Standards, better computational power and VFDs are creating more efficient and customized solutions.

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.

Another element that has consistently contributed to pump technology improvement is the development of testing methods. The commercial availability of advanced vibrations measurement systems allows the acquisition of a considerable amount of data. Proximity probes and body velocity transducers enable testing engineers to execute detailed investigations on machinery vibratory behavior and find correlations between mechanical parameters, flow readings, performance, FFT spectrum harmonic components, orbit shapes, and specific phenomena such as cavitation or internal flow rotating stall.

The introduction of computational fluid dynamics (CFD) also influenced the development of the process pump industry. The hydraulic design of pump flow patterns using 3-D shapes is a delicate task. Increasing computational power and the availability of reliable codes allowed consistent improvements to the centrifugal pump design process and the integration of original equipment manufacturer (OEM) experience with reliable simulations and predictions. Modern CFD commercial codes have advanced capabilities for modeling viscid and inviscid fluids. They also offer several turbulence equations and non-stationary algorithms for turbomachinery rotating regions.

The full design loop still needs to be closed with final machine physical testing at OEM workshops, but the implementation of CFD methods leads to better designs and shorter turnaround times. The computer simulation allows OEMs to quickly develop hydraulic channel shapes optimized for specific goals (such as max efficiency or NPSHR) in the operative conditions specified by the end user. The importance of these advancements became even more evident in light of the possibility of manufacturing customized hydraulic parts obtained from CFD design by integration with modern computer-aided design or computer-aided manufacturing (CAD/CAM) technology.

Control Systems

Advancement in control systems is another important development area for pump technology. Data about European industries shows that most energy use is for pump drive applications. In industrial settings, pump drives use about 76 percent of the overall energy. Despite this sizable cost, the idea of a control system for low-power pumps has not been popular in the past, especially compared with larger pumps where advanced control systems are currently used. This is mainly because of low ratio benefits/costs for small pumps caused by the high impact that a control system would have on the overall pumping station cost.

One OEM approach in the past was to provide multiple spare parts along with installation. This kind of redundancy-based approach requires higher initial installation costs for multiple pumps. In this case, many pumps work until destruction. The cost of labor and replacement is not calculated in the preliminary plant design but is incurred during operation. Today, the installation of a dedicated control system may be beneficial even for low-power or small pumps. Large energy savings are possible by integrating a variable frequency drive (VFD) system. Additional considerable operational and maintenance savings are achievable by using advanced auto-diagnostic control systems.

Modern pump control systems implement the idea of complete pump automation using the minimum number of installed field sensors. This goal is achievable through the use of performance-based models (PBM), which are now possible thanks to the increased computational capability of commercial programmable logic controller (PLC) systems. This system provides machine-control, protection and auto-diagnostic capabilities.

Compared with an unmonitored installation, monitored systems with slightly higher installation costs can provide reduced maintenance costs, repair costs and production downtime.

The main characteristics and advantages of these kinds of systems are multivariable control capabilities, VFDs and PBMs. The user sets the process parameter to be controlled, and the system provides continuous monitoring of all relevant process parameters. It automatically shifts the control over parameters that require protection intervention. When compared with traditional throttling methods, VFDs allow large reductions in the amount of power used as well as higher energy efficiencies.

The availability of PBMs allows users to predict the expected performance in the actual operative conditions. Field measurements allow them to determine the actual performance. Comparison of expected performance and actual measurements enables the system to provide auto-diagnostic indications (particular anomalies such as cavitation upset, mechanical degradation or transmitters failure) and activate correlated protection actions.

Many trends in pump technology are continually evolving. Many OEMs are continuing to research and develop CFD and CAD/CAM integration with the goal of creating new high-efficiency pump designs. The adoption of these two methods will allow for optimized custom designs for each unique application. When compared with traditional pump selection methods based on catalogue selection procedures, this custom design process would allow end users to find the highest-efficiency solutions for their applications. The diffusion of new control systems with PMBs and the implementation of auto-diagnostic software algorithms are expected to provide additional benefits. The integration of VFDs for better operational power management will provide energy savings, maintenance cost reduction and increased uptime. This integration of these methods is key to successful operation of next-generation centrifugal pumps.