By Ashley Shepherd, UK Sales Manager, Watson-Marlow Pumps Group

While few dispute that positive displacement (PD) pumps remain the preferred choice for metering highly caustic and corrosive chemicals, a new trend is emerging regarding pump type selection. Historically diaphragm and progressive cavity (PC) pumps have dominated this area of the market, but today, increasing numbers of process engineers are turning to peristaltic technology which has become intrinsically suited to dosing, metering and transferring chemicals such as hydrochloric acid, sodium hydroxide, sodium hypochlorite and sulphuric acid, to name but a few.

Selection criteria
The challenge for process engineers is finding a pump that can withstand chemical attack and run reliably; can meter accurately to optimise chemical usage; and is quick and simple to maintain and operate. Meeting all three requirements, the latest peristaltic pumps are helping increasing numbers of plants reduce life cycle cost and drive gains in process efficiency.

A well known manufacturer of engineered plastics now uses a  peristaltic hose pump to meter hydrochloric acid precisely in the production of linear polyphenylene sulphide. Thanks to the pump’s metering capability, the plastics manufacturer does not require separate meters and valves to measure and control flow. Instead, feed rate can be adjusted simply by altering the pump speed using a variable frequency drive.

Elegantly simple
Based on the physiological principle of ‘peristalsis’, a term referring to the alternating contraction and relaxation of muscles around a tube (e.g. throat or intestine) to induce flow, a peristaltic pump’s operation is elegantly simple. A flexible tube or hose element is compressed between rotating rollers (tube pump) or shoes (hose pump) and a track. Between each roller or shoe pass, the tube or element recovers to create a vacuum and draw in fluid.

It is well documented that acids, caustics and solvents attack the valves, seals, stators and moving parts of diaphragm and progressive cavity pumps, causing disruptive downtime and high life cycle costs. By contrast, the use of peristaltic pumps allows engineers to mitigate these costs as they contain no valves, seals or glands and have no mechanical parts in the product stream. The fluid only contacts the inside of a hose or tube element, which is a low cost, low maintenance and easily serviceable component.

Hose and tube materials, which in years gone by were the only prohibitive factor standing against the widespread adoption of peristaltic pumps for caustic chemical applications, are today available in many different elastomers, formulated specifically to balance long mechanical pumping life with sustained resistance against concentrated acids, bases and solvents.

For example, an elastomer called Hypalon available for hose pumps, can handle toluene and other corrosive solvents used in the manufacture of products such as gaskets, aerosol spray paints, wall paints, lacquers, paint strippers, adhesives, printing ink, spot removers, cosmetics, perfumes and anti-freeze.

The latest chemical duty elements from my own company are capable of thousands of hours’ continuous service operating at flow rates up to 1325 l/min and 16.5 bar pressures. Furthermore, peristaltic pumps have the ability to self-prime, dry-prime and self-clean (they are reversible to dislodge blockages or drain lines).

Self-cleaning functionality makes peristaltic pumps attractive to industries such as paper and pulp. For instance, a peristaltic pump is being used to handle sodium hydroxide for bleaching pulp and disinfecting potable water at a high profile paper mill. Because the hose is self cleaning, it does not experience the crystallisation that builds up in other PD pumps, which can destroy seals.

Precise metering
The inherent accuracy of PD pumps is another reason they are chosen when exact chemical metering or dosing is required. In peristaltic pumps, where metering accuracy is better than 0.5%, flow is proportional to pump speed. Here, complete element closure gives the pump its positive displacement action, preventing flow drop or erosion from backflow and eliminating the need for check-valves, which are typically the primary source of metering inaccuracy.

Accuracy is particularly important at power plants, as demonstrated by a power generation servicing company that is using Watson-Marlow peristaltic pump technology to meter
246 l/min of sulphuric acid when cleaning out power plant condensers – driving pH down to less than 4.0 in order to break down scale. Precise control of acid flow is necessary to ensure that, after scale is dissolved, the discharge to the municipal wastewater system will have a neutral pH.

Conversely, diaphragm pumps have internal valves that can stick, clog and seat incorrectly, causing flow variation that can jeopardise product integrity.

With regard to turndown ratio, a high quality peristaltic pump is capable of a flow range greater than 2000:1 simply by controlling rotor speed. With the added versatility of integrating different tube sizes into a single pump, flow range expands to 1000000:1. Diaphragm pumps are normally limited to 20:1.

Low life cycle costs
While the initial cost of a peristaltic pump can be slightly higher than other PD pumps, a quick assessment of associated life cycle costs quickly tips the calculation in favour of peristaltic pumps. For instance, there are no expensive seals, ball valves, rotors or stators to replace, while hose/tube replacement usually takes only a few minutes. Furthermore, the low cost tube or hose can be replaced in-situ and without the need for special tools or skills, making the process extremely economical in comparison with conventional PD pumps where replacement parts can cost up to 75% of the pump’s initial purchase price and take several hours to fit.

Another added cost for many PD pumps is the necessity for a separate control panel or variable frequency drive to achieve variable flow metering, with incremental cost if high turndown is required. Peristaltic pump manufacturers however, build high turndown, closed loop speed control capability and expansive I/O connections for DCS, SCADA and PROFIBUS systems into standard pumps to simplify any required integration.

In fact, my own company is taking process-duty pumping to the next level by pioneering integrated PROFIBUS networking capabilities with the introduction of its Bp pump. With two-way, real-time communications, the Bp range offers increased diagnostic capability and faster response, helping optimise process control and minimise plant downtime.

Pumping abrasive fluids
Frequently, fluids contain not only corrosive, but also abrasive material. Here, peristaltic pumps stand up well to the challenge, as a US-based construction product manufacturing plant can verify thanks to its use of  peristaltic pumps in the manufacture of fibre cement siding products. The cement mixture has little effect on the hose, despite being highly abrasive and strongly alkaline.

Peristaltic pumps are also used for other abrasive fluids, including lime slurry and underflow in mining operations, alum in wastewater treatment and titanium dioxide for pigmented inks and paints.

In contrast with peristaltic pumps, abrasion takes a toll on other PD pumps. Abrasive fluids cause erosion or clogging of valves in diaphragm pumps, while the same effect in progressive cavity pumps widens clearances between the rotor and stator, causing internal slip.

First choice technology
The advantages peristaltic pumps offer mean they represent a rapidly growing percentage of the PD pump market – my company alone has now sold over a million peristaltic pumps worldwide. Plant managers tasked with reducing pump life cycle costs are increasingly embracing the functionality and benefits of peristaltic pumps, which are fast becoming first choice for all chemically aggressive and abrasive applications.


Watson-Marlow Bredel Pumps       
Falmouth                    
Cornwall

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