The Motor seal is installed below the intake and above the electric motor. It is also named: Equalizer, balance chamber, or Protector. Seal section types, functions, components, and applications are detailed in this article.
In addition to the main function of transferring the motor torque to the pump shaft, the seal section performs four primary functions (Equalization, Expansion, Isolation, & “Aabsorbsion”):
- Equalizes the pressure in the wellbore with the pressure inside the motor,
- Provides area for motor oil expansion volume (induced by temperature changes in the motor),
- Isolates the well fluid from the clean motor oil,
- Absorbs the pump shaft thrust load (it houses the thrust bearing that carries the axial thrust developed by the pump, it can either be upthrust or downthrust, depending on the pumping conditions – obviously, for fixed impeller type only).
PS: The motor, pump and seal are often submerged below several thousand feet of fluid. The seal section allows the pressure in the motor and the annulus to equalize, so that there is very little pressure across the shaft seals or the pothead connection.
PS: When selecting the protector, we need to be certain that the protector shaft is capable of delivering the full torque required without exceeding its yield strength which could result in a broken shaft.
The main components of protectors are:
- Mechanical Seals: prevent the well fluid from traveling down along the shaft, and into the next chamber. It is for this very reason the mechanical seal is often called the heart of the seal section.
- Bag(s): provides expansion volume and isolation for clean motor oil,
- Labyrinth Chamber(s): provides expansion and isolation volume in vertical or near vertical wells,
- Thrust Bearing: carries the thrust load of the pump shaft and stages (fixed impeller type only). The biggest enemies of the thrust bearing are excessive thrust, vibration, misalignment and foreign material or fluids (water).
There are two different types of seal sections:
1. Labyrinth type protector:
Labyrinth type seal uses the difference in specific gravity of the well fluid and the motor oil to keep them apart even though they are in direct contact. Labyrinth-style protector creates a tortuous path of protection from well ﬂuids.
The protector oil specific gravity is around 0,85. As long as well bore SG is heavier, labyrinth path work to isolate motor fluid from well bore.
Limitations of Labyrinth Protectors:
Labyrinth type protectors are not recommended to be installed in a deviated or cycling well:
- Ineffective in well bore deviations > 40 degrees from vertical, (some Western ESP manufacturers suggest it is not worth installing them even in wells with a deviation of 20 degrees),
- Excessive cycling / frequent start-ups: well and motor fluids are in direct contact. Each time the motor is stopped its oil cools, contracts, and a small amount of mixing occurs. If there are many start-ups (e.g. intermittent producers) well fluids can eventually reach the motor, especially in deviated wells.
- Also, when the well fluid is lighter gravity than the motor oil (i.e. about 0.85)
2. Bag type protector:
Bag type seal is designed to physically separate the well fluid and motor oil by the mean of elastomeric bags that function as a positive barrier. By definition, an elastomer is a rubber-like material which can stretch under low stress and return to its original shape when the stress is removed. Elastomer bag creates a positive barrier to well bore. Bag expands and contracts to meet motor demand. As shown by the Figure below, there is a check valve which opens at 3-5 psi to release pressures inside the bag.
Elastomer must be selected carefully to match well temperature and fluid properties. Unlike Labyrinth type seal, bag type can be installed at any well deviation (even in horizontal wells). It offers a great deal of flexibility and is useful in a wide variety of applications.
One area where care needs to be taken with this type of seal is with harsh well chemicals, as seal failure may be experienced (a bullheading acidizing job for example). The durability of the bag material depends on the temperature and the environment to which it is subjected.
The following video shows an excessive bag wear due to acid (results of the Dismantle Inspection and Failure Analysis). In the same video, a brand new bag is presented, so you can note the difference!
NB: It is common practice to run tandem or dual seal sections and the configuration can be labyrinth-labyrinth, labyrinth-bag, bag-labyrinth, or bag-bag. The more seal sections the more mechanical seals, which increases shaft isolation. Furthermore, The expansion volume in large motors may exceed that of the top chamber of a seal section. Therefore, seal sections with the top two chambers having expansion bags in parallel are used to provide the necessary volume.
NB: The seal sections are filled with the same oil used in the motors.
NB: The seal requires a small amount of horsepower. Generally, the horsepower requirements are small enough to be neglected when sizing the motor to a pumping application.