VALVES

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                                                  VALVES

1. GLOBE VALVE

                  The globe valve has a bulbous body, housing valve seat and screw down

 spindle with disc arranged at right angle to the axis of pipe. Turning the hand wheel clockwise closes the valve while turning in anticlock direction opens the valve.

            The spindle or stem have a square thread, below or above the stuffing box

and it works on bridge (bonnet). Leakage along the valve spindle is prevented by a

Stuffing box, packed with suitable material and a gland.

            In one type of valve, the disc is attached to spindle. Hence the disc positively moves up and down with  the stem, allowing fluid flow in both directions. While the other type of valve having disc unattached to the spindle allows fluid flow only in one direction is known as Screw down non return (SDNR) valve. These valves are used in places such as bilge suction lines to prevent any back flow.

           Globe valves are primarily used for flow control and throttling purpose.

Generally not used in place which requires large flow quantity.  Globe valve can be normal (standard) type or angle type. Angle valves changes the flow directions by  90 degrees. The valves are made of materials such as Mild steel, Cast iron, cast steel, etc.,

2. GATE VALVE:

  

Gate valve (sluice valve) gives full bore flow without change of direction i.e they offer very little resistance to flow. They consist of chests divided at mid-length by a double membrane having central circular opening, furnished with seats, tapered or parallel on their inner, ie facing sides. A gate shaped appropriately can be move in a direction at right angles to flow by a screwed spindle working in a nut. ( valve moves Vertically up and down, on operating the spindle).

          Gate valve is not suitable for partially open operation or flow regulation,

since wire drawing of the seat will occur ie, they should be fully opened or fully closed. Gate valves are used for large quantity of flow.

3. BUTTERFLY VALVE:

                A Butterfly valve consists basically of a disc, pivoted across the bore of a ring body having the same radial dimensions as the pipe in which it is fitted. The full bore straight through flow arrangement of this type, especially if combined with a carefully streamlined disc profile, gives this type of valve excellent flow characteristics and low pressure drop ie it offers very little resistance to flow. The valve is quick acting, as it requires only a quarter turn of spindle to move from fully open to fully close position. Widely used in cargo, ballast and cooling water lines.    

4. RELIEF VALVE:

          Relief valve consists of a disc held closed by a spring – loaded stem. The

compression of the spring can be adjusted using the adjusting screw so

 that the valves opens at the set pressure. When the relief valve opens the excess pressure is discharged to the suction side or atmosphere. Relief helps in preventing excess pressure in the system. Relief valves are available at different ranges.

 

5. PRESSURE REDUCING VALVE:

       It is sometimes necessary to provide steam or air at a pressure less than that of the boiler or air reservoir. To maintain the downstream pressure within defined limits over the range of flow and despite any changes in supply pressure, a reducing valve is fitted.

       The working principle of a reducing valves is, the spring load causes the valve to open this is balanced by the reduced steam pressure P₂  acting on the area of the top of the valve, A₂. As this area is constant, any decrease in outlet pressure, will allow the valve to open and any increase will close it.

       The area on the underside of the valve, and that of the piston are equal A₁ , both these areas are exposed to the inlet steam pressure P₂ and so the loads on them will be equal and opposite, therefore they will balance. A regulating screw is provided for setting the tension on the spring.  

                                     PACKING AND JOINTS

            To create tightness or sealing between machine parts, a plastically deformable material, termed ‘ packing’ is often used. The various types of packing or sealing can be divided into two main groups:

1)      sealing between reciprocating / rotary parts ( dynamic)

2)      sealing between static parts.

The object of using packing material is :

1)      to prevent fluid, eg water, lub oil, fuel oil from escaping from system.

2)      to prevent gases and vapours from escaping from a system

3)      to prevent undesirable entry of gases, fluids and dirt into a system.

The three most important requirements that a packing or seal must fulfil are:

1)      it must be made of the correct material

2)      it must be suitably dimensioned

3)      it must be correctly mounted

Among the characteristics to be taken into consideration when selecting a packing material are:

1)      strength

2)      elasticity

3)      resistance to chemical influences

4)      resistance to high (or low) temperatures

5)      coefficient of expansion

6)      thermal conductivity

7)      density

8)      frictional properties

1. JOINTING BETWEEN ASSEMBLED PARTS:

              The jointing used between assembled parts is in the form of thin sheets. While metal joints are used in case of high temperatures and pressures eg. Cylinder head gasket used in a diesel engine. The metals used are aluminium, copper, steel, brass, etc.,

      1. Cold water     --  natural or synthetic rubber with or without linen lining

      2. Hot water       -- natural or synthetic rubber with or without linen lining                    

                                     for temperature upto 100deg C.

      3. Lub. & Fuel   -- bonded cork, oil paper and fibre

        

For all the above materials we can use General purpose (G.P) packing.

      4. Steam             -- Packing with metal lining

    

            In recent years, the synthetic materials such as Teflon, mipolan, nylon, perlon, trekollan and silicone rubber have found great application in the manufacture of sheet joints. Teflon and silicone rubber in particular have proved to be very suitable where other materials could not withstand the conditions. These two materials being resistant against many fluids and gases at temperatures as high as 250 degree C.

2. SEALING BETWEEN RECIPROCATING / ROTARY PARTS:

                The relative motion between the surfaces to be sealed can be:

a)      rotary

b)      translatory

c)      combined rotary and translatory

        eg. Pump rotating shafts, piston rods, valve stem, etc.,  For sealing between these parts either gland packing / stuffing box or mechanical seal will be used.

                                                   Stuffing box arrangement

                 The above stuffing box/gland packing is designed for use with fluids. The operational life and good function of a stuffing box depends on the ability fluid lubricating between the packing and shaft. This condition is obtained by tightening the packing bush so that leakage from the stuffing box is insignificant. In case of, presence of solid particles in the fluid, they get settled between the shaft and packing and cause wear and leak through the gland. In this case the gland packing has to be renewed.

               In certain cases the stuffing box is provided with a latern ring and sealing water or sealing oil with inlet and outlet connections. The pressure fed to the latern ring is slightly higher than the pressure to be tightened against, thereby preventing solid particles from forming deposits in the stuffing box eg. Boiler water circulating pump having sealing water connections.