Line Sizing Calculation (Part 1)

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1.            DESIGN BASIS

1.1.    Flow Rate, Temperature and Pressure

Flow rate, pressure, and temperature for all process lines used for line sizing calculation will be taken from the material balance from Process Flow Diagram.

1.2.    Line Sizing Criteria

The criteria shown on Table 1 will be used to determine the line size. Such criteria are based on the engineering practice used to select the economic size of piping inside the plant battery limit.



Items
Criteria for Nominal Pipe Size Selection

1. Gas/Vapor Lines

Pressure Drop(3)
- DP per 100 m  < 0.117 bar (P < 34.3 barg)
- DP per 100 m  0.117-0.274 bar (34.3 < P < 137.2 barg)

Velocity(1)
-Velocity < 18.3 m/s

2. Liquid Lines

For Continuous & Non Continuous Lines:
- DP per 100 m  < 0.9 bar (2)
- Velocity: <4 .6="" m="" s="" sup="">(1)
and must below than Verosion



3. Two Phase Line

Velocity(4)
- Velocity 4.6-15.2 m/s
- Velocity must below than Verosion


Note:(1) Recommended Practice for Design and Installation of Offshore Production Platform Piping
                        System, API RP 14E.
                    (2) Flow of Fluids through Valves, Fittings, and Pipe     
                                             (3) Norsok Standard No. P-CR-001-Rev 2
                                             (4) Surface Production Operation Vol. 1

Table 1. Line Sizing Criteria (Main Process Line)

3.4.    Equations

3.4.1.  Liquid phase
Liquid velocity  can be determined from following equation:
……………………………(1)
where    Q          : liquid flow rate, kg/hr
                 r           : liquid density, kg/m3
                        d           : pipe ID, mm
              n           : mean velocity of flow, m/s
Pressure drop (DP, kg/cm2/100 m) is determined by fanning equation from API 14E:
……..………..…………(2)
       Where   f           : Moody friction factor
                                 Q          : liquid flow rate, m3/hr
                          S           : liquid specific gravity, dimensionless (water=1)
                          d        : inside pipe diameter, mm
Reynolds number (Re) of flow in pipe is calculated from the following equation:
.………………..…(3)

Where   d   : inside pipe diameter, mm
                             n   : mean velocity of flow, m/s
                             r   : liquid density, kg/m3
                             m   : viscosity, kg/m.s
Friction factor is calculated based on the following Colebrook's equation (ref: Oil field processing of petroleum vol.1
If Re>2000, use equation :
    
                                                                                                           ..……………….……(4)
Where              f           : Moody friction factor
                             e          : surface roughness, mm
                             D        : internal diameter, mm
          Re     : Reynolds number
3.4.2.  Gas phase
           Actual gas velocity can be determined by:
          
         
where  Qg  : gas flow rate, Std.m3/hr
T  : temperature, oK
P  : pressure, kg/cm2a
                        d  : inside pipe diameter, mm
v   : gas velocity, m2/sec
Z   : gas compressibility factor, dimensionless

For compressible fluid with total pressure drop less than 10% of its initial pressure, the pressure drop (DP, kg/cm2/100 m) is calculated using equation from API 14E as follows:
                          
                                               ……………….……….(6)

S            : gas specific gravity at standard condition, dimensionless
Qg         : gas flow rate, std.m3/hr
Z    : compressibility factor, dimensionless
          T1  : flowing temperature, oK
f    : Moody friction factor, dimensionless
   L            : length of pipe, m
P1          : upstream pressure, kg/cm2a
d    : inside pipe diameter, mm
3.4.3.  Gas/liquid Two-phase
The fluid erosional velocity (Ve, m/s) is calculated using the following equation (API 14E):
           
Total weight flow of fluids is the sum of weight flow of gas and weight flow of liquid. Weight flow of gas can be calculated by equation below:

                                                                           ……………………....…….(8)

Where:
           Gas flow           : mass gas flow (kg/s)
Qg                     : volumetric volume of gas, Std m3/hr
Sg                      : gas specific gravity
Note: standard gas volume 0.024 m3/gmole is taken at pressure 1 atm and temperature 15oC

Weight flow of liquid can be calculated by equation below:

        
                                                                                  …………………………..………...(9)

Liquid flow          : mass liquid flow (kg/s)
Ql                        : volumetric volume of condensate, m3/hr
Sl                                        : gas specific gravity
                         
                     Total fluid mass flow is the sum of gas flow and liquid flow:

                                                                                                                            …..…………....(10)
                                                                  
The total volume flow would be:

                                                                                                   ….……..……...(11)
         Where :
Volume flow       : volumetric flow, m3/s
Total flow           : fluid mass flow, kg/s
rmixture            : gas/liquid mixture density, kg/m3

Mixture velocity could be determined from following equation:

                                                                                                          ……….…….…..….(12)
Where :
V  : velocity, m/s
d   : inside diameter of pipe, mm

The pressure drop (DP, kg/cm2/100m) in a two-phase steel piping system is estimated using simplified Darcy equation (API 14E):

                                                                                  ………………………..…(13)
f    : Moody friction factor, dimensionless
d   : inside pipe diameter, mm
rm : gas/liquid mixture density, kg/m3
W : total liquid plus vapor rate, kg/h


Please go to part 2 for example of calculation .

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