Standards, better computational power and VFDs are creating more efficient and customized solutions.
by Massimiliano Di Febo & Pasquale Paganini
pressure contours/velocity vectorsFigure 1. The pressure contours/velocity vectors in an impeller (Courtesy of IPC)

Rotating machinery technology plays an important role in the downstream industry. Plant process conditions are often severe. A typically sized topping unit may provide output capability of 1,500 tons per hour. Critical process points, such as those for furnace charge pumps that face temperatures near 800 C, require special technological solutions. A fluid catalytic cracking (FCC) plant requires the feed to be mixed with the catalyst and then pumped to the reactor with temperatures around 500 C. Air required by the process is normally supplied by centrifugal or axial compressors with normal flow around 50,000 normal cubic meters per hour (Nm3/h). Power requirements also may reach several megawatts. Rotating machinery for compression and pumping applications plays a critical role in determining the overall robustness of the plant. centrifugal pumps, being present in almost all plant processes, are especially important.

The centrifugal pump market covers a range of downstream process applications. Many centrifugal pump configurations have been developed for different applications, including single- or multi-stage, axial or radial split, and horizontal or vertical rotor position.

Horizontal pumps are the most common in downstream applications. They are used in a wide range of pressures (up to 400 bars) and temperatures (up to 450 C), while vertical pumps are used for a more restricted range of applications, mainly in low net positive suction head available (NPSHa) conditions and very low temperatures.

Technical regulations and industry standards are primary actors in the rotating machinery industry. American Petroleum Institute (API) 610 and Hydraulic Institute codes provide wide and comprehensive guidelines that drive the whole pump production process. These standards cover most aspects from early engineering to the shop manufacturing stage. API 610 has been subject to revisions, and criteria for evaluating pump efficiency moved from previous tolerances on the efficiency to actual tolerances imposed on the overall adsorbed power. Vibration limits had analogous modifications, reflecting efforts to achieve longer machine life cycles and better energetic efficiencies.

Centrifugal Pump Improvements

Plant management’s continuous search for final market competitiveness has pushed centrifugal pump technology development in terms of performance and overall cost reduction. The evolution of metallic material technology has played a major role. During operation, pumps are subject to corrosion, erosion and fatigue. From a design perspective, appropriate material selection is necessary for satisfactory machine life.

Today’s material technology promotes better resistance to harsh conditions. Actual metallurgical processes have better control of the chemistry of materials, as is the case for duplex steels (CD4MCu) or special steel grades such as precipitation hardening 17-4-PH often used for shafts. In other cases, innovative processes such as plasma or laser coatings for sleeve surface hardening allow for better wear resistance. Development of computer finite element methods (FEM) also promotes a better understanding of the stress field inside pump components. Modern rotating machinery design takes advantage of the availability of software tools that allow delicate evaluations in topic mechanical design points such as bearing selection, balancing pistons and sleeve clearances sizing. These elements, and the boundary design operative conditions, contribute to the final stability of the rotor.

Another well-known problem that affects centrifugal pumps is cavitation. This phenomenon is characterized by fluid vaporization that occurs when the net pressure becomes lower than the vapor pressure. Downstream pressure recovery causes vapor bubbles to suddenly implode, resulting in pitting that can damage the impeller. Cavitation effects may be dramatic, ranging from performance degradation to complete failure.

In the past, expert pump operators used to detect cavitation by recognizing the characteristic noise. Recent advances in vibration study show that incoming cavitation can also be detected using harmonics (fast Fourier transform, or FFT), opening the door to an automated diagnostic. The traditional approach to the problem of cavitation is based on plant sizing with a static safety margin to net positive suction head required (NPSHr) in the operative point.

It is possible to monitor cavitation by measuring pressure, temperature and fluid speed at the nearest machine suction point. Permanent installed monitoring systems were used in the past in a limited number of cases. Some determined the cost of the monitoring system to be excessive, especially for small machines. The arrival of affordable logic controllers on the market has changed this, and today even small centrifugal pumps may benefit from such control and monitoring systems.