Optimize pump life cycle

Understanding an operating system reduces costs and improves reliability. Getting the most out of your pump demands a well-thought-out, holistic strategy.

By Tom Carsten and Barry Erickson, Flowserve

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Getting the most out of your equipment is becoming more and more important in the chemical industry. Lean staffs, high energy costs, and market pressures are driving manufacturers to carefully look at the ubiquitous pump.

Equipment manufacturers formerly devoted to product development and distribution of process equipment are transforming into enterprises that provide services throughout the product life cycle. This trend has accelerated in more recent years as equipment users refine internal functions and increasingly outsource activities.

Today, the rapidly expanding service industry touches virtually every element of the equipment life cycle. What is driving this evolution? Owners have recognized that the overall cost of ownership is much greater than the acquisition cost — decisions made during the acquisition phase can dramatically affect lifetime ownership cost.

In the pump industry we recognized this a result of a joint effort between a major chemical manufacturer and one of Flowserve’s heritage companies, Durco International. That research was focused on chemical process pumps (ASME /ANSI B 73.1) and showed that the initial acquisition cost was as low as 5% of the life cycle cost (LCC). The LCC is not the sum of its parts.

In 2001 the Hydraulic Institute and Europump published a comprehensive guideline for evaluating LCC for pumps. As one would expect, these costs include design, purchasing, installation, operation and maintenance. Also included was the disposal cost, which has become more relevent today than ever before.

Although users are beginning to recognize the importance of LCC, how the various LCC elements interact with each other is not well understood. For example, optimizing installation costs could have a negative effect on equipment maintenance expenses. What is required is a comprehensive holistic approach to LCC where the optimization process is tailored for specific business variables and objectives.

This means looking at all components of LCC as a whole and considering their interactions and conflicts. Pump reliability is not defined by identifying the least reliable component. A seal, with a service life of one year, may leak after a few months if a suction hose leaks; the two parts are linked.

In a new installation most of the components affecting LCC can be considered as variables to be holistically optimized. For existing installations however, these factors are often fixed. The solution is to break down the LCC into two segments: LCC to date and future LCC.

Generally, the evaluation of the future LCC can be simplified by considering only the most significant costs: operation, energy and maintenance.

False perceptions

A common misconception is that energy costs are synonymous with pump efficiency. This isn’t the case in most situations. Typical pumping systems utilize a control valve to regulate flow rate to the desired amount. Although it’s necessary for a control valve to be partially closed at the design condition to maintain flow control, sizing the control valve for an excessive pressure drop is common — usually as a result of compounded design safety factors.

Figure 1 illustrates a pumping system designed to deliver 680 cubic meters per hour.

Figure 1. The control valve is sized for a 17-meter pressure drop at design flow rate.

Figure 1. The control valve is sized for a 17-meter pressure drop at design flow rate.

To maintain the design flow rate a control valve was selected with a 17-meter pressure drop. Additionally, when fully opened the control valve would permit 150% of design flow rate in the system. A properly-sized pump could operate with a control valve sized for a 3-meter pressure drop at design flow.

The overcapacity of the pumping system and inappropriate design of the control valve increased energy costs by more than 50%. Taking advantage of this savings requires the pump impeller diameter to be reduced as shown in Figure 2.

Figure 2. Optimize impeller and re-size control valve for the lowest operating cost.

The argument often made for using an undersized control valve is that it allows the system to operate as intended even if the pump deteriorates. While this is true, there is an effect on energy cost. Additional attention to maintenance of the equipment would easily pay for the reduction in energy cost and would dramatically reduce total costs.

For comparison Figure 2 also illustrates the effect of a higher efficiency pump (i.e., 80% versus 75% efficiency). The higher efficiency pump decreases energy costs by 7%; while not insignificant, this example illustrates the importance of considering the total system instead of focusing only on pump efficiency.

Although efficiency is vital, in an existing installation, matching the impeller to the system curve is where the payoff is (Figure 2). Trimming the impeller and re-balancing are relatively inexpensive and can be performed during routine maintenance. In many cases “right-sizing” the impeller diameter delivers an added benefit of improved reliability and lower vibration.

Variable frequency drives (VFDs) are often proposed as a means of energy reduction. In systems with widely varying flow rate requirements, and where system friction is a significant component of the system head, VFDs offer a viable solution for reducing LCC. In some applications VFDs also can eliminate the need for control valves and their associated maintenance expense. Unfortunately, drives often involve significant installation and set-up costs that may offset any impact on savings. Other costs associated with VFDs should be considered.

For example, conditioning of the power supply may be required to ameliorate high frequency harmonics. The type of process control is important. There are differences between a flow rate-regulated system and a flow quantity-regulated system. Most pumps are flow rate-regulated with a control valve or VFD. Some pumps are quantity-regulated by means of timers or level. Examples of these systems are municipal potable water systems that supply stand tanks and waste treatment lift pumps. In a quantityregulated system energy consumption is primarily affected by pump and motor efficiency; piping system changes to existing installations are usually impractical.

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