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Visual Assessment of Factors Affecting Reverse Pressure Gradients in Liners Using a Simulated Milking Device

Masafumi Enokidani1,*, Yoji Inui2, Hitoshi Kondo2, Kazuhiro Kawai3, Yasunori Shinozuka3, Kento Imao3


Published in Journal of the ASABE 66(3): 627-635 (doi: 10.13031/ja.15305). Copyright 2023 American Society of Agricultural and Biological Engineers.


1Hokkaido Dairy Management Services, Tsurui, Hokkaido, Japan.

The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative commons.org/licenses/by-nc-nd/4.0/

Submitted for review on 14 August 2022 as manuscript number MS 15305; approved for publication as a Research Article by Associate Editor Prof. Bulent Koc and Community Editor Dr. Heping Zhu of the Machinery Systems Community of ASABE on 28 February 2023.

Highlights

Abstract. The backflow of milk resulting from reverse pressure gradients in liners can cause mastitis. In this study, we hypothesized that liner internal diameter, vent location, and pulsation setting can induce backflow. We therefore attempted to clarify the mechanism of backflow using a flow simulated device. We visually analyzed the effects on backflow of the liner junction point (JP) (i.e., the border from the liner to the short milk tube), the liner inlet point (IP) (i.e., a distance of 63 mm from the end of the short milk tube), vent location, and pulsation setting, using high-speed video recordings of water flow in clear silicone liners at seven flow rates (1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 kg min-1/quarter). The actual flow rate was approximately half of the flow meter set value due to the effect of the pulsation closing phase (C+D phase) on quarter level. The results showed that the degree of backflow in liners increased considerably with an increase in flow rate and a decrease in liner internal diameter. The degree of backflow was affected considerably more by JP internal diameter than by IP internal diameter, pulsation setting, or vent location. The degree of backflow was not affected significantly by short milk tube vents and mouthpiece vents when JP internal diameter was larger. However, the degree of backflow was very strong when simultaneous pulsation was used, even at low flow rates. The findings showed that the JP internal diameter should be >13.0 mm, IP internal diameter should be >11.0 mm, the pulsation rate should be 60 pulses/min, and the pulsation ratio should be 60:40 when the alternative pulsation setting is used. This information might be useful for preventing intramammary gland infection.

Keywords. Liner internal diameter, Pulsation setting, Reverse pressure gradients, Simulated milking device, Vent location.

Mastitis can cause significant economic losses on dairy farms. Given the wide variety of factors that contribute to the incidence of mastitis, mastitis prevention requires that all areas of the dairy farm be assessed for mastitis risk and measures be implemented to reduce it. Numerous factors can contribute to mastitis outbreaks, and many studies have been published on milking preparations (Galton et al., 1988; Enokidani et al., 2013a, b) and milking systems (MS) (Baxter et al., 1992; Enokidani et al., 2016, 2017, 2019; Enokidani, 2022; Mein, 2012; Mein and Reinemann, 2014; Rasmussen and Madsen, 2000; Spencer et al., 2007). These studies reported that inadequate milking preparations and MS with poor settings (for example, location of pulsation and regulator, corner angle and diameter of vacuum line, vacuum pump capacity, etc.) (Enokidani et al., 2019) and poor maintenance increase the risk of mastitis.

An MS is the most frequently used machine on dairy farms and the only piece of equipment that comes into direct contact with the teats, which are the entry point for mastitis-causing bacteria. Therefore, inadequacies associated with MS are considered to more directly affect mastitis than other factors (Bade et al., 2009; Baxter et al., 1992; Besier and Bruckmaier, 2016; Enokidani et al., 2017; Enokidani, 2022).

New mastitis infections involving MS have been considered to arise mainly due to the droplet phenomenon from liner slips (Baxter et al., 1992; Spencer and Rogers, 1991; Thomas and Pearson, 1983). However, Mein (2012) reported that factors other than liner slips may be responsible for new mastitis infections. In addition, Itagaki (2002) reported that starch grains were detected in udder milk when starch grains were injected into the short milk tube. Their findings demonstrated that the starch grains entered the quarter due to backflow in the liner and that this was caused by something other than the droplets generated by liner slip. Furthermore, Ishida et al. (2006) reported that the volume change caused by the liner pulse induced the backflow phenomenon. They also reported that the ingress of backflow water into the teat cave was more likely to occur when the flow rate was low, i.e., when the negative pressure generated inside the teat cave was synchronized with the liner pulse at low flow rates.

Liners can be classified into round, clover-shaped, triangular, square, or round-with-ribs types based on the liner bore shape. Furthermore, depending on the location of the air vent, the vent location is classified as being a mouthpiece vent (MPV), a short milk tube vent (SMTV), or a claw vent (CV). Liners are composed of either silicone or natural rubber and are comprised of either one or two pieces. Although a wide variety of liners are used in the dairy industry, it is still unclear which types are optimal for preventing backflow and reducing the risk of mastitis.

The aim of this study was to analyze the factors visually that cause backflow inside the liner using a flow simulated device. This information might be useful for preventing intramammary gland infection. This study is the first step that will provide a relationship between liner internal diameter and backflow.

Materials and Methods

Test Outline

Test Milking System Outline

In 2018, one milking stall in the milking parlor (double-8, parallel parlor) of a commercial dairy farm in Japan was used to test a simulated milking device. The settings used for the MS at the dairy in this study wereas follows: operating vacuum: 44.0 kPa; effective reserve amount: 2972 L/min (105 CFM; cubic feet/min); pulsation setting: back and front alternating method; rate: 60 pulses/min; pulsation ratio: 60:40; test claw: E-type claw (Orion Company, Nagano, Japan) with a volume of 415 mL and abody weight of 600 g, with a claw inlet internal diameter of 10.5 mm and a claw outlet internal diameter of 14.5 mm.

A nipple was attached to the claw to measure the claw vacuum during testing, and a vacuum cut-off device (Viso-flow Take-off device, Orion Company, Nagano, Japan) was used in this study.

Overview of the Simulated Milking Device

The components of the simulated milking device are shown in figure 1.

Figure 1. Flow simulation milking device, artificial teat, measurement points, and vent location.

Setup of the device in the stall. The measurements from the base of the cow stall to the different parts of the stall are given below in brackets. The height from the base of the cow stall to the claw outlet (20 cm), mouthpiece vent (58 cm), flow meter outlet (85 cm), flow meter inlet (67 cm), water tank outlet (100 cm), Viso-flow inlet (-48 cm), milk line inlet (-100 cm).

The simulation milking device developed for this study consists of a water tank (50 L capacity, 379 mm in diameter, 673 mm in height), four independent liquid flow meters (KZ-7003-07L, Az-one Inc., Osaka, Japan), connected to four water tubes branching from the water tank, and connected to four independent artificial teats.

The water flow rate was adjusted so that the amount of water flowing through the four liners was the same. The height of the water tank relative to the flow meter was adjusted so that the effect of gravity on the teat was negligible.

The simulated teats were prepared using commercially available liner plugs, and the tube at the end of the simulated teat (rubber part) had a length of 40 mm, an internal diameter of 6.0 mm, and a total length of 90 mm. The total length of the simulated teat was determined by measuring the teatlength that was typically sucked into the liner during milking.

The test liners N-III and J225 (Orion Company, Nagano, Japan) were used in this study. Junction point internal diameters were 13 mm and 14 mm, and inlet point internal diameters were 11 mm and 11 mm, respectively. The milk tube had a total length of 222 cm; a 41 cm section with an internal diameter of 25 mm, and a 181 cm section with an internal diameter of 16 mm.

Investigation of Liner Internal Diameter

The 24 liners used on dairy farms in Japan were collected, and the internal diameter of the junction point (i.e., the border from the liner to the short milk tube; JP) and the liner inlet point (i.e., a distance of 63 mm from the end of the short milk tube; IP) were measured. A handmade diameter-measuring rod was used to measure the internal diameters of liners. The internal diameters of the JP and IP were measured by inserting the measuring rods into the liner with a liner shell.

Test Conditions

Test claw and liner

For the test claw, a CV with an E-type claw (Orion Co., Nagano, Japan) with a vent diameter of 1.0 mm and a nipple for vacuum measurement was used. A custom-made one-piece translucent silicone N-III liner (Orion Co.) (JP internal diameter: 14.0 mm, IP internal diameter: 11.0 mm) was used according to the vent position, and the test was conducted using a transparent plastic clear shell.

Placement of MPV and SMTV

A single translucent silicone J225 liner (Orion Co.) (JP internal diameter: 13.0 mm, IP internal diameter: 10.5 mm) was used with a handmade vent.

Water Flow Rate

In each test, the water flow rate was set to 1.0 kg/min for each liner and tested at increments of 0.5 kg/min up to 4.0 kg/min. Due to the effect of the pulsator closing phase, the measured flow rate was approximately half of the total flow rate displayed on the flow meter. The maximum flow rate was set to 16 kg/min (4.0 kg/min/quarter) using a total of four flow meters. Enokidani (2022) reported that high-producing cows produced over 8 kg min/4 quarters. After adjusting the flow rate, 30 s was used as the introduction time, and the movement of water in the liner was filmed using a high-speed (500 frames/second) video camera (Casio EX100F, Casio Inc., Japan) from 30 s to 60 s. The vacuum in the claw was measured using a vacuum measurement device (TriScan; Westfalia·Surge, Babson Bros. Co., Naperville, IL, USA).

Selection of Liner Aperture Internal Diameter

In the liner survey, all liners recommended by the manufacturers had been measured at the dairy farm. The selection of the aperture internal diameter was determined based on a survey of the internal diameter of liners used on Japanese dairy farms in this study.

The JP internal diameter was changed by inserting a test handmade aperture part (fig. 1) measuring 10 mm width using a measurement rod, and the internal diameter range was tested in 1.0 mm increments from 8.0 mm to 13.0 mm.

The IP internal diameter was changed by inserting a test handmade aperture part (fig. 1) measuring 10 mm width, and the internal diameter range was tested in 1.0 mm increments from 7.0 mm to 10.0 mm. The JP and IP internal diameters were varied independently.

Vent Location

To examine the effect of vent location, CV-, MPV-, and SMTV-type vents were tested in test liners. An E-type claw was used for the CV-type vent. However, for the MPV- and SMTV-type vents, test liners were used with the E-type claw vent closed. In the case of the MPVs and SMTVs, MPVs (1.0 mm internal vent diameter) and SMTVs (0.5 mm internal vent diameter) were installed in all four test liners.

Pulsation Setting

For the pulsation tests, a CV E-type claw and a J225 liner were used. The flow tests were conducted with different pulsation settings using JP internal diameters of 9.0 mm and 13.0 mm. In the case of the alternative pulsation tests, the following parameters were used:

Measurement of Vacuum in the Claw During Testing

The vacuum in the claw during the tests was measured using a vacuum measurement device for 30 s, i.e., from 30 s to 60 s, to check for air leaks after the water flow rate was adjusted.

Method for Assessing Reverse Pressure Gradient Score (RPGS)

A high-speed video camera was used to capture water movement through two liners at 500 frames/second for 30 s after the flow rate of the water was adjusted. The captured video was played back slowly (0.5 × speed) to check the flow in the two liners, and to assign RPGSs. The average RPGS for the two liners was calculated for each test flow rate.

The RPGS was assigned using the following criteria:

  1. No reverse flow.
  2. Slight backflow in the form of mist.
  3. Small water flow in the short milk tube is causing backflow.
  4. Reflux occurs with large water flow in the liner.
  5. Teats are submerged in the liner.

Combinations of Each Factors

The experiments were conducted at the three vent locations (CV, MPV, and SMTV) and showed 49 test combinations of JP diameters ranging from 8.0 mm to 14.0 mm (1.0 mm intervals) and flow rates ranging from 1.0 kg/min to 4.0 kg/min (0.5 kg intervals). The IP diameter ranged from 7.0 mm to 11.0 mm (1.0 mm intervals), and the flow rate ranged from 1.0 kg/min to 4.0 kg/min (0.5 kg intervals), for a total of 35 test combinations. In addition, 35 x 4 combinations of pulsation settings (Rate and ratio), 7 steps flow rates, and 2 JP diameters (9.0 mm and 13.0 mm) were used for a total of 140 combinations.

Results

Survey of Internal Liner Diameter

Table 1 shows the JP and IP internal diameters of the 24 liners investigated in this study. For the JP internal diameters, five were 9.0 mm, four were 10.0 mm, four were 11.0 mm, six were 12.0 mm, three were 13.0 mm, one was 14.0 mm, and one was 15.0 mm. Likewise, for the IP internal diameters, nine were 9.0 mm, nine were 10.0 mm, three were 11.0 mm, and three were 12.0 mm.

Table 1. Internal diameters of junction points and inlet points of liners from different companies.
NumberCompanyLiner ShapeVent LocationMaterialInternal Diameters of Junction and Inlet Points (mm) [a],[b],[c],[d]
8 mm9 mm10 mm11 mm12 mm13 mm14 mm15 mm
1ARoundClaw ventRubber??
2ARoundClaw ventSilicone??
3ARoundClaw ventRubber??
4ARoundClaw ventRubber??
5ARoundClaw ventSilicone??
6ARoundClaw ventSilicone??
7ARoundClaw ventSilicone??
8BRoundClaw ventRubber??
9BRoundClaw ventRubber??
10BRoundClaw ventRubber??
11BRoundClaw ventRubber??
12CRound with ribShort milk tube ventRubber??
13CRoundShort milk tube ventRubber??
14DRoundClaw ventRubber??
15DCloverClaw ventRubber??
16ERoundClaw ventRubber??
17FRoundClaw ventRubber??
18GEllipseClaw ventRubber??
19HRound with ribMouthpiece ventSilicone??
20ISquareClaw ventRubber??
21JTriangleMouthpiece ventRubber??
22KRoundClaw ventRubber??
23KRoundClaw ventRubber??
24LRoundClaw ventRubber??

    [a] Junction point internal diameters with the liner shell setting were measured using a measuring rod.

    [b]Inlet point internal diameter was measured at distance of 63 mm from the end of the liner.

    [c]? = Inlet point internal diameter.

    [d]? = Junction point internal diameter,

Average Claw Vacuum During the Test

The average claw vacuum during each test ranged from 41.0 to 42.0 kPa, regardless of the flow rate, and the all-flow tests had no air leakage due to liner slip. Regarding the average claw vacuum in the simultaneous pulsation setting, the claw vacuum decreased from 42.0 kPa to 36.7 kPa as the flow rate increased, and the range in the claw vacuum was larger than the range under the alternating setting.

Changes in RPGS Over Time

Figure 2 shows the backflow of water in frames No. 3 and No. 4, which are part of a series of images that were captured over the course of 1 second.

Relationship Between RPGS and Aperture Internal Diameter, Flow Rate and Vent Location

Table 2 shows the relationship between the RPGS and JP internal diameter, flow rate, and vent position. In the case of CVs, a high flow rate and narrow aperture diameter increased the RPGS; the RPGS for a JP internal diameter of =9.0 mm was high, and the RPGS for a JP internal diameter of =12.0 mm was low. In the case of MPVs, the RPGS increased at JP internal diameters of 8.0 mm and 9.0 mm, as the flow rate increased. However, at JP internal diameters of =10.0 mm, the RPGS remained at 1.0, even at a flow rate of 4.0 kg/min.

In the case of the SMTVs, the RPGS increased above 4.0 when the JP internal diameter was =10.0 mm and flow rate increased. However, at JP internal diameters of =11.0 mm, the RPGS remained low, even when the flow rate was high. The mean RPGS remained at 1.0 at a JP internal diameter of 13.0 mm, even at a flow rate of 4.0 kg/min.

Table 3 shows the relationship between the IP internal diameter and vent location on RPGS. For CVs, the RPGS obtained when using an IP internal diameter of =8.0 mm was greater than 2.5, even at a flow rate of 1.0 kg/min, and the RPGS increased as the flow rate increased. On the other hand, when the IP internal diameter was 11.0 mm, the obtained RPGS remained at 1.0, even when the flow rate was 4.0 kg/min. In the case of MPVs, the RPGS remained at 1.0 regardless of the IP internal diameter and water flow rate. In the case of SMTVs, the RPGS was 1.5 only when the IP internal diameter was 7.0 mm and the flow rate was =3.0 kg/min, but the RPGS remained at 1.0 for all other internal diameters and all flow rates.

Reverse Pressure Gradient Scores Obtained Using the Alternating Pulsation Setting

Table 4 shows the RPGS results obtained using the alternating pulsation setting in conjunction with a CV. The pulsation ratio was fixed at 60:40, and the pulsation rate varied.

In the case of a JP internal diameter of 9.0 mm, there was no significant difference in the RPGS due to the change in pulsation rate frequency, but the effect of a JP internal diameter of 9.0 mm was marked and the RPGS was 4.0.

Figure 2. Analysis of reverse pressure gradients over time.
Backflow was observed in frames of No. 3 and No. 4 (yellow inset).

When the JP internal diameter was 13.0 mm, there was no difference in the RPGS (1.0) due to the change in the pulsation rate frequency. The JP internal diameter had a greater effect on RPGS than the pulsation rate frequency.

Table 2. Reverse pressure gradient score for each junction point's internal diameter and vent location.
Flow Rate
(kg/min)[a]
8.0
mm
9.0
mm
10.0
mm
11.0
mm
12.0
mm
13.0
mm
14.0
mm
Claw
Vent
1.0[b]4.0[c]1.5 1.0 1.0 1.0 1.0 1.0
1.54.0 2.0 1.0 1.5 1.0 1.0 1.0
2.04.0 2.0 1.0 1.5 1.0 1.0 1.0
2.54.0 2.0 1.5 1.5 1.0 1.0 1.0
3.04.0 4.0 1.0 1.5 1.0 1.0 1.0
3.54.0 4.0 2.0 3.0 1.5 1.0 1.0
4.04.0 4.0 3.0 3.0 2.5 2.0 2.0
Mouthpiece Vent1.0[b]1.0[c]1.0 1.0 1.0 1.0 1.0
1.51.5 1.0 1.0 1.0 1.0 1.0
2.02.5 1.5 1.0 1.0 1.0 1.0
2.52.5 1.5 1.0 1.0 1.0 1.0
3.03.0 2.0 1.0 1.0 1.0 1.0
3.53.0 3.0 1.0 1.0 1.0 1.0
4.04.0 2.5 1.0 1.0 1.0 1.0
Short Milk Tube Vent1.0[b]2.0[c]1.0 1.0 1.0 1.0 1.0
1.52.0 1.5 1.0 1.0 1.0 1.0
2.04.0 2.0 1.0 1.0 1.0 1.0
2.54.0 4.0 1.0 1.0 1.5 1.0
3.04.0 4.0 4.0 1.0 1.5 1.0
3.54.0 4.0 4.0 1.0 1.5 1.0
4.04.0 4.0 4.0 2.0 1.5 1.0

    [a]Flow rate: kg/min/quarter measured using a flowmeter.

    [b]Due to the effect of the pulsator closing phase, the total flow (4 quarters) displayed on the flow meter was approximately half the actual flow volume.

    [c]The reverse pressure gradient score is the average score of two liners.

In the case of a JP internal diameter of 9.0 mm, the RPGS was 4.0 at a flow rate of =2.5 kg/min and pulsation ratios of 60:40, 65:35, and 70:30. On the other hand, in the case of a JP internal diameter of 13.0 mm, the RPGS was 2.5 when the flow rate was >3.0 kg/min, only at pulsation ratios of 65:35 and 70:30; however, the RPGS remained at 1.0 under all other conditions. The JP internal diameter had a greater effect on the RPGS than the pulsation ratio (table 5). These results indicate that the JP internal diameter has a greater effect on RPGS than the pulsation setting.

Table 3. Reverse Pressure Gradients Score at each claw inlet point internal diameter and vent location.
Flow Rate
(kg/min)[a]
7.0
mm
8.0
mm
9.0
mm
10.0
mm
11.0
mm
Claw Vent1.0[b]2.5[c]2.5 1.5 1.0 1.0
1.53.0 3.0 1.5 1.0 1.0
2.03.5 3.0 1.5 1.0 1.0
2.53.5 3.5 1.5 1.0 1.0
3.03.5 3.5 1.5 1.0 1.0
3.53.5 3.5 2.0 1.0 1.0
4.03.5 4.0 3.5 1.5 1.0
Mouthpiece Vent1.0[b]1.0[c]1.0 1.0 1.0 1.0
1.51.0 1.0 1.0 1.0 1.0
2.01.0 1.0 1.0 1.0 1.0
2.51.0 1.0 1.0 1.0 1.0
3.01.0 1.0 1.0 1.0 1.0
3.51.0 1.0 1.0 1.0 1.0
4.01.0 1.0 1.0 1.0 1.0
Short Milk Tube Vent1.0[b]1.0[c]1.0 1.0 1.0 1.0
1.51.0 1.0 1.0 1.0 1.0
2.01.0 1.0 1.0 1.0 1.0
2.51.0 1.0 1.0 1.0 1.0
3.01.5 1.0 1.0 1.0 1.0
3.51.5 1.0 1.0 1.0 1.0
4.01.5 1.0 1.0 1.0 1.0

    [a]Flow rate: kg/min/quarter measured using a flowmeter.

    [b]Due to the effect of the pulsator closing phase, the total flow (4 quarters) displayed on the flow meter was approximately half the actual flow volume.

    [c]The reverse pressure gradient score is the average score of two liners.

Reverse Pressure Gradient Scores Obtained Using the Simultaneous Pulsation Setting

Table 6 shows the RPGSs obtained using the simultaneous pulsation setting with a CV, a pulsation rate of 60 pulses/min, and pulsation ratio of 60:40. In the case of a JP internal diameter of 9.0 mm, the RPGS was high even when the flow rate was low, and it became higher as the flow rate increased. In the case of a JP internal diameter of 13.0 mm, the RPGS remained at 1.0 at a flow rate of =2.5 kg/min, but the RPGS was as high as 5.0 at a flow rate of =3.5 kg/min.

When comparing the simultaneous pulsation setting and the alternating pulsation setting using a back-and-front pulsation method, it was found that RPGSs obtained using a simultaneous pulsation setting were higher than those obtained using an alternating pulsation setting, even when flow rates were low.

Discussion

The average claw vacuum measured during the simulated milking test ranged from 41.0 kPa to 42.0 kPa for the alternating pulsation setting, regardless of the liner internal diameter and the water flow rate. From the results of the claw vacuum tests, it was confirmed that the MS used in this study had a high milking performance, as the claw vacuum was not affected by water flow rate (Enokidani et al., 2017) and no air leakage occurred during the test.

Table 4. Reverse pressure gradient score for 9.0 mm and 13.0 mm junction point internal diameters at different pulsation rates (alternative pulsation setting, pulsation ratio fixed at 60:40).
Pulsation Rate (pulse/min)[a]5055606570
Flow rate (kg/min)[b]RPGSRPGSRPGSRPGSRPGS
Junction point
internal diameter:
9.0mm
1.0[c]3.0[d]3.0 3.0 3.0 3.0
1.5 4.0 3.5 3.5 3.0 3.0
2.0 4.0 4.0 4.0 3.5 4.0
2.5 4.0 4.0 4.0 4.0 4.0
3.0 4.0 4.0 4.0 4.0 4.0
3.5 4.0 4.0 4.0 4.0 4.0
4.0 4.0 4.0 4.0 4.0 4.0
A phase time ms155ms (13%)[e]157ms (15%)153ms (15%)150ms (16%)145ms (17%)
D phase time ms376ms (31%)333ms (31%)297ms (30%)263ms (29%)243ms (28%)
Junction point
internal diameter:
13.0mm
1.0[c]1.0[d]1.0 1.0 1.0 1.0
1.5 1.0 1.0 1.0 1.0 1.0
2.0 1.0 1.0 1.0 1.0 1.0
2.5 1.0 1.0 1.0 1.0 1.0
3.0 1.0 1.0 1.0 1.0 1.0
3.5 1.0 1.0 1.0 1.0 1.0
4.0 1.0 1.5 1.0 1.0 1.0
A phase time ms154ms (13%)[e]154ms (14%)153ms (15%)153ms (17%)151ms (18%)
D phase time ms374ms (31%)331ms (30%)295ms (30%)265ms (29%)239ms (28%)

    [a]The pulsation ratio was fixed at 60:40.

    [b]Flow rate: kg/min/quarter measured using a flow meter.

    [c]Due to the effect of the pulsator closing phase, the total flow (4 quarters) displayed on the flow meter was approximately half the actual flow volume.

    [d]The reverse pressure gradient score (RPGS) is the average score of two liners.

    [e]Values in brackets indicate percentage of A phase time or D phase time.


Table 5. Reverse Pressure Gradient Score for 9.0 mm and 13.0 mm junction point internal diameters and different pulsation ratios (alternative pulsation setting, pulsation rate fixed at 60 pulses min-1).
Pulsation Ratio 50:50[a]55:4560:4065:3570:30
Flow rate (kg/min)[b]RPGSRPGSRPGSRPGSRPGS
Junction point
internal diameter:
9.0mm
1.0[c]2.0[d]2.0 3.0 3.5 3.0
1.5 2.0 3.0 3.5 3.5 3.5
2.0 3.0 3.0 4.0 4.0 3.5
2.5 3.0 3.5 4.0 4.0 4.0
3.0 3.0 3.5 4.0 4.0 4.0
3.5 3.0 3.5 4.0 4.0 4.0
4.0 3.0 4.0 4.0 4.0 4.0
A phase time ms142ms (14%)[e]145ms (15%)153ms (15%)152ms (15%)145ms (15%)
D phase time ms396ms (40%)351ms (36%)297ms (30%)249ms (25%)200ms (20%)
Junction point
internal diameter:
13.0mm
1.0[c]1.0[d]1.0 1.0 1.0 1.0
1.5 1.0 1.0 1.0 1.0 1.0
2.0 1.0 1.0 1.0 1.0 1.0
2.5 1.0 1.0 1.0 1.0 1.0
3.0 1.0 1.0 1.0 2.5 2.5
3.5 1.0 1.0 1.0 2.5 2.5
4.0 1.0 1.0 1.0 2.5 2.5
A phase time ms147ms (15%)[e]148ms (15%)153ms (15%)163ms (16%)152ms (15%)
D phase time ms393ms (39%)346ms (35%)295ms (30%)246ms (25%)197ms (20%)

    [a]The pulsation rate was fixed at 60 pulses/min.

    [b]Flow rate: kg/min/quarte measured using a flow meter.

    [c]Due to the effect of the pulsator closing phase, the total flow (4 quarters) displayed on the flow meter was approximately half the actual flow volume.

    [d]The reverse pressure gradient score (RPGS) is the average score of two liners.

    [e]Values in brackets indicate the percentage of the A phase time or the D phase time.

The survey of liner shapes used on Japanese dairy farms revealed that farmers used liners of different shapes. The liner internal diameters ranged from 9 to 15 mm, with JP internal diameters tending to be larger than IP internal diameters.

Numerous studies have been conducted on liners to date (Penry et al., 2016, 2017a, 2017b; Rasmussen and Madsen, 2000; Upton et al., 2016). While these studies focused on pulsation settings, milking yield, milking time, milk flow, vacuum settings, and teat conditions, none examined whether liner internal diameter affected backflow, which was the objective of this study. To the best of the author’s knowledge, this is the first study to examine the relationship between liners internal diameter and backflow.

Table 6. Reverse pressure gradient score for different junction points internal diameters using the simultaneous pulsation setting (pulsation rate fixed at 60/min, ratio fixed at 60:40).
 JP internal diameter
9.0mm
JP internal diameter
13.0mm
Flow rate (kg/min)[a]RPGSRPGS
1.0[b]2.0[c]1.0
1.5 2.0 1.0
2.0 4.0 1.0
2.5 5.0 1.0
3.0 5.0 3.0
3.5 Unmeasurable5.0
4.0 Unmeasurable5.0
A phase time (ms)[d]144ms (14%)[c]144ms (14%)
D phase time (ms) 238ms (24%) 238ms (24%)

    [a]Flow rate: kg/min/quarter measured using a flow meter.

    [b]Due to the effect of the pulsator closing phase, the total flow (4 quarters) displayed on the flow meter was approximately half the actual flow volume.

    [c]The reverse pressure gradient score (RPGS) is the average score of two liners.

    [d]The pulsation rate was fixed at 60 pulses min-1 and the pulsation ratio was fixed at 60:40.

    [e]Values in brackets indicate the percentage of the A phase time or the D phase time.

Bade et al. (2009) examined the effects of operating vacuum, pulsation b-phase time, and liner compression on milking speed and found that peak milk flow rate could be increased by increasing the operating vacuum and liner compression. Penry et al. (2017b) also examined the balance between milking speed and gentleness on teat tissue. They reported that liner compression, teat end vacuum, mouthpiece camber vacuum, and duration of each pulsation phase all affected milking flow rate at peak milk flow. The balance among factors in combination is thus considered more important than each factor in isolation.

The JP and IP internal diameters on the dairy farms ranged from 9.0 mm to 15.0 mm and 9.0 mm to 12.0 mm, respectively. The IP internal diameter was considered to be narrower than the JP internal diameter in order to prevent air inflow from the liner mouth when the liner is attached to the teat, and to prevent the liner from falling out when the claw inlet is inserted.

As for the RPGs, in the JP internal diameter test, when the JP internal diameter was as small as 8.0 mm and 9.0 mm, RPGS was higher even when the water flow rate was low, indicating that the JP internal diameter had a greater effect on RPGS than the IP internal diameter. There must be a difference between the JP and IP internal diameters because the water that is sucked from the teat during the liner opening phase must pass through the JP point instantaneously, which requires a larger internal diameter than the IP point. On the other hand, if the JP internal diameter is large, then the reverse pressure gradient (RPG) cannot form unless the water flow rate is increased. Therefore, the high RPGS could be observed only when the JP internal diameter was small and the flow rate was high. The findings showed that the JP internal diameter should be =13.0 mm to prevent backflow.

In the IP internal diameter test, RPGS is higher when the IP internal diameter is small and the flow rate is high. When the liner is opened, the water in the liner flows towards the teat due to the change in the liner bore volume. The volume of the short milk tube is considered to be the reason why the IP internal diameter can be smaller than the JP internal diameter. An IP internal diameter of =11.0 mm is desirable to prevent backflow.

As for the vent location, in the case of the MPV, the RPGS was 1.0 when the JP internal diameter was =10.0 mm, and backflow did not occur. This is because air flowed in from the MPV in the opposite direction of the flow of the water backflow, reducing the vacuum generated by the change in the liner bore volume. In the case of a JP internal diameter of =9.0 mm, backflow occurred when the flow rate increased. For a JP internal diameter of =10.0 mm, backflow did not occur at any of the IP internal diameters tested.

On the other hand, in the case of CV, RBGS was relatively high; unlike MPV, the air that was sucked in from CV only served to immediately pump the water (milk) in the claw into the milk tube, creating milk slag and sending milk to the milk line. In fact, the measured claw vacuum was always stable, and vacuum fluctuation did not affect the RPGS when the alternating pulsation setting was used.

In the case of the SMTV, the air drawn in from the SMTV improved the flow of water in the short milk tube, and the RPGS remained low even when the IP internal diameter was small. However, the air flow from the SMTV did not reach the JP, and it is thought that the air flow did not improve the flow of water in the liner at the JP.

As for the effect of the pulsation setting on RPGS, when the JP internal diameter was large (13.0 mm), there was no effect on the pulsation rate, only on the pulsation ratio. When the pulsation ratio was 65:35 or 70:30, the pulsation opening time (A phase + B phase time) was longer. Conversely, the pulsation closing time (C phase + D phase time) for water flow in the short milk tube decreased, which means that an RPG could easily form due to the water remaining in the short milk tube at the time of the next liner opening. The short duration of flow through the liner is thought to explain why backflow was more likely to occur. In the simultaneous pulsation setting, the claw vacuum fluctuated more than in the alternating pulsation setting due to the intermittent flow of water and the vacuum in the liner bore caused by the liner opening being higher than the claw vacuum due to milk flow. It was clear that this had a significant effect on backflow. It is assumed that the milk yield per opening liner is reduced because the suction for the next time of liner opening is started at the claw vacuum that is momentarily reduced by milking. Indeed, in this study, the RPGS was 5 or unmeasurable at flow rates of 3.5 kg/min or higher, and the simultaneous pulsation setting is considered to reduce MS performance and is prone to inducing backflow.

In this study, the effects of JP internal diameter size, IP internal diameter size, vent position, and pulsation setting on backflow were as follows: JP internal diameter size had the largest influence, followed by IP internal diameter size. For the vent location, it was clearly shown that MPV is most effective for preventing backflow, regardless of the JP and IP internal diameters, but a certain amount of air flow from the MPV must be guaranteed at all times during milking. Although ideal, using an MPV is often impractical, and requires that large JP and IP internal diameters be used. In terms of pulsation settings, the alternating setting is recommended rather than the simultaneous setting, and the ratio should be set to 60:40. In terms of the pulsation rate and pulsation ratio, it is necessary to allow time for the water to flow through the short milk tube to prevent backflow; to ensure that this occurs, the pulsation setting should be set to 60 pulses min-1 and the pulsation ratio to 60:40. If MS performance is to be improved, it is desirable to improve factors that affect claw vacuum rather than changing pulsation settings (Enokidani et al., 2016).

Ishida et al. (2006) found that the instantaneous volume change caused by the liner opening induces backflow and hypothesized that reducing the volume change inside the liner may prevent backflow. Their study also reported that the backflow of water into the teat cave is more likely to occur when the milk flow is low, and is affected by the vacuum in the teat cave generated by the liner opening. In the case of round liners, the liner opening causes a large volume change inside the liner, which is considered to generate a large amount of backflow. In the future, it will be necessary to conduct similar tests with non-round liners, which experience less volume change inside the liner during the opening phase.

In this simulated milking test, backflow was frequent when the liner internal diameter was small and the flow rate was high. The smaller the internal diameter of the liner, the more irregular the flow of water in the liner, and the more water flowed toward the vacuum created by the liner bore opening after the momentary blockage. Whether or not the water flowed completely through the short milk tube during the liner closing time was related to the liner internal diameter, the length of the short milk tube, and the amount of water flow.

Based on the results of this study, the previously used irregular MS settings were considered to affect backflow and increase mastitis risk. Further research is needed to perform backflow analysis with other types of liners. We need to study more about the phenomenon that is happening inside each type of liner at milking times, not only for milking performance (Bade et al., 2009; Penry et al., 2016), but also for mastitis prevention and milk quality improvement.

Conclusions

In this study, it was found that liner internal diameter, pulsation setting, and vent location all affect the backflow that occurs in the liner during pulsation opening time. A JP internal diameter of =13 mm, an IP internal diameter of =11 mm, a pulsation rate of 60 pulses/min, and a pulsation ratio of 60:40 are recommended for MS. The liner JP internal diameter had the greatest effect on backflow, and the vent location had no effect on backflow if the liner internal diameter was large.

Acknowledgments

The authors thank the dairy farmers for their cooperation in this study.

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