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ESP centrifugal pump
The electrical submersible pump (ESP) is a multistage centrifugal type. A cross section of a typical design is shown in Fig. 1. The pumps function is to add lift or transfer pressure to the fluid so that it will flow from the wellbore at the desired rate. It accomplishes this by imparting kinetic energy to the fluid by centrifugal force and then converting that to a potential energy in the form of pressure.
In order to optimize the lift and head that can be produced from various casing sizes, pumps are produced in several diameters for application in the most common casing sizes. Table 1 lists some common unit diameters, flow ranges, and typical casing sizes in which they fit.
The shaft is connected to the seal-chamber section and motor by a spline coupling. It transmits the rotary motion from the motor to the impellers of the pump stage. The shaft and impellers are keyed, and the key transmits the torque load to the impeller. As was mentioned earlier, the diameter of the shaft is minimized as much as possible because of the restrictions placed on the pump outside diameter. Therefore, there are usually several shaft material options available, depending on the maximum horsepower (HP) load and corrosion protection required.
The housing is the pressure-containing skin for the pump. It holds and aligns all the components of the pump. There are several material options available for different application environments. For additional corrosion protection, there are several coatings that can be applied.
Discharge head/tubing connection
The discharge head provides a female threaded connection to the production tubing. There are usually several thread forms and sizes to select from.
Several different styles of intakes can be selected. They allow for entrance of the fluid into the bottom of the pump and direct it into the first stage. Integral intakes can be threaded directly into the bottom of the housing during the manufacturing assembly process, while others are separate components, which are bolted on to the bottom pump flange.
A standard intake has intake ports that allow fluid to enter the pump. It is used when the fluid is all liquid or has a very low free-gas content. The intake shown in Fig 1 would be a standard intake if the reverse-flow screen were omitted.
A reverse-flow intake is used when the free-gas content in the fluid is high enough to cause pump-performance problems. The pump in Fig. 1 is shown with a reverse-flow design. The produced fluid with free gas flows up the outside of the reverse-flow intake screen, makes a 180° turn to enter through the perforations or holes at the top of the screen, flows back down to the intake ports and then back up to the first pump stage. These reversals in direction allow for a natural separation of the lighter gases from the liquid. The separated gas travels up the casing annulus and is vented at the wellhead. Another style is shown in the right-hand graphic of Fig. 2, which has a longer reversing path than does the intake with the screen.
Fig. 2-Separator intakes for pumps (after RP11S1).
The next step in handling free gas with an ESP involves downhole mechanical separation devices such as separator intakes. These devices take the fluid that enters its intake ports, impart a centrifugal force to it, vent the lighter-density fluid back to the annulus, and transfer the heavier-density fluid to the first pump stage. The heavier-density fluid, which is routed to the pump, has been either fully or partially degassed. Two of these devices are shown in the left-hand and center graphics of Fig. 2. The first device is the vortex-type separator. The produced fluid, which has already undergone some natural annular separation, is drawn into the unit through the intake ports. These can be straight intake ports, as already mentioned, or a reverse-flow-intake style. The fluid is then boosted to the vortex generator by the positive-displacement inducer. The vortex generator is generally an axial-type impeller. It imparts a high-velocity rotation to the fluid. This causes the heavier fluids (liquids) to be slung to the outer area of the flow passageway and the lighter fluids (free-gas laden) to mingle around the inner area and the shaft. The fluid then enters a stationary flow-crossover piece. The crossover has an outer annular passageway that takes the heavier-density fluids that enter it and directs them to the entrance of the pump. The lighter-density fluid that enters the inner annular passageway of the crossover is directed to the separator vents, where it exits to the casing annulus and flows up the wellbore.
The second device is a rotary centrifuge-type separator and is shown on the left in Fig. 2. It is similar in design to the vortex style, but it has a rotating chamber instead of the vortex generator. The chamber has several radial blades that are enclosed by an outer shroud or shell. The fluid that enters the chamber is centrifuged at very high g forces over the length of the chamber. Upon exiting the chamber, the fluid enters the flow crossover and follows the same processing as already described in the vortex style.
Flanged connection to seal-chamber section
The bottom flange of the pump bolts to the flange of the seal-chamber-section head. It maintains axial alignment of the shafts of the two units. It also allows the floating pump shaft to engage the end of the seal-chamber-section shaft so that the axial thrust produced by the pump is transferred to the thrust bearing in the seal-chamber section.
The stages of the pump are the components that impart a pressure rise to the fluid. The stage is made up of a rotating impeller and stationary diffuser. The stages are stacked in series to incrementally increase the pressure to that calculated for the desired flow rate. A graphic of the fluid flow path is illustrated in Fig. 3. The fluid flows into the impeller eye area and energy, in the form of velocity, is imparted to it as it is centrifuged radially outward in the impeller passageway. Once it exits the impeller, the fluid makes a turn and enters the diffuser passageway. As it passes through this passageway, the fluid is diffused, or the velocity is converted to a pressure. It then repeats the process upon entering the next impeller and diffuser set. This process continues until the fluid passes through all stages, and the design discharge pressure is reached. This pressure rise is often referred to as the total developed head (TDH) of the pump.
There are two styles of stages for the range of flow rates in which ESPs operate. The first is a radial stage. The impeller is shown in Fig. 4 and the diffuser in Fig 5. Its geometry has the flow entering the impeller or diffuser parallel to the axis of the shaft and exiting perpendicular to the shaft, or in a "radial" direction. They are sometimes referred to as "pancake" or "mushroom" stages, respectively, because of the impellers’ flat shape and the diffusers’ mushroom-shaped downthrust pedestal. A cross-sectional schematic of a radial stage is shown in Fig. 6.
The second is a mixed-flow stage; a typical impeller is shown in Fig. 7, and the diffuser is shown in Fig. 8. Its geometry has the flow exiting the impeller at an angle less than 90° to the shaft. A graphic of this flow path is shown in Fig. 9. Generally, this angle changes from near perpendicular to near axial, as the design flow rate of the stage increases for a particular-diameter unit. This relationship is shown in Fig. 10 .
A key feature for both styles of stages is the method by which they carry their produced axial thrust. Usually, the pumps that are under a 6-in. diameter are built as "floater" stages. On these, the impellers are allowed to move axially on the pump shaft between the diffusers. Contrary to the name given to this configuration, the impellers never truly float. They typically run in a downthrust position, and at high flow rates, they may move into upthrust. To carry this thrust, each impeller has synthetic pads or washers that are mounted to the lower and upper surfaces, as shown in the previous figures. These washers transfer the thrust load from the impeller through a liquid film to the smooth thrust pad of the stationary diffuser.
Three forces are involved in determining whether the impeller runs in downthrust or upthrust. The first is the downward force, and it is a result of a portion of the impeller discharge pressure acting on the area of the top impeller shroud. Two forces act in the upward direction. One is a result of a portion of the impeller discharge pressure acting against the bottom shroud of the impeller. The second is the force produced by the momentum of the fluid making its turn in the impeller passageway. A graphic description of the thrust forces on an impeller is shown in Fig. 11. Because the shaft is allowed to move axially and positions itself by contact with the seal-chamber section shaft, the fluid pressure causes a thrust load through the shaft to the seal thrust bearing. The thrust is the result of the force on the top end of the shaft (discharge pressure multiplied by the end area of the shaft) minus the force on the bottom end of the shaft (intake pressure multiplied by the end area of the shaft).
Fig. 11-Thrust forces on impellers(after Centrilift).
On 6-in. and larger pumps and on specially built smaller pumps, the impellers are usually fixed or locked to the shaft. These pumps are referred to as "fixed impeller" or "compression" pumps. In this configuration, all the thrust is transferred to the shaft and not to the diffuser. Therefore, the seal thrust bearing carries the load of all the impellers plus the shaft thrust. Particular care should be exercised in selecting the proper seal thrust bearing to match the fixed impeller pump conditions because these loads can be very high.
To maintain the optimum flow-path alignment between the impeller and its diffuser, the impeller is designed to maintain a downthrust position through its operating range. Usually, the impeller does not transfer into upthrust until its operating point is to the right of its maximum recommended point. Stage-specific thrust characteristics should be available from the manufacturers.
The manufacturers state the performance of their pump stages on the basis one stage, 1.0 specific gravity (SG) water at 60- or 50-Hz power. A typical performance curve for a 4-in.-diameter radial-style pump, with a nominal best-efficiency performance flow of 650 B/D, is shown in Fig. 12. A mixed-flow style with a nominal flow rate of 6,000 B/D is shown in Fig. 13. In these graphs, the head, brake horsepower (BHP), and efficiency of the stage are plotted against flow rate on the x -axis. Head, flow rate, and BHP are based on test data, and efficiency is calculated on the basis of
where Q is given in gal/min, TDH is given in ft, and C = 3,960; or Q is given in m3/d, TDH = m, and C = 6,750.
The head/flow curve shows the head or lift, measured in feet or meters, which can be produced by one stage. Because head is independent of the fluid SG, the pump produces the same head on all fluids, except those that are viscous or have free gas entrained. If the lift is presented in terms of pressure, there will be a specific curve for each fluid, dependent upon its SG.
The dark (highlighted) area on the curve is the manufacturers recommended "operating range." It shows the range in which the pump can be reliably operated. The left edge of the area is the minimum operating point, and the right edge is the maximum operating point. The best efficiency point (BEP) is between these two points, and it is where the efficiency curve peaks. The shape of the head/flow curve and the thrust characteristic curve of that particular stage determines the minimum and maximum points. The minimum point is usually located where the head curve is still rising, prior to its flattening or dropping off and at an acceptable downthrust value for the thrust washer load-carrying capabilities. The location of the maximum point is based on maintaining the impeller at a performance balance based on consideration of the thrust value, head produced, and acceptable efficiency.
API RP11S2 covers the acceptance testing of ESP pumps.  It also recommends the performance tolerance limits and describes the test procedure. One should pay particular attention to the method of calculating the acceptable limits of the head/flow curve. A good layman’s description of the method is given in Lund. The limit is calculated by a combination of ± 5% head and ± 5% flow.
Several parameters are used to relate the characteristics of stages of different size, under dynamically similar conditions. They show that head (H) is a function of diameter (D) to the second power and also of rotating speed (N) to the second power. Flow (Q) is a function of diameter to the third power and also a direct function of rotating speed.
The BHP curve shows the power required to drive the stage. The power is lowest at shutoff or zero flow and increases with flow. The HP also follows the relationship that is given in Eq. 4 for different-sized pumps under dynamically similar conditions.
Another performance-altering technique is to reduce the diameter of an impeller by trimming or cutting back its outside diameter. When this is done, the head, flow, and power are changed by the relationships shown in Eqs. 5 through 7.
For any particular-diameter-pump series, there is generally an overlap region between the radial and mixed-flow styles. A typical relationship of a family of similar-diameter stages is shown in Fig. 14. Notice that each style increases in efficiency as the flow rate increases, until the efficiency peaks and begins dropping off.
|C||=||constant = 3,960, where Q is in gal/min, and TDH is in ft [= 6,750, where Q is in m3/D, and TDH is in m]|
|D||=||diameter, in. [cm]|
|H||=||head, ft [m]|
|Q||=||flow rate, B/D [m3/d]|
|N||=||rotating speed, rev/min|
- API RP 11S1, Recommended Practice for Electrical Submersible Pump Teardown Report, third edition. 1997. Washington, DC: API.
- Electrical Submersible Pumps and Equipment, 11. 2001. Claremore, Oklahoma: Centrilift.
- API RP 11S2, Recommended Practice for Electrical Submersible Pump Testing, second edition. 1997. Washington, DC: API.
- Lund, R. 1983. Acceptance Tests for Mixed Flow, Axial and Centrifugal Pumps. World Oil 206: 398.
Noteworthy papers in OnePetro
Camilleri, L. A. P., Brunet, L., & Segui, E. (2011, January 1). Poseidon Gas Handling Technology: A Case Study of Three ESP Wells in the Congo. Society of Petroleum Engineers. doi:10.2118/141668-MS
King, D. G., Traylor, F. T., & Stewart, R. E. (1983, January 1). Abrasion Technology for Electric Submergible Pumps. Society of Petroleum Engineers. doi:10.2118/12199-MS
Pessoa, R., & Prado, M. (2003, February 1). Two-Phase Flow Performance for Electrical Submersible Pump Stages. Society of Petroleum Engineers. doi:10.2118/81910-PA
Takács G. (2009): Electrical submersible pumps manual. ISBN 978-1-85617-557-9. Gulf Professional Publishing, An Imprint of Elsevier, 440p.
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Jose Caridad, BSME & MSc ME