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 (High Shear) Pumps
I did consider this type of pump.
They are popular in North America and utilised on many spray rigs over there. The problem is that broadcast spraying & liquid furrow banding are poles apart in terms of ground speed, product to be applied & swath widths.
Also, of important consideration are the differences in mixes. Quite often, spray mixes have an SG (Specific Gravity) of slightly above 1 (Water) – they are heavily diluted. By contrast, liquid fertilisers that are commonly banded (eg. UAN), are often applied at close to neat (undiluted) & some liquids can have an SG approaching 1.5. They are generally far more viscous brews than those used in spraying.
Spraying is generally performed at much higher ground speeds than those typically used in furrow banding at seeding & their swaths (boom widths) are generally far greater than the widths of tillage bars. What this means is that the flow requirements are quite different.
A large SP Spray Rig can empty a 3,000litre tank in as little as 7 minutes (flow of 428 lpm). By comparison a 50 litre/hectare rate furrow banded with tillage equipment (eg. 18 metre bar at 9.0 kph) is a mere 13.5 lpm flow requirement! Factor in also, that to cater for varying ground speeds & the range of applied rates, the pump will have to supply product at pressures ranging from say 0.5 - 7.0 bar.
The problem with high shear pumps is that to achieve these higher pressures they must run extremely fast. This is not a real problem with spray rigs but it can be with furrow banding as you must be able control miniscule flows. Yes, you can do it by severe throttling however this can result in cavitation (the formation of bubbles or a vacuum within a liquid). Heat is the result of cavitation & it can result in burnt out pump seals & other nasties such as foaming, particularly if surfactants are involved. Under extreme circumstances it can even change the properties of a liquid. Microbial type liquids could therefore be at risk.
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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