Process Pipe Design & Sizing

When we walk around to any processing facility, one of the dominant item that we could easily find is a pipe. Pipes are the vein of a chemical process plant that is critical to ensure a chemical substance could be transported to one place to another. Process pipe design and sizing is an activity performed by a process engineer to define the required pipe size e.g. pipe diameter for a specific process condition and fluid service.

Figure 1 - Process pipe inside a processing facility (Source: indiamart.com)

Fluid Service in Oil and Gas Facility

In the typical oil and gas facility, typically, we will meet 3 type of fluid flowing at the process pipe, as follow:

• Vapor phase fluid
• Liquid phase fluid
• Mixed phase / Two phase fluid (mixture of Vapor - Liquid phase)

These different type of fluid phase will be naturally formed from the mixed phase production wellfluid as this wellfluid flowing through an inlet separation facility, as shown in the Figure 2 below:

Figure 2 - Different fluid type in the typical inlet receiving production facility

To check the thermodynamics data for the fluid, a process simulator that have thermodynamics library for chemical component could be used. The most popular one in oil and gas industry (which i used since in the university) is ASPEN HYSYS.
With the software, we could easily find following required properties for line sizing calculation. Please note that all of these parameters below is the function of fluid composition, operating pressure, and operating temperature :

• Vapor phase : Molecular Weight (MW), Z-factor, Cp/Cv, and Viscosity. This parameter is required to calculate gas density using gas correlation PV = ZnRT
• Liquid phase : mass density (ρ) and viscosity
• Mixed phase : all described parameter above

Process Pipe Sizing Specification

Each type of fluid will have different pipe design specification and this could be different for each oil and gas company.

Generally, some critical specifications of the flow inside a pipe are:

• Fluid velocity inside the pipe (v)

Mean velocity of the fluid inside a process line. This will be the function of fluid mass flow (m), fluid mass density (ρ), and inside diameter of a pipe (ID).

Figure 3 - Fluid flow inside a pipe

This parameter will be critical to ensure the operability of the pipe for the long term. High velocity in a long haul could lead to pipe erosion and leak. 
To calculate the fluid velocity, inside pipe diameter shall be used instead of outside diameter (OD). Approach to calculate the actual ID will be discussed separately as the pipe size in market is characterize by nominal pipe size (NPS), which is kinda different with pipe actual ID.

• Fluid momentum ( 𝞺v² )

Closely related with the vibration inside the piping. High fluid momentum will lead to phenomenon called flow induced vibration (FIV). If the piping segment has a weak point like a dead leg, thermowell, or small bore connection, this issue could be critical. For example, the small bore connection shall be well support in order to mitigate the integrity issue.

• Segment pressure drop (𝞓P / L)

High segment pressure drop could lead to inefficiency of a hydraulic loop and could lead to excessive size of compressor or pump.

To perform a correct line sizing, those three line sizing specifications shall be checked based on the limit value. This limit value usually will depend on the Oil and Gas or EPC Company standard. The limit value will also differ for different location of piping. For example: 

• pressure drop and velocity limit at the pump suction shall be lower than the limit for the pump discharge in order to prevent cavitation issue (related to available NPSH calculation). This will typically resulted to larger pipe diameter at the suction side.
• Velocity and momentum limit will go up with the higher operating pressure of a gas service. This is why gas pipe with high operating pressure is relatively small.
• Special consideration for mixed fluid phase will be covered in other article.

To give a better understanding, in the separate article, i will share a tutorial for performing a simple line sizing analysis using ASPEN HYSYS v10. 

I think that's all i could share about the basic of process pipe design and sizing. Hopefully, this article could help you to understand about the concept and philosophy of this activity. See you on the next thread :)

Latest update : 29 June 2020