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Selection of Diameter and thickness: I.

Pipe sizing and Pressure drop Calculations: Pipe Sizing:

Before proceeding beyond a preliminary / design of piping system, it is necessary to determine the pipe inside diameter which allow reasonable velocities and friction losses. The maximum allowable velocities of the fluid in pipeline is that which corresponds to the permissible pressure drop from the point of supply to the point of consumption or is that which does not result in excessive pipe line erosion. Trade Practice – Steel pipes are designated by their OD or their Nominal ID. 

Due to manufacturing conditions, OD is constant.



Slight deviations from normal wall thickness, modify only the ID also called clear width.



Why a pipe is generally not referred to by its ID.



Common Engineering practice to use nominal bore NB to indicate the proper size of the individual parts employed in a pipeline (pipes, flanges, fittings and valves).



Nominal bore = actual inside diameter.



Selection of the diameter (flow rate anticipated pressure head available).



Pressure head (provided by booster pumps, compressors, natural head as in the case of gravity main).



Pressure head is necessary for transmission to overcome losses in the flow rate due to internal friction in the moving fluid or to rough inside surfaces of pipe.



Pressure drop increased through turbulence and separation of flow of bends or in branch connections, fittings, valves and similar parts (reduce the economy of any pipe line.

Velocity profile in Different System: The mean velocities of steam and water in different system shall be as follows: Medium

Mean velocity – M/Sec.

Superheated steam Main steam 140 bar 500 / 530°C 180 bar 530 / 550°C 250 bar 530 / 550°C

Nb – 150 – 200 – 400 Nb – 40 – 50 – 60 Nb – 30 – 40 – 50 Nb – 25 – 35 – 45

Hot Reheat 25 bar 530 / 540°C 40 bar 530 / 540°C 60 bar 530 / 540°C

Nb – 300 – 500 – 800 Nb – 40 – 50 – 60 Nb – 35 – 45 – 55 Nb – 30 – 40 – 50

Cold Reheat 25 bar 300°C 40 bar 340°C 60 bar 380°C

Nb – 300 – 500 – 800 Nb – 30 – 40 – 50 Nb – 25 – 35 – 45 Nb – 20 – 30 – 40

Extraction Steam 10 - 25 bar 0 - 5 bar

Nb – 150 – 200 – 400 Nb – 30 – 40 – 50 Nb – 35 – 45 – 55

Saturated Steam High pressure 80 – 100 bar Medium pressure 12 – 20 bar Low pressure 4 – 8 bar

Wetness

Condensate Intake of condense (before the condensate pumps) Intake of feed water tank Pump discharge Discharge of pipe (MC)25-35 bar Discharge of pipe (FW)100-150 bar Discharge of pipe (FW)200-400 bar

0.5 – 1.0 1.5 – 2.5 Nb – 100 – 200 – 400 2.5 – 2.5 – 3.0 3.0 – 3.5 – 4.0 3.0 – 4.0 – 5.0

Q

= ρAW

A

π = --------- d2 4

d

354025 x Qv = -------------------w

Where A

= Area, mm2

d

= inside diameter, mm

Q

= flow rate, Tonnes/hr.

w

= Velocity, m/sec

ρ

= Volume of medium, Kg/m3

Pressure drop calculation: The pipe sizes calculated based on the above recommended velocities do not relieve the designer to check the adequacy of pipe size from the flow friction consideration. Pressure drop calculations are of prime necessity in determining: a)

The selected inside diameter meets the available pressure drop in the case of main steam, cold reheat, hot reheat and auxiliary steam lines and miscellaneous water lines.

b)

The discharge pressure of the pump (boiler feed pump and condensate extraction pump).

For finding the frictional pressure drop in pipelines Darcy’s Formula can be universally used for almost all the fluids. With suitable restrictions for gases and vapours. As long as the pressure drop is around 10% of starting point pressure (which is true in most of the steam lines in thermal power station). Darcy’s formula for pressure drop can be used since the specific volume change in the line due to pressure loss will have little effect on calculated pressure drop. Calculation to determine the pressure drop in the pipe is made according to formula: a)

For straight pipe

flw2 ∆P = ----------------- kg/cm2 20000 g c dv b)

For bends, elbows, tees, valves, etc. Kw2 ∆P = ----------------20000 g c v

kg/cm2

Where, f=

Friction factor found from a graph between Reynolds No. and Relative roughness.

K=

resistance coefficient for fittings there are established based on experiments and are available in a standard table in various books.

l=

length of pipe in meters

V=

velocity in m/sec

gc=

gravitational constant – 9.81 m/sec2

d=

inside diameter of pipe in meter

v=

specific volume in m3/sec.

c)

Water (non-expansive flow) in compressible fluids.

∆P=

l w2 x λ γ ---- x ------------ ± h x λ di 2g

∆P=

absolute pressure in

l=

length of pipe line in ft.

di=

inside diameter of pipe in ft.

w=

velocity of flow in ft/sec

γ=

specific gravity in lb/cu.ft (water = 62 lb/cu.ft)

lb/ft2

g=

acceleration due to gravity (=32.2 ft/sec2)

h=

geodesic height in ft for lines other than horizontal

λ=

friction factor number dimension

+=

ascending lines

−=

descending lines

0

=

for horizontal lines.

Pressure decreases in linear perspective with the length of the line, while the velocity remains unchanged. Reynolds Number: • *

dimension less ratio characterizing the dynamic state of fluid. The inertia forces present in the fluid passing thro’ the pipe Re = --------------------------------------------------------------------------------forces of viscosity w di λ Re = -------------ζxg

ζ=

lb – sec * independant of Pr dynamic viscosity -----------ft2 * influids ζ↓ with T↑

w=

velocity, ft/sec

Re=

Vxλ G 11 x --------- = 11 x -----------104ζ di 104ζ di

V=

cu.ft/hr.

G=

weight of blow in lb/hr.

λ=

f(Re)

Variation of friction factors λ with relation di ---K

K = roughness of pipe interior natural wall roughness through formation of rust, sediment and similar influences.

Suggested Approximate values for the Roughness Coefficient K Kind of pipe

Drawn steel pipe

Condition of pipe wall

Values of roughness coefficient (K)

New Pipes

0.0008 to 0.002

New pipe, bitumen coated

0.002 to 0.006

Used pipes, with rust spots or moderately 0.006 to 0.02 encrusted Welded steel pipe After some years of service (main value for 0.02 to 0.04 cross counting gas supply lines) Pipes with big nest spots or badly 0.04 to 0.12 encrusted Pipes for water supply lines with big rust 0.06 to 0.14 spots Riveted steel pipe Varying riveting conditions

Cast Iron pipe

Laminar Flow:

0.04 to 0.32

New pipe, bitumen coated

0.004 to 0.006

New pipe, non-bitumen coated

0.01 to 0.02

Used pipes, with rust spots

0.04 to 0.06

Pipes showing incrustations

0.06 to 0.12

Pipes cleaned after some years of service

0.06

Re ≤ 2300 λ depends only on Re without the effect of the roughness of the pipe wall entering into its value.

Turbulence:

Transition zone between hydraulically smooth and rough behaviour. λ varies both with Re and di/K.

Turbulence:

Hydraulically rough behaviour.. λ depends only on di/K and is independent of Re.