Chemical Pump Selection
Principles of chemical pump selection
To choose a pump, we must first understand and master the principles of chemical pump selection.
1. Make the type and performance of the selected pump meet the requirements of process parameters such as flow, head, pressure, temperature, cavitation flow, and suction.
2, Must meet the requirements of the media characteristics.
For pumps that transport flammable, explosive, toxic or valuable media, shaft seals are required to be reliable or leak-free pumps, such as magnetically driven pumps, diaphragm pumps, and shielded pumps.
For pumps that transport corrosive media, corrosive materials such as AFB stainless steel corrosion resistant pumps and CQF engineering plastic magnetic drive pumps are required for convection components.
For pumps containing solid particulate media, it is required that the convection component be made of wear-resistant material, and if necessary, the shaft seal should be flushed with a cleaning liquid.
3. High mechanical reliability, low noise and low vibration.
4. The economic consideration should be based on the lowest total cost of equipment, operating, maintenance and management fees.
5. Selection of chemical pumps under common occasions:
(1) Centrifugal pump has the characteristics of high speed, small volume, light weight, high efficiency, large flow, simple structure, no pulsation of infusion, stable performance, easy operation and convenient maintenance.
(2) When there is a measurement requirement, the metering pump is selected.
(3), the head requirements are very high, the flow rate is very small, and there is no suitable small flow rate high lift centrifugal pump can be used when optional, reciprocating pump can be used, if the cavitation requirements are not high, the vortex pump can also be selected.
(4), the head is very low, when the flow is very large, the axial flow pump and the mixed flow pump can be selected.
(5) When the viscosity of the medium is large (more than 650~1000mm2/s), consider the choice of rotor pump or reciprocating pump (gear pump, screw pump)
(6) When the medium gas content is 75%, the flow rate is small and the viscosity is less than 37.4mm2/s, a vortex pump can be selected.
(7) For occasions with frequent start-up or inconvenient pumping, pumps with self-priming performance, such as self-priming centrifugal pumps, self-priming vortex pumps, pneumatic (electric) special pump pumps.
Flow and head of chemical pump
The basis for pump selection should be considered in five aspects according to the process flow and water supply and drainage requirements.
1. Flow rate is one of the important performance data of the pump selection, which is directly related to the production capacity and conveying capacity of the entire unit. For example, the normal, minimum and maximum flow rates of the pump can be calculated in the design of the design institute. When the pump is selected, the normal flow rate is based on the maximum flow rate. When there is no maximum flow rate, 1.1 times the normal flow rate can be taken as the maximum flow rate.
2. The head required for the system is another important performance data for the pump. It is generally selected by amplifying the lift by 5%-10%.
3. The nature of the liquid, including the name of the liquid medium, physical properties, chemical properties and other properties, which relate to the lift of the system, the calculation of the effective cavitation allowance and the type of suitable pump, the choice of pump material and the type of shaft seal selected. .
4. The piping arrangement conditions of the equipment system refer to the data of the liquid feeding height, the liquid feeding direction, the suction side, the lowest liquid level, the highest liquid level on the discharge side, and other data and pipe specifications and their lengths, materials, pipe fittings specifications and quantities. Etc., in order to carry out the calculation of the combing process and the checking of the NPSH.
5. Determination of operating conditions, such as liquid operation T saturated steam force, suction side pressure (absolute), discharge side vessel pressure, altitude, ambient temperature operation is gap or continuous, pump position is fixed or movable of. They are all important basis for selection.
Pipe arrangement of chemical pump
When designing the layout of the pipeline, you should pay attention to the following items:
A. Reasonably choose the diameter of the pipe, the diameter of the pipe is large, the flow velocity is small, the resistance loss is small, but the price is high, and the pipe diameter is small, which will lead to a sharp increase in the resistance loss and increase the lift of the selected pump. With increased power, both cost and operating costs increase. Therefore, it should be considered from a technical and economic perspective.
B. The discharge pipe and its pipe joints should take into account the maximum pressure that can be withstood.
C. Pipe arrangement should be arranged as straight pipe as much as possible, minimize the attachments in the pipe and minimize the length of the pipe. When turning, the bending radius of the elbow should be 3 to 5 times the diameter of the pipe, and the angle should be as large as 90. °C.
D. The discharge side of the pump must be equipped with a valve (ball valve or globe valve, etc.) and a check valve. The valve is used to adjust the operating point of the pump. The check valve prevents the pump from reversing when the liquid is reversed, and prevents the pump from hitting the water hammer. (When the liquid flows backwards, it will generate a huge reverse pressure, which will damage the pump).
Chemical pump sealing options
No leakage is the eternal pursuit of chemical equipment. It is this requirement that has led to the expansion of the application of Magnetic Pumps and shielded pumps.
For static sealing, there are usually only two types of gaskets and seals, and the seals are the most widely used O-rings;
For dynamic sealing, chemical pumps rarely use packing seals, mainly mechanical seals, mechanical seals have single-end and double-end, balanced and unbalanced types, and balanced type is suitable for high-pressure medium sealing (usually refers to pressure greater than 1.0MPa), double-face machine seal is mainly used for high temperature, easy to crystallize, have viscosity, contain particles and toxic volatile medium. Double-end machine seal should inject spacer liquid into the sealed cavity, the pressure is generally higher than the medium pressure 0.07~0.1 MPa.
2. Sealing material
The material of the chemical pump static seal is generally made of fluororubber. In special cases, the PTFE material is used. The material arrangement of the mechanical seal static and dynamic ring is more critical. It is not the hard alloy to the hard alloy, the price is high, on the one hand, two It is not reasonable to have no hardness difference, so it is best to treat them differently according to the characteristics of the medium.
Selection of chemical pump special media
Sulfuric acid with different concentrations and temperatures has a great difference in corrosion of materials. For concentrated sulfuric acid with a concentration above 80% and temperature less than 80 °C, carbon steel and cast iron have good corrosion resistance, but it is not suitable for high-speed flowing sulfuric acid.
Not suitable as a material for pump valves:
Ordinary stainless steel such as 304 (0Cr18Ni9), 316 (0Cr18Ni12Mo2Ti)
The pump valve for conveying sulfuric acid is usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (20 alloy). Fluoroplastics have good sulfuric acid resistance, and the use of a fluorine-lined pump (F46) is a more economical option.
Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum-containing high-silicon iron can only be used for hydrochloric acid at 50 ° C and 30%.
Contrary to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choice for transporting hydrochloric acid.
Generally, most metals are quickly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid-resistant material. It has good corrosion resistance to all concentrations of nitric acid at normal temperature. It is worth mentioning that it contains molybdenum-containing stainless steel (such as 316 and 316L). The corrosion resistance to nitric acid is not only not superior to ordinary stainless steel (such as 304, 321), and sometimes even worse. For high temperature nitric acid, titanium and titanium alloy materials are usually used.
It is one of the most corrosive substances in organic acids. Ordinary steel is severely corroded in acetic acid of all concentrations and temperatures. Stainless steel is an excellent acetic acid resistant material. Molybdenum-containing 316 stainless steel can also be used for high temperature and dilute acetic acid vapor. . High-alloy stainless steel or fluoroplastic pumps are available for demanding requirements such as high temperature, high concentration acetic acid or other corrosive media.
Alkali (sodium hydroxide)
The corrosion resistance of ordinary stainless steel to lye has no obvious advantages compared with cast iron. Stainless steel is not recommended as long as a small amount of iron is allowed in the medium.
For high-temperature alkaline liquids, titanium and titanium alloys or high-alloy stainless steels are often used. The company's general cast iron pumps can be used for low-concentration lye at normal temperature. For special requirements, all kinds of stainless steel pumps or fluoroplastic pumps can be used.
Ammonia (ammonium hydroxide)
Most metals and non-metals are slightly corrosive in liquid ammonia and ammonia (ammonium hydroxide), and only copper and copper alloys are not suitable.
Ordinary steel is not too corrosive in sodium chloride solution and seawater or salt water, and generally needs to be protected by paint. All types of stainless steel also have a very low uniform corrosion rate, but may cause local corrosion due to chloride ions, usually 316 stainless steel is better.
Alcohols, ketones, esters, ethers
Common alcohol media are methanol, ethanol, ethylene glycol, propanol, etc., ketone media are acetone, methyl ethyl ketone, etc., ester media have various methyl esters, ethyl esters, etc., ether media are methyl ether, ether, Butyl ether, etc., they are basically non-corrosive, and commonly used materials can be applied. When selecting specifically, they should also make reasonable choices according to the properties of the medium and related requirements.
It is also worth noting that ketones, esters, and ethers are soluble in a variety of rubbers and avoid errors when selecting sealing materials.
Effect of viscosity temperature
The viscosity of the medium has a great influence on the performance of the pump. When the viscosity increases, the lift curve of the pump decreases, and the head and flow rate of the optimal working condition decrease, and the power increases, so the efficiency decreases. The parameters on the general sample are the performance when the water is delivered. When the viscous medium is transported, the conversion should be performed (the correction coefficient for different viscosities can be found in the relevant conversion chart). For the delivery of higher viscosity pulps, pastes and viscous liquids, screw pumps are recommended.
For media with a temperature below 120 °C, a special cooling system is usually not provided, and the medium itself is used for lubrication and cooling.
For mediums above 120 °C and below 300 °C, a cooling chamber shall be provided on the pump cover. The sealing chamber shall also be connected to the coolant (with double mechanical seal). When the coolant is not allowed to penetrate into the medium, Take the medium to cool it and connect it (can be realized by simple heat exchanger).
For high temperature medium above 300 °C, not only the pump head part needs to be cooled, but also the suspension bearing chamber should be equipped with a cooling system. The pump structure is generally in the form of a central support. The mechanical seal is preferably a metal bellows type, but the price is high (the price is More than 10 times the ordinary machine seal).