Variable frequency drives offer advantages as final elements of control

In offshore oil and gas applications, above surface or subsea, the issues of availability, reliability, safety, and efficiency of rotating equipment are crucial.

Manuel R. Suarez
Kumana & Associates

In offshore oil and gas applications, above surface orsubsea, the issues of availability, reliability, safety, and efficiency of rotating equipment are crucial. Finding the best option for drivers of that equipment is an important decision. Electric motors with variable frequency (or variable speed) drives (VFD) offer many advantages over most alternatives, but their successful implementation is often precluded by a common misconception about their characteristics.

Developments in power transmission, like high voltage direct current, converters, and semi-conductor technology facilitate the use of electric motors and VFDs in offshore applications. Voltage source inverters (VSI) are now possible in power ranges of more than 20 MW. These offer much better dynamic response than earlier load commutated inverters such as those used to drive the 48-MW compressors ofOrmen Lange, the gas line between Norway and England. VFDs are fundamental in modern FPSO vessels and even more so on floating LNG processes where they are used in applications from ship propulsion (e.g. azipod systems) to gas processing.

Yet, the VFD is often perceived as an "energy savings device" and not as what it truly is: a final element of process control. This is perhaps due to the fact that energy savings are far easier to quantify than the benefits of improved process. The former view lumps together the VFD with other equipment of strictly electrical nature like transformers, capacitors, switchgear, etc., therefore, the sole responsibility of electrical engineering. In general, process-chemical and mechanical engineers have little or no knowledge about VFDs, their principles of operation, functionality, and applications. This is perhaps the reason why it is seldom perceived as a "final element of control."

The successful implementation of a VFD system, especially in retrofit cases, requires the close work of those three disciplines from the start of a project. From conceptual design to sizing, selection and implementation, each of the three must contribute important information for the success of the project. Electrical engineers are more concerned with strictly electrical issues such as transformers-filters requirements, distance allowance between equipment, and cabling. Mechanical engineers are the interface between process and electrical (e.g. rotating equipment inertia, torque and speed constraints and limitations, and adequate head-flow characteristic for VFD service). Process will evaluate control strategy and the effects of speed control such as constraints imposed by mass transfer requirements.

The following illustrates the point. Discharge from a surge vessel was handled by a train of centrifugal pumps in parallel (25-hp units) with their respective control valves. To save energy on the pumps, a VFD system retrofit was considered to replace the control valve. The responsibility for its sizing, selection, and installation was given to electrical engineers.

Soon after installation, the level control loop was difficult if not impossible to tune and the pump developed leaks. The VFD was blamed, deemed inadequate for the purpose, and removed. Detailed investigation showed the existing level setpoint control previously used with a control valve was used to control pump speed with the VFD. Putting a level setpoint in a surge vessel is simply defeating the purpose; it just turns the vessel into a large diameter segment of pipe.

This error had been somewhat camouflaged by the slow acting control valve. The fast response of the VFD made this inadequacy very evident. The pump had a type of seal that required a minimum flow velocity for proper performance. In the low speed region of the VFD, flow velocity was too low and the seal leaked.

VFDs can work with synchronous or induction motors driving roto-dynamic or positive displacement rotating equipment (especially pumps and compressors) from low power ranges up to about 100 MW. Their use to control mass transfer operations offers considerable advantages:

As a final element:

  • Very fast loop response time. Torque loops response is in the order of fractions of millisecond. Speed loops are also very fast but depend on the moment of inertia of associated rotating equipment (i.e. proper dimensioning of the VFD for a given rotating equipment and required response time).
  • Absence of many non-linearities present in other final elements.
  • In compressors: SURGE prevention-mitigation. Changing the speed moves the operating point away from the surge line. In modern control systems with decision making at the local level and improved communications between elements, applications such as Bently Nevada incipient surge detection can work with the VFD to control compressor speed and avoid the onset of surge (e.g. vibration probes are just detection devices, they still require a final element for action to be taken).
  • Vibration/mechanical resonances of motor and rotating equipment: prevention and avoidance. A VFD can be programmed to "jump" (dead-band) over frequencies at which there are resonances, excess vibration, etc.
  • Process stability: "power loss ride-through" capabilities and compensation of voltage and frequency fluctuations keep process equipment operating in a stable manner (no rpm or torque flickers).

Maintenance/availability and reliability:

  • No moving parts and literally no maintenance. Basically all maintenance on a VFD is "remove and replace" on site. Average values compared to gas turbines: minor and major maintenance cycles are five to six times longer while repair time is five to six times shorter. Also, the operational efficiency is about double that of gas turbines while mean time between failures is again five to six times longer in the VFD.
  • Equipment protection: in medium voltage, an integrated gate commutated thyristor (IGCT) is 100 times faster than a power fuse and, unlike the fuse, it can be reset, no need to replace it when it trips.

The bottom line is VFDs offer significantly longer up time compared to other options.

The motor and the network:

  • The motor delivers full nominal torque at -0- rpm.
  • The startup and shutdown rpm ramps of the motor can be programmed with the desired slope to prevent, for instance, hammer pressures or undue stresses in equipment and piping.
  • Electric Network Stability: no current peaks at startup of motors, constant and near unity power factor over the entire speed range regardless of the load.
  • Possibility to "regenerate" power back to the grid in cases where the load drives the motor (e.g. a pump "pushed" by a gas slug in an oil well).
  • Four-quadrant motor operation capability offers new alternatives for process rotating equipment.

In addition, there is of course the energy savings potential of the VFD.

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