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Field Performance Testing of Centrifugal Compressors

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Field Performance Testing of Centrifugal Compressors FIELD PERFORMANCE TESTING OF CENTRIFUGAL COMPRESSORS by Roy B. Pais and Gerald J. Jacobik Dresser Industries, Inc. Dresser Clark Division Olean, New York Roy B. Pais is a Head Product Engi­ (flow to speed ratio) . Techniques for varying the � ratio neer at the Clark Division of Dresser for both fixed and variable speed drives are discussed. Industries, Inc., Olean, N.Y., where he has worked since 1970. He has A method of calculating horsepower consumed and the responsibility for the design and de­ capacity of the compressor is shown. Limitations of a field velopment of centrifugal compressors performance test are also commented upon. as well as associated instrumentation and control systems. Field Performance testing of Centrifugal compressors He graduated from the University with meaningful results is an area of considerable interest of Bombay with a B.S. in Mechanical to the process and gas industry. Although much time and Engineering in 1968. In 1970 he ob­ money are spent on a field test, one must remember that tained an M.S. in Mechanical Engineering from the State the accuracy of the results is not always in proportion to University of New York at Buffalo. the amount of effort put into the test. The preferred meth­ He is an active member of the A.S.M.E. and is currently od, of course, is to perform the test in the Manufacturer's Chairman of the Olean Section. Shop where facilities are designed for accurate data ac­ quisition, and subsequent performance computations. Gerald].]acobik is the Test Super· visor of the Dresser-Clark Division Proper planning is essential for conducting a meaning· Test Department, Dresser Industries, ful test. A test agenda must be drawn up to cover the Inc., Olean, New York. He is respon­ scope of the test. It should discuss the kind of test to be sible for the supervision of required performed and also define the basic objectives. The agen· personnel to test assigned equipment da should describe the instrumentation to be used and manufactured by Dresser-Clark in­ also list the data required. When the compressor manu· cluding centrifugal compressors, pipe· facturer gets involved in a field test, it is essential that the line boosters, Pacific pumps, Clark test conditions be discussed and an agreement on the I sopac compressors and ensure that acceptance of the test agenda by both the manufacturer they meet contractual requirements and user be made prior to the test. and customer specifications, both in house and in the field. His main concern is to establish mechanical and aerody­ DATA REQU IRED TO DETERMINE namic suitability of equipment by test set-up methods, COMPRESSOR PERFORMANCE instrumentation, data acquisition and analyzation. Mr. ]acobik received an A.A.S. degree in mechanical Evaluation of compressor performance involves the de­ technology from Erie Count)' Community College, Wil­ termination of the inlet capacity, pressure ratio and horse­ liamsville, New York in 1962 and has been associated power consumed for the specific conditions of the test, with Dresser Industries, Inc. for 12 years. which include inlet pressure and temperature, discharge pressure, compressor speed and gas properties. The fol­ Symbols listed at the end of the paper. lowing observations and measurements are required: ABSTRACT 1) Inlet Temperature The paper describes the type of data that must be rt'· 2) Discharge Temperature corded in order to performance test a centrifugal com­ pressor in the field. The instrumentation required to 3) Inlet Pressure measure different test parameters is also discussed. The 4) Discharge Pressure need for accurate measurements, as well as the effect errors in the data have on the calculated performance, is 5) Barometric Pressure explained. 6) Compressor Speed In order to obtain the characteristics of the compressor 7) Differential Pressure across flowmeter. Temperature and Pressure at the flow measuring device are also it is necessary to record data for different values of Q N required. 84 FIELD PERFORMANCE TESTING OF CENTRIFUGAL COMPRESSORS 85 8) Specific Gravity, thermodynamic properties and poles and line frequency. For other drives, speed can be analysis of the test gas measured by an electric counter actuated by a magnetic pulse generator or a 60 tooth gear. The latter method is TEMPERATURE MEASUREMENT preferred and most driver manufacturers install a gear on the driver shaft for this purpose. Accurate temperature measurements are very necessary in evaluating performance. Unfortunately, accurate meas­ urement is not easily achieved. Different kinds of instru­ FLOW MEASUREMENT mentation are available to measure temperature and the choice of any one particular kind depends on the accu­ The following are the various ways by which flow can racy desired and accessibility at the point of measure­ be measured: ment. Thermocouple and glass thermometers are common­ ly used devices. Thermocouples are more advantageous to a) Flow Nozzles use because of the following: b) Concentric Orifice Plates a) They are simple in basic design and operation. c) Venturi Tubes b) They can attain a high level of accuracy. d) Elbow Flow Meters c) They are suitable for remote reading. e) Calibrated pressure drops from the inlet flange to d) They are relatively inexpensive. the eye of the first stage impeller. A calibration curve is used for this purpose. When using the e) They can be installed in relatively inaccessible areas curve, it is important to ensure that the Reynold's and can also be inserted into the gas flow. number during the field test is identical to the one Since the output signals of thermocouples are small, during the calibration test. sensitive measuring devices must be used to measure Usually one of the above flow measuring techniques is them. incorporated as a part of plant piping. The choice of any Though glass thermometers are available in wide ranges one system depends on the cost, allowable pressure drops of sensitivity and accuracy, they are not recommended and accuracy required. for field testing. They are fragile and also have the dis­ advantage of not being adaptable for remote reading. Also, thermometers are placed in a fluid bath, and since LOCATION OF INSTRUMENTATION heat transfer losses are difficult to account for, tempera­ Figure l shows the location of the pressure and tem­ tures measured may not be very accurate. perature probes. Instrumentation at the compressor inlet must be placed downstream of any valves, elbows or PRES SURE MEASUREMENT screens. Similarly, instrumentation at the discharge Pressures are generally measured with high quality should be placed upstream of any valves or elbows. Bourdon tube test gauges which must be calibrated against Temperature and Pressure measurements for the pur­ a deadweight tester in their normal operating range. In pose of calculating flow must be measured right at the view of the considerable amount of time involved in read­ flow measuring device and not at some place hundreds of ing different pressures, deadweight gauges are rarely used feet away. for field testing. When selecting a pressure gauge it is im­ portant to note that the measured value is above the mid­ It is recommended that provisions be made right at the point of the scale. If one reads a 2 PSIG pressure with a design stage for location of instrumentation in the process 0-100 PSIG gauge the reading will be very inaccurate. piping. Sometimes a set of different pressures are measured with one gauge by manifolding and valving. This approach eliminates gauge errors. However, reading too many pres­ sures with one gauge has a disadvantage in that flow con­ ditions may vary during the time interval between record­ ing the first and last pressure measurements. DISCHARGE TEMPERATURE INLET TEMPERATURE 4 PROBES - 90° APART Differential pressures and subatmospheric pressures are 4 PROBES - 90° APART generally measured with manometers with suitable gravity fluids to permit easy reading. A fluid that is chemically stable when in contact with the test gas, as well as one which does not attack the materials of the compressor or process piping, should be selected. Precautions should be INLET STATIC PRESSURE - o taken to prevent foreign matter from blocking the sensing 4 TAPS 90 APART DISCHARGE STATIC PRESSURE lines and mercury traps should be used to prevent the 4 TAPS - 90o APART manometer fluid from entering the process piping. t SPEED MEASUREMENT In the case of a synchronous motor drive, the syn­ chronous speed can be calculated from the number of Figure 1. Location of Instrumentation 86 PROCEEDINGS OF THE THIRD TURBOMACHINERY SYMPOSIUM ACTUAL TEST FIG. 2. ALLOWABLE DEPARTURE FROM SPECI­ FIED OPERATING CONDITIONS After selection, calibration and installation of instru­ mentation, the test procedure can get underway. In se­ VARIABLE DEPARTURE % lecting a particular procedure, consideration is given to plant operation and the kind of variables available. 1. Inlet Pressure, psia 5 2. Inlet Temperature, oR 8 In order to obtain the characteristics of the compressor, 3. Specific Gravity 2 it is necessary to record data for different values of Q/N 4. Speed, rpm 2 (flow to speed ratio). The different techniques used for 5. Capacity, cfm 4 varying the Q/N ratio are dependent on the flexibility of the process and whether or not the compressor has a fixed NOTE: or variable speed driver. 1. The combined effect of 1, 2, and 3 shall not produce more than 8% departure in inlet gas density. For fixed speed drives the compressor is run near its 2. See paragraph on Care Required in Taking Test Data. overload condition. By throttling either the suction or dis­ charge valve the flow is decreased in increments and measurements are made at each value of flow until a FIG. 3. ALLOWABLE FLUCTUATION OF TEST minimum flow operating point is reached. READINGS DURING A TEST RUN For variable speed drives, the flow can be varied by MEASUREMENT FLUCTUATION% changing the speed of the compressor.
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