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Category: Plant Basics
Question: What is an effective way to dry out a piping system after hydrotest?
Keywords: v1i15,pipe,drying,after,hydrotest,nitrogen,pressure,depressure
Answer: We found the following advice on the internet from Sean Shepard of SASOL North America in Lake Charles, LA. Sean explains:"About 15 years ago an operator taught me a good trick for drying out piping. We were drying a new line for ethylene service by blowing it with hot nitrogen and testing the dewpoint at the other end. The drying process was taking a long time and costing more than we had anticipated. The operator recommended 'pressure and depressure' and then proceeded to close the outlet valve. When the gage read a positive pressure, he quickly opened the 8" gate valve. Each time he did this a quart or more of residual hydrotest water came pouring out, and the pipe was quickly dried. Since then the 'pressure and depressure' method of drying has been my method of choice. I'm not sure why it works so good (even without heat). It may be that this action forces the gas to fill the entire cross section of the pipe and 'lift' water from places that the free flowing gas misses."


I think this is basically a JT effect in action. This effect is used during startup in a cryo plant. It consist of letting down a high pressure to a lower pressure across a valve. I think this is basically what the smart operator tried to achieve. Closing the valve builds up the pressure hence the positive pressure reading on the gage and the sudden opening of the valve reduce the pressure and cause a the state of the fluid in to liquid+ gas state hence the liquid that forms.