Chapter 13: Valves – Machine Drawing with AutoCAD

Chapter 13

Valves

Chapter Outline

A valve is a device to check or control the flow of fluid through a passage. Valves are essential components for any fluid power and control circuit. Though there are many variants, yet all valves are alike in concept despite the present-day trend for greater simplification, and standardisation.

Valves are operated by hand, by the pressure of the fluid they handle, or by some external mechanism. Valves that are operated by the pressure of a fluid are called non-return valves. The name itself suggests that they allow passage of fluid only in one direction just like a diode in an electrical circuit. Relief valves and safety valves also operate by the pressure of fluids and play an important role in safeguarding equipment. Junction valves and stop valves, and various types of cocks are operated by hand in general.

COCKS

Undoubtedly, the cock was the very first fluid control device conceived by man. It has sufficed for well over 2000 years and will continue to do so owing to its simplicity in design and operation. Its origin is attributed to the Roman workers. By rotating a conical plug within a conical shell or body, open and close conditions can be created (Fig. 13.1).

 

FIG. 13.1   cock with conical plug

 

Cocks are mainly designed empirically although the concept of a conical plug is a distinct ‘breakthrough’ on the part of its originator. The included angle of the plug is generally 10°. A substantially smaller angle may prove difficult to turn. This type of plug-cocks (known as isolating cocks) are still being used by the Indian Railways for their air-brake system. They are generally intended for low pressure service where fluid tightness is ensured on metal-metal affinitive contact between plug and body. The correct wedging effect between the plug and the body is achieved by tightening the base nut.

A more refined version is the asbestos groove packed cock shown in Fig. 13.2. This was originally intended for boiler blowdown duties. Here, asbestos is tightly packed in two U-grooves cut on the body and circumscribing each of the two parts in the body. The sectional view of the cock body, developed in solid model, is presented in Fig. 13.3.

 

FIG. 13.2   Typical asbestos groove packed cock

 

FIG. 13.3   Sectional view of the cock body

 

There is no direct contact of the plug surface to the valve body. A stuffing box and a gland is also provided to prevent leakage of fluids through the clearance between the plug shank and the cover plate. The detailed drawing of the stuffing box and its working principle is described in Chapter 16. The plug is kept in position by a cover plate, generally oval in shape, by means of a stud and nut as illustrated in the figure.

FLAP VALVE

It is a non-return valve of swinging or flap type, mainly employed in water works practice. It consists of a valve (swinging door or flap) and a valve seat placed within the flap valve body to separate the inlet and outlet in a closed position as illustrated in Fig. 13.4. The amount of swing given to the valve controls the flow of the fluid. A commendable feature of this design is the articulated arrangement of hinge and flap that ensures that the latter will sit squarely on the seating and thus stopping the flow. A cover is attached to the body by means of nuts and bolts. The valve is usually made of gunmetal, brass, bronze, or steel in the form of a circular disc, beveled off at an angle of 45° at its lower edge.

 

FIG. 13.4   Flap type non-return valve

STOP VALVE

The cock has sufficed as a closure device for more than 2000 years. In 1768, Watt invented the rotating steam engine and, as a consequence, boilers found increasing demand. The commonly used plug cock soon proved inadequate to deal with the increasing steam pressure, although by present day standard, these pressures would not appear very high. Thus, we get the screw-down stop valve, whose function is to regulate (and completely shut off) the steam passing through the steam pipe.

The screw down stop valve is available in different shapes. It may take the form of a junction stop valve (Fig. 13.5) when a right angle change in the direction of steam flow is necessary or to sit directly on the stand pipe of a boiler. Another variety may be the globe valve (Fig. 13.6) for use in horizontal or in-line connection. As such, there is no fundamental difference in the design or working principle of these two types of valves.

 

FIG. 13.5   Typical screw-down junction valve, two-piece stem pattern

 

FIG. 13.6   Globe valve

 

Both figures show the sectional orthographic projection view of the stop valve. The particulars of the different components of the valves are tabulated in the part list (Table 13.1). In order to have a better understanding of the inner details, the entire globe valve is created in solid model and its exploded view is presented in Fig. 13.7. The figure clearly indicates the shape, position and orientation of the gland and the bridge with respect to the spindle. Some important components of the valve are discussed below.

 

Table 13.1

 

Body   The steam from the pipeline enters the valve body through the inlet port and exits through the outlet port. The mechanism controlling the steam flow is also housed within the valve body by means of the valve and valve seat. The body of a globe valve, cut into half, is shown in Fig. 13.7. The position of “Valve and Seat” that separates the inlet and outlet is clearly displayed in these figures.

Fig. 13.8 highlights the formation of the inside cavity and the flow path of the fluid through the valve body. A projection view of the valve body is also presented in Fig. 13.9. It may be noted that although the direction of the steam flow is horizontal at the two ports, it becomes vertical while flowing through valve and seat. The inside walls of the body should be such that the area presented to flow at the throat portion (Fig. 13.9) should be at least equal to that of the area presented at the inlet and outlet. This requirement poses an intricate problem of solid geometry during design work. From the viewpoint of moulding, thickness ‘t’ will be constant throughout the casting.

 

FIG. 13.7   Globe valve

 

FIG. 13.8   Body in half-section

 

FIG. 13.9   Projection view of the body

 

Valve and valve seat   The exploded view of the valve (Fig. 13.7) indicates the relative position of the valve and valve seat of a globe valve. The flow of steam is closed when the valve (Fig. 13.5) rests on a relatively narrow raised annulus portion of the valve seat fixed to the valve body. The annulus portion must be ground and lapped to a mirror finish for better performance. Another variation is the conical disc valve (Fig. 13.10) in which the lower edge is bevelled at an angle of 45°. The surface thus formed is called the face of the valve and it rests on the corresponding matching surface of the valve seat. The valve is made of metal such as brass, gunmetal, steel, which do not rust. It is usually provided with three or four wings that serves as a guide during the rise and fall of the valve. A groove is provided at the junction of the wings and the disc to facilitate the flow of steam and the movement of turning tool while machining the face of the valve.

 

FIG. 13.10   Conical disc valve

 

The seat is a fixed part on which the valve rests. The seat material is, in general, similar to that of the valve. The seat must be properly secured to the passage so that it does not move upwards along with the valve due to fluid pressure. This may be achieved by one of the following methods.

  1. Seat screwed into the passage (Fig. 13.6).
  2. By set screw from the side as shown in case of Ramsbottom safety valve (Fig. 13.18).
  3. By cap screw from the top (Fig. 13.11).

The annulus region of the seat is not made very narrow as the contact pressure may prove to be excessive and induce ‘galling’. If it is too wide, the contact pressure will be insufficient to ensure pressure tightness across the seating face.

Other parts   The spindle is attached to the valve by means of a split coller and nut as shown in Fig. 13.5. In case of a globe valve (Fig. 13.6), this is done by a coller and a pin. Leakage of steam through the annular space between the spindle and the cover is prevented by the stuffing box as illustrated in both figures. The gland of the stuffing box is pressed by a nut screwed on the cover. The hand wheel is fixed to the spindle by a nut (Fig. 13.6) screwed to the end of the spindle. Movement of the valve is controlled by the up and down motion of the spindle which is screwed in the bridge supported by two pillars.

 

FIG. 13.11   Valve seat attached to the body by cap screws

 

The detailed drawings of a junction stop valve is also shown in Fig. 13.13. The particulars of the different components are described in Table 13.2. The assembly drawing of the same is also presented in Fig. 13.12. Note that the fixing arrangement of the seat is done by inserting two cap screws from the top along the periphery of the seat. The valve used here is the flat face type and is screwed on the spindle and secured by a pin. This spindle is made hollow so that it may be connected to the top spindle by means of a coller as illustrated in Fig. 13.12. This design modification is necessary to provide the feature of ‘rotate spindle’ arrangement. The valve is operated by rotating the hand wheel which in turn rotates the screwed spindle mounted on the top spindle, providing axial movement of the valve. There is another hand wheel keyed to the top spindle. This hand wheel enables the valve member to be rotated on its seating without producing any axial displacement and thus clears the seat of any scaling that may be deposited on it. The stuffing box has a circular gland that slides on a gland ring (Fig. 13.12).

 

FIG. 13.12   Assembly drawing of junction stop valve

 

FIG. 13.13a   Junction valve details

 

Table 13.2   Particulars of Parts

 

FIG. 13.13b   Junction valve details

FEED CHECK VALVE

This is essentially a non-return valve required for the functioning of all steam boilers and some other pressure vessels as well. The valve permits feed water to be pumped into the boiler against steam pressure, the return flow being prevented by the check valve housed in the branch elbow shown in Fig. 13.14. The proportion of different parts, as obtained from design calculations and empirical relations, are also mentioned in the drawing.

A feed check valve is a combination of a screw down manually operated stop valve and an automatic non-return or check valve. With the screw down feature, it is possible to close the upper valve completely for overhauling the boiler itself or enabling the check valve portion to be examined or repaired even when the boiler is under steam pressure.

 

FIG. 13.14   Feed check valve

 

It may be noted that the length of the spindle projected underneath the screw down stop valve serves to limit the lift of the lower check valve. When the screw down valve rests on its seating, the lower check valve should still be permitted a small amount of lift, say 3 mm (Fig. 13.14). This ensures that the stop valve positively sits at all times without hindrance from the check valve. If this provision is not made, it would be impossible to remove the elbow when the boiler is under steam.

The feed check valve is very versatile as it may be converted to a ‘left valve’ or ‘right valve’ simply by removing the blank flange (Fig. 13.14) on the boiler branch and attaching it to the opposite end of the branch pipe. The inlet elbow may also be oriented as per requirement.

Fig. 13.15 shows the detailed drawing of a feed check valve with all dimensions. The particulars of the different components are furnished in Table 13.3.

 

FIG. 13.15a   Feed check valve details

 

FIG. 13.15b   Feed check valve details

SAFETY VALVE

The function of any safety valve or relief valve is to prevent an unwanted rise in pressure in the pressure vessel to which it is fitted, for example, a steam boiler or any other pressure vessel. The terms safety and relief are synonymous. Conventionally, valves employed to protect any vessel from exploding and thus endangering life and limb are known as safety valves. The term relief is applicable to valves that protect vessels from excessive pressure but without fear of violent explosion, for example, valves attached to cold water main.

 

Table 13.3   Particulars of Parts

 

The underlying working principles and operation of both types of valves are almost identical. They operate without any human assistance, that is, the valves operate automatically. The valves must open promptly when the pressure increases beyond a preset value and close quickly when normal pressure is restored after the discharge. There are various types of safety valves. A few of them are described below.

Dead Weight Safety Valve

It is the simplest type of safety valve consisting of a seating with a mass resting upon it. The mass should have sufficient weight to balance the force tending to lift the valve member.

A simple dead weight safety valve of open discharge type is illustrated in Fig. 13.16. The valve is loaded through the screwed spindle by weight placed on the suspended weight carrier. The escaping steam is discharged to the atmosphere directly via louvres shown in the figure. The valve member, in general, has a spherical contour for the self centering feature of the suspendary deadweight system.

 

FIG. 13.16   Simple dead weight safety/relief valve—open discharge pattern

 

It is to be noted that in this type of valve, the center of gravity of the suspended dead weight must be located well below the seating surfaces, thereby ensuring that it will be self centering. The deadweight safety valve is not meant for high pressure on account of the huge bulk of deadweight which would be required.

Lever Safety Valve

This is one of the simplest safety valves. The valve is loaded by a dead weight mounted on a lever to counter the upward thrust of the steam. The fulcrum of the lever is screwed to the cover of the valve. The weight effect is thus magnified by the leverage provided. This arrangement, along with other components of the valve, is illustrated in Fig. 13.17. The lever is usually made of rectangular section mild steel bar. The size of the valve is limited, to a great extent, by the size and weight of the suspended object.

 

FIG. 13.17   Lever safety valve

Spring Loaded Safety Valve

The dead weight and lever safety valves were in use in the early locomotive and marine boilers. Due to the jolting and rolling of the vehicle, on which they were mounted, these valves would exhibit erratic behaviour resulting in unsatisfactory performance. Over the years, several attempts were made by engineers to improve the performance of the valves with the introduction of spring. Thus we have several versions of spring loaded safety valves. Fig. 13.18 shows the arrangement of a Ramsbottom safety valve, suitable for locomotive boilers.

The particulars of the various parts are shown in Table 13.4. The two valves are equally loaded by a single tension spring and lever body. The attachment of the valves with the lever is achieved through pivots, one pivot being forged on the lever and the other connected by means of a pin. This dual arrangement is preferable to employing a single valve of increased diameter to give an equivalent area. The shrouded valve seats are fixed with the help of cap screws. The purpose of shrouded seats is to deflect the discharged steam clear of the driver's look out windows.

 

FIG. 13.18   Ramsbottom spring loaded safety valve

 

Two flat steel restraining members housed within the coils of the spring prevent the valve members from being blown out of their seats along with the lever in case the spring breaks. Either of the valves can be relieved manually by lifting or pressing the lever end. Normally, however, the lift is automatic depending upon the requisite steam pressure available in each valve. The length of the spring is adjusted by the coller placed under the shackle to which one end of the spring is hooked.

 

Table 13.4   Particulars of Parts

EXERCISE
  1. What is the difference between a cock and a flap valve? What is a non-return valve?
  2. Prepare a detailed drawing of a globe valve shown in Fig. 13.6 giving sectional views, if necessary.
  3. Reproduce the drawing of the globe valve shown in Fig. 13.6. Add sectional side view.
  4. The detailed drawing of a junction valve is shown in Fig. 13.13. Prepare an assembly drawing of the same with the following views: i) Half-sectional front view; ii) Sectional side view and iii) Top view.
  5. From the assembled drawing, generate the blown up view of the valve and seat portion using paper space.
  6. Develop the solid models of all the individual parts of the junction valve shown in Fig. 13.13 and thus create the solid model of the assembled valve.
  7. Fig. 13.7 exhibits the solid model view of the globe valve shown in Fig. 13.6. Develop the globe valve in solid model. Take the necessary dimension from Fig. 13.6.
  8. Prepare a 3-D drawing of the cock shown in Fig. 13.1. Create the orthographic projection views from the solid models thus developed.
  9. Develop the solid model of the valve and seat shown in Fig. 13.10, considering three wings, equally placed, instead of four. The dimensions of different parts may be taken from Fig. 13.6.
  10. The detailed drawings of different parts of a feed check valve is shown in Fig. 13.15. Prepare an assembly drawing of the valve with all the necessary dimensions. Show the sectional front view, side view, and top view.
  11. Explain the working principles of a dead weight safety valve and spring loaded safety valve.
  12. Develop the solid model of the elbow of the feed check valve shown in Fig. 13.14.
  13. Develop the solid model of the body shown in Fig. 13.14. Show a cut section of the same so that the inside of the body is visible.