Evaluating Pressure Rise and Reverse Flow Scenario

Process Ecology Inc. was requested by an oil and gas client to study overpressure at a complex gas injection system during a reverse flow scenario through centrifugal compressors. This study was conducted to evaluate the impact of check valve failure in their compression system. The objective was to review dynamics of reverse flow and overpressure around compressors and ensure associated shutdown valves close before overpressure occurs. The study aimed to aid in decision-making on maintenance requirements for either the check valve or shutdown valves, validate load-sharing of compressors, and report the dynamic settle-out pressure of the system after an event has occurred. The company's compression system comprises two sets of compressors, ABC and BCD, and multiple vessels that process gas, oil, and water. 

The study revealed that check valve failure resulted in high reverse flow through centrifugal compressors, not designed for large reverse flow. OSD cases generally resulted in higher pressure throughout the system because the compressors took ten minutes to ramp down from initial RPM to 80%. Turning down compressor D resulted in a much faster pressure rise since its recycle valve is much larger than unit ABC's recycle valve.

The anti-surge valve opening in two seconds would result in full recycle flow contributing to pressure rise in the system. In case of 10% leakage across all single compressor turndown cases, primary separator pressure PAHH 120 psig was reached 139 seconds after the recycle valve opens, and Stage 1 Compressor Discharge Pressure PAHH 590 psig was reached 48 seconds after the recycle valve opens. In case of 10% leakage across all multiple compressor turndown cases, primary separator pressure PAHH 120 psig was reached 12 seconds after the recycle valve opens, and Stage 1 Compressor Discharge Pressure PAHH 590 psig was reached 66 seconds after the recycle valve opens.

The study suggested several mitigations that could help reduce the pressure spike during the compressor turndown, including ensuring that the check valve does not leak, using PSVs in service and operate as intended, introducing a speed controller on the compressors to reduce pressure build-up in the system when load is transferred to the running units, and reducing the inlet flow rapidly for multiple unit turndown.

Process Ecology Inc. provided valuable insights into the overpressure at the system during a reverse flow scenario through centrifugal compressors. The study demonstrated that implementing the suggested mitigations could help reduce the pressure spike during compressor turndown and ensure that the system operates without process instabilities.

Process Ecology established a steady-state model based on the initial state of the system. The model showed the following parameters: Primary Separator Pressure: 93-97 psig, 3 Phase Flow to Primary Separator: 2260 MMSCFD, Gas from Primary Separator: 274-275 MMSCFD, 1st stage discharge pressure: 540 psig, and 2nd stage discharge pressure: 1425-1444 psig. 

To study the impact of full check valve failure, Process Ecology performed two cases: Case 1, compressor A turndown and Case 2, single compressor D turndown. The results demonstrated that the check valves on the compressor discharge are safety critical since the peak reverse flow is near to the compressor reverse capacity and pressure rise is rapid.

After these results were presented to the oil and gas company, Process Ecology was asked to conduct studies to evaluate the pressure rise considering 10% check valve failure. The compressor performance and pressures in the system were monitored.

The study showed that with 10% check valve leakage, the reverse through the stage 1 and 2 units is small and varies from -30 to -3 MMSCFD and occurs when the outlet ROV is open. The power calculated during steady-state and dynamic after stabilizing to new condition is within the design limits. The compressor turndown and anti-surge opening rates (quick opened in 2 seconds) were provided, therefore, the compressor operating point in some cases crossed the surge line, which could be detrimental to the compressor.

The study also showed that the vessels' pressure rise is higher for OSD cases compared to ESD cases because the compressor turndown is slower during OSD, and the recycle flow valve is open for a longer period. Also, the pressure rise is quicker when D turndowns compared to others due to the fact that the recycle valve Cv of unit D is larger than the others. The pressure rises to 147 beyond the 120 psig PAHH and is just under the design pressure of 150 psig.

In conclusion, the study conducted by Process Ecology for the oil and gas company showed that the check valves on the compressor discharge are safety critical. The study also provided valuable insights into the impact of check valve failure and how to avoid potential damage to the compression system.

Conclusions and Recommendations

Check valves are safety critical and it's important to make sure check valve leakage does not occur.

Pressure rises to PAHH in all cases and in some cases exceeds the design pressures of vessels. Therefore, it is important to make sure the relief devices are functioning properly and are always in service.

Here are some recommendations that could help reduce the pressure spike during the compressor turndown:

1. Use a more realistic starting flow specification of 250 MMSCFD (24 hour rolling average rate) instead of 275 MMSCFD used in this study.
2. Introduce a speed controller on the compressors to make sure when load is transferred to the running units, speed could be ramped up to meet the increased load (considering the power limitation). Doing so, will reduce the backpressure in the system.
3. In case of a single compressor turndown, reduce the inlet flow to the design limit of 2 units under operation.
4. In the case of multiple compressor turndown, the inlet flow is ramped down in 80 seconds, quicker reduction of the inlet flow would decrease the pressure rise in the system.
5. In all cases, it is recommended to review the anti-surge valve Cv calculation and resize since its Cv dictates the amount of flow recycled which in turn causes pressure rise in the compressor inlet and primary separator.