P. 4 ~ Continued - Independent recycle valves improve gas compressor turndown ratios

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Limitations, capabilities

The compressor control logic described thus far can operate both compressors in parallel with necessary control of suction and discharge pressure and flow for both LP and HP stages when it operates at 55% capacity or more. The control logic equally distributes flow between trains by modulating turbine speeds when two trains are in operation. It also prevents any surge condition in LP or HP stages by opening the appropriate antisurge valve for low gas flow.

The limitations of this control system, however, come into play when it must balance the varying flow requirements between two stages (LP and HP) of each compressor. Attempting to do so limited the compressor's turndown ratio to 55%, as one stage would reach surge limit and off-load at lower gas flow rates.

With recycle

New LP and HP compressor-stage recycle valves entered service to rectify the flaw in the existing compressor system design as a result of which antisurge valves were used for both surge protection and recycle.

• FV-7223: Train No. 1 LP Compressor Stage.

• FV-7224: Train No. 1 HP Compressor Stage.

• FV-7221: Train No. 2 LP Compressor Stage.

• FV-7222: Train No. 2 HP Compressor Stage.

The new recycle valves operate in parallel with the existing antisurge control valves and are controlled through the LP-HP compressor control system's PLC.

Recycle control is independent of the normal LP-HP compressor bias control described earlier, acting in a support role to stabilize the compressor operation. In its simplest form, recycle control is a means of balancing flows between the LP and HP compression stages of each train.

Recycle control

Per the existing compressor bias control (Fig. 1), a higher selector compares signals coming from the LP compressor-stage suction flow controller FIC-5134 and HP compressor-stage suction flow controller FIC-5255 A. The signal of the maximum value between FIC-5134 and FIC- 5255 A travels via FY-5612 A to gas turbine KT-1500 combustors and adjusts speed to accommodate the highest flow demand, allowing adjustment of flow to the LP or HP sections of the compressor but not satisfying the flow demand of both sections of the compressor. Instead, one of the compressor stages will have to use its new recycle valve.

When FIC-5134 signal is lower than FIC-5255 A, for example, the control logic sends a signal to LP compressor-stage recycle valve FV-7223, forcing it open. When, conversely, FIC-5134 signal is higher than FIC-5255 A, the control logic sends a signal to recycle valve FV-7224, forcing it open.

Observations

Installation of an additional recycle valve at each stage of the compressor has given flexibility to balance the varying flow requirements at each stage and improve the turndown ratio of the compressor from 55% to 20%, facilitating continuous smooth gathering center operations at lower production rates. The dedicated recycle valves have also prevented frequent offloads of compressors at lower throughputs, resulting in reduced gas flaring.

The authors