Reducing valve generated noise through fluid energy and turbulence controlling designs Laurence R. Strattona Sanjay V. Sherikarb Nihon Koso Co., Ltd. 1-16-7 Nihombashi Chuo-ku Tokyo 103-0027 Japan

ABSTRACT High speed turbulent flow inside the limited confines of valve boundaries can create high noise, unless the process of energy dissipation is controlled. Once noise is generated, solutions such as absorption technology, acoustic lagging, thicker pipes, etc. can be uneconomical in many cases of practical interest. As a result, excessive noise from control valves is still frequently seen as a problem. The primary building block in the understanding of control valve noise mechanism is the classical turbulent jet. If there is a significant expansion from the valve outlet into the downstream pipe and the energy of the fluid jet leaving the valve body is high, it may act as an additional noise source. This paper first describes such sources. The difference between turbulence characteristics of high velocity fluid and those of high velocity fluid jets as it relates to noise generation is explained. Industry standard noise prediction equations are used to estimate the effect of fluid kinetic energy on noise. One of the benefits of controlling volumic fluid kinetic energy at the throttle point and exit of the valve is lowered noise production. Rules for control valve selection, and design, to reduce the risk of high noise generation are provided.

1. INTRODUCTION Valves are potentially a significant source of excessive noise in power plants. The high noise from a valve may be a result of turbulence generated in a pressure letdown process (compressible media) or cavitation (liquids). The noise generated by turbulence generated in a pressure letdown process (compressible media) is considered aerodynamic noise. Very often, high noise is accompanied by high vibrations which are associated with the turbulence in the flow path and/ or damaging phenomenon like cavitation. High vibrations of the trim, by themselves, can be the dominant source of noise. This is considered mechanical noise.

                                                            a Email address: lrstratton@kosovalve.com b Email address: sanjay.sherikar@kosovalve.com

This phenomenon is not discussed further in this paper; it suffices to say that, in such cases, eliminating the excessive vibrations is the only practical solution. The following sections address the aerodynamic noise generation in valves and demonstrate that it can be reduced by controlling the characteristics of turbulence in the flow path. Control valves that are selected and designed to follow the criteria for fluid kinetic energy leaving the valve trim as outlined by Miller1 are less likely to have noise problems than those that do not follow this guideline. This provides the designers a first screen in the design process. This approach is dependable, easy-to-understand and verifiable for the stand-point of the end-users. As a result, it empowers practicing engineers to make good selection of control valves without getting smoked by the details of noise prediction, which is a complex subject.

A. ISA Guidelines for Control Valves There is no industry standard as far as how best to achieve pressure reduction in valves. However, the guidelines in Table 1 serve as good screening criteria, especially for severe service valve applications, and have been recommended for practice1. Several well documented analysis of practical experiences have demonstrated that limiting fluid kinetic energy (rV 2/2) exiting the valve trim is a key requirement in assuring good performance in severe service applications2 3.

Table 1: Trim Outlet Kinetic Energy Criteria1

Service Conditions Kinetic Energy Criteria

Equivalent Water Velocity

psi kPa ft/s m/s Continuous Service, Single-phase Fluids

70 480 100 30

Cavitating and Multi-phase Fluid Outlet

40 275 75 23

Vibration-sensitive System 11 75 40 12 This unified criterion addresses several problems that are chronic in high fluid energy applications; it is simple to understand, easily specified and verifiable. Most of all, it maintains the key essence of the physics of the phenomena being addressed in the selection and design process4.

2. NOISE GENERATED BY FLOW Turbulence in the flow is a primary source of noise in the case of valves and piping systems. The nature of turbulence varies in different flow; hence, the characteristics of noise generated by different sources also vary accordingly. In this context, the flows can be broadly categorized as: (1) jets, and, (2) flows with no separation, or with a closed separation bubble. The major difference between these two categories is the nature of the shear flow development downstream. The nature of turbulence varies in each case. Typical values of turbulence intensity, which is the ratio of the turbulent velocity of the fluid to its mean flow velocity, and length scales vary widely. The turbulence intensity for jets in about 25% to 30%, while it is only 5% to 8% for fully developed turbulent pipe flows (flow with no separation). The large length scales in jet flows are of the order of the jet diameter; for pipe flows, the length scales are of the order of the pipe diameter. Such marked differences have significant impact on noise generation. Jets may be confined or unconfined (free). From the stand-point of noise generation, they are considered to be very similar. The turbulence level in the shear layers of jets is high and is a

primary source of noise. Flows separated from the wall behave similar to jets as far as noise production. In the same manner, the action in a closed separation bubble is very local and its impact on noise generation can be neglected, i.e. it may be treated like flow with no separation. At times, resonance in the system is the source of high noise; however, that is not the focus of the discussion here.

A. Basic Features of Valve Construction and Resulting Flow Path Typical construction of globe style, linear control valves is illustrated in Figure 1. These valves can be constructed with different configurations of trims, some of which are shown in Figure 2. Predominant noise source is aerodynamic fluid jet(s) exiting the throttling point in the valve trim.

Figure 1: Basic construction of a linear sliding stem globe valve

Figure 2: Different trim types used in control valves; “At” denotes throttling areas that create jets.

The multi-stage trim forms of the type shown in Figure 3 were specially developed for severe service applications; in such designs, the total pressure drop is broken down into many stages. This causes pressure drop across individual stages to be much smaller; most importantly, pressure drop across the last stage, which is the key for noise generation, can be controlled through design.

Figure 3: Multi-stage, multi-path trim for severe service applications

Some valve manufacturers use orifice plates and/or pressure-reducing diffusers to limit the pressure drop across the trim. They are placed downstream of the valve throttling element and force the flow through many small jets. So, taking a significant pressure drop is a primary function of such elements. Flow distributors, when used, are located in the regions of abrupt area transitions downstream of the valve throttling element. Their primary function is to avoid large jets, or flow separation, that would otherwise take place in the flow path. Pressure drop associated with flow distributors is generally small, and incidental – this feature differentiates flow distributors from orifice plates and pressure-reducing diffusers. A typical geometry of a flow distributor is shown in Figure 4.

B. Noise Generating Flow Features in Valves and Associated Piping The noise generation takes place within the physical envelope of the valve and also in the piping systems. The flow characteristics of major sources that cause noise generation are described below. Valve trim – Predominant noise source is aerodynamic fluid jet(s) exiting the throttling point in the valve trim. High speed fluid jet(s) at the trim exit are in place by design for the purpose of the pressure reduction process; the resulting noise may be described as collateral damage. The turbulence and noise generation characteristics of this source are modeled after jet theory.

Figure 4: Typical geometry of a flow distributor.

. Transitions at valve-piping joints – Another predominant noise source may be “expansion” noise caused by the flow exiting the valve outlet into much larger outlet piping (as illustrated in Figure 5). It is a problem only when flow separates from the pipe wall to form a jet and the fluid energy is high. Valve sizes are often not the same size as the pipe lines in which they are located. Severe service control valves take a significant percentage of the system pressure drop. As a result, they often have reduced-size trims or are smaller in size than the surrounding pipes. The transition at the valve inlet is rarely a problem since the area is decreasing and tendency for flow separation is suppressed. On the other hand, a number of factors are at play at the valve outlet. One, the density is lower at the valve outlet (compared to the inlet) – so the velocities, and by extension the kinetic energy, are higher at the valve outlet. Also, any area increase in the transition from valve outlet to the line size has high potential for flow separation. The net result is that the turbulence intensity in this region can be high and, if uncontrolled, can be a major source of high noise.

 

Figure 5: Valve outlet expansion into downstream piping

Orifice plates and/or pressure-reducing diffusers – The turbulence characteristics of these devices are similar to that of a multi-path valve trim, i.e. combined contribution of the jets. Given the fact that these devices take a significant pressure drop, the noise from these elements is comparable to that from the valve trim and it cannot be ignored.

Flow distributors – Typically, noise associated with flow distributors is much smaller since the pressure drop associated with them is incidental. It can, usually, be ignored. The following sources are usually considered secondary in nature as far as noise generation. They are not part of standard noise prediction methods yet. The contributions of these sources are kept low through conservative design practices, which limit turbulence in the flow. However, on occasion, they appear as root cause of high noise. Valve gallery (Impingement of jets from the trim exit on the walls of the valve body or piping) – In this case, the resulting flow structure consists of a primary flow upon which secondary patterns, which depend on the detailed flow geometry, are imposed. There may or may not be any flow separation. However, the turbulence intensity and local pressure fluctuations may be high. In some cases, a feedback mechanism can cause resonance of the jets and, consequently, high noise. Pipe noise - High speed flows in piping by themselves are often considered a potential source of noise. However, it can be shown that the noise generated in the straight pipe sections are significantly lower in power than any fittings that may be in this line. As long as such fittings are considered in the analysis, the noise due to straight pipe sections may be ignored. Pipe fittings – Elbows, tees and other such fittings in piping systems result in disturbances in flow. There may or may not be flow separation, but the flow disturbances result in higher turbulence levels – and hence, higher noise.

C. Fluid Kinetic Energy and Predicting Noise Generation Methods for predicting noise generation from the sources in a system vary from somewhat sophisticated, but still quite inexact, to practically non-existent. “Non-existent” is okay when it is known from experience that the source contribution is minor. However, it is necessary to define boundaries that cause the contribution of these sources to be minor. In classical terms, attention has been primarily on velocity, or Mach number, raised to a high power in the prediction of noise; fluid density is not considered. However, fluid density varies by orders of magnitude depending on the application; so, it makes sense to factor it into rules for controlling valve noise. This is achieved by using the term fluid volumic kinetic energy, which is equivalent to the kinetic energy density and includes both fluid density and velocity. The adjective volumic is used to modify the name of a quantity, or the term density is added to indicate the quotient of that quantity and its associated volume6. In this case, the quantity is kinetic energy (mV 2/2), where m is the mass of the fluid and V is the velocity of the fluid through the given flow area. The volumic Kinetic Energy is represented by (ρV 2/2), where ρ is the fluid outlet density. This is the same term that is sometimes referred as “dynamic head” or “velocity head”. The adjective “volumic” is understood and often dropped in writings or discussions. Industry experience with severe service valves has demonstrated adequately that fluid kinetic energy at the throttle point is of fundamental importance. It can readily be seen that this is the dominant parameter in defining allowable limits to assure low noise from a system. Noise generated by jet-type flows – The noise produced by the flow in the trim is modeled as jet flow, and is of the form:5

na McmW 2 .    (1)

In this equation, Wa is the noise power, m is the mass flow rate, c is the speed of sound and M is the jet isentropic Mach number. Exponent ‘n’ depends on the flow regime; but it is never less than 2.

The above equation can be cast in a form:

2.. na MEKW .    (2)

This form illustrates the huge importance of fluid kinetic energy in noise generation in valves. The fluid kinetic energy term appears explicitly in noise generation equations. In addition, it is related to the Mach number for given conditions of local pressure and temperature. Lower kinetic energy means lower Mach number. As a result, lowering of fluid kinetic energy has a multiplier effect on noise reduction. When these relationships are applied to the jet(s) exiting the valve trim, the benefit of controlling kinetic energy at the trim exit is evident. The close relationship between noise power and the jet kinetic energy is also evident in Figure 64. It shows the measurements made for a 3 mm diameter jet issuing into ambient air for isentropic Mach numbers of up to 2. Also shown in this plot are predictions based on standard equations for jet noise.

Figure 6: Sound power and the kinetic energy at the exit for a jet issuing into ambient4.

Noise produced by other (non-jet) types of flows - The noise produced by flows that remain attached to the boundary walls is proportional to the pressure drop in approximate terms. Since pressure drop is proportional to fluid kinetic energy, the noise thus produced is also proportional to fluid kinetic energy. Disturbances in the flow path add to noise. Disturbances in piping systems may be caused by abrupt changes in fluid path. The effect of fittings and obstructions in these flows is similar. They result in strong secondary motion and/or increased turbulence and mixing, which become additional sources of noise. The increase in noise due to these effects has also been shown to be roughly proportional to the pressure drop, and hence to fluid kinetic energy.

3. APPLICATION TO CONTROL VALVE DESIGN AND SELECTION It is evident that the primary noise sources in valves are from jets flows exiting the trim and, in some cases, the jet from the valve outlet. From the basic noise equations for predicting noise, it is possible to derive the effect of trim exit kinetic energy on valve noise generation.

Trim noise – Equation (2) can be written in the form below to compare the difference in noise power from two different types of valve trims:

112212 log2log210....log10dB MnMnEKEKWa .  (3)

Subscripts 1 and 2 in the equation above refer to the two trims being compared. The first term in this equation may be considered as the effect of different fluid kinetic energy levels. The trim exit jet Mach numbers, M1 and M2, in the second term are also related the respective fluid kinetic energies K.E.1 and K.E.2. The variation of second term with fluid kinetic energy depends on the outlet pressure and is a bit complex; however, its contribution is in the same direction as the first term. Calculations were made for a high pressure, steam turbine bypass valve which reduces pressure from 18 MPa to 4 MPa. A conventional valve in such service would be very noisy and selection of sufficient number of pressure reducing stages in the trim is important to keeping noise low. Figure 7 illustrates the dependence of noise on trim exit kinetic energy; using 1030 kPa as a reference, this plot shows the increase in noise as the trim exit kinetic energy increases.

Figure 7: Dependence of trim exit kinetic energy on noise for a high pressure turbine bypass application. Figure 7 illustrates that limiting trim exit kinetic energy to 1030 kPa will always result in a quieter valve when compared to a design where this parameter exceeds that value. In general, the effect of trim exit kinetic energy on noise generation becomes increasingly significant as the valve outlet valve outlet pressure gets higher. For a the range of control valve designs seen in high pressure (HP) turbine bypass (TB) to cold reheat service, Figure 7 suggests that the difference in noise generation can be over 15 dB between a conventional valve and one in which a 1030 kPa limit is set on the fluid kinetic energy exiting the valve trim. Fluid kinetic energy is a controllable parameter in valve design which strongly influences noise generation. Therefore, it becomes an easy, and useful, screening criterion in selection of valves for users. An estimate of expected difference in noise for different design options based on Figure 7, or on Equation 3, is useful in comparing various design options. Although such a check is approximate, it is: (a) independent of suppliers’ biases, and, (b) does not require detailed noise calculations.

A limit on trim exit kinetic energy is intended as a screening criterion only – not as the only, and an absolute, criterion. Good engineering practices and standards must be followed as well. Valve outlet jet noise – It is a significant noise source from two different perspectives. First, the jet noise adds to the noise from other sources in the valve thereby increasing the overall noise power. Second, the jet noise is distributed in low frequency regions because of the large diameter of the jet; low frequency components attenuate less through the pipe walls and also have lowered benefit of A-weighting as far as the noise propagated to the outside. In addition, high energy valve outlet jets have a potential to cause serious vibration in the system. As a result, a high energy jet at the valve outlet is undesirable. Flow distributor is preferred over high pressure drop devices (for e.g., orifice plates, flow diffuser, pressure-reducing cylinders etc) to avoid any significant additional noise generation. Piping noise – Noise produced by high speed flow in piping, and through fittings such as elbows, tees etc, is often a concern. It is often addressed by limiting the Mach number of the flow in the pipe. However, these noise sources are also strongly dependant on fluid kinetic energy, and in the similar way as the trim noise. As a result, a limit on the fluid kinetic energy in the piping makes a superior criterion for pipe size rather than just the Mach number. For low pressure, pipe selection based on a fluid kinetic energy limit has successfully resulted in the selection of smaller pipe sizes than that based only on a conservative Mach number limit. Therefore, it is recommended as a design criterion in the selection of pipe size.

4. CONCLUSIONS Fluid kinetic energy at the trim exit is closely related to valve noise generation. Noise requirements are more likely to be met when ISA Guidelines for trim exit kinetic energy are followed than otherwise. The recommended fluid kinetic energy exiting the trim for intermittent service, high pressure turbine bypass valves is 1030 kPa. Following these guidelines results in noise levels that are over 15 dB less when compared to conventional single-stage pressure drop designs. The high energy jet from the valve outlet into a much larger piping should be avoided. It is a source of high noise generation. To eliminate this large, low frequency jet, flow distributors are preferred over orifice-plates, or pressure-reducing diffusers which are themselves a source of noise.

REFERENCES 1 Herbert L. Miller, “Control Valve Applications,” Chap.12, Pgs 434-438, in Control Valves – Practical Guides for

Measurement and Control, edited by Guy Borden, Jr. and Paul G Friedmann (Instrument Society of America, North Carolina, 1998).

2 Herbert L. Miller and Laurence R. Stratton, “Kinetic Energy as a Selection Criteria for Control Valves”, ASME Fluids Engineering Division, Summer Meeting, Paper FEDSM97-3464, Vancouver, B.C., Canada, June 22-26, 1997.

3 Herbert L. Miller, Laurence R. Stratton, and Mark A. Hollerbach, “The Case for a Kinetic Energy Criterion in Control Valves – Part 1,” ISA EXPO 2005, Paper ISA 2005-P133, The Instrument, Systems and Automation Society, Research Triangle Park, North Carolina, Chicago, October 25-27, 2005.

4 Sanjay V. Sherikar and Vinay K. Nagpal, “Fluid Kinetic Energy Based Limits in the Design of Control Valves and Valve-Related Systems”, 5TH ASME/JSME Joint Fluids Engineering Conference, Paper FEDSM 2007-37690,San Diego, CA, USA, July 30-August 2, 2007.

5 Noise considerations – Control Valve Aerodynamic Noise Prediction Method, Part 8-3, IEC Standard 60534-8-3, Second Edition, International Electrotechnical Commission, Geneva, Switzerland (2000)

6 Barry N. Taylor, “Guide for the Use of the International System of Units (SI),” NIST Special Publication 811, 1995 Edition