ON CAVITATION AND CAVITATIONAL DAMAGE AT BUTTERFLY VALVES.pdf

Scientific Bulletin of the Politehnica University of Timisoara

Transactions on Mechanics Tom 52(66), Fascicola 6, 2007

2nd IAHR International Meeting of the Workgroup on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems

Timisoara, Romania October 24 - 26, 2007

ON CAVITATION AND CAVITATIONAL DAMAGE AT BUTTERFLY VALVES

Gheorghe BRAN *

University Politehnica of Bucharest, Romania Florentina D. BUNEA

INCDIE ICPE-CA Bucharest, Romania Gabriela I. OPRINA

INCDIE ICPE-CA Bucharest, Romania *Corresponding author: Splaiul Independentei 313, 060032 Bucharest, Romania Tel.: +40214029486, fax +40214029865, E-mail [email protected]

ABSTRACT

The diagram of a butterfly valve D100 operating in cavitation is assigned; six evolution stages of cavitation are identified; choking flow rates were determinate. The diagram can be utilized to deter-mine the operating conditions in different stages of cavitation evolution. The cavitational damages are identified downstream, on the inferior half of the pipeline, in the shape of some macro-rugosities.

KEYWORDS Cavitational coefficient, butterfly valves, stage of cavitations development

1. INTRODUCTION The problem of cavitation apparition and evolu-tion in pipes, fittings (bent pipes, valves, et al.), in flow rate metering devices (diaphragms, Venturi tubes) is present in literature after 1960, when experimen-tal installations with this destination are beginning to be made [1]. Ulterior (1978), the study of cavita-tion at butterfly valves constitute the subject of PhD thesis. In older treatises [2], the mentioned problem is not approached; the new ones [1] add in chapters regarding this subject. Also, new research centers are appearing in this domain [3]. The cavitation problem at valves consist in de-termining the upstream flow rates and pressures, corresponding to certain development stages and in the case of admitted existence, the cavitation at valves is or not a cavitation generator, downstream or on all the circuit. Preliminarily we have to define more or least exact, the cavitation studies.

The solution proposed by Tullis [4] filled in with the authors experience and with the method chose for the evaluation of cavitation stages is adopted in this paper [5], [6]. In the cavitation evolution we propose the fol-lowing stages of cavitation development, acoustically characterized: 1. NON (N) stage, 2. INCIPIENT (I) stage characterized by specific

noises (snaps) quasi random, hardly determinable with the ear,

3. LIGHT (L) stage, low intensity, continuous noises, undeterminable at distance,

4. MODERATE (M) stage, continuous noises, de-terminable at distance,

5. HEAVY (H) stage, acute, continuous noises, de-terminable in all the laboratory,

6. VERY HEAVY (VH) stage, very acute, continuous noises, with different intensities,

7. SUPERCAVITATION (S) stage, difficult to sono-rous evaluate/differentiate from the VH stage, in which at the rise of the pressure drop on the valve, the flow rate remains constant (choking flow).

The cavitation coefficients are associated to each stage 2-7. The purpose of this paper is to analyze these coefficients and to experimentally rise the flow rate Q (m3/h), pressure drop on the valve p (bar) diagram in all the 7 stages mentioned, in order to pick the admissible working domain.

2. THE CAVITATION COEFFICIENTS AT VALVES For evaluating the quality of the valves, the flow rate coefficients Kv, the hydraulic resistance coeffi-cients or those of cavitation are utilized. The first two are defined by the relations:

Proceedings of the 2nd IAHR International Meeting of the Workgroup on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems 42

vr

pQ K = , 202 p

v = (1)

with Q [m3/h] the liquid flow rate, p [bar], the net pressure drop on the valve, v0 the average upstream velocity, the liquid density, r the relative density comparing to water (if the working liquid is water, then r = 1). The coefficient is dimensionless. In Anglo-Saxon units, with Q in USGPM and p in PSI, the flow rate coefficient is Cv. If Kv1> Kv2, valve 1 is hydrodynamic superior and similar if 1 < 2. The determination of the values Kv and is made in stalls/installations especially cre-ated and in conformity with certain regulations (for example [7]). For cavitation are used many coefficients presented below.

2.1. The cavitation coefficient , defined by Thoma

0 20

( )/ 2vp p t

v = , (2)

if p0i and/or v0i are the values at which the cavitation appears, then is the incipient cavitation coefficient i. If i is obtained by reducing the pressure p0, the results have a great dispersion and in order to avoid the hys-terezis phenomena, a developed cavitation study is realized and p0 is diminished to the value of pd when the cavitation disappears and his value is now d [1].

2.2. The cavitation coefficient of a solid in a liquid current (I. Anton)

The method generally establish in [1] for a but-terfly valve is applied. Considering the Bernoulli relation in permanent movement between the points M0 and M (fig. 1), it obtains:

pMMM hzg

vpzg

vp +++=++22

2

0

200

. (3) Defining the reserve coefficient at cavitation, rez

min20 / 2

vrez

p pv = , (4)

the installation cavitation coefficient

0 20 / 2

vinst

p pv = (5)

and the valve cavitation coefficient, in the hypothe-sis that minMp p= and maxMv v=

2max 0

2 2 20 0 0

1/ 2 / 2

pMc

hv z zv v g v g

= + + , (6) From the relations (4), (5) and (6) we obtain

rez inst c = . (7) So, from relation (7) results the following stages [1]:

0,rez inst c > > , non - cavitation; 0,rez inst c = = , incipient cavitation; 0,rez inst c < < , developed cavitation

(industrial); 0,rez inst c , super - cavitation.

Relation (7) has a theoretic base and differenti-ates ins from c, which at constant temperature de-

pends on Froude number, Fr, by 020

22 Fr

Mz zv g

= , on Reynolds number, Re and on the relative roughness k/D by hp, of distance LMM0

( )Fr, Re, / ,c f k D L = (8) In the case of butterfly valve (fig. 1) determines that between the cavitation coefficients associated with the section from M, M and the section from M1, M1 exists the relation

1rezM rezM < (9) respectively the cavitation begins in section M. Between the c and Kv coefficients, respectively exist a natu-ral dependence: the valves with big Kv, respectively small are, at the same flow rate, less exposed to cavitation.

Figure 1. The defining scheme for the cavitation coefficient


2.3. The cavitation coefficients with the up-stream and downstream parameters

The following cavitation coefficients (numbers) [1, 8] are proposed for valves:

;fam v

pKp p

= ;am v

dav v

p pKp p

= m av vpK

p p= (10)

In order to correlate the hydrodynamic qualities evaluated by with cavitation numbers, the relations (1) (5) are used and with av amp p p= it obtains

20

1 ;/ 2

am v am v inst

f

p p p pK p v

= = = f instK = , (11)

1

1inst

df rez

KK

= = , (12)

1

fm

f inst

KK

K= = . (13)

can be expressed, for example, in function of Kf. Also Kd Km = 1. Concurrently, in relations (10), the flow rate intervenes implicitly by p. At inst = ct. and superior hydraulic valves (small ), the Kf and Km coefficients have small values, at lest for the incipi-ent cavitation and they rise at developed stages if is changing. The experimental research accomplished before and partially presented here ( 2.4) show that the Kv and values at = ct. are not modifying prac-tically at stages (I) (H). So, it results that Kf and Km are not modifying with the development of cavitation and can be fruitful at determining of stages (I) or (L). Referring to Kd, the last two terms from relation (6) are considered negligible comparing to the others; this hypothesis is admissible, at lest in the case of small valves (D 100 mm) and in the following case

2max

20

1cvv

, (14) Relation (14) suggest the approximation of c with a resistance coefficient e and Kd becomes

inst instdrez inst e

K = = . (15) At the development of cavitation e is modifying (increases) so that, theoretically, Kd should augment.

2.4. The cavitation coefficients given by manufacturers

A small number of manufacturers give in prospects information concerning cavitation [8]. The condition for operating without cavitation is

;F y Fam v

pX Z Xp p

= (16)

and the parameter Zy is given in tables for different diameters and types of valves. For butterfly valve D100, at = 0, Zy = 0.06 and at = 40, Zy = 0.33. It can be seen that XF = Kf and the maximum admis-sible drop on the valve is obtained for non cavita-tion

( )F am vp X p p , (17) respectively ( )0,06 am vp p p at = 0 and p 0.45 ( )am vp p at = 40. Information con-cerning the flow rate results indirectly from the Kv coefficients:

2

;vv

QQ K p pK

= =

and from relation (16)

2

1y

v am v

Q ZK p p


Figure 2. Stand sketch

Table 1. Kv and values in function of and Q

degree Q

m3/h p bar

Kv m3/h

-

0 111.33 0.0074 1294.18 0.113 10 110.82 0.0163 868.01 0.251 20 109.28 0.073 404.46 1.155 30 66.24 0.083 229.92 3.567 40 53.15 0.154 135.60 10.160

The obtained values are same greater then those given by manufacturers [9].

4.2. The diagram for running in cavitation Based on the studies presented in 1, the diagram in figure 3 was built-up. It is useful in designing as well as in exploitation. So, for flow rates of 7080 m3/h, the cavitation is in the stage (M) at = 40, etc. The relative dispersion of the results is explained by the inevitable human error. In table 2 are given the values of the flow rates that have been experi-mentally determined for different stages. The values Kv and are not modifying at stages (I) (VH).

0

0.1

0.2

0.3

0.4

0.5

0.6

20 30 40 50 60 70 80 90 100 110 120

Q [cm/h]

delta

p [b

ar]

teta=30 teta=40

I

L

M

S

I L M H

H

VH S

VH

Figure 3. The running in cavitation diagram for a butterfly valve D100, Pn6


Table 2. The water flow rates at the development of cavitation and = 40 Cavitation

stage Q [m3/h] Medium

I 58.2 58.8 55.7 51.57 56.07 L 66.36 65.24 62.79 64.12 63.17 64.33 M 71.27 72.46 69.19 70.97 H 80.57 76.99 83.24 84 81.62 81.28

VH 87.29 87.2 91.09 90.47 90.38 89.28 S 99.39 93.54 94.64 94.44 96.51 96.51 95.84

4.3. Cavitational damages After ca. 2000 hours of discontinuous running, in stages (H) and (VH) cavitational damages have been detected on the downstream pipe, on the inferior half; the damages are in the form of some protuber-ances of different sizes (0.5 2 mm).

There were no erosions on the valve flap. This situation can be explained considering the flow scheme in figure 5. The contracted sections (geometrical) Sc and Sc1 are equal, as well as the minimum ones, Scm and Scm1. The implosion of the bubbles is producing downstream of Sc and the flow spectra encourage the implosion next to the solid wall, in the case of Scm1.

Figure 4. Cavitational damages downstream the valve

Figure 5. The scheme of the flow on the butterfly valve

CONCLUSIONS 1. The cavitation coefficients defined with the up-

stream and downstream parameters (11), as well as those given by the manufacturers (16) are useful

for determining only the incipient cavitation in de-pending of the essential parameter in that installa-tion (pam, respectively pav). On the other part, the relation (11) has a theoretic base and defines three stages of cavitation development,


2. Defining and identifying the 7 stages of evolution, the diagram for running in cavitation for a butter-fly valve Dn 100, Pn 6, at opening angles = 30 and = 40 was determined in the laboratory. This give to user the possibility of choosing the running conditions,

3. Cavitational damage has been observed downstream of the valve and is explicated by the flow scheme in figure 4.

NOMENCLATURE Cv flow rate coefficient (with Q in USGPM and p

in PSI) D (m) pipeline diameter Fr (-) Froude number Hl (mH2O) level difference at the manometer with

water Hm (mHg) level difference at the manometer with

mercury Kd cavitation coefficient defined with the upstream

and downstream parameters Kf cavitation coefficient defined with the upstream

parameters Km cavitation coefficient defined with the down-

stream parameters Kv flow rate coefficient L (m) the length on which the linear pressure drop is

measured LMM0 (m) the distance between the points M0 and M Q (m3/h) flow rate Re (-) Reynolds number XF cavitation parameter [9] Zy cavitation coefficient [9] g (m/s2) gravity hp (m) the pressure losses between the points M0 and M k (m) roughness p0 (Pa) absolute pressure upstream the valve, in the

point M0 p0i (Pa) the pressure at which the cavitation appears pam (Pa) the pressure upstream the valve pav (Pa) the pressure downstream the valve pd (Pa) the pressure at which the cavitation disappears pM (Pa) pressure in the point M pmin (Pa) the minimum pressure on the valve pv (Pa) vaporization pressure at temperature t t () temperature v0 (m/s) average velocity upstream the valve, in the

point M0

v0i (m/s) the velocity at which the cavitation appears vM (m/s) velocity in the point M vmax (m/s) the maximum velocity on the valve z0 (m) height of the point M0 zM (m) height of the point M p (bar) pressure drop on the valve pl (bar) the linear pressure drop on the l1 and l2 lengths

(fig. 2) pm (bar) the pressure drop read on the manometer

with mercury () butterfly valve opening angle (-) coefficient of the local pressure drop e (-) equivalent coefficient of the local pressure drop r (-) relative density comparing to water (-) Thomas cavitation coefficient c (-) cavitation coefficient of the valve d (-) cavitation coefficient at the pressure pd i (-) incipient cavitation coefficient inst (-) cavitation coefficient of the installation rez (-) the reserve coefficient at cavitation rezM (-) the reserve coefficient at cavitation in the

point M rezM1 (-) the reserve coefficient at cavitation in the

point M1

REFERENCES [1] Anton I., 1985, Cavitaia, Vol.II, Editura Academiei,

Bucureti [2] Knapp, H., Daily, J., Hamitt, F., 1970, Cavitation -

Mc. Graw Hill, New York. [3] Koivula T., 2000, On cavitation in fluid power, Proc.

Of 1st FPNI-PhD Symp., p. 371-382, Hamburg. [4] Tullis P., 1978, Choking and Supercavitation Valves

ASCE, Journal of Hydraulics Division, p. 19311945. [5] Bran Gh., Ianculescu C., 1995, The Function Dia-

gram for Cavitation of Valves, Conferina Interna-ional Sisteme Hidropneumatice de Acionare vol. 1, p. 73-83, Timioara.

[6] Bran Gh., 2001, Cavitaie i eroziune cavitaional, Editura Tehnic, Bucuresti.

[7] CEI/IEC 60534-2-1 Industrial process control valves, part 2.1, Flow capacity Sizing equations for fluid flow under installed condition.

[8] *** Application of butterfly valves for free discharge, minimum pressure drop and for chocking cavitation, AWWA 2002.

[9] *** EBROARMATUREN Product Programme ERHARD VALVES (prospect)

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