A seal-gas interruption drives gas flow and pressure below the values needed to keep the dry gas seal clean and its faces separated. The number that matters is not elapsed time alone. Damage exposure depends on residual flow, differential pressure, rotational speed, gas composition, contamination, condensate, groove geometry, and the seal arrangement.
No universal safe runtime follows from the information given. A seal may be exposed to unsuitable gas immediately after the clean supply disappears, while the resulting damage may not become visible until leakage rises or the machine returns to service. Treat the event as an operating-envelope excursion, not a timer-only problem.
Flow, Lift, and Thermal Limits
A dry gas seal operates with an extremely thin gap between its faces. Hydrodynamic features in the rotating face generate lift from the gas entering that gap. Adequate lift depends on speed, face geometry, gas properties, and the pressure conditions for which the seal was selected.
Loss of the specified supply creates two related hazards. First, particles or condensate can enter the sealing gap and score or contaminate the faces. Second, a change in gas molecular weight or operating pressure can alter lift enough to reduce face separation. Reduced separation increases frictional heating and can progress to face contact. This is heat and surface damage, not logic: the protection system may report only the initiating low-flow or low-pressure condition while the mechanical consequence develops inside the cartridge.
| Quantity or condition | Why it matters | Where to read or verify it |
|---|---|---|
| Seal-gas flow | Shows whether clean gas continued moving through the intended path | Flow transmitter trend and seal-gas panel instruments |
| Supply and process pressure | Determines the pressure relationship across the seal and whether process gas could enter | Pressure trends, panel gauges, and compressor records |
| Rotational speed | Affects hydrodynamic lift and the severity of rubbing if separation collapses | Machine speed history |
| Gas composition | Molecular weight affects lift; heavy compounds and condensate increase contamination risk | Supply-gas and process-gas analyses |
| Leakage | A change can indicate loss of sealing integrity | Primary leakage, flare, or vent measurements provided by the installed system |
| Acceptable limits | Defines whether each recorded value left the approved envelope | Compressor and dry gas seal OEM documentation; applicable project specification |
Operating Cases and Damage Potential
The event must first be classified. “No seal gas” can mean zero measured flow, insufficient pressure, insufficient flow, loss of the normal source with another gas still entering the seal, or loss while the compressor is stopped. These conditions impose different loads.
| Operating case | Mechanism | Recommended disposition |
|---|---|---|
| Rotating with no clean primary supply | Process gas may pass through the primary seal. Particles, condensate, or unsuitable gas properties can damage the faces or reduce lift. | Stop using elapsed time as the release criterion. Review trends and obtain an OEM disposition before restart. |
| Rotating with low flow or pressure | Some protective flow may remain, but the pressure relationship and cleanliness margin can be lost. | Compare the minimum recorded values with the OEM operating envelope and examine leakage behavior. |
| Stopped and pressurized | Hydrodynamic lift is absent at standstill. Pressure and gas migration can still expose the seal to contamination. | Limit the condition to the OEM-approved duration and pressure state. |
| Stopped and depressurized | Mechanical exposure is generally different from a rotating, pressurized event, but the seal may still have experienced contamination during rundown or depressurization. | Reconstruct the full sequence, including coastdown, before accepting the cartridge. |
For a tandem arrangement, loss of the primary clean-gas source can allow unfiltered process gas to traverse the primary seal and leak toward the flare or possibly the vent path. Actual routing depends on the installed arrangement. Single, tandem, and double seals therefore require separate decision paths based on their piping and instrumentation.
Inspection Method Comparison
A borescope, a static test, and cartridge removal answer different questions. None converts exposure duration into proof of face condition.
| Method | Useful finding | Key limitation |
|---|---|---|
| Borescope inspection | May reveal accessible deposits, liquid, debris, or damage around visible components | Cannot reliably inspect the critical mating surfaces or quantify microscopic scoring, distortion, groove blockage, or face flatness |
| Static test | Can identify leakage under the specific test pressure, gas, temperature, and shaft condition | A passing stationary result does not prove correct dynamic lift, clean grooves, or acceptable behavior at operating speed |
| Leakage trend review | Shows whether the event produced an immediate or persistent change in installed performance | Normal-looking leakage does not exclude latent face or groove damage |
| Cartridge removal and specialist inspection | Provides direct access for face, groove, deposit, and component assessment | Requires an outage and the acceptance criteria of the seal supplier |
The recommended approach is event reconstruction followed by OEM review, with cartridge removal when the exposure cannot be cleared against approved limits. If damaged seal faces are identified, replace the dry gas seal. A borescope or static test alone is not a defensible substitute for examining the sealing surfaces.
Event Reconstruction Procedure
- Preserve the sequence. Export seal-gas flow, supply pressure, process pressure, speed, leakage, flare or vent indications, alarms, trips, and valve states for the complete loss, coastdown, standstill, and depressurization period.
- Define the failed function. Record whether the event was loss of source, zero flow, low flow, low differential pressure, or an instrument-only indication. Confirm the condition using independent pressure, flow, and valve evidence where available.
- Identify the arrangement. Use the seal system drawings to establish whether the cartridge is single, tandem, or double and trace where process gas, leakage, flare, and vent streams could travel.
- Establish the gas actually present. Compare normal seal-gas composition with process-gas composition. Check for particles, condensate, heavy components such as C5+ compounds, or a molecular-weight change that could affect lift.
- Separate rotating and stationary exposure. Calculate the duration for each speed and pressure state from recorded timestamps. A total event duration hides the interval with the greatest rubbing and contamination risk.
- Compare against approved limits. Obtain minimum flow, pressure relationship, gas-quality requirements, permissible stationary exposure, leakage limits, and restart criteria from the compressor and seal OEM documentation. API 614 may be part of the project specification, but the installed equipment documents decide the operating envelope.
- Choose the inspection scope. Use borescope findings and static leakage only as supporting data. Remove the cartridge when recorded conditions exceeded approved limits, contamination entered the seal, leakage changed materially, or the OEM cannot release the seal from the event data.
Restart and Verification Criteria
Before restart, correct the supply failure and prove the complete seal-gas path. Confirm the source is available, valves are in their intended positions, filters or separators are serviceable, instruments respond, and the required pressure and flow relationship can be established. Remove any identified liquid or particulate source rather than merely restoring pressure.
During startup, trend speed, seal-gas flow, pressures, and leakage together. Compare leakage with the accepted pre-event baseline and the OEM limit through acceleration and stable operation. Stop the startup if leakage rises abnormally, the required gas relationship cannot be maintained, or the seal system shows renewed contamination. A stationary pass followed by abnormal dynamic leakage indicates that the static test did not reproduce the operating mechanism.
Recurring Assessment Errors
- Assigning a universal survival time: exposure severity changes with speed, pressure, gas quality, composition, and seal geometry.
- Treating source loss as zero gas flow: process gas may replace the clean supply and continue through the seal.
- Using one duration for the entire event: rotating, coastdown, pressurized standstill, and depressurized standstill impose different conditions.
- Accepting a clear borescope image as proof: the critical face interface may remain inaccessible.
- Accepting a static leak result as a dynamic release: stationary testing does not recreate hydrodynamic lift or full-speed heating.
- Ignoring gas properties: a cleaner-looking process gas can still have unsuitable molecular weight or condensation behavior.
FAQ
What happens if a dry gas seal runs with no seal gas?
Process gas may enter the seal, carrying particles or condensate and changing the lift generated between the faces. Damage can begin through contamination, reduced separation, rubbing, and heat; no universal safe runtime applies.
What happens if the dry gas seal passes a static test after supply loss?
The result supports leak-tightness only under that stationary test condition. It does not prove clean grooves, undamaged faces, correct lift, or acceptable leakage at operating speed.
When should a dry gas seal assessment stop and escalate?
Stop the restart decision when the event record cannot establish the rotating exposure, the process gas may have carried particles or condensate, leakage changed, or recorded values cannot be compared with approved limits. Escalate to the compressor or dry gas seal manufacturer's official support channel for cartridge disposition; remove and replace the seal when face damage is identified.