At a blocked outlet, motive flow becomes inventory: mass continues entering while the discharge path passes little or no flow. Pressure then rises toward the weakest allowable limit, and expansion from 40 barg can also create a low-temperature constraint. The protection design must control relief load, reverse flow, thermal effects, and isolation timing as separate problems.
Wrong fixes and their failure modes
Closing the motive-fluid valve looks like the direct fix, but a shutdown valve is an additional safeguard rather than the complete overpressure solution. When motive flow stops, pressure at the ejector discharge can still drive reverse flow through the suction connection toward equipment operating at 0.09 barg. The motive shutdown therefore limits further high-pressure inflow but does not isolate the low-pressure waste-gas unit from the recycle unit.
Adding relief valves to every ejector pipe segment can also miss the design opportunity. If the suction and discharge piping are fully rated for every credible pressure they can experience, separate piping-segment relief valves may be unnecessary. That choice does not remove the vessel relief devices: each vessel still needs protection for the scenarios that can pressurize it.
Sizing the discharge-vessel PSV from normal motive consumption is another recurring error. Blocked-discharge load must use the maximum credible motive supply pressure and the corresponding flow through the motive nozzle. A pressure alarm or trip cannot replace this capacity because its success depends on detection, logic, valve travel, and the pressure-rise time.
Pressure, reverse-flow, and thermal mechanisms
The number that matters is maximum nozzle mass flow at the maximum credible supply condition, not the ejector's normal entrainment rate. With the outlet blocked, that motive mass accumulates in the ejector, connected piping, and any vessel volume still open to it. The relief system must pass the resulting load before protected equipment exceeds its allowable pressure.
The fluid identity must be resolved before calculating nozzle flow. The project information identifies the motive stream as C2=, or ethylene, while one proposed calculation refers to steam. Steam and ethylene require different thermodynamic properties and nozzle calculations. Use the confirmed fluid composition, upstream pressure and temperature, nozzle geometry, and manufacturer performance data.
Reverse flow follows the pressure gradient. It may occur during a blocked outlet and can also occur after loss of motive fluid. If the suction-side system has a much lower pressure rating, the reverse path can become the governing overpressure case even after the 40 barg supply has been isolated.
Expansion from 40 barg toward the 2.5 barg discharge system can lower fluid and metal temperature. This is heat and phase behavior, not logic. Determine the outlet temperature with a suitable property method, check hydrate or solid-formation risk, compare the result with material minimum design temperature, and evaluate a motive-stream preheater where required.
Quantities and design boundaries
| Quantity | Project value or status | Engineering use | Where to read or confirm |
|---|---|---|---|
| Waste-gas pressure | 0.09 barg | Starting pressure and reverse-flow vulnerability of the suction system | Low-pressure unit datasheet and operating cases |
| Recycle-vessel design pressure | 2.5 barg | Protected-equipment pressure boundary | Vessel nameplate and mechanical datasheet |
| Motive supply pressure | 40 barg | Basis for maximum credible nozzle inflow | Motive header data and ejector datasheet |
| Relief set point mentioned in the load derivation | 7 barg; protected location not identified | Cannot be assigned to the stated 2.5 barg vessel until the protected section is identified | P&ID, relief-device datasheet, and pressure-boundary schedule |
| Maximum nozzle mass flow | Calculate | Blocked-discharge relief load | Ejector performance data and motive-nozzle calculation |
| Connected free volume | Measure or calculate | Pressure-rise time and shutdown response requirement | Vessel drawings and piping model |
| Minimum expansion temperature | Calculate | Hydrate check, preheater duty, and material suitability | Process simulation and material datasheets |
A 7 barg relief set point cannot be applied to equipment with a stated 2.5 barg design pressure without a documented, code-compliant pressure basis. First map the set point to the exact pipe or vessel it protects. Then check the permitted accumulation and every component inside that protected envelope.
Protection procedure
- Map pressure envelopes. Mark the ejector suction, motive inlet, discharge piping, low-pressure equipment, and recycle vessel on the P&ID. Record the design pressure and temperature of every component, including valves, flanges, instruments, and temporary connections.
- Define credible cases. Include blocked ejector outlet, motive-supply valve failure open, loss of motive flow, reverse flow from the recycle unit, and combinations created by valve positions. Identify which equipment receives flow in each case.
- Establish maximum motive inflow. Calculate flow through the motive nozzle at maximum credible supply pressure and temperature using the confirmed fluid. Use manufacturer ejector data where nozzle geometry or performance coefficients are proprietary.
- Size the relief paths. Apply blocked-discharge motive flow to the discharge-vessel PSV calculation. Where reverse flow can reach the suction system, calculate the upstream vessel relief case using the maximum credible reverse-flow source and account for relief-system backpressure.
- Review pipe ratings. Compare the suction and discharge piping ratings with every pressure exposure. Fully rating those segments may remove dedicated piping relief valves, but only after the rating review includes all inline components.
- Provide reverse-flow protection. Select a physical barrier appropriate to the service and include its leakage, response, and failure mode in the analysis. A check valve may reduce reverse flow, but the relief design must follow the credited reliability and the defined failure case.
- Add motive isolation. Place a shutdown valve on the motive line and consider a safety instrumented function that closes it on the selected high-pressure condition. Calculate process safety time from connected volume, initial inventory, incoming nozzle flow, relief response, and allowable pressure rise; then verify that sensing, logic, and valve closure fit inside that time.
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Complete the thermal review. Calculate expansion temperature, screen for hydrates, select low-temperature-compatible materials where needed, and determine whether preheating the high-pressure
C2=stream is required.
Verification and recurring pitfalls
Close the design by reconciling the P&ID, pressure-boundary schedule, ejector datasheet, vessel datasheets, relief calculations, and shutdown cause-and-effect. The same motive-fluid basis, maximum supply condition, fluid composition, and valve lineup must appear in each document.
Verify the PSV calculation against maximum nozzle flow rather than normal flow. Confirm that the relief destination can accept the calculated load and backpressure. Check that the upstream relief case includes reverse flow when the motive valve is closed or motive supply is lost.
For the instrumented safeguard, document the pressure trip source, voting arrangement if used, final-element action, loss-of-utility position, proof-test method, and total response time. Compare measured or documented response with calculated process safety time. Never validate protection by deliberately blocking a live ejector outlet; use calculation, approved functional tests, and controlled simulation.
Typical closeout failures are an unidentified 7 barg pressure boundary, a nozzle-flow calculation using the wrong motive fluid, a shutdown valve credited as a reverse-flow barrier, and a pressure-only review that omits expansion temperature. Each one changes either the protected equipment, the required relief area, or the material-temperature basis.
Frequently asked questions
What happens if an ejector outlet is blocked?
Motive fluid continues through the nozzle and accumulates in the connected volume, raising pressure. Size the discharge relief device from maximum credible motive pressure and corresponding nozzle flow, then evaluate reverse flow into the suction system.
What happens if the motive shutdown valve closes?
High-pressure motive inflow stops, but discharge gas can still flow backward through the ejector toward the 0.09 barg suction system. Motive isolation therefore does not replace reverse-flow protection or upstream relief analysis.
What happens if 40 barg ethylene expands toward 2.5 barg?
The temperature can fall enough to create hydrate or low-temperature material concerns. Calculate the expansion state, check the confirmed composition, and evaluate preheating plus minimum design metal temperature.
When should engineering stop and escalate the ejector design?
Stop when the motive fluid, maximum nozzle flow, protected boundary, allowable pressure, thermal endpoint, or process safety time remains unresolved. Request the missing performance data and blocked-discharge case from the ejector manufacturer's official support channel, and have the responsible pressure-relief and process-safety engineers approve the final protection basis before operation.