Understanding Pump Fundamentals for an Energy Efficient World (Part Five)

Click here for Part One, Part Two, Part Three, Part Four and Part Six.

Pumping systems play a crucial role in the operation of most industrial processes. The initial cost of a new pumping system is a small portion of the cost to operate a system over its life, which can be more than 15 years. Therefore, optimum efficiency in designing and operating pumping systems is in the best interest of all facilities.

New pumping systems provide a number of cost reduction opportunities that are typically cost prohibitive when applied to an existing system. At the design stage, the pump design engineer has control of more cost-reducing variables and may anticipate and account for cost reduction opportunities that would normally present themselves after the pump has been operating for several years. Design requirements are a critical aspect of both the initial and life cycle costs of a pumping system. Therefore, the design stage is an opportune time to review requirements to ensure excessive margin does not exist.

Initial Cost Versus Life Cycle Costs

Those firms that specialize in the construction market are typically heavily focused on initial cost, since contracts are not written in a way that rewards the engineer or contractor for optimizing life cycle costs. This approach ultimately increases the life cycle costs of the pumping system because compromises on the technical side leave some equipment with average or below average performance. The initial cost is not the significant portion of the total costs, so any new project should focus on the minimization of life cycle costs, not initial cost.

Competing Effects of Initial and Life Cycle Costs

Initial, energy and maintenance costs depend directly on the diameter of the pipework and the components in the pipework system. The pipework diameter is selected based on the following factors:

• Economy of the whole installation (pumps and system)
• Required lowest flow velocity for the application (e.g., avoid sedimentation)
• Required minimum internal diameter for the application (e.g., solids handling)
• Maximum flow velocity to minimize erosion in piping and fittings
• Plant standard pipe diameters

Increasing pipe run size will increase some costs while decreasing others. Thus, an optimum pipe run size may be found based on minimizing costs over the life of the system.

Operating Points Have Costs

Pump selection, including impeller size and operating speed, will affect the operating point of the system by shifting the pump curve. An important part of designing an efficient system is selecting a design with an operating point that is close to the BEP.

In a system that requires control features, the pumping system design engineer may choose between controls that are pump-centric (such as VFDs) or valve-centric (or use a bypass line). Both control methods will change the operating point, but pump-centric controls provide a much greater opportunity to change the pump curve and operating point in such a way as to reduce cost. Automation or intelligent VFDs can be used in conjunction with a programmable logic controller to direct the pumps to operate at an optimized efficiency. For a system with multiple pumps in parallel, the composite pump curve can be altered by turning some pumps off, thereby shifting the operating point.

Shifting the system curve via system design can also affect the operating point in such a way as to reduce initial or life cycle costs. The piping size and material will heavily influence the system resistance and hence the system curve.

Figure 1 shows all possible directions in which the pump and system curves can be shifted, and the corresponding effects on the operating point of the system.

The Optimal Operating Point

For every pump system, there exists an operating point that achieves the lowest cost option. This concept is known as the optimal pumping system operating point (OPSOP). The pump designer has significant influence over the operating point and attempts to move the design towards the OPSOP. However, the vast number of uncertainties associated with actual system requirements, equipment characteristics and how the system is operated and degrades make it difficult to identify and impossible to achieve the optimum state.

Finding the Optimal Design

Pumping systems can have a seemingly infinite number of design variants. Numerical optimization allows the pump system designer to find a design with an operating point near the OPSOP by sifting through a small fraction of possible designs. Numerical optimization uses the following steps:

• Numerically perturb the engineer's design
• Repeatedly reanalyze the system hydraulically
• Define the shape of the design space
• Identify combinations of design parameters that reduce cost
• Present the lowest cost option to the engineer

More importantly, numerical optimization allows for the cost of pump and system oversizing to be quantified before the design is finalized, which typically leads to more informed decisions during the design stage.

Margin in System Design

All pump systems have a certain amount of overdesign, known as design margin. In general, design margin is intended to cover three broad areas:

• Design imperfection or error
• Pump and system degradation
• System requirements that may change over time

When optimizing a new pump system design, a common concern is that the optimization process will eliminate the design margin. Without margin, various events may render the pump system unable to meet requirements. Design margin may be introduced to the system through intentional and unintentional sources. Intentional margin is introduced into the design through the design requirements or by the designer; unintentional margin is introduced because of imperfect design methods and unwarranted assumptions. Since unintentional margin is not quantifiable and may be conservative, the role of optimization should be to eliminate all unintentional margins. This will produce a system with a quantifiable design margin capable of meeting both initial and anticipated requirements.

How Pumping Systems Change Over Time

The conditions that exist at initial start-up of a pump may change over time due to passive or active influences. Table 1 lists some of the passive and active influences that can change the pumping system in ways that can have serious consequences on the pump's operation. A change in the pumping system can affect the pump's flow, efficiency, power consumption, maintenance needs and its useful service life. Therefore, it can be advantageous to design a flexible or resilient system that can adapt to varying conditions over time.

As the system ages or operational requirements change, the head generated by the pump may need to increase. If the pump impeller was selected with some trim, the pump head may be increased by substituting an untrimmed impeller into the pump. This will increase the power requirements of the pump, so consider the cost effectiveness of specifying an oversized driver for initial duty.

Selecting for Maximum Attainable Efficiency

The expected efficiency of a centrifugal pump varies as a function of specific speed and BEP rate of flow, and can be influenced by a wide range of available construction and design features for specific pumping applications. The theoretical values of maximum obtainable efficiency shown in Figure 2 provide a good model for what is normally achievable in the centrifugal pump industry with good design and manufacturing practices.

The figure shows that for a given specific speed, an increase in pump BEP flow rate results in an increase in expected best efficiency of that pump. Similarly, for a given pump BEP flow rate, an increase in the design specific speed of the pump results in an increase in expected pump efficiency, up to a point. Figure 2 should be used as a guide to identifying the pump configurations that will yield the highest basic efficiency. However, the pump manufacturer's efficiency values based on actual applications and construction features should be used.

Pump Corrosion, Erosion and Fouling

The pumped liquid can significantly affect the pump efficiency and reliability over time. Corrosion, erosion and fouling will reduce the pump efficiency due to increasing surface roughness within the pump. Suggested solutions to these problems are shown in Table 2 below.

Pump Design Speed

Typically, pump system designers may select either a small pump running at a higher speed or a larger pump running at a slower speed. The larger pump will have a higher initial cost. However, larger pumps tend to be more robust and lightly loaded than small pumps and therefore have significantly lower maintenance costs. In general, it can be assumed that pump wear and maintenance costs increase with pump speed. Large differences in pump efficiency are possible between the two designs. The user is cautioned that the energy cost of selecting the less efficient pump may far outweigh any expected maintenance or service life improvement costs.

Pump Style: Single Stage or Multistage

If suitable single stage and multistage pumps are available at the required duty point, the initial cost and energy costs are the key factor in pump selection. Single stage pumps usually have an end suction configuration, while multistage pumps are radially or axially split. This means that multistage pumps have a higher initial cost. Experienced maintenance personnel agree that pumps with fewer stages have lower maintenance costs. If a multistage pump is selected, a further consideration is whether it should be an in-line or a back-to-back arrangement. In-line arrangements tend to be more resilient to the effects of internal wear of system transients because the residual thrust load carried by the thrust bearing is lower.

Control Scheme Optimization-Commissioning the Pumping System

Take great care in defining the control limits and parameters of a pumping system during the start-up phase to ensure that once set into motion, it will operate within limits defined by the designer and equipment manufacturers. In particular, the following parameters should be closely considered during start-up:

• Liquid levels
• Speed range
• Verification of flow and pressure
• Control device parameters
• Variable speed drive/system control options

Learning how to build better new pumping system designs and optimize existing pumping systems is only part of the skills that the future engineers need. With the rising costs of energy and the pressing need for climate change, industrial plants are looking for new ways to lower operating costs and reduce greenhouse gases. Helping management see the financial justification of pumping systems opportunities is critical for projects to move ahead. Pumping system economics will be explored in greater detail in the next installment of this series of articles based on Optimizing Pumping Systems: A Guide to Improved Energy Efficiency, Reliability, and Profitability.

Reference

• ANSI/HI Pump Standards (available at www.pumps.org)

Pumps & Systems, October 2008