Designing straight pipes and piping systems: Some considerationsTemitayo Oketola | September 02, 2022
Pipes and piping systems are essential components used in various industries today. For instance, in the oil and gas industry, they are used to transport crude oil from oil wells to tank farms.
However, while seemingly simple, there is more to piping systems than meets the eye. Engineers must design piping systems to meet the required fluid flow and pressure integrity requirements. In addition, the ideal choice of engineering material for a piping system will depend on the type of fluid the pipe will transport.
Essential design calculations and considerations for piping systems
Pipeline design typically involves determining the pipe's inside diameter and wall thickness.
#1 Inside diameter of the pipe
The ideal inside diameter of the pipe depends on the quantity of the fluid to be delivered. It can be calculated using:
di = inside diameter of the pipe (m)
Q = volume flow rate of fluid (measured in cubic meters per minute)
V = velocity of fluid flowing per minute (m/min)
So, consider a scenario where a straight mild steel pipe is to be used to carry 50 m3 of steam per minute. If the steam is expected to flow at 1,600 m/min, then an ideal inside diameter of the pipe would be 0.118 m (or 118 mm), as shown below:
#2 Pipe wall thickness
The determination of the ideal pipe wall thickness is an important step to take during piping design. An ideal pipe wall thickness ensures the pipe can withstand the internal pressure from fluid flow and in-line components (like valves and fitting). It also ensures that the pipe material can withstand the fluid's chemical properties and working temperatures.
A simplified formula for calculating pipe thickness is:
P = internal pressure of the pipe (MPa)
di = inside diameter of pipe (mm)
σh = hoop stress in the pipe wall (MPa)
C = constant, according to Weisback (mm).
The value of this constant depends on the choice of pipe material. For instance, C = 4 for copper and zinc, C = 3 for mild steel, and C = 9 for cast iron. Assume the cast iron steel pipe (in the previous scenario) is used to carry steam at 50 m3/min. Suppose the pipe is expected to carry this fluid at a pressure of 1.5 MPa and the hoop stress of the pipe is 35 MPa. In such a scenario, an ideal pipe wall thickness will be obtained as 5.53 mm, as shown below:
However, keep in mind that this formula was developed based on several assumptions and simplifications. It is assumed that the hoop stress across the section of the pipe is uniform, the ratio of the pipe’s internal diameter to its wall thickness is greater than 20, and the ratio of allowable stress to the internal fluid pressure is greater than 6. While this equation might give an idea about suitable pipe wall thickness dimensions, engineers typically rely on several standards and design codes to specify pipe wall thickness for applications.
The ideal choice of standard and design depends on the country where the piping system will be used. For instance, for piping systems used in the U.S., engineers generally rely on design codes created by the American Society of Mechanical Engineers (ASME). These design codes include:
- The ASME standard B31.1 presents requirements for the design of pipes used in industrial plants, geothermal heating systems and electric power generating systems.
- The ASME standard B31.3 presents requirements for the design of pipes used in chemical plants and petroleum refineries.
- The ASME standard B31.4 presents requirements for the design of piping systems used in onshore oil facilities to transport hydrocarbons, anhydrous ammonia, liquid petroleum gas and alcohol.
- The ASME standard B31.8 presents the requirements for the design of pipelines transporting and distributing gases in onshore facilities.
While this article presents helpful information about sizing piping systems, there are several other things that engineers must consider when sizing piping for an application. Engineers must choose the ideal piping material depending on the strength requirement and environment where the piping systems will be used. In addition, engineers must choose the ideal valves, flanges and fittings for the required application.
All of these have been covered in different piping standards and design codes like the ASME 31.