When designing equipment, the design institute must determine the purpose and performance of the pump and select the pump type. This selection must first start with the type and form of the pump. So what principles should be used to select the pump? What is the basis?
Pump selection basis
The basis for pump selection should be considered from five aspects based on the process flow and water supply and drainage requirements, namely liquid delivery volume, device head, liquid properties, pipeline layout and operating conditions.
1. Flow rate
The flow rate is one of the important performance data for pump selection, which is directly related to the production capacity and delivery capacity of the entire device. For example, the design institute can calculate the normal, minimum and maximum flow rates of the pump in the process design. When selecting a pump, the maximum flow rate is used as the basis, taking into account the normal flow rate. When there is no maximum flow rate, 1.1 times the normal flow rate can usually be taken as the maximum flow rate.
2. Head
The head required by the device system is another important performance data for pump selection. Generally, the head after enlarging the margin by 5%-10% is used for selection.
3. Liquid properties
Liquid properties include the name of the liquid medium, physical properties, chemical properties and other properties. Physical properties include temperature c, density d, viscosity u, solid particle diameter and gas content in the medium, etc. This involves the system head, effective cavitation margin calculation and the type of suitable pump: chemical properties mainly refer to the chemical corrosiveness and toxicity of the liquid medium, which is an important basis for selecting pump materials and which type of shaft seal to choose.
4. Pipeline layout conditions
The pipeline layout conditions of the device system refer to the liquid delivery height, liquid delivery distance, liquid delivery direction, the lowest liquid level on the suction side, the highest liquid level on the discharge side and other data and pipeline specifications and their length, materials, pipe fittings specifications, quantity, etc., in order to calculate the system head and check the cavitation margin.
5. Operating conditions
The operating conditions contain a lot of content, such as the liquid operation T, saturated steam force P, suction side pressure PS (absolute), discharge side container pressure PZ, altitude, ambient temperature, whether the operation is intermittent or continuous, and whether the pump position is fixed or movable.
The petroleum and chemical industries occupy a very important position in the national economy. As a key supporting equipment, chemical process pumps are also attracting more and more attention. Due to the complex characteristics of chemical media and the increasing requirements for environmental protection, what aspects should be paid attention to when selecting chemical pumps?
01. The impact of corrosion
Corrosion has always been one of the most troublesome hazards of chemical equipment. If you are not careful, it will damage the equipment at the least, and cause accidents or even disasters at the worst. According to relevant statistics, about 60% of the damage to chemical equipment is caused by corrosion. Therefore, when selecting chemical pumps, you should first pay attention to the scientific nature of material selection.
There is usually a misunderstanding that stainless steel is a "universal material". It is very dangerous to use stainless steel regardless of the medium and environmental conditions. The following is a discussion of the key 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. Sulfuric acid of different concentrations and temperatures has a great difference 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 good corrosion resistance, but they are not suitable for high-speed flowing sulfuric acid and are not suitable for use as materials for pumps and valves.
Ordinary stainless steel such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited uses for sulfuric acid media. Therefore, pumps and valves for conveying 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 using fluorine-lined pumps (F46) is a more economical choice. The company's applicable products include: IHF fluorine-lined pumps, PF (FS) highly corrosion-resistant centrifugal pumps, 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 molybdenum-containing high-silicon iron 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 transporting hydrochloric acid. The company's applicable products include: IHF fluorine-lined pumps, PF (FS) strong corrosion-resistant centrifugal pumps, CQ polypropylene magnetic pumps (or fluoroplastic magnetic pumps), etc.
3. Nitric 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 nitric acid of all concentrations at room temperature. It is worth mentioning that molybdenum-containing stainless steel (such as 316, 316L) is not only not better than ordinary stainless steel (such as 304, 321) in corrosion resistance to nitric acid, but 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 pumps, DFL (W) PH shielded chemical pumps, DFCZ process pumps, DFLZP self-priming chemical pumps, IH chemical pumps, CQB magnetic pumps, etc., made of 304.
4. Acetic acid
It is one of the most corrosive substances among organic acids. Ordinary steel will be 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. For demanding requirements such as high temperature and high concentration 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 solutions below 80℃ and within 30% concentration. There are also many factories that still use ordinary steel at 100℃ and below 75%. Although corrosion increases, it is economical.
Ordinary stainless steel has no obvious advantage over cast iron in corrosion resistance to alkali solution. As long as a small amount of iron is allowed to be added to the medium, stainless steel is not recommended. For high temperature alkali solution, titanium and titanium alloys or high alloy stainless steel are mostly used. The company's general cast iron pumps can be used for low concentration alkali solution at room temperature. When there are special requirements, various types of stainless steel pumps or fluoroplastic pumps can be used.
6. Ammonia (ammonia hydroxide)
Most metals and non-metals are slightly corroded in liquid ammonia and ammonia water (ammonia hydroxide), only copper and copper alloys are not suitable for use. Most of the company's products are suitable for the transportation of ammonia and ammonia water.
7. Brine (seawater)
The corrosion rate of ordinary steel in sodium chloride solution, seawater and salt water is not very high, and generally requires coating protection; various types of stainless steel also have a very low uniform corrosion rate, but may cause local corrosion due to chloride ions, and 316 stainless steel is usually better. All types of chemical pumps of the company are configured with 316 materials.
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 commonly used materials can be used. When selecting, a reasonable choice should be made based on the properties of the medium and related requirements.
It is also worth noting that ketones, esters, and ethers are soluble in many types of rubbers, so avoid mistakes when selecting sealing materials.
02. Influence of other factors
Generally, the leakage in the pipeline system can be ignored in the process flow of industrial pumps, but the impact of process changes on flow must be considered. If agricultural pumps use open channels to transport water, leakage and evaporation must also be considered.
Pressure: suction tank pressure, drainage tank pressure, pressure difference in the pipeline system (head loss).
Pipeline system data (pipe diameter, length, type and number of pipeline accessories, geometric elevation from suction tank to pressure tank, etc.).
If necessary, a device characteristic curve should also be drawn.
03. Influence of pipelines
When designing and arranging pipelines, the following matters should be noted:
(1) Reasonable selection of pipeline diameter. A large pipeline diameter means a small liquid flow velocity and small resistance loss at the same flow rate, but the price is high. A small pipeline diameter will lead to a sharp increase in resistance loss, increase the head of the selected pump, increase the power, and increase the cost and operating expenses. Therefore, it should be considered comprehensively from the technical and economic perspectives.
(2) The discharge pipe and its pipe joints should take into account the maximum pressure they can withstand.
(3) The pipeline should be arranged as straight as possible, and the number of accessories in the pipeline and the length of the pipeline should be minimized. When a turn is necessary, 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 possible.
(4) Valves (ball valves or stop valves, etc.) and check valves must be installed on the discharge side of the pump. The valve is used to adjust the operating point of the pump. 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 back, a huge reverse pressure will be generated, causing damage to the pump)
04. Influence of flow head
Determination of flow
(1) If the minimum, normal, and maximum flow rates are given in the production process, the maximum flow rate should be considered.
(2) If only the normal flow rate is given in the production process, a certain margin should be considered.
For ns100 large flow and low head pumps, the flow margin is 5%, for ns50 small flow and high head pumps, the flow margin is 10%, for 50≤ns≤100 pumps, the flow margin is also 5%, for pumps of poor quality and bad operating conditions, the flow margin should be 10%.
(3) If the basic data only gives weight flow, it should be converted into volume flow.
05, the influence of temperature
The transportation of high temperature medium puts higher requirements on the structure, materials and auxiliary systems of the pump. Let's talk about the requirements for cooling under different temperature changes and the applicable pump types of the company:
(1) For media with a temperature below 120℃, a special cooling system is usually not set up, and the medium itself is mostly used for lubrication and cooling. Like DFL(W)H chemical pumps, DFL(W)PH shielded chemical pumps (the protection level of the shielded motor should be H level when it exceeds 90℃).
The DFCZ ordinary type and IH chemical pumps can reach the upper temperature limit of 140℃~160℃ due to the suspension structure; the maximum operating temperature of the IHF fluorine-lined pump can reach 200℃; only the CQB ordinary magnetic pump has an operating temperature not exceeding 100℃. It is worth mentioning that for media that are easy to crystallize or contain particles, a sealing surface flushing pipeline should be provided (interfaces are reserved during design).
(2) For media above 120℃ and within 300℃, a cooling chamber must generally be provided on the pump cover, and the sealing chamber should also be connected to the coolant (a double-end mechanical seal must be provided). When the coolant is not allowed to penetrate into the medium, the medium itself should be cooled and then connected (this can be achieved through a simple heat exchanger).
Currently, the company has DFCZ chemical process pumps, GRG high-temperature pipeline pumps and HPK hot water circulation pumps (under development) for selection. In addition, the CQB-G high-temperature magnetic pump can be used for high-temperature media within 280℃.
(3) For high-temperature media above 300℃, not only the pump head needs to be cooled, but the suspension bearing chamber should also be equipped with a cooling system. The pump structure is generally a center support type. The mechanical seal is preferably a metal bellows type, but the price is high (the price is more than 10 times that of ordinary mechanical seals). At present, the company only has DFAY centrifugal oil pumps that can reach a temperature of 420℃ (under development).
06. Impact of sealing performance
No leakage is the eternal pursuit of chemical equipment. It is this requirement that has led to the increasing application of magnetic pumps and shielded pumps. However, there is still a long way to go to truly achieve no leakage, such as the life of the magnetic pump isolation sleeve and the shielding sleeve of the shielding pump, the pitting problem of the material, the reliability of the static seal, etc. Now let's briefly introduce some basic information about the sealing.
Sealing form
For static seals, there are usually only two forms: sealing gaskets and sealing rings, and the O-ring is the most widely used sealing ring.
For dynamic seals, chemical pumps rarely use packing seals, and mainly use mechanical seals. Mechanical seals are divided into single-end and double-end, balanced and unbalanced types. Balanced type is suitable for sealing high-pressure media (usually refers to pressure greater than 1.0MPa). Double-end mechanical seals are mainly used for high-temperature, easy-to-crystallize, viscous, particle-containing and toxic volatile media. Double-end mechanical seals should inject isolation liquid into the sealing cavity, and its pressure is generally 0.07~0.1MPa higher than the medium pressure.
Sealing materials
The material of chemical pump static seals is generally fluororubber, and polytetrafluoroethylene materials are used in special cases; the material configuration of mechanical seal dynamic and static rings is more critical, and it is not the best for cemented carbide to cemented carbide. The high price is one aspect, and it is not reasonable that there is no hardness difference between the two, so it is best to treat them differently according to the characteristics of the medium.
(Note: The eighth edition of API 610 of the American Petroleum Institute has detailed regulations on the typical configuration of mechanical seals and piping systems in Appendix D)
05. Effect 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, and the head and flow rate of the best working condition decrease accordingly, while the power increases, so the efficiency decreases.