Industry News

How to choose a chemical pump?

2022-03-26

When designing equipment, the design institute should determine the purpose and performance of the chemical pump, and select the type of pump.

This choice starts with the type and form of the pump, so what principles should be used to choose a pump? On what 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 pumps; pumps that transport corrosive media require corrosion-resistant materials for the convection part. Such as AFB stainless steel corrosion-resistant pump, CQF engineering plastic magnetic drive pump. For the pump that transports the medium containing solid particles, wear-resistant material is required for the convection part, and the shaft seal is cleaned with cleaning fluid if necessary.

3. High mechanical reliability, low noise and small vibration.

4. Economically, the total cost of equipment cost, operation cost, maintenance cost and management cost should be comprehensively considered.

5. Centrifugal pumps have the characteristics of high speed, small size, light weight, high efficiency, large flow, simple structure, no pulsation in infusion, stable performance, easy operation and convenient maintenance.


Therefore, centrifugal pumps should be used as much as possible except in the following cases:

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 lift is low and the flow is large, axial flow pumps and mixed flow pumps can be used.

When the viscosity of the medium is high (greater than 650~1000mm2/s), a rotor pump or a reciprocating pump (gear pump and screw pump) can be considered.

When the air content of the medium is 75%, the flow rate is small, and the viscosity is less than 37.4mm2/s, the vortex pump can be selected.


For occasions with frequent starting or inconvenient pumping, pumps with self-priming properties, such as self-priming centrifugal pumps, self-priming vortex pumps and pneumatic (electric) diaphragm pumps, should be selected.


Selection basis of chemical pump

The basis for the selection of chemical pumps should be considered from five aspects: liquid transportation, device head, liquid properties, pipeline layout and operating conditions.

According to the technological process and water supply and drainage requirements.

1. Flow

Flow rate is one of the important performance data for 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 selecting a pump, base on maximum flow and consider normal flow. 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, the lift after the margin is enlarged by 5%-10% is selected. 

3. Liquid properties

Liquid properties, including the name, physical properties, chemical properties and other properties of the liquid medium, physical properties include temperature C, density D, viscosity U, diameter of solid particles in the medium and gas content, etc. It is related to the calculation of system head, effective cavitation allowance and suitable pump type: chemical properties, mainly referring to the chemical corrosiveness and toxicity of liquid medium, are an important basis for selecting pump materials and shaft seal types.

4. Pipeline layout conditions

The piping arrangement conditions of the system refer to some data such as the liquid inlet height, the liquid inlet direction, the suction side to the low liquid level, the discharge side to the high liquid level, etc., as well as the pipe specifications and their lengths, materials, pipe fittings specifications, and quantities. In order to calculate the comb head and check the NPSH. 

5. Manipulation

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


Due to the complex characteristics of chemical media and increasingly high environmental protection requirements, what should be paid attention to when choosing a chemical pump? 


01 Corrosion Effects

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 chemical equipment damage is caused by corrosion, so when choosing chemical pumps, we must first pay attention to the scientific nature of material selection. There is often a misunderstanding that stainless steel is a "universal material", and it is very dangerous to stick out of stainless steel regardless of the medium and environmental conditions. 


The following are some key points for the selection of 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. The corrosion of materials by sulfuric acid at different concentrations and temperatures varies greatly. For concentrated sulfuric acid with a concentration of more than 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 valves. The use of common stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) in sulfuric acid media is also limited. Therefore, pump valves that deliver sulfuric acid are usually made of high-silicon cast iron (difficult to cast and machine) and high-alloy stainless steel (20-gauge alloy). Fluoroplastics have good sulfuric acid resistance, and it is a more economical choice to use a fluorine-lined pump (F46). Our products include: IHF lined fluorine pump, PF (FS) centrifugal pump with strong corrosion resistance, CQB F fluorine plastic magnetic pump, etc.

2. Hydrochloric acid Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials). 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 good choices for delivering hydrochloric acid. The company's applicable products are: IHF fluorine-lined pump, PF (FS) centrifugal pump with strong corrosion resistance, CQ polypropylene magnetic pump (or fluorine plastic magnetic pump), etc.

3. Nitric acid Generally speaking, most metals are rapidly corroded and destroyed 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 no better than that of ordinary stainless steel (such as 304, 321), but sometimes even worse. For high temperature nitric acid, titanium and titanium alloy materials are usually used. The company's applicable products are: 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. Material is 304.

4. Acetic acid It is one of the most corrosive substances in organic acids. Ordinary steel is severely corroded in acetic acid of various concentrations and temperatures. Stainless steel is a good acetic acid resistant material. Molybdenum-containing 316 stainless steel is also available for high temperature and dilute acetic acid vapors. 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 solutions with concentrations below 80 and 30%. Many mills still use 100 and less than 75% ordinary steel. Although the corrosion is intensified, the economy is good. Compared with cast iron, ordinary stainless steel has no obvious advantage in lye corrosion resistance. As long as a small amount of iron is allowed in the medium, stainless steel is not recommended. High-temperature lye is mostly made of titanium and titanium alloys or high-alloy stainless steel. Our company's general cast iron pumps can be used for low-concentration lye at room temperature, and various stainless steel pumps or fluoroplastic pumps can be used when there are special requirements.

6. Ammonia (ammonium hydroxide) Most metals and non-metals corrode slightly in liquid ammonia and ammonia water (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, seawater and salt water is not too high, and coating protection should be used. The uniform corrosion rate of various stainless steels is also low, but local corrosion may be caused by chloride ions, usually 316 stainless steel is better. All kinds of chemical pumps of the company are equipped with 316 materials.

8. Alcohols, ketones, esters and ethers Common alcohol media include methanol, ethanol, ethylene glycol and propanol, ketone media include butanone, ester media include various methyl and ethyl esters, ether media include methyl ether, ethyl ether and butyl ether, basically non-corrosive properties, common materials can be used. Reasonable selection should be made according to the properties of the media and related requirements. It's also worth noting that ketones, esters, and ethers are soluble in many types of rubber, so mistakes can be avoided when choosing a sealing material.


02 Other factors influence

Generally, industrial pumps can ignore the leakage of the pipeline system in the process flow, but the influence of process changes on the flow rate needs to be considered.

If agricultural pumps use open channels to deliver water, seepage and evaporation also need to be considered.

Pressure: suction pool pressure, drainage pool pressure, piping system pressure difference (head loss). Piping system data (pipe diameter, length, type and number of pipe accessories, geometric elevation from suction tank to pressure tank, etc.).

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


03 The effect of pipes

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

(1) Reasonable selection of the pipe diameter. Under the same flow rate, the pipe diameter is large, the liquid flow rate is low, and the resistance loss is small, but the high price and small pipe diameter will lead to a sharp increase in the resistance loss, thereby increasing the head, power distribution and efficiency of the selected pump. cost, operating cost. Therefore, it should be considered comprehensively from the technical and economic perspectives.

(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 diameter of the pipe, 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 globe 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, preventing the pump from being hit by water hammer. (When the liquid flows back, huge back pressure is created, which can damage the pump)


04 The effect of water head

Determination of flow velocity

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

(2) If only normal flow is given in the production process, a certain margin should be considered. For ns100 high flow and low lift pump, the flow margin is taken as 5%, for ns50 low flow and high lift pump, the flow margin is taken as 10%, for 50ns100 pump, the flow margin is also 5%. For pumps in poor condition, the flow margin should be 10%.

(3) If the basic data only gives weight flow, it should be converted to volume flow.


05 The effect of temperature

The transportation of high temperature medium puts forward higher requirements on the structure, material and auxiliary system of the pump.

Let's talk about the cooling requirements of different temperature changes and the pump types applicable to the company: 

(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 pump and DFL(W)PH shielded chemical pump (when the temperature exceeds 90, the protection level of the shielded motor should be H). However, the DFCZ ordinary pump and the IH chemical pump can make the upper temperature limit reach 140~160 due to the suspension structure. The highest operating temperature of IHF fluorine-lined pump can reach 200; only CQB ordinary magnetic pump 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 the medium of chemical pump above 120 and below 300, generally a cooling chamber should be set on the pump cover, and the sealing chamber should also be connected with the cooling liquid (double-end mechanical seal is used). When the cooling liquid is not allowed to penetrate into the medium, the medium itself should be cooled and connected (this can be done with 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 medium within 280.

(3) For chemical pumps with high temperature medium above 300°C, not only the pump head 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 should use metal bellows, but the price is high (the price is more than 10 times that of ordinary mechanical seals). Currently only DFAY centrifugal oil pump can reach 420 (under development).


06 The influence of chemical pump sealing

No leakage is the eternal pursuit of chemical equipment, and it is this requirement that promotes 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 service life of magnetic pump isolation sleeves and canned pump shielding sleeves, material cavitation, reliability of static seals, and so on. Now briefly introduce some basic information about sealing.


Chemical pump seal form

For static seals, there are usually only two forms: gaskets and seals, and O-rings are widely used seals. For dynamic seals, chemical pumps rarely use packing seals, mainly mechanical seals, which can be divided into single-end and double-end, balanced and unbalanced. Balanced type is suitable for sealing high pressure medium (usually refers to pressure greater than 1.0 MPa). Double-ended mechanical seals are mainly used for high temperature, easy to crystallize, viscous, granular, toxic and volatile media. The double-ended mechanical seal should inject isolation fluid into the seal cavity, and its pressure is generally 0.07 higher than the medium pressure.


Chemical pump sealing material

Fluorocarbon is generally used for static sealing materials of chemical pumps, and PTFE is only used in special cases. The material configuration of the mechanical dynamic static ring is critical. 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.



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