What are the precautions when selecting a screw pump?

Petroleum and chemical industries occupy a very important position in the national economy, and three-screw pumps and single-screw pumps, as key supporting equipment, have also attracted more and more attention. Due to the intricate characteristics of chemical media and the continuous improvement of environmental protection requirements, how should we select chemical pumps? It is particularly important to focus on which aspects and so on.


selecting a screw pump


1. Corrosion resistance


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


Let’s talk about 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 with different concentrations and temperatures has a great difference in the corrosion of materials. For concentrated sulfuric acid with a concentration above 80% and a temperature below 80°C, carbon steel and cast iron have better corrosion resistance, but it is not suitable for high-speed flowing sulfuric acid. It is not suitable as a material for pump valves; ordinary stainless steel such as 304 (0Cr18Ni9), and 316 (0Cr18Ni12Mo2Ti) has limited use for sulfuric acid media.

Therefore, pumps and valves for transporting sulfuric acid are usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (alloy No. 20), but they are difficult to process and expensive, so they are not favoured by people.

The fluoroplastic alloy has good resistance to sulfuric acid. It is a patented material of the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences. It has been proved by the experiment of the Chinese Academy of Sciences that there is no chemical medium that can react with it. Therefore, the fluorine-lined pump (F46) is used. A more economical option.

2) Hydrochloric acid

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

3) Nitric acid

Most general 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 stainless steel containing molybdenum (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, fluoroplastic alloy materials are usually used.

4) Acetic acid

It is one of the most corrosive substances in organic acids. Ordinary steel will be severely corroded in acetic acid of all concentrations and temperatures. Stainless steel is an excellent material for acetic acid resistance. 316 stainless steel containing molybdenum is also suitable for high-temperature and dilute acetic acid vapour.

High-alloy stainless steel or fluoroplastic pumps can be selected for high-temperature, high-concentration acetic acid or other corrosive media. Such as CQB magnetic pump, and CQ stainless steel magnetic pump.

5) Alkali (Sodium Hydroxide)

Generally, the corrosion is not very strong, but the general alkali solution will produce crystallization, so the FSB-type fluorine alloy alkali pump equipped with a mechanical seal made of siliconized graphite 169 can be selected.

6) Ammonia (ammonia hydroxide)

The corrosion of most metals and nonmetals in liquid ammonia and ammonia water (ammonia hydroxide) is very slight, only copper and copper alloys are not suitable for use. At this time, it is better to choose the CQF engineering plastic magnetic pump and FSB fluorine alloy centrifugal pump.

7) Saltwater (seawater)

Ordinary steel has a low corrosion rate in sodium chloride solution, seawater, and salt water, and generally must be protected by coatings; various types of stainless steel also have a very low uniform corrosion rate but may cause localized corrosion due to chloride ions, and 316 stainless steel is usually 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, diethyl ether, Butyl ether, etc., are basically not corrosive, so ordinary stainless steel can be used, and a reasonable choice should be made according to the properties of the medium and related requirements.

It is also worth noting that ketones, esters, and ethers are soluble in various rubbers, so avoid mistakes when choosing sealing materials. It is recommended to choose a fluorine plastic magnetic pump without a seal.

There are many other media that cannot be introduced here one by one. In short, you must not be random and blind when selecting materials, and you should consult more relevant materials or learn from mature experience.


2. Sealing problem


No leakage is the eternal pursuit of chemical equipment, and it is this requirement that has led to the increasing application of magnetic 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, the pitting corrosion of materials, the reliability of static seals, and so on.

Some basic information on sealing is briefly introduced as follows:

1) Sealing form

For static seals, there are usually only two forms of gaskets and seal rings, and O-rings are the most widely used seal rings.

For dynamic seals, chemical pumps seldom use packing seals, mainly mechanical seals, which can be divided into single-end face and double-end face, balanced type and unbalanced type. 1.0MPa), the double-end mechanical seal is mainly used for high-temperature, easy-to-crystallize, viscous, granule-containing, and toxic and volatile media. The double-end mechanical seal should inject spacer fluid into the sealing cavity, and its pressure is generally 0.07~0.1 higher than the medium-pressure MPa.

2) Sealing material

The material of the static seal of the chemical magnetic pump is generally fluorine rubber, and the polytetrafluoroethylene material is used in special cases; the material configuration of the dynamic and static ring of the mechanical seal is more critical, and it is not that hard alloy is the best for hard alloy, and the price is high. It is unreasonable that there is no difference in hardness between the two, so it is best to treat them differently according to the characteristics of the medium.

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

3. 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 will decrease, and the head and flow rate in the best working condition will decrease accordingly, while the power will increase accordingly, so the efficiency will decrease.

Generally, the parameters on the sample are the performance when transporting clear water and should be converted when transporting viscous media.

For the transportation of high-viscosity slurry, paste and viscous liquid, it is recommended to use a screw pump.



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