Centrifugal compressors are frequently used in the oil and gas industry in higher pressure applications. One of the characteristics of applications in the oil and gas industry is the requirement to use the compressors over a wide range of operating conditions. The machines, usually speed controlled, are expected to operate from low flows near surge to very high flows near or in choke.
The potentially damaging effects of surging compressors are widely acknowledged. Operating compressors in the high flow region, i.e. at higher flows than at the best efficiency point, often referred to as overload, choke or stonewall, has been identified by many manufacturers as a region of operation that needs to avoided. Many compressor manufacturers will limit the operation of the compressors at high flows by defining a ‘choke limit’ or ‘sonic limit’ on a compressor map, prohibiting the operation of the compressor at low pressure ratios. This can require installation of additional throttle valves, or more complicated recycle and anti-surge valve selections, and thus impacts the cost of operation.
This article contains excerpts from the paper, “Operation of centrifugal compressors at choke conditions” by Rainer Kurz, Russell K. Marechale, Edward J. Fowler, Min Ji and J Cave of Solar Turbines Incorporated at the 2016 Asia Turbomachinery & Pump Symposium.
Unfortunately, the setting of choke limits is somewhat arbitrary in many instances, and may be subject to questions, in particular if it limits operational flexibility. To be very clear: Prolonged operation in the choke region should be avoided, because if nothing else, the efficiency of the compressor is very low. Whether the compressor operates in choke or not is not as clearly defined, and not as obvious from an operational standpoint as the operation of a compressor in surge.
A compressor map showing a single speed line will usually show a more or less steep drop in head and efficiency when the compressor is operated at flows higher than BEP. This behavior is more pronounced at Mach numbers approaching 1, and high mole weight gases. From an aerodynamic standpoint, choke refers to a situation where flow passages become blocked either due to the occurrence of compression shocks or due to massive flow separation. In centrifugal compressors, these flow passages can either be the impeller inlet, or the inlet to diffuser vanes. Both operation near the surge limit and in choke leads to flow conditions that are severely different from the flow conditions at the compressors design point.
While operation at high flow is unattractive to the user due to the associated drop in efficiency, there are operational situations where it can be encountered: -During start-up when the recycle valve is opened too much.
-Process upsets, for example if two compressors operate in parallel, and one of them has to be shut down
-Compressor undersized for the desired operating conditions. -Performance degradation due to fouling
Besides the performance penalties, the study indicates that operating in choke is often not a problem for the compressor provided: -The balance piston is sized correctly to provide adequate thrust load balance over the entire operating range -Issues like blade strength to deal with alternating stresses are considered, or the occurrence or strength of alternating stresses is reduced.
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