Protector

The Protector has four primary functions:

1. Couples the torque developed in the motor to the pump via the protector shaft.
2. Prevents entry of well fluid into the motor.
3. Provides pressure equalization.
4. Houses the bearing to carry the thrust developed by the pump.

Keep Out Well Fluids :

One of the main functions of the Protector is to keep well fluids out of the motor where they could potentially do harm.

Some types of water well motors are actually open to the well fluid and no effort is made to keep it out.

In the oil well environment, the produced fluids are much more aggressive than typically found in water environments and, if allowed to contact the inside of the motor, may cause a premature failure.

Protector Chambers ：

Labyrinths, Bags, Metal Bellows.

Chambers can be connected in series or parallel.

Every series can have different section combination.

Labyrinth Protectors :

 One such Protector type is the "Labyrinth" type.  

The labyrinth design 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. 

There will be cases where a labyrinth will simply not work.  In cases where the well fluid is  lighter gravity than the motor oil (i.e. about 0.85), the motor oil will go to the bottom of the chamber  rather than the top causing the motor fluid to be displaced by well fluid pretty quickly.

If the well fluid is about the same gravity as the motor fluid or lighter, this type of Protector should not be used.

Labyrinth Protectors also will not work in horizontal or highly deviated wells.  The gravity separation design requires the unit to be mostly upright. This Protector can operate in some amount of deviation but the expansion volume is somewhat reduced.

Many times the labyrinth protector is "checked" in the field to determine if water got into the bottom of the unit.  If this is done,always do it before the unit is laid down on its side. Once on its side, water initially in the bottom of the chamber may redistribute itself since the U-tube restriction is effectively removed.

Advantages :

• Excellent separation in vertical wells with high WC.
• Easily serviceable and re-usable
• Inexpensive. 

Disadvantages :

• Deviations over 45° are questionable;
• Deviations over 70° render the labyrinth practically useless.
• Oil well fluid density problems depend on the type of motor oil, but typically anything lighter than 0.85SG (35 °API) is dangerous

Positive Seal Protectors :

For applications where the well fluid and motor oil gravities are similar or where a well is highly deviated, a different Protector design uses a "positive seal" or "bag" to physically separate the two fluids.

This is similar to the "bellows" found in some water well motors except that the bag has a much greater capacity for expansion and contraction than a typical bellows.

The bag is made of a high temperature / high performance elastomer which can withstand the harsh Downhole environments typically encountered.  The bag keeps the well fluid on the outside and the clean motor oil on the inside.

When the motor oil expands or contracts, the bag simply flexes to accommodate the necessary volume change.

Advantages :

• High Temp / High Performance Elastomer;
• Accommodate volume changes

Disadvantages :

• Limited operating temperature as rubber 
• Aflas bags are rated to only 400 degF
• Rubber bags are exposed to chemical attacks
• Gas migration through rubber bags
• Hardening and loss of elastomer flexibility

Thrust Handling :

Another function of the Protector is to absorb the thrust generated by the pump whether it is upthrust or downthrust.  The Protector thrust bearing gives a very large surface over which to absorb thrust.  In addition, it does so in a nice clean oil environment which greatly prolongs the ESP unit life.

Care should always be taken to select a Protector series which has a thrust bearing rating greater than the maximum thrust the pump will generate.

When sizing a Protector for an application, it is sometimes necessary to consider the power used by the Protector when sizing the motor.  

In larger diameter / higher thrust load applications, failure to do so could result in an overloaded motor.

For smaller diameter and/or lower load conditions, the additional power consumption is usually insignificant.

Motor Torque :

One other function which the Protector carries out is transmission of the motor torque to the pump since it is physically located between the two. Although this may seem a little trivial, in the selection process 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.

Pressure Equalization :

The ESP motor, unlike a surface motor, undergoes very wide swings in pressure.  

When the unit is first installed in the well it goes from atmospheric pressure up to the very     high bottom hole pressure of the well.

When the unit is operated, internal heating causes the motor oil to expand which would add even more pressure if it did not have some place to go.  If the unit is stopped and allowed to cool down, the oil will contract which would create a vacuum in a sealed system.

The Protector simply keeps the pressure on the inside of the unit the same as the pressure on the outside of the unit regardless of what the external pressure is.

Since the bottom of the Protector is open to the motor, the motor pressure remains the same as that in the well annulus around it.

By maintaining a negligible pressure difference, there is no tendency for the well fluid to penetrate into the motor.

Connection Types :

A protector will always have multiple chambers connected together.

• Connected in series (designated with an “S”), redundant seal
• Connected in parallel (designated with a “P”), larger capacity

Series Configuration :

A series connection is simply one in-line after the other just like series resistors in an electrical circuit.  

For well fluid to enter the lower chamber, it must completely displace all the oil in the upper chamber first.

So having two bags in series means the lower bag is a redundant seal to the upper bag and is there as "insurance" in case the upper bag fails for some reason.

There is no additional expansion capacity in "series" systems.

Parallel Configuration :

Two bags in parallel, on the other hand, do double the available expansion volume.  This is useful in higher horsepower applications (larger motors hold more oil) where the natural expansion and contraction might exceed the capacity of one bag.

If only one bag is used and the capacity is exceeded, the unit will allow well fluid penetration so two bags eliminates this possibility.

Protector Design :

The previous illustration showed some of the possible combinations.  Regardless of what configuration is chosen, the Protector function is essentially the same.

The naming convention for modular Protectors is simple and easily shows how a protector is designed.

In naming, always start from the top and move downward.