The piston pump is again enjoying increasing popularity with liquid manure pumps. In particular, with high working pressure and high annual output, it can promote manure more favorably and more reliably than the widely used displacement pumps.
The good old piston pump experiences its second spring. When, in the 1990s, large technical advances were made in the rotary piston and screw pumps, the piston pump was somewhat obliterated. The high purchase price as well as problems with the intake and with clogging of valves were at that time with a reason, the piston pumps more and more by displacement pumps to replace. The piston pump could only be held in the hill and mountain area; screw and rotary piston pumps can not achieve the same delivery rates as piston pumps.
Larger farms – more pressure
But now, too, piston pumps are being bought again in the lowlands. This is mainly due to two reasons: on growing agricultural farms, the distances are also increasing, which must be pumped through the manure. If the existing pipelines and hoses are to be used further and thus even higher hours are required, pressures in the lowland of 15 to 20 bar and more are quickly reached. Exactly in this pressure range the screw pump reaches its limits, the rotary piston pump much earlier. In addition, biogas plants are increasingly being laid with longer floor conduits which require a high discharge pressure.
More inter-company application
The second reason is the increasing use of Gülletechnik. For example, contractors or machinery companies use their slurry pumps during an increasing number of operating hours per year: around biogas plants, it can quickly exceed 1000. This is combined with a high working pressure. It can come so far that the stator is a screw pump has to be replaced once or twice a year. The costs for the replacement of wearing parts are correspondingly high. Together with the higher energy consumption, this means that the price advantage of a positive displacement pump is quickly exhausted when it is purchased. On the other hand, users of modern piston pumps report that they would be able to operate their machines over a period of more than 1000 hours per year without great wear and tear.
The double acting pistons run in a steel cylinder whose inner wall is protected against wear by a specially hard layer.
Disadvantages have been eradicated
Last but not least, the already mentioned disadvantages of the 90’s were able to be eliminated by a further development of the piston pumps: Modern, lying piston pumps suck up by themselves even up to 7 m depth without problems thanks to new valve technology. Also with new valve technology and if necessary with an upstream cutting chopper the blockability with long straw was greatly improved. Maintenance has also made great progress since then.
The crank drives and rods run today in all models in the closed oil bath and are therefore practically maintenance-free except for an oil change every two to three years. Furthermore, the remaining manufacturers have worked on an improvement in the wear resistance. The cylinders are now equipped with specially hardened walls.
These also have better properties in terms of wear resistance and lubricity. Which is expected to have a positive impact on performance.
Positive displacement pumps and forced transport
But why are piston pumps more suitable for high working pressures? Piston pumps differ fundamentally from screw and rotary piston pumps, which are grouped under the designation of positive displacement pumps, from the functional principle. Both the worm and the rotary piston pumps have the same volume during transport. The pumping chambers “migrate” from these pumps from the suction side to the discharge side and are caused by the contact of rotors
(Worm or rotary piston) and stators (housing).
These chambers are only very dense as long as the rotor and stator fit together exactly in such a way that the pressure of the rotor on the stator is so great that the gaps between them are sealed off. However, it must not be too large as otherwise the energy expenditure increases and the wear increases. In the case of displacement pumps, the contact surfaces between the rotor and the stator, which frictionally rub against one another, are the only barrier between the suction side and the pressure side. As soon as, with these pumps, the sealing rubber parts have somewhat worn or deformed due to the high liquid pressure, the contact pressure drops: so that liquid flows back through the resulting gap from the pressure side. Thus, with increasing pressure, the delivery rate can drop relatively quickly to less than half, and in extreme cases even to zero.
The escape of liquid manure does not only result in a small flow rate: the escaping manure is strongly accelerated at the contact points (high-pressure cleaner effect), which in turn increases the wear on the pump components disproportionately. Finally, components must be replaced. To break the devil’s circle.
The screw pump (up to 18 bar) can work with a higher pressure than the rotary piston pump (up to 8 bar) for a long time, because, inter alia, the distance between the suction and discharge valves is not affected pressure side is larger and two or three stages are connected in series. So it is more difficult for the liquid to escape.
In the case of the piston pump, on the other hand, the delivery rate remains almost constant up to the maximum possible operating pressure. This is a real forced conveying, in which the suction and discharge sides are separated by valves, similar to a combustion engine. The conveying chambers do not move further, as with the displacement pumps, between two moving parts of a constant size, but are increased and reduced by the movement of the piston in the cylinder. The pressure has no effect on the sealing action of the piston collars. On the contrary, the higher the pressure, the faster the plastic collars on the piston ensure a seal on the cylinder wall. With this operating principle, a pressure of more than 500 bar can theoretically be generated as in the case of a high-pressure cleaner, but this would of course not make sense with a slurry pump. In the pressure phase, it is almost impossible for liquid to escape when the valves and pistons are tight. The piston pump therefore does not reach its limit on the application pressure because of the escape of liquid, but because of the limited stability of various components, such as crankcases, valves or hoses.
Large volumes amortize the additional investment
Piston pumps are almost twice as expensive to purchase as a screw pump with the same (maximum) flow rate. The larger the amount of manure required per year and the higher the working pressure, the faster the amortization can be made into a piston pump and from then on even money can be saved. Stöckli Pro AG produces two models with horizontal, double-acting pistons with 45 and 60 m3 / h and a pressure of up to 20 bar, as well as smaller models with single, single-acting pistons.
- Posted by Administrator
- On 23. June 2017
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