as opposed to an oil seal?
if you have a N2 grid available then I would go for a gas seal (opex=N2 usage and maybe requirement for flare connection depending on procesgas pressure)
oil seal might be a bit more capex intensive and maybe some opex for degassing oil equipment
You are going to need to provide some additional information.
1. What type of equipment are we talking about (centrifugal pump or compressor)?
2. What is the application/service?
3. What are the fluid properties (will depend on the type of equipment)?
4. Can the process handle a small amount of N2, i.e. is there a place for it to get out later in the process or will it build up over time?
5. What are the site utility connections to support the potential gas seal, pressure, quality, etc?
Depending on the answers to these questions you can start to get a sense of if a gas seal makes sense or not.
If you can determine that it will work then I am a big proponent of using a gas seal...there are just some places where you cannot apply them due to process/site limitations.
I suggest that you go back and do some additional digging and rephrase your question with some specifics before the community can help.
I'm sure there are others, but below here are a few of the advantages/disadvantages that jump to mind:
Advantages
1. Non-contacting and don't generate heat
2. Low power consumption
3. Generally use inert N2, which generally doesn't contaminate fluids
4. Can be a lot longer lasting if operated appropriately
Disadvantages
1. Maybe higher initial costs, especially if there isn't a N2 source already
2. Less robust that oil lubricated seals, they need to be operated correctly or its easy to damage them
3. Harder to service/repair yourself
Your replies doesn't reflect the problem to the core of definition.
Question is when to go for GAS seals means in what situations we can't go for liquid & contacting type of seal plans and condition in which it is more of mandate than option to go for GAS SEALS only.
I'm sorry my answer didn't help you. I'm afraid I find your phrasing confusing. If you have a specific application you are thinking about, then maybe that would be easier to offer help on.
Would say gas seals may be the only option if both of the following are true:
a)The process fluid cannot be used as seal barrier fluid ( due to flammability, toxicity or other HSE reasons, corrosivity, suspended solids, high vapor pressure at rotating seal contact surface temperatures which can lead to loss of cooling and local seal chamber vapor lockup,there may be other reasons also)
b)Contamination of the process fluid with even small amounts of any external "inert" barrier fluid (as in plan 53C or pressurised plan 53B) is also not acceptable.
You would go for Gas Seal when your Pump has Cofferdam. Cofferdam is normally used in Vertical Pumps for cryogenic application. It converts liquid to gas to prevent icing on seal area.
Gas Seal also is used for gas compressors where wet Seals will surely fail due to dry running.
