How to design a Treated Water Transfer Pump?
Sewage Treatment Plant (STP) is an essential facility that is used to treat wastewater and make it safe for discharge into the environment. The STP consists of several units, and one of the critical units is the Treated Water Transfer Pump. The purpose of this pump is to transfer treated water from the STP to the required location for discharge or reuse.
Here we will discuss how to design a Treated Water Transfer Pump.
Design Considerations:
Before designing the Treated Water Transfer Pump, it is essential to consider the following factors:
· Flow rate:
The flow rate of the treated water is the primary consideration for designing the pump. The flow rate will determine the capacity of the pump required.
· Total dynamic head (TDH):
TDH is the total energy required to move water from one point to another. The TDH is the sum of the static head and the friction losses in the pipes.
· Pipe diameter:
The diameter of the pipe is essential to determine the friction losses in the pipe. The friction losses will affect the TDH, and hence the pump's capacity.
· Specific gravity:
The specific gravity of the water will affect the pump's capacity since it determines the weight of the water to be lifted.
· Efficiency:
The efficiency of the pump is essential to ensure that the pump does not consume more energy than required.
Designing Steps:
Designing a treated water transfer pump in a sewage treatment plant (STP) involves several steps, such as:
1. Determining the quality and quantity of the treated water :
Determining the quality and quantity of the treated water that needs to be transferred from the STP to the point of use or disposal. This may depend on the type and level of treatment provided by the STP, the characteristics of the receiving water body or distribution network, and the regulatory standards and requirements.
2. Selecting a suitable pump type and configuration
Selecting a suitable pump type and configuration that can handle the treated water characteristics, such as viscosity, specific gravity, solids content, corrosiveness and abrasiveness. Some common pump types used for treated water transfer are centrifugal pumps, positive displacement pumps, submersible pumps and self-priming pumps. The pump configuration may include single or multiple stages, parallel or series arrangement, horizontal or vertical orientation, and fixed or variable speed operation.
3. Calculating the total dynamic head
Calculating the total dynamic head (TDH) of the pump system, which is the sum of the static head (the vertical distance between the pump inlet and outlet), the friction head (the head loss due to friction in the pipes, fittings and valves), and any other minor losses (such as entrance, exit, bend and expansion losses). The TDH represents the total pressure that the pump needs to overcome to transfer the treated water from one point to another.
4. Sizing the pump
Sizing the pump based on the required flow rate and TDH of the system. This can be done by using a pump selection software or consulting a pump manufacturer’s catalog or curve. The pump curve shows how the flow rate, head and efficiency vary with different impeller sizes and speeds. The pump should be selected such that it operates near its best efficiency point (BEP) to avoid cavitation, vibration and excessive wear. The pump size should also match the pipe size to minimize losses and ensure smooth flow.
5. Calculating the power requirement of the pump
Calculating the power requirement of the pump using the formula:
Power = Q x TDH x SG x g / 3960 x η
where
Q is the flow rate in gallons per minute (gpm),
TDH is the total dynamic head in feet,
SG is the specific gravity of the treated water
(usually 1 for fresh water),
g is the gravitational constant (32.2 ft/s2), and
η is the pump efficiency (expressed as a decimal). This gives the power requirement in horsepower (hp).
The power requirement should also include a safety factor to account for any losses or inefficiencies in the pump and motor.
6. Selecting a suitable motor
Selecting a suitable motor that can provide enough power to run the pump. The motor should have a rated power equal to or greater than the power requirement of the pump. The motor should also match the voltage, frequency and phase of the power supply. Other factors to consider are motor enclosure, insulation class, service factor and efficiency.
Case Study:
Lets take an example, if you need to design a treated water transfer pump that can deliver 150 m3/day of water at a TDH of 30 m from an STP to a storage tank, you can follow these steps:
The flow rate is given as 150 m3/day, which can be converted to 1.04 gpm using a conversion factor of 0.000694 gpm/m3.
· The TDH is given as 30 m, which can be converted to 98.4 ft using a conversion factor of 3.28 ft/m.
· A centrifugal pump is a common choice for transferring treated water, as it can handle a wide range of flow rates and pressures. However, other types of pumps may also be suitable depending on the application.
· A centrifugal pump curve shows how the flow rate, head and efficiency vary with different impeller sizes and speeds. You can use a pump selection software or consult a pump manufacturer’s catalog or curve to find a suitable pump model that can meet your requirements. For illustration purposes, let’s assume that you find a pump that can deliver 1.04 gpm at 98.4 ft of head with an impeller diameter of 4 inches and a speed of 1750 rpm. The pump efficiency at this point is 60%.
· The power requirement of the pump can be calculated by plugging in these values into the formula: Power = 1.04 x 98.4 x 1 x 32.2 / 3960 x 0.6 = 0.014 hp
· The power requirement should also include a safety factor to account for any losses or inefficiencies in the pump and motor. A typical safety factor is 1.2.
· The power requirement with the safety factor is 0.014 x 1.2 = 0.017 hp
· A motor with a rated power of 0.017 hp or higher can run this pump. You can choose a standard motor size such as 0.025 hp or 0.033 hp depending on availability and cost.
Summary:
So the Treated Water Transfer Pump is an essential unit in the STP, and it is crucial to design it correctly to ensure efficient and reliable operation. The design formula and technical calculations formula mentioned in this article can be used to design the pump based on the specific requirements. Proper consideration of the factors mentioned above can help in designing an efficient and reliable Treated Water Transfer Pump.
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