Learn - Mar. 30, 2023
We have sometimes learnt of amazing projects that involve amazing equipment. Imagine that, in the middle of a vast desert, has a huge concentrating solar power plant, which is made up of many heliostats, absorbing sunlight, storing it in central tower of molten salt, and eventually converting it into electricity. It's like something out of a magical world. For large scale molten salt storage tank, you need a magic stick to stir molten salt, magic stick is a vertical high temperature molten salt pump.
If there is a demand for super head of a molten salt pumps, then a multistage molten salt pump is needed. Here we introduce the vertical multistage molten salt pump which is suitable for handling high temperature and high head medium.
A pump is used for fluid transportation. The demand for multistage pumps, molten salt pumps and various high temperature pumps is increasing with the progress of modern industry. In chemical and renewable energy industry, molten salt pumps are used to convey high temperature nitrate, nitrite and other multiple molten salt medium. Pumps operate temperature between 250℃ ~ 700 ℃, and most of them are vertical submerged structure.
Providing vertical high temperature molten salt multistage pumps with superior performance and stable operation is very important for renewable energy customers. We have always play the role of: having a state-of-the-art pump design laboratory, manufacturing pumps with excellent performance and high efficiency, manufacturing pumps that operate consistently over long periods of time, and above all, customizing pumps for customers to solve various fluid transportation problems.
A molten salt pump is a single stage vertical submerged structure. Different from the single stage type, the vertical multistage molten salt pump has multi-stage impellers, its head could be higher and pressure could be larger. Our vertical multistage molten salt pump is leading in three aspects:
The traditional molten salt pump adopts vertical long shaft submerged pump, which could be divided into single tube type and double tube type; which could be divided into volute guide vane, runner guide vane and spatial guide vane.
High temperature multistage molten salt pumps adopt runner guide vane or spatial guide vane. These two forms of extrusion chamber avoid the increase of radial force of impeller caused by thermal deformation, which leads to the increase of bearing loss of unit.
The spatial guide vane is characterized by long axial direction and short radial direction. In the design, the bladeless transition segment structure similar to radial guide vane is cancelled, so the hydraulic performance is more superior. Secondly, the spatial guide vane structure is symmetrical, which can greatly improve the problem of uneven heating deformation under the liquid part, and can significantly reduce the asymmetric load at the bearing. The picture below shows the lower part of a multistage molten salt pump with the spatial guide vane. The characteristics of long axial direction and short radial direction can be clearly seen.
Multistage molten salt pumps with spatial guide vane should meet the following requirements:
1. The wet circumference of the blade passage section should be reduced as far as possible, preferably round or square
2. The shape of the runner changes smoothly
3. The Angle of each part should conform to the flow law
4. All kinds of speed changes should be uniform
The complex working condition of a multistage molten salt pump requires high performance. Thus, the hydraulic optimization of the impeller and guide vane of the low specific speed multistage molten salt pump is very important.
2.1. Impeller inlet diameter The vertical high-temperature multi-stage molten salt pump is multistage type, the pump shaft would be through the impellers, in determining the impeller inlet diameter to consider the hub diameter. Combined with strength requirements, the diameter of impeller hub can be appropriately increased.
2.2. Blade outlet setting Angle of impeller The blade outlet placement Angle is usually 18° ~ 40°. Pump efficiency is high when the blade outlet placement Angle ranges from 22° to 30°. However, if the outlet placement Angle is too large, the relative flow of liquid between blades will be seriously diffused. Through numerical simulation, the impeller outlet placement Angle is between 25 ~ 30°.
2.3. The choice of blade number Choosing the proper number of impeller blades would improve the pump head and efficiency, in the design of impeller to determine the appropriate number of blades is very important. When selecting the number of blades, the extrusion between blades and the length of impeller runner are considered to ensure the stability of the fluid in the runner. Considering the specific speed, inlet diameter of impeller and series of impeller comprehensively, the number of blades is determined to be 6 by numerical simulation comparison.
2.4. Blade wrap angle The blade wrap angle of the pump is closely related to the length of the blade passage. If the blade wrap angle is too large, the friction between the blade and the fluid in the passage will increase, resulting in the decrease of head and efficiency. Through numerical simulation, the blade wrap Angle of the impeller is determined to be 180°.
Pump in operation would generate complex pressure pulse phenomenon, especially for the vertical high temperature multistage molten salt pump, because of the high temperature environment, long axis, multistage impellers and spatial guide vane complex factors, the internal flow of the pump generate various pressure pulsation. Excessive pressure pulsation may cause vibration and noise of the whole unit and reduce pump performance. It is necessary to analyze the source of pressure pulsation and reduce the influence of pressure pulsation on pump performance.
For a multistage molten salt pump, the main factors that cause the pressure pulsation of the internal pump is the static and static interference between the impeller and guide vane and the periodic rotation of the pump shaft. Here, take a multistage molten salt pump as an example, its working medium temperature is 580℃, molten salt specific gravity is 1.89, pump speed n=1475 r/min, impeller blade number Z=6, guide vane blade number Z1=8, shaft frequency is 24.583Hz, blade frequency is 147.5Hz.
Pressure pulsation of impeller. The pressure pulsation signal of multistage molten salt pump propagates obviously between stages. The pressure pulsation in the impeller is dominated by high-frequency signals whose frequency is greater than or equal to the main frequency. With the increase of flow rate, the low-frequency fluctuation in the impeller recedes and the intensity of high-frequency fluctuation decreases. The pressure pulsation in the impeller of the pump listed is the axial frequency multiple frequency pulsation, the main frequency of the pulsation is half of the blade frequency, and the secondary frequency is the product of the integral multiple of the axial frequency and the number of guide vane blades. The pressure pulsation in the impeller is affected by the wake action of the impeller, the dynamic and static interference between the impeller and guide vane, and the stage coupling of the pulsation. Impeller phase interleaving can improve the amplitude of pressure pulsation inside the impeller.
Pressure pulsation of guide vane. The pressure pulsation in the guide vane is also dominated by high frequency signals. The pressure pulsation is manifested as the frequency doubling of the axial frequency. The main frequency of the pulsation is half of the blade frequency, and the main secondary frequency is an integral multiple of the blade frequency. Impeller phase interleaving can improve the amplitude of the pressure pulsation inside the guide vane, eliminate the low frequency fluctuation and reduce the high frequency pulsation intensity.
The relationship between pressure pulsation and phase interleaving of impeller provides data support for the sequential arrangement of impeller.
In this article, we take this multi-stage molten salt pump with a low specific speed of 42 to expand the explanation, and maybe it contains some mechanical professional knowledge, seeming to be a little difficult to understand. Of course, the manufacture of superior performance of the multistage molten salt pumps, cannot do without the expertise and in-depth research of the engineering department, we all admire these engineers. Our vertical high temperature multistage molten salt pumps with excellent performance also benefit from this. The enthusiasm for technological progress and innovation about pumps, through impeller hydraulic optimization, monitoring pressure pulsation, the use of numerical simulation software, we give everything to provide excellent performance of multi-stage molten salt pumps.
