Keywords: pressure-reducing valve, vibration, nobe. INTRODUCTION. Control valves handling compressible fluids can generate unpleasant noise, particularly ...
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A Practical Solution to the Problem of Noise and Vibration in a Pressure-Reducing Valve A. Amini I. Owen University of Liverpool, Department of Engineering, Liverpool, United Kingdom
-.The mechanical vibration that is occasionally found in gas pressure reducing valves can be eliminated by careful design of the valve plug and seat. A pressure-reducing valve was found to be excessively noisy, producing a sound pressure level of 117 dB when throttling air at an inlet-to-outlet pressure ratio of 15; as a result the valve suffered wear and vibration damage. By changing the design of the original flat plug and seat, the problem was significantly reduced. A 60° conical plug and seat produced a noise reduction of 12 dB (a factor of 4 in actual sound pressure level), the mechanical vibration was eliminated, and the flow capacity was increased by about 25%. Keywords: pressure-reducing valve, vibration, nobe
INTRODUCTION Control valves handling compressible fluids can generate unpleasant noise, particularly when exposed to high pressure differentials. The problem arises from the jets of fast-moving gas on the downstream side of the pressurereducing valve, which mix with slower moving gas, causing the loss of kinetic energy and hence a pressure drop across the valve. At certain critical conditions, the conversion of energy into noise can sometimes reach about 150 dB. In stating the problem it needs to be recognized that there are two categories of noise. The first and most c o m m o n is aerodynamic noise resulting from the Reynolds stresses or shear forces created in the flow stream as a result of rapid deceleration, expansion, or impingement. The second category of noise, and the one that is of particular concern in this paper, is the mechanical noise and vibration that can be stimulated in the moving parts of the valve by the fluid-dynamic pressure fluctuations. This vibration, as well as being aurally unpleasant, can very quickly lead to damage of the valve stem and seat [1]. The source of these two categories of noise is usually the same, with the principal area of noise generation being the recovery region immediately downstream of the vena contracta formed when the flow discharges through the gap between the valve plug and seat and where the flow field is characterized by intense turbulence and mixing. When there is a large pressure drop across the valve, the noise is also affected by the choked-flow shock formation in the gap between the valve plug and seat [2, 3]. Therefore, although the main concern of the present paper is the potentially harmful mechanical vibration in a pressure-reducing valve, the two types of noise are directly
coupled. Figure 1 shows a pilot-operated pressure-reducing valve in which the area of interest is the main valve plug and seat. The valve plug is essentially a flat disk that sits on a flat, raised annular face, the two being lapped together. Of concern in this valve were the wear on the push rod in its guide and the damage to the valve seat and plug. Analytically, the prediction of aerodynamic noise emanating from the control valve is based on two principles [2]: first, the noise created by the control valve during the throttling process, which is a function of the mass flow and fluid Mach number across the control valve restriction; second, the transmission of the sound into the downstream pipe and then radiation of the sound by the pipe exterior into the surrounding environment. Formulations predicting the level of control valve noise have been proposed [4, 5]. The mechanical vibration in a pressure-reducing valve was investigated by Nakano et al. [6], who showed how the flow separating from the valve plug can be unstable and oscillatory in nature. Various seat and plug geometries and various flow regimes were investigated. An important conclusion from the study was that the level of flow instability was dependent upon the geometry of the flow passage between the plug and the seat. This conclusion is widely accepted and is the basis for the design of "quiet valves" such as those described by Seebold [7]. The principle of these designs is not to throttle the flow through a sudden expansion as in a flat plug and seat design but to expand it through a number of tortuous passages so that pressure loss occurs not only by turbulent dissipation but also by viscous effects. The narrow flow passages normally associated with these designs, however, lead to blockages and to severely reduced flow capacity.
Address correspondence to Dr. A. Amini, Department of Mechanical Engineering, University of Liverpool, P.O. Box 147, Liverpool L69, 3BX, United Kingdom.
Experimental Thermaland Fluid Science 1995; 10:136 141 © Elsevier Science Inc., 1995 655 Avenue of the Americas, New York, NY 10010
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Figure 1. Pressure-reducing valve. Pressure-reducing valves handling gas flows will suffer from aerodynamic noise. To a large extent, the level of noise, and whether or not it will stimulate mechanical vibration, depends upon the pressure reduction across the
valve. A pressure-reducing valve on a compressed air system, for example, will normally have a limited pressure reduction because the air is usually required at moderately high pressures for use in actuators, etc. In this case the associated noise, although it may be unpleasant, is limited and can be reduced by judicious use of silencers and insulation [4, 7]. Steam pressure reducing valves, on the other hand, can be required to reduce the steam from mains pressure to atmospheric pressure, or even below, depending on the temperature required by the process. (The temperature of saturated steam is governed by its pressure, and for processes that require heating at 100°C, for example, the steam might be throttled from a mains pressure of 10 bar to a process pressure of 1 bar.) Because of these high pressure ratios, mechanical vibrations are more prevalent in steam pressure reducing valves. Furthermore, to obtain such a large pressure ratio, the gap between the valve plug and seat will be small; any vibration in the valve will therefore cause the plug to "rattle" on the seat and will lead to damage. The study reported in this paper was carried out because under certain operating conditions it was found that the steam pressure reducing valve shown in Fig. 1 suffered from mechanical vibration and wear. The critical operating conditions were mainly dictated by the inlet and outlet pressures. The study focused on modifying the design of the valve plug and seat to provide a valve with reduced
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noise and no mechanical vibration but at the same time having no significant reduction in its flow capacity. For ease of experimentation, the vibration tests were carried out using air, with the results being finally confirmed using steam. Flow capacity tests were also carried out using steam. E X P E R I M E N T A L APPARATUS AND PROCEDURE The four basic designs of valve seats and plugs are shown in Fig. 2. The standard design is a fiat disk mating with a fiat thin-annulus seat. The cone design is a matching conical plug and conical seat; the design shown is for a 60° angle; other angles between 15 and 75 ° were also tested. The stepped design is one that was recommended by Hutchinson [1]; this was included in the tests to provide a comparison as it employs the "tortuous-path" principle of "quiet valves." A number of variations of this stepped design were also tested, but the results are not reported here because the design is not a good one, as will be shown later, and the inclusion of the additional results would only confuse the issue. Figure 3 shows a schematic diagram of the equipment used to measure the noise and vibration of the valve working with high-pressure air. The upstream pressure to the pressure-reducing valve was varied from 13.6 barg to 1.7 barg at intervals of 1.7 barg. For each valve seat, the desired setting to generate sound was achieved by turning the adjustment screw (to obtain critical condition; see Fig. 1). Sound pressure level measurements were obtained by use of a condenser microphone coupled to a frequency analyzer. The microphone was placed on the centerline of the outlet at a distance of 1 m downstream of the pressure-reducing valve. An accelerometer was placed at the top of the pilot diaphragm to record the mechanical vibration in the valve. The output of the vibration spectrum was recorded on a transient recorder. The capacity tests, which were carried out for the standard plug and seat and for two of the valve plugs and seats that gave the lowest sound pressure level measurements, were made using the steam flow measurement rig shown in Fig. 4. In these tests the upstream pressure, Pu, was kept constant at 7 barg while the downstream pressure, Pd, was increased from 1 barg to 5 barg. The uncertainty in experimental results was calculated in accordance with [8] and reflects the reliability of the instrumentation and the accuracy with which it could be read. The uncertainty was determined using Student's t distribution at the 95% confidence level. In most cases only one value of the uncertainty in the variable was calculated, which represents the total uncertainty accounting for readability, unsteadiness, instrument calibration, and any estimated fixed errors. All the error contributing to the uncertainty in the variable was identified and quantified and then combined to obtain the uncertainty in the final results. The range of experimental error in the basic data in the pressure measurement is +__0.5% of the reading, the mass flow rate error was calculated to be ___1%, the error in sound pressure level measurement is about + 2%, and the error in frequency and amplitude is + 1.5%. The uncertainty in seat angle is due to uncertainty of measuring equipment and is about 0.5%.
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RESULTS AND DISCUSSION The results of sound pressure level measurements for the conical valve seats are shown in Fig. 5. This figure shows how the valve generally produces the most noise (a combination of aerodynamic noise and mechanical vibration noise, where it occurred) for pressure ratios in excess of 8. The reduction in noise between pressure ratios of 4 and 8 is believed to be due to a change in the flow regime within the valve. Nakano et al. [6] showed similar effects and related them to the flow regime using schlieren photography. It can be seen that all the conical designs produced lower noise than the standard design, and, referring to Fig. 6, it can be seen that the 60° seat appears to be the optimum. The reduction in sound pressure level is about 12 dB at an upstream pressure of 14 bar. It should be pointed out that this is a significant reduction, since a change of 12 dB, being on a logarithmic scale, represents a reduction in the sound pressure level by a factor of 4. The reduced noise level of 104 dB is the measurement of the jet noise exhausting into the atmosphere. This is obviously still very high, but in practice it should be borne in mind that the valve will be exhausting into a pipeline, not to the atmosphere. How the sound pressure level of the valve fitted with the 60° cone plug and seat compares with the stepped and standard designs is shown in Fig. 7. The standard design is clearly the noisiest, with the stepped and conical designs being very similar at pressure ratios in excess of 8. The sound pressure level measurements discussed above are very illustrative. However, they do not distinguish between aerodynamic noise and mechanical vibration, al-
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though an increase in the total noise in the standard valve design was usually associated with the onset of mechanical vibration. An accelerometer was placed at the top of the pilot diaphragm to measure directly the mechanical vibration. The valve pressure ratio was set to about 14, a position at which the greatest noise and vibration occurred, and the vibration was recorded on a transient recorder. The results of this are shown in Fig. 8. The amplitude of vibration for the standard design is very much higher than for the stepped and cone designs, with a number of resonant frequencies being found. What this shows, and indeed what was easily discernible during the tests, is that the stepped and conical seats eliminated the mechanical vibration. The levels of vibration amplitude shown for these designs correspond to fluid-dynamic vibrations and not mechanical ones. The final tests carried out with the valve were to measure its flow capacity when fitted with the different designs of plug and seat. The results of these are shown in Fig. 9. As expected, the capacity of the stepped design, due to its tortuous flow path, is significantly lower than that of the standard design. What was not expected, however, was the result that the flow capacity with the 60 ° design is significantly better than that of the standard design. This is believed to be due to the flow passage being more suitable for pressure recovery than that of the standard design. Thus for a given pressure drop there can be a higher flow rate, since for a given valve lift the geometry of the plug is such that the resultant flow areas are similar. It is clear, therefore, that the cone design of plug and seat offers a solution to the problem of mechanical vibration without incurring any penalties regarding flow rate normally associated with "quiet valves." The laboratory
Noise and Vibration in Pressure-Reducing Valve 300
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Pressure-reducing valves are a c o m m o n feature of all high-pressure gas systems. These valves all have the capability of producing undesirable and excessive noise and vibration. In a steam system the pressure of the steam is selected to provide saturation t e m p e r a t u r e with the process requirements. Therefore pressure-reducing valves throttling steam often o p e r a t e over a much greater pressure ratio than in the gas systems and consequently can suffer from greater noise and vibration problems. This p a p e r has shown that by careful design of the valve seat and plug the acoustic noise can be considerably reduced and the vibration can be eliminated. Standard valves that suffer from noise and vibration can have their seat and plug replaced, in service, by the conical design recomm e n d e d in this paper, thus saving time and expense for the operator.
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Figure 9. Pressure-flow characteristics of pressure-reducing valve with various plug and seat designs with steam flow. results were confirmed by modifying a problematic valve on an industrial plant. Notwithstanding these results, however, it is possible that different operating conditions may require different valve designs to provide an acceptable solution. The results shown in this p a p e r were for the valve venting to atmospheric pressure; when the pressure downstream of the valve was increased, it was found for the standard seat design that the onset of mechanical vibration was slightly different and did not correspond to a particular pressure ratio or pressure differential. U n d e r laboratory conditions, the 60 ° conical valve was quiet c o m p a r e d with the other designs, but it is possible that under conditions that could not be p r o d u c e d in the laboratory the vibration p r o b l e m may reappear. It is believed, however, that the solution will still be found by modifying the design of the valve plug and seat. A n o t h e r consideration that should be borne in mind is the direction of flow through the valve. In Fig. 1 it can be seen that the flow is tending to push the plug onto the seat against the action of the push rod. F o r this design it has been shown that a conical design of plug and seat is better than a fiat one. N a k a n o et al. [6] tested a valve in which the flow was in the opposite direction, that is, tending to push the plug off the seat. In their case they suggested that the fiat geometry is b e t t e r than the conical.
It has been shown how the mechanical vibration in a pressure-reducing valve can be eliminated by changing the design of the valve plug and seat. F o r the pressure-reducing valve used in the present work, a 60 ° conical plug and seat was found to be optimal. The sound pressure level was reduced by 12 dB (a factor of 4), mechanical vibration was eliminated, and flow capacity was increased by about 25%. REFERENCES 1. Hutchinson, J. W., ISA Handbook of Control Valves, 2nd ed., Pittsburgh, 1976. 2. Reed, C. L., Noise Created by Control Valves in Compressible Service, Third Control Valve Symposium, ISA, 79-83, 1977. 3. Reethoff, G., Turbulence Generated Noise in Pipe Flow, Ann. Reu. Fluid Mech. 10, 333-367, 1978. 4. Schuder, C. B., Control Valve Noise--Prediction and Abatement, in Noise and Vibration Control Engineering, M. J. Ceroker, Ed., 90 94, Purdue Univ., West Lafayette, IN, 1972. 5. Jenvey, P. L., Gas Pressure Reducing Valve Noise, J. Sound Vibration 41(4), 506-509, 1975. 6. Nakano, M., Outa, E., and Tajima, K., Noise and Vibration Related to the Pattern of Supersonic Annular Flow in a Pressure Reducing Gas Valve, J. Fluids Eng., Trans. ASME 110, 55-61, 1988. 7. Seebold, J. G., Control Valve Noise, Noise Control Eng. J. 24(1), 6-12, 1985. 8. Leaver, R. H., and Thomas, T. R., Analysis and Presentation of Experimental Results, Macmillan, London, 1974.
Received November 25, 1993; revised July 29, 1994
