The seal on a screw compressor follows from how the machine is lubricated: an oil-flooded screw takes a contacting mechanical seal fed by the compressor’s own oil, an oil-free process-gas screw usually takes a dry gas seal, and the gas, the oil and the maximum sealing pressure then set the arrangement and the materials.

Only an open-drive machine has this seal to select: a hermetic or semi-hermetic screw has its motor inside the gas envelope and no shaft seal.

Which Type of Seal a Screw Compressor Takes

An oil-flooded screw compressor takes a contacting mechanical seal, and an oil-free one usually takes non-contacting seals. Some screw compressors carry a lip seal on the drive shaft instead of a mechanical seal.

Oil-Flooded Screw Compressors

An oil-flooded screw takes a contacting mechanical seal on the drive shaft, lubricated by the compressor’s own oil.

By the mid-1990s the edge-welded metal bellows seal, qualified on screw machines, had replaced pusher and rubber-bellows seals as the preferred type in refrigeration compressors.

Oil-Free Screw Compressors

An oil-free screw usually takes non-contacting seals, and which kind depends on what it compresses.

On oil-free air, the usual set is a gap seal such as a labyrinth against the air and a threaded shaft seal against the bearing oil, so there is no mechanical seal to choose.

On oil-free process gas, the seal to choose is a dry gas seal. Faces whose groove pattern works in both directions of rotation fit every shaft end and survive the machine running in reverse. The seal also needs clean, dry seal gas whenever the compressor runs.

Single, Tandem or Double Arrangement

On process gas, a dry gas seal goes in a single arrangement when the gas is non-toxic and non-flammable, and in a tandem or double arrangement when it is toxic, flammable, wet or dirty:

  • Single: non-toxic, non-flammable gas such as air, nitrogen or carbon dioxide.
  • Tandem: toxic or flammable gas where a small leak through the vents is acceptable; it is the standard arrangement for hydrocarbons.
  • Tandem with intermediate labyrinth: the same duty, with inert gas fed between the two seals, so the secondary seal runs on inert gas and the primary leakage leaves through the primary vent.
  • Double: gas that must not reach the atmosphere at all, wet or dirty gas, gas carrying abrasive particles, and low sealing pressure. It needs an inert barrier gas held at least 3 bar above the process side.

A plain tandem needs no inert gas, so it suits a site without a nitrogen supply.

In the mid-1990s most refrigeration compressors, ammonia machines among them, ran a single oil-lubricated seal. Where escaping refrigerant is an emissions or cost problem, a dual seal adds a dry-running containment seal behind it, vented to recovery and able to hold full pressure if the primary seal fails.

What Pressure the Seal Has to Hold

The seal has to hold the maximum sealing pressure, which ISO 10440-1 (API 619), the process screw compressor standard, defines as the highest pressure the seals must hold in any static or operating condition, start-up and shutdown included.

At standstill the pressure to check is the settle-out pressure: the highest pressure the compressor reaches when not running, once pressures have equalised. It is usually above the design suction pressure; take the figure from the plant or the compressor maker.

While running, the pressure at the seal depends on the machine: on an oil-flooded screw, take the normal sealing pressure from the compressor maker instead of assuming suction pressure.

A single-ply welded metal bellows handles dynamic pressure up to about 20 bar, and a two-ply bellows roughly doubles that. A welded bellows is hydraulically balanced by its construction and a dry gas seal is a balanced design; a pusher seal offered for the same duty should be balanced as well.

A dry gas seal needs a positive pressure difference across it while running; vacuum or reverse pressure on the process side is acceptable only at standstill. A screw whose suction runs under vacuum, or whose suction-to-discharge differential drops low at start-up, shutdown or low demand, needs a seal gas supply or an arrangement that keeps that difference.

Seal Face Materials

An oil-lubricated screw compressor seal runs blister-resistant carbon against a cast-iron or silicon carbide seat, or silicon carbide against silicon carbide where carbon blisters in the oil. Faces that run in contact on gas, as in a dry-running containment seal, are paired carbon against silicon carbide.

Silicon carbide against silicon carbide ends blistering but has none of carbon’s tolerance for a poor film: in published seal-cavity tests a silicon carbide pair in thin ISO VG 32 polyol ester oil wore and roughened.

Designs for up to 50 m/s may need hard faces, and so may the higher face loads of a two-ply bellows at high pressure.

On ammonia, keep bronze and other copper alloys out of the faces and seal hardware.

Elastomer Materials

The elastomer has to be proven in the gas-and-oil mixture the seal runs in, at the seal cavity’s temperature, because a compound that tolerates the oil or the gas alone can still shrink or swell in the two together.

Fluorocarbon (FKM), the usual choice for hot oil, is rated poor for ammonia in the common compatibility charts, so ask for the compound’s record in ammonia with the plant’s own oil.

A field record outweighs a laboratory rating. A lubricant maker’s lab tests advised against neoprene in polyol ester oil with HFC refrigerants, yet by the mid-1990s neoprene seals were in successful use in nearly all major compressor makes, at a lower price than the FKM and HNBR compounds the lab had rated higher.

Mid-1990s refrigeration seals ran their standard elastomers to 90–95 °C at the seal cavity, and fluorocarbon to 120 °C.

On high-pressure gas, such as at a dry gas seal, rapid gas decompression becomes a concern from about 40 bar, so specify a compound tested for decompression under ISO 23936 or NORSOK M-710.

Cartridge or Component Seal

A dry gas seal comes as a cartridge. For the oil-lubricated seal on an oil-flooded screw, the format depends on the existing seal cavity. On some refrigeration screws the stationary seat sits in the compressor’s own seal cover with the seal head on the shaft, a component layout the machine was built around, and a screw’s seal cavity leaves little spare room for a cartridge’s sleeve and gland.

Where a cartridge seal fits the cavity without changes to the shaft or cover, it arrives preset, so the faces do not have to be set during installation.

What Data the Seal Supplier Needs

The supplier needs the full duty, start-up and shutdown included, and the dimensions of the seal cavity:

  • Gas: the full composition with its variations, or the refrigerant; whether it is toxic or flammable, and how much may reach a vent.
  • Oil: type and viscosity grade, the flow and temperature reaching the seal chamber, and any planned change of oil.
  • Pressures: the normal sealing pressure from the compressor maker, the maximum sealing pressure including settle-out, and any vacuum at suction.
  • Temperatures: at the seal cavity, each paired with the pressure it occurs with.
  • Speed and rotation: the maximum shaft speed and whether the machine can run in reverse.
  • Transients: start-up, shutdown, and standstill with and without pressure.
  • Seal cavity: the shaft and cavity measured for diameter, bore and depth, the port positions, and the original seal’s drawing or the seal itself.
  • Inert gas: whether nitrogen is available on site.

For a replacement, an OEM-compatible seal that matches the old one’s dimensions has proved only that it fits; the duty data above shows whether it suits the machine.