Vacuum Extraction from Grass Forage

Objectives

1)  Rapid removal of all Oxygen from grass forage

2)  Carbon Dioxide atmosphere for fermentation

3)  Compression of forage

4)  Storage of silage in a pickled state in a Carbon Dioxide atmosphere.

Back to home page Yalland Vacuum Compressed Silage 

Principles

Grass forage needs to be sealed and air-tight.  Some sort of perforated pipe needs to be sealed into the stack and coupled up to a vacuum pump.  There also needs to be a stop valve to shut off when the gases have been extracted.  The valve needs to be between the  pump and the stack.

A vacuum pump needs to be able to extract all gasses from a grass stack within 4 hours of sealing, ideally 2 hours.  The vacuum achieved should be about 9 Inches of Mercury (300 mbar) to create the clamp of silage.

Sealing of Grass Forage

Bales of grass can be bagged and sealed at the neck or put in pods and sealed in a sheet.

Stacks require large silage sheets below and above to envelope the forage.  Ideally Sheet Seam should be used to seal all edges and joins.  Tape is required to seal stack pipes into stacks through sheets.

Other methods of sealing such as folding and weighting, burying edges, adhesive sealants, heat fusing, and extensive taping are used to lesser effect.

Sealing needs to be quick, easy, reliable and long-lasting (months).  It also needs to hold against a vacuum or pressure of 300 mbar.

Vacuum Pumps

The required size of pump depends on the volume of forage.  The fastest extraction on farms is using a slurry tanker and that would be needed for yard clamps and full size One Day Silage Bays.  At the other end of the scale one bagged bale of silage can be extracted using an industrial vacuum cleaner.  One field clamps and pods only require low power pumps.

A typical 10,000 litre slurry tanker extracts at up to 8000 litre per minute (240 cubic metres per hour) and can draw down to 0.5 bar (500 mbar) vacuum (see table) or further.  A tanker of this size would pump down a One Day Silage Bay in 2 to 4 hours.

Yalland Vacuum Compressed Silage used a vacuum extractor that was an Alpha-Laval milking machine pump driven from a tractor PTO shaft on one field stacks of 25 to 50 tons of uncompressed grass forage. That resulted in a clamp that was about a quarter of its starting height within 4 hours.

This moderate size of clamp can also be serviced by an electric pump (see below).   Silage Bale Pods can also be extracted using electric pumps.

Required Vacuum

The vacuum achieved should be about 9 Inches of Mercury (300 mbar) to create the clamp of silage.  More vacuum is unnecessary.  Less vacuum may still do most of what is required in terms of Oxygen extraction, but will achieve less compression.

A fully evacuated clamp should feel firm to the touch and it should be possible to walk in socks on the clamp without making an impression.  This is the guide if a vacuum gauge is not used.

Slurry tankers can extract to greater vacuums than required, and may challenge the sheet strength and seals, so a vacuum gauge and visual monitoring of the clamp is required to shut off at the right time.

Electric pumps are usually supplied with relief valves that can be set to blow at 300 mbar.

Power Required

Tractors can be set up wherever the silage clamp is sited and coupled up by P.T.O. shaft to  a pump.  The pump can be on a slurry tanker or a mounted pump suitable for milking machines. 

Slurry tankers may require large tractors to move them, but the operation of the pump only requires less than a 45 kW tractor. 

Electric pumps can be found that only draw 1.5 kW and can be run off long extension leads, or generators, and produce vacuums of up to 300 mbar.  These may be suitable for one field clamps (up to 70 tonnes) and for use in covered storage areas and on bales and pods.

Stack Pipes

The Yalland VCS system used a 4m long rigid alkathene pipe of 50 mm (2 inch) nominal bore size.  That was perforated by 8 mm (¼ inch) holes in four directions every 200 mm (8 inches).  It had a closed end.  The end that projected from the top sheet was sealed to the top sheet and connected to the pump by another rigid pipe with no perforations.

Larger and longer pipes can be used on clamps of more than 100 tonnes, or multiple pipes can be used of the same size, to handle the larger gas volumes and greater size of clamps. 

Smaller pipes can be used for electric pumps and used with pods and bales.

Stack Pipes should reach to within 4 m of the edges of clamps.  So an 8 m wide clamp should only need one Stack Pipe and it would need to be long enough to reach within 4 m of each end of the clamp.  Wider clamps would need additional Stack Pipes.

Any stop valves or connections are the same as would be used for water pipes.  Anything tested to more than 2 bar pressure can be used.

New Developments

The use of milking machines and slurry tankers to draw vacuums has been well tested and described and there are photo images to support successful use of them.  A new development being tested this year (2022) is the use of an electric pump.

The BL42 Side Channel Blower can draw a vacuum of 280 mbar within 4 hours on clamps of up to 110 tonnes of silage from uncompacted or mechanically compacted clamps. It can evacuate up to 145 cubic metres per hour (2400 litres per minute). An image of the unit is below and updates will appear on this page. Preliminary trials have found that it can work for sustained lengths of time at 280 mbar and that almost matches the vacuum requirement for Yalland Vacuum Compressed Silage. This may be useful for farmers in remote areas.