After the seal-gas pressure is referenced to the actual seal-chamber pressure, the correct control target becomes a positive differential at the injection point—not a pressure copied from either the 9.5 bar suction or the 12.6 bar discharge. Confirm the dry gas seal supplier’s required differential and flow before sizing the orifice.
Reference-pressure identification
- Locate the pressure point identified as the primary-seal reference in the compressor and dry gas seal documentation. Use the seal chamber, cavity, or another explicitly designated reference connection; do not select suction or discharge pressure from the process diagram merely because it is nearby.
- Measure that reference pressure during each relevant operating state. Record the pressure at startup, minimum normal load, normal load, maximum normal load, shutdown, and any pressurized standstill condition used by the installation.
- Measure seal-gas pressure at the injection point, downstream of the filters, restriction, control hardware, piping losses, and fittings that can reduce pressure.
- Calculate
ΔP = Pseal-gas,injection − Preference. Compare that differential and the measured flow with the seal supplier’s required operating range.
Do not move on until the reference tapping point and the injection measurement point are unambiguous. A reading taken at the discharge header or filter inlet does not represent the pressure available at the seal face.
Pressure-reading decision tree
| Reading | Meaning | Next check |
|---|---|---|
| Injection pressure is above the designated reference pressure, and flow meets the supplier requirement | The seal receives the intended clean-gas displacement under that operating condition. | Repeat the check at the lowest discharge-pressure condition and during transients. |
| Injection pressure is above reference, but flow is low | A blocked filter, undersized orifice, closed valve, piping restriction, or inadequate available pressure may be limiting flow. | Measure differential pressure across each filter and across the restriction. |
| Injection pressure equals or falls below reference | Process gas can oppose or reverse the intended flow path into the seal cavity. | Check discharge pressure, filter loss, restriction loss, regulator or valve position, and the actual reference pressure. |
| Filter inlet is near 12.6 bar, but injection pressure is low | Pressure is being consumed between the discharge takeoff and injection point. | Segment the system with readings before and after the filters and orifice. |
| Reference pressure tracks neither 9.5 nor 12.6 bar | Internal compressor pressure distribution governs the seal chamber. | Use the measured chamber pressure and confirm the designated reference with the compressor documentation. |
Seal-pressure mechanism
The compressor discharge supplies the H2S-containing seal gas at a reported 12.6 bar. The filters operate near discharge pressure minus losses in the takeoff piping and filter elements when they are upstream of the orifice. The orifice then consumes part of the available pressure to establish flow. It does not hold a fixed downstream pressure as a pressure regulator would.
The reported 9.5 bar suction pressure and 12.6 bar discharge pressure define the compressor’s external process conditions, but neither automatically equals the primary-seal reference pressure. Internal leakage paths, balance arrangements, shaft-end geometry, and seal-cavity connections determine the pressure adjacent to each primary seal. The two ends can therefore require separate measurements even on a single-stage compressor.
Flow through a fixed orifice changes with upstream pressure, downstream pressure, gas density, temperature, and restriction area. As compressor discharge pressure falls, the pressure available to overcome filter, piping, orifice, and seal-system losses also falls. That makes minimum normal discharge pressure the controlling case for confirming delivery capability. Maximum credible pressure remains the separate case for checking the mechanical pressure rating of filters, housings, piping, and instruments.
Loss-location checks
- Read pressure at the compressor discharge takeoff. Confirm that the seal-gas source is available in every state where the primary seal requires gas.
- Read pressure immediately before and after the active seal-gas filter. A rising filter differential indicates contamination loading or blockage. Compare the reading with the filter’s specified service limit rather than inventing an allowable value.
- Read pressure immediately upstream and downstream of the orifice. This identifies how much of the available differential the restriction consumes.
- Read pressure at each primary-seal injection connection. Separate readings reveal unequal branch losses, obstructed tubing, valve-position errors, or different seal-chamber pressures.
- Read the designated reference pressure for each seal at the same time. Non-simultaneous readings can produce a false differential while compressor load is changing.
- Confirm seal-gas flow with the installed flow measurement. Pressure differential alone does not prove that the required quantity of clean gas reaches the seal.
Because the supplied gas contains H2S, keep the system closed and use the site’s approved isolation, depressurization, detection, and personal-protection practices before opening filters or instrument connections.
Orifice and filter assessment
Use the minimum normal compressor discharge pressure as the inlet boundary for the delivery calculation. From it, subtract the measured or calculated losses through the takeoff, dirty filter, valves, tubing, fittings, and orifice. The remaining injection pressure must exceed the designated seal reference by the supplier-required differential while delivering the required flow.
| Item | Selection basis | Confirmation |
|---|---|---|
| Filter flow performance | Required seal flow at minimum normal discharge pressure, including the permitted dirty-element loss | Pressure and flow remain acceptable with the limiting filter condition |
| Filter mechanical rating | Maximum credible system pressure and gas service | Nameplate and documentation match the design conditions |
| Orifice | Required flow using actual upstream and reference conditions | Measured branch flow and differential meet the seal requirements |
| Branch piping | Loss at required flow and compatibility with the process gas | Injection-point pressure agrees with the pressure-loss assessment |
Do not size the orifice from the difference between 12.6 bar discharge and 9.5 bar suction. That 3.1 bar process difference is not the seal-gas driving differential unless the documented seal reference happens to be the suction pressure. Use the gas properties and sizing method specified for the restriction, then validate the result with field pressure and flow readings.
Resolving procedure and acceptance test
- Mark the discharge takeoff, filter inlet, filter outlet, orifice inlet, orifice outlet, each seal injection point, and each designated seal reference point on the system drawing.
- Calibrate or cross-check the pressure and flow instruments used for the test. Use simultaneous readings where operating pressure changes with load.
- Operate at the lowest normal discharge-pressure condition. Confirm that pressure after the filters remains high enough to drive the required flow through the restriction.
- Calculate the differential for each end from its own readings:
ΔP1 = Pinjection,1 − Preference,1andΔP2 = Pinjection,2 − Preference,2. - If either branch fails, identify the loss by comparing pressure before and after the filter, restriction, and branch piping. Clean or replace a loaded element, correct valve alignment or piping obstruction, or have the orifice recalculated from the required flow and actual boundary conditions.
- Repeat the readings across the normal operating range and required transition states. Accept the arrangement only when each seal maintains the supplier-specified positive differential and flow without exceeding any component rating.
Frequently asked questions
What happens if I use 9.5 bar suction pressure as the seal reference?
The calculated margin can be wrong if the seal chamber does not operate at suction pressure. Measure the designated reference connection and calculate differential against that reading.
What happens if seal gas enters the filters at 12.6 bar?
Filter inlet pressure will be near compressor discharge minus upstream piping losses. Injection pressure will be lower because the filter, orifice, branch piping, and fittings consume pressure.
What happens if the filter differential pressure rises?
Less pressure remains available across the orifice and seal branch, so flow or injection differential can fall. Compare the filter differential with its service limit and service the element when required.
What happens if seal injection pressure falls below reference pressure?
The clean seal-gas flow can be lost or reversed, allowing process gas toward the seal cavity. Locate the pressure loss and restore the specified differential and flow before accepting operation.
What happens if both seal branches show positive pressure differential?
Complete the final verification by confirming that both measured flows also meet the seal supplier’s requirement at minimum normal discharge pressure and through the required operating transitions.