By Aref Vaezpour, Mechanical Rotating Equipment Engineer
Water Treatment Plant
Water is vital for living. Development and sustainability of the world’s population strongly depend on access to clean water. One of today’s urgent concerns is locating, treating, and conveying the required supplies of clean water. As the energy requirements to provide water treatment and conveyance activities are substantial, poor application of the pumps, flow control, and motors can lead to inefficiency and wasted energy consumption. Optimizing pumping applications and operations is increasingly important to achieving energy efficiency goals.
Surface water, water located on the Earth’s surface, becomes treated water after the physical removal of solids and contaminants in the water; this is followed by a disinfection process which kills microorganism and viruses.
The purpose of a water treatment system is to provide water of sufficient quality to meet the goals and standards for safe consumption by a community. These goals and water quality standards define the allowable amount of constituents in treated and potable water, and are established by the community through its regulatory agencies.
Process Treatment Scheme
Typical treatment processes for surface water depend on the quality of the surface water. Four common treatment trains for treating surface water are identified below:
- Conventional water treatment
- Direct and in-line filtration treatment
- Membrane filtration treatment
- Reverse osmosis treatment
The flow path of water, and other related flow streams, through a conventional water treatment plant is depicted in the Process Flow diagram, see Figure 1. The Process Flow diagram identifies the processes used, how the processes relate to each other, how pumps are applied, and lists the most common pump types suitable for each application in a facility.
Figure 1 and Table 1 are designed to highlight how various treatment plant process components interact, and how the pumps transfer flow between unit processes.
Pump Selection Procedures
To ensure that the correct pump is chosen for each process in a water treatment plant the following steps should be followed:
1. Establish the design criteria
- Determine the maximum, average, and minimum flows, as well as the minimum and maximum static heads, suction, and discharge piping configuration,
- Determine friction losses,
- Determine fluid characteristics,
- Develop operational scenarios.
2. Develop the system head curve
The system head curve chart should contain the following:
- Dynamic friction losses for new and old pipes static heads,
- Design point (flow and head), and
- Operating range.
3. Determine the pump type
Refer to schematic process diagram and table to determine:
- Type of pump,
- Installation type (vertical or horizontal), and
- Process fluid characteristics.
4. Determine the pump specific speed versus impeller characteristics
5. Calculate pump maximum speed
- Check if any limitation should be considered.
6. Select pump curve
Refer to pump manufacturer’s catalog. To select pump curve:
- Use flow and head range,
- Use maximum allowable speed and range,
- Select best efficiency point,
- Optimize efficiency at operating points where pump operates most of the time,
- Check pump operating within MCF, AOR and POR, and determine if variable speed drive is required.
7. Determine if variable speed pump is required
8. Superimpose plot pump curve over system head curve to develop system and pump curve chart
9. Confirm the following information
- Pump selection with pump manufacturer,
- Design point and range of operation, MCF, POR and AOR, NPSHR versus NPSHA Maximum operation speed, minimum operating speed, non-overloading hp and motor hp, and
- Pump layout dimensions.
10. Determine pump material of construction
- Impeller,
- Casing,
- Shaft,
- Shaft sleeve,
- Ear rings,
- Seal type, and
- Fasteners, see Table 1.
11. Pump selection is complete, prepare the equipment data sheet
Final Thoughts
It should be noted that other factors and requirements such as: contract’s standards and codes, site location and ambient conditions, installation space, and efficiency are important and could affect the pump selection; they should be carefully reviewed during pump selection phase. By following the suggested guidelines the reader will be able to optimize pump applications and operations making them more effective and efficient at achieving energy efficiency goals.
References
1. Hydraulic Institute: Water Treatment Plant Pumps: Guidelines for Selection, Application, and Operation, 2017.
2. MWH’s Water Treatment Principles and Design, 3rd edition, 2012.
About the Author
Aref Vaezour is a Mechanical Rotating Equipment Engineer located in Toronto, Canada. He has over 15 years of experience in the engineering design, and supply of rotating equipment for EPC consulting and operating company both in the Middle East and Europe. He has a M.Sc. in Mechanical Engineering from Shiraz University.