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Compressible Fluid Notes


Introduction

The notes below relate primarily to compressible fluids flowing in pipes.  The notes are of a basic level sufficient for a mechanical engineer to be able to estimate operating conditions in a pipeline transferring vapours or gases.

It is much more difficult to determine the operating characteristics of compressible fluids (vapours and gases) as the density is not constant under flowing conditions.

The extremes conditions encountered are adiabetic flow (PVγ = constant ) or isothermal flow (PV = constant).     Adiabetic conditions occur when no heat is transferred across system boundaries.  Isothermal conditions occur when the system changes occur at constant temperatures.  For short insulated pipes adiabetic conditions can be assumed.  For long pipes with reasonable levels of insulation isothermal conditions provide good approximations to real conditions...  Operating conditions occuring at some point between the extremes can often be related to the polytropic process (PVn = constant)

Compressible fluid flows also have a maximum velocity which is limited by the speed of propogation of a pressure wave travelling at the speed of sound for the fluid under consideration.   If the differential pressure along a pipe is such that the fluid velocity approaches sonic speed then any further increase in differential pressure will not be accompanied by an increase in fluid velocity.

For many real life pipe flow conditions it is possible to use the Darcy equations and factors as provided on webpage Pipe Flow Calcs subject to the following restrictions.

1).. If the calculated (estimated ) pressure drop (upstream (P1)  -  downstream (P2 ) is less than 10% of the inlet pressure (P1) then reasonable accuracy is achieved if the specific volume used is based on the known conditions. (Upstream or downstream).

2)..If the calculated (estimated ) pressure drop is greater than 10% but less than about 40% of the inlet pressure P1 then reasonable accuracy is achieved by using a specific volume based on the average of the downstream and upstream pressures.

3)..If the calculated (estimated) pressure drop is greater than 40% of P1 then methods as identified on this page should be used.


Symbols

a = Acceleration (m/s2
A = Area (m2)
a = Speed of sound (m/s)
F = Force (N)
g = acceleration due to gravity (m/s2 )
h = fluid head (m)
K = Bulk modulus (MPa )
L = Pipe length (m )
m = mass (kg)
m = mass flow rate (kg/s)
M = mach number u /a
M = Molecular weight
P1 = Inlet fluid pressure (gauge) (N /m2 )
P2 = Outlet fluid pressure (gauge) (N /m2 )
P1 = Inlet fluid pressure (abs) (N /m2 )
P2 = Outlet fluid pressure (abs) (N /m2 )
P - Absolute pressure (N /m2 )
pgauge - gauge pressure (N /m2 )
patm - atmospheric pressure (N /m2 )
p s= surface pressure (N /m2 )
Q = Volume flow rate (m3 /s)
q = Heat transfer /unit mass (J/kg)
R = Gas Constant (J/(kg.K)
Ro = Universal Gas Constant (J/(kg.mol.K)
ρ = fluid density (kg /m2 )
s = specific volume (m3 /kg)
u = fluid velocity (m/s)
v = specific volume (m3/kg)
v1 = specific volume at inlet conditions(m3/kg)
x = depth of centroid (m)
β = Compressibility (1/MPa)
θ =slope (radians)
ρ = density (kg/m3)
ρ r = density (kg/m3)
τ = shear stress (N /m2)
μ = viscosity (Pa.s)
ν kinematic viscosity (m2Ěs-1)
υ = Specific volume (m3 / kg)
γ= Ratio of Specific Heats



Compressible Fluid Flow equations.

The flow of compressible fluids in long lines approximates isothermal conditions.   The flow rate in a pipe under isothermal conditions is provided by the equation below...

This equation has been developed on the basis of a number of assumptions including: Isothermal flow, steady flow, perfect gas laws apply, constant friction value, straight and horizontal pipe.   The equation below is a simplified version and assumes no acceleration along streamlines.




Limiting Flows.

The equations above do not take into account the fact that , for a particular fluid, there is a maximum speed which cannot be exceeded in the compressible fluid flowing in a pipe.

The maximum velocity of a compressible fluid is limited by the velocity of a pressure wave travelling at the speed of sound in the fluid. Reference Sonic velocity.    Clearly the maximum velocity will be at the downstream end of the pipes as the velocity will progressively rise as the pressure falls resulting in a increase in the specific volume.  Now if the pressure drop is sufficiently high such that sonic velocities is about to be exceeded the resulting pressure decrease and hence driving force will not be transmitted upstream and consequently there will be no increase in flow rate.

The sonic velocity which cannot be exceeded is expressed as

vs = (γRT ) = (γP v )



More Notes to follow -March -2007


Table of Air Flows through sched 40 Piping.

Notes : Factors for other conditions..

1) For inlet pressures  (p o) other than 7 bar gauge... [    Multiply table pressure drop value by 8,013/(p o + 1,013)     ]
2) For inlet temperatures  (t o) other that 15 deg C... [   Multiply table pressure drop values by (273 + t o ) / 288    ]
3) For Pipe sizes (d o ) other than sched 40 ( d 40 )... [   Multiply table pressure drop values by (d 40 /do ) 5    ]
4) Pressure drop is proportional to length. For pipe lengths l o other than 100m.... [   multiply table presuure drop by l o /100   ]
It is important to note that this table should only be used for crude estimates. For serious work then detailed calculations should be used.



Pipe Sizes 1/8" to 2"

Pressure drop of air in bars per 100m of schedule 40 commercial pipe
Air Flow
m3/min
15 Deg C
1,013 bar abs
 Pipe Size (Sched. 40)
1/8" 1/4" 3/8" 1/2" 3/4" 1" 1 1/4" 1 1/2" 2" Inches
3 6 10 12 20 25 32 40 50 mm
6,8 9,2 12,5 15,8 21 26,6 35,1 40,9 52,5 ID(mm)
0,03 0,093 0,021 0,0045 -          
0,06 0,337 0,072 0,016 0,0051          
0,09 0,719 0,154 0,033 0,011          
0,12 1,278 0,267 0,058 0,018          
0,15 1,942 0,405 0,087 0,027 0,0067        
0,2 3,357 0,698 0,146 0,047 0,011 0,0035      
0,3 7,554 1,57 0,319 0,099 0,024 0,0073      
0,4   2,71 0,548 0,17 0,041 0,012      
0,5   4,1 0,842 0,257 0,062 0,018      
0,6   5,9 1,19 0,37 0,088 0,026 0,0066    
0,7   8,03 1,62 0,494 0,117 0,035 0,0086 0,0041  
0,8     2,12 0,634 0,15 0,044 0,011 0,0053  
0,9     2,64 0,803 0,187 0,055 0,014 0,0065  
1,0     3,26 0,991 0,231 0,067 0,017 0,0079  
1,25     4,99 1,55 0,353 0,102 0,026 0,012  
1,5     7,2 2,19 0,499 0,147 0,036 0,017 0,0048
1,75     9,79 2,98 0,679 0,196 0,047 0,022 0,0064
2,0       3,82 0,871 0,257 0,062 0,029 0,0082
2,25       4,84 1,1 0,325 0,076 0,036 0,01
2,5       5,97 1,36 0,393 0,094 0,045 0,012
3,0       8,6 1,92 0,565 0,135 0,063 0,018
3,5         2,61 0,754 0,184 0,086 0,024
4,0         3,41 0,984 0,236 0,11 0,03
4,5         4,32 1,25 0,298 0,136 0,038
5,0         5,34 1,54 0,368 0,164 0,046
6         7,68 2,17 0,518 0,236 0,066
7           2,95 0,689 0,321 0,09
8           3,85 0,9 0,419 0,115
9           4,88 1,14 0,53 0,145
10           6,02 1,41 0,64 0,179
11           7,29 1,71 0,774 0,217
12           8,67 2,02 0,921 0,252
13             2,38 1,08 0,295
14             2,76 1,25 0,343
15             3,13 1,44 0,393
16             3,57 1,64 0,443
17             4,01 1,85 0,5
18             4,49 2,07 0,558
19             5,01 2,31 0,618
20             5,49 2,53 0,685
22             6,65 3,07 0,825
24             7,91 3,61 0,982
26             9,28 4,22 1,15
28               4,86 1,33
30               5,62 1,52
32               6,39 1,73
34               7,22 1,94
36               8,09 2,17
38                 2,41
40                 2,67
45                 3,36
50                 4,15
60                 5,98
70                 8,14
Inches 1/8" 1/4" 3/8" 1/2" 3/4" 1" 1 1/4" 1 1/2" 2"
mm 3 6 10 12 20 25 32 40 50
ID(mm) 6,8 9,2 12,5 15,8 21 26,6 35,1 40,9 52,5


Pipe Sizes 2 1/2" to 12"

Pressure drop of air in bars per 100m of schedule 40 commercial pipe
Air Flow
m3/min
15 Deg C
1,013 bar abs
 Pipe Size (Sched. 40)
2 1/2" 3" 3 1/2" 4" 5" 6" 8" 10" 12" Inches
3 80 90 100 125 150 200 250 300 mm
62,7 77,9 90,1 102,3 128,2 154,1 202,7 254,5 303,3 ID (mm)
2,25 0,0042                
2,5 0,0051                
3,0 0,0073                
3,5 0,0097                
4,0 0,012                
4,5 0,016 0,0051              
5,0 0,019 0,0063              
6 0,027 0,009              
7 0,036 0,012 0,0059            
8 0,047 0,015 0,0075            
9 0,058 0,019 0,0094            
10 0,072 0,023 0,011            
11 0,085 0,028 0,014 0,0073          
12 0,101 0,033 0,016 0,0085          
13 0,119 0,039 0,019 0,0098          
14 0,138 0,045 0,022 0,011          
15 0,158 0,051 0,025 0,013          
16 0,178 0,058 0,028 0,015          
17 0,2 0,065 0,031 0,016          
18 0,223 0,072 0,035 0,018          
19 0,247 0,081 0,039 0,02          
20 0,266 0,089 0,043 0,022 0,0072        
22 0,328 0,107 0,052 0,027 0,0086        
24 0,388 0,126 0,061 0,032 0,01        
26 0,455 0,148 0,071 0,037 0,012        
28 0,525 0,171 0,082 0,043 0,014 0,0054      
30 0,603 0,197 0,094 0,049 0,016 0,0061      
32 0,682 0,222 0,106 0,055 0,018 0,0069      
34 0,77 0,251 0,119 0,062 0,02 0,0078      
36 0,863 0,28 0,134 0,07 0,022 0,0087      
38 0,957 0,312 0,148 0,077 0,024 0,0096      
40 1,05 0,346 0,164 0,086 0,027 0,011      
45 1,33 0,435 0,207 0,107 0,034 0,013      
50 1,65 0,534 0,254 0,132 0,042 0,016      
60 2,37 0,765 0,363 0,188 0,059 0,023 0,0058    
70 3,23 1,03 0,495 0,254 0,08 0,031 0,0077    
80 4,22 1,35 0,639 0,332 0,104 0,04 0,01    
90 5,34 1,7 0,808 0,418 0,13 0,051 0,013 0,0041  
100 6,59 2,1 0,992 0,513 0,16 0,062 0,015 0,005  
110 7,97 2,54 1,19 0,621 0,192 0,075 0,019 0,006  
120 9,49 3,02 1,42 0,739 0,228 0,089 0,022 0,0071  
130   3,55 1,67 0,862 0,267 0,103 0,026 0,0082  
140   4,12 1,93 1 0,308 0,12 0,029 0,0095  
150   4,73 2,22 1,15 0,353 0,138 0,034 0,011 0,0045
200   8,4 3,94 2,03 0,628 0,243 0,059 0,019 0,0078
250     6,16 3,17 0,975 0,378 0,09 0,029 0,012
300     8,88 4,56 1,4 0,54 0,129 0,041 0,017
350       6,21 1,9 0,735 0,174 0,056 0,023
400       8,11 2,48 0,96 0,227 0,072 0,03
450         3,14 1215 0,286 0,091 0,037
500         3,88 1,5 0,352 0,112 0,046
550         4,69 1,82 0,424 0,134 0,055
600         5,58 2,16 0,504 0,16 0,066
650         6,55 2,54 0,592 0,188 0,076
700         7,6 2,94 0,686 0,218 0,089
750         8,72 3,38 0,788 0,248 0,101
800           3,84 0,896 0,282 0,115
850           4,34 1,01 0,319 0,13
Inches 2 1/2" 3" 3 1/2" 4" 5" 6" 8" 10" 12"
mm 3 80 90 100 125 150 200 250 300
ID (mm) 62,7 77,9 90,1 102,3 128,2 154,1 202,7 254,5 303,3




Useful Links
  1. Free Calc-On-Line Compressible Flow Pressure Loss.. Online easy to use calculator
    Free Calc-On-Line Compressible Flow Friction Loss..
  2. Compressible Fluids ..pdf Download - Relatively advanced approach to the topic of compressible fluids
  3. Compressible Fluids ..Short but relevant notes
  4. Fundamentals of Compressible Fluid dynamics ..~A large downloadable document -very comprehensive


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Last Updated 28/01/2013