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Steam Pipes Sizing: Correct sizing of steam lines

Steam Pipes Sizing or Correct sizing of steam lines reduces pressure drop and radiation loss. Steam pipes that are undersized cause high pressure drop and result in steam starvation at the point of usage. Conversely, over sized steam pipes, are in no way detrimental to the operation of the plant but lead to an unnecessary increase in the capital cost of installation. In addition over sized pipes also increase the running costs due to radiation losses.

The steam pipe sizing can be done on the basis of two methods namely;

A. Velocity

B. Pressure Drop

A. Pipe Sizing based on Velocity

Steam pipe sizing calculations are based on the specific volume of steam while using the velocity method. For dry saturated steam lines, the velocities considered range between 25-35 m/s. or 80-120 ft. /sec. For superheated steam, the velocities can be up to 40m/sec.

Illustration

Consider a steam line with steam passing through it at a pressure of 5 bar g and the flow rate of 2000kg/hr. The density of steam at 5 bar g is 3.11 kg/m3. Consider that the steam is travelling at a velocity of 25 m/s. Hence the pipe size can be calculated by using the formula:

D = 4 XQ / (π X V X ρ)

Where,

D = Diameter of the pipe (mm)

Q= Flow rate of steam (kg/hr)

V= Velocity of Steam (m/s)

Ρ = Density of Steam (kg/m3)

Using the formula,

The pipe size = 100 mm

However, sizing steam pipes on the basis of velocity does not capture the length of steam piping network and travel. Longer the network, greater will be the pressure drop, and hence this will lower the available pressure at the point of usage.

B. Pipe Sizing on the basis of Pressure Drop

Steam pipe sizes can be calculated on the basis of pressure drop by using the following formula

(P 11.9375 - P 2 1.9375)/L = Q 1.853 / 0.0103063 * D4.987

Where,

P1= Initial Pressure (bar a)

P2= Final Pressure (bar a)

L= Length of pipe (m)

Q = Steam Flowrate

D= Internal Pipe Diameter (mm)

Illustration

Consider for example a steam line having an upstream pressure of 8 bar a and downstream pressure of 7 bar a. Consider that the length of the pipe is 3 m and it accommodates a steam flow rate of 3000kg/hr. Applying the formula mentioned above the pipe size equals 80 mm