On Vacuum Pumps and Milking Machines

Today, vacuum pumps are indispensable to the dairy industry, where they are used in milking machines and other common dairy equipment.  The integration of the vacuum pump in milking machines is an interesting evolution.

History

The creation of the modern milking machine actually took place over many years of trial and error. As the industrial revolution of the 19th century swept through industry, many recognized the potential efficiencies of mechanical milking machines, but farmers were resistant to the idea. And no wonder!  Milking machines that would be horrific by modern standards were used as late as the 20th century. They used catheters and tubes made of “pure silver, gutta percha, ivory, and bone”.  The first true vacuum pump milking machines were developed in 1851 in England and 1859 in America, however they did not use the large, industrial-size vacuum pumps of today.  Instead, they used a rudimentary “hand cranked suction pump [that] drew milk from all four teats at once”.  Just picture Greg Focker explaining this at Thanksgiving dinner!  Fortunately, in 1898, the USDA approved a foot-pedal-powered milking machine that allayed farmer’s concerns about harming their valuable livestock.  As you can see, it took about 50 years for an acceptable mechanical milking machine to be developed.

Modern Milking Machines

The modern milking machine has become a standardized device.  Its goal is to collect milk from a cow by vacuum suction. These milk extraction systems are comprised of several components including vacuum pumps, reserve tanks, vacuum regulators, pipelines, and pulse tubes.  In addition, pulsators that alter the force of vacuum on the cow are used, as are various valves, which control the flow and suction.   No proper system would be complete without milk tubes and receivers that transport the milk from the vacuum pump system to a bucket, pump, pipeline, and so on.

The Role of Vacuum Pumps

Essentially, vacuum pumps are used to evacuate (remove air) the pipelines and tubes connecting the milking machines to the cows.  By removing the air molecules from the system, its overall pressure is reduced well below atmosphere*.  Then simple vacuum physics takes over and the higher pressure milk flows through the lower pressure pipelines from the cows to the milking receptacle.

Interestingly, a key component of the vacuum pump milking machine is the use of an interceptor.  The interceptor is fitted to the primary vacuum line and acts to prevent solid and liquid material from getting sucked into the vacuum pump.  As one can imagine, the absence of the interceptor would be detrimental to the performance and longevity of the vacuum pump, and thus to the milking machine.

As with any vacuum system, limiting downtime on a milking application relies on the use of both the proper ancillary equipment (interceptors, particulate traps and filter, etc.) as well as proper system maintenance including vacuum pump oil changes, pipeline preventative maintenance/cleaning, and leak detection and prevention.

Atmosphere or atmospheric pressure (the pressure all things are exposed to in everyday life) is one level of vacuum or pressure.  Typically pressures above atmospheric pressure are expressed in common measurements, such as pounds per square inch (PSI), whereas pressures below atmosphere are expressed in atmospheres (atm), inches or mm of mercury, pascal (Pa), or torr.  Stay tuned for our next blog where we’ll discuss pressure measurements in greater detail.

If you have questions about how to keep your vacuum based system in tip top shape, feel free to contact us at United Vacuum for assistance.

The way vacuum pumps actually work, if you have never considered it, is by lowering the pressure to below-atmospheric levels. When the vacuum pumps are switched on, the air pressure inside the lines and tubes drops, making a difference between the atmospheric pressure and the pressure inside the lines. This difference is call the “vacuum level”, and that’s measured by a mercury manometer. (You may recognize the word “manometer” from getting your blood pressure checked.) The unit of measurement for pressure is sometimes still referred to as mm of mercury (a literal description, if you think about it), but the international standard to measure the vacuum level of milking equipment is the Kilo PAscal (Kpa). For point of reference, 1 mm Hg = 0.1333 Kpa.

That basically covers the role of vacuum pumps in milking machines and dairy operations. Hopefully, that clears up any questions you might have.

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What is a Vacuum Vent Valve and Why Do I Need One?

A lot of smaller rotary vane vacuum pumps come equipped with an anti-suck back valve and this is built into the pump to stop migration of vacuum pump oil into the process chamber and vacuum lines.  But many larger rotary vane pumps and rotary piston pumps do not have an anti-suck back valve.  It’s just not feasible to have it built into the pump the way they are designed.  So these pumps need to be equipped with a vacuum vent valve.  It is a very simple mechanism that is very easy to install, but is critical in keeping your vacuum process from being contaminated by the vacuum pump oil.

Here’s an example.  Let’s say you have a Univac 200 series vacuum pump connected through piping to a 6’ x 4’ vacuum chamber.  Once the vacuum pump pulls the chamber down to its ultimate base pressure (say 50 microns) what we have is a 6’ x 4’ chamber and attached piping at 50 microns.  Now the vacuum pump, as long as it’s running, is OK because it can maintain the level of vacuum equal to or less than the level of vacuum in the chamber.  Therefore, the oil will stay in the vacuum pump.  But if there’s an interruption in the power supply or if the vacuum pump is shut off, the levels of vacuum in the system would change rapidly.  Here’s how they would change.  The vacuum chamber actually becomes a large reserve of vacuum – its volume is many times greater than that of the vacuum pump on the other end, and it’s also sealed so it maintains its vacuum.  Meanwhile the vacuum pump only has a set of exhaust valves separating the vacuum integrity of the pump and atmosphere, and these valves are in no way even close to being airtight.  So what will happen is that air will pass from the atmosphere, through the exhaust valves into the pump through the piping into the vacuum chamber.  When this migration takes place from the higher to lower pressure, the oil will also follow and migrate into the chamber.  (It’s like when you drink soda through a straw – it goes from a higher pressure in the glass to a lower pressure in your mouth through the straw.)  The oil has now contaminated the piping, the vacuum chamber, and all of the product in the chamber.  This is now a considerable clean up with lengthy down time for the system.  Not to mention that when you start up your vacuum pump again it won’t have oil in it, and when it sucks the oil from the chamber back in, it will bring with it all of the contaminants from the chamber and piping.

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Remanufacturing of Vacuum Pumps

At United Vacuum, when we remanufacture a rotary piston vacuum pump (e.g., UniVac 400, Stokes 412), we bore the hinge bar area and fit the pump with oversized hinge bars and hinge bar spacer.  We replace the shaft.  On the piston slide we check the ID bore and if it has more than .003” wear or loss of concentricity it is replaced with a new one.  We also check the tongue of the piston slide.  If it has any scoring wear greater than .002” it is replaced with a new one.  We will not sleeve or flamespray these areas.

At United Vacuum we check the hollow and solid eccentrics.  If the OD has wear greater than .003” or loss of concentricity it is replaced with new.  We also check the faces where the shaft and shaft shoulder ring contact; if worn or undersized, the eccentric is replaced.  We also check the keyway; if rounded or deformed, it is replaced.  We will not sleeve the OD of the eccentrics.  If the center bearing ring is worn greater than .004”, it is replaced with new.  We will not sleeve the center bearing ring.

We also inspect the bearing well and endplate.  If it is worn, it is replaced.  We will not sleeve this area.  We inspect the pump side of the endplate for scoring and galling.  If  any is found, we Blanchard grind the endplate to remove all markings and scores.  We then measure the amount of material removed and remove this amount from a new shaft shoulder ring on a lathe.  This ensures that the eccentric is properly located during the operation of the pump.

We replace all four valve bases with new ones.  We do not grind or sand the old ones.  We totally disassemble the solenoid coil valve, and if there are any signs of wear, or if the rubber is hard or cracked, the valve is assembled with new pieces and fitted with a new solenoid coil.  We also check the motor for smooth operation and correct amperage draw.  If the motor cannot be repaired with new bearings, it is replaced with a new motor.

At United Vacuum we feel taking that these steps in the remanufacturing process ensure that the pump will be as good as new and will give the customer reliable operation for a long time.

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Remanufacturing vs Rebuilding of Vacuum Pumps

What is the difference between remanufactured and rebuilt vacuum pumps?  Basically, remanufacturing returns the pump to OEM specifications, while rebuilding may not.  See the chart below to see the differences in processes performed in remanufacturing and rebuilding of rotary piston vacuum pumps and rotary lobe boosters/blowers. Continue reading

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New or Remanufactured Vacuum Pump?

The most important factors to consider when deciding whether to purchase a new or remanufactured vacuum pump are 1) the types of vacuum pump and, 2) the company that is doing the remanufacture, particularly their warranty and reputation.

You should be cautious when purchasing anything other than a new pump in certain types of pumps.  One of those is a dry claw pump.  Dry claw pumps work well until they are worn to the point of needing rebuilding or remanufacturing.  Wear on areas of these pumps can shorten the time of acceptable operation.  Many times a new dry claw will last for 2 years, but after rebuild, will only last for 9 months before requiring another rebuild or replacement.  Another of these types of vacuum pumps is higher pressure rotary vane pumps with oil cooling systems.  Unless the entire oil cooling system is replaced during remanufacturing, pump life and performance could be compromised.

You can get away with remanufactured vacuum pumps in other types of vacuum pumps.  The more durable and heavy duty a vacuum pump is, the better suited it is for remanufacturing; two examples being rotary piston pumps and rotary lobe boosters.  In our next blog we will discuss the remanufacturing process for these vacuum pumps.

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Conditions of Industrial Vacuum Pumps

When looking for a vacuum pump for your application, there are several choices available with respect to the condition of the pump: new, remanufactured, rebuilt or used.  Following is a description of each.

New: All components are brand new and all of the dimensions and specifications are that of a new pump.  It is important that new pumps are assembled and thoroughly tested by a reputable company.  Warranty and reputation for standing by the warranty are critical because even new industrial vacuum pumps have problems.  Be cautious of pumps that are manufactured and tested offshore and never run before entering your facility.

Remanufactured: A remanufactured vacuum pump is one that was used or had a problem during new pump testing that required major rework.  The pump is completely disassembled and inspected.  All parts that don’t meet OEM wear specifications are replaced with new parts.  A quality remanufacturing job should not bastardize the pump by overboring, sleeving or overspraying because these things shorten the lifespan of the pump between rebuilds and could lead to catastrophic failures.  All consumable parts should be replaced with new.  Electrical components should be replaced, e.g., solenoid coils.  New motor bearings should be installed and tested to amperage.  If motors are questionable or outdated they should be replaced with new.  Belt guards, sheaves and pulleys should be new or in excellent condition.  Pump should be in “as new” condition when completed.  Warranties should be very similar to those of new industrial vacuum pumps.

Rebuilt:  Rebuild is similar to remanufacture except that some questionable parts are reused or machined and fitted with sleeves.  Flame spraying is acceptable here.  The vacuum pump is cleaned, consumable parts are replaced.  The pump is tested.  Warranties typically cover a shorter period of time than new, and the pump no longer meets OEM specifications.  Some critical wear areas may be scored and compromised.

Used:  A used vacuum pump is just what it says; it was used by someone else prior to your receiving it.  The condition could range from almost new to ready for the garbage.  Only buy a used pump if you plan on having it rebuilt or remanufactured because if the pump was good the company would still be using or keeping it.  If the company selling the used pump went out of business, chances are that money was tight and the pump was not properly maintained.

In our next blog we’ll discuss things to consider when choosing new versus remanufactured industrial vacuum pumps.

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PROPER OPERATION OF VACUUM PUMPS IN SPECIALTY GAS APPLICATIONS

It is important to properly install and start up your vacuum pump, whether it’s new or re-manufactured.

First, wire the pump and be sure to check rotation prior to connecting to the manifold/system. Only bump the pump; do not let it run as this could damage the pump.  Next, fill the pump with the appropriate vacuum pump fluid. Then put a vacuum gauge directly on the inlet of the vacuum pump, and run the pump to check the level of vacuum; this will check the integrity of the pump. Finally, connect the pump to the system using either a suitable sealant on threaded connections (NEVER use Teflon tape on threaded connections), or a high vacuum grease on KF fittings.

Sizing of the vacuum inlet lines has a big effect on vacuum pump performance.  Keep all connections to the inlet of the pump as large as possible. This will allow the pump to achieve its maximum pumping speed.

The exhaust side of the vacuum pump is just as important. Try to make the piping on the exhaust side the same size as the exhaust port on the vacuum pump. If the pump exhaust line is too small, back pressure will be created in the pump. This will cause the vacuum pump to overheat and will also cause premature seal failure. Also, make sure that there are no shut-off valves in the exhaust line because if a valve is closed while running the pump it will cause severe damage to the pump.

It is highly recommended that routine maintenance schedules be followed for your vacuum pump. It is critical that the oil be changed on a regular basis. The best gauge to determine if the oil needs to be changed is by examining its clarity; if the oil is discolored or milky, change it as soon as possible. On pumps equipped with exhaust filters, examine the filters quarterly for discoloration and saturation.  When filters are saturated or discolored, change them as soon as possible. Also, consult your owner’s manual for suggested routine maintenance.

By properly installing and maintaining your vacuum pump, it will remain in service for many years.

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SELECTING A VACUUM PUMP for SPECIALTY GAS APPLICATIONS

When choosing a vacuum pump for Specialty Gas applications, it is important to consider many factors, including pump type, size and vacuum level required.

There are 3 major types of pumps that may be used:

  • Single stage oil sealed rotary vane vacuum pumps
  • Dual stage oil sealed rotary vane vacuum pumps
  • Single stage oil sealed rotary piston vacuum pumps

Choosing the right type of vacuum pump for your application depends partly on the vacuum level required.  A single stage rotary vane vacuum pump is well suited when working in the range of 27” to 29.9” Hg.  Airtech H Series pumps and Leybold SV Series pumps work well.  Also, though they are capable of much deeper vacuum, Kinney KC and KD series rotary piston pumps are suitable.

When working in the range of 100 microns or less, a two stage rotary vane vacuum pump or single stage rotary piston vacuum pump works best.  United Vacuum’s UniVac 100 series, Edwards E2M and RV series, Leybold D series, Kinney KC and KD series, and all Alcatel vacuum pumps are acceptable.

The size of the vacuum pump should also correspond to the size of the manifold.  The larger the manifold, the larger the pump needed.  To determine the pumping speed for your pump, the following information is needed:

1.     the annular volume of the manifold and bottles on the manifold.  Note: never base your volume on pressurized cylinders!

2.     the ultimate vacuum that must be achieved.

3.     the desired pump down time from atmosphere to desired vacuum.

Once all three specifications are gathered, call United Vacuum to determine the correct pumping speed.  A couple of things to note, the larger you can keep the manifold and all of the connecting pipes, tubing and hoses up to your restricting orifice (which is usually the cylinder valve), the faster your pumping speeds (pump down time) will be.  Also, always be sure to factor in the size of your smallest orifice as this will be your restricting factor.

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METALLURGY of the HINGE BAR BORE in a VACUUM PUMP

The hinge bar bore area of a rotary piston vacuum pump is made up of 3 parts: the block, the piston slide and the hinge bars.  Each part is detailed below.

The block of a rotary piston vacuum pump is made of cast iron which is a porous material.  This is beneficial because it can hold the oil in the pores of the metal.   Cast iron also very good at holding its size and is very resistant to contorting/warping under heat and stress.  This is the stationary portion of the hinge bar area.

The piston slide, which is also made of cast iron, is sandwiched by the hinge bars inside the hinge bar bore area.  It translates back and forth in the hinge bar area.

Therefore, the hinge bars are subjected to translating forces on one side from the piston slide, and rotational forces from the block on the other side.  Wear is coming from the both sides, so the hinge bars have to be made out of a material that is tough and will resist wear and galling.  It is also important to hold tight tolerances on these parts.

While it would be simple to make the hinge bar out of a very hard material that would not wear, the consequence would be that the block and piston slide would wear very rapidly.  So what we have to do is select the appropriate material and heat treating processes to ensure that the hinge bars:

1.     will not wear

2.     will not stress or contort under heat and stress

3.     will not wear out the block and piston slide

4.     will not be affected by corrosive processes or particulate

The hinge bar and the hinge bar bore are probably the most critical elements of a rotary piston vacuum pump.  As mentioned in our earlier blog, this is where all of the sealing takes place, and therefore, this is where the vacuum is made.  Proper metallurgy of the hinge bars is crucial.  We have seen vacuum pumps come in with hinge bars made from everything from plastic, to brass, to cast iron!  The results of running pumps with these bars have been catastrophic.  Check what your hinge bars are made of, and be sure to use hinge bars that are designed to work in your vacuum pump!

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How To Bore a Block in a Vacuum Pump

If you have a rotary piston vacuum pump, whether it is a UniVac, Stokes, Edwards or Kinney, it is important to bore your block.  Why?  Because the hinge bar bore of a rotary piston vacuum pump is critical to both base pressure and pumping speed.  The hinge bars and the piston slide, along with the block of the vacuum pump all meet in this area.  Wear takes place when the hinge bars rotate and translate.  For every hour the pump operates, this rotation and translation takes place 72,000 times.  If the byproduct of your process contains and corrosives or particulate, this wear will be accelerated exponentially.

When a pump is new, or it has been freshly bored with new hinge bars, the clearances between the pieces in the hinge bar bore are between 0.0015” and 0.002”.  Tight clearances are used because the vacuum pump oil used in the pump has to seal this area.  If the clearances become too large, the oil can no longer seal, and the gases will jump from the compression side to the vacuum side.  This slows your pumping speed and eventually prevents you from reaching your specified ultimate base pressure.  Other consequences of a worn hinge bar bore are:

  • the pump will exhaust more air
  • oil will be lost through exhaust
  • exhaust filters will become clogged
  • the pump will run hotter
  • the interval between oil changes will be shorter because the oil will break down quicker
  • backstreaming of oil into the vacuum system will be increased

When a worn pump is bored, the hinge bar bore area will be bored until it is concentric and free of grooves or wear.  When done properly, it will be bored in increments of 1/32” – 1/16”.  The tolerance on the hinge bar bore must be held to +0.002”, -0.000”.  The freshly bored block should get new hinge bars and a hinge bar spacer, if necessary.  These new hinge bars must also have tight tolerances, and their finish is critical – a ground finish is required.  Perhaps the most important part of the hinge bars is their metallurgy and the process under which they are produced.  Come back for our next blog to learn more about hinge bars.

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