How to choose a chemical pump?
When designing equipment, the design institute should determine the purpose and performance of the chemical pump, and select the type of pump. This choice should start from the type and form of the pump, so what principles should be used to select the pump? What is the basis?
Chemical pump selection principle
1. Make the type and performance of the selected pump meet the requirements of process parameters such as device flow, head, pressure, temperature, cavitation flow and suction.
2. Need to meet the requirements of medium characteristics.
Pumps that transport flammable, explosive, toxic or precious media require reliable shaft seals or non-leakage pumps, such as magnetic drive pumps, diaphragm pumps and canned motor pumps; pumps that transport corrosive media require the use of corrosion-resistant materials for the convection part. Such as AFB stainless steel corrosion-resistant pump, CQF engineering plastic magnetic drive pump.
For pumps that convey solid particles, wear-resistant materials are required for the convection part, and the shaft seal should be cleaned with cleaning fluid if necessary.
3. High mechanical reliability, low noise and low vibration.
4. Economically, the total cost of equipment cost, operating cost, maintenance cost and management cost should be considered comprehensively.
5. The centrifugal pump has the characteristics of high speed, small size, light weight, high efficiency, large flow, simple structure, no pulsation of infusion, stable performance, simple operation and convenient maintenance.
Therefore, except for the following conditions, centrifugal pumps should be used as much as possible:
When there is a metering requirement, choose a metering pump.
When there is a high-lift, high-lift, low-flow centrifugal pump, a reciprocating pump can be used, and a vortex pump can be used when cavitation is not required. When the head is low and the flow is large, axial flow pumps and mixed flow pumps can be used.
When the medium viscosity is high (greater than 650~1000mm2/s), rotor pumps or reciprocating pumps (gear pumps and screw pumps) can be considered.
When the medium has an air content of 75%, a small flow rate, and a viscosity less than 37.4mm2/s, a vortex pump can be used.
For occasions with frequent startup or inconvenient pumping, pumps with self-priming performance should be selected, such as self-priming centrifugal pumps, self-priming vortex pumps and pneumatic (electric) diaphragm pumps.
The selection basis of chemical pump
The selection of chemical pumps should be based on five aspects: liquid transportation, device head, liquid properties, piping layout and operating conditions. According to technological process and water supply and drainage requirements.
1. Flow
Flow rate is one of the important performance data of pump selection, which is directly related to the production capacity and conveying capacity of the entire device. For example, in the process design of the design institute, the normal flow, minimum flow and maximum flow of the pump can be calculated. When choosing a pump, take the large flow rate as the basis and consider the normal flow rate. When there is no maximum flow rate, 1.1 times the normal flow rate is usually taken as the maximum flow rate.
2. Elevator
The head required by the system is another important performance data for pump selection. Generally select the lift after the margin is enlarged by 5%-10%.
3. Liquid properties
Liquid properties, including the name, physical properties, chemical properties and other properties of the liquid medium. The physical properties include temperature C, density D, viscosity U, diameter of solid particles in the medium, and gas content. It is related to the calculation of the system head, effective cavitation allowance and the appropriate pump type: chemical properties, mainly refer to the chemical corrosiveness and toxicity of the liquid medium, and are an important basis for selecting pump materials and shaft seal types.
4. Pipeline layout conditions
The piping layout conditions of the system refer to some data such as liquid inlet height, liquid inlet direction, suction side to low liquid level, discharge side to high liquid level, etc., as well as pipe specifications and their length, materials, pipe fitting specifications, quantity, etc. In order to calculate the combing head and check the cavitation allowance.
5. Handle pieces
There are many operating conditions, such as liquid operation T, saturated steam power P, suction side pressure PS, discharge side container pressure PZ, altitude, ambient temperature operation is intermittent or continuous, and whether the pump position is fixed or movable.
Petroleum and chemical industries occupy a very important position in the national economy, and chemical process pumps, as key supporting equipment, are receiving more and more attention. As the characteristics of chemical media are complex and environmental protection requirements are getting higher and higher, what should be paid attention to when choosing chemical pumps?
01
The effect of corrosion
Corrosion has always been one of the headaches of chemical equipment. A little carelessness may damage the equipment, or even cause an accident or disaster. According to statistics, about 60% of the damage of chemical equipment is caused by corrosion, so when choosing a chemical pump, we must first pay attention to the scientific nature of material selection.
There is usually a misunderstanding that stainless steel is a "universal material", regardless of the medium and environmental conditions, it is very dangerous to extend stainless steel. The following are some key points for selecting materials for common chemical media:
1. Sulfuric acid
Sulfuric acid, as one of the highly corrosive media, is an important industrial raw material with a wide range of uses. Different concentrations and temperatures of sulfuric acid have great differences in the corrosion of materials. For concentrated sulfuric acid with a concentration above 80% and a temperature below 80, carbon steel and cast iron have good corrosion resistance, but they are not suitable for high-speed flowing sulfuric acid, nor are they suitable for pump valve materials.
304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) and other ordinary stainless steel in sulfuric acid medium is also very limited. Therefore, pump valves that transport sulfuric acid are usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (alloy 20). Fluoroplastics have good resistance to sulfuric acid, and it is more economical to use a fluorine-lined pump (F46). Our products include: IHF fluorine-lined pumps, PF (FS) centrifugal pumps with strong corrosion resistance, CQB F fluorine plastic magnetic pumps, etc.
2. Hydrochloric acid
Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and high ferrosilicon containing molybdenum can only be used for hydrochloric acid below 50% and 30%. Contrary to metal materials, most non-metal materials have good corrosion resistance to hydrochloric acid, so rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) lined with rubber are good choices for transporting hydrochloric acid. The company's applicable products include: IHF fluorine-lined pump, PF (FS) centrifugal pump with strong corrosion resistance, CQ polypropylene magnetic pump (or fluoroplastic magnetic pump), etc.
3. Nitric acid Generally speaking, most metals will be corroded and destroyed rapidly in nitric acid. Stainless steel is a widely used nitric acid resistant material and has good corrosion resistance to various concentrations of nitric acid at room temperature. It is worth mentioning that the corrosion resistance of molybdenum-containing stainless steel (such as 316, 316L) to nitric acid is not only not better than that of ordinary stainless steel (such as 304, 321), and sometimes even worse.
For high temperature nitric acid, titanium and titanium alloy materials are usually used. The company's applicable products include: DFL (W) H chemical pump, DFL (W) PH shielded chemical pump, DFCZ process pump, DFLZP self-priming chemical pump, IH chemical pump, CQB magnetic pump, etc. The material is 304.
4. Acetic acid
It is one of the most corrosive substances in organic acids. Ordinary steel will be severely corroded in various concentrations and temperatures of acetic acid. Stainless steel is a good anti-acetic acid material. Molybdenum-containing 316 stainless steel can also be used for high temperature and dilute acetic acid steam. High-alloy stainless steel or fluoroplastic pumps can be used for high-temperature, high-concentration acetic acid or other corrosive media.
5. Alkali (sodium hydroxide)
Steel is widely used in sodium hydroxide solution with a concentration of less than 80 and 30%. Many factories still use 100 and 75% ordinary steel. Although the corrosion is intensified, the economy is good.
Compared with cast iron, ordinary stainless steel has no obvious advantage in corrosion resistance to lye. As long as a small amount of iron is allowed in the medium, stainless steel is not recommended. Titanium and titanium alloy or high-alloy stainless steel are mostly used for high-temperature lye. Our company's general cast iron pumps can be used for low-concentration lye at room temperature, and various stainless steel pumps or fluorine plastic pumps can be used when there are special requirements.
6. Ammonia (Ammonia Hydroxide)
Most metals and non-metals have slight corrosion in liquid ammonia and ammonia (ammonium hydroxide). Only copper and copper alloys are not suitable for use. Most of our products are suitable for the transportation of ammonia and ammonia water.
7. Salt water (sea water)
The corrosion rate of ordinary steel in sodium chloride solution, sea water and salt water is not too high, and coating protection should be adopted. The uniform corrosion rate of various stainless steels is also low, but local corrosion may be caused by chloride ions, and 316 stainless steel is usually better. The company's various chemical pumps are equipped with 316 materials.
8. Alcohols, ketones, esters and ethers
Common alcohol media include methanol, ethanol, ethylene glycol and propanol, ketone media include methyl ethyl ketone, ester media include various methyl and ethyl esters, and ether media include methyl ether, ethyl ether and butyl ether, which are basically non-corrosive Sex, ordinary materials can be used. A reasonable selection should be made according to the attributes of the media and related requirements.
It is also worth noting that ketones, esters, and ethers are soluble in a variety of rubbers, so errors can be avoided when selecting sealing materials.
02
Other factors
General industrial pumps can ignore the leakage of the piping system in the process flow, but the impact of process changes on the flow rate needs to be considered. If the agricultural water pump uses open channels for water delivery, leakage and evaporation also need to be considered. Pressure: suction pool pressure, drain pool pressure, piping system pressure difference (head loss).
Pipe system data (pipe diameter, length, type and quantity of pipe accessories, geometric elevation from suction tank to pressure tank, etc.).
If necessary, the device characteristic curve should also be made.
03
The influence of the pipeline
When designing and arranging piping, pay attention to the following items:
(1) Choose the pipe diameter reasonably. Under the same flow rate, the pipe diameter is large, the liquid flow rate is low, and the resistance loss is small. Cost, operating cost. Therefore, it should be considered from a technical and economic perspective.
(2) The maximum pressure that the discharge pipe and its joints can withstand should be considered.
(3) The pipeline layout should be as straight as possible, and the accessories and pipeline length in the pipeline should be minimized. When turning, the bending radius of the elbow should be 3 to 5 times the pipe diameter, and the angle should be greater than 90 as much as possible.
(4) The discharge side of the pump needs to be equipped with a valve (ball valve or stop valve, etc.). ) And check valve. The valve is used to adjust the working point of the pump, and the check valve can prevent the pump from reversing when the liquid flows back, and prevent the pump from being hit by water hammer. (When the liquid flows backwards, it will produce huge reverse pressure, thus damaging the pump)
04
The effect of head
Determination of flow rate
(1) If the minimum, normal, and maximum flow rates have been given during the production process, the maximum flow rate should be considered.
(2) If only the normal flow rate is given during the production process, a certain margin should be considered.
For ns100 high flow and low head pumps, the flow margin is taken as 5%. For ns50 low flow and high head pumps, the flow margin is taken as 10%. For 50ns100 pumps, the flow margin is also 5%. For poor pumps, the flow margin should be 10%.
(3) If the basic data only gives a weight flow rate, it should be converted to a volume flow rate.
05
The influence of temperature
The transportation of high temperature medium puts forward higher requirements on the structure, materials and auxiliary system of the pump. Let me talk about the cooling requirements for different temperature changes and the company's applicable pump types:
(1) Chemical pumps usually do not have a special cooling system for the medium with a temperature lower than 120, and the medium itself is used for lubrication and cooling. For example, DFL(W)H chemical pumps and DFL(W)PH canned chemical pumps (when the temperature exceeds 90, the protection level of the canned motor should be H).
The DFCZ ordinary pump and IH chemical pump can make the temperature upper limit reach 140~160 due to the suspension structure. IHF fluorine-lined pumps can be used at a maximum temperature of 200; only CQB ordinary magnetic pumps can be used at a temperature not exceeding 100. It is worth mentioning that the medium that is easy to crystallize or contains particles should be equipped with a sealing surface flushing pipeline (the interface is reserved in the design).
(2) For chemical pumps with a medium above 120 and below 300, a cooling chamber should generally be set on the pump cover, and the sealed chamber should also be connected to the coolant (double-ended mechanical seals are used). When the coolant is not allowed to penetrate into the medium, the medium itself should be cooled and connected (this can be achieved by a simple heat exchanger). The company currently has DFCZ chemical process pump, GRG high temperature pipeline pump, HPK hot water circulation pump (under development) to choose from. CQB-G high temperature magnetic pump can be used for high temperature media within 280.
(3) For high temperature medium above 300, not only the pump head needs to be cooled, but also the suspension bearing chamber must be equipped with a cooling system. The pump structure is generally in the form of center support, and the mechanical seal should be made of metal bellows, but the price is higher (the price is more than 10 times that of ordinary mechanical seals). At present, only DFAY centrifugal oil pump can reach 420 (under development).
06
The influence of chemical pump sealing
Leak-free is the eternal pursuit of chemical equipment, and it is this requirement that promotes the increasing application of magnetic pumps and canned motor pumps. However, there is still a long way to go to achieve no leakage, such as the service life of magnetic pump isolation sleeves and shielding pump shielding sleeves, material cavitation, and the reliability of static seals. Now briefly introduce some basic information about sealing.
Chemical pump seal form
For static sealing, there are usually only two forms: gasket and sealing ring, and O-ring is a widely used sealing ring.
For dynamic seals, packing seals are rarely used in chemical pumps, mainly mechanical seals, which can be divided into single end face and double end face, balanced type and unbalanced type. The balanced type is suitable for sealing high-pressure media (usually the pressure is greater than 1.0 MPa). Double-ended mechanical seals are mainly used for media with high temperature, easy crystallization, stickiness, particles, and toxic volatile. The double-ended mechanical seal should be filled with isolating fluid in the sealed cavity, and its pressure is generally 0.07 higher than the medium pressure.
Chemical pump sealing material
Chemical pump static sealing materials generally use fluorocarbon, and PTFE is only used under special circumstances. The material configuration of the mechanical dynamic static ring is very important. It is not that cemented carbide is a good cemented carbide. On the one hand, the high price is unreasonable, because there is no difference in hardness between them, so they should be treated differently according to the characteristics of the medium.