Gas Seals or Rod Packing

Gas Seals or Rod Packing. This is the system of seals on each piston rod. There are normally 2 seals, each with its own function.

The first seal or “Gas Packing” serves to minimise the leakage of process gas along the moving piston rod as it passes out of the cylinder. The normal arrangement is a sequential set of pressure-activated split rider-type seals, each of which settles itself to produce a very close fit to the rod and thus forms a close labyrinth seal. Each seal consists of flat sections lapped to each other with a required accuracy down to a few wavelengths of light, these slide to create a near- gas-tight package. There will always be some leakage across such seals. The seals are typically made of bronze alloys, for relative softness, toughness and thermal conductivity. Other materials are PTFE, carbon, iron, cast iron. Sintered or cast materials are often chosen for machineability and lubricant retention. The seals are stacked in nested disc-shaped carriers, bolted together to from a top hat shaped assembly. Normally, dry nitrogen gas under pressure is blown in part way along the stack, with a process gas / nitrogen mix being vented to flare. If the seal is working correctly, what emerges from the crosshead end of the seal is nitrogen gas only, at low pressure and flow. There will also be a drain for cylinder lubricating oil / condensate wiped off the rod.

Figure 4,4 – 11 Piston Rod Gas Packing Cut-Away.

Physically, the first seal is located inside the distance piece, which is a cylinder with access covers. The seal is normally pre-assembled on a mandrel, which is a dummy piece of piston rod. The mandrel is loaded into position and screwed onto the end of the piston rod. The seal can then be eased into position in the cylinder end and the piston rod should slide through, replacing the mandrel. The mandrel can then be unscrewed, the seal aligned, tightened up and

tested. Earlier compressors had non-cartridge seals but working conditions inside the spacer are so bad that it was common for 6 hours’ work to be spoiled by a single displaced seal ring. Cartridge designs, which can be built up and even static tested in workshop conditions, are far superior.

If the gas is toxic, particularly soluble in lubricating oil, or known to be difficult to seal, a backup gas seal may be installed, in a partition in the distance piece. This arrangement, known as a double distance piece, prevents any gas leaking from the first seal from contacting lubricating oil. Thus the crankcase remains uncontaminated.

The second type of seal is the oil wiper seal, this serves to retain crankcase lubricating oil. The intent is that no part of the piston rod sees both crankcase oil and cylinder oil. The primary purpose of the oil wiper seal is to scrape off crankcase oil and return it to the sump. A secondary purpose is to act as a backup gas seal. In the event that that main gas seal fails, process gas will enter the first spacer. This space will be vented, but will still reach some pressure. To minimise the amount of process gas entering the crankcase and contaminating the oil or possibly causing an explosion, the oil wiper seal also contains gas seal ring(s). The oil wiper seal is fitted in the same way as the gas seal.

The oil wiper seal is fitted in the bulkhead between the distance piece and the crankcase extension, which contains the crosshead and crosshead bearing.

4.4.6.6 Valves

Reciprocating compressors require suction and delivery valves at each working end of each cylinder. Machines often have multiple valves. Each valve is a set of discs held together, normally, by a through-bolt. The valves are self-actuated (opened by differential pressure, closed by a combination of differential pressure and spring force) with multiple gas passages closed by lightweight valve elements for rapid operation. The moving valve elements may take the form, typically, of slotted discs, shaped rings, or poppets. Springs may be slotted discs, or coil springs.

Valves fit into shaped seats in the cylinder end, with a very close clearance to the moving piston. Suction and delivery valves are different shapes, they must be designed not to fit into the wrong seat ( e.g. by being different diameters) as mechanical impact with the piston can eject the valve. Similarly, the valve retaining nut is designed to be outside the cylinder, a failed nut can jam the piston and cause major damage.

In some modern machines a common valve is used for both suction and delivery duty, though in theory they fulfil the requirement to avoid incorrect fitting extreme care is needed to understand the required installation and position of such valves.

Each valve is retained in place by a ported cylindrical retainer or "lantern", this in turn is held in by a bolted cover. The bolted cover has to contain the relevant gas pressure and carry mechanical loads from the valve.

Valves can fail closed (by coking up or collecting sticky deposits) or, more usually, fully or partly open (by breakage of valve elements of jamming in of debris). Failed valves immediately affect the process operation, and typically cause increased gas temperatures, but do not pose a direct safety risk.

The more significant safety risk relates to the failure of the valve retainer and cover, failure of the cover opens a large diameter hole to atmosphere, there is a significant possibility of also ejecting the valve and retainer. Although the cover is a robust component, it and its fasteners should be subject to rigorous inspection and fitting practice. Fasteners should only have rolled threads, be bought from approved sources, and should never have threads re-cut. Tapped holes must be inspected for wear and checked with thread gauges if there is any doubt.

Figure 4,4 – 12 Modern Valve Design Options.