Chapter 12: Pipe Joints – Machine Drawing with AutoCAD

Chapter 12

Pipe Joints

Chapter Outline

Pipes are extensively used in process plants for carrying fluids (oil, gas, chemicals, and so on) from one place to another. Pipes are generally made of cast iron, steel, brass, copper, reinforced concrete, plastic, or fibre reinforced plastic depending on the type of fluid or fluid/solid mix to be conveyed. Steel pipes are used for high temperature and high pressure application such as boiler tubes. Cast iron or concrete pipes are widely used for underground sewerage. The recent developments in materials have helped in the manufacture of plastic pipes that are highly resistant to corrosion. They are, therefore, suitable for carrying chemically active fluids.

Pipes are usually specified by their internal diameter. In general, pipes are available in standard lengths (5.0 to 6.0m) for ease in transporting, storing and handling. A long pipeline is laid by connecting two or more pipes of standard dimensions. Various forms of pipe joints are used depending on the material of pipes, material to be conveyed through the pipes, and the resulting pressure and shock to be withstood by the joint. At times, pipe joints have to fulfil additional requirement such as easy disassembly, allowance for expansion, and so on. In the following section, we shall discuss some of the commonly used pipe joints.


These are most widely used forms of connections. They may or may not have a socket or a ferrule (Fig. 12.1). A socket is a short cylindrical sleeve that is threaded inside whereas a ferrule is threaded on the outside. All the threads are pipe threads, with a taper of 1 in 200 to ensure tight connection. If made properly, these joints are economical and satisfactory for water as well as for steam and air. The only difficulty is in disassembling the joint. To make the joint leakproof, a few strands of jute coated with red lead are wound round the bottom of the thread on each pipe.


FIG. 12.1   Threaded joints in pipes


This type of joint is used to connect coaxial pipes with large diameters, conveying high pressure fluids. The faces of the flanges integral to the pipes are machined square to the axis of the pipes so that there is no misalignment of the pipe axes when joined. Packing in the form of gasket made of rubber, canvas, and so on is placed between the flanges to make the joint leakproof. The flanges are ultimately joined together with the help of bolts and nuts as shown in Fig. 12.2. Cast iron flange joints are widely employed in Tee pipe and Angle Bend connections.


FIG. 12.2   Flanged joint


This joint is used where slight flexibility is essential from the functional point of view as in the case of underground gas and water mains and sewer pipes. The flexibility of the joint accommodates small changes in the level due to earth's settlement. The spigot end of one pipe enters the socket end of the other. The Solid model view shown in Fig. 12.3 clearly exhibits the annular space created between the spigot and the socket. The annular space between them is partly filled up with yarn cord. A ring of lead is cast in the remaining space near the mouth of the socket and is retained in position by the groove.


FIG. 12.3   Solid model of spigot and socket joint


Neat Portland cement may also be used instead of lead. Fig. 12.4 illustrates the projection view in section of the joint with dimensions of the different parts.


FIG. 12.4   Spigot and socket joint


In pipelines that carry high pressure fluid, it is essential to make provision for longitudinal expansion or contraction of metal pipes due to large variations in temperature. For a change of temperature of about 150° C, a pipe 6 m long will change in longitudinal dimension by 10 mm. Expansion joints are designed to allow for a considerable elongation or contraction of the pipeline.

A gland type expansion joint, suitable for steam pressure joint, is shown in Fig. 12.5. A stuffing box is also provided to make the joint leakproof.


FIG. 12.5   Gland and stuffing box expansion joint


FIG. 12.6   Support system for steam pipes


A pipe (indicated by X in the figure) is free to slide in the stuffing box. Leakage of steam is prevented by asbestos packing P placed at the right place by the gland G and stud S as shown in the figure. A brass bush B and brass liner L is also put between the gland and the outside diameter of the pipe to prevent corrosion. The steam pipes are never clamped rigidly to allow free expansion. Instead, they are suspended on hangers or freely supported on rollers. The arrangement is illustrated in Fig. 12.6.


The screwed joint discussed earlier cannot be disassembled easily. The union joint is used for pipes of small sizes and where rapid disassembly is necessary.

A nut with threads on both sides is screwed at one end of the pipe.

Similarly, another nut (Y) is also screwed on the other pipe. A coupler C is used to connect the two flanges as shown in Fig. 12.7. A packing ring (gunmetal or brass) is placed between the two pipes to prevent leakage.


FIG. 12.7   Union joint


The outer surface of the coupler is hexagonal to help tighten the joint with a spanner.


One hundred years ago, water was the only fluid that was transferred from one point to another by means of pipes. Now, almost all types of fluids (oil, water, acids, liquor, and so on) are conveyed through pipes during its production, processing, transportation, or utilisation. Even liquid metals, such as sodium, potassium, bismuth as well as liquid nitrogen, oxygen and so on are being handled in pipes for transportation from one place to another. For any process plant, petrochemical and fertiliser plants, piping drawings are an essential component of the system design. Therefore, draftsmen and engineers need to have sound knowledge about pipe drawings.


Different components, known as fittings, are connected to a pipe to make a complete pipe network. Pipe fittings are specified by the nominal pipe size, the names of the fittings, and the material. Fig. 12.8 illustrates a pipe drawing along with commonly used fittings. The purpose of each fitting is explained below.


FIG. 12.8   Double line pipe drawing


Gate valves    They are used to control the flow of liquid. They are used where operation is infrequent and close control is not essential.

Globe valves    These types of valves are used to control the flow of air, steam, or compressibles where very fast on and off operations are required.

Check valves    They allow the flow of fluid in one direction only. They are operated by the pressure of the fluid, with no external assistance.

Tees, crosses, and elbows   They are used to connect different pipes as per the fluid flow requirement. Elbows are essential when the pipe has to bend.

Reducers    They are employed to connect two pipes of different nominal sizes.


The major objective of pipe drawings is to communicate the size and location of pipes, their fittings and valves. Most of these items are standardised. They are used a number of times in a large pipe drawing. In order to avoid repeated drawing of complicated features in true shape, a set of symbols has been developed to portray them on a drawing. As a result, there have been two types of pipe drawings in use–single line and double line pipe drawings.

Single Line Drawings

As the name implies, in a single line drawing, the pipes, valves, pumps, and other fittings are represented by standard symbols. A single line is used to display the entire network of pipes and the fittings. Fig. 12.9 shows a single line drawing of a pipe network. The center line of the pipe, regardless of its size, is represented by a heavy line. Different fitting symbols representing various pipe components are appended to the line. The size of the symbols are selected in proportion to the size of the drawing.


FIG. 12.9   Single line pipe drawing


Single line drawings are presented in orthographic (Fig. 12.10a) or isometric (Fig. 12.10b) or oblique (Fig. 12.10c) form. In most installations, some pipes are placed vertically and some horizontally. It is a standard practice to show the entire drawing in one plane either by revolving the vertical pipes in a horizontal plane or the horizontal pipes in a vertical plane. This type of drawing is referred to as developed drawing (Fig. 12.10d). Quite often a pictorial view is drawn in conjunction with the orthographic and developed view.


FIG. 12.10   Various projection systems used in Pipe Drawing

Double Line Drawings

When lengths and positions of the pipes and their fittings are critical or when a pipe is precut and shipped to the site before assembly, then double line drawings are preferred. Since the entire drawing is generated on a definite scale, it is very helpful to show the details required. This type of drawing is usually drawn in orthographic projection as shown in Fig. 12.10a. In fact, the single line drawing shown in Fig. 12.9 is again developed as a double line drawing to highlight the differences between the two drawing styles. The meaning of each double line drawing symbol is also mentioned in the drawing.


As shown in the figures, the dimensions for pipes and pipe fittings are generally mentioned from outer to center. The lengths are guided by the pipe fitter at the time of assembly and hence they are not provided in the drawing.


Pipe drawing has now become easy with the aid of AutoCAD or any other advanced CAD software. Be it a single line drawing or a double line drawing, it is now possible to develop a library (using Block command) for frequently used symbols or the outer configuration of the fittings. Once the library function is ready, an expert AutoCAD draftsman will not take much time to develop even complicated pipe drawings. Fig. 12.8 and 12.9 illustrate the application AutoCAD in pipe drawing.


We have already mentioned that the development of any complicated piping layout cannot be restricted to one plane only. As a result, sometimes it may be quite difficult to visualise and understand the drawing from its orthographic or developed views. Therefore, the present trend is to create the entire network in 3-D using the solid model features of AutoCAD. A typical example of such a drawing is presented in Fig. 12.11. The underlying drawing methodology is similar to that of the previous cases, that is, individual fittings are first generated in solid models. Then they are assembled as per requirement to obtain the full drawing.


FIG. 12.11   Pipe drawing in 3-D

  1. Draw two views with dimensions of a flanged joint to connect two C.I. pipes, each having a diameter of 80 mm.
  2. Draw views of a socket and spigot joint to connect pipes having diameter equal to 125 mm. Where is this joint used in general?
  3. What is the function of an expansion joint? Explain its working principle with sketches.
  4. Draw the full sectional view and a side view of the expansion joint shown in Fig. 12.5.
  5. Why are steam pipes not rigidly connected? What kind of supporting arrangements are used in practice?
  6. Draw two views with dimensions of a union joint for 25 mm diameter pipes. Add the side view.
  7. Develop the solid model of the double line pipe drawing shown in Fig. 12.8.
  8. Develop the solid model of a spigot and socket joint shown in Fig. 12.3. The dimensions of various parts may be taken from Fig. 12.4.
  9. Show the solid models of a spigot and socket with one quarter removed so that the inside is clearly visible.
  10. Generate the orthographic projection views from the solid models described in Question 8.
  11. Develop the solid model of the gland and stuffing box expansion joint shown in Fig. 12.5.
  12. Why are hangers used in pipe construction? Explain their uses with sketches.
  13. What is meant by oblique or isometric pipe drawings?
  14. What are the advantages of AutoCAD over manual mode in the case of pipe drawing?