Line sizing by 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. 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, data sheets, and line lists
- To determine pump head requirements
- To meet design process parameters such as flow, velocity & pressure.
This article will highlight few of the factors which must have to be considered during line sizing.
Factors affecting Line sizing decisions:
Line sizing decisions are affected by:
- 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 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.
- In compressible 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 fully turbulent flow to avoid stagnant areas in the piping system that can promote bacterial growth.
- Pressure Drop Effects:
- 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.
- Line Holdup:
- In the biopharm 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.
- Typically, space requirements are not a significant concern in the biopharm 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.
- 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.
- 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.
Normal Steps in Line sizing procedures:
Step 1 – Assume line size
Step 2 – Calculate velocity by
V = Q / (Flow Area)
Step 3 – Calculate pressure drop by proper method.
Step 4 – Check whether calculated velocity & pressure drop falls within recommended ranges.
- If YES then select the line size
- If NO then select new line size and repeat step 2 to 4.
Recommended velocities & max ΔP:
|Sr. No||Types of Service||Velocity (m/sec)||Maximum ΔP
|1||General Recommendation||1.5 – 4.6||0.92|
|– Boiling Liquid||0.6 – 1.8||0.115|
|– Non – Boiling Liquid||1.2 – 2.4||0.23|
|– 0 – 57 m3 / hr||1.8 – 2.4||1.38|
|– 57 – 159 m3 / hr||2.4 – 3.0||0.92|
|– >159 m3 / hr||3.0 – 4.6||0.46|
|4||Bottom Outlet||1.2 – 1.8||0.14|
|5||Reboiler Trapout||0.3 – 1.2||0.035|
|6||Liquid from Condenser||0.9 – 1.8||0.11|
|7||Liquid to chillers||1.2 – 1.8|
|8||Refrigerant Lines||0.6 – 1.2||0.09|
|9||Gravity Lines||0.2 – 0.5||0.04|
|10||Drain Lines||0.5 – 1.2|
|11||Boiler feed||2.4 – 4.6|
|12||Liquid with suspended solids||0.9|
Table – 1 Liquid Lines in Process & Equipment service
|Sr. No||Types of Service||Velocity (m/sec)||Maximum ΔP
|1||General 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|
|2||Compressor piping Suction||0.12|
|3||Compressor piping discharge||0.23|
|4||Refrigerant suction lines||4.6 – 11|
|5||Refrigerant discharge lines||11 – 18|
|Pressure (P > 50 psia)||12 – 18||0.046 – 0.12|
|Atmospheric||18 – 30|
|Vacuum (P < 10 psia)||38 – 61||0.012 – 0.023|
|P > 3.5||0.06|
|3.5 < P < 10.5||0.115|
|10.5 < P < 21||0.25|
|P > 21||0.35|
Table – 2 Vapour Lines in Process & Equipment service
|Friction Loss for Lines Less||Friction Loss for Lines between|
|than 50m (kPa/100m)||50m and 150m (kPa/100m)|
|760mm Hg to 2.0 bar a||3.0||2.5|
|2.0 to 5.0 bar a||10||5|
|5.0 to 8.0 bar a||12||7|
|8 to 12 bar a
|12 to 15 bar a
|15 to 35 bar a
|Over 35 bar a
Table – 3 Pressure Drop Criteria