Unsteady Pressure and Velocity Measurements in Pumps

The objective of this paper is to report the state-of-the-art of unsteady pressure measurements in pumps. Though optical techniques seem in some aspects more attracting, unsteady pressure measurements can give a detailed insight in static as well as dynamic operating behaviour of pumps. A brief description of wall mounted pressure transducers and of probes fitted with miniature pressure transducers will be given. The main topics of this report are: • Miniature pressure sensors • Resolution in time and space (response characteristics) • Data acquisition • Ensemble/phase averaging • Measurements in stationary and rotating frame • Detection of non-synchronous components • Pressure and velocity measurements by means of high response probes (2D and 3D) 1.0 INTRODUCTION Modern design methods like CFD give detailed insight into the flow through turbo machinery. Some years ago when CFD became popular many people thought measurements would become dispensable. Today we know, it is not. Indeed, we have a growing demand for detailed measurements of the flow in turbo machinery to compare this to the obtained CFD results. Particularly with regard to the calculation of losses and efficiency pressure measurements are indispensable. Unsteady pressure measurements are widely used to examine the unsteady behaviour of pumps, for example part load pumping or operation under cavitation. Unsteady pressure measurements are carried out in the field of research as well as condition monitoring. Wall mounted pressure sensors are used for the investigation of stator and rotor flow near casing walls (Fig. 1, b and c). Measurements of pressure distribution on rotor blades are carried out with miniature transducers and telemetry systems for data transmission from rotating rotor to stationary frame of reference. Probes fitted with miniature pressure transducers are used to examine the rotor exit flow (Fig. 2) and will give complete information on the 2D or 3D flow field including velocities and pressure distribution. Wulff, D.L. (2006) Unsteady Pressure and Velocity Measurements in Pumps. In Design and Analysis of High Speed Pumps (pp. 4-1 – 4-34). Educational Notes RTO-EN-AVT-143, Paper 4. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 01 NOV 2006 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Unsteady Pressure and Velocity Measurements in Pumps 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) TU Braunschweig Institut für Strömungsmaschinen Langer Kamp 6 D-38106 Braunschweig GERMANY 8. PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, distribution unlimited 13. SUPPLEMENTARY NOTES See also ADM002051., The original document contains color images. 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT UU 18. NUMBER OF PAGES 34 19a. NAME OF RESPONSIBLE PERSON a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 Unsteady Pressure and Velocity Measurements in Pumps 4 2 RTO-EN-AVT-143 Figure 1: Unsteady Pressure Measurements by Means of Pressure Sensors Mounted in Stationary and in Rotating Components. Figure 2: Measurement of Rotor Flow using High Response Probes. Unsteady Pressure and Velocity Measurements in Pumps RTO-EN-AVT-143 4 3 2.0 SENSORS FOR UNSTEADY PRESSURE MEASUREMENTS 2.1 Design Since electronic data acquisition is common practice today, electrical signals proportional to the measured pressure become essential. These signals can be generated by electromechanical pressure transducers. When pressure is applied, the force on the sensing element due to the pressure results in a deformation of the sensing element. This deformation changes the electrical properties of the element and therefore the electrical output of the sensor. Sensors for unsteady pressure measurements need a high natural frequency. Fig. 3 shows the idealized frequency response of a pressure transducer, the output is essentially independent of frequency below one-fifth of the resonance frequency [7]. Generally, a high natural frequency is achieved with small sensors with low mass and front-sided diaphragm. Figure 3: Idealized System Frequency Response. Fig. 4 compares the size of a common pressure transducer suited for steady measurements with a miniature transducer for unsteady pressure measurements. The natural frequency of the common transducers is in the range of 100 Hz while the miniature transducers is around 100.000 Hz (rising with increasing pressure range, see table 1). Miniature pressure transducers are commonly available with three pressure options: gage, absolute and differential. While in common transducers metal strain gages are used, in miniature transducers silicon semiconductor bridges are employed, therefore these are called piezoresistive transducers. Advantages of the latter are: lower weight, smaller size, higher sensitivity and higher frequency response. The primary disadvantages are greater thermal sensitivity and zero shift. Fig. 5 shows a typical layout of a silicon semiconductor bridge. Most piezoresistive transducers on the market employ a fully active Wheatstone bridge consisting of four gages, which are diffused, in a silicon diaphragm. The silicon integrated chip is itself the diaphragm. Therefore a protection against the measured fluid is necessary. While for short-time service in water a coating is sufficient, for long-time service a welded steel diaphragm is advisable. The rear side of transducer is commonly sealed with epoxy that will give no adequate sealing against water intrusion. Therefore standard transducers cannot be submerged in water without damage. Fig. 7 shows a transducer with custom-made tube fitting on the rear side to provide watertight long-time service. Installation in an impeller is shown in Fig. 8, tubing on the rear side is required for routing sensor cables to amplifier and telemetry. Unsteady Pressure and Velocity Measurements in Pumps 4 4 RTO-EN-AVT-143 Figure 4: Pressure Sensors with Four-Arm (Wheatstone) Bridge for Steady (LUCAS SCAEVITZ, top) and Unsteady (KULITE, bottom) Pressure Measurements. Battery (middle) to compare size. Table 1: Specification of KULITE XTM-Type Piezoresistive Pressure Transducer with Steel Diaphragm Pressure Range 1,7 bar 7 bar 35 bar Input Impedance 650 Ohms Output Impedance 1000 Ohms Sensitivity (FSO) 7,5mV/V Natural Frequency 75 kHz 125 kHz 300 kHz Residual Unbalance ± 3% FSO Combined Non-Linearity and Hysteresis < ± 0,5% FSO Thermal Zero Shift ± 2% FSO Figure 5: Layout of Four-Arm Bridge of Piezoresistive Pressure Sensor. All strain gage and piezoresistive transducers are passive devices and require an external excitation (Fig. 6) to provide an output signal. The energy sources must be stable and well regulated to avoid error signals at the output. The excitation causes a finite current to flow through the bridge, which results in an increase in temperature. Water will assure good heat conduction, so this effect can in most cases be Unsteady Pressure and Velocity Measurements in Pumps RTO-EN-AVT-143 4 5 neglected. Common power sources are DC or AC carrier power supplies. In most cases DC power supplies are preferred because of their higher frequency response. Figure 6: Schematic Diagram of Full Bridge Transducer with AC-Amplifier. Figure 7: Piezoresistive Transducer with Custom Made Sealing on Back (KULITE XTM-XX-190M). Figure 8: Piezoresistive Transducer (KULITE XTM-XX-190M) Fitted in Pump Impeller. Unsteady Pressure and Velocity Measurements in Pumps 4 6 RTO-EN-AVT-143 If only information on the unsteady component of pressure is required, piezoelectric sensors can be used. Unlike strain gage transducers, piezoelectric devices require no external excitation. Because their output signal levels are low, special signal conditioning (charge amplifier) is required. Since measurements on pumps often deal with cavitation and therefore information on absolute pressure is required, piezoresistive transducer will be the common choice. 2.2 Response Characteristics In any dynamic measurement, the frequency response of the transducer and the data acquisition system must be considered. The essential requirement to pressure measurements is that one must consider the fluid coupling to the transducer from the measuring point. A transducer mounted flush with the wall (Fig. 9a) will give the highest frequency response while installation according to Fig. 9c) can severely limit the response of the system. On the other hand an installation according to Fig. 9c will give better spatial resolution than a flush-mounted transducer. Obviously there is a trade-off between spatial and temporal resolution. 2.2.1 Sonic Speed Sonic speed of a free propagating wave in a liquid is: