Energy-Saving Strategies for Fluid Handling Systems

David Spitzer, PE

Where to look for wasted power, and how to get it back

People and events throughout the world are interrelated as never before. Technical developments in far-off places can have dramatic effects here at home. Fluid handling systems may seem independent, but a malfunctioning system may affect the bottom line of a major corporation. Inefficient fluid handling system operation can cause a distant power plant to generate more electricity. The plant might then emit more greenhouse gases and potentially increase global warming that might affect us all. Or it might trip off and cause a major outage.

Fluid handling systems are generally composed of mechanical equipment that generates hydraulic energy for a process that consumes that energy. The fundamental strategies to improve the efficiency of fluid handling systems are to reduce (or eliminate) the need for the hydraulic energy, to generate useful hydraulic energy more efficiently, or to use the process to produce useful energy.

The control engineer may or may not have direct responsibility for implementing these strategies, but his ability to influence the implementation of these strategies should not be underestimated.

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Pay Attention to the Process

Reducing the need for hydraulic energy often entails a technical analysis of the process, which results in beneficial process changes. Control engineers are often ill prepared to perform this analysis because their training and experience lie in other areas. However, they can influence the decisions of others by simply asking thought-provoking questions:

* Can we eliminate the pump by locating the vessel upstairs and feed using gravity? This would free up space and allow us to...

* Chilled water is used to cool a liquid to keep the flowmeter operational, but if we use a different flowmeter, which does not require cooling the fluid, then we can reduce the size of the chiller and...

* What is the maximum load required by the process? Is the equipment oversized? Smaller equipment would occupy less space, be less expensive, and cost less to operate...

* Can the fluid be obtained from another fluid handling system and eliminate the need for this equipment? That would simplify the process and make space for...

Most of the benefits cited in the above examples are not energy savings. Instead, they represent a win-win situation where the need for hydraulic energy is reduced, so energy costs are reduced. As a byproduct, the process and/or installation is improved. Reducing or eliminating the need for hydraulic energy offers one of the best strategies to reduce energy costs, but it usually requires pragmatic insight into the process and utility operation.

Minimize Pressure Drops Through Instrumentation

Another method to reduce the need for hydraulic energy is to reduce its consumption. Hydraulic energy is proportional to flow rate and pressure. In many processes, the hydraulic energy requirements to move the fluid can be reduced if the fluid flow and/or the fluid pressure are reduced.

In most applications, the flow rate must be maintained. In many applications, fluid pressure requirements can be reduced by careful instrumentation and control valve selection/sizing.

For example, it is common practice to size pumps using assumed pressure drops across the flowmeter and control valve in the piping system. In many applications, the flowmeter technology can be selected to have a negligible pressure drop, and the control valve can be sized using a pressure drop that is smaller than assumed. Such designs can potentially reduce the hydraulic requirement of the pump, in turn reducing its size and energy requirements.

Because pumps are often specified and purchased prior to specifying and purchasing instrumentation (or are part of an existing installation), these opportunities to reduce equipment and energy costs are often lost because the difference between the assumed pressure drop and the actual pressure drop is dissipated by the control valve. It is important to understand that reducing the pressure drop across the flowmeter and/or sizing the control valve for a lower pressure drop will not result in energy savings when the mechanical equipment is at the same operating point.

Consider Efficiency of Mechanical Equipment

Many influencing factors enter into the process of purchasing fluid handling equipment. One of these factors should be the efficiency of the mechanical equipment under its intended operating conditions. Note that the mechanical equipment may not (and usually does not) actually operate near its best efficiency point. For example, it is not uncommon to design centrifugal pumps to provide high pressure at relatively low flow rates. In these applications, the mechanical efficiency of the pump could be a small fraction of the efficiency at the best efficiency point of the pump.

Figure 1: Throttling Wastes Energy

In an unthrottled system, the hydraulic energy generated is equal to the hydraulic energy required (point 1). Decreasing the flow by throttling (point 2) increases the pressure above the system requirements and wastes energy.

Some equipment can use custom controls that allow the equipment to be throttled with varying degrees of energy efficiency. For example, a compressor could be controlled by bleeding off excess gas, loading and unloading the compressor, or by internal means that reduces the compression ratio. Each control strategy cited is generally more efficient than the previous strategy, however, the energy savings may or may not justify their incremental costs.