PISTONS play an important role in the homogenizer, pressurizing the product in the pump block in the same way a bicycle pump creates pressure in tyres. The environment in which pistons operate is a demanding one, with pressure up to 630 bar and temperatures as high as 140°C.
A number of factors can affect how long a piston lasts: air in the product, water/steam quality and cleaning-in-place solutions can all reduce a piston’s lifetime by a quarter to a half. Wear or damage to the piston or compression ring, or product-related problems can shorten it by as much as 75%.
With a number of different materials to choose from – each with their own characteristics, benefits and drawbacks – how do you know which piston is right for your application?
Think process, think product
Nicole Uvenbeck, Line Product Manager at Tetra Pak, says the importance of choosing the right piston material is often overlooked. “Material selection is important for the running economy of your homogenizer. There are a number of factors to consider: the right piston can depend on your process – whether you are upstream or downstream – but also on the product you are making. Does it have particles that make it abrasive, or is it acidic?”
Your running schedule should also be considered. To stop production to change parts is costly and must also be considered when material is selected. As a rule, the longer piston lifetime you want, the more expensive material has to be used, and there is a break-even between price and lifetime.
At one extreme you can find cheap pistons on the market with a lifetime of only a few hundred hours. At the other end of the scale you have ceramic pistons from Tetra Pak which, when handled with care, will last for as long as 50,000 hours.
Ultimately, Uvenbeck says, for standard products like white milk the best advice is to start with one of the least expensive pistons. “If it wears out quickly, talk to your Tetra Pak representative about other pistons available from Tetra Pak,” she says. “If you get corrosion, select a case hardened piston. I wouldn’t advise anyone to start with the most expensive material.”
A solution for every application: Tetra Pak’s piston portfolio
Stainless steel: These are standard on a Tetra Pak Homogenizer. They are Ideal for milk and other non-abrasive products (abrasive products are those that cause wear and tear on parts, because they contain particles, for example).
Chromium plated: Last longer than stainless steel and have twice the surface hardness. There is a risk of corrosion if the cooling water contains high chloride levels.
Case hardened: Treated with a secret hardening procedure that achieves a hard surface, these pistons last long and will never corrode. Both case hardened and tungsten carbide coated pistons have higher surface hardness than chromium plated, but case hardened pistons are especially suitable for abrasive and corrosive products.
Tungsten carbide coated: Even more wear-resistant than chromium plated, but not recommended for products of low pH such as fruit juice due to the risk of corrosion. Also be careful with acidic cleaning in combination with high temperatures.
Solid ceramic: Many homogenizer suppliers offer solid ceramic pistons. However, bear in mind that there is a variety of ceramic materials and they vary in quality and endurance. Tetra Pak solid ceramic pistons have been developed to have optimal wear resistance balanced with the best possible mechanical properties. Therefore the material used in Tetra Pak’s solid ceramic pistons is more advanced than that used by other homogenizer brands.
Tetra Pak solid ceramic pistons are the Rolls Royce of pistons and will last for years in normal use. But because ceramic is brittle, they need to be installed properly and handled with care. When selecting solid ceramic pistons for a new homogenizer, a set of stainless steel pistons is also included with the order. The idea is that the stainless steel pistons should be used during commissioning, and then the solid ceramic can be installed when commissioning is completed and the production line is up and running smoothly.
Benefits of solid ceramic pistons:
- The surface hardness of solid ceramic pistons is more than three times that of stainless steel pistons.
- High wear resistance
- No risk for corrosion