Pumps for Corrosive Acids:
How to Choose the Right Material and Type
How to Choose the Right Material and Type
Transferring acids is one of the most critical operations in any chemical, electroplating or pharmaceutical plant. Choosing the wrong material or pump type does not simply result in mechanical failure — it can lead to hazardous spills, costly downtime and, in the worst cases, serious safety risks for operators.
The challenge is that no single pump works for every acid. Concentrated sulfuric acid at 98% demands a completely different approach than a dilute 5% hydrochloric acid solution. Temperature, concentration, the presence of suspended solids and required flow rate all influence the decision.
This technical guide covers the fundamental criteria for selecting the most suitable pump for each type of corrosive acid, comparing thermoplastic materials and construction types.
Why metal pumps fall short with acids
Stainless steel pumps (AISI 316) are often regarded as the “safe” choice for chemical duty. In practice, however, stainless steel has significant limitations when handling acids.
- Hydrochloric acid attacks stainless steel even at very low concentrations, causing pitting corrosion that rapidly deteriorates the impeller and pump casing.
- Concentrated sulfuric acid at temperatures above 50°C causes generalised corrosion even on the most resistant grades.
- Hydrofluoric acid is incompatible with any iron-based alloy.
Furthermore, metallic corrosion contaminates the pumped fluid with metal ions — an unacceptable problem in pharmaceutical manufacturing, potable water treatment and semiconductor production.
For these reasons, thermoplastic pumps represent the most reliable and often most cost-effective solution for handling corrosive acids.
The three thermoplastic materials compared: PP, PVC and PVDF
Polypropylene (PP)
the most versatile option
Polypropylene is the most widely used thermoplastic in the manufacture of pumps for corrosive fluids, and for good reason. It offers excellent resistance to most dilute acids, saline solutions and bases, with a maximum operating temperature of 80–90°C.
PP is the ideal choice for dilute acids and bases in non-oxidising environments, galvanic solutions based on dilute sulfuric acid, saline solutions and pickling baths, and processes where material cost is a critical factor.
The main limitation of polypropylene is its poor resistance to strong oxidising agents (such as concentrated nitric acid and high-concentration hydrogen peroxide) and to chlorinated organic solvents.
PVC
the low-temperature specialist
PVC offers chemical resistance similar to PP for many acids, with one specific advantage: it is the material of choice for sodium hypochlorite, a reagent widely used in water treatment and electroplating plants.
However, its maximum operating temperature is limited to 60°C, restricting its use to processes at ambient temperature or slightly above. PVC is not suitable for organic solvents and aromatic hydrocarbons.
Typical applications include fume scrubbers, containment tanks and sodium hypochlorite dosing circuits.
PVDF
superior chemical resistance
Polyvinylidene fluoride is the highest-performing thermoplastic for aggressive acid applications. The presence of fluorine atoms in the polymer chain ensures more stable chemical bonds, providing exceptional resistance to concentrated strong acids.
PVDF withstands sulfuric acid up to 98% concentration, hydrochloric acid at any concentration, concentrated nitric acid and strong oxidising agents. Its maximum operating temperature reaches 100°C, with good dimensional stability under load.
The higher cost compared to PP and PVC is justified by longer service life in extreme environments and reduced downtime for maintenance.
PVDF is the mandatory choice for magnetic drive pumps handling concentrated acids, high-temperature concentrated sulfuric acid transfer, pharmaceutical processes where purity is critical, and applications involving hydrofluoric acid.
Quick comparison table
Property | PP | PVC | PVDF |
Max. operating temp. | 80-90°C | 60°C | 100°C |
Dilute sulfuric acid | Excellent | Good | Excellent |
Concentrated sulfuric acid | Poor | Poor | Excellent |
Hydrochloric acid | Good | Good | Excellent |
Nitric acid | Poor | Poor | Good |
Sodium hypochlorite | Good | Excellent | Good |
Organic solvents | Poor | Poor | Fair |
Relative cost | Low | Low | High |
CNC machinability | Excellent | Buona | Buona |
Pump types for acids: which configuration to choose
Horizontal centrifugal pumps with mechanical seal
Horizontal centrifugal pumps are the most common type for transferring corrosive fluids. The fluid enters the impeller axially and is accelerated outward by centrifugal force, generating flow and head.
The mechanical seal is the most critical component: it is the interface between the rotating part (shaft) and the stationary part (casing), and must prevent any fluid leakage. Single seals are adequate for low-hazard fluids, while a double flushed seal adds a safety barrier with a barrier fluid between the two seal faces.
Nuova Darimpianti’s PMC series pumps feature casings and impellers machined from solid blocks of polymer on 3-axis and 5-axis CNC machining centres — a method that delivers tighter dimensional tolerances and higher internal pressure resistance compared to moulded pumps. The PMC-1 series is fitted with a single mechanical seal, while the PMC-2 series features a double flushed seal for maximum safety when pumping concentrated acids and hazardous liquids.
Magnetic drive pumps: zero leakage
Magnetic drive pumps completely eliminate the mechanical seal. Motion is transmitted from the motor shaft to the impeller through a pair of magnets separated by a containment shell. There is no physical contact between the drive side and the fluid.
This design guarantees absolute zero leakage — a decisive advantage when pumping concentrated acids, toxic solvents or fluids prone to crystallisation that would damage a traditional mechanical seal.
Nuova Darimpianti’s HTM series is a horizontal centrifugal magnetic drive pump available in PP, PVC and PVDF, designed specifically for strong acids, hazardous fluids and applications where safety is the top priority.
The limitations of magnetic pumps include sensitivity to dry running (which can demagnetise the magnets) and generally lower head than mechanically sealed pumps at the same power rating.
Vertical pumps: ideal for tank-mounted installation
Vertical pumps are installed directly on the edge of the tank or vessel, with the shaft and impeller submerged in the fluid. This configuration eliminates the mechanical seal problem entirely, as there is no fluid passage through external gaskets.
Nuova Darimpianti’s VSK series is a cantilever vertical pump without guide bushings: the absence of wetted sliding parts dramatically reduces wear, making it ideal for fluids containing suspended solids or abrasive particles.
Vertical pumps are particularly suited to galvanic tanks, acid storage vessels, scrubber recirculation systems and applications where floor space is limited.
Drum transfer pumps
For emptying drums and containers, portable and easy-to-handle solutions are required. PP and PVDF drum transfer pumps enable safe transfer of acids and bases from industrial drums without the need for tilting, reducing the risk of spills.
How to size the pump: key parameters
Selecting the right material and type is not enough — the pump must be correctly sized for the system’s operating conditions.
The key parameters are:
- flow rate, the volume of fluid to be transferred per unit time, expressed in litres per minute or cubic metres per hour
- head, the energy the pump must impart to the fluid to overcome the system’s friction losses and static height difference, expressed in metres of liquid column
- and NPSH (Net Positive Suction Head), the parameter that determines whether the pump can draw fluid without cavitation occurring.
Cavitation is a destructive phenomenon that occurs when suction pressure drops below the fluid’s vapour pressure, causing the formation and violent collapse of vapour bubbles on the impeller. In a plastic pump, cavitation can erode the impeller very rapidly.
A common mistake is sizing the pump based solely on motor power in HP or kW. The critical parameter is actually the torque transmitted to the shaft, which determines the pump’s ability to overcome fluid resistance. For viscous or dense fluids, a pump with a powerful motor but insufficient torque will not perform correctly.
Quick selection guide: which pump for which acid
Per semplificare la scelta, ecco le combinazioni materiale-tipologia più comuni per i principali acidi industriali:
- \Sulfuric acid (H₂SO₄):
For concentrations up to 70% and temperatures up to 80°C, the choice is PP with a horizontal centrifugal pump (PMC series). For concentrations above 70% or elevated temperatures, PVDF is necessary, preferably with a magnetic drive pump (HTM series) to eliminate leakage risk.
- \Hydrochloric acid (HCl):
At any concentration, PVDF offers the best resistance. For dilute solutions at ambient temperature, PP is a valid and economical alternative. The vertical pump (VSK series) is ideal for direct suction from tanks.
- \Nitric acid (HNO₃):
As a strong oxidiser, nitric acid attacks PP. PVDF is mandatory for concentrations above 40%. For dilute solutions, PVC may be adequate up to 60°C.
- \Sodium hypochlorite (NaClO):
PVC is the preferred material for sodium hypochlorite. Vertical pumps for dosing from storage tanks or horizontal PMC pumps for distribution circuits.
- \Hydrofluoric acid (HF):
Requires exclusively PVDF with a magnetic drive pump (HTM series) for maximum safety. Hydrofluoric acid is highly toxic and no leakage is acceptable.
The advantage of solid-block CNC machining
An often overlooked aspect when choosing an acid pump is the manufacturing method of the pump casing. Most manufacturers use injection moulding or rotational moulding — processes that can introduce internal stresses in the material and non-uniform wall thickness.
Nuova Darimpianti manufactures pump casings by solid-block machining: every component is machined from a solid block of polymer on 3-axis and 5-axis CNC machining centres.
This method guarantees no residual internal stresses, millimetre-precision dimensional tolerances, uniform and controlled wall thickness throughout the pump casing, and higher internal pressure resistance compared to moulded parts.
The result is a stronger, more precise pump with a longer service life — particularly in applications with concentrated acids where chemical stress is at its highest.
Frequently asked questions
Which material is better for sulfuric acid: PP or PVDF?
It depends on the concentration. PP performs well up to approximately 70% at ambient temperature. For higher concentrations or elevated temperatures, PVDF is essential due to its superior chemical resistance to strong oxidisers.
Can magnetic drive pumps run dry?
No, dry running is the main risk for magnetic pumps. The absence of fluid to lubricate and cool the internal bearings can cause overheating and demagnetisation of the magnets. Dry-run protection devices are essential.
How often should the mechanical seal be replaced on an acid pump?
Seal life depends on the fluid type, temperature and duty cycle. Under typical conditions, a mechanical seal on an acid pump lasts between 6 and 18 months. With the PMC-2 double flushed seal, the barrier fluid significantly extends seal life.
What happens if I choose the wrong material?
Chemical corrosion can manifest as polymer swelling, stress cracking, loss of mechanical strength or, in the worst cases, sudden casing failure with fluid spillage. This is why consulting chemical compatibility charts before selecting the material is essential.
Can Nuova Darimpianti build custom pumps?
Yes. Thanks to solid-block CNC manufacturing, Nuova Darimpianti builds pumps in custom configurations for material, dimensions, connections and accessories. Every pump can be engineered to the specific requirements of your plant.
Choose the right pump for your process
Need technical advice on choosing the right pump for your process?
Contact our engineers for a personalised quotation.
Correct selection of a pump for corrosive acids requires careful analysis of four factors: the type of acid and its concentration, the operating temperature, the required flow rate and head, and the safety level demanded.
Nuova Darimpianti designs and manufactures horizontal centrifugal pumps (PMC-1 and PMC-2 series), magnetic drive pumps (HTM series) and vertical pumps (VSK, VGA, VL series) in PP, PVC and PVDF, all machined from solid blocks using CNC technology.