by Chris Denus, Denus TurboDesign Solutions

Computer-aided engineering enables the improvement of pump hydraulic performance in power plants.

Cooling water pumps for power plant condenser cooling systems are large flow-rate, low- to medium-head units that operate at low rotational speeds. For these pumps’ parameters, a typical hydraulic design stage configuration, optimal from the efficiency point of view, is either a mixed-flow or axial-flow single stage unit. 

These pumps are developed and delivered by several leading pump manufacturing companies and are operational worldwide in both fossil-fuel and nuclear power stations (in the latter case, in a secondary cooling loop).

Reference 1 and Reference 2 outline pump hydraulic design procedures which are applicable to develop hydraulic shapes of both the impeller and vane diffuser, which are typical components of these pumps, shown in Figure. 1A, while Reference 3 gives a good example of the development and construction of the cooling water pump for Cumberland power station in the U.S.

The use of computer-aided design (CAD) based geometry modellers in the industry (see Reference 4) and computational fluid dynamics (CFD) (see Reference 5 and Reference 6) allows for the definition and optimization of the shapes of the through-flow channels and blades. Designers arrive at design solutions that have improved performance features, when compared to machines designed using earlier and, to a large extent, test-based methods outlined in References 1 and 2. 

The pump performance and the design systems’ features, which become increasingly important are:

  • The desired position of the best efficiency point (BEP)—BEP parameters prediction
  • The shape of performance characteristics, such as H = f(Q), η = f(Q)—overall performance prediction
  • Slope of the H = f(Q) characteristics—CFD prediction of 
instability regions
  • Achievable efficiency at the BEP and shape of the efficiency = f(Q) characteristics
  • Net positive suction head (NPSH) performance—CFD prediction of cavitation
  • Hydrodynamic, including vibration performance—CFD studies of stage components’ interaction

This article addresses some technical capabilities of computerized, pump design and redesign systems, which are now in use in performance upgrade projects. It also discusses how these technologies’ methods allow for the analysis of existing components even under conditions with limited access to the original part’s geometrical design data, such as what is available from the existing pump hardware.

Figure 1. Case 1 pump's digital design model

Computer Aided Engineering

To satisfy the operational requirements of cooling water pumps and ensure that the high efficiency values of these pumps, their hydraulic layout results in the design solutions in the high specific speed mixed-flow and axial-flow ranges. Early generations of these pumps were designed using basic engineering approaches, which combined one-dimensional modeling of flow properties with the empirically derived design factors and coefficients. 

The inaccuracy of these methods and the limited validity range of the empirically derived design factors had to be compensated for by the cost- and time-intensive test-bed optimization effort at the manufacturers’ sites to achieve performance goals and meet project specifications. 

The development of advanced, computer-aided engineering (CAE) and, particularly, the numerical flow simulation technologies makes it possible for manufacturers to further develop the existing pump ranges and optimize them for each application, including the design solutions for large capacity water cooling pumps. 

Original equipment manufacturers (OEMs) now use   CAE technologies when designing new pumps. In addition, specialized pump engineering service companies, their consulting engineering groups and/or external partners are able to further enhance the performance of existing, already installed pumps that were, until recently, considered state-of-the-art solutions in terms of efficiency, cavitation and reliability. 

A prime example of such a redesign effort is a completed project aimed at upgrading the Edmonston pump station (Tehachapi) units, (see Reference 7). The key required capabilities of technologies used in the redesign and performance improvement project were:

  • The quality of the CFD (numerical flow prediction) used to support the redesign process
  • The capability of the geometric modellers to represent and interactively modify the pump components and pump stage geometries
  • The balance between the computer-based design/optimization effort and the model testing, which is the ultimate source of information on the actual achieved overall performance of the developed design solution

Figure 2. Case 1 pump's CFD solutions at BEP

Figure 3. Case 1 pump's CFD solutions at the partial-load operation