Understanding the Motor Seal in Submersible Pumps

Submersible pumping systems, often found operating thousands of feet below the surface in challenging wellbore environments, rely on a series of components working in harmony to ensure efficient and reliable operation. Among these critical elements, the motor seal, also known by various other names such as the equalizer, balance chamber, or protector, plays an indispensable role. This unassuming yet vital section is strategically installed between the intake and the electric motor, acting as a crucial barrier and facilitator for the entire system's longevity and performance. Its primary purpose extends beyond simply transferring the motor's torque to the pump shaft; it undertakes a multifaceted role that is essential for the protection and optimal functioning of the submersible motor.

The Multifaceted Functions of a Motor Seal

The motor seal section is far more than a simple intermediary. It is engineered to perform several primary functions, each contributing significantly to the overall integrity of the submersible pumping unit:

• Equalisation: One of its key responsibilities is to equalise the pressure within the motor with the ambient pressure of the wellbore. This equalisation is paramount as it significantly reduces the pressure differential across the motor's shaft seals and the pothead connection, thereby mitigating the risk of leakage and ingress of well fluids.
• Expansion Volume: The motor's operational temperature can fluctuate, leading to the expansion and contraction of the motor oil. The seal section provides a dedicated space, or expansion volume, to accommodate these changes in oil volume, preventing over-pressurisation or vacuum conditions within the motor housing.
• Isolation: In the harsh environment of a wellbore, the fluids present can be highly corrosive or abrasive. The seal section acts as a robust barrier, isolating the clean, specialised motor oil from the potentially damaging well fluids, thus preserving the lubrication and cooling properties of the motor oil.
• Thrust Load Absorption: For fixed impeller type pumps, the seal section houses the crucial thrust bearing. This bearing is designed to carry the significant axial thrust loads generated by the pump's operation. This thrust can be either 'upthrust' or 'downthrust,' depending on the specific pumping conditions, and the bearing's ability to manage it is vital to prevent damage to the pump and motor shafts.

The importance of effective pressure equalisation cannot be overstated. When a submersible motor and pump are thousands of feet submerged, the external wellbore pressure can be immense. By equalising this pressure with the internal motor pressure, the seal section ensures that the delicate shaft seals and electrical connections (pothead) are not subjected to extreme pressure differentials, which could lead to catastrophic failure.

Key Components of a Motor Seal Section

To effectively perform its diverse functions, the motor seal section is comprised of several key components, each with a specific role:

Mechanical Seals

Often referred to as the 'heart of the seal section,' mechanical seals are the primary defence against well fluid migration. They are meticulously designed to prevent any downhole fluid from travelling along the shaft and entering the motor's internal chambers. The integrity of these seals is paramount for maintaining the purity of the motor oil.

Bag(s)

These flexible, elastomeric bags serve a dual purpose: providing the necessary expansion volume for motor oil and acting as a physical barrier for isolation. Their ability to expand and contract with changes in motor oil volume is crucial for maintaining stable internal pressures.

Labyrinth Chamber(s)

In certain seal designs, labyrinth chambers are incorporated. These chambers create a convoluted, or tortuous, path that well fluids must navigate to reach the motor oil. This design leverages the difference in specific gravity between the well fluid and the motor oil to achieve a degree of separation, particularly effective in vertical wells.

Thrust Bearing

As mentioned, for fixed impeller pumps, the thrust bearing is a critical component housed within the seal section. Its primary function is to absorb the axial thrust generated by the pump. However, this bearing is sensitive to several factors that can lead to premature failure, including excessive thrust loads, operational vibration, shaft misalignment, and the presence of foreign materials or fluids like water.

Types of Motor Seal Sections

The design and application of motor seals vary, leading to two primary types of seal sections:

1. Labyrinth Type Protector

The labyrinth type protector operates on a principle of fluid separation without direct physical contact between sealing elements. It utilises the difference in specific gravity between the well fluid and the motor oil. The protector oil typically has a specific gravity of around 0.85. As long as the wellbore fluid's specific gravity is higher, the tortuous path created by the labyrinth design effectively prevents well fluids from mixing with the motor oil.

Limitations of Labyrinth Protectors:

While effective in certain conditions, labyrinth protectors have significant limitations:

• Wellbore Deviation: They are generally not recommended for deviated wells. Deviations exceeding 40 degrees from vertical can compromise their effectiveness, with some manufacturers suggesting that deviations as low as 20 degrees render them questionable.
• Frequent Start-ups: In wells with frequent start-stop cycles (intermittent production), the well and motor fluids come into direct contact during each shutdown. As the motor cools and its oil contracts, a small amount of mixing can occur. Over many cycles, especially in deviated wells, well fluids can eventually penetrate to the motor.
• Fluid Specific Gravity: Labyrinth protectors are also ineffective when the well fluid has a lighter specific gravity than the motor oil.

2. Bag Type Protector

The bag type seal offers a more robust and versatile solution by creating a physical separation between well fluids and motor oil. This is achieved through the use of elastomeric bags, which are made from rubber-like materials capable of stretching and returning to their original shape. These bags act as a positive, impermeable barrier. The bag expands and contracts to accommodate the motor's oil volume requirements, and a check valve is typically incorporated to release internal pressure at around 3-5 psi.

Advantages and Considerations for Bag Type Protectors:

• Versatility: Unlike labyrinth protectors, bag type seals can be installed in wells with any deviation, including horizontal wells, offering significant flexibility in application.
• Material Selection: The durability of the elastomeric bag is critical and depends heavily on careful selection of the material to match the well's temperature and fluid properties. Harsh well chemicals, such as those encountered during acidising jobs, can lead to premature seal failure.
• Environmental Resistance: The longevity of the bag material is directly influenced by the temperature and chemical environment it is exposed to downhole.

Tandem and Dual Seal Configurations

In many applications, particularly for larger motors or those in particularly demanding environments, it is common practice to run tandem or dual seal sections. These configurations can include:

• Labyrinth-Labyrinth
• Labyrinth-Bag
• Bag-Labyrinth
• Bag-Bag

Running multiple seal sections increases the number of mechanical seals, thereby enhancing shaft isolation and providing an additional layer of protection. Furthermore, for motors with substantial oil expansion requirements, seal sections featuring two top chambers with parallel expansion bags are employed to provide the necessary volume capacity.

Operational Considerations

It is important to note that the seal sections are filled with the same specialised oil used in the submersible motors. While the seal section itself requires a small amount of horsepower to operate, this requirement is generally so negligible that it can be disregarded when sizing the motor for a specific pumping application.

Protecting Your Investment

The motor seal is an often-overlooked but fundamentally critical component in the reliable operation of submersible pumping systems. Understanding its functions, the types available, and the factors affecting its performance is essential for proper selection, installation, and maintenance. By ensuring the correct type of seal is chosen for the specific well conditions and that it is properly maintained, operators can significantly extend the life of their submersible motors and pumps, minimise downtime, and maximise production efficiency.

Frequently Asked Questions

What happens if the motor seal fails?

Seal failure can lead to well fluids entering the motor, contaminating the motor oil, potentially causing overheating, bearing damage, and electrical insulation breakdown. This can result in premature motor failure and costly downtime.

Can a labyrinth seal be used in any well?

No, labyrinth seals are not suitable for highly deviated wells or wells with frequent start-stop cycles. Their effectiveness is also dependent on the specific gravity of the well fluid relative to the motor oil.

What is the most common type of seal used today?

Bag type seals are generally considered more versatile and reliable for a wider range of well conditions, especially in deviated or challenging environments.

How much horsepower does a motor seal use?

The horsepower requirement for a motor seal is very small and is typically considered negligible when sizing the main motor for the pumping application.

What are the main enemies of the thrust bearing in a seal section?

The primary enemies are excessive thrust loads, vibration, shaft misalignment, and contamination from foreign materials or fluids like water.

Can I use a different type of oil in the seal section?

No, the seal sections must be filled with the same specialised oil used in the motors to ensure compatibility and proper functioning.