How to Handle Excessive Centrifugal Compressor Recycling Part 2
Previously in Part 1, I began to look at the issue of excessive, continuous, and unwanted recycling of centrifugal compressors in oil and gas operations. Some recycling is needed for the safe and smooth operation of centrifugal compressors. This is mainly to protect the compressor from surges. Centrifugal compressors are however not meant to operate continuously in recycling as a normal operating regime. If this is this case, this means there is a problem that needs addressing due to its significant impact on overall production efficiency. In order to resolve the problem, one has to first understand the likely causes.
In the prequel to this article, I identified five causes of excessive centrifugal compressor recycling. In this article, I will highlight another five causes and then look at possible solutions.
These are as follows:
MORE CAUSES OF EXCESSIVE RECYCLING OF COMPRESSORS
6. Poor Compressor Design & Selection: Centrifugal compressors used in oil and gas applications are bespoke equipment, they are not off-the-shelf machinery. This means that the design and selection must be suited to the process requirements. This should not just be the case at the start of field life but further down the line when the process duty may have changed. If the selected design is not a good fit for the process requirements, it can result in excessive compressor recycling due to a variety of reasons.
7. Overly Sensitive Anti-surge Control System: An anti-surge control system can become overly sensitive to small process changes. This can lead to a situation in which the compressor oscillates in and out of recycling. This is a control issue and is usually a result of an inadequately sized/specified anti-surge control valve system.
8. Incorrectly Set Anti-surge Control System: this is similar to the cause given above but the focus here is more on the anti-surge control algorithm settings and their impact on the surge control line. If the anti-surge control settings are not properly set up, you could have a conservative set system and this could lead to continuous excursions into recycling.
9. Process Changes: Significant and sudden process changes or perturbations can lead to oscillations in and out of recycling or even continuous operation in recycling. These would normally be caused by process phenomena upstream of the compressor which will result in significant changes in inlet pressure or flow.
10. Inefficient Configuration: this can be a problem where there is more than one compression train or more than one compressor section/stage. This cause can manifest in a variety of ways. In the case, of multiple compression trains in parallel, issues with load-sharing controllers can mean that the balance is not right between all operational compression trains. This can lead to one or more trains operating within or around the recycle region.
Another way this can be an issue is when you have too many operational compression trains for the current duty. This tends to happen when the facility has been operating for some time with a certain number of compression trains, and an adjustment needs to be made for the reduced flow encountered in later life.
The third way the configuration can be inefficient is when you have a hybrid configuration system. For example, a flexible parallel/series compression train configuration system to enable the compressor sections to be switched between parallel and series by means of a complicated valve and piping system – I have seen this used in some gas storage applications in a bid to tackle the often vastly different production and injection compression duties. These flexible systems can lead to a situation where it is optimized for one configuration and not for the other; this can lead to excessive recycling operation. These complex systems are also inefficient for the huge pressure losses that result from the complex piping and valve systems.
RESOLVING THE PROBLEM
By now, those of you that are logical thinkers as I am would be thinking – broadly speaking, we have design, control, operational, and process issues as possible causes of excessive compressor recycling; surely there would need to be significant changes in these areas to resolve the problem.
I agree with you, so depending on the diagnosis of the problem, one or more of the following may prove effective:
1. Redesign & Rewheel of the Centrifugal Compressor: This is when a modification to the existing aerodynamic design of the compressor is achieved – usually through the modification or replacement of the aerodynamic assembly. Normally, the compressor bundle/internals is replaced with a modified design while the existing casing is retained. Such changes may lead to changes in the geometry of numbers of inlet guide vanes, impellers, diffusers, return channels, etc.
The effect of this is a change to the performance envelope of the compressor to be better optimized for its current and future duties. A lot of major problems around excessive compressor recycling can be resolved this way. However, the devil is in the detail as it is one thing to say that a compressor wheel needs to be done and another to establish what exactly needs to be done. This needs to be determined by a proper engineering study carried out by an experienced engineering consultant.
2. Modification of Control Algorithms: Here the control parameters such as gains, biases, surge control margins, and other tuning parameters can be adjusted to better optimize the system. This should however be carried out carefully from a good understanding of where the actual surge position is. This is normally established during surge mapping tests. There also needs to be a good understanding of how the anti-surge control system works. I would recommend that the controls vendor is significantly involved in such adjustments. Similarly, load sharing control algorithms can be adjusted in the event that the recycling is caused by an imbalance between the compressor trains.
3. Modification of the Anti-Surge Control Valve System: The anti-surge control valve size, response times and stability has a big impact on the overall stability of the compressor. These can be investigated by means of dynamic modeling and simulation. The results of which can be used to make adjustments to better optimize the recycle valve system.
4. Changes to Operational Philosophies & Configuration: Inefficient configurations can be handled by making necessary operational adjustments. This may mean operating with fewer trains or a single compression train. This may be combined with a redesign and wheel to better optimize the system, especially for later field life scenarios. Complex configurations such as parallel/series switching compression trains should be avoided where possible. Where they exist and are contributing to inefficiencies, one mode of operation should be chosen and optimized for current and future duties.
5. Changes to Process Gas Inlet Conditions: Small adjustments to the inlet process conditions of the compressor can have a significant impact on its operating point and performance envelope. For example, adjusting process equipment upstream of the compressor to achieve a slightly higher inlet pressure at the compressor may sometimes be sufficient to significantly alleviate the load on the compressor, giving it more favorable operating conditions. This can lead to the elimination or reduction of recycling.
6. Turbomachinery upgrades, overhauls & maintenance: Due to technological advancements, most gas turbine manufacturers periodically release new variants of their existing product line. Usually, these have higher power capability and better efficiency. This presents upgrade opportunities for compression facilities struggling with recycling problems due to insufficient driver power. This is especially relevant if such facilities are operating with an older variant of the gas turbine driver.
A gas turbine upgrade may provide the necessary additional power to increase the throughput and pull the compressor out of recycling. With overhauls and other maintenance activities, the in-service performance of gas turbine drivers and centrifugal compressors can also be recovered. This improves the operation of the compressor and reduces the risk of unwanted recycling operations.
