizes the complexity of setting clearances and takes up about 25 percent as much space in the plant. Sizing the pump properly at the outset is critical. This is accomplished by advanced analytics and computer-tailored hydraulics, which place the best efficiency point (BEP) at or slightly below the rated point, resulting in optimum efficiency and reduced end-of-curve horsepower (hp).
This enables smaller drive sizes to be used, which can still deliver the required output while saving energy. Because process conditions change over time, it is important for users to monitor pump performance and identify when pumps need to be re-rated to ensure the most efficient and reliable operations (Image 1).
Keeping the pump operating within the API limits for high-energy pumps—ideally between 70 and 100 percent of BEP—is critical. Running at flows above or below design can result in cavitation, bearing damage, seal failures, excessive shaft deflection, high vibration and motor overloads.
Pump failures of any kind result in poor mean time between repair (MTBR), plant downtime and increased maintenance costs. Maintenance and operational costs can make up as much as 80 percent of a pump’s total life cycle costs (Image 2).
Many multistage centrifugal pumps used in chemical and petrochemical plants have been in service since the late 1980s. This longevity can be attributed to proper manufacturing, quality materials and diligent maintenance. But there is another factor—a modular design that makes it easy to swap out parts when necessary.
The adage, “if it isn’t broke, don’t fix it,” often applies to pumps and other equipment. But this does not mean that pump companies are not seeking (and making) improvements. Advances in analytics such as finite element analysis (FEA), computational fluid dynamics (CFD) and multiphase modeling have greatly improved both hydraulic and mechanical designs. Coupled with improved materials, increased reliability is readily achieved, particularly if upgrades are coupled with re-rates to match pump flows to the required process conditions.
Enhancements in manufacturing, combined with extensive customer feedback from dozens of chemical, refining, fertilizer and PTA manufacturers, have identified seven key areas for optimizing pump performance, extending maintenance intervals and reducing total cost of ownership.