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Research on Diaphragm Pumps for Water-Coal Slurry Transportation Abstract: Based on the results of the product evaluation for the low-pressure coal slurry pump project as part of major scientific and technological initiatives, as well as on the findings from research and development efforts and industrial operation assessments, this paper discusses the key core technologies related to diaphragm pumps. It also outlines the approaches to achieving localization of diaphragm pumps for water-coal slurry applications. Keywords: water-coal slurry; diaphragm pump; diaphragm flexure; wear 1. Introduction Diaphragm pumps are among the pump types that present the greatest technical challenges and offer the highest added value in the pump industry, both domestically and internationally. It uses a diaphragm to separate the medium to be transported from the driving medium (oil), preventing any leakage of the medium to be transported. More importantly, it ensures that the particulate medium in question does not come into contact with the moving parts of the pump piston, thereby avoiding severe wear on the pump caused by solid particles and significantly improving the pump’s performance stability and reliability. The product developed as part of the **“Eighth Five-Year Plan” major scientific and technological research project “Development of Low-Pressure Coal Slurry Pumps”, carried out by Hefei General Machinery Research Institute, is innovative in nature, and it was approved by the **Mechanical Industry Bureau at the beginning of 1999. The following introduces the results of the product evaluation for the low-pressure coal slurry pump, as well as the design of the diaphragm fluid control system for the coal-water slurry diaphragm pump and the outcomes of its industrial tests. 2. Diaphragm control system design 2.1 Introduction to foreign diaphragm control technologies The fundamental difference between diaphragm pumps and ordinary reciprocating pumps lies in the presence of a diaphragm; the core of this technology is diaphragm control, with the aim of ensuring a long service life for the diaphragm. Currently, the technical solutions adopted by major foreign manufacturers of reciprocating diaphragm pumps include: the diaphragm pump produced by the Dutch company GEHO controls the deformation of the diaphragm through a control rod made of metal and non-metal, which is integrated with the diaphragm. When the deflection becomes excessive and the diaphragm deviates from its central equilibrium position, moving forward or backward beyond the design limits, an electromagnetic valve is used to remove any excess driving fluid or to add the fluid that is missing, thereby ensuring pressure balance on both sides of the diaphragm and keeping its deflection within the design limits. This product features multiple solenoid valves and an oil circuit system, requiring a high level of sensitivity from electrical components as well as skilled operation. The British company FELUWA uses a hydraulic system that combines hose-type diaphragms with sheet-type diaphragms to provide dual safety guarantees. This design requires a high level of balance in the oil volume within the hydraulic chamber, and it is primarily used for transporting coal slurry, ash slurry, and mineral slurry in power plants and smelting facilities. 2.2 Design and Features of the Diaphragm Control System The low-pressure diaphragm coal slurry pump developed by Hefei General Machinery Research Institute adopts a completely different technical approach from those used abroad, namely hydraulic diaphragm control. To ensure the service life of the diaphragm and to trigger an alarm at the moment of its rupture, the diaphragm control system features the following technical advantages: This pump uses a purely mechanical hydraulic compensation mechanism; it lacks any magnetoelectric conversion control process, as well as any corresponding solenoid valves or related hydraulic components. The mechanism is simple, has few components, is easy to operate, and tests have proven its practicality and reliability. (1) Ensure dynamic pressure balance on both sides of the diaphragm. Although the discharge pressure of a diaphragm pump is lower than that of a high-pressure reciprocating pump, the forces acting on both sides of the diaphragm are quite large (for a diaphragm with a pressure of 116 MPa and an effective size of 400 mm, the forces on both sides are approximately 20 tons). In other words, it is necessary to maintain a very small pressure difference, or one that is approximately zero, between the two sides of the diaphragm (one side containing the driving liquid and the other containing the liquid to be conveyed), so as to achieve a nearly balanced pressure. If the pressure is unbalanced, the diaphragm will be destroyed in an instant. Due to leaks at the piston rod seals, the intake of gas, insufficient or excessive compensation from the compensation system, as well as too much or too little oil being injected for the first time, the amount of oil inside the piston cylinder is constantly changing, and all of these factors disrupt the pressure balance on both sides of the diaphragm. We utilize the patented technology of the “mechanical automatic control mechanism for maximum diaphragm deflection in hydraulic diaphragm pumps”. The design incorporates a comprehensive compensation system that enables automatic adjustment regardless of changes in the driving fluid (oil), thereby ensuring dynamic pressure balance on both sides of the diaphragm at all times. And ensure that when the diaphragm bends outward from its central balanced position, the maximum degree of bending of the diaphragm does not exceed the value permitted by the design; (2) Use a pre-deformed diaphragm to minimize the internal stresses generated by bending. The diaphragm is a double-layer rubber composite diaphragm that has been pre-deformed. The so-called pre-deformed diaphragm refers to a diaphragm whose shape is not that of a flat plate, but rather one that has been pre-formed into a curved shape. Rubber material resistant to oil and wear, along with good elasticity, is used in its construction; this rubber is then pressed into the desired curved shape using molds, ensuring that the two layers fit together well to form a complete diaphragm. The diaphragm also has a certain thickness to ensure a sufficient service life. When the diaphragm bends from its central equilibrium position to either side up to the maximum allowable degree of deflection, its curved surface assumes a bowl-like shape, resulting in no additional tensile stress – the internal stress being almost zero. As a result, even though the diaphragm continuously bends back and forth from its central equilibrium position during operation, the very low or near-zero internal stress allows for a significant extension of its service life. Obviously, the design and manufacturing of such diaphragms are much more complex than those of ordinary flat diaphragms, but prototype tests have shown that these diaphragms are more resistant to fatigue and have a longer lifespan. (3) Application of diaphragm rupture alarm technology Although the above measures can ensure a long service life for the diaphragm, it is still subjected to repeated bending and fatigue at a rate of 1–2 times per second. Coupled with the wear caused by the highly abrasive slurry, failure eventually occurs. Once the diaphragm ruptures, the slurry may enter the piston cylinder, causing severe wear on the piston and the cylinder liner. This requires not only the replacement of the diaphragm but also the piston and cylinder liner. To prevent this from happening, this pump is equipped with a double-layer \"sandwich\" diaphragm, along with a diaphragm rupture alarm system based on pressure-sensitive technology. Whether it is the layer in contact with the liquid being transported or the layer in contact with the driving fluid that ruptures, the liquid pressure is quickly transmitted to the micro-pressure sensor and micro-switch, allowing for the generation of audible, visual, and electrical alarm signals within 0.12 seconds of the rupture. This alerts operators that the diaphragm has broken, enabling them to immediately shut down the pump or start a backup pump right after shutdown, thus ensuring the safe operation of the equipment and the processing process. (4) Optimal design of diaphragm shape and membrane cavity curvature For hydraulic diaphragm pumps, if the piston stroke volume is designated as Vs, the maximum allowable deflection volume of the diaphragm as Vm, and the volume formed by the diaphragm cavity as Vq, then the following design principle should be followed: Vq ≥ Vm > Vs. In the case of non-metallic diaphragms such as those made of rubber or plastic, even a slight pressure difference on either side can generate tensile forces on the diaphragm that exceed the material’s capacity to withstand them. We generally consider it to be a flexible diaphragm; the effect of bending on the balance of the diaphragm can be ignored when the bending stiffness is approximately zero, so that elastic deformation is negligible. Based on the theory of deflection of circular flexible diaphragms, an approximate equation for the elastic surface is given: W = W0. The equations for calculating the pressure difference across the diaphragm and its maximum deflection are as follows: where P represents the pressure difference on both sides of the diaphragm, E is the elastic modulus of the material, R is the effective radius of the diaphragm, W0 is the deflection at the center of the diaphragm, h is the thickness of the diaphragm, L is Poisson’s ratio of the material, and r is the distance from the symmetry axis of the diaphragm to the point on the deflected surface. Using these formulas along with empirical considerations, the maximum allowable working deflection W0 is determined to be 35 mm, while the effective diameter of the diaphragm is 2R = 400 mm. The maximum allowable deflection volume Vm of the diaphragm (the volume enclosed away from the central position) can be calculated accordingly. To minimize the internal stresses resulting from the deflection of the diaphragm, pre-deformation is applied, taking into account the curve of the diaphragm cavity as determined by the calculations. 3. Performance and Industrial Testing of Water-Coal Slurry Diaphragm Pumps The low-pressure diaphragm-type coal slurry pump (Q=50 m3/h, P=116 MPa) developed by Hefei General Machinery Research Institute was installed and tested at the Beijing Water-Coal Slurry Demonstration Plant in September 1998; it operated for a total of 46 hours during the testing period. Full-load continuous testing officially began on September 23, and by 16:00 on November 10, the pump had operated continuously for a total of 1,008 hours at a discharge pressure of 116 MPa; including 46 hours of commissioning operation, the total operating time was 1,054 hours. During operation, there were no abnormalities in terms of oil temperature, noise, or the diaphragm. The viscosity of the water-coal slurry was 2064 mPa·s, and the actual flow rate was 54.12 m3/h; the acceptable viscosity range for this water-coal slurry is 1000±200 mPa·s. From September 23 to November 10, a total of 13 tests were conducted at the water-coal slurry plant; the ash content and concentration of the water-coal slurry remained relatively stable. However, due to the continuous pumping and circulation, the viscosity of the medium varied significantly, with the highest viscosity reaching 4648 mPa·s. After treatment with the additive, the viscosity decreased again, with the lowest viscosity reaching 504 mPa·s. Tests have shown that this pump has a strong ability to adapt to various operating conditions, and it can still function properly even when transporting slurries with a viscosity as high as 4648 mPa·s. 4. Conclusions (1) The low-pressure diaphragm coal slurry pump described in this paper is a product developed independently using the patented technology of \"mechanical automatic control mechanism for maximum diaphragm deflection in hydraulic diaphragm pumps\", and it possesses originality; (2) Long-term experimental studies as well as practical tests of prototype units in industrial settings have shown that the low-pressure diaphragm coal slurry pump discussed in this paper has a simpler structure and more reliable operation. (3) By leveraging the key patent technologies of existing diaphragm pump units and the achievements in low-pressure diaphragm slurry pumps, **it will accelerate the process of local production of diaphragm pumps in China.