Sizing Centrifugal Compressor Blocked-Outlet Relief

The relief load becomes a defined compressor operating-point and mass-balance problem once the protected boundary, credible valve positions, recycle path, and trip actions are fixed. For this installation, the whole circuit, including the suction drum, has a 900 psig design pressure, normal recycle is about 90% of compressor flow, and compressor capacity is about ten times process flow. Those facts make normal flow split a poor substitute for a blocked-outlet relief case.

Commissioning decision sequence

1. Mark the protected boundary. Include every connected item that can remain pressurized together: suction drum, compressor, cooler, recycle piping, discharge piping, and isolated export section as applicable. Record each component's design pressure and relief-valve set pressure. Do not move on until the lowest allowable pressure within the boundary is identified; a common 900 psig design pressure does not by itself establish the relief set point.
2. Define the initiating closure. Identify the export valve that blocks flow, its fail position, closure signal, and whether it closes automatically following a compressor trip. Confirm the actual position feedback used to prove closure.
3. Define every recycle path. For each recycle valve, bypass, and cooler path, record whether it is open, closed, or moving when the export path becomes blocked. Analyze a closed process-recycle path for the blocked-outlet case unless an independently justified protective function guarantees another state.
4. Confirm shutdown functions. Determine whether a discharge PSHH trips the compressor, whether that trip is independent of the normal control loop, and whether its response is credited by the protection design. A normal recycle controller or inlet pressure controller is not automatically a safety function.
5. Check reverse-flow isolation. Locate the check valve between the compressor and export system, if fitted. With no effective check valve, include possible reverse flow from the export system in the overpressure assessment.
6. Select the sizing branch. Use the closed-recycle compressor operating point for the conservative blocked-outlet branch. Use a recycle-open branch only to evaluate a fixed, continuously available path or to study the transient before shutdown.

Boundary and valve-state checks

The phrase “blocked in” must refer to a specific boundary. Closing the export line does not necessarily stop feed into the compressor. The upstream feed enters at suction pressure, while the blocked export pressure acts on the discharge side. An upstream source pressure below 900 psig therefore does not prove zero inlet flow; the inlet control valve, upstream pressure, suction pressure, and connecting resistance decide the feed rate.

Record commanded state and proved state separately. A controller may demand full recycle during a pressure excursion, but valve travel, actuator capacity, instrument lag, and the compressor pressure rise determine whether the path opens soon enough.

Symptoms and deciding measurements

Pressure trends alone cannot distinguish compression heating from added mass. Rising pressure with rising temperature and nearly fixed mass is a different mechanism from rising pressure caused by continuing feed or reverse flow. Build the event trend on one time base before assigning the relief source.

Compressor operating-point mechanism

A centrifugal compressor does not deliver a fixed volumetric flow against every discharge pressure. For the blocked-outlet case, select the applicable maximum permissible speed curve; the cited assessment considered a speed that could be 105% of maximum operating speed, associated with the high-speed alarm setting. Use 105% only when the machine data and protection philosophy make that speed credible.

Set suction pressure to the applicable PSHH case and use the highest credible feed-gas density or molecular weight. Set discharge pressure to the relief-valve relieving pressure. The cited method used 110% of set pressure, while also noting that some cases require the operating point at 100% of set pressure. Select the governing pressure from the relief basis and applicable design rules; do not transfer either percentage into another design without checking it.

Read the intersection of required polytropic head and the compressor head-versus-actual-flow curve at the selected speed. That intersection supplies compressor throughput. Neglecting cooler friction increases the pressure available across the compressor and can provide a conservative screening case. Then read or calculate compressor power at the same point and compare it with available driver power. For a gas-turbine driver, include the applicable unfouled delivery capability when that defines the maximum compressor input.

If the calculated point falls outside the published curve, do not extrapolate silently. Obtain the manufacturer's extended operating data or use a validated compressor model. Also check whether the point crosses an operating limit that causes a separate trip or unstable operation.

Feed, recycle, and inventory mass balance

Compressor throughput is not automatically the relief rate. With an open recycle line, the compressor can circulate the same gas repeatedly. The relief valve handles the net mass entering the protected boundary, plus any credible reverse flow, minus mass retained as inventory while pressure rises. At a sustained condition, accumulation approaches zero and mass discharged through relief equals net external mass entering the boundary.

The ten-to-one ratio between compressor capacity and process flow explains why compressor throughput can greatly exceed fresh feed during normal operation. It does not cap the compressor's internal circulation at the available feed rate. Conversely, it does not justify sizing the relief valve for full circulating flow when a permanent recycle path returns a demonstrated portion to suction.

If the process recycle path remains open after export closure, compression work raises gas temperature while cooler duty removes heat. Suction pressure may rise as trapped inventory heats or as feed continues. A dynamic hydraulic and thermal model is appropriate when the required result is time to relief, peak transient pressure, controller effectiveness, or vessel-temperature response. The model needs vessel and piping volumes, gas properties, compressor maps, cooler duty, external feed behavior, valve characteristics, actuator travel, and relief-valve flow.

For relief capacity at an established relieving state, first solve the credible compressor operating point and the external mass balance. Add dynamics where the transient determines whether a trip or valve action completes before the allowable pressure is exceeded.

Protective-path alternatives

A fixed restriction orifice across the recycle valve can preserve recycle flow even when the control valve is closed or too slow. One proposed basis assigns approximately 80% of compressor throughput at relief to the fixed recycle path, leaving approximately 20% to the export relief valve. Treat those percentages as a design target, not a universal split.

Size the orifice from the detailed upstream and downstream pressures at the relief condition, with suction at the selected PSHH, discharge at the selected relieving pressure, highest applicable gas molecular weight, and compressor flow taken from the matching curve point. Check choking, gas temperature, noise, vibration, and cooler duty using the selected orifice method.

Do not install a block valve in this bypass if relief sizing depends on its availability. Mark the P&ID and operating manual so the orifice cannot be removed or isolated without engineering review. After sizing, calculate bypass flow at normal suction and discharge pressures. A path sized for relief may recycle too much gas during normal operation, reduce export capacity, increase power consumption, or impose excess cooler duty.

An instrumented shutdown provides another branch only when its pressure sensor, logic, final elements, response time, and proof testing are part of the credited protection design. Check whether discharge PSHH trips the compressor and whether the export block valve automatically closes on that trip. Normal pressure control and process recycle remain control functions unless the protection design explicitly assigns and verifies a safety duty.

Resolving-branch sizing and verification

1. Freeze the case definition. Document export closed, recycle state, inlet state, check-valve behavior, compressor speed, suction PSHH, gas composition, and relief pressure. Confirmation: every state has a signal, mechanical position, or approved design assumption.
2. Calculate compressor throughput. Intersect the selected speed curve with the head required between suction and relief pressure. Confirmation: the point lies on valid compressor data and has a corresponding power value.
3. Apply the power limit. Compare point power with driver delivery. Confirmation: the adopted flow cannot require more power than the driver can supply under the sizing condition.
4. Calculate permanent recycle flow. For a restriction orifice, use its actual relief-condition pressure differential and gas properties. Confirmation: calculated recycle flow does not exceed compressor throughput and the assumed pressure pair is hydraulically self-consistent.
5. Form the relief balance. Determine flow reaching the relief location from compressor discharge after fixed recycle, then reconcile it with continuing feed, reverse flow, and inventory accumulation. Confirmation: each boundary mass flow has one sign and one physical path.
6. Check normal operation. Recalculate orifice flow at normal suction and discharge pressure. Confirmation: export capacity, compressor operating margin, power, cooler duty, and controllability remain acceptable.
7. Validate the transient where protection timing matters. Model or test sensor delay, logic delay, valve travel, compressor coastdown, pressure rise, and heat removal. Confirmation: the credited action completes before the protected equipment reaches its allowable pressure.
8. Verify the installed configuration. Inspect the orifice identification and bore, confirm that no isolating valve defeats the path, stroke the recycle valve, prove the discharge trip and valve sequence, and trend suction pressure, discharge pressure, temperature, speed, valve positions, and trip state during an approved functional test.

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