Line Clearing Purge Requirements

[gte682n]

Hello,

I would like to size a N2 purge to clear a 3" diameter line, 500 ft in length. The material that I am trying to push out of the line is a molten rosin. Viscosity is 15 cP, and will solidify at ambient temperatures. This line is steam traced, but the preferred practice it to blow this line out with steam to remove any chance of a plugged line. There is concern over the use of steam due to potential condensate and hot rosin mixing.

What methods exist to size the flow rate required for this purge? Is it just a matter of determining the time in which I would like to clear the volume, and ensure I have enough N2 supply pressure to drive the material out?

Is there a way to ensure I don't blow through the liquid, leaving the line to still contain the rosin, and eventually plug?

The steam is able to satisfactorily clear this line. Do I just ensure I have the same volumetric flow and pressure of N2?

Thank you.

 

[LittleInch]

gte,

"Purge" is normally associated with clearing one gas by means of another, typically an explosive gas like methane by use of an inert gas like N2.

What you're talking about looks more like "clearance".

With a material at some unknown temperature ( 80-90C?) it is 15 Cp which is not too bad, but still quite "thick".

I don't know what rosin is, but I can only assume that the steam is doing two things - moving the liquid material until it blows pas and then keeping the remainder quite liquid due to heat input and blowing it out as a two phase flow.

N2 will do the former, but not the latter...

Unless you can introduce some form of pig or plug / gel which will prevent the N2 blowing past then you risk leaving more of the material in the line.
How clear does is need to be?? This is the crucial question
My guesstimate for a 500 ft (150m) long 3" line is that ~ 90-95% will be removed as a liquid before the N2 blows past it. Then it depends on you how long you keep blowing to remove the last 5-10%, but unless you heat the N2 to the same temp as your steam supply then it is likely to solidify your liquid.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Even though the nitrogen is not at the same temperature as steam, you probably can expect the same performance with nitrogen with an equivalent gas volume and velocity because the pipe is heated.

How clean does this pipe have to be?

Not sure of the details but it should be noted that this is a potentially hazardous operation and that nitrogen is an asphyxiant.

Purging tips

 

[Compositepro]

Very good tips above. I process molten thermoset resins. We generally avoid using pipes because of the issue you face. When we do clear pipes we use compressed air. We do not process at so high a temperature that flammability of the resin is an issue. Our resin is more like 500 cP, so air is used to clear a channel through the resin so we can do a solvent wash later at room temp. If you want to clear the pipe you need a gas velocity of 100 ft./sec or more.
You might want to consider turning off the steam tracing as you clear the line so that there is less chance of residual rosin draining to a low point and creating a plug. On the other hand, if you improve your heat tracing you might not need to clear the line at all. Re-heating the pipe would take much longer, though, if the pipe is full of frozen rosin.