Large Pipe: When to change to two smaller ones.

[Leclerc]

Current plant design is a single 60"NB inert offgas line (mainly nitrogen and water vapour as a result of desuperheating to an operating pressure/temperature of 1.1 bara/ 50 deg.C) to a single branch inlet to the final offgas scrubber (caustic solution) before discharge to atmosphere. Design pressure/ temperature of the Vessel is "0.5 barg+ full of liquid"/250 deg. C. Line material is class 150 316L stainless.

Proposed plant will be double sized, i.e. line/ vessel dias will be 1.414 x current, making for a 90"NB line.

What factors should I consider in deciding whether go for one large pipe or two smaller ones?. What engineering factors would prohibit such a large dia. line?

 

[epoisses]

I suppose the downtime to replace existing 60" by 90" versus adding a 60" would be prohibitive, or wouldn't it?

 

[Ashereng]

Cost.

Flexibility. With 2 x 60", you now have the ability to run at 50% in case of problems to one of the trains.

Do you have space in your piperack/run for a second 60"? If not, you can always build a 90" new and still keep the 60" for spare.

Cost is always a consideration.

Future expansion?

There are many lines with much larger diameters than 90" and at higher pressures. I believe the engineering and construction should not prohibit using the size you are considering.

 

[vzeos]

Leclerc:

You might want to look at your required size for the new off gassing line. I get 78.5” pipe to replace (2) 60” pipes working in parallel.

My calculation method is as follows:
1) Using a gas flow calculator (or any calculation method); arbitrarily set the length at 1000 ft and the pressure drop at 10 psig. Now setting the diameter at 60” calculate the flowrate….I get 156730.878 mcfh.
2) Two 60” pipes would have a flow rate of 2 x 156730.878 = 313461.756 mcfh.
3) Now, using the same arbitrarily set the length of 1000 ft and the pressure drop of 10 psig, set the required flowrate at 313461.756 mcfh and solve for the diameter. I get 78.5”.

The actual magnitude of the flowrate does not matter. What matters is the relative carrying capacity of the pipes involved, all of which are operating under the same conditions of length and pressure drop.

 

[Ashereng]

Leclerc,

Correction. There are not many pipelines larger than 90". Actually, the largest that we have heard of is 60" diameter. The largest we have seen is 48" diameter. It's Friday, sorry.

How long is the current line?

How big is your valve? if any?