Pumps for liquids containing solids.
Introduction.
The process of pumping mixtures of solids and liquids is common in industry, especially in industries such as:
- hard coal mining,
- metal ore mining,
- power engineering,
- metallurgy,
- cement and lime industry,
- construction aggregates industry,
- sugar industry.
Pumping mixtures of solids and liquids has its own specificity and differs in many respects from the process of pumping homogeneous liquids, e.g. clean water. A significant problem is the diversity of mixtures in terms of their characteristics, including the way they affect the pumping installation. Depending on the type of solid, its granulation (granulometric composition), concentration, shape and grain size, the pumped mixture may have different erosion properties, density and viscosity.
The set of these features is important for the selection of pump installation elements and construction materials. The pump, as the most important device in the process of pumping mixtures of solids and liquids, is subject to particularly high operational requirements. The basic criterion for pumps is their highest possible durability and the longest possible period of failure-free operation (so-called operational reliability). The second most important criterion when selecting a pump is energy efficiency. The purchase of relatively quickly wearing out elements of the flow system and relatively frequent repairs and renovations are the basic costs when operating this type of pumps. For this reason, the criterion of durability and failure-free operation has the greatest impact on the design, pump parameters and materials used.
Design features of slurry pumps.
To achieve high durability in extremely difficult operating conditions, it is not enough to use more resistant materials while maintaining the design of the pump intended for clean liquids.
Pumps for mixtures of solids and liquids, referred to as slurry pumps, are characterized by certain specific design features. Most often, these are stationary, centrifugal, single-stage pumps.
In the past, tests were carried out using multi-stage pumps to achieve higher lifting heads. However, when pumping sludge, multistage pumps have significant operational shortcomings. The durability of interstage seals and guide vanes, exposed to the impact of solids flowing from the rotor at high speed, turns out to be unsatisfactory. There are also problems with blocking the flow system due to the smaller cross-sections found in multistage pumps. Moreover, disassembly and renovation of multistage pumps are more complicated and expensive. For these reasons, if there is a need to obtain significant lifting heads, when pumping sludge, it is more advantageous in practice to connect single-stage pumps in series.
Most slurry pumps are designed to operate with high inflow and high internal pressure. Slurry pumps usually have low speed characteristics due to the low rotational speeds used. These pumps use rotors of a special design with large cross-sections of inter-blade channels, enabling pumping of mixtures containing larger grains.
Figure 2. PH pump.
Figure 1. HC type pump.
The discs and rotor blades have significant thicknesses that determine durability. In order to enable pumping of mixtures containing large grains, single-sided open impellers or free-flow impellers are also used.
Blade rotors usually have a small number of blades, most often from l-5. The rotors are sealed with a front gap, regulated by the movement of the entire bearing assembly together with the rotor. The remaining elements of the slurry pump flow system, i.e. the casing, stub pipe and protective walls, have increased thicknesses, taking into account wear allowances and the increased pressure in the pump casing due to the possibility of working with inflow. Specially designed submersible pumps are often used to pump sludge. This is justified in cases where, for location reasons, it is impossible to create conditions for the stationary pump to work with inflow, which is necessary when pumping mixtures with a density above 1100 kg/m3. The flow system of these pumps has a similar structure to that of stationary pumps and is made of similar materials.
Construction materials used in slurry pumps.
When pumping clean liquids, even chemically aggressive ones, a passivation layer is formed on the surfaces of the flow system, preventing further corrosion. When pumping sludge, the protective layer is constantly removed due to erosion and the construction materials are constantly exposed to the corrosive effects of the flowing medium. The combined impact of corrosion and erosion places high demands on the materials used to make flow systems of slurry pumps. Special materials are used, most often high-alloy cast steel or cast iron, subjected to appropriate heat treatment, which ensures hardness in the range of 50-65 HRC and the required material structure.
Some non-metallic materials, e.g. rubber or polyurethane, combine high abrasion resistance with good anti-corrosion properties. Their weakness, however, is low mechanical strength and susceptibility to damage by larger bodies and sharp shapes. This limits their scope of use to slurries with small granulation and pumps with a moderate lifting height. In most slurry pumps, special materials are used for the impellers and replaceable linings located in an additional, external casing. This simplifies repairs and also frees materials exposed to erosion from stresses caused by internal pressure that are transmitted through the hull.
Conditions related to the selection of slurry pumps.
When selecting slurry pumps, different principles should be applied than when selecting pumps for clean liquids.
The service life of slurry pumps decreases quickly with increasing rotational speed, so you should not strive to use a direct drive with the synchronous speed of an electric motor, which is a common practice when pumping clean liquids. For slurry pumps, precise selection of rotational speed is required.
When there is variability of parameters over time, it is preferable to use variable speed drives, while for constant operating parameters, optimal adjustment of the pump to the system can be achieved by using an appropriate belt transmission and making a rotor with the appropriate diameter.
The natural tendency of users is to use single pumps even for high lifting heights, which requires high rotational speeds. Meanwhile, when pumping sludge, it is often more economical to connect pumps in series because, despite increased investment costs, savings are achieved thanks to longer periods between renovations.
The characteristics of slurry pumps provided by manufacturers are usually prepared for clean water. The parameters of the sludge pump are usually reduced compared to operation on clean water. There are no universal methods for converting characteristics from clean water to sludge. Individual slurry pump manufacturers use their own methods for this purpose, which may provide uncertain results for unusual mixtures.
An additional problem is the difficulty in determining the required lifting height, because users and designers are not always able to predict what the flow resistance in the installation will be, which results from the fact that the rheological properties of some sludges are difficult to predict. Please remember that when the flow rate drops below a certain value, solid particles settle and block the pipeline.
Taking into account the limited accuracy of the analyzes performed, it is advisable to equip the user with some options for regulating the parameters of the installed pumps. Often, it is sufficient to install a belt transmission in the pump unit, which allows, by changing the gear ratio (e.g. replacing the pulley), to adjust the pump's rotational speed to obtain the required parameters. The above-mentioned complications when selecting a sludge pump require close cooperation between the designer, user and pump manufacturer in determining all data regarding the pumped mixture, pumping installation, pump and drive selection. Slurry pumps manufactured by POWEN SA
The leading manufacturer and supplier of slurry pumps on the domestic market is POWEN SA, formerly operating under the name Zabrzańska Fabryka Maszyn Górniczej POWEN. The company has several dozen years of experience in the construction of slurry pumps, in the selection of appropriate construction materials and in the selection of pumps for hydrotransport systems. Over the last decades, pumps of the following series: KA, PŁP, PŁK, PŁS, PC, OŁ, PG, PŻ and PH produced by POWEN have satisfied the vast majority of the needs of the Polish economy in the field of sludge pumping.
Many years of experience have allowed us to develop a simple and safe design of PH pumps, equipped with flow system elements made of special alloy cast steel, guaranteeing their long service life. Produced in basic sizes: PH-65, PH-80, PH-100, PH-150, PH-200, PH-250, PH-300, these pumps have numerous versions and design variants.
The manufacturer's efforts were aimed at expanding the operating range of the pump series. The result was the PH-100W pump (Qn=100 m3/h, Hn =95 m), especially popular in the metallurgical industry.
Another direction of work was to increase the cross-sections of the flow channels. The result was pumps with free-flow impellers, type PH-100S. PH-150S and a PH-250M pump with a special vane impeller.
The PHP-150 vertical shaft pump was created based on the flow system of the PH-150 pump. PH pumps were adapted to pumping liquids at elevated temperatures, resulting in the PH-G series of pumps. The great popularity of PH pumps among users and the ongoing interest in these pumps are the basis for the continuation of their production by POWEN SA
In recent years, work has been carried out on a new series of slurry pumps. As part of a targeted project financed by the KBN in cooperation with the Central Mining Institute in Katowice and the Silesian University of Technology in Gliwice, the design of a series of HC pumps was developed.
When developing the design of these pumps, the need to improve some nodes to meet the operational requirements of users was taken into account. A new bearing unit was constructed without water cooling, and an oil filter was introduced. A specially designed stress reduction unit is used between the shaft and the rotor, facilitating disassembly. A design of the flow system was introduced, enabling various positions of the discharge port. A new, more durable construction material was used for the elements of the flow system.
Thanks to this, HC pumps are currently an attractive market offer of POWEN SA, successfully competing with the offers of foreign companies.
Another achievement of POWEN SA in the field of slurry pumps is the development and introduction into production of a submersible slurry pump type P-370. Used to pump a mixture of water and slag in one of the heat and power plants, it successfully competed with other foreign pumps of this type in terms of the durability of the flow system elements.
Summary.
The article discusses the issues of selection and operation of slurry pumps. The basic problems faced by users, suppliers and designers cooperating with them have been highlighted. Using the example of the main manufacturer of slurry pumps in Poland, POWEN SA, the tasks that the manufacturer must meet in order to provide modern slurry pumps that meet user requirements are discussed. Attention was paid to the advisability of cooperation between the user and designer with the pump manufacturer. POWEN SA, as a manufacturer of slurry pumps with many years of tradition and extensive experience, invites all interested parties to such cooperation.
MSc. Mark Coghen
The article was published in issue 6 of the "Pompy-Pompownie" magazine in 2000.
Author's comment after 15 years:
“The basic issues regarding pumping mixtures of liquids and solid particles presented in the article still remain valid. The products described 15 years ago, then produced by POWEN SA, are currently produced by the company - Powen-Wafapomp SA Group. The offer of slurry pumps has been extended, among others: o a series of MF pumps with rubber linings, HRC pumps for installation on suction dredgers, a series of HZ submersible pumps. The Powen-Wafapomp SA Group continues to work on further pump solutions, adapted to the increasing requirements of users.”
