A magnetically coupled plastic centrifugal pump with full plastic housing from MUNSCH. (Pictures: MUNSCH)
Today’s chemical process industry is hard to imagine without pumps from plastic materials. However, today a plastic pump does not always mean a tiny centrifugal or dosing pump. Today’s centrifugal plastic pumps have become almost equal with their metallic relatives. Very strict safety standards are in place for almost all equipment at chlorine production sites – for good reasons. Read on to learn how non-metallic pumps have been able to capture this application and what the crucial factors are when selecting pumps for this application.
Pure metals for high demands
Varieties of fluids in chlorine production plants are highly corrosive and in addition, the production process requires a high level of purity. This has never made it an easy task for equipment manufacturers, including pump suppliers. Areas where elementary (wet) chlorine is present require equipment from pure or palladium stabilized titanium. The associated constraints resulting hereof are not just economical. Purchasing and production of goods in these materials requires vast expertise and specialization and pump suppliers are rare in this category.
Fully fluorinated plastics such as PFA or PTFE were always known to withstand the chlorine in the electrolyzer cells. Eventually the membranes – key elements for the process – are partially made of it. However, plastic and plastic lined pumps were originally banned from these places to prevent any risk of permeating chlorine from inside the pump and creating a subsequent flow of iron ions from the metal exterior to the electrolyzers where it would damage the Cathode and reduce the overall efficiency.
Concepts of plastic pumps
Pumps from titanium (for anolyte) and from pure nickel (catholyte) have become increasingly difficult to find on the world’s pump market. A very volatile pricing and a long delivery times makes maintenance an expensive and unpredictable task. Hence, growing availability and quality of plastic pumps has been the key factor for them to take over most of the critical pump places in today’s alkali chlorine electrolysis plants.
While some pump manufacturers are specialized in the concept of lining, others are specialists in solid plastics. In lining, the liquefied plastic (i.e. PFA) is applied and mechanically cramped to the inside of the pump’s metal parts. From a manufacturer’s point of view, this production method offer advantages in terms of cost efficiency and production quantity. Usually, lined pumps will have a wall thickness between 3 to 5 millimeters.
Manufacturers of solid plastic pumps produce stable, thick plastic parts – often 10 mm or more in thickness. In order to withstand high pressures even at high temperatures these pumps normally do also have a cast iron armor. Now, why does wall thickness matter?
Safety against permeation
When dealing with fluids that contain elementary chlorine wall thickness is a crucial factor. Both chlorine and water vapor are able to permeate through fluorinated plastics, independent of its type.
The speed of permeation is substantially influenced by pressure and temperature – both parameters that cannot be negotiated in chlorine production processes. The only way left to protect the pump from being corroded on its metallic exterior and to protect the cathode from being contaminated is – you guessed it – wall thickness. Common literature does not provide any recommendations about wall thickness of pumps in alkali chlorine electrolysis. Furthermore, pressures do vary within the pump, making it even harder to provide general advice. Experience with anolyte and catholyte pumps have shown that PFA or PTFE layers of 7 mm or more are effective in preventing corrosion during normal operation periods (2-3 years).
Seal concepts · safety concepts
Mechanical seals as well as magnetic drives have been proven safe in chlorine alkali electrolysis. Now, which one is superior? In fact, that question will stay unanswered but we will provide you with some crucial information that could help answer your question on which seal concept will be the most appropriate for your application.
The pump size: Non-metallic centrifugal pumps with magnetic drive are available for flow rates up to 600 m³/h (2’600 GPM). However, suppliers for mag drive pumps of this size are very limited and one should consider this in terms of maintenance, operational safety and support. Even if the initial investment represents just a fraction of the overall pump cost, it does play a major role as budgets, not only in maintenance, usually have limits. However, magnetic force is costly. 50 to 150 kW of power transmission normally have a serious impact on the pump price. Pumps with double mechanical seals are less expensive, are available for even larger flows and offered by more than just a few suppliers.
Operating mechanical seals: Dangerous fluids such as chlorinated brine definitely require a double mechanical seal for safety. In general, these seal concepts consists of two dynamic seals on a common shaft. A barrier fluid is applied in between the two seals with pressure to make sure that in case any of those seals fails it is only the barrier liquid getting mixed with the pumped fluid or escaping the pump but never the dangerous fluid. You have probably guessed it – the barrier fluid needs to be circulated and monitored properly to achieve the required level of safety here. Running a double mechanically sealed pump requires significant effort and knowledge (including operators) and should be considered when doing pump selections.
Character of damages: Most centrifugal pumps, regardless of their concept, can be protected with reasonably efforts. However, pump manufacturers often learn the opposite. While no or inadequate liquid supply to mechanical seals sometimes causes severe damage to the surrounding plastic parts the same scenario could easily create a total loss for a mag drive pump. A thorough risk assessment is truly worth the time as a large mag drive pump sometimes costs as much as a mid-range car.
Modular, growing pump concepts
The decision, which pump concept is the best fit for the individual circumstances has to be made on site. Some pump manufacturers offer flexible concepts with standardized concepts in sizes or seals. This can help operators a lot, if demands in chlorine capacities are varying and flows have to be adapted.
Best and safest solutions will be achieved if process engineers, pump supplier, operators and planners are involved. “We often learn that pump protection was already incorporated in the plant concept but not being used as such” says Frank Pauly from plastic pump specialist Munsch.
Everything but sensitive
Plastic pumps are everything but sensitive. For a pump in the caustic evaporation plant (NaOH, 180°C) a special heavy-duty PFA mag drive pump was selected. Non-metallic pumps, especially those with mag drive, were banned for decades as the high temperature and the (magnetic) nickel content were considered a too big risk. Together with operators and maintenance team on site Munsch engineers developed a concept for safe operations and cleaning that prevented damages to this pump from day one.
Munsch centrifugal pumps from solid plastics are available with various options as mag drive as well as with mechanical seals and can be found in chlorine production sites globally. The German plastic pump specialists have learned that operational safety and reliability do always go in hand with a close co-operation of pump supplier, operator, process engineers and maintenance. Together with the members of Star Pump Alliance, the toolbox has been extended for further solutions.