Gas Compression: Boosting Capacity at a Compression Plant

BY TALAL AL-RASHIDI & HAMAD K. AL-RUZIHI

Two multistage 16,000 HP electrical motors are used to drive two onshore gas compressors feeding a gas plant commissioned in 1984 (Figure 1).

Figure 1: Onshore compressor skid.

These compressors needed to produce gas at a higher capacity. Unfortunately, the existing units were unable to meet the new demand. One option considered was purchasing and installing an additional compressor. Engineers also evaluated the possibility of utilizing the available capacity of adjacent offshore compressors. Although onshore and offshore gas compressors are located on the same platform and discharge to a common header, they were designed to process different gas molecular weights due to separate feeds for onshore and offshore units (Figure 2).

Figure 2: Compressor flow schematics.

The challenge was to ensure that offshore compressors could handle not only the new capacity but the new gas conditions. As the plant receives gas from two associated crude oil fields that produce different API oil grades, changing gas molecular weight is prevalent due to the number of online producing wells. Understanding the effect of changes in gas molecular weight can enhance performance prediction and capacity control of centrifugal compressors.

The original 1980s design assumed a molecular weight of 32 g/mole. But recent simulations and forecasts predicted a 12-18% reduction. This simulation was validated through test samples that showed a molecular weight of 27 g/mol. In addition, inlet temperature and pressure have significant impact on centrifugal compressor performance. The required system polytropic head and differential pressure are indirectly proportional to molecular weight. Generally, centrifugal compressors operating at higher molecular weight than design can deliver higher actual volumetric flow rate at the new gas condition. But at a lower molecular weight, more head needs to be developed. Therefore, the inlet flow rate would need to be decreased, moving to the left on the Head-Flow curve, to satisfy higher head requirements. In fact, varying centrifugal compressor operating conditions result in different dynamic behavior which in adverse cases become impossible to accommodate.

To assess existing compressors for the new conditions, a site performance test was conducted. It revealed that the onshore compressors could handle an additional 5% capacity. When added to offshore compressor capacity, this enabled the facility to meet the new requirements by expanding an existing jump-over line that connects the feed stream header.

MORE HEAD REQUIRED

During the conducted site performance test, the inlet conditions differed from the originally designed inlet parameters in terms of high pressure and low temperature. This change had a positive impact on compressor capacity and developed head. This is mainly attributed to the fact that onshore compressors are operated within different compression stages which allow for better control of any compression stage inlet conditions in case of any compression stage poor performance. It is evident that differential pressure is not maintained due to increased gas rates. The more pipeline compressors operating, the lower the required differential pressure and the lower the system head pressure required from onshore compressors. The compressor was operated at five different flow rates but stopped before the choke region as the targeted flow was achieved and to prevent any negative impact on the downstream compressor performance (Figure 3).

Figure 3: Performance test plot.

To estimate the impact of molecular weight changes in the available compressor flow, a correlation formula was devised between flow element design conditions and the measured gas conditions. Compensation or correction of flow measurements are common pitfalls that can be major sources of errors in site tests (Figure 4).

Figure 4: Plot of onshore compressor capacity showing the corrected flow of the onshore compressor based on the corrected flow measurement correction and molecular weight changes

CONCLUSION

The current onshore compressors at an average molecular weight of 27 g/mole can exceed the required new flow demand. Therefore, instead of an additional compressor, the plant fully utilized existing compression capacity at adjacent units. The new rated point per each compressor has conservatively 5% more capacity available than the design point. Also, the motor has enough horsepower to run at the new capacity and molecular weight.

 

Turbomachinery International