APPLICATION BULLETIN - 4880-040 | BACK TO INDEX

Argentinian donkey pumps operating in remote locations, in extreme climate , with unreliable power and the inherent stresses of deep well pumping.

PDL’s exclusive Argentinian distributor, Varitel Electronica, has pioneered one such efficient and cost-effective solution to a difficult problem on the oilfields of Argentina.

The oilfields are located in the remote high-altitude Neuquen desert region about 1500 km south-west of Buenos Aires. The climate in the region is extreme, with temperatures ranging from a summer high of 50 deg C to a winter low of -15 deg. Winter snow up to 2 metres deep and winds up to 100 km/h add to the problems of production from the 1100 oil wells owned by the giant Repsol YPF company, Argentina’s biggest producer, responsible for 75% of Argentina’s oil and gas output.

Oil well donkey pumps are a familiar sight on production fields around the world. The actual pumping is done by a sucker rod pump deep in the well. The rod is attached to a horsehead at one end of a walking beam. The other end of the beam is driven by a counterweighted crankshaft connected to a flywheel geared to a 56 to 130 kW electric motor. This mechanical arrangement produces the long, slow, powerful strokes necessary to pump crude oil from deep wells, but it means extraordinary stresses for the electric motor. Every up stroke of the rod means the motor is working hard; every down stroke means the rod and pump falls due to gravity and this energy has to be absorbed by the motor. Normally the motor regenerates.

Traditionally, this regenerated power is absorbed by dynamic braking resistors – expensive in both capital cost and the wastage of energy as heat. Another problem is the unreliable and fluctuating power supply to the oilfield. The power supply is AC which is rectified to DC and then fed through a chopper to produce a simulated AC waveform. The AC fed to the drive motors is therefore not synchronous with the main oilfield supply. External timers are fitted to conventional pump drives, to stop 1,000 pumps trying to restart simultaneously after a power outage and causing further outages through overloading. However, these timers can cause the ‘run’ signal to be lost. The motor then fails to restart after the pre-set time has elapsed.

Repsol YPF asked Argentinian industrial engineering contractors, Varitel Electronica, for an efficient solution, and Varitel selected the PDL Ultradrive Elite UE-140 with VYSTA Virtual Automation control software.

IP54 standard enclosures are used for the electronic equipment. The cabinets are outdoors. To cope with the extreme weather variations both heating resistors and cooling fans are fitted, controlled by the Virtual Automation software and an internal temperature sensor. A special building is not required.

The use of VYSTA Virtual Automation has produced immediate benefits:

• Instead of regenerating, the motor is set to provide zero torque during the down stroke. The motor speeds up as the pump mechanism moves down under gravity.
• At the moment the stroke goes from "down" to "up" a torque limit of 250% is applied. Energy from the speeding motor charges a capacitor and is then reabsorbed during the "up" stroke, producing a 30% energy saving.
• VYSTA software monitors powerline conditions and includes an internal timer for restarting after power failures. The control signals are stored and re-applied, ensuring a smooth adn reliable automatic start.
• Programming is easy, with only seven parameters to be set.

Leandro Lisjak, Applications Engineer for Varitel Electronica, lists the advantages of the VYSTA Virtual Automation and UE-140 installation, compared to other products, as: No external timers No external temperature sensors No external mains chokes No external filters No building required Easy programming Standard RS232 Modbus remote terminal unit A 30% energy saving Varitel Electronica and Repsol YPF, considered offerings by many internationally-known electronic control manufacturers before settling on the PDL Electronics product. According to Leandro Lisjak, problems with the competing products were: Company A: "They don't save energy, because when the motor regenerates they increase the speed to spend the energy regenerated by the motor having an irregular cycle." Company B: "They can't increase the speed more than 50 Hz" Company C: "Their drive works fine but the programming and calibration is very diffiicult." Other companies: "they said that it is impossible to make without a dynamic brake"