Technology’s present situation and the development prospects of energy efficiency monitoring as well as performance testing & analysis for process flow compressors

In this paper, the background of performance testing of in-service process flow compressors set in user field are introduced, the main technique barriers faced in the field test are summarized, and the factors that result in real efficiencies of most process flow compressors being lower than the guaranteed by manufacturer are analysed. The authors investigated the present operational situation of process flow compressors in China and found that low efficiency operation of flow compressors is because the compressed gas is generally forced to flow back into the inlet pipe for adapting to the process parameters variety. For example, the anti-surge valve is always opened for centrifugal compressor. To improve the operation efficiency of process compressors the energy efficiency monitoring technology was overviewed and some suggestions are proposed in the paper, which is the basis of research on energy efficiency evaluation and/or labelling of process compressors.


Introduction
Process flow compressors are key equipment of processing industries such as petrochemical engineering, coal chemical engineering, etc., and have huge shaft power and flux, complex pipe net and variable working conditions and gas components, which result in much difficulty in the performance testing before ex-factory and energy efficiency evaluating at user field. Processing compressors in petrochemical, coal chemical and metallurgical industries, for examples, the hydrogen compressor in oil refining, the cracking gas, propylene and ethylene compressors in ethylene production, the blower in blast-furnace and the large scale of axial compressor in air-separation system, consume huge energy and have lower operation efficiency [1].
Four types of compressor, reciprocating, screw, axial and centrifugal, are mainly used in the processing industry, and their routine tests in the manufacture's factory are very difficult because of the larger rate of flow and shaft power, complex pipe line, multiple gas constituents and their feature of inflammable, easy explosion and the toxicity. Normally the delivery test only include mechanical running, impeller accelerating and seal trial [2]. So, the compressor performance curve from manufacturer do not reflect their real quality. However, users need real performances of processing compressors to meet the demand of flow parameter variety. Reliability, higher efficiency and long term running are basic demands of user to processing compressor, but in real operation more attentions are paid to its safety. For an example, in the surge control system of centrifugal compressor the anti-surge valve is generally opened in advance and the compressed gas is forced to flow back to inlet pipe so that the operation efficiency of compressor set is decreased. On the premise of safe and reliability how to keep the compressors operating at higher efficiency at wider range of working conditions is main developing direction of process compressor technologies, to rate real efficiency of process compressor, to grade compressors and to constantly monitor their performances for energy saving operation are also the key point of development.
In this paper it will be introduced that the present situation and development trend of performance worksite test and energy efficiency monitor technique of processing compressors. Take centrifugal compressor as example, the reasons of performances difference between user field and manufacturer factory, surge control and energy consume increase are analyzed. Relative technologies of process compressor will be discussed in the paper too.

Performance curves and background of user field test of process compressor as well the results analysis
Energy efficiency evaluation of compressor is basis of energy saving and carbon emission reduction, and also an effect method of impartial assessment of real operation efficiency of compressor set. For general power and refrigeration compressor, the manufacturer and/or the third part can test their real performances at varies operation conditions with different working fluids by setting up test rig because of smaller compressor power, easier operation condition adjustment and simpler gas component. So, similar to the energy efficiency standards of power and refrigeration compressor of Japan, EU and US [3], China also drawn herself energy efficiency standards and began energy efficiency identification of product [4].
For process compressor, so far, there is no complete energy efficiency identification standard in the world because exist many indeterminacy factors in the performance test. The match of compressor performances with parameter change of the process system is a big problem, which results in lower operation efficiency of compressor set. So, it is significant to research performance test and energy efficiency monitoring technique, and to draw energy efficiency standards of process compressor.

Theoretical performance curves of centrifugal compressor
Delivery quantity, discharge pressure (pressure ratio), power and so on, are characteristic performance parameters of compressor, and are also the design basis of product. Performance curve of centrifugal compressor depend on four relative parameters, the inlet flow rate Qs、the inlet pressure ps, the inlet temperature Ts and the rotation speed of compressor n. At constant inlet states (ps, Ts) and n, the stage pressure ratio  and polytrophic efficiency pol ,which can be gotten by numerical analysis and/or theory calculation. The polytrophic efficiency pol  is calculated using equation (1) below [5].
Where k and m are isentropic index and polytrophic exponent respectively, and  is pressure ratio.
Change trend of theoretical performance curves of centrifugal compressor is shown in figure 1(a). Vary of pressure ratio and efficiency at certain rotation speed with inlet flow rate can be gotten. At certain rotation speed to decrease inlet flow rate the pressure ratio gets large, and as the inlet flow rate is reduced to some value the surge condition of compressor set occurs. Conversely, at certain rotation speed to decrease pressure ratio the inlet flow rate gets large, and as the pressure ratio is reduced to some value the blocking condition of compressor set occurs. The conditions between surge and blocking make sure safety operation of centrifugal compressor.  For positive displacement compressor, for example, the piston and screw compressors, whose theoretical efficiency is expressed as isentropic efficiency. The efficiency, power and delivery quantity (inlet flow rate) of piston and screw compressors also vary with rotation speed of compressor, see figure  1(b). At certain pressure ratio the maximum efficiencies of piston and screw compressors correspond to some rotation speed, but nothing to do with delivery quantity, so, the surge and blocking conditions do not occur for positive displacement compressor.

User field test and results analysis of process compressor
User field test of process compressor not only may get their real operation performance datum to resolve conflict between users and manufacturers or vendors, but provides references for manufacturer to improve product design to adapt real high efficiency and reliability operation demand of compressor.
M Sib Akhtar introduced a Joint Industry Project entitled "Pointing to the optimum performance of centrifugal compressors for oil & gas" [6]. In the project, design and operating data of more than 50 compressors and drivers were obtained, performance and history of the machine was analyzed over an 18-month period, and real performances varying with working conditions was gotten. For simplicity, the pointes expressing efficiency at different operation conditions in Ref. 6 were deleted, only fitted curves were retained, see figure 2.  Real operation performance curves of centrifugal compressor not only influence operation reliability of compressor set, for an example, the surge working condition will occur in advance as real flow rate is less than its demand at design conditions, but have a big effect on the energy consumption of compressor set, for instance, much more power is needed because real operation efficiency is lower than the designed value. Data from China petroleum and petrochemical energy saving monitoring centre is shown in table 1.

Difficult points of user field test of process compressor and discuss of energy efficiency labeling
The direct object of user field test of process compressor is to verify if the compressor performances guaranteed by manufacturers could be reached at design working condition and real gas operation, moreover, to provide users the actual performances curve of centrifugal compressor including curve trend, working condition points corresponding to smallest and largest flow rate for operation control and adjustment.
User field test of process compressor is different from lab or factory workshop test, field test is difficult and test result is uncertainty. Firstly, design of test system placing is subject to field environment, system parameters selection like that in laboratory is impossible. Pipe placing, operation condition change and gas compositions and so on will influence the accuracy of instrument installment and test, so, the special test plan for particular user field is essential. Secondly, it is difficult to choose the test instrument suited to the variable operation conditions. In addition, severe field environment and maintenance & calibration measure let measuring apparatus being damaged more easily, which influence test precision accuracy.
Field performances test is basis of efficiency evaluation and energy rating label of process compressor. The third part is responsible for the test and needs wide consultation on test plan from manufacturers, users and designing institute, on which typical compressor sets from petrochemical and metallurgy flow equipment can be selected for test, for examples, centrifugal and reciprocating compressor in oil refining factory, the axial compressor in the blast furnace, etc. The performances of the elected compressor sets can be tested according to ASME PTC 10 Performance Test Code on Compressors and Exhausters [7], the main parameters include inlet and outlet pressure and temperature, flow rate, gas compositions, speed, power and so on.
Main thermal parameters of process compressors can be gotten by field test and the performance parameters such as pressure ratio and efficiency of centrifugal compressor can be calculated, on which the curves of Qs- and pol  -Qs at given speed can be plotted. At same time, about one year operation data of the compressor sets were recorded and sorted out, which is reference value of efficiency evaluation and energy rating label, see figure 4.

Efficiency
Volumetric flow rate

Figure4. Grading of energy efficiency of process gas compressor
For the grading of energy efficiency of process gas compressor shown in figure 4, the fundamental performances curve should be confirmed and approved by vast majority of users and manufacturers first, then energy efficiency grad is formulated according to the deviation between real performance and fundamental performance, which is expressed in equation (2).
, calculate, monitor and record production capacity and efficiency that reflect performance parameters of compressor operation, and track the deviation situation and trend between real performances and the date from health state model and if it reaches the set threshold alarm will sound.
US OnSet Corporation summarized the defects of now available control system for compressor set [9], the efficiency control problem of application together of positive displacement and turbo compressors, the effect integration of different bands of compressor products, the advanced algorithm for best compressors combination, the complete performance monitoring, and the accurate surge condition forecast. For process compressor the indirect monitoring of power can be realized by direct testing of electric current, but more precise power calculation is necessary.
Ted Gresh introduced an on-line performance monitoring system [10], by which the health condition of compressor can be evaluated by comparing instant performance data with the parameters from factory. The performance calculation software they developed can be used to calculate the input power by using the original parameters such as pressure, temperature, speed, and flow rate of compressor. Operation state change of compressor could be monitored by the records for some time, which is used for specified maintenance and trouble founding, see figure 5.

Figure5. Comparison of compressor performance between design and actual in Flexware company
A. Suomilammi and J. Leppäkoski [11] introduced another on-line performance monitor system with fuzzy classification and merging technology. System can show the desired value by using fuzzy clustering prediction model as well acquisition of history data of outlet pressure, temperature and speed of compressor, and as the performance of compressor set seriously deviates from the desired value the maintenance order will be touched off.
ITCC, Integrated Turbine Compressor Control, is a new technique based on advanced technologies such as electronic, communication, control algorithm, and so on. Nowadays, the main functions of integrated control systems for large scale of process compressors include unit interlock ESD (Emergency Shutdown Device), SOE(Sequence of Events) recording, unit control PID (for examples, anti-surge control and speed adjustment control), and routine instruction record, fault diagnosis, etc. Widely used control system in the world include Micro Net TMR of Woodward Corp., Tricon of Triconex Corp., TMR of CCC, FSC of Honeywell, GMR of GE, Trusted of ICS Triplex, and H41/H51 of Hima. Speed adjustment control, process control and compressor control in conventional control system are separate and provided by different venders, while three function control modules and their interrelationship are utilized thoroughly in the ITCC, the positive actions are enhanced and negative actions are eliminated, on which the performance integrated control of increasing operation efficiency and reliability of compressor sets, saving energy consume and fund and minimizing operation and maintenance cost can be realized to the greatest extent. ITCC can decouple anti-surge control returns, and an application case of ITCC in propylene compressor is shown in figure 6 [12]. From it we can see that the compression process of compressor comprises of six sections with each anti-surge control return and share a shaft, and scope of stable operation working conditions can be widened by move left of the new surge curve. Irwin Stambler [13] introduced a complex array intelligent tools by which the general health situation of gas turbo system with single cycle and combination cycle can be inspected from points of mechanics and thermodynamics. For compressor performances the operation condition can be assessed by comparison of real data with desired one.
Under the assistance of fund from Chinese Academic of Engineering and National Nature Science Foundation of China authors participate in the present operation situation investigation of compressor sets in petrochemical and metallurgy industries, and mainly focus on sets heath operation and energy efficiency monitoring [14]. Most of large scale of centrifugal compressor and piston compressor sets in the process device in China have been installed on-line operation state monitoring system but no energy efficiency monitoring system.

Development prospects of Energy Efficiency Monitoring
Energy efficiency monitoring is an important measure to ensure high efficiency operation of compressor sets. The technique development trends of energy efficiency monitor system include: (1) to synchronously test much more parameters, to set reference data base, to create real performance curves of process compressors on the basis of history operation data, and to realize the function parameters