A submersible pump, also known as an electric submersible pump (ESP), is a device designed with a hermetically sealed motor closely coupled to the pump body. This entire assembly is submerged in the fluid it is intended to pump, offering the significant advantage of preventing pump cavitation—a problem often encountered when there’s a high elevation difference between the pump and the fluid surface. Unlike jet pumps that create a vacuum and rely on atmospheric pressure, submersible pumps actively push fluid to the surface. They are especially effective in heavy oil applications, using pressurized fluid from the surface to drive a hydraulic motor downhole instead of an electric motor.
The concept of the submersible pump dates back to 1928 when Armenian engineer Armais Arutunoff installed the first submersible oil pump. A year later, in 1929, Pleuger Pumps developed the design of the submersible turbine pump, laying the groundwork for the modern multi-stage submersible pump.
These pumps are typically multistage centrifugal pumps operating in a vertical position. Liquids, once accelerated by the impeller, lose their kinetic energy in the diffuser, converting it to pressure energy. This principle is central to radial and mixed flow pumps. Submersible pumps can also be screw-type, using a steel screw as a working element, making them suitable for water with high sand content and other mechanical impurities.
Submersible pumps are utilized in various applications, including drainage, sewage pumping, general industrial pumping, and slurry pumping. They are also used in pond filters, water extraction, oil wells, sewage treatment plants, seawater handling, fire fighting, deep well drilling, offshore drilling rigs, artificial lifts, mine dewatering, and irrigation systems. Their design is crucial in hazardous locations to prevent ignition of combustible liquids or vapors.
In oil production, submersible pumps offer an efficient form of “artificial lift,” capable of operating across a wide range of flow rates and depths. By reducing the pressure at the bottom of the well, they can significantly increase oil production compared to natural production methods. These pumps are typically electrically powered, though hydraulic versions are also available.
Read Also: Water Pump Types: Technical Complete Guide [PDF]
Working Principle of Submersible Pump
At the core of a submersible pump is the centrifugal pump mechanism. This type of pump uses rotational energy, typically from an electric motor, to move fluid. The pump and motor are enclosed in a single housing, submerged in the fluid it is pumping, thus the term “submersible.
The electric motor spins a series of impellers. Each impeller is essentially a rotating disc with vanes, designed to move fluid. As you’ve outlined, in a submersible pump, each impeller pushes water into the diffuser of the next impeller above it. This series of impellers is what allows the pump to move water from the source to the surface or through the piping system.
Pressure Generation:
The pressure generated by the pump is a function of the number of impellers and their design. Each impeller contributes to the overall pressure output of the pump.
In our example, a 10-stage submersible pump (with 10 impellers) would develop a pressure of about 90 psi, assuming each impeller adds approximately 9 psi.
Capacity and Pressure Considerations:
The capacity (or flow rate) of the pump is determined by the design of the impeller, particularly the width of the impeller vanes.
The pressure capability, on the other hand, is influenced by the number of impellers. More stages (impellers) mean higher pressure output but potentially lower volume, as seen in the comparison between a 7-stage and a 14-stage pump of the same horsepower.
It’s important to note that like all centrifugal pumps, an increase in the depth from which water is being pumped, or an increase in discharge pressure, will reduce the pump’s capacity.
Design and Application Considerations:
The selection of a submersible pump for a specific application requires careful consideration of both the required flow rate and the pressure head.
For deep wells or applications where a high head is needed, a pump with more stages (more impellers) is typically chosen.
Conversely, for applications requiring higher flow rates but at lower pressures, a pump with fewer stages and possibly larger impeller vanes would be more appropriate.
Advantages
The advantages of submersible pumps, particularly deep wells, are significant and contribute to their widespread popularity in various applications.
- High Efficiency:
- Submersible pumps are known for their efficiency in moving water. Because they push water to the surface rather than pulling it, they require less energy than surface pumps for the same amount of water movement. This efficiency is especially pronounced in deeper wells where the distance the water needs to be moved is greater.
- The motor’s close coupling to the pump body and its submerged operation contribute to reduced energy loss, enhancing overall efficiency.
- High Capacity:
- These pumps are capable of handling a large volume of water, making them suitable for various applications, including agricultural, industrial, and municipal water supply.
- The design of the impellers and the motor power can be adapted to achieve the desired flow rate, accommodating a wide range of capacities to match different requirements.
- Low Maintenance:
- Submersible pumps require less maintenance compared to their surface counterparts. Being submerged, they are less prone to problems like cavitation, which can damage pump components.
- The sealed design also protects the internal components from external elements, reducing wear and tear.
- Economical for Deep Wells:
- For wells that are 80 feet deep or more, submersible pumps are generally more economical. The deeper the well, the more advantageous a submersible pump becomes in terms of both operational efficiency and energy consumption.
- They eliminate the need for a series of pumps or a long suction pipe, both of which can add complexity and cost in deep-well applications.
- Reliability and Longevity:
- The submerged operation means these pumps are not subject to issues like pump priming or surface weather conditions, leading to more reliable performance.
- With fewer moving parts exposed to the external environment and consistent operating conditions underwater, submersible pumps often have a longer lifespan.
- Quiet Operation:
- As the pump operates underwater, the noise level is significantly reduced. This makes submersible pumps an ideal choice in residential areas or situations where noise pollution is a concern.
- Space-Saving Design:
- Unlike external pumps, submersible pumps do not require additional space for installation above ground. This can be a crucial advantage in areas where space is limited.
Submersible Pumps Limitations
Submersible pumps have specific limitations that are important to consider when selecting them for use. These include:
- Tolerance for Abrasives: Submersible pumps have relatively small tolerances between the impellers and diffusers, making them unsuitable for pumping water that contains sand or other abrasives.
- Cooling and Lubrication: These pumps are cooled and lubricated by the water they pump. They should not be installed in wells that are likely to run dry unless they are equipped with a liquid level control.
- Gas Locking Issue: Submersible pumps are not appropriate for pumping water with high concentrations of dissolved gases. In such conditions, the pump may become gas locked, impairing its function.
- Well Size Compatibility: Most submersible pumps are designed for wells with a minimum inside diameter of 4 inches. While there are pumps available for smaller wells (as small as 3 inches in diameter), these tend to be more expensive than the comparable 4-inch models.
- Pump Selection and Well Capacity: Careful consideration should be given to the selection of a submersible pump based on both the desired pump capacity and the safe pumping capacity of the well. It’s important to avoid over-pumping a well, as using a pump that exceeds the well’s capacity can damage both the well and the pump, and reduce their life expectancy.
- Pressure Concerns: If a submersible pump is capable of producing wellhead pressures exceeding 80-100 psi, it might be necessary to install an emergency pressure relief valve. This precaution is especially vital if there’s a risk of the pressure switch failing in the “on” position.
How to Install Submersible Pumps?
When installing a submersible pump, along with the associated pressure tank and fittings, there are several important considerations and steps to follow:
Before installation in the well, the pump should be wired, submerged in a container of water, and operated to ensure it’s working correctly.
The choice of drop pipe depends on several factors:
- The depth to the water and the pumping level.
- The pressure that the pump can develop.
- Allowable friction loss in the drop pipe.
- The weight of the pump itself.
Making Adaptor Connections: For drop pipe connections, use only nylon (with three serrations), bronze, or galvanized (with seven serrations) male adaptors. Warm the pipe with water below 65°C before making connections; never use a torch as it can damage the polyethylene. Ensure all connections are double clamped with 100% stainless steel clamps, with screw heads facing opposite directions.
Safety Rope: A nylon safety rope is recommended, tied with a bowline knot. After installation, this rope should be tightened and secured at the top of the well casing. This reduces the risk of the pump falling into the well in case of a pitless unit or pipe failure.
The electrical cable running to the pump should be inserted into a 3/4 inch plastic pipe to protect it from abrasion against the well casing.
Submersible pumps need protection from power surges and lightning strikes. Pumps with motors of 1 horsepower or less often have built-in protection, but this should be verified with the supplier. Alternatively, a lightning arrestor may be necessary, wired to the line conductor and back to the electrical grounding system using no less than No. 6 AWG copper wire or cable.
In some cases, the electrical grounding system is connected to a steel well casing. It’s important to confirm electrical grounding details with an electrical contractor or inspector, especially if you are doing the wiring yourself.
Following these guidelines carefully ensures a safe and efficient installation of the submersible pump, minimizing risks and maximizing the system’s longevity and performance.
Starting the Pump
The start-up process for a submersible pump involves a careful approach to ensure efficient operation and to minimize the risk of damage, especially from sandy water. Here are the steps to follow:
- Initially, before starting the pump, the restrictor valve should be almost completely closed. This valve controls the flow rate and pressure of the water being pumped.
- With the restrictor valve nearly closed, start the pump.
- Slowly open the restrictor valve. This should be done gradually to monitor the pump’s performance and the water quality. There are three scenarios to consider:
- Open the valve until the pump is working at the full yield of the well.
- Alternatively, open the valve until the pump reaches its rated capacity, as per manufacturer’s specifications.
- If the water starts to become sandy, this is an indication that the pump is drawing in sand from the well.
- Submersible pumps can quickly wear out if they pump sandy water. In such cases:
- If sandy water is being pumped, do not immediately turn off the pump. This could cause further complications or damage.
- Instead, adjust the restrictor valve to reduce the pump’s intake. This may prevent sand from being drawn up into the pump.
- Continue operating the pump on a restricted setting until the water clears up.
It’s important to note that continuously pumping sandy water can significantly reduce the lifespan of a submersible pump. Therefore, if sandy water is a frequent issue, it may be necessary to consider additional solutions such as well rehabilitation or the installation of a sand separator. Additionally, always refer to the manufacturer’s guidelines for specific instructions related to the start-up and operation of your particular pump model.
Related Article and References: