Why Compressors are Affected by More Than What Goes on Between its Nozzles
I am sure a few of you would have looked at the title of this article and thought that it was a bit of a strange or funny title. It is to me, but I could not think of a better way to uncompromisingly convey the essence of what this article is all about. I remember chatting to a friend who worked with one of the major oil and gas operators. I asked him, what they did to look after their centrifugal compressor systems and he said that they didn’t bother with it but left it all in the hands of the OEM.
I thought to myself, he is probably one of those who think that oil and gas centrifugal compressors are affected by only the things that go on between its nozzles. Don’t get me wrong, the OEMs do a great job designing, manufacturing, and servicing the centrifugal compressors. I have worked closely with many of them on projects and can personally attest to their quality and delivery and can therefore understand why some of my operator colleagues will want to leave everything in their hands when it comes to centrifugal compressors. However, what some of these colleagues don’t realize is that the OEM is primarily concerned with the machinery they have supplied and some supporting ancillaries.
They are not as concerned with the other systems around the asset that affect centrifugal compressor performance – they are not paid to be. This means optimizing your gas compression systems for your current operation’s needs significantly more than your OEM valuable contribution.
SOME COMMON MISCONCEPTIONS
From consulting with various oil and gas companies internationally, I have come across two common misconceptions some colleagues have in dealing with gas compression systems and they are somewhat linked. The first one is that gas compression systems are essentially a “black box” in a complex oil and gas producing facility. People who have this opinion basically view centrifugal compressor equipment as simple devices having little or no internal complexity or intricacy. Essentially these are seen as plug-and-play devices that can easily be replaced and will work well as long as they are adequately maintained.
People who have this view are usually surprised or shocked when compression equipment becomes the major bottleneck in a production facility as they underestimate the impact of gas compression systems on the overall deliverability of the asset. It tends to be the case that most people who have this view are outside the disciplines of rotating machinery and mechanical engineering. The second misconception is the one that this article is looking to address. These are those who feel that if there is a problem with the compressor then it has to be something inside or immediately around the casing that is causing it.
I can understand this viewpoint in the sense that an impending problem to the compressor usually shows up within the casing. However, as we will see from my discussions below, the root cause of these problems is often not in and around the compressor and therefore outside the remit of the compressor OEM.
OTHER ISSUES THAT AFFECT COMPRESSOR PERFORMANCE
Centrifugal compressors in oil and gas are bespoke to the particular application and are therefore designed and selected to suit a particular range of operating conditions. However, the world of oil and gas production is very dynamic and for a typical facility, it seems the only constant thing is change. It is therefore difficult to guarantee a fixed set of inlet conditions for the compressor as this is dependent on events that have occurred upstream of the compressor. The centrifugal compressor performance is therefore significantly affected by other issues and systems within the integrated production system. These external issues have a significant impact on the operating duty of the centrifugal compressor.
Some of the issues that frequently affect gas compression system performance are as follows:
Production Profile: the production profile that an asset is able to deliver is affected by a number of subsurface and surface factors. Reservoir structure, geophysics, and geomechanics affect the productivity of the field. Well, drilling schedules, choking strategies, and performance can also have a significant impact on the production profile that is achieved. All these combine to determine the duty (e.g. flow rates and pressures) at which the compressor will operate at. There is no guarantee that this will be an optimum operating point for the centrifugal compressor.
Flowlines: the flowlines which bring the good fluids to the processing facility can significantly affect the duty (and therefore the performance) of the centrifugal compressors. For example, pressure losses and temperature losses all have a potential bearing on the hydrocarbon arrival pressures and temperatures at the processing facility. In some cases, multiphase flow in the pipelines may mean unstable slug flow which will need to be mitigated by sufficiently sized slug catchers at the processing facility.
Process Equipment: process equipment such as slug catchers, separators, scrubbers, valves, etc. affect the operating conditions of the centrifugal compressors and therefore its performance. In addition, the efficiency and performance of the process equipment will determine the chemical and physical composition of the gas that is compressed. For example, an inefficient process system can lead to liquid carryover into the compressor which could jeopardize the performance of the centrifugal compressor
Process Changes: process changes can result in a significant deviation from the expected process gas composition of the compressor. As I have described in other articles, this can lead to significant suboptimal performance of the centrifugal compressor.
Control Systems: control systems determine the way the equipment and facility respond to production and operational changes. They determine the operating point of the equipment. Centrifugal compressors are significantly affected by control systems especially those in close proximity such as the anti-surge and load sharing controllers.
Mechanical Drive Gas Turbine or Electric Motor: Mechanical drive machinery determines the amount of useful power available to the centrifugal compressor and they can be a constraint to the operation of the compressor. These mechanical drive equipment are in turn affected by other external phenomena. For example, the power that is generated from gas turbines is dependent on ambient conditions such as ambient air temperature.
AN INTEGRATED APPROACH IS NEEDED
Having looked at some of the factors (external to the centrifugal compressor) that can affect its performance, it becomes clear that optimization of oil & gas compression systems requires a holistic integrated systems approach. In other words, it needs to go beyond just a narrow look at the centrifugal compressor. As the performance problem with the centrifugal compressors can often be the result of other extraneous factors leading to suboptimal and off-design performance, routine like for like overhauls will only deliver short-term benefits. An experienced integrated asset approach must be combined with advanced turbomachinery expertise to deliver lasting success.
Working at an early stage with a knowledgeable and integrated team that includes the OEM and specialist engineering consultants who understand centrifugal compressors, as well as the wider oil and gas systems in which they operate, is the best approach in my opinion to achieve lasting optimization success.
