Factors Affecting Line Sizing of Piping or Pipeline Systems

Line sizing by the Process department is one of the most important and critical activities for any project. Line size is determined with proper sizing calculations considering various parameters.

Purpose of Line Sizing

The main purpose of line sizing are:

  • The purpose of line sizing (for common, water-like liquids, gases, and applications) is to fill in appropriate data on P&ID’s, datasheets, and line lists
  • To determine pump head requirements
  • To meet design process parameters such as flow, velocity & pressure.

This article will highlight a few of the factors which must have to be considered during line sizing.

Factors affecting Line sizing decisions

Line sizing decisions are affected by:

Factors Affecting Line Sizing of Piping or Pipeline Systems

1. Economics:

Line sizing decisions have economic impacts, including:

  • cost of pipe
  • cost of pipe supports
  • operating pressure/power requirements

Liquid lines that are sized smaller will require a larger system supply pressure (due to higher friction losses), and possibly a larger pump and pump motor, increasing equipment capital cost and operating cost. Another factor to consider is the cost of the piping (including valves). The difference of the capital cost of ½” to ¾” piping is negligible, however, the cost of 3” to 2” piping is significant.

Therefore, line sizing involves multiple economic factors, as well as other impacts.

2. Effect of Velocity/Turbulence in Pipeline Sizing:

  • Incompressible fluids (air steam, nitrogen, etc.), the only design concern is to avoid sonic velocity (typically above 100 ft/sec). Usually, pressure drop guidelines result in pipe size selections that avoid sonic velocity.
  • In non-sanitary liquid applications, velocity and turbulence are typically not a concern, unless slurry flow is present.
  • For sanitary, non-compressible applications (our most frequent designs), velocity and turbulence are concerns. It is necessary to maintain a fully turbulent flow to avoid stagnant areas in the piping system that can promote bacterial growth.

3. Pressure Drop Effects for Line Sizing:

  • The effect of pressure drop within a system is closely correlated to the economics of the system. Smaller pipe sizes result in larger pressure drop requirements; economic considerations require a balance between pipe size and pumping/power requirements to overcome pressure losses through the system.

4. Impact of Line Holdup in Pipe Sizing:

  • In the biopharma industry, certain product liquids may have exceedingly high value, and the holdup in piping systems may be a primary concern. In addition to minimizing the length of piping systems and the recovery capabilities (air blows, sloped piping, etc.) of the piping systems, a smaller line size may be chosen to further minimize the loss potential of the liquid held up in the line. This becomes the governing criterion for sizing even though pressure drop exceeds the recommended range.

5. Space requirements for Line Sizing:

  • Typically, space requirements are not a significant concern in the biopharma industry, since most pipe sizes are less than 6 inches in diameter. Space may be a concern when existing pipe racks are used, where areas are tightly piped, or with gravity drainage systems. Interaction with the piping designer will identify critical areas where space is a consideration.

6. Effect of Expansion Effects in Line Sizing:

  • Header system sizing should consider the possibility of plant expansion. The incremental cost of a larger pipe size will avoid many headaches down the road if expansion is a distinct possibility.
  • Sizing of sanitary water loops must critically consider expansion possibilities, since oversizing the supply loop may reduce the velocity below the acceptable values to maintain full turbulence. Often, the supply pump needs to be oversized as well as the header to account for expansion. This oversizing should only be done if expansion is a distinct probability.

7. Line Sizing based on Equipment Nozzle Connections:

Equipment nozzle connections for piping tie-ins are typically undersized when compared to the recommended pipe size. Be sure to perform the sizing calculation, select the appropriate pipe size, and, if required use reducers to connect the pipe to the equipment nozzle.

Steps in Line sizing procedures

Step 1 – Assume line size

Step 2 – Calculate velocity by

V = Q / (Flow Area)

Step 3 – Calculate the pressure drop by the proper method.

Step 4 – Check whether calculated velocity & pressure drop falls within recommended ranges.

  • If YES then select the line size
  • If NO then selects a new line size and repeat steps 2 to 4.

Recommended velocities & max ΔP for Line Sizing:

Sr. NoTypes of ServiceVelocity (m/sec)Maximum ΔP   Kg /cm2/100m
1General Recommendation1.5 – 4.60.92
2Pump Suction  
 – Boiling Liquid0.6 – 1.80.115
 – Non – Boiling Liquid1.2 – 2.40.23
3Pump Discharge  
 – 0 – 57 m3 / hr1.8 – 2.41.38
 – 57 – 159 m3 / hr2.4 – 3.00.92
 – >159  m3 / hr3.0 – 4.60.46
4Bottom Outlet1.2 – 1.80.14
5Reboiler Trapout0.3 – 1.20.035
6Liquid from Condenser0.9 – 1.80.11
7Liquid to chillers1.2 – 1.8 
8Refrigerant Lines0.6 – 1.20.09
9Gravity Lines0.2 – 0.50.04
10Drain Lines0.5 – 1.2 
11Boiler feed2.4 – 4.6 
12Liquid with suspended solids0.9 

Table – 1     Sizing of Liquid Lines in Process & Equipment service

Sr. NoTypes of ServiceVelocity (m/sec)Maximum ΔP   Kg /cm2/100m
1General Recommendation for pressure level (Kg /cm2)  
                    P > 35.1 0.46
        14.1 < P < 35.1 0.35
        10.5 < P < 14.1 0.14
          3.5 < P < 10.5 0.069
             0 < P < 3.5 0.034
         Subatmospheric 0.023
2Compressor piping Suction 0.12
3Compressor piping discharge 0.23
4Refrigerant suction lines4.6 – 11 
5Refrigerant discharge lines11 – 18 
6Tower Discharge  
 Pressure (P > 50 psia)12 – 180.046 – 0.12
 Atmospheric18 – 30 
 Vacuum (P < 10 psia)38 – 610.012 – 0.023
7Saturated steam60 
8Superheated Steam75 
9Steam Pressure  
                    P > 3.5 0.06
         3.5 < P < 10.5 0.115
       10.5 < P < 21 0.25
                  P  >  21 0.35

Table – 2     Size of Vapour Lines in Process & Equipment service

Pressure      Friction Loss for Lines Less than 50 m (kPa/100m)Friction Loss for Lines between 50 m and 150 m (kPa/100m)
50mm Hg    1.51.2
100mm Hg    2.01.5
400mm Hg    2.52
760mm Hg to 2.0 bar a3.02.5
2.0 to 5.0 bar a105
5.0 to 8.0 bar a127
8 to 12 bar a    149
12 to 15 bar a    1610
15 to 35 bar a    1912
Over 35 bar a    2514
       

Table – 3   Pressure Drop Criteria for Line Sizing

Few more Resources for you…

Offshore Pipeline Design
Hydraulic calculation in piping networks
Process Engineering Related articles
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

3 thoughts on “Factors Affecting Line Sizing of Piping or Pipeline Systems

  1. Dear Anup, That’s very useful article you have have written. could you please give reference to the recommended velocities and pressure drops?
    Best Regards
    Zahid.

  2. Dear sir,

    All information provided by you is very useful.
    For more study purpose could you provide some data for natural gas pipeline line size calculation if discharge flow is known.

  3. Applied process equipment design by E E ludwig, volume 1, chapter 4 is n
    Best reference for piping. I can send a copy if email ID is sent to me. klpai49@gmail.com

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