What is Line Sizing and How it is done?

It's a very common problem for students and all fresher’s who start their careers as either GET or Management Trainees in Process Industry to get familiar with terminologies used in process industries. Here i have tried to simplify and provide most basic level information when one is at the beginning level of his/her career. 

Usually a Line refers to a Pipe and Line size refers to its ID (Internal Diameter) which can be calculated by using the continuity equation. Usually we have Fluid normal/standard flow rates & we are asked to calculate the line size. Now the question is how to do it.

We have a relation 

Actual Flow Rate(m3/s) = (Maximum Allowable Velocity)x(Cross Sectional area of pipe)

i.e. Q = a.v

Where Q = Actual Volumetric Flow Rate (m3/s)

           V = Maximum Allowable Velocity ≤ Erosion Velocity (m/s)

           A = Cross Sectional Area of Pipe (Pi/4 *ID²) (m2)

           ID= Internal Diameter of Pipe (m)

In most of the practical industrial problems you will have Volumetric Flow rate but you will not have the Maximum Allowable Velocity or Erosion Velocity (of course no question of Pipe ID, as you have to calculate this).

Rule of thumb says that maximum allowable velocity for Gases is 29 m/s while optimum is considered as 20 m/s. while for liquids minimum velocity should be kept as 0.9 m/s and maximum is 4.5 m/s. Optimum velocity is considered between 1.5 – 2.0 m/s.

Kindly note I have not considered pressure drop in this line sizing. However; one can control the pressure drop within the range by increasing or decreasing pipe Diameter on excel file.

Now the important part is that why to do Line Sizing?

Of Course to control the pressure drop to keep our pumping cost low & to make sure that a Line has such a diameter that velocity of fluid flowing inside the line do not crosses maximum allowable limit (normally Erosion Velocity) to prevent damage of line due to Erosion.

Now in industries, one of the major problems is that you will never get actual flow rate. Your flow rate will be given as either standard flow or normal flow for gases. Some of the common Units are as follows:

1.     MMSCFD             : Million Standard Cubic feet per day

2.     MMSFH                : Million Standard Cubic Feet Per Hour

3.     SCFM                   : Standard Cubic feet per minute

4.     Sm³/hr                  : Standard Cubic meter per hour

5.     Nm³/hr                  : Normal Cubic meter per hour

6.     Kg/hr                    : Mass Flow Rate

Now important point is to note that these all flow rates are not actual Volumetric Flow rates.

For 1 to 4 units, convert it to Sm3/hr   and then convert it to actual flow rates by the help of following Relation

(P1V1)/T1 = (P2V2)/T2             i.e. V Actual     V2 = (P1V1T2)/P2T1

                                                               

V Actual =  (V1 Standard x Standard Pressure   x Actual Temperature (Max.)) divided by  (Actual Pressure (Min.) x Standard Temperature)

Here V stands for Volumetric Flow Rate, Pstd. 1 atm, Tstd. 25 Degc / 298 K

Similarly for Flow rate in Nm³/hr be converted to Actual Flow Rate using above formulae. Only T Normal will reduce to 0 Deg C or 273 K.

It is interesting to note that in actual life problem we will be given a pressure range. For line Sizing one shall consider minimum pressure and maximum temperature in that range which will enable us to know largest Pipe ID.

Kindly note that all units should must be in same convention to cancel each other to give the unit of calculated parameter.

Let me share an example for simple Line sizing, as next examples very complex.

You have been given Mass Flow Rate of Gas 69084 Kg/hr. 
Molecular Weight of Gas is Given as              16.48 kg/kgmol
Temperature is Given as                                60 Deg C
Pressure                                                        11.2 bar

Solution: Now remember the formulae for Sizing a Line, You require Volumetric Flow Rate which can be obtained by Dividing Mass flow by Density.

Now the point is how to Calculate Density, Firstly convert your temperature in Rankine which is 599.67 for given case & Pressure in PSI which is 162.4 in this case.

Now if you remember basic chemical equations,
Density= (Mol. Wt x Pressure) / (R x T) which is =(16.48*162.4)*(10.73*599.67)
  = 0.416 lb/ft3 which is 6.66 kg/m3.

Hence your Volumetric Flow Rate is = (69084)/(6.66*3600)= 2.88 m3/s

Now take Allowable velocity for gas = 20 m/s

Now your Line Size = Sqrt((Actual Vol. Flow Rate) / (velocity*pie/4)) in m
                             = SQRT((2.88)/(20*.785)) in m
                             = 0.428 m
                             = 428 mm

Hence Appropriate line Size is 450 NB

Given Below are some complex problems, usually encountered in Natural Gas Industry. Kindly refer Rule of Thumb for Pipelines before arriving any conclusion.

If Pressure drop is also to be considered for Gases case then

When Flow rate is given as mass flow rate then it can be multiplied by density to get the flow rates.  Even if Molecular weight is given, Density can be calculated at given pressure and Temp.

For liquid fluids, there is not standard or normal flow. Any Volumetric flow is actual flow. So it can be used directly in above formulae.

Now it is very obvious that by knowing actual flow rates of fluid, we can calculate the approximate line size if pressure drop is not considered.