Last month’s column explained how to make sure the entire team understands the pump system’s operation with an energy cost balance sheet. This column discusses using those techniques to work through the assessment process.
First, develop an energy cost balance sheet based on how the system currently operates. Then identify system improvement options, and generate an energy cost balance sheet for each. The balance sheets provide the plant’s financial management team with the data required to decide on an action plan based on the potential risks/rewards.
Current System Assessment
In the demineralized water system that has been used in this column, a typical process system is assessed. This system consists of a supply tank, centrifugal pump, process component, destination tank and two control loops (see Figure 1). Downstream from 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 runs through the process component, flow element and onto the destination tank. The level in the destination tank is maintained at 5 feet by a level control valve.
Figure 1. The pump curve shows the location of the original flow, the flow of Option 1, and the flow and head of Option 2.
The flow rate into the system’s destination tank averages 2,500 gallons per minute (gpm) to maintain the tank level. This system is currently operates 8,000 hours per year to meet the plant’s production needs, and the system has operated this way since it was commissioned five years ago. An assessment was performed because of the system’s continuous operation, the position of the level control valve and because bypass control is used to maintain the pump’s downstream pressure.
The system currently operates with two circuits, a process circuit and a bypass 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 calculation of the pump operating cost (see Table 1) is critical.
Table 1. The energy cost balance sheet for the operating system—notice that the total pump energy and power cost is the same as the sum of the process circuits.
The manufacturer’s pump curve is key to understanding the pump’s operation (see Figure 1). The flow element shows the flow rate to the process tank required to maintain the level, and it does not include the bypass flow. The pump head of the process pump will be used to determine the flow rate through the pump. The installed pressure gauge on the pump discharge will be used in the calculation. The suction pressure will be calculated by using the liquid level in the supply tank, elevation of the pump suction and the estimated head loss in the suction pipeline.
A calculated pump head of 235 corresponds to a flow rate of 4,000 gpm and an efficiency of 83 percent according to the pump curve. A fluid density of 62 pounds per cubic foot (lb/ft3) was obtained from the process engineer, and the average power cost of $0.05 per kilowatt hour (kWh) was established by the energy assessment team. The annual pump operating cost (AOC) is calculated, using the equation on page 25, at $90,243 per year.
Because the process pump is the sole source of fluid energy, this cost represents the system’s energy cost. The energy cost balance sheet for the system under its current operation is provided in Table 1.
The head consumed by each element in the system and the associated power cost were calculated using the operating data from the installed plant instrumentation and the pump curve, process equipment data sheets, and control valve data sheets provided by their respective manufacturers. The calculation details will be explained in future columns.
In the energy cost balance sheet, approximately one-third of the total energy is used by the bypass circuit. It appears that the system is an excellent candidate for reducing operating cost and improving plant profitability.
System improvement opportunities can be analyzed. The primary focus is the elimination of the bypass circuit. Four alternatives are suggested:
- Change the plant’s operation procedure to eliminate the need to maintain a continual bypass for pressure regulation.
- Eliminate the bypass control and trim the impeller to reduce excessive head across the level control valve (LCV).
- Install a variable speed drive (VSD) to establish level control in the destination tank by varying the pump speed.
- Purchase a new pump to meet the current system’s needs.
The decision to implement any option should not be made quickly. Its effects on the system operation should be well understood by all stakeholders. However, identifying and sharing the cost of operation for each alternative helps encourage communication between the different plant groups.
After reviewing the design documents, the team learned that the bypass was originally installed to ensure a minimum flow through the process pump during plant startup and shutdown. Over time, the bypass was operated regardless of plant load to eliminate the possibility of low flow rate through the pump during large changes in plant load.
Plant operating records indicate that the flow rate through the system has never been below the manufacturers minimum flow value for the pump. After a discussion with the operations, instrument departments and the pump supplier, the team determined that continual bypass was not required. As a result, a systems assessment on the proposed changes was conducted.