by Ray Hardee (Engineered Software, Inc.)
October 1, 2014

Last month’s column discussed the pre-screening process that identifies and prioritizes systems with a high probability of significant energy savings. After this process is complete, the actual system assessment begins.

During the pre-screening process, readily available design and operating data are collected to help develop a priority. This column will follow the assessment of a system selected as a priority. A batch tank fill system had a high rating following the pre-screening because of the motor’s size, 150 horsepower (HP); the hours of operation (continuous); and the control method (on/off).

A baseline for current operation is determined during the assessment. After this, the plant management team can identify improvement opportunities, decide on the viable options, determine the financial requirements of each option and develop an action plan.

During the system evaluation, the assessment team will collect detailed system information, analyze how the system is currently operated, identify any potential existing problems and determine what plant operating data are needed to determine the operating cost. This column begins with developing a system description and determining how the system is currently operated.

System Description

The batch tank fill system was designed to stock a 60,000-gallon batch tank in 45 minutes. The system includes four batching tanks. Once they are filled, completing the batch process takes approximately three hours. After the batch process ends, the tanks are drained, and the fill process restarts.

During operation, as one tank is filling, the three remaining batching tanks are in their batch operation. A 15-minute buffer was established in the filling process to provide the operators with an operating margin. The batch fill pumps (BTP-01 in Figure 1) start manually and are automatically shut down when the tank liquid level reaches the desired set point.

Batch tank fill system piping schematicFigure 1. Piping schematic of the batch tank fill system with the data collection needs displayed (Graphics courtesy of the author)

System Walk-Down

During the system walk-down, the assessment team completes the following tasks, which may be assigned to different members of the assessment team:

  • Identify the equipment in the system
  • Establish the system boundary
  • Determine the accuracy of the piping system schematic
  • Assign the instrumentation required for the assessment
  • Develop the test method

First, the team must compare the available design documents to the installed system. This is done by confirming the pipe routing and verifying the installed process equipment’s nameplate data. Estimating the pipe lengths and the number of valves and fittings is also helpful.

During the walk-down phase, discussions are conducted with the operations department to determine how the system is actually being operated and controlled. If the system is operating differently than originally designed, the reason for the changes should be documented.

Discussions with maintenance personnel can identify chronic maintenance problems that may indicate potential system problems. In addition, the maintenance department can identify any outstanding maintenance issues that could adversely affect the outcome of the assessment. For example, if a pump has a worn impeller, the assessment should be delayed until it is repaired or replaced.

During the walk-down, the data and instrumentation that are required to perform the assessment are identified. Does the plant have adequate instrumentation installed? Will temporary instrumentation need to be installed? The operating data are used for calculating energy use and cross validating the results using different data. Finally, if the system operates under different conditions, separate data collection for each of these conditions may be required.

After completing the fieldwork and personnel discussions, the design documents are collected for use during the assessment. These documents include the manufacturer’s pump curves, the control valve data and the flow versus pressure drop curves for any operating equipment. The elevations of the supply and destination tanks are determined and documented. This information should be available in design documents, equipment specifications and documentation for any system modifications made since the system was originally designed and installed.

Walk-Down Results

This section includes the information collected during the walk-down process for the batch pump system. The assessment team discussed the information collected, beginning at the supply tank and working toward the batch tanks. Each component’s function was defined: