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# Pressure Purge

## Pressure Purge

(OP)
Hi
please i need some help here
i want to ask a few questions about pressure purge method used for inert a vessel:
1-Does The Material/ Gas inside the vessel affect the purge technique ?
2-What about an inlet natural gas filter ( with elements ), does the element affects the purge ?
3- why in some books when implement the equation to calculate number of purge cycle- required for the vessel to be complelty purged- they starts with initial concentration of oxygen after first pressureization not 0.21?

### RE: Pressure Purge

"Pressure purge" is not really on the list of purge categories that actually make sense. People often use that term for a "dilution purge", and sometimes for a "clearing purge".

With a dilution purge you raise the pressure in a closed vessel until the gas mixture contains a sufficiently low percentage of the undesirable gas (usually air) to reduce the risk of problems (e.g., fire, explosion, and oxygen-related corrosion modalities) to an acceptable level. With this kind of purge you raise the pressure to achieve the desired mix and then blow down to slightly above atmospheric pressure. Many people do this in stages (often the first stage is with an inert gas, subsequent stages are with process gas). The benefits of pressuring up, blowing down, pressuring up 4-8 times is pretty minimal and should only be done in cases where there is a risk of a reaction that could have catastrophic consequences.

A clearing purge is done with a vent opened and you introduce process gas at a high enough rate to induce choked flow out the vent. Generally the target pressure is around 1 atmosphere gauge. Once you reach target pressure you purge 3 pipe volumes.

The mix of gases you start with doesn't have much impact on the purge procedure. There are a lot of pretty marginal discussions of purging in the literature. Some a just dumb. Others are downright dangerous. I was part of a team that tried to get our heads around the literature a few years ago. The Purge Manual is on my website, I'm not sure if it will help clear up some of the conflicting processes or just add to your confusion, but many people have looked at it and implemented procedures based on it that worked with appropriate costs and perfect safety records.

I can't tell you why someone would say that you have 21% oxygen after the first purge unless you are "purging" an air filled system with air. I think I would avoid any book that advocated that.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: Pressure Purge

(OP)
first of all thank you for your response :)

actually your answer just add another confusion to me but it really will help me to looks up to the purge system with another Point of view.

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