Piping inside box culvert vs buried pipe

[kag2078]

Hi All,

I am currently designing uPVC Sch80 chemical dosing lines containing Sodium Hypochlorite (0.8%) and am trying to understand how to properly derate the PVC (and find the temperature rise in the pipe) for the following options:
[ul]
[li]Buried concrete box culvert - nominal depth 450mm[/li]
[li]Direct buried - nominal depth 750mm[/li]
[/ul]
The fluid temperature leaving the building will initially be 25degC, but will be pipe approximately 200m to a dosing point, with the pipe being laid in either of the two options.

The ambient temperature will be around 49degC, so I am currently unsure how to calculate the rise in temperature inside the box culvert. Am I right to assume that the temperature in the box culvert will eventually rise to the ambient air temperature, or will the box culvert alleviate the temperature slightly?

I have been quoted that the soil at the 750mm burial depth will be a constant 35degC. So believe that the max fluid temperature the second option would rise to is also 35degC.

Any help or direction in this matter would be greatly appreciated.

 

[BigInch]

Depends on how long the 25[°]C fluid is in contact with the 35[°]C environment. Obviously a very quick trip through a higher temperature environment won't hardly change the fluid's temperature at all. Theoretically the fluid will only reach 35[°]C if it remains in the 35[°]C ambient temperature for an infinite amount of time.

Soil temperatures at 1m below ground will not reach maximum air temperatures, unless the sun doesn't go down for about a month. "750mm burial depth will be a constant 35degC." so at 450mm depth, expect roughly a maximum tempearure of around the average of 35 and 49, or 42 deg. The minimum temp at 450mm depth will be roughly the average of 35C and the minimum surface temperature you can read at night.

Reaction to change doesn't stop it

 

[bimr]

PVC is not completely resistant to sodium hypochlorite. However, PVC is used for sodium hypochlorite because it is inexpensive and can easily be replaced once it fails. PVC sometimes leaks prior to failure.

Sodium hypochlorite will also outgas.

Assume that is the reasoning for the 0.8% solution as well as your installation questions.

Have you considered using PVDF pipe? Have you considered adding insulation?

https://plasticpipe.org/pdf/tr-19_thermoplastic_pipe_for_transport_of_chemical.pdf

https://www.zeusinc.com/materials/pvdf-polyvinylidene-fluoride/chemical-resistance-chart-pvdf

750mm burial depth will be a constant 35degC. Agree at 450 mm depth, expect the temperature to be the average temperature between daytime and nighttime.

 

[cvg]

given that the fluid will be in the pipe for about 2 - 3 minutes total and that pvc (and plastics in general) does not conduct heat very well, I would estimate the amount of heat transfer to be very low. For design purposes, I would estimate the maximum temperature of the pipe with no fluid flow and de-rate it for that.

 

[LittleInch]

cvg - we have no idea what the flow rate or velocity is?

Might / should be < 5 minutes for that sort of distance, but might be 2 hours, might start and stop - we haven't been told.

worst case is 49 C - if de-rating doesn't impact design then use that and move on.

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

 

[cvg]

200 meters distance at even a very low velocity is just a few minutes. unlikely that flow rates would be so low to be in the hours. either way, my recommendation is the same, i still suggest using the worst case temperature for design regardless of the actual flow rate or heat transfer

 

[bimr]

LittleInch - A newby engineer would find it simple to arrive at an economical velocity. The burial depth, chemical being used, and distance would lead someone to think this piping is less than 2-inch.

Without knowing the consequences of failure, taking the worst case is not unreasonable.

 

[kag2078]

Hi All,

Thank you for all the responses. I'm in Australia, so I apologise that I could not reply in a timely manner.

To add more information, the rate of dosing is approximately 10L/min, so the velocities in the line are extremely low. The dosing is expected to be continuous, as this is dosing into a main supply line.

The piping is envisioned to be double contained, though am currently unsure about insulation. Will double containment of pvc-pvc, have a resounding effect on this temperature?

 

[BigInch]

Its not going to get any hotter than 42 no matter how much you insulate.

Reaction to change doesn't stop it

 

[LittleInch]

Pipe size?
Velocity?

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

 

[BigInch]

small
slow

Reaction to change doesn't stop it

 

[JohnGP]

It's all a bit of a guess without actual design data, but double containment will have an effect, but I don't think a "resounding" effect. After all it is basically another layer of "insulation" with low thermal conductivity. Concrete will have a similarly low thermal conductivity depending on design.

According to the linked article-

http://www.pipa.com.au/sites/default/files/document/attachment/tn003.pdf

even at 40^oC, the derating factor would be 0.7. Is that sort of pressure derating a problem? If it is then best to go for the buried option where the derating factor would be 0.78.

 

[cvg]

KAG, we are still guessing at this. keep getting bits of information, but not all the pertinent bits. we do not know how small the pipe is or how slow the flow is.

 

[bimr]

With the flow rate of 10L/min, the pipe would be approximately 20 mm.