by Wallace Wittkoff

Indirect factors can directly impact the true efficiency of the product-transfer process, if not considered and monitored.

For months, the focus of the Pumps & Systems "Efficiency Matters" column has been on pump design, installation and operation, factors that directly affect the energy consumption of the pump itself. For example, the April and May 2010 articles on pump performance bands discussed how normally, when a positive displacement pump has less slip, it will more efficiently pump the fluids. The amount of product pumped per unit of energy used would be considered a direct measure of efficiency.

However, the decision-maker who is tasked with optimizing energy use and reducing costs must also consider indirect costs. The global energy impact decisions must also be made. This article explores how pump design can affect three indirect efficiency areas:

  • Use of seal coolant (water) with associated cost and water treatment
  • Pump design that affects efficiency of product recovery
  • Pump design that reduces loss and waste treatment energy usage and costs

These indirect factors often result in energy creep. This is when indirect efficiency issues are not monitored and unintended waste occurs.

Energy Efficiency of Mechanical Seals

While seal cooling or flush only applies to a subset of pump applications, it serves as a good example of an indirect efficiency issue for those analyzing the total energy footprint of pump selection. Frequent applications can be found in the food, beverage and pharmaceutical industries where transferring sweeteners that tend to crystallize on seal faces can cause premature seal failure. (See Figure 1.) Traditionally, the common solution has been to use advanced seals (most of which are not permitted or adaptable for hygienic applications) or using mechanical seals with water or other fluid flush.

Transfer line from sweetener storage

Figure 1. Transfer line from sweetener storage

However, seal water usage on pumps is a classic case in which energy creep can occur. It is typical over time that the volume of seal water is increased to be safe. In fact, some experts in the industry note that we typically see 10 times the amount of water used for seal flush than what is necessary.

Benefits of Eccentric Disc Design

Negating the use of seal water altogether can help to avoid this cost (and possible creep). The solution is to use pumps that have totally sealed pumping chambers and do not require seal flush. Diaphragm and magnetic-drive pumps may be familiar options. However, new to the field are eccentric movement pumps that better fit some applications that are not suitable for the former pump styles.

Most processors realize that water is becoming a valuable (and increasingly expensive) natural resource. Water is a visible expense as the county, city or other sources that provide it are passing onto the processor the costs to supply and then treat it. If the processor treats the water, he can determine the energy usage and costs for this. As an example, a processor who handles sweeteners in the confectionary industry calculated that his plant's total cost for water used in flushing seals was more than $10,000 per year/per pump.

In another case, a processor that makes sauces in the Southeastern U.S. was faced with a permit cost of more than $400,000 if additional water was to be used in the plant. This hurt growth. In addition, if water is used over and above limit, the county must expand its water-treatment capacity. Whether it is a per-pump water use cost or permit cost, new options to negate the use of water means less energy used to supply and treat the water, as well as other costs that may be incurred.

The eccentric movement or eccentric disc design for sealing pumps is an alternative to the magnetic drive or diaphragm, no-flush options. The eccentric movement sealed pumps do not use mechanical seals and, therefore, seal flushing is not applicable or needed. Compared to magnetic drives, the eccentric movement can be implemented so that it is sanitary/hygienic and also withstands semi-abrasives better. Finally, the eccentric movement does not produce heat buildup.

This pump also offers efficiency because of low slip (a direct efficiency parameter). With this pump, the example of indirect cost through water consumption is eliminated, and the global efficiency of the pumping solution within the application is realized.

The eccentric movement pump negates the use of dynamic seals. In most cases, this pump is driven by standard rotating drives. This drives the shaft within the pump with a coupling. However, unlike most pumps, the shaft is machined on different planes so that the drive end of the shaft is on a different plane than the tip that is driving the pumping mechanism (See Figure 2.).

Eccentric Disc Design Pump Cutaway

Figure 2. Eccentric Disc Design Pump Cutaway

Attached to the shaft are bearings and both are enclosed by a hermetically sealed metal bellow or rubber boot. The shaft rotates, the metal bellows or rubber boot does not rotate thanks to the bearings. Instead, it flexes in an eccentric motion. This flexing is minor and within the elastic range of the stainless steel so that preventive maintenance inspection is recommended at 150-million duty cycles.