VACUUM STABILITY IN A MILK LINE OF A TUBE MILKING MACHINE OF VARIABLE LOADING

Danuta Skalska, Józef Szlachta, Mariusz Nejman
Institute of Agricultural Engineering, Agricultural University of Wrocław, Poland

ABSTRACT

The study analyzed changes of vacuum and the range of vacuum amplitude in a milk line of a tube milking unit. Two manners of connecting six milking machines to the installation of the milking machine were used. Milking machines were connected successively, beginning with the first stand tap lying the closest to the final unit, and then beginning with the sixth stand tap found the furthest in relation to the final unit for two configurations of the milk line (arm and arm with the wash supporting line) and for three systems of vacuum regulation. The studies showed a significant influence of the total liquid stream from the milking machines Q_mr, the mean velocity of the flowing liquid v_cśr and a configuration of the milk line on the drop and range of vacuum amplitude aP14 in the transport zone of the milk line. The decrease of vacuum in the transport zone for the configuration of the milk line - arm, with full loading of the installation was 5 kPa, and the range of its amplitude was about 8 kPa. For the configuration of the milk line arm with a wash supporting line the drop of vacuum was 2 kPa, and its range of amplitude - about 6 kPa. In the section of the milk line with the working cluster, drops of vacuum were significantly dependent on the quantity of the flowing liquid Q_mr and on the appearing "thresholds of liquid" at the stand taps. The range of vacuum amplitude aP14 in the milk line in the section with the working cluster changed from 1 to 9 kPa.

Key words: tube milking machine, configuration of the milk transfer line, drop of mean vacuum, range of vacuum amplitude.

INTRODUCTION

A stable level of vacuum is the basic parameter ensuring the proper course of milking and directly affecting the healthiness of milk cows. In order to obtain stable vacuum at the end of the teat, attention should be paid above all to the stable value of vacuum in the milking installation of the milking unit. Incorrectly selected or badly functioning vacuum installation has a negative effect on the milking process, leading to drops and irregular amplitude of vacuum accompanying the milking process [6, 7, 9]. A properly designed milk line requires accurate determination of the ability of the milk stream in order that the "milk corks" should be prevented since they contribute to considerable changes and drops of vacuum in the milking installation [5, 10]. The occurrence of "milk corks" is connected with a number of factors that are closely related to each other. The most important [2, 3, 4, 5] include the mean diameter of the milk line, the number of working clusters, the time between installing the milking machines and the peak stream of milk from the milking machines (the cows´ efficiency). In the process of milking the relations occurring between the structure of vacuum and the milking system are important, especially with a very high-placed milking line and with the milking of high efficiency cows, when the resistance of the liquid and air flow increases in the whole milking system. A simultaneous work of a cluster is the reason for a considerable "loading" of the milking system with the liquid, which is in close relation to its fulfillment as well as the variability of pressure conditions, both within the whole milking installation and the milking machines [5]. When tube milking machines are used, requirements concerning the following increase:

• installation of vacuum (efficiency and reserve of vacuum pump, diameter of the main and circular vacuum line, length of the loop, efficiency and characteristics of the vacuum regulator),

• installation of the milking machine (diameter of the milking line, length and drop of the milking loop (without unnecessary increases), the optimum number of simultaneously working clusters),

• type and parameters of the work of milking machines (kind of pulsation, shape and construction of collectors and their capacity, shape and characteristics (TPD elasticity) of teats, the diameter of short milking tubes, the diameter of long milking tubes).

The above requirements towards the milking parameters of tube milking machine, especially with increasing milking efficiency of cows, justify the analysis of the effect of the delivered stream of liquid on the changes of vacuum and the range of its amplitude in the milk line (in the transporting part and in the working clusters area), in the tube milking unit. The studies referred to the operation of six milking machines connected alternately to the installation (beginning with the stand tap located the furthest from the final unit) for two configurations of the milk line (arm and arm with the wash supporting line) and three systems regulating the system vacuum.

MATERIALS AND METHODS

The studies were conducted in the laboratory of the Institute of Agricultural Engineering of the Agricultural University in Wrocław. The measurement stand was constructed on the basis of a tube milking machine (two variants of producing the vacuum and three systems of regulating the vacuum (fig. 1 and 2):

I - vacuum aggregate consisting of a vacuum pump RPA, with a so-called water ring with the efficiency of 850 l.min^-1 cooperating with the valve regulating the vacuum Vacurex,

II - vacuum aggregate consisting of a vacuum pump VP-76, lubricated with oil of the efficiency 900 l.min^-1 cooperating with the valve regulating the vacuum VRM900 (optionally with two weight valves regulating the vacuum VV40).

Besides, the milk installation was equipped with an interceptor with the capacity of 20 l, a container of the final unit with the capacity of 25 l, and stand taps type Combi DE 52. The studies used milking machines of Harmony type (fig. 2); (a collector´s capacity of 450 cm³), pneumatic pulsators with alternate pulsation, artificial teats and a system of delivering the liquid mass within the range of 0-8 l.min^-1 in a closed circuit (fig. 2). The installation of vacuum consisted of a pipeline with the diameter of Ø = 0.05 and the length l = 16.5 m (made of steel) and a pipeline with the diameter of Ø = 0.07 m and the length l = 4.5 m (made of PCV). A milk line (made of glass and suspended at the height of 2 m), with the diameter of Ø = 0.05 and the length l = 12×12 m, sloping towards the final unit at 0.05% on its whole length, was connected to the vacuum system through the final unit. The milk line had a shape of an arm and arm with the wash supporting line. Measurements of the changes of vacuum in the chamber of the milk collector, in a short pulsation tube, in sub-teat chamber, at the end of the teat, in the vacuum pipeline and in the milk line were performed by means of vacuum sensors PS-SM-100 by Vigotor. A 15-channel recorder used in the studies ensured a simultaneous recording from all sensors with the frequency of 100 trials per second. The recorded data were analyzed by means of a specialist software "Grafakw" used in this type of studies. The stream of the liquid in the pipeline as well as its loading were recorded by means of Video-Kamera Recorder SONY CCD TR840E. The studies were conducted using a substitute of milk (distilled water), where the measured parameters of milking did not significantly differ from the parameters that appear when milk is used [8].

Fig. 1. Scheme of the measuring stand to set vacuum parameters: 1 - vacuum pump, 4 - regulator VRM 900, 6 - milking vacuum line, 7 - milk transfer line, 8 - interceptor 20 l, 9- vacuum gauge, 10 - receiver, 11 - sanitary tap, 12 - long pulse tube, 13 - pulsator, 14 - milk tap, 15 - releaser milk pump, 16 - delivery line (Fig. 2), 17 - claw HARMONY 450cm3, 18 - milking unit HARMONY, 19 - long milk tube, 24 - recording; PO, P2, P3, P13, P14, PKS, PKK, PKP, PKM - vacuum sensor

The parameters of a tube milking unit were chosen according to the norms ISO5707 and 6690. All the working clusters had the same dosage of liquid Q_m in the range from 0 to 8 l.min^-1, every 2 l.min^-1.

An important functional element of a milking machine is the pipeline transport part, which has influence on the milking units connected to it. The function of the milk line is to keep the proper vacuum in the milking machines and to transport the milk to the final unit. The fulfillment of both these tasks is difficult since the flowing milk and the air disturb each other´s stream. It is important not only to keep the mean value of vacuum but also to keep its possibly small changes. The range of vacuum is described as amplitude of deviations from the mean value.

Fig. 2. Scheme of the measuring stand - system of dosage of liquid Qmr: 6 - milking vacuum line, 7 - milk transfer line, 12 - long pulse tube, 13 - pulsator, 14 - milk tap, 15 - releaser milk pump, 16 - delivery line, 17 - claw HARMONY 450 cm3, 18 - milking unit HARMONY, 19 - long milk tube, 20 - rotameter, 21 - liquid tank, 33 - tube with ruffs, 23 - artificial teat

The milk line was divided into zones, where vacuum drops and its changes were determined:

- vacuum drop per one transport section from the first tap to the final unit,
- vacuum drop in the pipeline at the stand length with the working cluster.

RESULTS AND DISCUSSION

Vacuum drop (dP3-dP2) in the transport zone of the milk line (fig. 2A) changes in a non-linear manner and it grows together with the growth of the total dosage of the stream Q_mr. The maximum value of the drop takes place for six working milking units and Q_mr equal to 48 l.min^-1 in the configuration of the milk line (arm), and it is over 3 kPa. On the other hand, the configuration of the milk line (arm with a wash supporting line) the value of the vacuum drop reaches 2 kPa. The vacuum drop between the final unit and the first stand tap (for the configuration of the milk line arm) is dependent on the total liquid stream in the milk line from the milking machines Q_mr, on the quantity of air delivered to the milk line through the milking machines Q_pam and on the length of the milk line. On the other hand, the vacuum drop between the final unit and the first stand tap for the configuration of the milk line arm together with the wash supporting line depends on the total liquid stream Q_mr and the quantity of air flowing together with milk Q_pam1 and on the length of the line, assuming that the whole liquid flows through one part of the arm. The vacuum drop in the other part of the arm, i.e. in the wash supporting line depends only on the total liquid stream Q_pam2 and the length of the line where the liquid does not flow. Equalization of vacuum between the arm and the wash supporting line takes place in the section of the milk line between the first and last milking machines. The place where the vacuum is equalized depends on the number of the milking machines and their total liquid stream Q_m. The air in this place will flow in both directions of the milk line to the final unit (through the part of the arm and through the part of the arm with the wash supporting line).

Fig. 3. Effect of total liquid stream in milk line Qmr on : A - vacuum drop (dP3 - dP2), B - range of vacuum amplitude aP14 and effect of liquid velocity vcśr on: C - vacuum drop (dP3 - dP2), D - range of vacuum amplitude aP14 in transport milk line area (for configuration of milk line arm and arm with line wash supporting)

The vacuum drop between the final unit and the first stand tap (dP3 - dP2) affects the mean value of the working vacuum of the milk line at the stand taps P14. The cause of these changes is resistance of the air flow in the milk line. The resistance of the air flow is influenced by the amount of liquid delivered to the milk line Q_mr and the amount of the air delivered to the milk line Q_pam by the working clusters. The interaction of both values Q_mr and Q_pam influences the velocity of the liquid stream.

The range of vacuum amplitude aP14 in the transport part of the milk line is presented in fig.3B for Q_mr from 2 to 48 l.min^-1, when the milking units are connected to the milk line successively from the first tap. The vacuum range aP14 grows in a linear manner together with the increase of Q_mr. Two straight lines represent the configuration of the milk line: arm and arm with the wash supporting line. The character of changes for both configurations is close to the whole liquid stream Q_mr from 2 to 24 l.min^-1. Above the value 24 l.min^-1 the range of vacuum aP14 amplitude in the configuration arm is slightly bigger than for the configuration arm with the wash supporting line, which is caused by a considerable increase of the mean velocity of the liquid v_cśr in the configuration arm. Therefore, it is justified to present aP14 and (dP3 - dP2) depending on the mean velocity of the liquid v_cśr (fig. 3C and D). The analysis of the effect of mean velocity of the flowing liquid v_cśr on the vacuum drop (dP3 - dP2) for the configuration of the milk line arm and arm with the wash supporting line (fig. 3C) showed that the maximum value of these changes is 5 kPa for arm and 2 kPa for arm with the wash supporting line. The effect of the mean velocity of the flowing liquid v_cśr on the changes of aP14 is presented (fig. 3D) for two configurations of the milk line and for Q_mr ranging from 2 to 48 l.min^-1. The range of vacuum amplitude aP14 grows together with the growth of the mean velocity of the liquid. The character of the course of the curves is convergent in the section from 0.25 to 1.5 m.s^-1 for Q_mr from 2 to 36 l.min^-1 (for two configurations of the milk line).

Fig. 4. Effect of milk line fulfillment w on: A - vacuum drop (dP3 - dP2), B and C - range of vacuum amplitude aP14 in transport milk line area (for configuration of milk line: arm and arm with milk line wash supporting)

Sometimes one can find suggestions in literature that vacuum drop and its range in the milk line depend on its fulfillment. That is the reason why measurements of the mean fulfillment of the milk line were performed. Vacuum drop (dP3 - dP2) in the transport milk line is presented depending on its fulfillment w for the configuration of the milk line arm and arm with the wash supporting line (fig. 4A). Such a relation is not explicit, which is similar to the vacuum range aP14 in the transport milk line and this, in turn, cannot be referred to the fulfillment w in an explicit way either (fig. 4B and C). Hence, it is difficult to present the effect of the milk line fulfillment on pressure parameters in an explicit way. On the other hand, there exists a close correlation between the mean velocity of the liquid v_cśr and pressure parameters in the milk line (fig. 3 C and D).

A multi-factor analysis pointed to a significant effect of the three analyzed sources of variability (milk line configuration, number of connected milking machines, quantity of the liquid stream from the milking units Q_mr) on the range of vacuum amplitude aP14 in the transport milk line.

The section of the milk line where the milking machines are connected is of special importance since it is there where the greatest disturbances of vacuum stability occur. It is first of all associated with the working cluster. That is why it is important to determine the range of vacuum amplitude in the milk line depending on the number of the working milking units and the sequence in which they are connected. Figures 5A, B present changes of the range of vacuum aP14 in the milk line, in the part of six working units. Figure 3B presents the range of vacuum aP14 along the pipeline between the six working milking units in the configuration of the milk line: arm with the wash supporting line. The first tap (apparatus) is the closest to the final unit. A quality change of aP14 takes place between the values 12 l.min^-1 and 24 l.mim^-1 of the total liquid stream Q_mr. The value of the range of vacuum aP14 amplitude increases from Q_mr equal to 24 l.min^-1 together with the distance of the measurement point from the first unit towards the last milking machine. On the other hand, the value of the range aP14 for the milk line in the configuration arm (fig. 5A) for Q_mr increase from 24 l.min^-1 symmetrically from the centre of the section with the working cluster, which only partially confirms the suggestions contained in the literature [1, 4] that additional equipment of the milk line in an arm with the wash supporting line ensures smaller fluctuations of vacuum aP14. Such dependence takes place only for the two first milking units, while at the fifth and sixth machines the vacuum aP14 range in the configuration of the milk line arm with the wash supporting line are higher than for the arm. A considerable amplitude of the range of vacuum aP14 in the configuration arm with the wash supporting line behind the sixth milking unit are caused by the fact that all the liquid flows through one part of the milk line, namely the arm to the final unit. The air from the cluster area spreads to two parts of the milk line, from the arm to the wash supporting line (the part of the pipeline that is not filled). The spreading air disturbs the liquid stream in the pipeline at the final milking units. This disturbance causes temporary changes of the fulfillment w of the milk line, and together with the spreading air causes fluctuations of vacuum aP14. From the point of view of vacuum stability in the milking system of the tube milking machine, it is interesting to observe the effect of the total liquid stream Q_mr in the milk line on the range of the amplitude of vacuum aP14 measured at the first stand tap (fig. 3B). This means that such a level of vacuum range at the beginning of the transport milk line has a disturbing influence on the part of the milk line with the working cluster.

Fig. 5. Effect of all working clusters and total liquid stream in milk line Qmr (change Qmrfrom 0 to 48 l.min-1) on: range of vacuum amplitude aP14 in milk line area for configuration of milk line: arm (A) and arm with milk line wash supporting (B), in working clusters area (at stand taps)

The relation of aP14 in the area of six working clusters for three regulators (Vacurex, VRP 900, VV 40) and two configurations of the milk line (fig. 6) points out that the values of the aP14 vacuum amplitude are only slight.

Fig. 6. Effect of milk line configuration and vacuum control system (Vacurext, VRM 900, VV 40) on the range of vacuum amplitude aP14 in milk line at the area of the working cluster

This was confirmed by a two-factor analysis for all combinations of connecting the milking units to the milk line for three regulators (along the working cluster area). It showed that the range of vacuum amplitude aP14 varies slightly (0.5-1.5) kPa. This means that the analyzed systems of vacuum regulation ensured comparable results. On the other hand, a of two-factor variance analysis showed a significant effect (p = 0.000) of the configuration of the milk line and all the working combinations of the six working clusters on the range of vacuum amplitude aP14 in the milk line at the stand taps. The measured values of the range of vacuum amplitude aP14 were higher by about 4 kPa at the sixth tap for the configuration of the milk line - arm with the wash supporting line. On the other hand, the values of vacuum range aP14 in the milk line at the first tap were higher by about 2 kPa for the configuration of the milk line - arm.

CONCLUSIONS

1. Values of the range of vacuum amplitude in the milk line for the three analyzed regulators vary only slightly. A multi-factor variance analysis pointed to a significant effect of the three analyzed sources of variability (configuration of the milk line, number of connected clusters and the liquid stream from the milking machines Q_mr) on the range of vacuum amplitude and the drops of vacuum in the transport milk line and in the area of the working clusters.

2. Vacuum range in the transport milk line changes in a linear way and it grows together with the change of the total liquid stream Q_mr (from 0 to 48 l.m^-1). The value of aP14 changes within the range from 0 to 8 kPa. The vacuum drop in the transport milk line changes in the function of total liquid stream Q_mr in a non-linear manner for both configurations of the milk line and in a linear way together with the growth of the mean liquid velocity v_cśr.

3. The range of vacuum in the milk line in the working clusters area change in a non-linear manner and reaches its minimum between the third and fourth units with sixth working clusters. The value of the range aP14 changes from 1 to 9 kPa. Vacuum drops in the milk line in the area of the working clusters are non-linear and depend on the changing amount of the flowing liquid Q_mr and the appearing "liquid thresholds" at the stand taps.

4. The drops of vacuum and its range in the transport milk line area cannot be presented in the function of fulfillment w since these are not explicit dependencies.

REFERENCES

1. Brzozowski T., 1980. Wpływ niektórych parametrów aparatu udojowego na przepływ mleka [The effect of some parameters of the milking machine on the milk stream]. Maszynopis (a type-written copy) IBMER, Warszawa [in Polish].

2. Józefowicz E., 1995. Algorytm symulacji przepływu mleka w dojarce jednostanowiskowej z rurociągiem [Algorithm of a simulation of milk stream in a one-stand milking unit with a pipeline]. Rocz. AR w Poznaniu 46, 25-32 [in Polish].

3. Kupczyk A., 1986a. Spadki i wahania podciśnienia w rurociągu mlecznym dojarek przewodowych. Cz. I. Podstawy struktury przepływu mleka i powietrza w rurociągu mlecznym dojarki [Drops and changes of vacuum in a milk line of tube milking machines. Part I. Basis of the milk stream and air structure in the milk line of a milking machine]. Rocz. Nauk Rol. T. 77-C-1, 25-33 [in Polish].

4. Kupczyk A., 1986b. Spadki i wahania podciśnienia w rurociągu mlecznym dojarek przewodowych. Cz. II. Spadki podciśnienia w poziomym rurociągu mlecznym [Drops and changes of vacuum in a milk line of tube milking machines. Part II. Drops of vacuum in a horizontal milk line]. Rocz. Nauk Rol. T. 77-C-1, 35-41 [in Polish].

5. Szlachta J., 1999. Normy międzynarodowe ISO a sprzęt udojowy [International norms of ISO and the milking equipment]. Poradnik Hodowcy 5, 1 [in Polish].

6. Tan J., Janni K. A., Appleman R. D., 1989. Dynamic variations in milking vacuum systems. ASAE Paper No. 89-6065. St. Joseph, MI: ASAE.

7. Tan J., Janni K. A., Stelson K. A., 1992. Mathematical modelling of milking machine vacuum systems. Transactions of the ASAE 35(1), 327-332.

8. Wiercioch M., 1998. Ocena intensywności wypływu mleka ze strzyka krowy w poszczególnych cyklach przy użyciu różnych systemów (aparatów) udojowych [Estimation of the intensity of a milk stream from a cow´s teat in particular cycles using various milking systems (machines)]. Maszynopis (a type-written copy), IIR AR Wrocław [in Polish].

9. Woolford M. W., 1974. Milking machine design: Factors effecting vacuum stability. Agricultural Research In New Zealand 79, 1972-1973.

10. Woyke W., 1995. Międzynarodowa Konferencja Naukowa IBMER "Podstawowe problemy w technice i technologii produkcji zwierzęcej z uwzględnieniem aspektów ekologicznych" [International Scientific Conference IBMER "Basic problems in the technology of animal production considering ecological aspects]. Warszawa 1995, 217-220 [in Polish].

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Danuta Skalska
Institute of Agricultural Engineering,
Agricultural University of Wrocław, Poland
37/41 Chełmonskiego Street; 51-630 Wrocław, Poland
phone: (+48 71) 32 05 731
fax: (+48 71) 348 24 86
email: skalska@imr.ar.wroc.pl

Józef Szlachta
Institute of Agricultural Engineering,
Agricultural University of Wrocław, Poland
37/41 Chełmonskiego Street; 51-630 Wrocław, Poland
phone: (+48 71) 32 05 731
fax: (+48 71) 348 24 86
email: szlachta@imr.ar.wroc.pl

Mariusz Nejman
Institute of Agricultural Engineering,
Agricultural University of Wrocław, Poland
37/41 Chełmonskiego Street; 51-630 Wrocław, Poland
phone: (+48 71) 32 05 731
fax: (+48 71) 348 24 86
email: nejman@imr.ar.wroc.pl

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Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.

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