DAIRY ENGINEERING

Module 1. Sanitary pipes, fittings and milk handling equipment

Lesson 2
SANITARY PIPES AND FITTING, GLASS, PLASTIC TUBING AND GASKETS

2.1 Introduction

Design and selection of pipes for handling milk and fluid milk products has to be done with same care and attention to sanitary features as is done for the dairy equipment, where the surfaces are coming in contact with the product. The type of material has to be Stainless Steel of grade 304 or 316. As milk flows with high velocity hence, the corrosion due to erosion as well as pitting has to be factored in while designing. In some of the instances, instrumentation for measuring the properties of the flowing fluid will be required, and need special type of pipe fittings. Care also has to be taken regarding the surface finish, both inside and outside of the pipe lines. In general, the following aspects are important in design of sanitary type of pipes.

2.2 General Points to be Considered While Designing and Layout of Sanitary Pipe Lines ere Given Below

1. Pipes should be seamless and preferably cold drawn
2. They should softened, descaled and pickled
3. Polished internally and externally
   
4. Clean, smooth and free from surface defects, straight and free from longitudinal grooving both inside and outside
5. Material for fittings to be same as that of pipe
6. The condensate from the surface of milk pipe lines should not be allowed to fall on places where unhygienic conditions or milk contamination is likely to occur
7. The capacity of the pipe line should be designed to allow the flow within the limits, without much pressures loss
   

Table 2.1 Outer diameter, thickness and carrying capacities of sanitary pipes

O.D	Thickness	Approx. Capacity (Kg/h)
25.4 mm	1.25 mm	< 2000
38.1 mm	1.25 mm	<7200
50.8 mm	1.25 mm	7200 – 15000
63.5 mm	1.5 mm	15000 – 22700
76.2 mm	1.5 mm	22700 – 36000

• Next larger size for heavy liquids, like cream, Ice cream mix.
  
• Similarly, next larger size if the flow is by gravity
• Criteria for selection is flow velocity and cost

2.2.1 General piping design considerations are also relevant here

1. Piping lengths kept to a minimum
2. Piping layout should not be a hazard or restriction to operator
3. All valves and instruments located so as to be both easily operable and maintainable
4. Pipe layout should be conducive to adequate individual support, and group supports wherever possible.
5. Piping system designed to be drainable with few if any pockets, and provided with adequate drain valves.

2.3 Glass Piping

It is generally used where the visibility of the product is important. Due to its brittleness and fragile nature, it has to be again protected by external piping of suitable metal. It is also used in the level gauges, sight and light glasses for milk storage tanks. In certain situations, however, it can be used as an inner liner for the storage tanks or pipelines. The glass should not be of splinter type.

2.4 Plastic Tubing

Plastics, in addition to being used as packing material, are also extensively used in manufacture of some of the pipes and flexible tubing. Plastic pipes and tubes have to be of food grade, and should not impart off flavours. Its smoother surface gives lower resistance to the fluid flow, and is light in weight. It should have higher resistance to detergents.

However, it has lower resistance to temperature, and the maximum continuous exposures to heat for various plastics are given below:

Table 2.2 Temperature resistance of various plastic materials for tubing

1	PVC	50°C
2	Polystyrene	65°C
3	Polyethene (linear)	60- 90°C
4	Polypropylene	70 - 100°C
5	Polytetrafluoroethylene	325 - 330°C

While it is less easily wetted by aqueous solutions, it is more easily wetted by fats. This can cause fat to penetrate into the surface. It can be removed by process of extraction and if it is not removed, a rancid odour develops on the surface of the plastic material due to fat decomposition. The consequences can be discolouration, crevice formation, irreversible distortions of shape, bacterial contamination in fine crevices, taints in milk due to released plasticizer.

2.5 Gaskets

The role of Gaskets is to prevent leakage between two metal surfaces. In addition to being flexible, the dairy industry demands them to be fat-resistant, resistant to detergents and sanitizers. They will have to resist, certain degree of heat too in applications like Pasteurizer gaskets. The various applications in dairy industry are SS pipe fittings, manhole gaskets, milk pumps, cream separators, homogenizers, diaphragm valves etc.

The following are the different types commonly used in dairy and their heat tolerance limits.

• Nitrile rubber (NBR): 130°C
• Butyl rubber(Butyl): 140°C
• Ethelene propelene (EPDM): 165°C
• Silicon rubber:175°C
• Viton:180°C

To keep the gaskets in place, various methods are used. Glues, gasket cements, mechanical methods are more commonly used. In case of glues and gasket cements, special procedure is needed to fix the gaskets in place. This some times limits their applications for high temperatures.

The gaskets have to be stored in cool and dry place. It should not be exposed to ozone like in electric arc welding applications.