SIZING OF THE MAIN VACUUM SUPPLY AIRLINE (CMFBMP DAIRY GUIDELINES) .

Definition:

In systems with milklines, the main vacuum supply is defined as the airline between the vacuum pump and the sanitary trap near the receiver jar. In weigh jar systems, the main vacuum supply line is defined as the airline from the vacuum pump through to the connection to the line that supplies milking vacuum to the top of the jars.

Improper sizing of the main vacuum supply airline from the vacuum pump through to the milking and pulsation sides of a milking system can have serious implications on both system airflow and vacuum stability, and on herd production and udder health. If the past eight(8) years of conducting milking system performance tests in Manitoba is any indication, approximately 60 to 70% of all dairies could improve system performance by upgrading main vacuum airline size to current recommended standards.

Common problem areas with respect to the main vacuum supply airline include: undersized (i.e. too small a diameter line), installation of excessive line length, installation of an excessive number elbows and/or tees, rusted/corroded galvanized lines, and undersized/restrictive tapered fittings/connections to vacuum pumps, vacuum tanks and sanitary traps.

During the past year new industry standards were published for recommended minimum pipe sizes for a system's main vacuum supply airline. The following table, which summarizes these new standards, was included in a paper presented at the Second Annual Western Large Dairy Herd Management Conference in Las Vegas, Nevada (April 6-8, 1995) by Dr. Graeme Mein, Professor of Milking Research at the University of Wisconsin-Madison.

Recommended minimum pipe size (inches of internal pipe diameter) for the main vacuum supply airline of a milking system:

• Footnote #1: CFMASME stands for cubic feet per minute (of airflow), American Society of Mechanical Engineers' Standard.
• Footnote #2: The above figures include an allowance for the equivalent length of straight pipe (measured in feet) for one(1) vacuum tank, one(1) sanitary trap and eight(8) elbows.
• Footnote #3: The airline sizes above were derived based on extensive airflow studies that indicated minimal to no negative effects on airflow characteristics for each vacuum pump size and length of airline.
• Footnote #4: In systems with two(2) receiver jars, the theoretical maximum airflow rate in the two(2) separate airlines between the distribution tank and the sanitary traps may be halved. As a result, the recommended size of these split lines can be reduced according to the values in the table corresponding to half the total vacuum pump capacity.

Source: Dr. Graeme Mein, Professor of Milking Research, University of Wisconsin - Madison, April 1995.

The concept of proper airline sizing is easy to grasp if you think of it in the following terms:

Moving air through a vacuum line is similar to moving water through a water pipe. Regardless of how much pull (i.e. on air) or pressure (i.e. water) is created, all line/pipe sizes have limits to how much air or water they can physically move. Think of a herd drinking from a watering tank that is supplied water through a half inch (0.5 inch ) diameter line. The replacement rate of water could be greatly increased at the same water pressure level by simply increasing the diameter of the line to three-quarters of an inch (0.75 inches) between the pressure system and the watering tank. The same analogy can be applied to vacuum airlines. An undersized main vacuum supply airline can create serious restrictions to system airflow unless airline size is matched to the airflow capacity of the vacuum pump and the length of line required to connect the vacuum pump to the milking side of the system.

Evaluate the capacity and size of your milking system's main vacuum supply airline as follows:

1. Draw a schematic of you system's main vacuum supply airline starting from the vacuum pump. Include all elbows and tee connections, line sizes and variations in line sizes, line length, and locations of any tapered fittings.
2. If you don't have an accurate reading on CFM of airflow output from your vacuum pump, estimate it by using the rule of thumb that every one(1.0) horsepower (HP) from the motor powering the vacuum pump is equivalent to about 10 CFM ASME of airflow.
3. Using the table of recommendations on the previous page, compare the size of your system's main vacuum supply airline to what is required to suit the airflow capacity of you vacuum pump and the actual length of line. In the case where your system has an excess number of elbow and tee connections (i.e. greater than eight), increase total line length by about 5.0 feet for every 1.5 or 2.0 inch 90 standard elbow and 3.0 feet for every 1.5 or 2.0 inch 45 standard elbow.
4. If you determine that airline size is inadequate, immediately contact your dealer and have airline size upgraded to meet at least minimum recommended standards. Note: If there's a chance that vacuum pump capacity will have to be increased to suit future herd expansion, then, size the main vacuum supply airline to reflect the anticipated future vacuum pump capacity of the system.
5. When upgrading airline size, attempt to minimize the number of elbows installed, keep line length as short as possible and stay away from tapered fittings unless it is right at the connection to the inlet port of the vacuum pump.
6. In all cases when upgrading the main vacuum supply airline, install PVC airlines since PVC is easier to clean, it requires less maintenance, it offers less resistance to airflow than galvanized pipes, and it won't rust/corrode with age like galvanized lines.

In summary, milking system performance is highly dependent on having all components properly sized and in balance with one another. Airflow restrictions caused by an undersized main vacuum supply airline can have serious implications on both herd performance and system vacuum stability.

Don't sacrifice herd and milking system performance. Make sure your system's main vacuum supply airline is sized to meet current recommended standards.

Article written by:

Tom Droppo, Manitoba Dairy Specialist Animal Industry Branch, Manitoba Agriculture Phone: (204) 945-7670; Fax (204) 945-4327

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