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Pumping Fluids Containing Solids

PROCESS Pump Meeting Gives Practical Guidelines

06.04.2007 | Editor: Dr. Jörg Kempf

The more difficult the medium, the harder it is to select and design the right pump. 70 delegates at the PROCESS meeting discuss possible solutions on the basis of specific problems.
The more difficult the medium, the harder it is to select and design the right pump. 70 delegates at the PROCESS meeting discuss possible solutions on the basis of specific problems.

As ever, the PROCESS pump meeting did not focus on the advantages a certain pump series has to offer from a manufacturer‘s point of view. For two days, (March 22 and 23), 70 delegates discussed the issue of which pumps can best pump fluids containing solids.

Gone are the times when as a speaker and delegate at a seminar you could rely on the intimacy of the small group. Everything is different now, at Vogel Industrial Media at least: The pump meetings initiated by PROCESS are meticulously recorded on digital camera and are available to the whole world within a very short period of time as webcasts that can be downloaded from the journal’s website. And a good thing too, as speakers put a lot of work into their presentations. This expertise is therefore available to a considerably larger group of experts courtesy of the webcasts.

The only snag with this is that if a speaker is not on good form that day, this is recorded for ever… Friedrich-Wilhelm Hennecke does not have to worry about any possible goofs: he was as commanding as ever as he led the meeting as presenter.

He laid down the line of approach right at the outset: “We are not looking at what can be pumped with a certain pump. We are starting from the operator, the task and the medium and looking for suitable solutions for these.” This is of course not an easy matter when it comes to the task of “pumping fluids containing solids”. Both centrifugal pumps and displacement pumps are possible contenders. The table shows the limits to what individual pump types can achieve and which are better or worse for pumping fluids containing solids. In each case, the following questions must be answered: What medium is to be pumped (hazardous substance, viscosity, gas or vapor volume, solids content and grain size, tendency to crystallize out, polymerize, agglomerate)? What operating states need to be taken into account (continuous or batch operation, cleaning and rinsing processes)? What parameters have been predefined (throughput, pressure, temperature)?

Professor Helmut Jaberg from the Technical University of Graz/Austria gave in his eloquent way an overview of the possibilities, and limits, of centrifugal pumps for pumping media containing solids. According to him, centrifugal pumps are extremely well suited to transporting solids if the medium is in the form of a suspension. This could even be sand and excavated earth from mining or dredging, or ash in power stations: “So long as the suspension arrives at the suction fitting of the pump, it will be pumped,” said Jaberg.

Of course, the manufacturer has to take precautions against abrasive wear in the flow-carrying components, for example by using ductile metals with a high hardness, using cast mineral composite or by applying rubberized surfaces to metallic substrates. “If you’re clever, you can find a way around it,” proclaimed Jaberg, explaining the advantage of elastic materials where there is contact with solids.

What can the operator do? He can, for example, make sure that the solids suspension consists of a mixture of smaller and larger grains; the fine grains will then aid the pumping of the larger parts.

Where possible, the pump should not be operated under partial load, as this according to Jaberg will cause additional abrasion due to secondary flows. In particular, it is important to avoid separation. It is therefore a question of maintaining a sufficiently high flow rate in the pipelines.

Unchokable pumps: the impeller is often crucial

The plethora of company contracts was started by Richard Layes von Emile Egger, a manufacturer whose production range includes unchokable pumps. “We prefer the open impeller in 99 percent of cases,” confirmed Layes. Compared to a pump with a closed non-clogging impeller, the efficiency is reduced from 75 to around 60 percent, but this is offset by the other advantages offered by the unchokable impeller, such as the low shearing rate of the product and the extremely good resistance to wear. This is due to the fact that only around 15 percent of the medium comes into direct contact with the impeller, the rest is moved exclusively by means of hydraulic coupling.

The pumps presented by Dieter Lau from Hermetic-Pumps do not provide an obvious solution with respect to solids in the pumping medium: magnetic drive pumps and canned motor pumps are not what would be described as the first choice for such tasks. He was quite clear that “the primary concern with these pumps of course is to avoid solids.”

However, what do you do if the medium can only be pumped with a leakage-free pump but contains solids? If solids are unavoidable, explained Lau, the grain size should not exceed 0.5 mm and the solids volume should be limited to 2-3 percent. “The quantity and nature of the solids are the critical factors.” If the pump is damaged, the bearings are involved in 70 percent of cases. As a design measure, the operator can work with initial filters on the suction side. Hermetic also offers a slurry configuration in which a pure fluid is added in metered doses on the drive side.

Displacement pumps: not all of them are tolerant of solids

Rainer Stettin from Knoll presented a type of pump ideally suited to pumping media containing solids in the shape of the eccentric screw pump. A particular feature of MX eccentric screw pumps is the design in EvenWall technology: the stator jacket is adapted to the inner contour of the elastomer, increasing the pressure stability considerably. Vacuum-resistant screw pumps from the KTSV series can be used both for free-flowing and highly-viscous media.

Rotary pumps are also extremely good for pumping media containing solids, as Hans Georg Steinmann from Börger showed. For abrasive media, there are rotors that have been rubberized all over in adjustable designs. As the rotor wears, the tips of the piston can be adjusted by means of an integrated mechanism so that a sealing line is restored to the housing surrounding the rotor or the rotor working in the opposite direction. A special feature is the rotary piston rotors with individually replaceable rubberized blade tips and movable metallic blade edges.

Gear pumps cover a broad viscosity range, are insensitive to high differential pressures, meter extremely precisely and are characterized by low pulsation rates during pumping: it is therefore not surprising that these pumps are often used for metering and mixing tasks. However, they are extremely sensitive to solids, as Holger Kremer from Witte explained. What does Kremer suggest? First of all, the hardness, shape and nature of the foreign bodies must be known. The concentration of solids should be less than five percent, the particles smaller than the smallest gap in the gear pump. It is always advisable to reduce the pump speed and mate hard materials for the plain bearings and shafts. In the case of suspensions, it must be ensured that there cannot be any segregation.

Compressed-air diaphragm pumps: robust universal pump

The pumps presented by Detlev Ammon from Crane on the other hand definitely come under the category of “universal pumps”. Compressed-air diaphragm pumps where specially designed for pumping abrasive, viscous and contaminated media. The design is robust and configured for heavy duty. Simply connect the pump to the compressed-air ductwork system, and the pump will pump using a dry, regenerative mechanism. Thanks to the option of selecting from a wide range of housing materials, membranes and elastomers, the pump can be used universally in nearly all industrial applications, confirmed Ammon.

He explained, however, that due to the limited hydraulic efficiency, these pumps are only built up to certain sizes as otherwise the drive would be too large in relation to the pump body. The working principle of hose pumps is similarly simple: A rotating rotor continually compresses an elastomer hose and displaces the medium towards the outlet on the pressure side. In this way, hose pumps pump media of practically any viscosity — sewage water and slurry, or equally acids and alkaline solutions. There are of course also limits and constraints on the use of hose pumps, for example if the solvent content of the medium is too great.

Piston diaphragm pumps offer high delivery pressures

Gunnar Stork from Verder introduced piston diaphragm pumps of the HydraCell type with a swashplate drive. These pumps represent a symbiosis between the classic piston pumps for high pressures and the diaphragm pumps for viscous and abrasive media. According to Stork, these pumps come into their own in applications where difficult products have to be pumped and metered under high pressure. Thanks to the diaphragm, which separates the hydraulic part from the pump chamber, solids can easily be pumped without causing wear to the piston or cylinder.

One of the pumps presented by Matthias Sauter from Lewa was the ecoflow piston diaphragm pump. In conjunction with the DPS (diaphragm protection system), the M900 diaphragm pump head with a PTFE diaphragm ensures high operational reliability. The DPS controls the movement of the diaphragm and ensures that the membrane always moves within the optimum working range. Regardless of external factors (difficult operating conditions, blatant operating errors), it is ensured that the movement of the diaphragm does not exceed prescribed limits.

Sauter advises never to install the suction pipes at the lowest point of the container and to ensure that the average flow velocity is sufficiently high. He believes that hermetically sealed piston diaphragm pumps are predestined for conveying media containing solids, although fluids with solids could affect the valve closing delay and valve pressure loss. Choosing the appropriate valve configuration requires considerable experience, he explained.

Feluwa leader Heinz M. Nägel believes there is a significant worldwide trend towards also transporting suspensions containing coarse impurities hydraulically. This is not a problem for hose-diaphragm piston pumps, which pump media of differing viscosities and consistencies (chemically aggressive or mechanically abrasive, liquid or highly viscous). Depending on the medium, the dry substance content can be 80 percent, as it is in coal gasification applications, for example.

The “Multisafe” double hose-diaphragm piston pumps are new: in this pump design, the flat diaphragm principle previously applied was replaced with two flexible hose diaphragms which perform the displacement and at the same time provide hermetic separation between the area which comes into contact with the media and the hydraulic actuation system. The pumping medium only comes into contact with the inside of the hose diaphragms and the delivery valves. The pumping medium cannot enter the hydraulic chamber.

Hans-Jürgen Haubold from Putzmeister brought the meeting to a close by presenting hydraulic piston pumps for pumping thick matter with dry substance contents of up to 60 percent and more. What thick matter can be pumped? The mixture of liquid and solid contents of a thick matter must produce a plastically ductile mass, according to Haubold. Bleeding, i.e. excretion of liquid constituents, must be prevented, he explained. The following are examples of substances that cannot be pumped: sand, pebbles or thick matter with a high gas content.

Conclusion: The more complex the medium, the harder it is to select and design the right pump. The meeting “Pumping fluids containing solids” provided the delegates with practical guidelines on this topic. “If I want to choose a pump, I need an overview of the alternatives available. This seminar gave me this overview.” This was how one delegate accurately described the special role of PROCESS pump meetings. The series of events will continue in the autumn.

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