Once-through systems have system resistance curves where the head varies with the square of capacity. This makes them a good fit for variable speed drives, since the pump head-capacity varies similarly—the pump remains at good efficiency and above minimum flow at all speeds (see Image 4). Reduced pump speed is usually obtained via variable frequency input to the motor but can also be obtained with a fluid or magnetic drive or a multi-speed motor.
Some units have low capacity factors, and may be cost constrained from major capital investments. The choice is to keep the costs modest or shut down the unit.
Flow metering can be difficult on once-through systems due to buried pipe, pipe materials (concrete or concrete lined) and lack of accessibility. Several intrusive and non-intrusive methods can be employed to provide adequate flow metering results.
Circulating pump systems have minimal instrumentation, often lacking basis data such as:
- discharge pressure
- intake water level
- vibration monitoring
- motor amps
- motor thrust bearing temperature
Vibration data below grade (wetted transducers) should be considered to improve the ability to monitor circulating water pump health and provide early indications of distress (see Image 6). Plants need a reliable method of estimating and reporting water usage to the regulatory agencies. This can be determined by direct measurement of velocity or calculated from other data. Due to the potential large expenditures for some of the possible options, it is beneficial to start with a system analysis to evaluate the original design, current operations and options for desired performance.
Analysis of the circulating water system involves multiple disciplines and includes the pumps, motors, condenser, valves, piping and ancillaries. An investigative process that has been successfully used at various locations includes the five key steps listed here.
- Visit the site and conduct a system walkdown. Collect data, drawings, curves, etc. Discuss plant goals and constraints (see Image 5).
- Develop a test plan. Discuss testing plan with site personnel.
- Perform system testing. Collect flow and pressures including pump discharge pressures, condenser inlet and outlet pressures, motor data. (There are many options for flow metering to determine existing pump and system performance. If possible, non-intrusive flow metering is performed. If this is impractical, there are alternate testing means to determine system capacity.)
- Perform an analysis of data and issue report and conclusions.
- Present options and cost/benefit analysis.
In conclusion, there are many options for compliance with 316b. Plants are different, and each one has a different set of constraints and cost/benefit situations. The lowest cost solution is typically to use existing hardware and modify operations to achieve compliance. If inadequate, various hardware changes can be made to pumps, motors (including VSDs), piping system, and/or intake screens to achieve compliance at an acceptable cost to the plant.