Massimiliano Di Febo and Pasquale Paganini

Control systems are the brain of the Process Plant. Their importance is well recognized, and, for this reason, the design and implementation of plant control systems are considered from the first design until the final start up stages. Medium and large plants may include many different process areas with different kind of machines. Each one of the involved processes may require special attention and precautions in order to render the best outputs and contribute to maximizing the overall plant performance. Despite their importance, it often happens that the plant control system may not be fully optimized.

In this paper we’ll restrict the focus to centrifugal compressors plants, introducing some main concepts applicable to the compression station control system. Furthermore, a case history shall be presented about the optimization applied by the IPC team on the plant control system.

Also, this paper will provide some interesting evaluation of benefits and estimation of the environmental impact.

Compression station control systems elements

Each compression station includes a station control system. This system is the array of all the devices devoted to managing, control and protecting all the plant physical assets, including machinery, piping, drivers, instrumentation, auxiliaries etc.

Controlling a compression station requires at least a couple of main tasks: assure an adequate machinery protection, and assure plant stability, meaning that the main plant parameters should be fluctuating, during operational time, within the expected operative range. Plant stability is then the result of the control system's ability to coordinate all the machinery and devices during the operational time. Plant stability allows to avoid unwanted phenomena as recycles and shutdowns that shall be better described later.

The following paragraph will give a quick recall of the main areas that make up a compression station control system.

API std 617 provides detailed indications about the compressor instrumentation. Onboard equipment should include vibration and temperature probes. Sensors are normally located in proximity of rotor bearings where vibration and temperature readings are taken and compared to alarms and trips. Field instrumentation assures that the machine will not operate outside the permitted range. Not only machinery but also process items (headers, scrubber, coolers, etc) are often equipped with sensors to detect potentially anomalous or even dangerous conditions. Instrumentation represents the base layer of the control system upon which all logic and control actions are based.

One of the main tasks is “load control”. This task provides for maintaining a mass balance between the gas coming into the station on the gas outgoing. This may appear as a simple task. The reality is more complex, and many problems are normally faced especially when the station has a less simple architecture such as in stations with many compression trains in parallel. The load control is usually carried out by acting on the speed of the driving motor (frequently a gas turbine for turbo-compressors) or by acting on the "throttle" valve in the case of constant speed drivers (frequently asynchronous motors for motor-compressors). It should be noted that the load control has the suction header pressure as process parameter. But the same suction pressure affects the flow in the anti-surge valve which is controlled by the anti-surge controller. If these two control actions should occur at the same time, they would influence each other, and the station could be destabilized by persistent oscillations, potentially triggering recycles and shut downs.

The situation is complicated by the fact that, in some circumstances, the anti-surge valves are used for load control and therefore adequate strategies are needed to accommodate these events, thus assuring overall station stability.

Other factors must also be considered for the purpose of system stability, such as the closing speed of the anti-surge valve, when and under which conditions the anti-surge valve closure should be permitted, how to manage the single compressor recycle when working in a compression train, how to manage more trains in parallel, and how to manage a compressor ramp up into the parallel and so on.

These considerations show how delicate and important is the compression station control system duty, and how it may require a high level of sophistication in case of non-trivial plant configurations.

Among critical operative conditions the surge deserves a special mention: It consists of a very quick inversion of the flow inside the compressor, which happens when the characteristic curve becomes unstable, i.e. when the derivative of the discharge pressure vs the flow approaches zero. Surge has a very destructive nature: the inversion of the flow causes high vibrations and violent axial displacements that overload the bearings. During the surge cycles the gas may achieve very high temperatures causing damage to the compressor parts.

Most of the times the actual operative conditions are different from design ones, consequently the identification of the compressor surge point becomes complicated because it requires the adjustment of the compressor performance curves at the actual off-design conditions. While this capability is provided by IPC software CMap, traditional systems are often still based on approximated methods that when implemented turn out to be quite conservative and set the protection thresholds quite far from the surge point. This often causes the compressor to enter the recycle condition even when not strictly necessary.

While this approach goes in the direction of machine safety, it causes, as a corollary, a higher impact both on financial and environmental point of view. In fact, during operation, plant instabilities may trigger the protection actions. The effect is that the compressor goes off-line and the gas incoming to the station is inevitably burned in the flare.

Shutdowns are critical events that require the machine to be immediately stopped. This happens usually when some of the critical operative parameters exceed alarm values hitting the trip point. Of course, shutdowns represent a loss of money due to lost production during time intervals necessary to reset the system and re-start.

Re-cycles are recirculation of the compressed gas back from the machinery discharge to the suction side. The recycling of a centrifugal compressor is most often due to the triggering of the anti-surge valve emergency opening. This protection action is actuated by the control system when the operative flow becomes very near to the surge point (normally 1,03 of surge flow).

Also recycles have a great impact on the overall station productiveness since they require system reset, although compressors are not stopped. System reset requires human intervention for acknowledgment and problem understanding and is therefore time consuming. Recycles have a first step where the machine enters into protection mode (anti-surge emergency opening) and remains in this status until operator acknowledgement, and a second step were the machine recovers its load. Also recycles represent a loss of production because the gas continues to be flared until the completed machine reloading, causing a neat loss of gas production, a loss of energy for unnecessary compression power, a CO2 equivalent impact for this wasted energy and a very high impact for the CO2 injected in the atmosphere.

In the case history considered in this paper, the compression station was made up of three independent compression trains (independent gas sources), with each train including two compressors in series. Each train was driven by a single fixed speed electrical driver. Each single compressor was equipped with its own anti-surge valve.

Before the beginning of the plant optimization project, the compression plant was affected by a high number of critical events, mainly recycles and shutdowns.

After an observation period dedicated to understanding the problems and the behaviour of the system, the IPC team decided to proposed to the plan management to move along the following main directives aimed at boosting plant stability:

Evaluation of compressor performance, by comparing operational values to the OEM expected performance.

Observation of plant dynamical behaviour with an understanding of the linke between the impact of plant parameters and machine operative performance

Execute detail analysis of all the involved control system tasks and design of control tasks modification aimed at reducing or minimizing the appearance of critical events.

The directive A) was applied using the available field measurement data and applying to that the capabilities of CMap software, IPC commercial software for the prediction of centrifugal compressor performance.

CMap contributed largely to identifying compressor performance non-conformity and then to update control system settings for effective compressor operative conditions. The flexibility, reliability and quickness of the software contributed to obtaining a better picture of the overall system operational status.

Cooperation with the supervising personnel was decisive in obtaining an understanding of the observed dynamics and infer the dependencies of causes and effects in a reliable way. These activities helped in increasing the stability level of the compression system, reducing the number of recycles and shutdowns to a few events per year.

Financial and environmental impact of the optimizations

Before the launch of the optimization project, the plant was affected by an average of 1 recycles per day per train with a duration of about half an hour per recycle.

In these transients each compression train was recycling about 40,000kg/h, with the overall station flaring about 72,000kg/day. The estimated loss through flared gas was about 4.6 million euros per year, including loss of production and accounting for cost of energy spent on recycles.

During recycles the train were wasting about 3 MW with an estimated loss of about a quarter of million euros per year due to unnecessary adsorbed electrical power.

Total loss: about 4.9 million euros per year.

From an environmental point of view the situation was even worse: the CO2 equivalent introduced in atmosphere for the unnecessary recycling power was about 1160 tons per year while the CO2 flared was about 70400 tons per year. CO2 equivalent for heat wasted by flaring was about 13500 tons per year.

Just for giving to the reader a rough idea of how much important this impact is, we could recall that this amount of CO2 would be equivalent to the amount of CO2 produced in a year by 42720 passenger cars. Another interesting comparison could be the one with wind turbines: about 18 medium size wind turbines (2.2MW) would be necessary to avoid the introduction in the atmosphere of the same quantity of CO2, at a cost of 40 million euros.

Following the history and data presented in this paper, one consideration emerges: all efforts towards any possible improvements of process plants control systems and for compression plant controls optimization strategies are justified from both a financial and environmental impact.

The adoption of advanced strategies for machinery monitoring and station control systems represents, from the financial point of view, a justified investment.

From the environmental point of view benefits are also more than evident: the gas industry, beyond the financial goals, bears also the moral responsibility for environmental compliance, and is therefore even more spurred to apply improvements and innovations on the control system area.

Knowledge of compression systems, application experience and the support of specialty machinery software tools as CMap can be therefore considered as valid support in reducing cost and impact on the environment.

Articles

Optimizing compressor control systems

Assuring safe and optimal operative conditions for centrifugal compressors is often understood as a primary goal for the process plant maintenance management teams. In the course of the last few decades, the approach to process plant machinery maintenance has progressively, but radically, changed. The initial corrective approach, with repair interventions after failures, was followed by the preventive approach, with parts replacement at fixed work hours.

Though the last approach represents a considerable improvement, especially for important machines, it has not been considered optimal since it requires the execution of maintenance interventions and parts replacement even in the absence of failure signals. An optimal method should then be able to understand when the component is approaching critical status and provide a notification with enough advance advice to organize the necessary logistics and resources for maintenance interventions.

2-Actual trends and compressor predictive maintenance

For many and most common centrifugal machines the predictive techniques commonly implemented are, today, connected to vibrational conditions and mechanical indicators (mainly bearing vibrations and temperatures). But in many cases the appearance of defects is subtle, such as in situations where a degradation of the machine appears without being accompanied by comparison of evident mechanical signals. In such cases poor machine performance becomes progressively worse and starts to affect plant performance without triggering mechanical indicator alerts.

In these situations, the problem source continues to operate inside the machine, cumulating its adverse effects, until it appears with high intensity. A typical case of this subtle behaviour is fouling, which introduces a progressive loss in performance and efficiency despite the absence of mechanical indicators.

Another aspect of the actual practice connected to the reading on vibration is that the measurement changes with the machine operational point while the threshold remains fixed. Above considerations highlight the importance of machinery performance analysis.

The underlaying machine modelling paradigm is today called “digital twin”. The quantitative approach at the base of this  method may allow the connection to AI (artificial intelligence) solutions. Machine capability to deliver expected performance is then a root signal of machine health, and lack of performance is the earliest external indicator of the presence of some problem. For compressors, the calculation of expected performance requires a state-of-the-art approach involving aeromechanical machine model and gas thermodynamics and consequentlythe use of adequate calculation tools.

3-New generation monitoring systems, the proactive approach

The next generation of machinery monitoring systems should therefore embed the capability to dynamically evaluate machine performance according to effective operative conditions, which “dynamically” indicates that the limits should be adjusted to the machine's actual operative condition. Also, these systems should be able to anticipate in time the identification of the problem, shifting the focus from the failure effect to the failure cause and provide possibly indications about the time scale in which the problem is expected to develop. This is a proactive approach and represents the effort to push the preventive maintenance philosophy to its inherent limits.

Process computers offer the computational capability required for the implementation of an accurate model-based performance monitoring methods bringing the traditional predictive systems into the proactive maintenance dimension. Advanced diagnostic capabilities can therefore arise from the coupling of machine performance analysis capability with the predictive methods based on the analysis of mechanical parameters (vibrational, thermal and tribological), thus not replacing but embedding and extending the previous system's capabilities.

4-Performance-based diagnostic method and definitions 

At the core of a performance monitoring system should be the capability to compare the actual performance to some reference value. This reference value should represent the optimal machine performance, i.e., the performance of a new machine or a machine that has been just shop-restored to conditions like a new one. Therefore, design performance or as-tested performance may be used as valid reference point.

It should be noted that while the measurement of actual performances is quite straightforward, the evaluation of the design performance may be quite complex. In the case of centrifugal compressors, in order to perform a comparison between homogeneous values, the design performance needs to be adjusted to actual operative conditions. This adjustment step is not trivial and, as said, requires the availability of adequate computing tools. A complex machine aeromechanic and thermodynamic is necessary to get precise numerical results and allow a correct comparison with field measured parameters.

Today, thanks to the diffusion of more powerful computational tools and machinery performance prediction software (as Camp software for centrifugal compressorsperformance), such complex machine modelling capabilities are available in the market and allow to implement diagnostic strategies based on the analysis of performance.

The introduction of the concept of performance evaluation within the machinery maintenance theme signifies also the irreversible step from qualitative evaluation to numerical and model-based methods, allowing the alignment of machinery maintenance to other departments such as engineering and operations. This process of integration is sometime called “Performance productive maintenance” and allows a projection into overall plant productivity.

The starting point of the method is the availability of a centrifugal compressor performance curve, the relevant gas mix composition, and thermodynamic conditions (pressure and temperature). Having this input data available, the software will perform all complex calculations in a fully automated way and will produce the expected compressor performances for inlet pressures, inlet temperatures and gas mix compositions for design/reference ones. It's possible to obtain a valid comparison term for the evaluation of compressor performance in actual conditions, referred here as expected performance. The method is compliant with the ASME PTC10 (Performance Test Code on Compressors and Exhausters) standard

Compressor map in reference condition at variable speed

Inlet reference condition i.e. inlet pressure, inlet temperature and gas mix composition

Selected EOS for gas properties calculation

Measured parameters (actual) used as inputs are:

The system will provide the following outputs:

Compressor performance curve at the actual speed and inlet condition

Once these performance values are obtained, it’s possible to compare the actual values to the expected ones. The difference is an indicator of how much the compressor behaviour deviates from the normal/optimal behaviour. These indicators shall be referred as “performance deviations”.

A value, in whatever operative condition, will be “normal” when its deviation is zero. Deviation zero consequently represents an operative status were the machine is operating just as expected according to reference (or design) values, in the underlaying hypothesis that the design performance is relative to an optimal (as new) machine condition or represents the reference term for any evaluation. These parameters may be trended in time. The change in time of a deviation parameter shall represent a measure of how quickly the observed performance moves away from its “normal” value.

When a performance deviation becomes irreversible in time it is possible to say that the machine has suffered performance degradation. A stable degradation is the sign of a defect that is non-critical and compatible with machine operations and affecting mainly performance. Degradations increasing in time will lead to critical events if not interrupted. Time trends of performance deviations, deviation rates and degradations will provide a very powerful indication of machine health status and may be used along with mechanical indicators, with specific diagnostic logics to address possible failure causes.

In this paragraph a real case will be presented. A compressor was running largely under off-design inlet conditions. A performance-based monitoring system was implemented. The system included a calculation server devoted to run the machine model (CMapsoftware) to obtain the expected or “normal” performances in actual (off-design) conditions. PC/PLC based architecture allows to bring to machinery panel level all the computational power necessary to predict machinery behaviour as per OEM expectations and use this information to continuously evaluate the machinery healthy status.

Automated application of CMap can provide continuous monitoring of the machine performance and exert an automated surveillance and diagnostic. Also, the compressor protection from surge can be automatically and continuously updated to actual inlet conditions, overcoming limitation of actual systems.

The availability of expected performance allowed the comparison to measured field values and evaluation of performance deviation and degradation. The following picture shows a comparison of the actual efficiency to expected efficiency in actual condition. As previously mentioned, this parameter is a powerful indicator of compressor operative condition and healthy status: eventual deviation between expected and actual parameters is an indicator of the occurrence of malfunctions.

The same trends are available for discharge pressure, discharge temperature, polytropic head and power. Machine performance analysis can generate diagnostic advice on the nature of the possible failure directing the user towards some main defect areas such as mechanical, wear/fouling, and instrumentation.

Machinery monitoring systems will be the next generation of machinery surveillance systems. The quantitative performance evaluation approach allows to implement predictive maintenance strategies optimizing the schedules of interventions for maximizing production and minimizing costs.The proposed method could provide benefits especially for centrifugal compressors that work in high and very high-pressure ranges and under rapidly varying process conditions.

Methods proposed and described in this paper allow to achieve the following goal: Predict the performance of a centrifugal compressor in off-design condition. The prediction of compressor performances is accurate even at high pressures, where the ideal gas theory commonly used introduces considerable errors.

During experiments done on existing machinery the described methods allowed to implement a more accurate surveillance of the machines and detect failure indicators earlier than what is possible with traditional methods. The availability of a real time machine model continuously predicting the expected performance has been used to detect possible anomalies in instrumentation reading. When such error conditions are detected, the calculated expected flow may be temporarily used to replace the measured one allowing the system to continue to run without interruption while replacing or repairing the faulty transmitter.

Compressor operation: Continuous monitoring of possible failure modes

BY MASSIMILIANO DI FEBO & PASQUALE PAGANINI

Assuring the availability of centrifugal compressors is one of the primary tasks for maintenance teams. Quantitatively evaluating machine performance plays a key role in understanding if a compressor is working as expected. Software tools, such as Cmap, allow the user to predict compressor performance and obtain a quantitative indication of deviating parameters.

Compressor performance depends on gas composition and the operating inlet pressure and inlet temperature. Gas composition affects the way gas moves through the compressor.

Any assessment of compressor performance, therefore, requires the ability to predict pressure, temperature and gas composition. Computational models must incorporate compressor aeromechanics and compressed gas thermodynamics.

Field inlet conditions are sometimes different than the conditions given in machine specifications. An adjustment of design performance maps to actual operative conditions, then, is necessary to quantitatively assess the performance.

This approach to numerical analysis results shows how variations of individual gas mixture components effect compressor condition and can aid in performance prediction. All numerical evaluations were developed using the most recent thermodynamic theories and machine aeromechanical models. These were done according to the prescriptions of the ASME PTC10 (Performance Test Code on Compressors and Exhausters) standard.

The starting point is the reference centrifugal compressor performance map. This can be taken from OEM datasheets or from “as tested” maps. The software combines the gas mix composition, pressure and temperature parameters with available reference performance maps to perform complex automated calculations to predict compressor performance.

That makes it possible to determine the consequence of each variation in gas components with regard to overall compressor performance. The study also looked at machine performance for gas mixes with the same molecular weight but with a different gas composition.

Four different variations of gas mixtures were considered. Inlet conditions were kept constant as follows:

This analysis obtained the expected performance for the compressor in each of the four off-design conditions considered. Performance changes were mapped against discharge pressure, discharge temperature and power. These charts are also useful in noting shifts in the surge line.

It is possible using advanced computational tools, therefore, to accurately determine centrifugal compressor performance under varying inlet conditions. These performance curves aligned well with field measurements of real world equipment and conditions.

This method of predicting performance maps for a centrifugal compressor in off-design conditions, therefore, has been found to be accurate even at high pressures. It enables engineers to predict adjustments based on inlet pressure, temperatures and gas composition to implement advanced protection from surge. It also can be used for early identification of machinery trouble before other indicators, such as vibration or other mechanical parameters, appear.

RAISING PERFORMANCE

DETERMINING THE PERFORMANCE OF CENTRIFUGAL COMPRESSORS IN OFF-DESIGN CONDITIONS

by Massimiliano Di Febo & Pasquale Paganini

Centrifugal compressors are widely used in upstream as well as middle-stream process plants. The ability to detect early indicators of eventual malfunction plays a crucial role in determining plant efficiency and profitability.

Traditional predictive maintenance techniques relate to the vibrational and structural dynamic aspects of rotor operation. But predictive strategies based on performance analysis help diagnosis machine status during operation.

For centrifugal pumps, for instance, machine models are being used to compare measured performance to design performance. The same process is more complex for centrifugal compressors due to the dependency of compressor performance upon the gas mix composition and inlet conditions (pressure and temperature). Therefore, a more complex model is required.

One of the primary difficulties in the performance analysis of centrifugal compressors arises from the need for a performance map adjusted to actual inlet conditions. One approach is to reduce the complexity of the problem by considering compressor head as invariable and applying simplified machine formulas. This works for low-pressure ranges and constant gas mixes.

However, error is introduced as soon as pressures rise or the gas mix changes. In these situations, it is necessary to adjust the design performance to operational conditions.

The starting point is the availability of a centrifugal compressor performance curve, the relevant gas mix composition and thermodynamic conditions (pressure and temperature). With these input data are available, expected compressor performance can be calculated.

The elements necessary to achieve this include: Reference and design compressor maps available from OEMs; reference input conditions and off-design input conditions. The Cmap software tool is used to analyze this data and produce an accurate performance model.

Take the case of a compressor operating with off-design inlet conditions. The model calculated at actual flow, the values of expected pressure, temperature, head and efficiency in the off-design conditions and compared them to the measured ones. This highlighted an error of about 9% for discharge pressure compared to its original design expectation. This approach is field tested to predict compressor performance. Different equations of state can be included in calculations based upon variations in the gas mix.

These results provide a useful analytical basis for compressor maintenance decisions. In most cases, deviations indicate the early stages of a problem. This proves especially useful for those centrifugal compressors that operate in high-pressure ranges, as well as under time-varying process conditions.

This method also opens the door to continuous monitoring of machine performance. Surge protection can be strengthened by automatic updates concerning actual inlet condition due to different inlet pressures and temperatures, as well as varying gas compositions.

Authors: is operation manager at IPC. IPC offers technical services and

technology solutions for machinery, system, plant devices such as turbo machinery and relevant, compressors, pumps and relevant monitoring, control and protection systems. Pasquale Paganini is technical manager at

IPC, which has developed the IPC SPS system powered by Cmap software. For more information, visit www.ipc-eng.com or www.compressormap.com

COMPRESSOR PERFORMANCE

Compressor test, usually performed at OEM facilities, are in general performed to validate the performance as stated by the OEM in the machinery specification sheets.

Centrifugal compressor shop test represents one of the main step in the machine manufacturing process. The importance of this phase is also attested by the fact that, in almost all cases, the project quality requirements includes a witnessed test, to be executed in order to assure that the manufacturer predictions meet the specification (design) performance.

Shop testing for centrifugal compressors is than a quite delicate task. In general it may happen that inlet conditions, during the test, are different from specification conditions defined in the machines data sheets.

Sometime the need for different inlet pressure and temperature may occur, sometime the test could require a gas different from the one specified in the data sheet; in some cases both conditions may turn up simultaneously. The ASME PTC10 (Performance Test Code on Compressors and Exhausters) standard identifies these possible different test cases and refers to Type1 Test for test with same specification gas and specification conditions, and Type2 test for test with different gas and/or inlet conditions.

In those cases where inlet conditions during the test are different from the ones recalled in the specifications, the OEM provides usually the compressor performance maps adjusted to test conditions in order to superimpose on that the compressor performance measured during the test.

Similar consideration may apply also to field testing where, moreover, the inlet conditions, at test time, may not be under full control and may represent an independent variable for the test process. Also in certain circumstances inlet conditions may vary in time in unpredictable way. In these last particular cases the availability of computational tools for the prediction of compressor performance under changed (off-design) inlet conditions may be of valuable help.

This paper is aimed to show the results of a numerical simulation of centrifugal compressor test performance for a type 2 test, with test gas different form specification gas, where test gas is R134a. The input for this analysis consists in the compressor performance maps provided by the OEM in specification conditions i.e. with a specification gas, in the considered case an hydrocarbon.

A numerical analysis is executed, using the IPC software CMap, in order to predict the compressor performance when running in test conditions with a test gas. The output of this analysis is a performance map referenced to specific test conditions with R134a refrigerant (simulated test map). The obtained simulated map (predicted performance adjusted to the test gas R134a) may be than used as reference for a comparison to the test performance measured during the shop test.

The main parameters used to assess the performance of a centrifugal compressor are:

Some additional conditions are needed to allow a significant comparison of these parameters in the two different referenced conditions (specifications/test). From a fluid dynamic point of view a strict similarity of flow at each performance point is necessary in order to compare the predicted performance with tested performance. For this reason the non-dimensional parameters head coefficient, flow coefficient and Mach number must be conserved primarily (some additional conditions are requested on, the machine Reynolds number and the Machine Mach number and volume ratio).

This equality of non-dimensional parameters among test and specification conditions, eliminates the requirement to test the centrifugal compressor at the same speed as OEM predicted performance in specification conditions and allow, within a certain range , to test with gas mixes different form the design one.

In this paper the test performance has been predicted considering Freon R134a as test gas. R134a is in fact a typical gas used in many performance test.

As said the starting point is the availability of an OEM centrifugal compressor performance curve in specification conditions, along with relevant gas mix composition and thermodynamic conditions (pressure and temperature). Having this input data available, the CMap software will perform all complex calculations in fully automated way and will produce the predicted compressor performances for test inlet pressures, inlet temperatures and Freon R134a different from the design / reference ones.

CMap allows to perform the compressor performance prediction using different equations of state (EOS) depending on gas mix considered in the calculation. For hydrocarbon gas mixture Lee-Kesler or PREOS can be used. For Freon R134a the MBWR EOSshall be selected to determine the thermodynamic properties of the operating fluid.

After this first calculation step it will be possible to proceed to the comparison of the predicted tested performances to the actual tested performances. The next table shows the inlet condition of predicted performance and inlet condition of tested performance:

Starting from the centrifugal compressor performance map in specification conditions, the tested performance curves have been calculated.

The following picture shows the predicted tested performances curves obtained.

 Fig 2: discharge pressure in Specification condition[/caption]

 Fig 3: polytropic head in Specification condition[/caption]

 Fig 4: discharge pressure in test condition (with Freon R134a)[/caption]

 Fig 5: polytropic head in test condition(with Freon R134a)[/caption]

 Fig 6: compressor surge in actual operative conditions[/caption]

As stated in the beginning of this paper, a first application mode of this method consists in the execution of a comparison of the test predicted maps to the measured test performance. Using software the OEM specification (Design) maps are in fact quickly adjusted to the test conditions with a test gas and different inlet pressure and temperature. Thanks to the software this operation is quick and straightforward. Obtained predicted maps can be easily superimposed to measured test maps. This comparison will allow to check the capability of the machine to match the specified conditions.

The proposed method can be profitably used also to predict the centrifugal compressor overall performances when running with operative inlet conditions different from specification ones.This may be the case of a compressor running in operative conditions different form specification ones. In this case the proposed method will allow to predict the expected compressor performance when running in actual inlet conditions. The availability of predicted performance (i.e. design performance adjusted to actual operative conditions) will also allow a new kind of diagnostic capabilities arising from the possibility of comparing actual performance to expected ones: a deviation of the measured (actual) performance from the expected (adjusted) ones, beyond a certain threshold, may be read as a first indication of some of phenomena or problem affecting the machine and causing its operative parameters to deviate from expected values.

The detection of a deviation of actual efficiency form the expected efficiency (i.e. design adjusted in actual condition) is a meaningful indicator of compressor healthy status. This is a powerful feature that permits to have continuous indication about how much the machine behavior is aligned with design expectations. These evaluation can be than used to have historian trends and to build pictures of the machine status along the operation period. Collected data will be useful to support decision making on predictive maintenance activities plans an operations.

Availability of such kind of compressor advanced protection feature increases the level of machinery protection and allows to avoid potential critical failures due to the catastrophic effect of the compressor surge as well as the dangerous effect of possible surging events of short duration but with repetitive occurrence, whose cumulative action may cause an important reduction of the machine lifetime if not detected.

Simulation of compressor performance test

Today machinery maintenance works require improvements in the evaluation of the machine conditions. In fact, often one critical task for the plant management consists in taking the decision to stop the machine or run until the next planned plant outage. These evaluations and decisions are in many cases still based on an expert’s indications, which are often experiential and subjective.

Software can help assess centrifugal compressors' health conditions through quantitative comparison of field operative parameters to ones predicted from machine computer model.

While intended not to replace human experts but rather to support them better, the method has been applied jointly with mechanical best practices, vibrational machinery analysis, accurate mechanical inspections and high quality repair processes and procedures.

This approach, aimed to exceed the actual best practices, can support plant management in the maintenance decision-making steps and maximization of plant ROI.

Quantitative assessment of compressor performance and health status needs the comparison of measured performance to design ones adjusted at actual operative conditions. Starting from OEM design or test performances maps, software allowed to predict compressor expected performances in the actual inlet conditions (actual suction temperature and pressure and gas mix composition).The software calculates, at the actual running speed, the following expected curves, valid for actual inlet conditions:

Availability of above maps allows to calculate, at the actual flow, the values of expected pressure, temperature, Head and efficiency and then compare to the measured ones. A comparison of these values can detect deviations and provide quantitative information about the actual compressor health status and possible operational problems.

The following picture shows a typical process screen

With reference to above process screen, the following table compares the pressures and temperatures from PT/TT transducers to the value predicted by software:

In the previous tables it can be noted how the maximum error calculated is less than 1 %.The same comparison has been executed for Head and Polytropic efficiency.

Measured errors have been considered tolerable for the purpose of the evaluation of compressor field performance.The analysis developed obtained an evaluation of the efficiency deviation (difference between the actual compressor efficiency and the expected efficiency in the actual operative conditions). Time trends of calculation results provided a useful analytical basis for compressor maintenance decisions.

The method has been profitably used to predict compressor performances and support planning of machinery maintenance activities.

Numerical prediction of compressor performance allows a quantitative evaluation of compressor health status. The applied methods allowed also to:

predict the performances of a centrifugal compressor under varying thermodynamic conditions of the inlet gas. The prediction of compressor performances is accurate even at high pressures, where the ideal gas theory and simplified formulas approach is commonly used, introduce considerable errors.

analyze the performance of the compressor during operation and compare them with those expected provided by the OEM.

implement advanced protection from surge, overcoming limits of the current technology

having useful indications on the health of the compressor (diagnostics) based on the capability to analyze the performances and efficiency of the machine in a simple and immediate way

support decisions and planning of maintenance and activities for optimization of plant productivity

Quantitative assessment of compressor performance

The downstream oil and gas industry is celebrating its first century. Developed in the USA in the early 1900s, the refining and gas processing business of today has gigantic dimensions. It possesses a high degree of technological sophistication, with rotating machinery playing a vital role.

Plant process conditions, after all, can be severe: an average-sized topping unit can provide an output capability of 1500 tons/hour. At critical process points such as furnace charge pumps, temperatures are close to 800°C.

A Fluid Catalytic Cracking (FCC) plant, for example, requires the feed mixed with the catalyst and pumped to the reactor at temperatures of around 500°C. Air required by the process is normally supplied by centrifugal or axial compressors with a flow rate of around 50000 Nm3/h and with a power output of several megawatts. Rotating machinery for compression and pumping applications often has a critical role in determining the robustness of the plant. Centrifugal pumps, in particular, are present in almost all of these plant processes.

The centrifugal pump market offers downstream process applications a wide range of size options. A large number of centrifugal pump configurations have been developed for various applications from single to multistage, axial or radial split, with a horizontal or vertical rotor position (Figure 1).

(Figure 1: A wide range of size options are available for centrifugal pumps)

The most common pump type is the horizontal one used in pressures of up to 400 bars and temperatures of up to 450°C.Vertical pumps, on the other hand, are used across a more restricted range of applications – mainly in low Net Positive Suction Head (NPSH) conditions and very low temperatures.

Technical regulations and industry standards for process industry pumps include API 610 and Hydraulic Institute codes. These provide comprehensive guidelines that drive the whole pump production process. Developed by a board of manufacturers and customers, these standards cover everything from early engineering to manufacturing.

API 610 has been subject to several revisions. For example, the criteria for evaluating pump efficiency moved from previous tolerances on efficiency to actual tolerances imposed on overall absorbed power. Vibration limits had analogous modifications,too, reflecting increasing attention on longer machine lifecycles and better energy efficiency.

Material technology has also undergone an evolution. As pumps are subject to corrosion, erosion and fatigue, material selection is a primary task in the achievement of a satisfactory machinery lifespan. Metallurgical processes, these days, enable greater control of material chemistry as in the case with duplex steels (CD4MCu) or special steel grades with precipitation hardening (17-4-PH), often used in shafts. Innovative processes such as plasma and laser coatings for sleeve surface hardening also allow better wear resistance.

The development of the computerized Finite Element Method (FEM) has allowed better understanding of the stress field inside pump components. This has been facilitated by the greater availability of computational techniques and adequate computational powers.

As a result, modern rotating machinery design can take advantage of dedicated software tools that allow better evaluation in areas such as bearing selection, as well as balancing piston and sleeve clearance sizing.These elements, along with boundary design operative conditions, have contributed to greater rotor stability.

The introduction of Computational Fluid Dynamics (CFD) has also exerted an influence upon the development of the process pump industry. The hydraulic design of three-dimensional pump flow patterns is a delicate task. More computational power and the availability of reliable software code has allowed consistent improvement in the centrifugal pump design process, as well as the ability to integrate OEM experience withreliable simulations.

Modern CFD commercial coding has opened the door to accurate modeling of viscid (viscous) and inviscid fluids, more turbulence equations, non-stationary algorithms for the rotating regions of turbomachinery, and better designs with shorter turnaround times.

The advancement of control systems is another area that deserves mention. Recent European Community studies have demonstrated how inefficient many pump drive applications are. In the low power range, about 76% of the overall energy is absorbed (Figure 2).

(Figure 2: CFD diagram showing pressure contours)

Despite this, the concept of improving control systems for equipment in the low power pump range has received little attention when compared to large pumps where advanced control systems are currently used. This is partly due to the low ratio in terms of benefits versus costs for small size pumps i.e. more sophisticated control system would have a major impact on overall costs for small pumping stations. However, this is due to the large energy savings possible by integration of variable frequency drive (VFD) systems and auto-diagnostic control systems (Figure 3).

(Figure 3: Energy savings via improved controls can prove economic in even small scale and low power pump applications.)

Modern pump control systems enable the implementation of pump automation using a small number of field sensors. This is achievable through the implementation of performance-based models and commercial PLC. Compared to an unmonitored pump installation, slightly higher installation costs are offset by reduced maintenance and repair costs, as well as a drop in downtime.

These systems offer multivariable control so the user can set the process parameters to be controlled. The system then provides continuous and automatic monitoring of all relevant parameters. By adding a VFD drive, higher energetic efficiencies are achievable (up to 40%) when compared to traditional throttling methods. Additionally, performance-based models allow designers to predict expected performance in operative conditions. By comparing this to actual performance, control systems can be adjusted to provide accurate auto-diagnostic indications – particular anomalies such as cavitation upset, mechanical degradation and transmitters failure.

Operation Manager at industrial Plants Construction (IPC), a firm that developed the IPC Proteo control system for pump applications. 

Pasquale Paganini, is Technical Manager at IPC. For more information contact 