S08 ORME 6 2016 - Technology 1_Layout 1 19/08/2016 08:50 Page 30
The buffer volume (2) lowers the gas pressure against which the oil has to act, allowing the oil pressure to be set just above the suction pressure of the cylinder and so reducing the loads on the oil seal rings. Without the buffer, the oil pressure would have to be set above the cylinder discharge pressure, so loads and oil consumption would be higher.
Almost all of the oil that leaks out of the packing is “pumped” back in by the motion of the rod” The buffer volume remains at the suction pressure thanks to the conventional packing rings (1) upstream. Any leakage past these rings during the compression stroke will increase the pressure in the buffer, but because the rings are singleacting, the pressure immediately falls again during the suction stroke. In practice, even worn rings are capable of holding the buffer volume at the suction pressure.
“Pump effect” minimises oil loss So far, so good – but since we require sealing rings to keep the oil in place, have we not simply exchanged one sealing problem for another? It is true that oil will always leak past the sealing rings, just as gas leaks from a conventional packing box. However, there are two important differences from the conventional setup. First, the much higher viscosity of oil compared to gas means that the rate of oil leakage is very slow. Second, almost all of the oil that leaks out of the packing is “pumped” back in by the motion of the rod. The idea of a seal that pumps oil seems counter-intuitive, but in fact it is a well-known property of hydraulic seals. The difference is that it has never before been applied to compressor seals, and for understandable reasons. One measure of the difficulty of a sealing problem is the product of differential pressure and mean rod speed, known as the load collective; in the case of the new seal this is much higher than for a typical hydraulic seal. The other reason is that the compressor seal must accommodate a much greater range of rod movement perpendicular to the main axis of motion. Designing an oil seal that will pump effectively requires an understanding of viscous flow, hydrodynamics and elasticity. The seal lip is designed so that the motion of the rod pulls the oil film into a narrowing gap. As the oil velocity increases, so too does the hydrodynamic pressure. If this pressure is large enough, it forces the oil back into the packing case. Since the shape of the seal lip deforms under pressure, the design calculation becomes an iterative process.
Issue 6 2016
Figure 3: Cross-section of the new zeroemission packing. The cylinder is to the left and the crankcase to the right
With several much larger-scale iterations of the design, HOERBIGER engineers have succeeded in developing sealing rings that return more than 99 per cent of the oil leakage to the packing case during the in-stroke of the piston. The resulting net oil consumption is no higher than that from a conventional lubricated packing: typically 0.5–1.5 litres/day per packing. And, since the sealing rings ride on a film of oil at all times, their wear rate is practically zero. The core of the new system therefore meets its three original design goals: zero gas leakage through the pressure packing along the piston rod, oil consumption according to market requirements and stable operation under a wide variety of operating conditions.
Ensuring a fail-safe system The pressurised oil for the packing box comes from a purposedesigned oil supply unit that is approved for use in explosive environments (Figure 4). This circulates oil at a defined flowrate and pressure through the channels in the oil barrier, where it picks up frictional heat released by the oil seal rings. On its return journey the
The new sealing system has been tested successfully at three plants handling natural gas” oil is cooled by an integral heat exchanger, so no additional packing cooling is required. Depending on rod size, speed and gas pressure, one oil unit can supply up to six packing cases.