I have posted a lot of excel spreadsheets on the Cheresources community related to process engineering calculations. I also have posted blog entries where I have outlined the steps along with equations to build an excel spreadsheet for various calculations.
The idea of posting blog entries with equations and calculation steps is to encourage young engineers to build their own spreadsheets and understand how excel can be a versatile tool to do all kinds of engineering calculations.
Let me share something about excel with you. I never had any formal training on excel. I am a self-taught person with bits and pieces of help from others who know excel equally well or more than me. The level of prowess that I have developed is due to practice and by making dozens of spreadsheets over the last few years. I still do not know how to write a macro or VB code in excel. I use the function library of excel and it has served me well till date. The conclusion I made over these years was the more you practice the better you get. Well it is a universal law applicable to any skill that anybody wants to acquire.
Enough of the lecturing on acquiring skills related to excel.
Today's blog entry is related to depressuring or blowdown time from a long-distance natural gas pipeline. Blowdown from a gas pipeline would be required under two circumstances. The first and most important would be an emergency blowdown for de-inventorying the pipeline due to an emergency such as fire, pipeline mechanical damage (rupture / displacement). The second would be a planned maintenance of the pipeline which would require the pipeline to be de-inventoryed before handing it over to the maintenance crew.
Normally permanent blowdown connections are provided for all long-distance pipelines along the pipeline route. Blowdown connections (piping, associated blowdown valves, restriction orifices) at dispatch and receiving stations are generally hooked up to the flare system at the station. However, most pipelines run through remote and uninhabited areas, where it is not practical to provide a flare system. Some sections of pipeline separated by block valve stations, which pass through remote areas, if required to be depressured or blown down, need to be vented to the atmosphere through cold vent lines provided with blowdown valves.
One exercise that a process engineer needs to do is to design or calculate the size of the vent line based on the time required to blowdown or depressure the system. Blowdown is a transient process where the pressure from the system to be blown down is reduced in time steps within an acceptable total time.
The blowdown process follows both the sonic process of pressure reduction and the subsonic pressure reduction. Sonic discharge refers to the initial stage during which the flow at the point of minimum effective vent area (usually the blowdown valve throat) is choked and the Mach number is necessarily unity. In other words, the ratio of pressure in the gas pipeline to atmospheric pressure is greater than the critical pressure ratio and thus flow is sonic. For high-pressure blowdown, sonic flow accounts for most of the blowdown time.
During blowdown the pressure in the gas pipeline decreases and the flow at the minimum area or restriction in the blowdown or vent pipe remains choked until the pressure ratio becomes less than critical. Then the flow is no longer choked and remains subsonic until the pressure in the pipeline reaches atmospheric at which point the blowdown or depressuring of the pipeline is complete.
Most process engineers well versed with HYSYS know about the HYSYS Depressuring Utility for doing depressuring or blowdown calculations to obtain blowdown time, flow rate, system / fluid temperatures.
Well what about those who do not have access to HYSYS and want to do depressuring calculations. Today's blog entry makes an attempt to remedy that to a certain extent by providing an excel spreadsheet to calculate the blowdown time from a long gas pipeline.
This excel spreadsheet however does not provide flow rates or system / fluid temperatures and is limited to providing the blowdown time from a given upstream pressure (pipeline pressure) to a downstream pressure (atmosphere) and for a given vent or stack line size. Despite its limitations, the calculated blowdown time is within 10% of the blowdown times as calculated by HYSYS for a number of cases and within the application limits of the equations provided in the spreadsheet. Further accuracy can be obtained if an iterative procedure is adopted for pressure reduction which has not been considered in the present version of the spreadsheet.
Before presenting the spreadsheet, many thanks to member "Alexsandres" who provided the link for the technical paper from which the spreadsheet was developed. Refer the link below where he has provided the link for the paper:
I am hoping that the Cheresources members will like this spreadsheet and specifically those who are constrained by not having a sophisticated process simulation software such as HYSYS. I look forward to comments from the knowledgeable forum members.
Download the Excel spreadsheet in the File Repository here: