Useful information on pumps for acids and alkalis

What are acids/alkalis and how to choose a suitable pump for them

This document provides some information on the differences between acids and alkalis and the considerations that must be made when selecting a pump for processes involving these chemicals.

What is an acid?

An acid is a substance that produces hydrogen ions when it is dissolved in water, resulting in a solution with a pH less than 7. The lower the pH, the stronger the acid. Some examples of common acids used in industrial processes are shown below:

Name	Formula	Typical Uses
Hydrochloric acid	HCl	Metal refining, steel pickling, chemical processes.
Sulphuric acid	H2SO4	Manufacturing processes, fertilizer production, petroleum refining, wastewater treatment, batteries
Nitric acid	HNO3	Fertilizer manufacture, metal processing
Acetic acid	CH3COOH	Polymer production, cosmetics, food processing, pharmaceuticals, and detergents
Citric acid	C6H8O7	Cleaning agent, cosmetics, dyes
Carbonic acid	H₂CO₃	Carbonated drinks, cleaning agent, fire suppression
Ferric chloride	FeCl3	Sewage treatment, etching circuit boards

 

What is an alkali?

An alkali (sometimes called a base) is a substance that dissolves in water to form hydroxide ions (OH−) resulting in a solution with a pH value greater than 7. The higher the pH, the stronger the alkali. Some examples of alkalis used in industrial processes are shown below:

Name	Formula	Typical Uses
Sodium hydroxide (caustic soda)	NaOH	Soap and detergent production, cleaning, paper production
Potassium hydroxide (caustic potash)	KOH	Making soft soaps, as an electrolyte in some batteries
Calcium hydroxide (slaked lime)	Ca(OH)2	Mortar production, sewage treatment, fungicide, food treatment and preparation
Aqueous ammonia (ammonium hydroxide)	NH4OH	Cleaning products, water treatment, food preparation
Magnesium hydroxide	Mg(OH)2	Antacids and laxatives, water treatment, fire retardant
Sodium carbonate (washing soda)	Na2CO3	Water treatment, toothpastes, dye and tanning processes
Sodium bicarbonate (bicarbonate of soda)	NaHCO3	Food additive, fungicide, neutralizing and cleaning agent, mild abrasive for surface cleaning
Sodium hypochlorite	NaOCl	Disinfectant and bleaching agent

What are the challenges with pumping acids and alkalis?

The major problem is corrosion or other damage to the wetted parts of a pump. Many of the materials commonly used in pump construction are susceptible to acid corrosion, including metals such as cast iron and some stainless steels. In addition, acids can also degrade many of the materials such as elastomers and plastics used to make seals and O-rings.

Alkalis, at high concentration or temperature, and especially strong ones like sodium hydroxide and potassium hydroxide, can also cause corrosion of metallic pump materials. Aluminium, zinc, tin and some stainless steels are particularly vulnerable to alkaline corrosion. Alkalis can also degrade or cause the swelling of common elastomers used in seals and gaskets, and this can lead to leaks.

Acids and alkalis are often toxic and hazardous chemicals which must be safely contained by the pump and associated equipment. Any leakages or spills could be dangerous for both workers and the environment. Particular attention must be given to the pump’s sealing system and, in many cases, sealless pumps with magnetic drives are required.

Reactions between an acid or alkali and exposed pump materials (including seals and shaft components) can result in impaired performance and increased maintenance costs. In extreme cases, corrosion can cause the premature failure of pumping equipment. All pump manufacturers carry out extensive tests to ensure the suitability of their systems. It is important to seek advice from specialists when choosing a pump for handling acids or alkalis.

Which pump materials are suitable?

Pump housings and components are often available in a range of construction materials:

• Cast iron or carbon steel when used in applications with aqueous solutions with pH values between 6 and 10, cast iron, which contains about 4% carbon, quickly acquires a protective graphite layer. Cast iron with a higher carbon content is more resistant to corrosion than steel.
• Aluminium is susceptible to corrosion in both acids and alkalis. In some processes, it is possible to dose inhibitors into the feed to maintain a protective coating on aluminium components.
• Stainless steel (particularly 316) offers good corrosion resistance to alkalis and some acids. Chromium and other trace elements such as nickel, manganese and molybdenum form a thin, inert oxide layer that binds strongly to the surface and protects the underlying metal in environments that would be highly corrosive for cast iron or steel. Under the right conditions, the protective oxide layer can be self-repairing if it is scratched or damaged.
• Nickel alloys are available in a range of compositions, these are resistant to corrosion from acids and alkalis. Nickel alloys are also strong, resistant to galling, and can withstand high temperatures and pressures. They are commonly used in the manufacture of the moving parts of gear pumps and circumferential piston pumps but can also be used for casings. However, nickel alloys are expensive and heavy.
• Thermoplastics, for example PTFE, are polymers that are resistant to most of the chemicals that corrode metals. Mechanical strength can be reinforced with fillers such as glass fibre. However, physical properties are inferior to those of equivalent metallic pumps.

A good compromise for dealing with corrosive chemicals is to line the wetted parts of iron or stainless steel casings with an inert thermoplastic such as PFA or ETFE or a ceramic coating. In addition to their use with corrosive chemicals, lined pumps are also employed in hygienic applications to prevent contamination in food and beverage processes.

Liquiflo Polyguard Series PFA-lined gear pumps are an excellent choice for pumping acids and alkalis such as hydrochloric acid, ferric chloride, sulphuric acid, hydrofluoric acid, sodium hypochlorite, nitric acid, sodium hydroxide and chromic acid.

Which pump seals are suitable for acids and alkalis?

Pumps contain two types of seal: shaft seals and stationary seals. Shaft seals prevent liquid escaping from a rotating or reciprocating shaft. They are particularly important in the case of centrifugal pumps and often several sealing options will be available. Stationary seals connect pump components but do not have to seal around any moving parts. They include O-rings, gaskets, and diaphragms.

For pumping acids and alkalis, no leakage can be tolerated, and so-called ‘sealless’ pumping systems, using magnetic drives, are recommended. Although such systems do not feature a shaft seal, some form of stationary seal is always necessary and its chemical compatibility with the pumped fluid needs to be considered.
Stationary seals are often made from elastomers (synthetic rubbers). These can be manufactured to have specific physical and chemical properties by the addition of fillers and other compounds and through special vulcanisation processes.

What is a chemical compatibility chart?

Pump manufacturers generally provide a chemical compatibility chart for their range of products. These describe the resistance of materials commonly used in pump construction to various chemicals. It is important to refer to these when selecting a pump to ensure that all wetted parts are chemically compatible with, and resistant to, the pumped medium. This can only be a guide as corrosion and, in the case of thermoplastics and elastomers, chemical damage such as softening or swelling, can vary widely with concentration, temperature, the presence of abrasives, impurities or entrained gases. It may be necessary to carry out tests under field conditions and to consult with pump manufacturers before committing to a system.

Section of a chemical compatibility chart for Finish Thompson’s range of centrifugal pumps
Numerical values listed are the maximum temperature limits in degrees Fahrenheit

What types of pump are used?

The common requirements for transferring acids and alkalis are:

• Non-metallic wetted parts (for example, polypropylene or PVDF) for corrosion resistance
• Seal-less design (magnetic drives) to reduce the possibility of leakage of any harmful chemicals

Most processes are continuous so it is important that the chosen pumps are robust and reliable. Centrifugal pumps are widely used for transferring acids and alkalis. Air operated diaphragm pumps are also often employed and other types of metering pumps, such as gear pumps, may be used for precise dosing.

For high pH alkalis, polypropylene pump construction with EPDM O-rings are suitable. Polypropylene construction may also be suitable for dilute acids. Stainless steel pumps are not recommended for low pH acids but are sometimes suitable for alkalis. For very strong acids, Alloy C/Hastelloy construction is an option although this can be expensive. PVDF-lined pumps fitted with FKM O-rings may be an alternative for low pH acids. If FKM is not compatible with the acid, then perhaps PTFE or Kalrez seals may be suitable. Pumps with an ETFE or PFA lining can be used for most acids and alkalis but are more expensive than polypropylene or PVDF options.

 

The Finish Thomson DB series and SP series are magnetic drive centrifugal pumps designed for handling acids such as sulphuric acid and phosphoric acid and alkalis such as sodium and potassium hydroxide. The pumps are available in polypropylene and PVDF.

 

The Finish Thompson UC series are a range of ETFE-lined centrifugal pumps constructed from tough ductile iron and designed specifically for handling acids and alkalis such as sodium hydroxide, sulphuric acid, hydrochloric acid, and sodium hypochlorite.

Wanner™ Hydra-Cell® Pro seal-less diaphragm pumps offer a wide range of construction materials including Hastelloy, PTFE, PVDF, FFKM and FKM suitable for pumping sulphuric, hydrochloric, nitric acids and other corrosive liquids.

The FTI-AIR range of air-operated double diaphragm pumps are available in polypropylene, PVDF, aluminium and stainless steel. The pumps offer a wide range of resistant diaphragm materials.

Summary

Acids and alkalis can cause corrosion of metallic components and damage to pump seals, particularly at elevated temperatures. They must be safely contained within the pump - any leakages or spills could be dangerous for both workers and the environment. Particular attention must be given to the pump’s sealing system and, in many cases, seal-less magnetic drives should be employed.

Pumps made from nickel-based alloys have good resistance to corrosion but are expensive. Systems constructed from, or lined with, thermoplastics are a viable and economical alternative.

Manufacturers produce chemical compatibility charts which show the resistance of pump construction materials to various chemicals. It is important to refer to these when selecting a pump to ensure that all wetted parts are resistant to corrosion or other chemical damage.

Further advice

Please contact us for advice regarding pump selection for any difficult application. We have extensive experience of pumping many acids and alkalis.