Falah Al-Azmi
Chirag A. Parikh
Kuwait Oil Co.
Ahmadi, Kuwait
Based on presentation to the 19th Annual GPA GCC Technical Conference, Kuwait, May 4, 2011. |
Installing a recycle valve in parallel with the antisurge valve of a turbine-driven multistage (low pressure/high pressure) centrifugal compressor, in combination with control system modifications, increases the compressor's turndown ratio.
The LP-HP centrifugal compressor for natural gas installed at Kuwait Oil Co.'s gathering center was equipped with an antisurge control valve and control system both to prevent surge and control capacity, limiting the compressor's lower operating range. The lack of alternately required high-pressure or low-pressure gas resulted in frequent opening of the antisurge valve at the compressor's offloading HP stage.
This article describes the field experience of the operating-range limits of running a compressor without a dedicated recycle valve for capacity control and provides guidance on designing and installing an independent recycle control system to increase the lower operating range of a centrifugal compressor.
The actual turndown ratio of the compressor improved from 55% to 20% after an independent recycle valve was installed for capacity control. This improvement allowed the gathering center to operate at lower production rates without gas flaring.
Background
Two main gathering centers at KOC's West Kuwait (WK) fields have a combined production, processing, and export capacity of 430,000 b/d of oil and 160 MMscfd of gas. Four multistage centrifugal gas compressors (two at each gathering center) export gas.
The gathering centers use two identical compressor trains, each containing a two-stage (LP and HP) centrifugal compressor. The two stages are connected to the same shaft and are driven by a single gas turbine powered by fuel gas.
The LP stage of the compressor mainly receives low-pressure gas at around 50 psig from LP separators and discharge from tank vapor compressors. The LP stage increases pressure to around 270 psig and sends gas to the suction end of the high-pressure compressor stage. The HP stage also receives gas from HP separators at the gathering center and increases pressure to around 920 psig.
The LP stage's capacity is 24 MMscfd and the HP stage's 47 MMscfd. The existing configuration of regulating surge and capacity control with an antisurge valve limited the compressor's ability to export gas at lower throughputs due to lack of proper capacity control. The compressor's antisurge control system was detecting compressor surge at lower throughputs and off-loading the compressor by opening the antisurge valve.
The addition of LP and HP compressor recycle valves rectified a flaw in the existing compressor system design, in which the antisurge valves were used for both surge protection and recycle. Adding the new recycle and thermostatic valves, as well as modified antisurge and load sharing programs, required replacing the existing MC5000 compressor controllers with new unit control panels (UCP).
Compressor surge
Surge is a momentary reversal of gas flow in a compressor. Instead of going from the inlet to the discharge, the gas momentarily goes from the discharge backward to the inlet. Surge often produces an audible boom sound and vibrations in the system. Since the gas actually moves opposite to the direction of the rotating impellers, the reversal also produces tremendous stress on the impellers and the shaft. Repeated surge can overheat or even totally destroy the compressor in a short time and presents a serious hazard to personnel working in the area.
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