Oil wiping technology for reciprocating compressors

Paul Boughton

A new Oil Film Dynamic (OFD) wiper ring from HOERBIGER improves compressor reliability and cuts operating costs by eliminating crankcase oil losses and oil contamination.

The design and operating principle of the new OFD oil wiper ring eliminates the leakage paths, oil pumping action and sharp edges which characterise conventional oil wipers.

As a result, it pushes oil back into the crankcase instead of outwards, as conventional oil wipers do. The OFD ring is made from hard-wearing polymer instead of metal, ensuring long life without damaging the piston rod or impairing lubrication.

Extensive testing, both in-house and on 25 field applications, has shown a huge reduction in oil leakage with almost no wear.

Oil wiper rings are used on the piston rods of reciprocating compressors. Their job is to stop crankcase oil from contaminating the gas being compressed in the cylinders, without compromising the lubrication of the machine.

Operators of reciprocating compressors typically waste oil and suffer reduced reliability because of the poor design of conventional metal oil wiper packings.

Outward leakage leads to crankcase oil losses as high as 5l (1 gallon) per day per compressor. At the same time, oil carried back into the crankcase can be contaminated with process gases – such as corrosive hydrogen sulphide – which can damage the compressor and require more frequent oil changes.

The factors characterise the design and performance of the HOERBIGER OFD oil wiper ring are:

* Geometry. Oil wiper rings are made in circumferential sections to fit the piston rod, and in conventional wiper rings the gaps between the sections offer paths for oil leakage. OFD wiper rings, in contrast, feature a two-piece design based on two concentric rings of L-shaped cross-section. Each ring covers the gaps in the other, closing off oil leakage paths. The inner ring has a carefully engineered profile for best oil control.

* Spring plate. A new spring plate which applies a side (axial) load to the OFD rings. This stops the rings from moving, eliminating the 'shuttling' effect that plagues conventional wiper rings. Shuttling is bad news because it pumps oil from the crankcase side of the ring to the outside.

* Material. The radial stiffness of conventional metal wiper rings stops them conforming accurately to the shape of the piston rod. In addition, their sharp edges can damage the rod. HOERBIGER OFD rings are made from a high-performance polymer that eliminates leaks because it follows the shape of the piston rod accurately. The polymer is durable yet soft, so there is no danger of rod damage.

These three design aspects combine to create the fourth key differentiator of the HOERBIGER OFD rings: their innovative working principle. Conventional wiper rings tend to pump oil out of the crankcase, while their sharp edges run the risk of scraping the oil film off the piston rod, leading to inadequate lubrication and damaging the rod surface. OFD oil wiper rings, on the other hand, use elasto-hydrodynamic effects to pump oil back into the crankcase without losing the vital lubricating film, while their soft material avoids rod damage.

HOERBIGER has tested the new OFD oil wiper rings both in-house and in customer applications. The company’s multi-purpose test compressor (MPTC), with a piston rod diameter of 28 mm (1⅛ inches), allows full monitoring of friction forces, oil flows and temperatures across a wide range of pressure ratios at speeds of 600–1500rpm.

Field tests on 25 customer applications confirmed the performance of OFD oil wiper rings, with practically zero oil leakage and no measurable wear.

HOERBIGER offers the new OFD oil wiper rings in several configurations to suit different industries. For air compressors in the PET bottle market, OFD guarantees an absolutely oil-tight solution that is also compact and affordable. For process gases and natural gas, the ability to combine oil wipers and pressure packing in the same housing gives reliable performance at high speeds, in vertical or horizontal configuration, and with rod diameters up to 130 mm (5 inches). Where condensation is an issue, a double seal arrangement – with oil wipers mounted each side of the pressure packing – keeps liquids out of the crankcase.

For more information, visit www.hoerbiger.com