For the U.S. food, beverage, and pharmaceutical industries, which typically base their verification procedures on 3-A SSI sanitary standards, the situation brings up an important question: Should end users recognize EHEDG as equivalent to 3-A on hygienic-approved flow measurement equipment?
Liquid flow measurement is an inherent aspect of many production processes in the food and beverage industries. For some manufacturers, the ability to obtain accurate flow measurements can make the difference between profit and loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious—even disastrous—results.
Many industries require a “sanitary” condition for their flow measurement equipment. In this case, the word sanitary refers to a highly clean and hygienic condition. The design requirements and specifications for sanitary flowmeters originated in the dairy industry, where the handling and packaging of perishable products like milk required equipment that did not contribute to the bacterial growth or decay of product.
Sanitary flowmeters are designed to ensure that flowing product or residue is not trapped in the meter body where it can spoil. For example, sanitary turbine flowmeters employ 316 stainless steel construction and a smooth finish that eliminates cracks and crevices where bacteria breed. Typical applications include measuring or batch controlling USP or deionized water, vaccines, pharmaceuticals, milk, vegetable oil, wine, beer, spirits, soft drinks, juices, and any other clean, consumable liquid. Other flowmeter designs, such as positive displacement meters, are suited for accurately measuring thick fluids in applications for which batch repeatability is desired.
Some sanitary electromagnetic flow-meters are manufactured from 304 stainless and lined with polytetrafluoro- ethylene for added durability in process plant applications. Mag meters, available with Hastelloy C electrodes, offer enhanced corrosion resistance, while a potted and sealed electrode housing helps to eliminate the damaging effects of humidity.
Liquid clamp-on ultrasonic flowmeters are an ideal solution for many different types of sanitary environments. These meters, which can be installed on the outside of existing process piping, use the ultrasonic transit time method to determine flow rate.
End users should become familiar with the key design considerations for flowmeters and other sanitary process instruments. These issues are detailed below with a comparison of 3-A SSI standards and EHEDG guidelines.
Food Contact Surfaces
One of the most critical considerations in the construction and design of sanitary equipment is the food contact surface. Why? Because contamination of these surfaces will more than likely contaminate the food itself. Therefore, all food contact surfaces should be smooth, impervious, free of cracks and crevices, nonporous, nonabsorbent, nonreactive, corrosion resistant, nontoxic, and cleanable. These standards also apply to any nonmetal coatings on a surface.
The standards established by 3-A SSI require that all surfaces and coatings maintain corrosion resistance and be free of surface delamination, pitting, flaking, chipping, blistering, and distortion under conditions of intended use. Likewise, EHEDG requires consideration of important issues:
- compatibility with food stuffs and ingredients;
- chemical resistance (cleaning and disinfectants);
- temperature resistance in use (upper and lower use temperatures);
- steam resistance (clean in place/steam in place);
- stress-crack resistance;
- hydrophobicity/reactivity of the surface;
- effect of surface structure and smoothness;
- residue accumulation;
- cold flow resistance; and
- abrasion resistance.
Stainless steel is by far the preferred metal for food contact surfaces due to its corrosion resistance and durability in most food applications. Not all stainless steel is equal, however. For most surfaces, 3-A SSI requires 316 stainless steel; 304 stainless steel can only be used for utility pipes, and 303 stainless steel is restricted.
EHEDG has similar guidelines:
“Where good resistance to general atmospheric corrosion is required, but the conditions of intended use will involve only solutions with a pH of between about 6.5 and 8, low levels of chlorides (say, up to 50mg/l [ppm]) and low temperatures (say, up to 25ºC), the most common choice would be AISI-304, an austenitic 18%Cr/10%Ni stainless steel, or its low-carbon version AISI-304L (DIN 1.4307; EN X2CrNi18-9), which is more easily welded.”