Frequently Asked Questions
Why do you use positive displacement, piston diaphragm pumps in your auto-rate systems?Obviously there are a number of different pumps to choose from on the market so we had to carefully analyse the pros and cons of each type before making a final choice. In reality though, the choice was relatively simple because many of the pumps are just not designed to handle liquid fertilisers let alone compliment a rate controller. Below, are some of the types reviewed.
Squeeze Pumps / Peristaltic Pumps
These pumps are generally associated with the movement of slurries, or laboratory applications, however in the agricultural industry peristaltic pumps are used to dose main lines with trace elements etc... They did not suit our application due to the aversion rate controllers have to pulsing - an inherent problem with this type of pump if being utilised for commercial operation.
Worm and Stator Pumps
The materials used in the construction of these pumps will handle corrosive and aggressive materials, however the stators are prone to meltdown if deprived of liquid.
Centrifugal Pumps
Certainly of consideration provided constructed of the correct materials, however we opted against this type of pump for several reasons. Firstly, damage to the seals if deprived of liquid and secondly a very small window of speed variation if used with an auto-rate / rate controller. This is because most centrifugal pumps purpose built to handle these liquids struggle to achieve anything above 3.5 bar pressure which means that an operator may only have a window of a few kph with full rate control. There is also the problem of high output/high pump speed when in reality only a fraction of this is required. - This can lead to cavitation within the pump and 'cooked' seals.
Variable Stroke Piston Pumps
There are variable stroke piston pumps, purpose built to handle liquid fertilisers however they are best suited to ground-drive systems.
Positive Displacement Piston Pumps
With this type of pump the liquid is in contact with the cylinder and piston. Certainly ceramic piston pumps can handle the aggressive liquids however they do not take kindly to running dry.
Positive Displacement Piston Diaphragm Pumps
This was the style of pump we opted for! The particular pumps we use are Bertolini 'Millennium' Piston Diaphragm Poly Pumps. The range we use is from 75lpm. To 210lpm., but larger (300lpm.) models are available. Even the smallest of these pumps is of a 3 piston configuration and extremely smooth in operation. This smooth operation means that 'pulsing', (the bane of flow meters), is almost indiscernible.
With piston diaphragm pumps the liquid never comes into contact with the actual piston, only the diaphragms, valves and porting galleries and with the 'Millennium' pumps all of these wetted surfaces are of 316 stainless steel and polypropylene. Both these materials are highly resistant to attack from the most aggressive of liquid fertilisers.
An added bonus of these pumps is that their moving parts sit in an oil bath and although not recommended for extended periods, can be run dry without damage occurring.
Max. pressure for these particular pumps is 15 bar. so there is not a problem with a narrow speed variation window.
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