Pipe Material for CIP Chemical in a Dairy Plant - 304 versus 316

Cleaning in place (CIP) has become standard practice in almost all dairy, beverage and processed-food production plants brought about the ever increasing pressure to increase uptime of the process. Over the past 10–15 years, the industry technology has seen increased demands from customers in terms of CIP verification and validation to provide improvements in plant hygiene, finished product quality, and related shelf-life and microbiological considerations.

The cleaning process will usually consist of a series of discrete stages or cycles, generally including:

• Removal of Gross Debris - refers to the removal of any residual product by mechanical means prior to introduction of a water rinse. It generally involves draining product from the system to be cleaned under gravity or physically displacing the product using various media, such as compressed air, water or a mechanical pigging device. Product recovery systems are becoming more sophisticated with the introduction of membrane plants that are designed to remove high levels of water from the effluent stream to allow the recovered solids to be sold for re-processing, reducing effluent loading and can form part of site pollution prevention and control (PPC) systems.
• Pre-Rinse - utilizing "recovered water" from the previous intermediate rinse stage, to reduce total water consumption (and effluent generation) and utilize any residual detergent solution carried into the recovered rinse tank during the rinse recovery stage. It may be heated to provide a greatly enhanced method of product residue removal
• Detergent Recirculation - where the main cleaning takes place, resulting in the soil being lifted from the plant surface and held suspended or dissolved in the detergent solution. Temperatures, Concentrations (or the use of more sophisticated / expensive detergent formulations), flow and duration is set by experimentation and experience
• Intermediate Rinse - removing all traces of the first detergent and any entrained soil from the plant being cleaned and in a partial recovery situation, to recover for reuse as the pre-rinse for the next cleaning cycle
• Second Detergent Recirculation - occasionally, an acidic detergent is used after an alkaline product in the first detergent stage. This removes the residual thin layer of mineral scale (usually of calcium or phosphates) deposit left behind by the first detergent cleaning
• Final Rinse - using cold potable good quality water to remove the remaining solution and preparing the line for production
•  Disinfection - using an oxidizing biocide or hot water / steam

Turbidity or conductivity sensors in the return line are normally used to control the sequencing, along with timers in the program acting as fail safe.

CIP of relies substantially on the action of chemicals on the soilage. Carbohydrates and proteins is removed by alkali solution. Fats and oils are insoluble in water and normally heated by the water temperature and then solubilized by the alkali with the help of polyphosphates to emulsify. Mineral deposits formed when milk is heated during the process, need to be dissolved by acids. The alkali most commonly used is sodium hydroxide with silicates and wetting agents added to inhibit its chemical attack on the equipment and to improve rinsing. Other additives may also be added to enhance protein and fat removal, sequestering, wetting, etc. The acid most commonly used was phosphoric acid because of its effectiveness, relative safety, and less corrosiveness versus other mineral (inorganic) acids. Additives (i.e. wetting agents) are often included in the formulation. Organic acids (e.g. citric, glyconic, gluconic, hydroxyacetic, etc.) are preferred as they are milder, relatively safer and less corrosive but widespread use is limited due to much higher costs.

However, if a factory processing dairy (milk) discharge treated process wastewater to surface water, it needs to comply with legal phosphorus limits levels. Phosphorous is one of the major nutrients contributing in the increased eutrophication of lakes and natural waters and controlling phosphorous discharged from the wastewater treatment plants is a key factor in preventing eutrophication of surface waters. For plants using phosphorus based cleaners (i.e. phosphoric acid), the CIP acid solution going to the sewers forms forty to fifty percent of the load with the remainder from product loss to the sewer.

Phosphorus can be removed in the waste water treatment plant by biological uptake or chemical precipitation. The use of non-phosphorus based substitutes is encouraged to help lower the discharge concentration. Nitric acid is an acceptable (and cost competitive) alternative. Through cleaning chemical substitution and control of product losses, most facilities should be able to reduce the phosphorus to a level of 15 to 20 mg/l (this is still 3 to 4 times the level in domestic wastewater), in the untreated effluent.

Assuming the decision is made to switch to nitric acid with a new piping system, the question is now posed as to what stainless steel material is better suited for the piping system considering caustic and nitric acid as the base cleaning chemical. Although other materials (e.g. plastics) can be used, particularly for the caustic, stainless steel is preferred due to risks mitigation.

There are many issues involved in the question of the advantages and disadvantages of 304 and 316 components. One of the basic reasons why we use stainless steel is because of their resistance to corrosion. The prevailing stainless steel materials being used are either SS-316 or SS-304. SS-316 stainless steel has 2% to 2.5% molybdenum compared to 304, which has about 0.5%. SS-316 has slightly more nickel and slightly less chromium than 304. These slight differences in chemical composition result in 316 stainless steel being substantially more resistant to corrosion and chloride pitting as compared to SS-304. Thus, it is often specified blindly if the application calls for better corrosion resistance and its being more expensive is not a problem.

However, there are exceptions and the Molybdenum additions, as found in SS-316 do not improve resistance to corrosion by nitric acid. For this reason, it is better to use type 304 for nitric acid up to the boiling point. The tabulation below is excerpted from the “Corrosion Resistance of the Austenitic Chromium Nickel Stainless Steels in Chemical Environments” report by The International Nickel Company and shows the corrosion rate for the different grades of stainless steel in nitric acid. Unmodified chromium-nickel stainless steels, when they contain precipitated carbides resulting from sensitization in the temperature range 800-1650 F, are susceptible to inter-granular attack in nitric acid. For this reason the columbium-stabilized alloys, Types 347, 309Cb, and 310Cb, are used in equipment fabricated by welding.

 

Consideration should also be noted on the effect of internal stresses, as these will influence the “stress” corrosion & surface cracking in conjunction with the chemical stress corrosion. As shown on the above tabulation, 316 steel can have marked differences (508 vs. 13,716 μmpy) in corrosion rates depending on the thermal treatment.

For caustic application, SS-316 is of course more resistant to corrosion than SS-304. However, the difference is insignificant even for highly concentrated solutions as can be seen in the tabulation (excerpted from the “Corrosion Resistance of the Austenitic Chromium Nickel Stainless Steels in Chemical Environments” report by The International Nickel Company) below and SS-304 is first choice for the application.

 

 

 

Thus, it is actually more advantageous to use SS-304 materials for the chemical handling from the delivery to the CIP chemical storage tanks and from there to the chemical detergent tanks based on corrosion rates and costs.

For materials in product contact, SS-316 is justified and commonly specified. The reason here is that food (in this particular case dairy) will form organic acids and contains salts / minerals that is better resisted by SS-316. Plus, the product being handled is FOOD and needs to be corrosion / contamination free.

It should also be noted that chromium-nickel stainless steels contain precipitated carbides and are susceptible to inter-granular attack in nitric acid. For this reason, the lower carbon 'variants' (e.g. 316L) were established as alternatives to the 'standards' carbon range grade to overcome the risk of inter-crystalline corrosion. If the carbon level is below 0.030% then this inter-crystalline corrosion does not take place, especially for the sort of times normally experienced in the heat affected zone of welds.

In order to prevent the chromium from forming carbides, Niobium is added to austenitic grades of stainless steel. This results in the “Cb” grades (e.g. 309Cb, 316Cb) of the low carbon stainless steels.