Dairy Engineering

Module 1. Sanitary pipes, fittings and milk handling equipment

Lesson 4
MILK STORAGE TANKS, SILOS, ROADS AND RAIL TANKERS

4.1 Introduction

Milk is stored in a great variety of storage tanks, first near to villages as Bulk milk cooling units, then transported by Road Tankers and then to Raw Milk Storage tanks in dairy plants. Further, it may be transported in Rail Tankers to long distances after processing, to large urban centers. Further, there may be requirement for processing operations like mixing, ripening, culturing, aging etc, which require further special features in storage tanks.

The requirement of modern quality standards, eliminate the use of open vessels. Further, the storage tanks are now made only of Stainless Steel, instead of Aluminum or other similar metals.

4.2 Milk Storage Tanks

They are usually cylindrical and mostly insulated. They may be horizontal or vertical types, depending on availability of flour space. For extremely large quantity of storage, Vertical Silo tanks are provided outside the Processing section, to avoid tall buildings to accommodate them.

The inner shell of storage tank is of Stainless Steel, and covered with suitable insulation, and then an outer shell, which may be Stainless Steel or Mild steel. Outer shell of mild steel is used to reduce cost. The outer mild steel shell will be painted to prevent corrosion, which may be of two coats of antirust and two coats of enamel paint of suitable color , especially cream yellow.

The inner shell of storage tank should be smooth in surface finish, and should not have any sharp corners, to aid in proper cleaning. The edges must have a knuckle radius of at least 25 mm. Any welds present should be properly ground, so as to have smooth finish.

The shell will have drain from lowest point, preferably from the center of the bottom. If this is not possible, the bottom of the vessel should be sloped towards the outlet valve side of the bottom to ensure complete drainage. The slope is at least 1:12.5 towards the outlet.

The shell will have other openings like for Agitator, non-foaming type milk inlet, inlet for cleaning solutions through a spray device, air outlet etc. It will also have fittings for sight glass and light glass, for inspection of inside level measurement, to ensure proper cleaning etc. The glass has to be of non splinter type. The opposite end of sight glass will have the level marked by food grade paint, or buffing of the surface to indicate level markings and values marked adjacent to it. The tank is fabricated usually 15% over capacity than the nominal capacity of the tank. The technology of using load cells for remote sensing of measurement of level is also being introduced in most of the modern dairies, linked to automated operation.

Fig. 4.1 Horizontal milk storage tank

Inner shell will also be supported by four legs, through load distributer MS angles, to avoid change of shape or damage to inner shell. The load distributors also will have a lifting eye at top to lift the entire storage tank with cranes. The thickness of shell depends on the size of the tank, to withstand the weight. Usually it is 2 to 3 mm thick for horizontal tanks, and 4 to 5 mm for silos. The ends of tanks are usually dish shaped to give strength and it also gives a greater knuckle radius at the joint of dish end.

Insulation is using thermocol or PUF for lower temperatures. For higher temperature, glass wool or mineral wool is preferred. The thickness should be such that, the increase in temperature of fluid when filled should not rise more than 2 °C in 18 hours, under a test condition of 35 °C temperature difference between inside and outside. Average thickness of thermocol insulation provided is between 5 to 10 cm. There should be no hollow space in between the two sheets of SS and MS. In some of the designs however, this gap is provided with a breathing outlet at the bottom, so that the gap does not build up pressure when hot cleaning solutions are used.

The legs may be four or six in number, with adjustable ball feet. Threads of the ball feet are not exposed.

The milk inlet should be non foaming type at the top, with bent tube projecting towards inner wall. For Silos the inlet is also at the bottom, with a non-return valve. The milk outlet will be at the bottom, with a flange type valve fitting, usually with a two way valve. It is usually of 51 mm or 63.5 mm, to allow rapid unloading.

To keep the milk properly mixed, and avoid cream separation an agitator at slow speed is operated, which may be either horizontal, vertical, and in some cases, inclined too. Most commonly, the agitator is vertical, with one or two sets of blades, and is supported at the bottom by bush, so that there is no lateral oscillation. The agitators are connected to motor through reduction gear mechanism, which has oil as the lubricant. It has to be perfectly sealed to prevent oil leaking along with the agitator into the tank.

Tanks are also provided with other essential fittings like manhole, sampling cock, thermo-well, ladder in front to watch through sight glass, CIP pipe line connection with adequate support on top. The manhole is of swing-back type, which is oval in shape and can swing both horizontally and vertically, so that it can be positioned in place from inside. There is a tightening arrangement to prevent leakage from manhole door, once the milk is filled. The sampling cock is positioned at a level such that sampling can be done even up to 5% of the capacity of the tank. It has spring loaded push type of lever, or a screw type of opening. Provision is also made of a Thermo-well, in which the sensing element of temperature indicator can be placed. It is an inclined SS tube, with lower end sealed, and positioned inclined to the inner shell, welded in that position. It is filled with oil or ceramic powder to have a better contact between the sensing element and the wall of thermo-well.

The air vent to the tank is provided to avoid build up of pressure during loading, and development of vacuum during unloading. It is provided with vermin proof cover to prevent dust and insects entering into the tank.

4.2.1 Vertical storage tanks / silos

The vertical storage tanks are mainly used, when the floor space is less, and the roof of processing hall is sufficiently high. Some of the high viscous products like cream or ghee are also preferably stored in this type of storage tanks.

Due to its shape, the manhole is at the top, and a ladder is provided for reaching on to the top. The top is also conical to prevent any accumulation of water or dust. The sight and light glass are also provided at the top. Due to its height, the agitator has to be long, and provided with more than two sets of blades for agitation. An internal ladder is provided to climb down for manual cleaning. Other fixtures like CIP cleaning attachment or spray ball, thermo well, sampling cock, lifting lugs or eye at the top, are provided as in horizontal storage tank. Both the inlet and outlet will be at the side at the bottom level of the tank. The slope is 1:10 towards the outlet so that there is free and complete drainage of liquid. The inner wall may be of Stainless steel sheet of 3mm thickness while outside can be 2 mm. Some of the floors have greater thickness of 5 mm. The tank is supported by four legs with SS ball feet and provision for height adjustment. Silos are vertical storage tanks of large capacity, usually more than 30,000 to even upto 1 lakh liters. The agitation is by compressed air in such case .

Fig. 4.2 Vertical storage tank

The compressed air is passed into it through a filter and a control valve. A pressure gauge is also provided to know the air pressure. The inlet and outlet are fitted to be operated from inside of the processing hall itself, through an opening called alcove. Inlet valve is provided with non-return valve. An overflow line extending to inside of the processing hall is also provided to know if the tank is full.

4.2.2 Road tankers

The milk once collected in milk collection centers, has to be transported to dairy plants by Road tankers. The tankers have to withstand great deal of stress due to vibrations, jerks, exposure to weather, while transporting milk. Hence, the essential features include, thicker SS sheet for inner and outer shells, better insulation, weather proof manhole doors covered with a hood, two or three compartments to prevent rolling over of milk during jerks. Further, the whole structure has to be mountable on the truck, or the articulated trailer, with all the limitations of safe maneuverability during driving. The capacity, mounting and length should be suitable for village roads, which is a severe limitation. The insulation will be a thermocol in two or three layers, with proper seal of bitumen to prevent moisture seepage. The newer road tankers are having PUF insulation material. The barrel of the tankers is preferably oval in shape to reduce the centre of gravity, and is supported by M.S. supports, to be fitted to the chassis of the vehicle. The vehicle will have a ladder at the rear and a walk way (of suitably checkered plate to avoid slippage) to approach the manhole during inspection, sampling and manual cleaning.

4.2.3 Railway tanker

Railway tankers are used for transportation to long distances and are of capacities 20,000 to 40,000, divided into two or more cells. The final structural dimensions should meet the requirement of Indian Railways, suitable for transport by passenger trains. The top of the tank has dust cover, pressure relief valve and vacuum relief valve, manhole door. Manhole door and internal ladder are also provided. The Manhole door is provided with gasket of Acrylonitrile butadiene copolymer or polychloroprene. A dip stick is provided to know the quantity of milk approximately. Insulation is 100 mm thick such that when full, the rise in temperature from 4°C is only 2°C in 24 hours, when temperature differential to outside is 35°C. Outlet size is 63.5 or 76.2 mm, located at centre bottom. Two outer ladders are also provided on each side. The inner vessel should withstand 0.35 kg/cm² pressure, and should be tested for Radiographic and dye penetration test to detect possible leakages.

4.2.4 Other kinds of tanks

There are other types of tanks suited to specific purposes like cream storage tank, ghee settling tank, etc, which are double jacketed. The jacket has SS perforated ring to allow a gentle flow of cold/hot/well water to maintain the temperature of the fluid in the tank. They are usually vertical types, with all the essential features as that of horizontal milk storage tank. Milk reconstitution tanks are again special purpose tanks connected with additional features like funnel with venturi and valve at bottom, circulation pump, potable water inlet to tank. This tank is not insulated, as the temperature is expected to be about 40 °C.

Fig. 4.3 Milk silo tank

Fig. 4.4 Reconstitution with funnel venturi system