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Difference between revisions of "Seal chamber section"
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==Noteworthy papers in OnePetro==
==Noteworthy papers in OnePetro==
Revision as of 14:50, 2 April 2014
The component located below the lowest pump section and directly above the motor, in a standard electrical submersible pump (ESP) configuration, is the seal-chamber section. API RP 11S7 gives a detailed description of the design and functioning of typical seal-chamber sections.  The content on this page repeats some of this information, but it is also intended to supplement the information contained in API RP 11S7. The seal-chamber section is basically a set of protection chambers connected in series or, in some special cases, in parallel.
Function of seal-chamber section
This component has several functions that are critical to the operation and run-life of the ESP system, and the motor in particular.
- It protects the motor oil from contamination by the wellbore fluid. The motor is filled with a high-dielectric mineral or synthetic oil for electrical protection and lubrication. Well fluid migrating into the motor can potentially cause a premature electrical or mechanical failure through the reduction of the motor dielectric or lubricating properties.
- It allows for pressure equalization between the interior of the motor and the wellbore. Its design allows for a breathing or equalization method that compensates for pressure variances caused by the submergence pressure encountered during the installation from surface pressure to downhole static pressure and the thermal expansion and contraction of the motor oil because of motor heat rise during operation.
- It also absorbs the axial thrust produced by the pump and dissipates the heat that the thrust bearing generates.
Fig. 1 shows the seal-chamber section of the ESP unit and its component parts.
Fig. 1-ESP seal-chamber section (after RP 11S1).
Usually, there are several shaft options available, and their selection is based on the fluid environment and the horsepower (HP) to be transmitted. Even though a majority of the shaft is exposed only to the clean, dielectric motor oil, the top end is exposed to the wellbore fluid. Therefore, the material must be an alloy that protects the integrity and function of the shaft. This could be the entire shaft or, at a minimum, the top section that is directly exposed to the wellbore fluid.
Labyrinth protection chambers
This chamber design features a direct fluid interface between the wellbore fluid and the motor oil. A typical design layout is shown in Fig. 2. It is commonly referred to as a "labyrinth"- or "U-tube"-style chamber. It is configured to have several concentric annular volumes that form a U-tube-type communication path for fluids coming in the top of the chamber to travel through to get to the exit point at the base of the chamber. This flow path is shown schematically in Fig. 3. In many mild applications, it is a very effective protection design. There are several application weaknesses that need to be considered. First, there is a direct fluid interface between the motor oil and the wellbore fluid in the top chamber. This allows the motor oil to be slowly wetted through a wicking action of the wellbore fluid, thereby, slowly degrading the dielectric strength of the motor oil. In some applications, high-density blocking fluids are used to retard or eliminate this motor oil. Second, gasses can permeate into the motor oil causing potential corrosion problems or burping and excessive loss of motor oil if there is a sudden decompression. Third, the labyrinth’s effective volume decreases as the chamber is inclined. Therefore, they are not generally recommended at deviations greater than 30° from vertical.
Positive-barrier protection chambers
This chamber incorporates a positive barrier between the wellbore fluid and the motor oil. The barrier is usually an elastomeric or rubber bag, which is also called a bladder. A typical design layout is shown in Fig. 4. The bag or bladder forms a seal between the motor oil inside the bag and the wellbore fluid between the bag and seal-chamber section’s housing. It also allows for pressure compensation by expanding and contracting in this annular area. The motor oil flow path is shown in Fig. 5. The barrier-style chamber is recommended for deviated-well applications. The bladder material should be resistant to the well fluids and any injected chemicals.
Fig. 4-Positive-barrier protection chamber.
Fig. 5-Positive-barrier-protection-chamber flow path.
Mechanical face seals
A rotating mechanical face seal is generally located at the top of each protection chamber. A typical design is shown in Fig. 6. The rotating part of the face seal is sealed to the shaft by elastomeric bellows. The stationary part is sealed into the stationary component of the seal-chamber section. A spring preload force then keeps the rotating and stationary seal faces in contact. Once the unit starts rotating, a hydrodynamic fluid film is developed on the face. This film then carries the load, prevents wellbore fluid from crossing the face by the pressure-differential setup, and cools the loaded face.
Fig. 6-Typical shaft mechanical face seal.
Axial thrust bearing
This bearing carries all of the axial thrust produced by the pump and seal-chamber section. Generally, sliding-shoe hydrodynamic types are used for this application because of their robustness and ability to function totally immersed in lubricating fluid. It is composed of two main components: a stationary pad and a rotating flat disk. The stationary part has pads finished to a very close flatness tolerance, connected to a base by a thin pedestal or flexible joint. The rotating disk is also finished to a very close flatness tolerance. Several different bearing designs are shown in Fig. 7. They represent standard-style cast bearings for normal applications and machined bearings for intermediate- and high-load applications.
When selecting the style and options of a seal-chamber section for an application, the user must consider the shaft torque, thrust-bearing load, volumetric motor oil expansion required, and the wellbore-fluid environment to which it will be subjected.
The shaft has to transmit, from the motor to the pump, the entire torque required by the equipment for its application. This not only includes the stabilized running torque but also the short-term torque spikes caused by unit startup and intermittent pump loads. Because the diameter of the shaft is constrained because of the maximum diameter of the unit, materials of differing mechanical properties must be used to provide different load capabilities. These materials must also provide protection from corrosive wellbore fluids.
The thrust-bearing performance is a function of the load that is transferred to it and the viscosity of its lubricating oil. The load transmitted from the pump can be calculated on the basis of the pump geometry and the total developed head (TDH) produced for the application. For "floater" pumps, the shaft load is always down and is equal to the cross-sectional area of the top of the shaft multiplied by the discharge pressure of the pump (Pdischarge) minus the cross-sectional area of the bottom of the shaft multiplied by the pump intake pressure (PIP). For "fixed" impeller pumps, the load is equal to the shaft force, as just calculated, plus the summation of all the impeller thrust forces. The impeller thrust forces can be roughly calculated, as previously described in the pump-stage section, or obtained from the pump manufacturer.
The hydrodynamic thrust bearing depends on developing and maintaining a fluid film between the stationary pads and the rotating disk. This fluid film actually carries the load, not the running of the disk against the pads. In fact, if contact is made between the two components, heat is generated and rubbing can become severe enough to start bearing failure, even seizure. To maintain the proper film thickness, both the viscosity of the lubricating oil and the operating temperature of the thrust bearing are critical. Most manufacturers provide a range of lubricating oils, so the proper viscosity range can be provided at the estimated operating downhole temperature.
The seal-chamber section also adds HP load to the motor. It is usually a low value and significant only on lower-HP applications. Because each style of seal-chamber section has its own characteristics, the manufacturer should be consulted for these values.
The seal-chamber section also has to handle the volumetric expansion and contraction of the motor oil. This volume includes everything from the top of the seal-chamber section to the bottom of the motor. This expansion and contraction is a result of the changing temperatures and pressures the unit undergoes during operation. During installation, the unit goes from surface ambient conditions to wellbore setting-depth conditions. The impact of the increase in pressure does not have a significant impact on the volume occupied by the motor oil, as long as the unit is vented of air properly during filling. The temperature, on the other hand, causes the volume to change significantly. As the motor oil heats up during installation, it expands, and the volume that cannot be contained in the seal-chamber section, whether labyrinth or bag style, is vented from the top chamber into the wellbore annulus. When the ESP is started, it undergoes further temperature rise until it reaches its stabilized operating point. During this stabilization, it continues to vent any expanding volume of motor oil. Once it reaches a stabilized operation, the venting stops and the seal-chamber section and motor run at almost equal pressure with the wellbore. The next significant event is when the ESP shuts down. At this point, the motor oil temperature starts dropping from the operating temperature back down to the wellbore ambient. The pressure also increases from wellbore flowing to static. Once again, the temperature change has the largest impact and, in this case, on fluid contraction.
On a labyrinth style, well fluid is pulled back into the first chamber as the motor oil contracts back along its communication paths (Fig. 3). As long as the contraction volume does not exceed the volume of the first chamber, well fluid is contained in the first chamber. If the fluid contraction exceeds the chamber volume, well fluid is drawn into the second or lower chamber. With multiple thermal cycles, the well fluid can slowly be drawn towards the seal-chamber section thrust bearing and motor where it can be fatal or, at least, reduce the total run-life of the equipment. Because of the method of breathing, the labyrinth style is not recommended for well deviations greater than 30°. When the labyrinth chamber is tilted or inclined, the effective length of the labyrinth or U-tube communication path is shortened, effectively reducing the volume of the chamber.
The bag style, with its positive barrier, maintains a physical separation of the motor oil and wellbore fluid during expansion and contraction. Upon contraction, the cylindrical bag collapses around the center of the chamber, absorbing the contraction. Then, on a recycle or temperature increase, the bag expands to its original position before any motor oil venting is allowed. A more detailed explanation of these processes is found in the API RP11S7 document. 
In recent years, many operators have begun to run multiple seal-chamber sections in series. This gains additional chambers or more protection between the well-fluid entry point and the motor. While this is true, it must be balanced with the fact that more motor oil fluid volume is also being added. More motor oil volume means more expansion and contraction. Because the first chamber volume is fixed, there is a better chance of operating over the capacity of this chamber. Therefore, the selection of which style seal-chamber section to use and how many to run is dependent upon the application. The proper selection is to choose the one in which the operational expansion cycle uses only a portion of the first chamber. In some very severe applications, seal-chamber sections with the first two bag chambers communicated in parallel instead of series have been used in an effort to handle the wellbore-fluid contraction volume.
- API RP 11S7, Recommended Practice for Application and Testing of Electrical Submersible Pump Seal Chamber Sections, second edition. 1999. Washington, DC: API.
- Bearden, J. and James, M. 1988. ESP Seal Assembly Design and Application. Paper presented at the 1988 SPE Gulf Coast ESP Workshop, Houston, 28–29 April.
- API RP 11S1, Recommended Practice for Electrical Submersible Pump Teardown Report, third edition. Washington, DC: API.
Noteworthy papers in OnePetro
Guerra, W., Moreno, L. F., Cuellar, H., Salazar, J. I., Pirela, J. J., Urdaneta, B., & Sarkis, N. (2013, May 21). Implementation of New Seal Design To Improve ESP Performance in Corcel Field. Society of Petroleum Engineers. doi:10.2118/165024-MS
Young, J., Kappelhoff, G. H., & Watson, A. (2003, January 1). ESP Run Life Improvement in Harsh Elastomer Environments, the Moomba Field. Society of Petroleum Engineers. doi:10.2118/80526-MS