Author: Ray Hardee, P.E.
Smart pumping is not just about buying a pump for your system that has a high best efficiency point or installing a variable frequency drive to reduce power consumption and energy costs. Successfully designing a smart pumping system allows the plant to operate in steady state conditions with minimal downtime for long periods of time, the key to reliability and profitability in any commercial or industrial facility. Operating in steady state improves product quality, reduces maintenance downtime and costs, maintains environmental compliance, and directly impacts the bottom line.
Designing and operating a smart pumping system requires a good understanding about the overall requirements of the system, the hydraulic performance of the equipment in the system, the expected range of operations, and the fundamental laws of physics that govern how the system will operate. Steady state is achieved when the fluid properties (including the flow rate, pressure, temperature, pH, and other critical quality parameters) at any given point remain constant with time.
Because the pump, process, and control elements all work together to achieve the design requirements of the system, it’s critical that the engineering design team knows how much flow is required, what pressure must be provided to a given piece of equipment, or what temperature must be achieved. The configuration of the system, the number of loops or end users, and the location of the critical path load (or most hydraulically remote loop) will come into the calculations. The presence and effect of a siphon must also be evaluated whenever there is a change in elevation from a high point to a lower elevation.
Considering the closed loop chilled water system shown in Figure 1, one of the heat exchanger loops will be the most hydraulically remote, or the critical path load, depending on the flow rates to each, the elevation differences, and the size and length of the pipelines. Sizing and selecting a pump to make this a smart pumping system will ensure that all users will have their required flow rates without excessive pressure drop across the control valves, regardless of which one is the critical path load. In addition, there may be a siphon present due to an elevation difference in the system. This siphon effect must be evaluated to ensure that cavitation or choked flow conditions don’t occur in the control valve or flow meter at the high point in the system.
When a system is initially designed, there may be many unknowns that must be estimated so that long lead equipment can be ordered and delivered in time for construction. The pipe routing, the number of valves and fittings, or the hydraulic performance of key equipment may not be known by the time the pump must be ordered. Sizing and selecting the pump then becomes problematic and often results in adding design margins and over-sizing the pump to ensure the required flow rate or pressure can be achieved.
Installing a larger pump than needed not only adds to the capital costs of the project, but may result in higher maintenance costs to repair cavitation damage in control valves, more downtime due to broken welds or leaking gaskets caused by high vibration, or reduced product quality due to poor process control.
Failure to achieve smart pumping may be because of the unknowns during the initial design phase of a system, but also due to changing the system requirements over time. This may be because a higher or lower production rate is needed, the temperature set points must be adjusted, or new environmental regulations must be met. This may move a pump operating close to its best efficiency point to one that’s operating near its minimum or maximum flow rate at an extremely low efficiency. To achieve smart pumping, the pump must match its system requirements at all times.
After the system is built and the true system requirements are determined, the system may need to be optimized to achieve smart pumping. Trimming the pump impeller or installing a variable frequency drive are two common options for ensuring the pump matches the system requirements and to take advantage of the pump affinity laws to save energy costs. For grossly over-sized pumps, replacing the pump with one that is better sized to meet the actual system requirements may be a better option than living with years of downtime, high energy and maintenance costs, poor product quality, or environmental non-compliance.
Smart pumping begins in the plant’s design phase by understanding what the pumping system is required to do, configuring the system to achieve these design requirements, and sizing and selecting the equipment to operate at their best efficiency in steady state conditions. When the plant is running smoothly in steady state, hazardous work conditions are avoided, environmental emissions are kept under control, and prime product output is increased.
Modeling and analyzing the design in a steady state hydraulic simulation software package is key to achieving a smart pumping system. A system model can facilitate clear communication between the various engineering design teams, owners and operators, and equipment manufacturers to minimize the impact of the unknowns in the design phase and more accurately size critical equipment. A model also helps the various working groups to understand how the system should operate in steady state, how to troubleshoot the system when it’s not operating properly, and how to modify the system as requirements change or optimization is needed over time.