LUOLAN Magnetic Pump Technical Description
1. Technical background
Unsealed centrifugal pumps are widely used in the petrochemical process to pump dangerous chemical liquids that are flammable, explosive, toxic and harmful due to their intrinsically safe characteristics. However, under the current conventional technology, there are two technical bottleneck problems in the hermetic pump. One is the eddy current loss problem, and the other is the sliding bearing problem.
Unsealed pumps usually only refer to "magnetically driven centrifugal pumps" (MDP) and "shielded motor centrifugal pumps" (CMP). The principle of the two pumps to achieve no seal is essentially the same. Therefore, there are eddy current losses and bearing problems. When driving at low power, the above problems are not obvious. But the problem is very prominent when driving with high power. This situation restricts the wide application of high-power unsealed pumps.
The shielded electric pump is a mature technology product with a large market application coverage. The technical progress of magnetic pumps in recent years has a tendency to replace the former. An important reason is that magnetic pumps have the advantages of convenient, fast and low-cost on-site maintenance compared to shielded electric pumps.
2. Eddy current loss of magnetic pump
2.1 Causes of eddy current loss
Magnetic pumps use permanent magnet coupling to achieve contactless power torque transmission. In this way, a pressure-bearing shell can be installed in the magnetic field air gap to isolate the working medium from the atmosphere, so that the normal shaft dynamic seal becomes a static seal. The pressure-bearing shell is commonly known as "isolating sleeve", and its installation position and working conditions determine the special requirements for the performance of the material. Such as high mechanical strength, non-ferromagnetic, high resistivity, corrosion resistance and so on. Among the conventional materials, there are no materials that satisfy all the conditions. In desperation, it is necessary to use non-ferromagnetic and relatively high resistivity metal materials. Such as austenitic stainless steel, Hastelloy, titanium alloy, etc.
When the magnetic pump is working, the metal isolation sleeve is in an alternating magnetic field, which will induce eddy current, which is the loss in the process of power transmission and is called "eddy current loss".
The empirical calculation formula of eddy current loss:
The empirical calculation formula of eddy current loss:
Where: PL—vortex loss K—constant T—housing wall thickness
L—Magnetization length N—Speed Bg—Magnetic flux
D—Isolated sleeve average diameter M—Number of magnet groups R—Resistivity
The magnetic pump is driven by high power, and the correct saying is that it is driven by high torque-the volume of the permanent magnet will also increase, the size of the metal isolation sleeve will also increase, and the eddy current loss will increase with the cube of the diameter of the metal isolation sleeve.
Table 1 Resistivity of common metal materials
2.2 Negative effect of magnetic pump eddy current loss
The eddy current loss of the magnetic pump reduces the efficiency of the unit. When driven by high power, the eddy current loss will increase geometrically, and the efficiency will be unacceptably low. This is the main technical bottleneck restricting the use of high-power magnetic pumps.
"Eddy current loss" is dissipated in the form of Joule heat, and the metal insulation sleeve will continue to rise in temperature, so it needs to be rinsed and cooled. The usual design is to establish an internal flushing circuit. Cooling with working fluid will bring multiple negative effects.
1) The temperature rise of the internal flushing fluid heat exchange usually returns to the pump inlet, which will worsen the suction conditions of the pump. When the effective cavitation margin (NPSHa) is low and the small flow adjustment operation is performed, cavitation is extremely likely to occur.
2) The temperature rise of the rinsing liquid is not suitable for the working liquid conditions such as easy vaporization or easy polymerization.
3) The flushing circuit must always be kept smooth. Therefore, the working condition that the working fluid contains impurities that may cause pipeline blockage is not applicable.
4) Generally for corrosive liquids, temperature rise will increase the corrosion rate.
5) Flushing backflow generates additional volume loss, which is especially obvious when driving with high power and large eddy current loss. Typical working conditions are small flow and high head.
3. The problem of sliding bearings
The working rotor support of the magnetic pump can usually only use sliding bearings, and rely on the pumped liquid for lubrication. The failure of the magnetic pump is concentrated in bearing damage. Therefore, the reliability of the sliding bearing is the key factor to ensure the continuous operation life of the magnetic pump unit.
As shown in Figure 1, the sliding bearing is essentially a viscous pump. It relies on lubricating fluid to generate pressure in the wedge-shaped gap formed by the displacement of the journal when the journal rotates. A liquid film is formed at the minimum gap to balance the vector load. Therefore, it is called "hydrodynamic sliding bearing".
The formation of the liquid film is mainly related to three parameters: lubricating fluid viscosity Z-cp (0.03pa·s), speed N (r/min), bearing pressure P-lb/in² (13.6bar), the general experience is ZN /P value is greater than or equal to 30 (0.44) to form a liquid film.
The working fluid of the magnetic pump is usually very low in viscosity. Under the condition of the specified rotation speed and high power drive, the bearing load increases (including additional vibration and shock load), the liquid film is difficult to form when the bearing works, and the bearing sliding will produce contact friction on the coupling surface, called "boundary lubrication" ".
The degree of wear of the sliding bearing "boundary lubrication" depends on the material's PV value characteristics.
Table 2 PV values of different bearing materials (from: Pump Handbook "Pump Manual", China Petrochemical Press)
P—Net load/projected area (minus the slotted area)，psi
V—Speed at shaft diameter or average diameter of thrust surface，ft/min
The high PV value of silicon carbide/silicon carbide is the preferred material for magnetic pump sliding bearings. However, the following problems exist in actual use:
1) As shown in Figure 2, due to machining and assembly accuracy errors and bearing inherent gaps and other reasons. When sliding bearings are used, it is difficult to maintain the parallelism of the axis line of the neck bearing (bushing sleeve) and the journal (joke bushing) and the thrust bearing, resulting in "bearing bias". At this time, as the load during high-power driving increases, the pressure concentration exceeds the PV value of the material combination, which will cause self-wear of the SiC material and even material fragmentation.
2) The thermal expansion coefficient of silicon carbide material is very low. For neck bearings, when the temperature is high, a large expansion difference is formed with the metal components that are installed together, which makes the fit gap excessive or enlarged. A serious result is the fragmentation of the ceramic material and the accompanying increase in vibration.
Figure 2 Schematic diagram of radial bearing eccentric load
6. Other related instructions
6.1 Efficiency comparison of Roland magnetic pump
In the small power range, the Roland magnetic pump has an efficiency of about 8-12% higher than comparable ordinary magnetic pumps and shielded electric pumps; it is about 2% lower than the mechanical seal pump, and the efficiency when it deviates from the BEP point by half It will be about 6% lower.
The overall efficiency of Roland high-power magnetic pump is basically the same as that of comparable mechanical seal pump.
(Explanation): The sliding bearing and the magnetic coupling in the magnetic pump produce friction loss, but the loss value is a relatively constant amount. When running at a small flow away from the BEP point, the shaft power decreases, and the ratio of friction loss to shaft power increases compared to the BEP point. When driving with high power, the ratio is very small and can be ignored.
6.2 Comparison of main advantages and disadvantages of canned pump
Affected by various factors, users may have trouble when facing the selection of magnetic pump and canned pump. In this case, there are many cases of type selection failure, and the losses caused by it often far exceed the purchase cost of the pump. Therefore, the basis of selection and control should also be a deep understanding of the principle of the equipment and a full understanding of some of the characteristics of use. At this time, the full exchange of information is particularly important.
Compared with ordinary magnetic pumps, the biggest advantage of canned pumps is that the allowable operating temperature is designed and the inlet pressure of the allowable pump is higher. The disadvantage of the canned pump is that there is a greater heat generation problem. The reason is that not only the metal shielding sleeve will produce eddy current loss, but also the copper loss (winding resistance) and iron loss of the motor itself (the eddy current loss and hysteresis loss caused by the alternating excitation of the rotor). In addition, at the same torque, the outer dimensions of the motor rotor are much larger than the rare earth permanent magnet rotor, and the friction loss generated is also larger. In this case, when working conditions such as low effective cavitation margin (NPSHa), easy vaporization, easy polymerization and low flow operation, the use of canned pumps should be especially cautious. Another disadvantage of canned pumps is the need for professional maintenance (shielding sleeves, rotor encapsulation and coaxial replacement). It is inferior to the magnetic pump in terms of maintenance convenience, time and cost.
6.3 Comparative advantages of Roland magnetic pump and mechanical seal pump
1. Roland magnetic pump has high safety level and great safety redundancy.
2. The overall efficiency of the Roland magnetic pump is basically the same as that of the mechanical seal pump.
3. Roland magnetic pump has no dynamic seal and auxiliary system, few faulty nodes, high reliability and no maintenance responsibility.
4. The inner rotor of the Roland magnetic pump allows free axial movement. Therefore, it is particularly suitable for installing balance plates. It can realize the dynamic full balance of axial force. This advantage is evident in multi-stage vane pumps.
5. The span of the rotor bearing of the magnetic pump is much shorter than that of the mechanical seal pump. Therefore, the stiffness of the shaft is stronger and the critical speed is larger.
6. The impeller rotor of the magnetic pump has no contact coupling with the driving rotor. Therefore, the vibrations of each other are not superimposed, and the overall vibration value is smaller.
7. The outer bearing box of the magnetic pump is under nitrogen protection in the secondary sealed cavity. Therefore, the lubricant does not have the problems of oxidation and moisture absorption denaturation.
6.4 About demagnetization of permanent magnetic materials
Modern permanent magnet materials, with their excellent magnetic properties, make high-power permanent magnet transmission possible. Rare earth permanent magnet materials are commonly used in two major categories: samarium cobalt 2:17 and neodymium iron boron, with different performances depending on the grade. As the permanent magnet drive of the magnetic pump is used, the selection of materials should be based on the principles of application and economy. Therefore, the difference in materials should be understood.
Comparatively speaking, the Sm2Co17 permanent magnet material (Sm2Co17) has a high Curie temperature, high coercive force, is not easy to demagnetize, and has good corrosion resistance. The maximum use temperature of different brands can reach 250~350℃, but the price is high and the material is brittle very large. NdFeB is characterized by a high magnetic energy product and a large magnetic field strength, about 1.5 times that of samarium cobalt, the price is lower than samarium cobalt, the maximum use temperature of different brands is from 80 to 200 ℃, poor oxidation resistance and corrosion resistance, the material surface needs Plating treatment.
There are many factors that affect the demagnetization of permanent magnet materials, including: temperature, geometry, mechanical vibration and shock, radiation, corrosion, external magnetic interference, etc. Before the emergence of rare earth permanent magnet materials, these factors were very easy to cause demagnetization of permanent magnet materials. However, the magnetic properties of modern rare earth permanent magnet materials are very high, especially the coercivity is high, and the preparation technology is also improving. In addition to temperature factors, other factors have little or no effect on the demagnetization of rare earth permanent magnet materials.
The high temperature increases the energy density of the thermal motion of the molecules inside the permanent magnetic material, and drives the magnetic moments to be disorderly arranged. The magnetic moments partially or completely cancel out, the ferromagnetism changes to paramagnetism, and the macroscopic whole shows part or all of demagnetization. The critical temperature that produces total and irreversible demagnetization is called "Curie temperature", which is an important parameter to measure the high-temperature stability of permanent magnetic materials.
Rare earth permanent magnet materials actually allow the maximum working temperature to be much lower than the Curie temperature. This is because as the temperature increases, the magnetic induction strength of the permanent magnetic material will be reduced, but at a certain temperature, the degree of reduction is limited, and is linearly reversible, so it is magnetically stable, repeatedly exceeding this The temperature will produce some irreversible demagnetization. This temperature is called the knee point or inflection point, which is the allowable working temperature of the permanent magnet material. Practice has also proved that the demagnetization phenomenon does not occur when the magnetic force pump using rare earth permanent magnet materials is operated below the knee point temperature.
6.5 The starting problem of magnetic pump
The permanent magnet coupling of the magnetic pump is generally synchronous, and there is no slip. The maximum torque is the decoupling torque. Once the maximum torque is exceeded, the coupling will slip off, and due to the inertial force, it cannot automatically re-couple in the process. The rotor inertia of the high-power magnetic pump is also large, and the instantaneous starting torque is often more than several times the rated torque. Due to cost and other factors, the permanent magnet coupling is generally designed to have a maximum torque of only 1.2 to 1.5 times the rated torque, which is lower than the locked-rotor torque of the motor. At this time, the direct power frequency start will cause the decoupling of the permanent magnet coupling, which will make the start invalid. Therefore, the high-power magnetic pump must slow down and delay the soft start.
When the permanent magnet coupling slips out, it will generate strong vibration, the driving power will be reduced by about half, and it will suddenly rise to the maximum allowable temperature within a few seconds to tens of seconds. Another reason for decoupling is the problem of "holding the shaft", such as impurity crystallization and polymerization. If this is possible, means of monitoring and interlocking are necessary.
6.6 Standards for magnetic pumps
Magnetic pumps and shielded electric pumps are collectively called unsealed pumps. Relevant standards are API685 "Unsealed Pumps in Petroleum, Heavy Chemical and Natural Gas Industries". The latest version is the second version (2011 version). The details of this standard are mainly aimed at the relatively mature medium and small power unsealed pumps. The high-power magnetic pump is still in the technical growth period, and its unique technical problems and solutions have not yet reached consensus. Therefore, the standard does not involve much on high-power magnetic pumps.
Roland magnetic pump implements API685-2011 standard. However, it should be noted that strict implementation of standards will significantly increase costs. Some non-essential clauses in the standard should be carefully requested. The basis for prudence is a full understanding of the operating conditions and equipment. Therefore, it is necessary to coordinate the technical exchange and plan with the manufacturer in advance.
6.7 Roland magnetic pump type
The types of centrifugal pumps are diverse, and most of them can be designed as magnetic pumps (except axial split pumps). In addition to the typical standard type (API610), Roland magnetic pumps can also be designed in many variants as needed. Strictly speaking, Roland magnetic pumps are customized products. The purpose is not only to meet specific needs, but also to take into account reliability, ease of maintenance and cost reduction.
The variant design of Roland magnetic pump generally does not involve changing the existing hydraulic model, and the effective solutions to various problems accumulated over the years, the performance of the product will remain consistent.
The successful selection of Roland magnetic pump depends on a full understanding of the operating conditions. Therefore, users should provide detailed and accurate data sheets.
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