Every plant that uses acids or alkalis has to move them somewhere. Sulphuric acid leaves a bulk tank and ends up in a neutralisation pit; caustic soda is drawn from storage and metered into a pH control loop; ferric chloride is pumped from an IBC to a flocculation tank. Chemical dosing and transfer systems are the pumps, tanks, pipework, valves and instruments that do this work. Specified well, they run for years with little attention. Specified badly, they produce a steady stream of failed seals, cavitating pumps, corroded floors and operators exposed to acid fumes.
This guide is written for process engineers, plant managers and buyers who need to specify, upgrade or replace a system for withdrawing, transferring and dosing corrosive liquids. It covers the main components, how to match materials to the chemical, the design choices that keep leaks under control, and the data you need before asking for a quote. The last section explains how Nuova Darimpianti, based in Flero near Brescia, Italy, designs and builds these systems to order, from a single pump to a complete skid.
What chemical dosing and transfer systems do
Three functions are usually involved, often in the same installation:
- Withdrawal: taking liquid out of a storage tank, pit or drum.
- Transfer and recirculation: moving it to a day tank, a process vessel or a treatment line, or keeping it circulating through a filter, heat exchanger or scrubber.
- Dosing: injecting a controlled quantity of chemical into a process, for example acid for pH correction, caustic for neutralisation, or coagulant and polyelectrolyte for water treatment.
A typical layout therefore includes a storage tank, a transfer pump, sometimes a make-up tank with an agitator where the chemical is diluted or dissolved, a dosing pump, the valves and instruments that protect each item, and secondary containment around the whole lot. Where the chemical gives off fumes, tank vents are routed to an abatement system.
The difference from an ordinary water circuit is the fluid. A concentrated acid attacks metals, swells the wrong elastomers and turns a small drip into a safety incident. Every wetted part has to be compatible with the chemical, and the layout should keep the number of potential leak points as low as possible.
The main components
Withdrawal from tanks: vertical immersion pumps
For drawing liquid from an open-top tank or pit, the usual answer is a vertical centrifugal pump. The pump casing sits submerged in the liquid while the motor stays above the rim, so the pump is always flooded, needs no priming and has practically no suction line. That removes the most common cause of cavitation (suction-side losses) along with a whole set of flanges and fittings that could leak.
Nuova Darimpianti builds several vertical pumps entirely in thermoplastic, with no metal parts in contact with the pumped liquid:
- The VSK vertical pump is made of PP, PVC or PE for temperatures up to 80 °C. Its cantilever motor shaft has no connections and no shaft guide, which keeps maintenance very simple.
- The VGA vertical pump is available in PP, PVC or PVDF for temperatures up to 90 °C, with a ceramic bush guiding the shaft end.
- The VL/VLK vertical pump has a mechanical support with aluminium lantern and bearing, an elastic coupling between motor and pump and a ceramic guide bush. Above 2000 mm length it uses a double ceramic guide, which makes it the model for deep tanks.
The vertical range also handles liquids carrying solids or with moderate abrasiveness, which is common in neutralisation pits and spent plating baths.
When large volumes of corrosive liquid have to be recirculated at low head, for instance on a scrubber or during bulk decanting, the EVFA-N external vertical flushed pump is worth considering. Built in polypropylene, it runs with a low-speed motor (1450 rpm) suited to continuous duty, and its mechanical seals are flushed with clean water so they stay in good condition even with very aggressive liquids. A single-seal version without external flushing is also available.
Transfer and recirculation: horizontal centrifugal pumps
If the pump must sit outside the tank, for example to feed several users from one storage tank or to draw from a closed vessel under flooded suction, a horizontal centrifugal pump is used. The key decision is how the shaft is sealed.
The HTM magnetic drive pump, in PP or PVDF for temperatures up to 90 °C, has no mechanical seal at all. Torque reaches the impeller through a magnetic coupling across a sealed separator, so there is no dynamic seal to wear and leak. It is the natural choice for hazardous chemicals where no emission is acceptable; our guide to magnetic drive pumps covers the trade-offs in detail.
The PMC-1 and PMC-2 horizontal pumps use a mechanical seal, single or double flushed; the PMC-2 is a close-coupled design for high flow rates. A double flushed seal puts a barrier between the process fluid and the surroundings.
Dosing and flow control
Accurate dosing is normally done with positive displacement metering pumps (diaphragm or piston), which deliver a fixed volume per stroke and are adjusted by stroke length or frequency. For larger, less critical flows, a centrifugal pump on a variable speed drive can do the job, with the control loop closed on a flow meter or directly on the process measurement, such as pH.
Whatever the technology, three points matter when dosing acids: chemical compatibility of the pump head and check valves, stable delivery across the range of back-pressures, and a pressure relief valve on the discharge. The metering pump itself is selected project by project, according to the chemical, the flow range and the injection point.
Tanks and make-up vessels
The tank is the largest item in most systems. Nuova Darimpianti manufactures polypropylene tanks reinforced with steel frames, including large-capacity units, intended for containing highly corrosive liquids.
When a chemical has to be diluted, dissolved or kept uniform, the make-up tank needs an agitator. Nuova Darimpianti’s mixers for corrosive environments are used in polyelectrolyte preparation, flocculation, neutralisation and physical-chemical water treatment plants, among other duties.
Valves, pipework and instruments
A dependable system needs at least:
- isolation valves on both sides of each pump, so it can be taken out of service safely;
- check valves on the discharge and, on dosing lines, back-pressure and relief valves;
- level indication or transmitters on each tank, with a low-level alarm to protect pumps from running dry and a high-level alarm to prevent overfilling;
- pressure gauges fitted with diaphragm seals, so the corrosive fluid never reaches the instrument;
- pipework and fittings in the same thermoplastic as the pump, or another compatible material, properly supported and allowing for thermal expansion.
Matching materials to the chemical
Chemical compatibility comes first. Polypropylene (PP), PVC and PVDF are the three thermoplastics found most often in dosing and transfer systems for corrosive fluids, and each has its own operating window. The table below gives common general guidance; final selection must always take concentration, temperature and any oxidisers or impurities into account.
| Chemical | Materials usually suitable | Design notes |
|---|---|---|
| Hydrochloric acid | PP, PVC, PVDF | Fumes readily: pay attention to vapour seals and tank venting |
| Dilute sulphuric acid | PP, PVC | Consider PVDF at higher concentrations |
| Concentrated sulphuric acid | PVDF | Dilution releases a lot of heat: design the mixing step carefully |
| Sodium hydroxide (caustic soda) | PP | PVDF is not always suitable with hot strong alkalis: check case by case |
| Sodium hypochlorite | PVC, PVDF | Oxidising: check elastomer compatibility |
| Ferric chloride, polyelectrolytes | PP, PVC | Typical in water treatment; watch for crystallisation |
| Nitric acid | PVDF | Strong oxidiser: polyolefins are limited at higher concentrations |
Each pump also has its own declared temperature limit, for example up to 80 °C for the VSK in PP, PVC or PE and up to 90 °C for the HTM in PP or PVDF. For a full comparison, read PP vs PVC vs PVDF: choosing the right material for chemical pumps and agitators.
O-rings and gaskets deserve the same care. As a rule of thumb, FPM suits many acids, EPDM suits alkalis, and PTFE is the safest all-round choice when the chemical changes or compatibility is uncertain.
Designing out leaks and exposure
Safety is far cheaper to build in at the design stage than to retrofit. The principles are straightforward.
Fewer leak points. Every flange, fitting and seal is a potential leak. Vertical immersion pumps eliminate the suction line; magnetic drive pumps eliminate the mechanical seal. Where a seal is unavoidable, a double flushed seal adds a second line of defence.
Secondary containment. Tanks and pumps should stand in a bund sized for the largest tank, made of or lined with a compatible material. For particularly hazardous chemicals, double-wall tanks with interstitial leak monitoring are common practice. Exact requirements depend on local regulations and on how the substance is classified.
Pump protection. Low-level alarms, dry-running protection and relief valves on positive displacement circuits prevent the most serious failures. A pump damaged by running dry is often where a leak starts.
Fume control. Hydrochloric acid, nitric acid and many plating baths release corrosive and toxic vapours. Tank vents and extraction hoods should be ducted to an abatement system. Nuova Darimpianti builds static-bed and floating-bed abatement scrubbers, from design through to installation.
Sizing: the data you need
Collect this information before requesting a quotation:
- Chemical and concentration, with the safety data sheet, any solids or crystals, and viscosity.
- Temperature: minimum, normal and maximum.
- Flow rate for withdrawal or transfer, and for dosing the turndown range required.
- Head: elevation differences, pipe length and diameter, pressure at the injection point. Our guide to pump head and flow calculation walks through the method.
- Tank geometry: depth, minimum draw-off level, nozzle arrangement. This sets the immersion length of a vertical pump.
- Duty: continuous or intermittent, number of starts per hour, need for a standby pump.
- Installation: indoor or outdoor, hazardous area classification, available footprint, power supply.
A common mistake is oversizing “to be safe”. In transfer duty this pushes the pump away from its best efficiency point, with more vibration and faster wear. In dosing it leaves the metering pump running near the bottom of its range, where accuracy is poorest.
Typical applications
| Industry | What the system does | Typical components |
|---|---|---|
| Water and wastewater treatment | pH correction, flocculation, neutralisation | Make-up tanks with agitators, dosing of acids, alkalis and polyelectrolyte |
| Electroplating and surface finishing | Bath top-up and recirculation, transfer to effluent treatment | Vertical immersion pumps, magnetic drive pumps, filters |
| Chemical processing | Reagent storage and transfer, reactor feed | PP tanks, horizontal pumps with double seals or magnetic drive |
| Air pollution control | Recirculating scrubbing liquor, reagent dosing | High-flow, low-head pumps, dosing of neutralising agent |
How Nuova Darimpianti builds custom systems
Alongside individual components, Nuova Darimpianti designs and builds complete skid-mounted plants for treating chemicals and water. Each skid is designed to the customer’s specification, on a stainless steel, painted iron or plastic-coated frame, and comes complete with pumps, tanks, agitators, filters, heat exchangers and pipework, with an electrical control panel on request. Because the system arrives pre-assembled, site work is reduced to a few connections.
Projects start in the in-house engineering office, which takes them from specification to construction, including systems for conveying and pumping aggressive and hazardous products. With 3- and 5-axis machining centres, lathes and milling machines, pumps, agitators and custom parts are machined from solid thermoplastic.
The wider special equipment range covers PP, PE, PVDF and PVC heat exchangers, filter units, oil separators and abatement scrubbers. Once the system is running, the in-house workshop provides routine and extraordinary maintenance on the pumps, agitators and parts supplied.
Frequently asked questions
What is the difference between a transfer system and a dosing system?
A transfer system moves liquid from one tank, pit or drum to another point, and its main requirement is flow. A dosing system injects a controlled amount of chemical into a process, and its main requirement is accuracy. In most plants the two are linked: a transfer pump fills a day tank, and the metering pump draws from it.
When should I use a vertical pump to withdraw chemicals from a tank?
Whenever you are drawing from an open-top tank or pit. A vertical pump works with its casing submerged, needs no priming and has no suction line, which reduces both cavitation risk and the number of leak points. Models such as VSK, VGA and VL are chosen according to tank depth, chemical and temperature.
Which material is best for dosing sulphuric acid?
It depends on concentration and temperature. PP and PVC are commonly used for dilute solutions, while PVDF is generally the safer choice for concentrated acid. Elastomers should be checked just as carefully as the pump body.
How do I prevent leaks in an acid transfer system?
Reduce the number of leak points (vertical pumps, magnetic drive pumps, double flushed seals), install tanks and pumps inside a bund, protect pumps from dry running with level alarms, and fit isolation valves so maintenance can be done safely.
Does Nuova Darimpianti supply complete systems or only components?
Both. Pumps, agitators, tanks and other components are available individually, and the company also designs and builds complete skid-mounted plants for chemical and water treatment, with an electrical panel on request.
Conclusion
Chemical dosing and transfer systems for corrosive fluids work best when every decision starts from the chemical: wetted materials, pump and seal type, containment and fume handling. Sourcing pumps, tanks, agitators and skid assembly from a single supplier also cuts down the interfaces you have to manage.
Planning a new chemical dosing or transfer system, or upgrading an existing one? The engineering team at Nuova Darimpianti can help you select pumps and materials and design the complete installation. Contact us for a no-obligation technical consultation.