LINE SIZING CRITERIA FOR VAPOR LINES

• Single-phase gas lines are normally designed based on the velocity criteria as specified in API 14E.
• The maximum recommended design velocity is 18 m/s,though higher velocities are also acceptable.
• Some  operating companies prefer to use (density*velocity^2) criteria.

          Operating pressure(gauge)                                                       (density*velocity^2) criteria
              P < 2000 kPa                                                                                   7,500 kg/m-s2

           2,000  < P < 5000 kPa                                                                        9,500  kg/m-s2

          5,000  < P < 8000 kPa                                                                        12,500 kg/m-s2

               P < 8000 kPa                                                                                 20,000 kg/m-s2

• COMPRESSOR DISCHARGE LINES                                                  VELOCITY
  

             P < 500 kPaG                                                                                    30 m/s

           500  < P < 1000 kPaG                                                                        30 m/s

          1,000  < P < 5,000 kPaG                                                                     30  m/s

               P < 5000 kPaG                                                                                30  m/s

•  COMPRESSOR SUCTION LINES   velocity is 20 m/s.    

• LINE SIZING CRITERIA FOR TWO-PHASE FLOW

 Maximum allowable velocity calculated for two phase-flow based on empirical relationship of erosion velocity.

                                       Erosion velocity=  C / density of two phase


 The value of empirical constant C is as follow:
      Type of service                                                                                    Value of 'C'

Continuous service                                                                                     122

Intermittent service                                                                                    152

For solid-free fluid where corrosion is not anticipated,or when corrosion is controlled by inhibitor , the value of C from 183 to 244 may be used for contentious flow, and value up to 305 can be used for intermittent service.

• RELIEF VALVE INLET LINE SIZING

           The inlet line to the relief valve is designed for a maximum pressure drop of 3 % of the relief valve set pressure.However the inlet line size should not be smaller then the inlet flange of the relief valve.

• RELIEF VALVE OUTLET LINE SIZING                                  
  

  The outlet line from the relief valve is designed based on the Mach number.Generally Mach number  in between 0.5 - 0.75 is maintained.However outlet line size should not be smaller then the outlet flange size of the relief valve. 
For calculating the velocity of sound at operating condition follow the link:

http://www.engineeringtoolbox.com/speed-sound-d_519.html

• LINE SIZING CRITERIA FOR GRAVITY FLOW LINES 

Normally, for gravity flow a slop of 1:100 is maintained ( 0.597degree) . For gravity flow, line with dimeter 200mm or less is designed for 50% liquid capacity and for a diameter more ten 200 mm, 75% capacity.
  

• DOWN PIPE SIZING

Down pipe sizing is an interesting  sizing requirement, particularly used to design distillation column internals.For distillation column the downpipes is designed in such a way that the pipes are self venting,i.e,while the liquid flows downward in the pipe, it disenggages the vapour which flow upward.
This type of downpipe is designed for a Froud number <0.3 .
  
If Froud number is < 0.3 , then the liquid flow will be self venting.
  
For  Froud number 0.3-2.0 ,the pipe will be full of liquid, with some possibility of vapor ingress at top of the pipe.
  
For Froud number > 2.0 , liquid will only flow through the pipe, and vapor diengagement sould occur before any liquid enters the downpipes. 

  Froud number = V/( ( g*D* (dl-dg)/dl)^0.5


where        g = 9.8 m/s
                  D= diameter of pipe,m
                 dl= density of liquid,kg/m3
                 dg= density of gas,kg/m3
                 V= velocity of fluid,m/s

or you can calculate the Froud number by following link:
http://www.engineeringtoolbox.com/froude-number-d_578.html