Best Way to Learn Process Design of Piping

The process design of piping involves finding a balance. This balance is between the size or diameter of the pipe and the pressure drop within it.

For a particular flow rate of fluid, selecting a larger pipe gives a lesser pressure drop. However, it increases the fixed cost of the pipe. On the other hand, a lower pressure drop means lower power consumption or lower operating cost.

Lower pressure drop reduce the size of the fluid-moving device (e.g. pump, fan, blower), so it reduce the fixed cost of the pump.

The fixed cost of the pipe increases with its diameter. Meanwhile, the pumping cost decreases as the diameter of the pipe increases.

Here, we need to balance the pipe diameter with the pressure drop in the pipe. We must find the optimum value of the pipe diameter.

Optimum Pipe Diameter

The optimum pipe diameter depends on the current cost of material. It also relies on the cost of power. Additionally, it depends on the rate of interest at a particular place and time. It changes with place and time.

Equation below is used to find the optimum diameter of the Carbon Steel Pipe.

This equation is valid for the turbulent flow of an incompressible fluid

Where, d_optimum = Optimum pipe size, (mm)
G = Mass flow rate, (kg/s)
ρ = Density of fluid, (kg/m³)

Equation below is used to find the optimum diameter of Stainless Steel Pipe

This equation is valid for the turbulent flow of an incompressible fluid

where, d_optimum = Optimum pipe size, (mm)
G = Mass flow rate, (kg/s)
ρ = Density of fluid, (kg/m³)

Standard Pipes

All standard pipes are available from 3 mm (1/8 in) to 600 mm (24 in) size.

Standard pipes are specified with three different diameters,

1. Inside Diameter,
2. Outside Diameter
3. Nominal Diameter.

For standard pipes with a diameter exceeding 300 mm (12 in), Nominal Diameters match the actual Outside Diameter. Still, for smaller pipes, there is no relation between Nominal Diameter and Inside Diameter or Outside Diameter.

Standard Pipe Size Chart

Wall Thickness of the Standard Pipe

The schedule number indicates it is as per the US standards.

where, p_sf = Safe working pressure, N/mm²
σ_s = Safe allowable stress, N/mm²

The thickness of standard pipe increases with increases in schedule number.

Seamless standard pipes do not have welding joints.

Fabricated pipes are made by rolling the plates. The ends of the plate are then joined by welding. These pipes are also known as Electric Resistance Welded (ERW) pipes.

Using the following equation the thickness of the pipe, subjected to internal pressure.

where, t = Thickness of pipe, mm
p = Internal design pressure, N/m²
r_i = Inside radius of pipe, mm
r_o = Outside radius of pipe, mm
σ = Allowable stress of pipe material at design temperature, N/m²
E = Joint efficiency; for seamless standard pipe, E = 1
CA = Corrosion allowance, mm

Suggested Fluid Velocities in Pipe

These are only for approximate calculations of pipe diameter. It can be used for the quick calculation of short-distance pipelines. It is also useful for estimating pipe size as a starting point for pressure drop calculations.

Table 1: Suggested Fluid Velocities in Pipe

Pressure Drop-in Pipe

Fanning or Darcy equation gives the relation between pressure drop and pipe diameter. It is derived for steady flow in uniform circular pipes running full of liquid under isothermal conditions.

where, Δp = Pressure drop, Pa
L = Length of pipe, m
ṁ = Mass flow rate of fluid, kg/s
ρ = Density of fluid, kg/m³
Di = Pipe inside diameter, m
v = Velocity of fluid, m/s
f = Fanning friction factor

The friction factor is a function of the Reynolds number (Re) and the roughness of the inside surface (ε).

Table 2 – Values of Surface roughness (ε) for various materials

A plot of Fanning friction factor as a function of Reynolds number (Re) and relative roughness, ε/D, is given as

A more accurate relationship between f and Re for turbulent flow is given by

where, Δp = Pressure drop, kPa
L = Length of pipe, m
Di = Pipe inside diameter, mm

Pressure Drop in Fittings and Valves

In addition to pipes, the piping system contains fittings and valves. These fittings and valves offer additional frictional loss or additional pressure drop. This additional frictional loss of a fitting or of a valve is expressed in two ways. One way is as an equivalent straight pipe length (Le). Another way is as several velocity heads (K) lost in a pipe of the same size and material.

Equivalent Length of Pipe (Le) for Fittings and Valves

The equivalent length of a valve or a fitting is the length of a straight pipe of the same size. This straight pipe creates the same friction loss as the fitting or the valve being considered.

Often, Le is expressed in terms of the inside diameter of the pipe.

Then Le = (Le/Di) Di,

where, Di = Inside diameter of the pipe.

Table 3 – Values of Le/Di for valves and fittings

Another way of calculating pressure drop through the fittings and valves is the use of factor K.

“Number of velocity heads (K) lost in pipe” for fittings or valves is defined by the equation

where, ΔF = Additional frictional loss, J/kg or N · m/kg
Δp = Additional pressure drop, N/m²
v = Average fluid velocity through the pipe of the same size as valve or fitting, m/s

Table 4 – Values of K for normally used fittings and valves are given in below

References

Sinnott, R. K. (2005). Coulson & Richardson’s CHEMICAL ENGINEERING VOLUME 6 FOURTH EDITION Chemical Engineering Design. Elsevier.

Thakore, S. B. and B. B. I. (2015). Introduction to process engineering and design. McGraw-Hill Education.

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