1、Key points of material selection

Acetic acid is one of the most corrosive substances in organic acids.Ordinary steel will severely corrode in all concentrations and temperatures of acetic acid.Stainless steel is an excellent acetic acid resistant material,and stainless steel chemical pumps containing molybdenum can also be suitable for high temperatures and dilute acetic acid vapors.For demanding requirements such as high temperature and high concentration acetic acid or containing other corrosive media,high alloy stainless steel chemical pumps or fluoroplastic pumps can be selected.

2.Alkali(sodium hydroxide)steel is widely used in sodium hydroxide solutions below 80℃and within 30%concentration.Many factories still use ordinary steel at temperatures below 100℃and 75%.Although corrosion increases,it has good economic efficiency.Compared with cast iron,ordinary stainless steel has no significant advantage in corrosion resistance to alkaline 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 alkaline solution,titanium and titanium alloys or high alloy stainless steel are often used.The general cast iron chemical pumps of the company can be used for low concentration alkaline solution at room temperature.When special requirements are met,various types of stainless steel pumps or fluoroplastic pumps can be used.

Ammonia(ammonia hydroxide)Most metals and non-metals have slight corrosion in liquid ammonia and ammonia water(ammonia hydroxide),and 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.

Salt water(seawater)

Ordinary steel has a low corrosion rate in sodium chloride solution,seawater,and salt water,and generally requires coating protection;Various types of stainless steel also have a low uniform corrosion rate,but may cause localized corrosion due to chloride ions.Usually,316 stainless steel is preferred.

Alcohols,ketones,esters,and 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,ether,butyl ether,etc.They are basically non corrosive and can be used as commonly used materials.When selecting them,reasonable choices should also be made based on the properties and relevant requirements of the media.It is also worth noting that ketones,esters,and ethers have solubility in various types of rubber,so errors should be avoided when selecting sealing materials.There are many other media that will not be introduced here.

Sulfuric acid,as one of the highly corrosive media,is an important industrial raw material with a wide range of uses.The corrosion of materials varies greatly with different concentrations and temperatures of sulfuric acid.For concentrated sulfuric acid with concentrations above 80%and temperatures below 80℃,carbon steel and cast iron have good corrosion resistance,but they are not suitable for high-speed flow of sulfuric acid and are not suitable as materials for pump valves;Ordinary stainless steels such as 304(0Cr18Ni9)and 316(0Cr18Ni12Mo2Ti)are also limited in their use for sulfuric acid media.Therefore,chemical pump valves for transporting sulfuric acid are usually made of high silicon cast iron(with high difficulty in casting and processing)and high alloy stainless steel(alloy 20).Fluoroplastics have good sulfuric acid resistance,and using a fluorine lined pump(F46)is a more economical choice.

Most metal materials,including various stainless steel materials,are not resistant to hydrochloric acid corrosion.Molybdenum containing high silicon iron can only be used for hydrochloric acid at temperatures below 50℃and 30%.Contrary to metal materials,the vast majority of 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.

Most metals in nitric acid are rapidly corroded and destroyed in nitric acid.Stainless steel is the most widely used nitric acid resistant material and has good corrosion resistance to all concentrations of nitric acid at room temperature.It is worth mentioning that stainless steel containing molybdenum(such as 316 and 316L)not only has lower corrosion resistance to nitric acid than ordinary stainless steel(such as 304 and 321),but sometimes even worse.For high-temperature nitric acid,titanium and titanium alloy materials are usually used.

2、Media issues

Medium temperature

We all know how important it is to transport the medium in pump selection.Let's start with the temperature of the conveying medium.The transportation of high-temperature media poses higher requirements for the structure,materials,and auxiliary systems of chemical pumps.Below,we will discuss the cooling requirements and applicable pump types for different temperature changes:

1)For media with a temperature below 120℃,a dedicated cooling system is usually not set up,and the medium itself is often used for lubrication and cooling.

2)For media above 120℃and below 300℃,a cooling chamber must generally be installed on the cover of the chemical pump,and the sealing chamber should also be connected to the cooling liquid(equipped with a double end mechanical seal).When the cooling liquid is not allowed to penetrate the medium,it should be cooled and connected(which can be achieved through a simple heat exchanger).For high-temperature media above 300℃,not only the pump head part needs to be cooled,but the suspension bearing chamber should also be equipped with a cooling system.The chemical pump structure is generally in the form of a central support,and the mechanical seal is preferably a metal corrugated pipe type,but the price is high(more than 10 times that of a regular mechanical seal).

Viscosity issues

The viscosity of the medium has a significant impact on the performance of chemical pumps.As the viscosity increases,the head curve of the chemical pump decreases,and the head and flow rate under optimal operating conditions also decrease,while the power increases,resulting in a decrease in efficiency.The parameters on the general sample are the performance when transporting clean water,and conversion should be carried out when transporting viscous media(the correction coefficients for different viscosities can be found in relevant conversion charts).For the transportation of slurry,paste and viscous liquid with high viscosity,it is recommended to use screw pump.The biggest feature of screw pump is its strong adaptability to the medium,stable flow,small pressure pulsation,and good self-priming energy,which can not be replaced by any other pump.

3、Sealing issues

No leakage is the eternal pursuit of chemical equipment,and it is this requirement that has led to the increasingly expanding application of magnetic pumps and shielded pumps.However,there is still a long way to go to truly achieve no leakage,such as the lifespan issues of magnetic pump isolation sleeves and shielding pump shielding sleeves,material pitting issues,reliability issues of static seals,and so on.Here is a brief introduction to the basic situation of sealing in chemical pumps:

Sealing form

For static sealing,there are usually only two forms of sealing gasket and sealing ring in chemical pumps,and O-ring is the most widely used sealing ring;For dynamic seals,chemical pumps rarely use packing seals,mainly mechanical seals.Mechanical seals can be divided into single and double face,balanced and non balanced types.Balanced types are suitable for sealing high-pressure media(usually referring to pressure greater than 1.0MPa).Double face mechanical seals are mainly used for high temperature,easily crystallized,viscous,granular,and toxic volatile media.Double face mechanical seals should inject isolation liquid into the sealing chamber,Its pressure is generally 0.07-0.1MPa higher than the medium pressure.

sealing material

The material for static sealing of chemical pumps is generally fluorine rubber,and in special cases,polytetrafluoroethylene material is used;The material configuration of mechanical sealing dynamic and static rings is crucial,and hard alloy is not the best choice for hard alloy.The high price is one aspect,and there is no difference in hardness between the two,which is not reasonable.Therefore,it is best to choose a reasonable sealing material for chemical pumps based on the characteristics of the medium.