During the last several decades, submersible wastewater pump manufacturers have improved the performance of their products to a high degree, optimizing design and manufacturing techniques. Today, submersible wastewater pumps are highly efficient, long-lasting machines, and have been used in a great percentage of the wastewater pumping stations designed and constructed in recent history.
Submersible wastewater pumping systems, which are often purchased as individual components, provide ideal service only when part of a well-designed system. The proper design of that system—a submersible pump lift station—relies on many mutually dependent factorsIt is the consulting engineer’s role, using the project specifications and drawings, to ensure that all components are selected to work together effectively and efficiently.
Submersible System Components
All submersible pumping systems are comprised of a pump, driver, sump, pipe system and operating controls. The pump’s task in the system is to deliver liquid through pipes or conduits to a selected point against the system pressure. For a wastewater pump station, or lift station, the pump installation is usually designed to handle a peak flow rate—which, in practice, often never occurs.
Care must be exercised by the designer to select pumps that will accommodate the realistic peak flow requirements of the station but are not dramatically oversized for the day-to-day flow requirements. Pumps that are oversized for the task will run inefficiently and will be unnecessarily costly for the owner. The challenge, therefore, is to design and install a pump station that is efficient across a broad range of flows while still meeting any local code requirements.
Most wastewater pumping stations operate in a fill-draw mode, starting and stopping as the level rises and falls in the wastewater sump. The energy consumed in accomplishing this task depends on the design of the pump, the design of the installation and the way the entire system is operated and maintained. Furnishing the pumping station as a complete system is a solution to this challenge. The system may include variable frequency drives to help modulate the flow during the course of the day or may include pumps of different sizes (a jockey pump system) to promote more efficient operation during widely varying flow conditions.
To properly assemble a relatively complex system such as a submersible wastewater pump lift station, the designer must realize that all the system components are interdependent and must be carefully matched to each other and remain so throughout their working lives. Those system components (wet-pit submersible station) typically include:
- Guide rails between the access hatch and the base elbow
- Base elbows mounted on the floor of the wet well
- Electrical control panels that control the starting and stopping of the pumps
- Level control devices, such as float switches
- Variable frequency drives (VFDs)
- SCADA, telemetry or alarm monitoring systems
- Discharge check valves
- Isolation valves
- Piping systems
- Access hatches for both the pumping station and valve fault (if used)
- The wet well itself (either concrete or fiberglass)
- Other small accessories—such as cable support grips, float brackets, junction boxes, etc.
What is the Systems Approach?
One important aspect of the systems approach is that a single entity is selected to supply and coordinate key pump station components such as pumps, controls, VFDs and alarm monitoring systems. The single entity ensures component compatibility and warrants the system for an initial period. The end user or pump station owner is provided with all the critical system components by this single entity, resulting in components that are guaranteed to work together.
The systems approach assists consulting engineers, contractors, users and others in gaining a better understanding of the interaction between the pumps, valves, control panels, VFDs, basins, mechanical seals and other components that make up the system. Considering these components and how they will work together will create optimum performance, minimum maintenance and long life.
Why would end users want to implement the systems approach? The primary reason would be to optimize the bottom-line performance of submersible lift stations. That bottom-line performance will include energy, maintenance, and initial construction and commissioning costs. The system should be evaluated and optimized for the lowest total life-cycle cost, and the systems approach aids in performance optimization.
Some pitfalls of not implementing a systems approach include:
• Control systems that are not designed properly to operate and monitor the submersible pumps in the system
• Level control devices that are incompatible with the controls
• Pumps that will not fit through the supplied hatch covers
• Check valves that are inappropriate for the intended service
These are just a few of the many possible example issues that will result in additional project cost and delays while they are being resolved. The common thread with these examples is that often the components were supplied by different independent suppliers, who may each interpret the project requirements differently. Add to this lack of communication between the suppliers and lack of oversight by a single entity, and disaster can result. The system approach aims to mitigate these issues by limiting the number of sub suppliers and creating strong contract documents which define the responsibility for coordination among the suppliers.
How does the lift station designer enforce system supply (a single entity supplying all major components)? There are many construction delivery methods for engineered facilities like submersible wastewater pumping stations. The systems approach can be used for each type.
Those methods include the classic design- bid- build, design-build or variations on either approach. Regardless, quality must be maintained via strong contract documents, including detailed technical specifications. The specifications should require a methodology to establish compliance with the contract requirements, which could include detailed shop drawings and certifications of unit responsibility. Also, at the end of the project the end-user should have a detailed warranty statement covering the entire system for an agreed upon warranty period.
Benefits of the Systems Approach
The systems approach helps create a system where multiple pieces of equipment work in harmony to achieve optimum performance. Some benefits are encouraging proper initial design, determining the system responsibility, optimizing system performance and establishing a standard format for submittals and approvals.
Encourage Proper Design
Before a component is added to the system, the whole system must be designed. Some decisions that must be made are:
• The proper horsepower and impeller sizing of pumps
• The proper power requirements
• The sizes of the wet well, hatch covers, valves and other ancillary equipment
Determine the System Responsibility
When a system has a problem, the cause should be readily known and corrected. Complex systems with multiple component parts such as pumps, control systems, valves and special start/stop units require unit responsibility. Unit responsibility benefits are:
• Elimination of questioning the cause of failure; the cause is isolated under the auspices of a single source
• Reduction of the time required for repairs
Optimize System Performance
The systems approach allows for a pump station in which all the components operate efficiently together. This assures that multiple pieces of equipment will operate to provide the best performance.
A systems approach can also help standardize different aspects of the planning process, including:
• Standard pump charts and curves
• Pump test standards
• Engineering, operations and maintenance manuals
A focus on the systems approach will encourage and promote better submersible pump system design through a more comprehensive understanding of the impact a component has on the other components and on the whole system. This results in improved work flow during construction and commissioning and a station that runs more efficiently, reduces the possibility of overflows and requires less maintenance.