50% caustic soda filtration: how to implement a custom self-cleaning filter

The company is one of the world leaders in the production of salicylic acid and its derivatives. The compounds it manufactures are used in particular by the pharmaceutical, cosmetics and food industries.

The company uses 50% caustic soda (and soda lye) to neutralise acids at certain stages of pharmaceutical compound synthesis. It wanted to filter the soda upon reception (delivered by rail tankers) to remove suspended solids. This unloading operation, lasting 2 to 4 hours, takes place daily.

The client wanted to use a self-cleaning filter to reduce maintenance on filtration equipment. The system had to meet the following operating requirements:

• Fluid to be filtered: Group I liquid – 50% caustic soda
• Maximum pressure: 2.85 bar
• Maximum temperature: 40 °C
• Maximum flow rate: 30 m³/h
• Maximum viscosity: 78 cPo
• Maximum density: 1,520 kg/m³

We recommended using a self-cleaning filter in 316L stainless steel with the following characteristics:

• Inlet/outlet: DN80 PN16 – raised face flanges
• Drain connection: DN50 PN16 – raised face flanges
• Closure: quick-opening door with swing bolts
• Basket: 316L stainless steel wedge wire strainer with a 500 µm filtration threshold
• Mechanism: shaft with rotating cleaning arm fitted with 4 brushes and 4 scrapers
• Sealing (FDA-certified): door O-ring in FPM encapsulated in PTFE, and basket and drain seal in FPM

We also selected several additional options:

1. Double-seal bearing, featuring a dual lip seal housing with leak detector and recovery between the two seals — providing robustness and ease of maintenance.
   The mechanism shaft was also modified to accommodate both lip seals, with the ball bearing positioned above them.
2. Specific materials used for the seal (EPROM), brush (PP–stainless steel) and scraper (PTFE–stainless steel)
   These were supplied with FDA certificates (seal certified by the seal manufacturer for EPDM, remaining items certified by ourselves).
3. Tubing fitted with flanges compliant with DIN standard

In addition to its technical performance (cleaning completed in 2.5 hours, whereas the specification required a duration below 4 hours), our solution was chosen primarily for its cost efficiency and ease of use.

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