Determination of Milking Machine System Vacuum

Tuesday, 14th October 2008
 
Recently, the MPTA Council was informed of some problems caused through system vacuum level being set to low for the physical configuration of the plant. The consequence of this was poor milking and washing to the point where equipment was disconnected to reduce the added static lift and friction. What had happened was system vacuum had been set to low or not adjusted to compensate for the extra friction loss and amount of extra lift from the cow floor. The result being insufficient vacuum at the cluster when the system was at flow.

It is important to understand there is one level in all plants that is a constant, this is the mean teat end vacuum, in the presence of flow, of around 40kPA. This vacuum occurs during the b phase, being the milk extraction part of the pulsation cycle when the liner is in the open position.

System vacuum set by the regulator or transducer is merely a product of mean teat end vacuum during phase b plus static lift plus friction. Friction, just as in water system design , is affected by flow rate, the higher the flow rate the greater the friction. Hence we often see European and American plants run at higher vacuum levels than we do, the reason being they have higher flow rates, therefore have to allow for more vacuum drop between system vacuum and mean teat end vacuum, also they often have more equipment between the claw and the milkline.

Lets take a few examples.

A Lowline, (many rotaries in NZ are set up this way); Start with the required teat end vacuum during phase b of about 40kPA then add static lift, of which there is none, add friction, a new plant will probably have high-flow milk tubes straight into a large milkline, so there won't be much. An appropriate system vacuum would be about 40+0+2= 42kPA. If this same plant had standard long milk tubes and small milklines, ie more friction, you might add one or two kPA. If this example plant had automation, depending on the type, this will add some or a lot of friction, so again you might add one or two kPa.

A Highline Herringbone; Again start with the required teat end vacuum during b phase of about 40kPA then add static lift, about 1.5kPA. In this case an appropriate system vacuum would be about 40+1.5+4=46kPA. As in the scenario above if this example had standard milk tubes and droppers and small milklines you might add one or two kPA to compensate for the extra friction loss.

Herd Test Meters; These are a good example of how added equipment will affect
Mean teat end vacuum. Some of these meters add quite a lot of friction and sometimes add to the static lift. You may have experienced that to achieve an accurate herd test the system vacuum needs to be lifted around 5kPA. The reason being, this higher vacuum level is required to overcome the extra losses and get the system back to the usual mean teat end vacuum of about 40kPA. ( Continued on next page)

Over Milking; We talk about over-milking, this also has an impact on our selection of system vacuum. Remember when the cow is milking the added friction associated with flow gives us our target mean teat end vacuum of about 40 kPA however, when the flow stops most of this friction is no longer present and the teat is now exposed to something close to system vacuum. For this reason, without cluster removers in a dairy where over milking is likely, you would set the vacuum level on the low side. If the machine was equipped with cluster removers you can afford to be a little more aggressive with the vacuum level because the cluster is removed at the end of flow thus not exposing the teat to long periods of system vacuum. There are of course other forces acting on the teat, however this article is focusing on vacuum level.

Much more is now understood about teat end vacuum, and all the things that can influence it. There will be an appropriate level for every plant configuration and flow rate. For instance, if we start with a given system vacuum, mean teat end vacuum will be higher with a large 2X2 claw than a small 2x2 claw. Conversely it will be higher with a small 4x0 claw than with a large 4x0 claw.

Summary

We must take time to look at an installation, access the amount of lift, the amount of friction offered by the components present and the degree of protection offered from over-milking, being ACR's or plenty of milkers.
From this you extrapolate a system vacuum. In all cases if the vacuum is to low it will likely cause cup slip and or slow milking, if it is to high it will cause teat end damage.

The Milking Machine Tester Course covers this well, however the tutors will in future put a little more emphasis in this area.
Some OEM companies are already using small scale simulators where a particular plants teat end vacuum during phase b can be measured and set accordingly thus removing the guesswork.

This is being driven by a requirement in the Manual, for specific milk flows, the desired average liner vacuum during the b phase and d phase of a pulsation cycle for their systems.

We invite any Questions or comments directed through MPTA Council.