Piping design involves preparation of the Process Flow Diagram (PFD). The process flow diagram indicates the following parameters in a single line diagram:

1. Flowing fluid (Example: water, steam, etc.)

2. Fluid temperature (Degree C)

3. Fluid pressure (kg / sq cm (g))

4. Fluid mass flow rate (tonne / hour)

5. Direction of fluid flow

a) Pipe diameter (mm)

b) Pipe thickness (mm)

c) Pipe material

d) Pipe design code

e) Pipe material specification

f) Fluid flow velocity (m / sec)

g) Fluid properties

A.1) Mass balance

A.2) Energy balance

A.3) Scheme

A.4) Identification of skids

   The shear stresses (? xy, ? yz and ? zx) are equal to zero for the afore-said case. The afore-said equations are valid only for thin pipes under internal pressure. In the real life, the external pressure is present. The external pressure on the earth's atmosphere is approximately equal to 0.1 MPa (a). Hence, the above formulas are inaccurate. The inaccuracies in these formulas are taken care-of by using the gage pressure (for P = internal pressure) in place of the absolute pressure.

1. Location of equipment

2. Tag numbers of equipment

3. Tag number of lines

4. Tag number of valves

5. Tag number of instruments

6. Tag number of motors

7. Location of vents and drains

8. Type of valves

9. Type of instruments

10. Purpose of instruments

11. Output signal from instruments

12. Flow measuring devices

13. Level indicating devices

14. Equipment interfaces

15. Scope of suppliers

The intelligent P & ID used in process industry indicates the complete particulars of different components and piping.

    The piping layout is usually prepared as plans at various elevations. Where equipment and devices are located in the vertical runs of the piping, elevation views at different locations are shown. The layout of piping indicates the following items:

1. Pipe size (Pipe diameter and thickness)

2. Pipe material

3. Pipe bends

4. Pipe elbows

5. Valves

6. Gages

7. Pipe fittings

8. Pipe supports

9. Pipe restraints

10. Pipe anchors

11. Access

12. Approach

13. Walk-way

14. Platforms

15. Maintenance requirements

16. Fluid flow direction

17. Expansion joints

18. Floor elevation

19. Interface with equipment

20. Field joints

The layout gives a single line diagram of piping. This gives enough information for planning of work and activities at field. Layout drawing is prepared by the piping designer, in consultation with the related agencies. The layout diagram indicates the extent of floor and space required for piping. From the layout diagram, the piping isometrics are prepared.

    The isometric of the piping is a diagram shown in isometric projection. Right-handed system of co-ordinate axes is used for this purpose. The plant North direction is shown in the isometrics. The isometrics of piping are single line diagrams. This shows in three dimensions, the locations of various equipment, valves, gages, instruments, supports, hangers, anchors and restraints. The isometric of piping is used for construction. The isometric drawing contains Bill of Materials (B O M, also known as B O Q). The total weight of all the items covered in a single system is indicated. The isometric, in its final form, is used for field work.

    Isometric diagram indicates the transportable segments of piping. Usually the valves are not shipped with the piping. Even-though three dimensional bends can be fabricated, these are rarely used in practice. Required shipping lists are prepared to identify, store, retrieve and lift the piping at field.

    The drains, vents, safety valves, relief valves and piping pre-setting particulars are shown in isometric diagrams. The drawing is an Engineers Language and represents the information in a codified form to the down-stream agencies.

    The isometric diagrams are used for giving inputs to the piping stress analysis computer programs like CAESAR II and CAEPIPE. The outputs of the piping stress analysis are used to up-date the isometrics. As the design is an iterative process (based on trial and error process), the design of the piping is done in several stages. The method presently used for piping design is known as Design by Analysis Method. The methods used in the past are known as Design by Rules.

    As safety is of paramount importance, several aspects of the design are reviewed. Isometrics give a Birds Eye View of the piping. The presently used Plant Design Systems (PDS) and Plant Design Management Systems (PDMS) computer programs assist in preparation of piping isometrics.

    The piping used in engineering applications is subjected to corrosion and erosion. Several methods are available to take care of the corrosion and erosion. The following methods are used to take care-of corrosion in piping:

1. Provide additional thickness

2. Limit fluid flow velocity

3. Provide anti-corrosive materials

4. Use corrosion preventives

5. Use suitable surface preparation and surface treatment

6. Use electro anti-corrosive methods

The following methods are used to take care-of erosion:

    a) Provide erosion shields

    b) Provide erosion resistant materials

    c) Use acceptable fluid velocities

    d) Provide suitable surface preparation and surface treatment

In the power plants and process plants the corrosion and erosion of piping can take place on the inside or the outside of the piping. The approaches to address these two problems are different. The life of the plant and piping depends on the methods used for corrosion and erosion preventive methods.

The methods used for corrosion and erosion prevention are based on empirical methods and past experience. The behavior of materials subjected to corrosion and erosion are non-linear. The loads and load combinations used induce several stresses. If the material is lost due to corrosion and erosion, the stresses induced will be higher. This will lead to pre-mature failure.

Failed samples of power plants and process plants indicate that corrosion and erosion can not be completely avoided. Hence, the life of components is expected to be lower than the life of the plant. This indicates hat the components need to be replaced or repaired at required intervals.

The power plants and process plants are shut-down every year for annual over-haul. During the annual over-hauls, the components are closely inspected. After a complete inspection, the engineer and the management is to decide whether to run or repair or replace.

    Insulation and refractory are used for safety and economy. Insulation is usually applied on the outside of piping. Refractory is usually applied on the inside of piping. The insulation on the outside of piping is provided with outer casing. The outer casing is usually made of Aluminum. The refractory is usually provided with inner casing. The material of the inner casing is usually stainless steel. For lower temperature applications, low-alloy steel is used for inner casing. Insulation is used for lower to medium temperature applications. Refractory is usually used for high temperature application.

    The materials used for insulation are given below.

    1. Mineral wool

    2. Glass wool

    3. Slag wool

    4. Cast-able insulation

   The materials used for refractory are given below.

      a) Fire clay

      b) Fire brick

      c) Cast-able refractory

    The density of insulation vary from 100kg/cu m to 150kg/cu m. The density of refractory varies from 2,000kg / cu m to 2,500kg/cu m.

    The thermal conductivity of insulation and refractory varies with temperature. This aspect is taken care- of in selecting the material and thickness of insulation and refractory.

    For cryogenic application, shinning surfaces, made of mirror or a polished surface, is used. A shinning surface has lower emisivity. This reduces the heat loss or gain due to radiation.

    The surface temperature permitted on the insulated and / or refractory applied surface is 60°c to 65°c. This is based on human safety considerations.

38. Liner and Casing

    Liners are provided for retaining the insulation and refractory. It also avoids damage to insulation and refractory. Supports, in the form of rods, are provided for insulation and refractory. The supports are made of carbon steel or low-alloy steel.

    Liners can be of plain sheets or corrugated sheets. The casing is used for pent house, wind box and economizer casing. The design procedure for liner and casing are different. The following loads are considered in the design of liner and casing:

1. Self-weight

2. Attachments

3. Wind load

4. Seismic load (Earth-quake load)

Suitable load combinations are to be considered in design. The design involves calculation of loads, calculation of load combinations, selection of materials, selection of thickness, layout of stiffeners, sizing of stiffeners and drafting.

The liner casing provided in the pulverized coal pipes inside is for erosion protection. The cement liners provided for the water lines inside is for hygienic design. The liners made of Aluminum have better heat radiating properties. This reduces heat loss. The liners are usually of 1.0 mm or 1.6 mm thickness. In calculating the loads on the liner, the imposed loads (live loads) are not considered. This is from economic considerations. However, where imposed loads are likely, the same are considered.

High temperature applications like the gas turbine outlet are provided with ceramic lining on the duct inside. Ceramic has better heat withstanding capacity. Ceramics are to be handled carefully, since, they are brittle.

    The inside and outside surfaces of piping are prepared to avoid corrosion and to ensure adequate life. The following surface preparations are popularly used:

1. Wire brushing

2. Sand blasting

3. Shot peen the surface

4. Pickling

5. Blackening

6. Nitride treatment

7. Cyanide treatment

8. Phosphate treatment

9. Galvanizing

10. Surface hardening

The surface treatment ensures life and aesthetics. The following surface treatments are popularly used:

1. Painting

2. Ceramic treatment

3. Cement lining

4. Coating

5. Wire netting

The surface preparation and surface treatment of the piping are selected based on the past experience and experiments. The science of surfaces is under constant research. New methods and procedures are under evolution. The cost of surface preparation depends on the types used. The surface preparation and surface treatment cost is one to three percent of the piping cost. Periodic inspection and application of surface coating is required assure required life.

The valves, gages, fittings and instruments require special care and attention. The environment of the piping plays a major role in selection of surface preparation and treatment.