Aug-2019

Piston design method makes recips

more reliable Gas leaking past piston rings and rider bands is a significant problem for many reciprocating compressors.

ANDREAS BRANDL, BRUCE HERMONAT and JOHN LADD
Hoerbiger

Article Summary

Leakage of gas past piston rings and rider bands – otherwise known as slippage or blow-by – is a significant issue for up to one in three reciprocating compressors in process gas service. For new or recently serviced compressors, piston leakage is rarely a problem. As time goes by, however, ring wear can give rise to serious leakage, especially in cylinders with bores smaller than 10 in (250 mm), high pressure applications, and non- lubricated compressors.

Cylinder leakage can have three consequences, depending on the setup of the individual compressor:
• Leakage may cause unacceptably high cylinder discharge temperatures
• Leakage can significantly reduce compressor capacity
• The rider bands and piston rings may wear quickly or fail altogether, necessitating frequent maintenance shutdowns.

Extensive field experience at Hoerbiger suggests that 30% of compressors have at least one cylinder where after a year of operation the additional temperature rise due to piston blow-by is at least 20°F (11°C). Depending on the compressor, a rise of 30°F (17°C) will typically trigger a high temperature alarm.

One study1 suggests that 9% of compressor damage reports relate to piston rings and a further 9% to rider bands. The combined figure of 18% makes cylinder rings the second most frequent cause of failure for reciprocating compressors, after valve failures.

The fact that cylinder leakage is so common, and often so harmful, implies a problem with the rules of thumb traditionally used to decide the type and number of piston rings and rider bands (also known collectively as cylinder rings). Clearly, we need a more rigorous engineering approach.

It turns out that modelling gas leakage to an acceptable degree of accuracy is not difficult. For a given compressor geometry and operating condition, we can approximate the leakage path as a series of orifices of known size. Combined with empirical models of the rate at which the piston rings wear, this allows us to predict which cylinders will be vulnerable to the effects of leakage, and to optimise the configuration of piston rings and rider bands so as to reduce leakage.

In this article, we explain how piston leakage occurs, what the consequences are, and how the new leakage model works. We also present several examples of how redesigned configurations of piston rings and rider bands have reduced previously serious leakage to acceptable levels.

In recent years, the scientific study of compressor valves has greatly increased the performance and reliability of recips. We believe that in turning our attention to cylinder rings we can bring a similar level of benefits, for both new compressor designs and retrofits.

Why leakage happens
Piston ring leakage occurs when the ring’s end gap opens up as a consequence of circumferential wear.

The job of the piston rings is to seal the compression chamber. In a typical double-acting compressor, the piston rings maintain a seal between the two ends of each cylinder (see Figure 1), while the rod packing stops gas escaping to the environment.

Piston rings fit into grooves in the piston and press outwards to create a sliding seal against the cylinder wall. Depending on the application, the number of piston rings is generally between two and eight. They are generally made from PTFE or PEEK based wear materials.

In normal operation, the outer surface of the piston ring wears away through contact with the cylinder liner; this can happen especially quickly in unlubricated cylinders. To accommodate this wear, and to allow the rings to be fitted and removed, each ring has a narrow gap in its circumference. Various designs exist (see Figure 2a-f), but the general principle is the same. When the ring is new, the end gap is designed to be as small as possible, consistent with thermal expansion and the need to prevent the two ends of the ring from touching (which can lead to premature failure).

As Figure 2 shows, both piston rings and rider bands can be equipped with grooves for pressure control. On piston rings, these are called pressure balancing grooves (see Figure 2d). Their purpose is not to eliminate differential pressure (which would defeat the purpose of the ring) but to reduce the contact pressure on the cylinder liner and hence the wear rate. On rider bands, they are known as pressure relief grooves (see Figure 2i and j), since they are added to eliminate differential pressure (see below).

More complex piston ring designs exist with a view to reducing leakage. One example is the so-called L-type (US) or twin (European) ring (see Figure 2e and f). Here a plain ring and a second ring of L-shaped cross-section fit together so that each ring covers the gap in the other. These designs are only suitable for single-acting cylinders, however.

In addition, complex designs (and this even includes the step-cut ring shown in Figure 2c) are not always reliable: they have narrow cross- sections prone to breakage and uneven wear, and corners that can act as stress concentrators.2 A further difficulty is that because these designs are novel they are generally unfamiliar to operators and even repair shops, so repairs are more difficult and time consuming.

To handle realistic wear rates, piston rings are normally rather deep in cross-section. On a large compressor, for instance, the rings can typically wear by up to 0.25 in (6.3 mm) before they need to be replaced. The problem is that as a worn piston ring expands to maintain its contact with the cylinder wall, the gap in the ring opens up (see Figure 3).

For 0.25 in (6.3 mm) of radial wear, the size of the end gap increases by 2π × 0.25 = 1.57 in (39.9 mm). Assuming a gap size of 0.1 in (2.5 mm) to begin with, and a clearance of 0.06 in (1.5 mm) to the cylinder wall, the resulting leakage area is 0.1 in2 (63.6 mm2), equivalent to  the area of a circular hole 0.36 in (9.0 mm) in diameter. It is no wonder piston leakage can be a problem.

Especially at risk are non-lube machines, because oil helps to seal gaps as well as reducing ring wear. High operating pressures and speeds also tend to wear out rings faster.

Small-diameter cylinders – 10 in (250 mm) and below – tend to suffer the worst effects of piston leakage. This is because for a given amount of radial wear on the piston rings, the size of the end gap – and hence the mass flow rate of gas lost to leakage – is relatively independent of the cylinder diameter, assuming a constant clearance between piston and cylinder. A certain leakage rate might account for just 0.5% of the capacity of a 25 in (635 mm) cylinder, for instance, but 20% of the capacity of a 4 in (100 mm) cylinder.