Heat required for steel piping

[Darkmatt]

Hi
I have a 16 inch steel pipeline that is above ground that I need to coat the weld joints with a warm climate two part epoxy coating. I would like to know how many BTU/H I would need to heat this pipeline to a temperature of 75 degrees F. The pipe is 16 inch ID 1/2 inch wall. It has a Fusion Bonded epoxy coating on it of 50 mils. The pipe is 700 feet in length and subjected to an ambient temp of 0 degrees F. I would need to raise the steels temperature by at least 75 degrees in order to coat the weld joints with the two part epoxy coating. I am just guessing that I would need a million BTU heater with a 4700 CFM fan in order to heat the steel over 24 hours to reach and maintain the required surface temperature. Any one have an idea is the heater will work?

 

[racookpe1978]

What are you going to do?

Blow the heater exhaust into one end of the pipe and heat all of it?
Move the heater from one joint to the next and wait for it to heat up the joint (evenly ????) by blowing hot exhaust air on the outside of the pipe from one side, then wait for it to cool off so you can handle it and apply the coating?
Use two heaters - one on both sides?

What do your heat transfer calc's assume for a pipe in mid-air from at 0 deg F to 75 deg F? If the hot air is being blown on from the outside, are the ends of the pipe capped so cold air won't flow though the middle?

 

[Darkmatt]

The intention was to blow heated air down the inside of the pipe and bring the entire string up to a manageable temperature. Both ends are open with only one heater operating. At present we are heating outside at the weld joint to 140 degrees with a torch and then coating the weld joint. Afterward we put a tent over the weld and continue to heat the enclosure to about 90 degrees, and allow the heater to run all night. My theory is that heating the pipe internally will accomplish the same thing but allow me to coat all the welds (16 total) at one time without moving the heater. I cannot figure out the formula to calculate required heat needed per hour to bring it to 75 degrees and hold it in a 24 hour period.

 

[racookpe1978]

I've got to get back to some fundamental equations myself on that - I'm not forgetting you, but I don't have them memorized.

1. Are you absolutely positive 75 deg F is required? Seems like most coatings allow application and drying over a 50-100 deg F range. That they 'want" 75 deg F is true, but what they actually get is almost always a wider range in the real world.

2. Is the 0 deg F outside air temperature constant, or was that a nighttime minimum?

3. If the air temperature is changing, where will you monitor the nominal (high, medium, low?) pipe surface temperature, and how will you reduce the heat into the pipe if it gets too hot at the inlet///too cold at the outlet?

 

[racookpe1978]

http://www.thermal-wizard.com/tmwiz/default.htm

Using the Maya Thermal Wizard (caution! you need metric units for their default calculator)

Horizontal pipe,
in natural convection air at -17 deg C (0 deg F)
entire pipe is at 23 deg C (75 deg F)
entire pipe is 213 meters long (700 feet) x .4 meters dia (16 inch),
then I estimate (with that calculator) the pipe would lose about 101,300 watts
or 345,000 btu/hr.

See attached file showing their tables, their prediction of 101,300 watts or about 345,000 btu/hr.

Now, I would break the pipe up into at least 4 sections - assuming you accept the assumptions and physical properties that the Maya calculator assigned ! for film temperature, still air properties, etc
thus:
0 - 50 meters is what assumed temperature (internal air temp ~ outside of pipe wall temperature over a 24 hour period.)
50 -100 meters is what assumed air temperature?
100 - 150 meters is what assumed air temperature?
150 meters - 213 meters is what assumed air temperature?

 

[SWDPUMichael]

Hi, I suggest you to use our one component polyurea, you do not have to heat the pipe, and it is easy to handle.