Why every gas compressor seal leaks
A compressor has to let a moving shaft or piston rod pass from the gas-filled casing into the atmosphere. No seal does that with zero leakage. Each design controls leakage to a predictable level and moves it to a predictable place: a vent, a flare, a recovery line, the suction of the machine or, for harmless gases such as air or nitrogen, the atmosphere.
That is why a seal is never “complete” just because a cartridge or packing set has been fitted. The seal system includes the seal itself, any buffer or barrier gas, the vent and drain lines, the instruments that watch leakage, and the trip logic that protects the machine when the seal degrades. If any of those parts is missing, a nominal leakage figure on a data sheet proves very little.
The consequences of leakage depend entirely on the gas. Nitrogen leakage costs money. Natural gas, biogas or hydrogen leakage creates a flammable atmosphere and needs hazardous-area thinking. Carbon dioxide can collect in pits and low rooms. Toxic or corrosive gases bring personnel exposure limits into play. Helium is expensive and escapes through paths that hold other gases. The same seal design may therefore be acceptable on one duty and unacceptable on another.
Which seal types are used on gas compressors?
The seal follows the machine. Reciprocating compressors seal a sliding rod; screw, centrifugal and rotary machines seal a rotating shaft; diaphragm compressors avoid a dynamic process seal altogether.
| Seal type | Typical machines | How it seals | Where leakage goes | Main weakness |
|---|---|---|---|---|
| Rod packing (segmented rings in a packing case) | Reciprocating process and CNG compressors | Stacks of segmented rings pressed onto the rod by gas pressure and garter springs | Packing vent to a vent, flare or recovery line; distance piece vent | Wear from rod run-out, dirty or wet gas and poor lubrication; leakage rises gradually |
| Labyrinth seal | Centrifugal compressors, some blowers | Series of fine clearances that throttle gas in steps | Balance line, suction, or vent | High inherent leakage; clearances open after a rub or surge |
| Carbon ring seal | Steam turbines, some centrifugal and screw machines | Segmented carbon rings with small running clearance, often with buffer gas | Vent or buffer system | Ring wear and breakage; sensitive to dirt |
| Oil-film (wet) seal | Older centrifugal and some screw gas compressors | Pressurised seal oil forms a barrier between gas and atmosphere | Gas dissolves in seal oil and is released at a degassing drum | Seal-oil contamination, gas loss through the oil, complex oil system |
| Mechanical seal | Screw gas compressors, blowers, pumps | Rotating and stationary faces held together, lubricated by oil or gas | Small drain or vent | Face damage from dry running, dirt or pressure reversal |
| Dry gas seal | Modern centrifugal and some screw compressors | Grooved faces lift on a thin gas film; clean, filtered seal gas is supplied | Primary vent to flare or vent; secondary vent with separation gas | Contaminated or wet seal gas, reverse pressure, slow-roll operation without support gas |
| Hermetic designs (diaphragm, canned) | Diaphragm compressors for high-purity or toxic gases | Metal diaphragm isolates the gas completely from the hydraulic side | No dynamic process leakage; leak detection between diaphragms | Diaphragm fatigue; limited flow |
For dry gas seals on centrifugal compressors, API 692 and the seal sections of API 617 are the usual references. For reciprocating packing and distance pieces, API 618 sets the arrangements most process buyers specify. We name these as the standards your consultant or licensor will usually call up; the purchaser’s specification decides which apply.
What the seal system needs around it
Most seal problems we see are not caused by the seal itself but by the utilities that support it. Before accepting any package, check who supplies and who owns each of these:
- Seal or buffer gas: filtered, dry and above its dew point at every operating state, including start-up and settle-out. A dry gas seal fed with wet gas will not last.
- Separation gas: usually nitrogen or instrument air between the seal and the bearing, to keep oil mist out of the seal and gas out of the bearing housing.
- Vent and drain lines: sized for the leakage at worst condition, free-draining, without pockets that collect liquid, and routed to the correct destination.
- Lubrication: on reciprocating packing, the right oil and feed rate; too little wears rings, too much carries oil into the process.
- Cooling: water-cooled packing cases need clean cooling water with flow confirmed.
- Power and instrument supplies: what happens to seal gas when power fails or the machine coasts down.
How much leakage is normal?
There is no single acceptable number. The seal maker states expected leakage at defined conditions, and that figure is only useful when you know the gas, pressure, temperature, speed and whether it was measured new or at the end of a service interval. Leakage figures should always be stated on a defined reference basis, for example normal cubic metres per hour at 0 °C and 1.013 bar(a), or as mass flow in kg/h.
A practical approach that works on real plants:
- Record a baseline leakage value for each seal or packing vent soon after commissioning or overhaul, at a known suction and discharge pressure.
- Log the same measurement at regular intervals and at the same operating point, so trends are comparable.
- Agree an alert level and an action level with the seal maker and the plant’s process safety owner.
- When parallel machines discharge into a common header, measure each machine separately; a shared vent reading hides which unit is failing.
- After any trip, surge, liquid carry-over event or loss of seal gas, take a fresh reading before returning the unit to normal service.
Rising leakage is often the earliest sign of trouble: worn packing rings, a scored rod, a damaged dry gas seal face or a failing O-ring. A sudden step change usually means a mechanical event; a steady rise means wear.
Where should seal leakage go?
The destination is a process-safety decision, not a mechanical afterthought. The plant’s hazardous-area classification, environmental permit and flare or vent design govern it.
| Destination | Suits | Points to check |
|---|---|---|
| Atmosphere at a safe location | Air, nitrogen and other non-hazardous gases | Asphyxiation risk in enclosed spaces, especially for nitrogen and carbon dioxide |
| Elevated vent | Small flammable leakage where permitted | Dispersion, ignition sources, local rules and the plant’s emissions policy |
| Flare header | Flammable and toxic gases at refineries, gas plants | Back-pressure from the flare header must not exceed what the seal vent can tolerate |
| Recovery to suction | Valuable gases such as helium, or where emissions must be minimised | Small recovery compressor or ejector; check the effect on seal differential pressure |
| Scrubber or treatment | Toxic or corrosive gases | Scrubber capacity during worst-case leakage, not only normal leakage |
Back-pressure deserves special attention. If a dry gas seal primary vent or a packing vent is tied into a flare header that rises in pressure during an upset, gas can be pushed the wrong way across the seal. Many seal failures follow exactly this sequence.
What happens when seal support gas is lost?
Every package needs a clear answer to this question before start-up. On a dry gas seal, loss of filtered seal gas lets unfiltered process gas, possibly with liquids or dust, reach the seal faces. On an oil-film seal, loss of seal oil pressure lets gas reach the bearing housing. On a reciprocating compressor, loss of packing cooling or lubrication accelerates wear quickly.
The cause-and-effect chart should define alarms and trips for low seal-gas differential pressure, high primary vent flow or pressure, high seal-oil level in the degassing drum, low separation-gas pressure and high packing temperature. Settle-out pressure after a shutdown matters too: a machine that stops with its casing full of gas still needs its seals protected while it is pressurised.
How do you monitor gas compressor seals?
- Flow meters or rotameters on dry gas seal primary vents, with differential-pressure transmitters across the seal gas filter.
- Packing vent flow or pressure measurement on reciprocating compressors, and temperature on each packing case where fitted.
- Rod drop or rod position probes on reciprocating machines, since rod wear and rider-band wear affect packing life.
- Fixed gas detectors in the compressor house for flammable or toxic gases, positioned for the gas density: high for hydrogen and natural gas, low for carbon dioxide and heavier hydrocarbons.
- Periodic portable leak surveys of flanges, valve stems, cylinder covers and instrument fittings, which often leak more than the seals themselves.
Data is only useful if someone reviews it. A monthly trend of each vent, compared with the baseline, is often enough to schedule packing replacement at a planned stop rather than a forced one.
Maintenance that keeps seals healthy
Seal life is decided largely by the condition of the parts around the seal. On reciprocating compressors, check rod run-out and rod surface finish whenever the packing is renewed; new rings on a scored or worn rod will not seal for long. Check that the packing case cups are flat and clean, that ring orientation is correct and that the lube point is delivering oil. On screw and centrifugal machines, check seal-gas filter elements, the condition of the separation-gas supply and the vibration history, since rubs and surge damage labyrinths and seal faces.
Gas quality matters as much as mechanical condition. Liquids, fine solids, hydrogen sulphide and polymers all shorten seal life. The suction scrubber, interstage separators and filters must work, and their drains must be checked. Our page on interstage cooling and separation covers that side in more detail.
Buying a gas compressor: questions to ask about seals
- Which seal type is offered, and why is it suited to this gas, pressure and speed?
- What is the expected leakage, on what reference basis, new and at the end of the service interval?
- What seal gas, buffer gas and separation gas are required, and who supplies them?
- Where are vents routed, and what back-pressure can each vent tolerate?
- Which alarms and trips protect the seal, and what happens during slow roll, start-up and settle-out?
- Which spare seals, rings and cartridges should be held, and how are they identified?
How Reddy Compressors helps
We sell (new and used/refurbished), rent and service gas compressors, including CNG, nitrogen, process gas and booster machines, and we supply their spares. We work as an independent multi-brand specialist from our office at Tarnaka, Secunderabad, with more than 20 service engineers and technicians covering Telangana and Andhra Pradesh, and supply equipment and spares across India.
For seal and leakage work we can inspect packing cases and rods, replace packing and rider rings, check lubrication and cooling to the packing, set up leakage baselines and trending, and identify replacement seal parts from the machine’s make, model, serial number and the old part, confirming fit before we quote. Where a dry gas seal cartridge needs factory repair or testing, we arrange it through the appropriate specialist route rather than claiming to do it ourselves.
Duties involving oxygen, hydrogen or toxic gases still follow the gas-specific codes and the compressor maker’s design rules; we work within the plant’s own hazardous-area classification and process safety procedures and do not approve seal designs or disposal systems on the plant’s behalf.
What to send us
- Compressor make, model, serial number and a nameplate photo.
- Gas name and composition, including moisture, hydrogen sulphide and any liquids or solids.
- Suction and discharge pressures (bar and psig), temperatures and running hours since the last seal change.
- Seal type, any seal drawings and the current leakage readings or vent observations.
- Photos of worn packing rings, rods or seal parts if they have been removed.
- Where vents go now, and any change you want to make.
Frequently asked questions
Is some leakage from compressor rod packing normal?
Yes. Reciprocating compressor packing is a controlled-leakage seal, so a small, steady flow from the packing vent is normal. What matters is the trend. Record a baseline after new packing is fitted, measure at the same operating point each time, and plan replacement when leakage rises towards the action level agreed with the manufacturer and the plant’s safety owner, rather than waiting for a visible failure or gas alarm.
Why do dry gas seals fail early?
Most early dry gas seal failures come from contaminated or wet seal gas, reverse pressure from a vent or flare header, or running at slow roll and start-up without proper seal gas support. Liquids and fine particles damage the grooved faces quickly. Check the seal gas filter and conditioning, the vent back-pressure and the start-up sequence before blaming the seal cartridge itself.
Where should compressor seal vents be routed?
That depends on the gas. Air and nitrogen vents can usually go to atmosphere at a safe location, though nitrogen is an asphyxiation risk indoors. Flammable or toxic gases normally go to a flare, vent stack, scrubber or recovery system, as decided by the plant’s hazardous-area classification and process safety design. Always confirm that the destination’s back-pressure cannot push gas back across the seal.
How often should gas compressor packing be replaced?
There is no fixed interval that fits every machine. Packing life depends on gas cleanliness, rod condition, lubrication, pressure and running hours. The most reliable approach is to trend vent leakage and packing temperature, inspect the rod at every change, and set the replacement point from your own machine’s history and the maker’s guidance, rather than following a calendar alone.
Can you supply seal and packing spares for any brand of gas compressor?
We supply gas compressor spares across many brands as an independent multi-brand specialist. We identify the part from the make, model, serial number, nameplate photo and the old part, and we confirm fit before quoting. For proprietary seal cartridges, we tell you whether a genuine or an equivalent part is available and arrange specialist repair where it is needed.
How do I find which gas compressor seal is leaking?
Measure each seal vent separately rather than reading a shared header. Use vent flow or pressure readings, packing case temperatures, gas detectors and a portable leak survey around the machine. Remember that flanges, valve covers and instrument fittings often leak more than seals. Compare readings with the baseline taken after commissioning or overhaul to see which point has changed.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.