Application Guide for HDESP Submersible Pumps

HDESP pumps offer the operator significant operating advantages for lifting fluids from oil and gas wells.  This particular type of pump is used extensively in the toughest applications ranging from mining to oil and gas.  As with all pump technologies, HDESP offers advantages and has limitations that must be considered when applying it to the field.

There are some situations that are not appropriate for HDESPs, and the operation of these pumps outside of the limits described in this application guide will result in pump failure.  Failures of this type are not caused by defects in the design, materials or workmanship, but are the result of using the pump in environments or situations for which it is not designed.  We are constantly improving our pumps, and are developing new pumps that better address some common environments for which HDESPs are not appropriate.  SmithLift warrants pumps to be free from defects in design, materials and workmanship and will replace defective pumps free of charge within the scope of the limited warranty (see attached warranty statement).  On the other hand, SmithLift does not warrant pumps that have been installed and/or operated outside of the conditions described in this application guide.  Please review this application guide carefully and consider your application to determine if HDESPs are appropriate.  Experienced field engineers are available to consult free of charge on your application. 

The biggest limitation of any downhole pump is the generation and transportation of power from the surface to the downhole pump.  For the Electrical Submersible Pump (ESP) this transportation is accomplished by transferring electrical energy through an insulated cable from the surface to the pump motor.  This method of energy transfer is extremely efficient and convenient, but electricity has the potential to damage or destroy downhole pumps and motors. 

The most common problems with electrical systems are 1) The generation of heat in the cable and the motor 2) Maintaining electrical insulation in the cable and the motor in the well environment and 3) Maintaining a constant, high quality supply of electrical power.  Failure to consider these problems in the application will lead to premature motor and/or pump failure in any type of submersible pump, including HDESP pumps.

Thermal Problems 

ESPs generate heat as a result of the passage of electrical current through conductors in the cable and the motor.  This heat creates a temperature rise in motor and the cable that will continue until the amount of heat generated comes to equilibrium with the amount of heat dissipated.  The temperature rise in the motor and the cable will depend on the size of the motor, the current draw of the motor, ambient temperature, the type of fluid in which the motor is submerged, the geometry of the well, and the flow of fluids through the geometry.  Every ESP has a temperature limit at which the insulation systems, bearings and other systems will fail.  Once this limit is reached, the failure will cascade due to additional heat that is generated by the failure of insulation and bearings.  The life of the pump is directly related to the temperature at which the pump is operated.  The higher the temperature, the shorter the life. 

To assure the pump operates at the lowest temperature possible, a flow of fluid should be maintained past the pump to cool the motor.  Locating the motor in a thermally insulated section of the well will result in the rapid failure of the motor due to excessive heat buildup.  This problem is most often encountered when the pump is placed in a stagnant portion of the wellbore, usually the rathole.  Overheating is best avoided by locating the inlet of the pump so that the source of the pumped fluid must flow across the motor.  In a conventional oil or gas well, ratholing should be avoided.  If the well design requires the pump be located below the perforations, then shrouds or similar devices are the best way to assure low temperatures are maintained in the motor.  HDESPs have been successfully  used in ratholes without shrouds, but other heat transfer methods are used to make sure the motor remains cooled.

Even with effective fluid flow across the motor, thermal problems can arise.  For example, high ambient temperatures such as those associated with steam floods can induce pump thermal failures.  Any loss of fluid flow, or loss of contact between the heat transfer fluid and the pump, will cause the temperature in the motor to rise.  For example, continuing to run a pump after it is pumped off, or when large amounts of gas are flowing has caused problems in the field.   Operating strategies, such as the use of flow switches and cycling the pump to provide cool off periods have been successfully employed in difficult applications.  Pump off, by itself, will not cause pump failure, but if fluid flow is required to maintain heat transfer, then pump off can result in thermal problems.

Electrical Insulation

Electrical power is a convenient way to get power from the surface to the pump, but it requires high voltages to be introduced into the well environment, therefore requiring attention to electrical system design to prevent pump failure.  Failures due to electrical system problems are generally caused by damage to the cables and splices which allow ground faults or line to line current leakage to occur over time or at the time of installation.  Damaged cables are the most common cause of extremely short run times.  Ground faults can be detected using a good quality mega ohm meter and systems should be checked prior to commissioning.  Electrical faults will often cause electrical controls at the surface to trip out, or generate excessive amounts of heat in the motor downhole.  The use of good quality materials in the cables, and adequate attention to detail at installation will prevent most damage to the insulation. 

Electrical System Design and Power Quality

Cables, motors and electrical controls must be designed to allow the pump to operate within specific limits.  If these limits are exceeded, then controls must detect the fault and cut off the flow of power to the pump to prevent damage.  These devices are commonly referred to as overloads or “motor savers”.  The mechanism of damage to downhole motors as a result of electrical problems is thermal destruction.  As discussed earlier, downhole pumps are designed to dissipate normal operating heat without creating excessive temperature rise.  Under abnormal conditions, the amount of heat generated dramatically rises, and destruction of the ESP can occur swiftly.  Abnormal conditions include phase imbalance, low or high voltage, ground faults, voltage surges, nearby lightning and harmonics.  A class 10 or class 5 overload device properly set to the appropriate amperage is the minimum acceptable protection.  This device and a more sophisticated motor saver will provide even more protection.  On average, a line fault will occur every 6 months in the field due to weather or other conditions.  Without protection, the ESP will often fail whenever a line fault occurs.  Normal oilfield fuses and circuit breakers are not adequate protection and will result in motor failure.  Often times,  variable speed drives will provide adequate protection, but only if properly set.  We have many examples of motor failure occurring under abnormal conditions where a variable speed drive and/or a class 10 overload is present but not properly set.

In some cases, these devices will not prevent motor failure due to the presence of harmonics or power factor problems.  Harmonics and power factor problems are usually caused by other equipment operating on the same electrical supply.  Some devices inject spurious electrical impulses or change the voltage current phase relationship as a result of normal operation.  Downhole motors are especially sensitive to heating caused by these problems, and overloads do not always protect the pump.  Variable speed drives and add a phase devices can also create harmonics and current imbalances.  A three phase pump running with an add a phase will run significantly hotter then a pump running on normal three phase power.  If pumps are failing due to thermal problems, and an add a phase is being used, better results can be obtained by using a variable speed drive and/or increasing motor cooling.  If pumps fail due to overheating, and proper electrical controls are in place, then we recommend a power analysis be conducted to determine if power problems exist.

Pump Issues

HDESPs have a pumping mechanism that allows it to pump aggressive fluids, including fluids containing high levels of solids, aggressive chemicals, and dissolved salts.  The pump also is very efficient due to the positive displacement, but this characteristic can lead to problems in some applications due to overload.  If a high efficiency pump is shut in, the pressure will build until either something breaks, the motor overloads or a relief valve operates.  This is true for any positive displacement pump.  The most common causes of shut in pumps are 1) Inadvertent closing of valves on the surface, 2) Solids interference in the tubing or the check valves or 3) plugging due to scale.  If a valve is suddenly closed at the surface, and the pump is shut in, the pressure will build until something breaks, or the pump overload shuts off the pump.  By this time, it is often to late to prevent thermal or mechanical damage to the pump or other components.  The best strategy is to avoid shut in.  If that is not possible, then relief valves should be provided at the surface. 

Solids interference is a very common cause of pump failure both for HDESPs and for other types of pumps.  Interestingly, it is rare to have solids interference in the pump itself because fluid velocities are relatively high in the pump.    Solids interferance can be a difficult problem to solve, but in general, the following suggestions have proven effective in the field.  1) maintain fluid velocities of greater then 1 ft/sec in the vertical portions of the well piping  2) Avoid cycling the pump, or if the pump is cycled make sure the pump is on enough time to clear the tubing of solids 3)  Set the pump away from the source of solids in the well to allow solids to settle before fluid enters the pump inlets 4) Use a bleeder check to prevent solids from falling back on the ball and seat valves and avoid introducing solid through the tubing string. 5)  Go to a higher flow rate pump or smaller diameter tubing to increase flow velocity. Pump cycling in sandy well environments can lead to problems because the sand settles on the check valve or the top of the pump, preventing these valves from opening correctly.  The worst type of solid interference occurs when solids are sticky and tend to conglomerate. 

Scaling, paraffin or chemical incompatibility are also problems that can lead to pump failure.  Often, scale or paraffin accumulation can choke off flow passages and lead to pump failure.  Effective chemical treatment programs for scale or paraffin can be applied with the pump in place.  Corrosion is less common, but can be found in highly acidic H2S or CO2 wells.  Again, chemical treatment is the most effective means of correcting the problem.  The diaphragms and rubber materials used in the construction of the HDESP are compatible with most common oilfield environments and chemicals.  Occasionally, environments are encountered that are incompatible with nitrile rubber, and can cause failure of the rubber materials in the pump. 

HDESP’s are very versatile and can provide long, trouble free operation when properly installed.  They are not applicable for every well, and in the wells they are applicable, care must be taken to assure they are properly applied.  In some cases a measure of trial and error may be needed to determine the proper formula for long pump life.