A typical submersible pumping consists of an electric motor, seal section, intake section, multistage centrifugal pump, electric cable, surface installed control panel and transformers. Additional miscellaneous components of installation will normally include means of securing the cable along-side the tubing, wellhead support, check and bleeder valves, etc. Optional equipment may include a downhole sentry for sensing bottomhole temperature and pressure.
In its operating position, the downhole equipment is suspended from disharge and submersed in well fluid. The setting depth or bottom hole pressure creates no problem as the seal section equalizes internal pressure in the motor with the submergence pressure in the well. Installations in crooked or directional holes are possible.
The submersible electric motor us the prime mover for the pump. It is a two-pole,
three-phased squirrel-cage induction motor and rotates at about 3450-3500 RPM at 60 HZ curretn (2875-2915 RPM at 50 HZ) at full load. It is designed in a wide range of operatinf voltiages and currents.
The motor stator laminations and windings are enclosed in a carbon steel housing. The rotating assembly consists of a series of rotors mounted on and keyed to a high-strenght steel shaft supported by intermediate bearings. The required housepower is obtained by increasing the motor lenght and number of rotors. The overall lenght of a single-section motor is usually limited to about 33 feet to facilitate proper assembly and ease in transportation.
The seal section, in general, performs the following basic functions:
- To house the pump thrust bearing to carry the axial thrust developed by the pump.
- To prevent the entry of well fluid into the motor.
- To equalize the pressure inside the motor with the wellbore pressure and thus to eliminate the pressure difference across shaft seals.
- To compensate for the expansion and contraction of motor oil due to heating and cooling of the motor when the unit in running or shut down.
The motor oil further protected from the well fluid by means of an expansion bag. The entry of well fluid along the shaft is eliminated by the use of two mechanical shaft seals. The seal sectin is equipped with a high-capacity tilting-pad thrust bearing.
Intake Section, Gas Separator:
The Intanke Section plays the role of a suction manifold feeding the well fluid to the pump impeller eye. Depending on well conditions, it can be in the form of a gas separator. In high GOR and Low-bottom-hole pressure applicaions, the well fluid may
contain significant amounts of free gas which may have a detrimental effect on pump performance. In such cases, the intake section is replaced by a gas separator.
The gas separator are designed to separate free gas from the well fluid before it enters the pump. As the well fluid (consisting of free gas and liquid) passes through the impeller. It is subjected to the action of centrifugal forces. The liquid being of higher density, is thrown towards the periphery of the impellerwhile gas forms a core near the center. The gas is vented to the annular space while the gas-free liquid enters the pump impeller eye.
The pump can be modified in the field to accept a gas separator or the basic intake section depending on the field conditions.
Multistage Centrifugal Pump:
Submersible pumps are multistage centrifugal pumps. A stage consists of a rotating part known as an impeller and a stationary part called a diffuser. Due to limited well casing diamter, the lift or head developed by a stage is relatively low and a large number of stages are stacked together to meet the high head requirements of artificial installations.
Depending on the way axial thrust is handled in a pump, it may be assembled as a floated or fixed impeller pump. In a floater pump, the impellers are free to move axiallt along the shaft. In operating position, the impeller rests either on the downthrust pad oton the upthrust pad depending on the flow rate. In doing so, the axial thrust developed by each impeller is taken on individual upthrust or downthrust washers provided on each impeller and diffuser pad. The axial force due to pressure differential across the pump and acting at shaft and is taken by the thrust bearing in the seal section.
In a fixed pump, the impeller is fixed to the shaft and cannot move axially. The total axial thrust, developed by impellers as well as due to pressure difference, is taken on the external thrust bearing installed in the seal section.
Check and Bleeder Valves:
A check valve is usually installed 2-3 tubing joints above the pump assembly and helps in maintaining a full column of fluid above the pump. If a check valve is not installed. Flow of fluid from the tubing through the pump can cause reverse rotation of the unit when the motor is shut down. Any attempt to start the unit during its reverse rotation may lead to severe damage to the motor, thrust bearing or shafts. To avoid this in those cases when a check valve is not installed, sufficient time must be allowed for the tubing fluid to flow back before making any attempt to restart the motor. The actual speed and duration of reverse rotation depends on many factors such as depth and tubing size, type and size of pump, tubing restrictions, etc. A minimum od thirsty minutes between restarts is recommended.
In installations with a ckeck valve, a blleder valve must be installed just above the check valve to prevent pulling a wet tubing string, When pulling the tubing, the break-off plug in the bleeder valve should be sheared as soon as fluid is observed in the tubing.
Power is transmitted to the submersible motor by an electric cable. it is available in a range of conductor sizes which permits efficient matching to motor requirements. It may have galvanized steel, bronze or monel armor capable of withstanding the hostile environments of an oil well.
Depending on the type of insulation used, cables may be installed in wells with temepratures in excess of 300·F. A round cable is normally recommended, but a flat cable can be used where clearance is a problem. The cable is secured along the pump and tubing by means of cable clamps or bands.
The surface equipment usually includes a juction box, control panel and transformers. The standard control panels are weather-proof and available in a range of sizes and accessories to accomodate various installations. They may range from simple units with push-button magnetic contactors and overload protection to more complex assemblies with fused disconnects, recording ammeter, underload and overload protection signal lights, timer for automatic restarts and instruments for automatic remote control, etc