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2016-10-03 - Gas Machinery Conference

Maximized compressor throughput for additional earnings - HOERBIGER automates VVCP capacity control

HOERBIGER announces the release of an world first electric, stepless variable clearance pocket (eVCP) (Figure 1). For example, a capacity increase of 1 mmscf/day can earn an additional $3.5 million over the course of five years (assuming a natural gas price of $2.00/mscf). The new innovation allows fine adjustments to compressor capacity and load by signaling eVCP to increase or decrease the volume pocket. Operators can now run compressor drives near maximum torque which also maximizes the compressor’s throughput.

“With eVCP, our customers are able to automatically adjust to field conditions to optimize engine load and compressor throughput,” said John Metcalf, Senior VP of Sales and Engineering at HOERBIGER Corporation of America

Traditional clearance pockets make optimization difficult

To match supply to demand, many natural gas operators use load step schedules to control capacity. In a multi-compressor installation, individual units are operated or put on standby. For finer control, individual cylinders are unloaded, either by holding the suction valves open (end deactivation) or by opening fixed-volume clearance pockets. The result is compressor operation in a series of steps, typically covering 25% increments of nominal throughput. These stepwise jumps usually result in capacity loss.

Most stepless variable volume pockets in the gas compression industry have a hand wheel that is operated manually (Figure 2). The hand wheel may be very difficult to operate when the compressor is at operating pressure. In such cases, the compressor may have to be shut down and depressurized before the clearance volume can be adjusted.

Innovation for automatic variability

The innovation that makes the eVCP possible is its ability to overcome the gas pressure to move the pocket piston. To make electric actuation possible, motor torque is converted to linear force using a gear box and lead (power) screw.

Designing a lead screw to withstand the high impact forces was the key challenge. Experiments showed that the repeated hammer blows generated by the compressed gas quickly destroyed conventional solid steel, ball screw and roller screw designs.

The HOERBIGER R&D Team developed a unique hybrid nut using a polymer sliding surface inside a steel casing. The polymer provides low friction and high durability in the face of repeated blows from the compressed gas. The steel casing prevents the polymer from extruding.

The unique screw mechanism is oil-free, and the polymer component of the hybrid nut operates with enough friction to make the system self-locking. The drive motor only operates while changing the volume of the clearance pocket. Hydraulic systems, in contrast, are not self-locking, and therefore operate continuously, with corresponding energy consumption.

Simple design, easy to handle

A standard 1.5 hp (1.1 kW) electric motor drives the lead screw spindle through a three-stage oil-lubricated spur gearbox. The actuator itself is a standard product; it attaches to the clearance pocket via a simple adaptor plate, which is tailor-made for each compressor type and clearance pocket arrangement. A visual indicator shows the position of the spindle at a glance.

HOERBIGER's eVCP systems have been undergoing field trials since April 2015 (Figure 3). The eVCP effectively automates the variable-volume clearance pocket. An inspection after more than 8000 hours of runtime has revealed no signs of wear on the hybrid nut.

Moving output to the maximum

For natural gas pipeline compressors, the return on investment is measured using additional gas available for sale or through effective horsepower use. Operators get more from their existing compressors, and may even allow the shutdown of other units.