by D. K. Shukla & D. K. Chaware, Essar Oil Limited and Anil B. Dhore, Essar Engineering Services Division

Predict efficiency using the Hydraulic Institute’s standards as guidelines.

Energy conscious centrifugal pump users are always interested in knowing the value of maximum attainable pump efficiency for the required capacity and head. This value can be used as a benchmark to facilitate:

  • Selecting and purchasing an energy efficient pump 
  • Conducting an energy audit of pumps in an operating plant/refinery to make decisions about the replacement of energy inefficient pumps 

In view of these advantages, developing a simple method to predict maximum attainable efficiency was needed. 

Historical Performance Curves

Approaches such as referring to pump hydraulic performance curves available in the archives of a refinery/plant or on manufacturers’ websites have limitations. These limitations include the availability of limited data and the retrieval of efficiency values from this data. This retrieval is mostly a manual process, which is time consuming and cumbersome, especially when the number of pumps is large. 

Further, the value retrieved from this limited data may not be a true representation of the maximum attainable efficiency. The possibility of using a single equation or charts available in literature [1, 2, 3, 4, 5 & 6] was checked. The search for a single, reliable equation covering a wide capacity range and pump types proved futile. The comparison of charts available in the literature [2, 3, 4, 5 & 6] for efficiency prediction revealed that those given in Figure 1A and 1B of Efficiency Prediction Method for centrifugal pumps [6], offered several advantages and were selected. 

Hydraulic Institute’s Method

In the seven graphs in Figure 1A from Reference 6, the efficiency (at optimum specific speed) is expressed as a function of capacity. The Hydraulic Institute’s (HI) presentation makes efficiency reading easy because pump users’ are accustomed to reading efficiency as function of capacity, which is always available. In other references [3, 4 & 5], the efficiency, at constant capacity, is expressed as a function of specific speed. This makes efficiency reading difficult because most of the time specific speed values require the operating speed [7] for calculations. This is usually not readily available to the end user. The seven graphs in Figure 1A cover a wide range of pump designs. Each of the seven graphs refers to a particular design. 

This segregation based on design coupled with the specific speed correction chart in Figure 1B helps the pump user generate more options for a particular requirement of head and capacity. More options help the user make informed choices as explained in “The Energy Audit” section. The charts in Reference 2 also had potential but were not considered because of the limited application range. 

Excel Spreadsheet to Speed Calculations

Based on the charts from Figure 1A, an interactive Excel spreadsheet was developed to help make the efficiency calculations faster. To check the reliability of the charts, the efficiency values calculated using them were compared with the pumps available in the market. This comparison, shown in Table 1, was carried out for most of the widely used pumps: API overhung center-line supported (OH2 Type) pumps designed as per API [7] in capacities ranging from 18 cubic meters per hour (m3/h) to 1,080 m3/h. The figures in Columns D and E in Table 1 reveal that reliability of the prediction is higher if the pumps’ specific speeds are closer to 2,500 rpm. 

Higher efficiencies are observed when the specific speed is near 2,500 rpm. This suggests that pump users should try to purchase pumps that have specific speeds close to the optimum specific speed of 2,500 rpm. Pump users can achieve this to some extent by selecting the right combination of speed, head/stage and type as indicated in Table 2. However, in actual applications, selecting a pump using these criteria is not always practical. In such cases, the pump user can use HI’s predicted efficiency values at optimum specific speed, as well as with specific speed corrections. This will give a reasonable understanding of efficiency for pump selection. These values provide a possible range of efficiency and power savings for making an informed decision. 

Please feel free to email the authors if you would like a spreadsheet for review.

The Energy Audit

Considering the limitations, an energy audit was conducted on the pumps in an operating refinery. Some of the audited pumps’ data are shown in Table 2. The efficiencies of the pumps (those predicted with the HI method for different design options and those per other approved vendors are mentioned. The possible energy savings are also noted. The power consumption details will facilitate life-cycle cost studies to decide about the replacement or upgrading of existing pumps. 

The column “Comments” provides possible actions. The interacting spreadsheet can be used to generate options (Table 2) to make more informed choices. These options (for example, Case 1) show the possibility of using cheaper, overhung pumps with almost the same efficiency (60 percent) or between bearing pumps with more stages with considerably higher efficiency (68 percent) for existing between bearing pumps. The market availability of pumps with efficiencies close to or better than what is predicted by spreadsheet proves the practical application of the spreadsheet. Most important, the spreadsheet indicates the possibility of using single-stage overhung pumps to replace costly between bearing pumps, which is evident from Case 6.