PIPING AND LINE SIZING IN CHEMICAL PLANTS

 Line sizing is one of the most important factor for preparing a P&ID and adequate efforts are to be put into for proper line size, Line rating and material of construction.

 Here we will discuss the basic engineering aspects of the pipe line sizing.

 CHECK LIST FOR PIPING IN P&ID

 1)     Prior starting detail designing it is necessary to put down all the existing related pipe lines on the paper including correct Line number, Rating & size. This will help to decide the line rating in the modification/design. 

 2)     Exact Tie In and Tie Out points to be defined in the drawing. These points should be indicated such that the whole PID is self explanatory.

 3)     First of all clearly mention the allowable Pressure drop that this system has. Secondly state the available pressure drop for the line based on velocity & length criteria. This is because if the allowable pressure drop is too high one should not design line for very high PD, instead a let down valve can be provided. Line is a tool for transportation and not for pressure drop.

 4)     HYDRAULICS

 If isometric drawing is available put down each and every element/fitting for pressure drop calculation. In absence of isometrics and line layout it is very difficult for an engineer to decide line size. For this following method can be followed for fairly correct results.

 Take the equivalent line length = 3 times the straight line length

 Velocity is another crucial factor for determining line size. Various limits for allowable velocity is listed at Annexure -1for user benefit.

 After taking into account the velocity and pressure drop criteria, a sensitivity analysis to be made for one size higher and one size lower. This will be helpful in optimizing line size.

•  Clearly indicate the fittings, valves (gate, globe, etc) , equipments in the PID.
•  Define the modifications clearly in either red lines / dotted lines / dark lines. These indications should be consistent throughout the   document.
•  Show the change of line rating in the drawing.
•  Ensure proper NRV or Blind provision where there is a connection of LP & HP fluids. Ensure change of Rating at the appropriate valve / NRV.
•   BASIS OF FLOW / PRESSURE / TEMPERATURE TO BE CHECKED AND CROSS CHECKED.
•  Drain & vent line to be provided with double isolation / one isolation + blind if operating pressure is above 30 BarG.

 ANNEXURE-1

VELOCITY CRITERIA FOR GAS & LIQUID SERVICES

 I- VENT LINE SIZING

Vent line size is determined as follows :

 LINE VELOCITY

Assume Mach number 0.2 to 0.5.

 Mach number = u / ((k x R x T/M)^0.5)

where

      R = 8.30873 x 10^3 kg.m2/(sec^2-°K-kgmol)

     T = Temperature in °K

     M = Molecular weight

     k = Cp/Cv

     u = Velocity in m/s.

 The pressure drop between PVC outlet and flare header is calculated and confirm the pipe rating.

If PCV back pressure is above the pipe rating, the line size shall be increased.

For more detail refer API RP 521.        

II – STEAM AND CONDENSATE & WATER LINE SIZING.

Services                                                  Velocity in m/s

Low Pressure Steam                                 15 ~ 30

Medium Pressure Steam                          20 ~ 40            

High Pressure Steam                               30 ~ 60

Liquid subcooled (Pump discharge)         1 ~ 3

Liquid sub cooled (Pump suction)             0.5 ~ 1.5

Liquid boiling (Pump suction)                    0.2~ 0.5

Gravity Flow                                             < 0.5

III – GAS (KU)

Gas (upto 2 Bar)              

               Wet (Below Dew point)              5 ~ 10

               Dry (Above Dew point)              15 ~ 25

Gas (above 2 Bar)

               Wet (Below Dew point)              1 ~ 10

            Dry (Above Dew point)              15 ~ 30

Compressed air / O2                                15 ~ 25

Compressed air (suction)                         ~ 10

Special care to be taken for O2/H2/C2H2 etc lines.