Developing a Pump System Assessment with Energy Cost Balance Sheets

In the April edition of the ESI News Brief, we talked about gaining a clear understanding of pump system operation by developing an Energy Cost Balance Sheet.  This month we’ll put those techniques to work by working through the assessment process. 

The first step in the assessment involves developing an Energy Cost Balance Sheet for the way the system is currently operating.  We can then identify system improvement alternatives and develop an Energy Cost Balance Sheet for each proposed alternative.  These results provide the plant’s financial management team with the data needed to develop a course of action based on the potential risk/ reward. 

The Current System

We’ll demonstrate by performing an assessment on a typical process system (figure 1) consisting of a supply tank, centrifugal pump, process component, destination tank, and two control loops.  Downstream of the process pump some of the flow is returned to the supply tank to maintain a 100 psi pressure.  The remainder of the system flow goes through the process component, flow element, and onto the destination tank.  The level in the destination tank is maintained at five feet by a level control valve.

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Figure 1 – Piping schematic of the example process piping system, with current operating parameters.

The operating data of the plant’s installed instrumentation is displayed in Figure 1, notice the flow rate into the destination tank averages 2,500 gpm to maintain the tank level in the system.  This system is currently running 8,000 hrs. per year to meet the plant’s production needs.  The system has been operating this way since it was commissioned five years ago. 

A system assessment was performed due to the system’s continuous operation, the position of the level control valve, and the fact that by-pass control is used to maintain the pump’s downstream pressure. 

The way the system currently operates consists of two circuits, a process circuit, and the by-pass circuit.  Both circuits are supplied by a common pump, so the energy (or head) consumed by each circuit equals the head produced by the pump.  The Energy Cost Balance Sheet is presented in Table 1.  A key element of the analysis is the calculation of the pump operating cost, the method to calculate this important value is demonstrated in the following paragraphs. 

The key to understanding pump operation is the manufacturer’s pump curve (Figure 2).  The flow element shows the flow rate to the process tank needed to maintain level and does not include the by-pass flow.  The pump head of the process pump will be used to determine the flow rate through the pump.  The installed pressure gage on the pump discharge will be used.  With no suction pressure gage installed, the level in the Supply Tank, elevation of the pump suction, and the estimated head loss in the suction pipeline will be used to calculate the pump’s suction pressure.

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Figure 2 – Pump curve showing the location of original flow, the flow of alternate 1, and flow and head of alternate 2.

A calculated pump head of 235 corresponds to a flow rate of 4000 gpm, and resulting efficiency of 83% read from the pump curve.  A fluid density of 62 lb/ft3 was obtained from the process engineer, and the average power cost of $0.05/kWh was established by the energy assessment team.

The annual pump operating cost is calculated using Equation 1 at $92,421 per year.