N2 utility - backflow prevention 2

[plumberet]

To prevent back flow of process (liquid, gas, flammable, non-flammable, etc )into N2 line, one can have block valve, check valve ( single or double ), double block and bleed, spade and PD TSOV. Any guidelines on which of the above isolation to use for different cases ?

Thanks.

 

[itdepends]

You need to determine what the likelyhood and consequences of backflow are. The higher the chance (and consequence) the higher the protections you'll put in place.

e.g. 24MPa N2 cylinders discharging into a vessel half filled with inert liquid with a PSV set at 5MPa- chance of backflow- very very small- consequence- also small

or- 100kPa N2 discharging into an acid process liquid that normally runs at 50kPa but can exceed 200kPa- chances of backflow (without protection) - high, consequences (depending on materials of construction) may also be high.

For a giggle- under a crossover scenario I've seen acidic process water forced back up a nitrogen line into the distribution system, back to a H2S generation plant- down the N2 purge line into an electric heater and back up the heaters electric cable into a MCC electrical cabinet. The "leak" was found when a sparky opened the cabinet to find out why it wouldn't reset- and water came out.

 

[OVERHEATED]

we use double block and check valves arrangement when N2 connection is connected to process lines /catalyst vessels or tanks.We normally indicate on P&ID's that the N2 connection shall be dismantled after use/start ups for safety reasons unless it is meant for continuous blanketing of hazardous storages.

 

[TD2K]

I just don't like having utilities hard piped to process systems. You can put double blocks, double checks, blinds but it's always going to be a potential issue.

Some companies utilize drop-out spools but they are suspectible also to not being removed after use and thus expose the system to backflow.

Hoses are frequently provided with double check valves but this is not assurance you can't get backflow if the process pressure increases above the N2 pressure. You basically need to rely on good operating procedures and practices when N2 is connected to a process system.

 

[JLSeagull]

These are all good points.

Online analyzer sample conditioning systems often include a decontamination connection that permits flushing with water, nitrogen or other solvent to permit safety. Typically the flushing fluid and the process connections include lockable block valves. The utility is normally plugged and hose connected when required. As the tubing is small, flexible tubing is available rated well beyond the maximum design pressure for the nitrogen, etc.