1. Basic basis

1. Flow rate is one of the important performance data for pump selection. It 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, minimum and maximum flow rates of the pump can be calculated. When selecting a pump, take the maximum flow rate as the basis and take into account the normal flow rate. When there is no maximum flow rate, usually 1.1 times the normal flow rate can be taken as the maximum flow rate.

2. The lift required by the device system is another important performance data for pump selection. Generally, the head should be selected by enlarging the head by 5%~10%.

3. Liquid properties. Including the name of the liquid medium, physical properties, chemical properties and other properties, the physical properties include temperature c density d, viscosity u, solid particle diameter and gas content in the medium, etc. Type of suitable pump. Chemical properties, mainly referring to the chemical corrosiveness and toxicity of the liquid medium, are an important basis for the selection of pump materials and the type of shaft seal.

4. The piping arrangement conditions of the device system refer to the liquid feeding height and the liquid feeding distance. Some data such as the lowest liquid level on the suction side and the highest liquid level on the discharge side, as well as the pipe specifications and their lengths, materials, pipe fittings specifications, quantities, etc., are used to calculate the head of the tie-comb and check the NPSH.

5. There are many operating conditions. Such as liquid operation T saturated steam force P, suction side pressure PS (absolute), discharge side container pressure PZ, altitude, ambient temperature operation is intermittent or continuous, pump position is fixed or movable.

The effect of corrosion

Corrosion has always been one of the most troublesome hazards of chemical equipment. A little carelessness can damage the equipment at light level, and cause accidents or even disasters at worst. According to relevant statistics, about 60% of the damage of chemical equipment is caused by corrosion. Therefore, when selecting chemical pumps, the first thing to pay attention to is the scientific nature of material selection. There is usually a misunderstanding that stainless steel is a "universal material", and it is very dangerous to hold stainless steel regardless of the medium and environmental conditions.

The following are the main points of material selection for some commonly used chemical media:

1. Sulfuric acid

As one of the strong corrosive media, sulfuric acid 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 of more than 80% and a temperature of less than 80 °C, carbon steel and cast iron have better corrosion resistance, but it is not suitable for high-speed flowing sulfuric acid. Materials not suitable for use as pump valves;

Ordinary stainless steel such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are also limited in use for sulfuric acid media. Therefore, 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 (No. 20 alloy). Fluoroplastics have better resistance to sulfuric acid, and it is a more economical choice to use a fluorine-lined pump (F46).

2. Hydrochloric acid

Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum-containing ferrosilicon can only be used for hydrochloric acid below 50 °C and 30%. Contrary to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so lined rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for delivering hydrochloric acid.

3. Nitric acid

Generally, most metals are rapidly 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 room temperature. It is worth mentioning that molybdenum-containing stainless steel (such as 316, 316L) ) The corrosion resistance to nitric acid is not only not better than ordinary stainless steel (such as 304, 321), but sometimes even worse. For high-temperature nitric acid, titanium and titanium alloy materials are usually used.

4. Acetic acid

It is one of the most corrosive substances in organic acids. Ordinary steel will be seriously corroded in acetic acid of all concentrations and temperatures. Stainless steel is an excellent acetic acid-resistant material. Molybdenum-containing 316 stainless steel is also suitable for high temperature and dilute acetic acid steam. . For severe requirements such as high temperature and high concentration of acetic acid or other corrosive media, high alloy stainless steel or fluoroplastic pumps can be selected.

5. Alkali (sodium hydroxide)

Steel is widely used in sodium hydroxide solution below 80°C and 30% concentration. There are also many factories that still use ordinary steel at 100°C and below 75%. Although corrosion increases, it is economical. Compared with cast iron, the corrosion resistance of ordinary stainless steel to lye has no obvious advantages. As long as a small amount of iron is allowed to be mixed into the medium, stainless steel is not recommended. For high-temperature lye, titanium and titanium alloys or high-alloy stainless steel are mostly used.

6. Ammonia (ammonium hydroxide)

Most metals and non-metals corrode slightly in liquid ammonia and ammonia water (ammonium hydroxide), and only copper and copper alloys should not be used.

7. Salt water (sea water)

The corrosion rate of ordinary steel in sodium chloride solution, seawater and salt water is not too high, and generally needs to be protected by paint; all kinds of stainless steel also have a low uniform corrosion rate, but local corrosion may be caused by chloride ions, usually 316 stainless steel is used. better.

8. Alcohols, ketones, esters, ethers

Common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc., ketone media include acetone, butanone, etc., ester media include various methyl esters, ethyl esters, etc., ether media include methyl ether, ethyl ether, Butyl ether, etc., they are basically non-corrosive, and common materials can be applied. The specific selection should also be based on the properties of the medium and relevant requirements. Make a reasonable choice. It is also worth noting that ketones, esters, and ethers are soluble in many rubbers, so avoid mistakes when choosing sealing materials.

3. The influence of other factors

General industrial pumps can ignore the leakage in the pipeline system in the process flow, but the influence on the flow rate when the process changes must be considered.

If agricultural pumps use open channels to transport water, leakage and evaporation must also be considered. Pressure: Suction pool pressure, drain pool pressure, differential pressure in the piping system (head loss). Pipeline system data (pipe diameter, length, type and number of pipe accessories, geometric elevation from the suction tank to the pressure tank), etc.

If necessary, the device characteristic curve should also be drawn.

Fourth, the impact of the pipeline

When designing and arranging pipelines, the following items should be paid attention to:

A. Reasonable selection of pipe diameter, large pipe diameter, under the same flow rate, small liquid flow rate, small resistance loss, but high price, small pipe diameter, will lead to a sharp increase in resistance loss, increase the head of the selected pump, and match As belt power increases, costs and operating expenses increase. Therefore, it should be comprehensively considered from the perspective of technology and economy.

B. The discharge pipe and its pipe joints should consider the maximum pressure it can withstand.

C. The pipeline layout should be arranged as straight pipes as much as possible, and the accessories in the pipeline should be minimized and the length of the pipeline should be minimized. When it is necessary to turn, the bending radius of the elbow should be 3 to 5 times the diameter of the pipeline, and the angle should be as large as 90 °C.

D. Valves (ball valves or globe valves, etc.) and check valves must be installed on the discharge side of the pump. 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 make the pump avoid the blow of water hammer. (When the liquid flows back, it will generate huge back pressure and damage the pump)

5. The influence of flow head

Determination of flow:

A. If the minimum, normal and maximum flow have been given in the production process, the maximum flow should be considered.

B. If only normal flow is given in the production process, a certain margin should be considered.

For ns100 large-flow low-lift pumps, the flow margin is 5%; for ns50 small-flow high-lift pumps, the flow margin is 10%; for pumps with 50≤ns≤100, the flow margin is also 5%. For pumps with inferior and poor operating conditions, the flow margin should be taken as 10%.

C. If the basic data only gives weight flow, it should be converted into volume flow.

6. The influence of temperature

The transportation of high temperature medium puts forward higher requirements on the structure, materials and auxiliary systems of the pump. Let's talk about the cooling requirements of different temperature changes:

1. For the medium with temperature lower than 120℃, usually no special cooling system is set up, and its own medium is mostly used for lubrication and cooling. Like DFL(W)H chemical pump, DFL(W)PH shielded chemical pump (the protection level of shielded motor should be H-class when it exceeds 90 ℃); while DFCZ common type and IH chemical pump use the suspension structure to make the temperature The upper limit reaches 140°C~160°C; the maximum operating temperature of the IHF fluorine-lined pump can reach 200°C; only the CQB ordinary magnetic pump can use the temperature not exceeding 100°C. It is worth mentioning that for the medium that is easy to crystallize or contain particles, it should be equipped with a sealing surface flushing pipeline (the interface is reserved in the design).

2. For the medium above 120°C and within 300°C, generally, a cooling cavity must be provided on the pump cover, and the sealing chamber should also be connected to cooling liquid (must be equipped with double-end mechanical seals). When the cooling liquid is not allowed to penetrate into the medium , the medium should be cooled and then connected (which can be achieved by a simple heat exchanger).

3. For the 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 central support, and the mechanical seal is preferably made of metal bellows, but the price is high ( The price is more than 10 times that of ordinary machine seals).

Seven, the effect of sealing

No leakage is the eternal pursuit of chemical equipment, and it is this requirement that has contributed to the increasing application of magnetic pumps and canned pumps. However, there is still a long way to go to achieve no leakage, such as the life of the magnetic pump isolation sleeve and the shielding pump shielding sleeve, the pitting corrosion problem of the material, the reliability of the static seal and so on. Now let's briefly introduce some basic conditions of sealing.

1. Sealed form

For static seals, there are usually only two forms of gaskets and seals, and O-rings are the most widely used seals; for dynamic seals, chemical pumps seldom use packing seals, mainly mechanical seals, and mechanical seals There are single end face and double end face, balanced type and unbalanced type. The balanced type is suitable for the sealing of high-pressure media (usually refers to the pressure greater than 1.0MPa), and the double-end mechanical seal is mainly used for high temperature, easy crystallization, viscosity, and particles As well as toxic and volatile media, the double-end mechanical seal should inject isolation liquid into the sealing cavity, and its pressure is generally 0.07~0.1MPa higher than the medium pressure.

2. Sealing material

The material of the static seal of the chemical pump is generally made of fluorine rubber, and the PTFE material is used in special cases; the material configuration of the dynamic and static rings of the mechanical seal is more critical. It is unreasonable to have no difference in hardness, so it is best to treat them differently according to the characteristics of the medium.

(Note: The typical configuration of mechanical seals and piping systems in the eighth edition of API 610 of the American Petroleum Institute is more detailed in Appendix D)

8. The influence of viscosity

The viscosity of the medium has a great influence on the performance of the pump.

When the viscosity increases, the head curve of the pump decreases, the head and flow rate of the optimal working condition both decrease, and the power increases, so the efficiency decreases. Generally, the parameters on the sample are the performance when conveying clean water, and should be converted when conveying viscous media (for the correction coefficients of different viscosities, please refer to the relevant conversion chart). For the transportation of slurries, pastes and viscous liquids with high viscosity, it is recommended to use screw pumps.