Freeze Protection Temperature Setpoint Value?
Freeze Protection Temperature Setpoint Value?
(OP)
We are installing heat trace on an overland tailing slurry line. The piping is about 2km long and the heat trace is strictly for freeze protection. It is not process related. We are trying to determine the temperature setpoint. Looking over the IEEE Heat trace standard 515, it doesn't explicit state this value. It does give an example where 4 Celsius is used.
For this application, the energy (kWh) is expensive so we are determining how close to 0C we should go. What is typically used as best practice? Why would one not use 2 Celsius over 4 Celsius? We are discussing that there maybe cold points along the way and 4C is more "conservative." But we are really guessing and I am curious if we can set the setpoint temp lower.
For this application, the energy (kWh) is expensive so we are determining how close to 0C we should go. What is typically used as best practice? Why would one not use 2 Celsius over 4 Celsius? We are discussing that there maybe cold points along the way and 4C is more "conservative." But we are really guessing and I am curious if we can set the setpoint temp lower.





RE: Freeze Protection Temperature Setpoint Value?
1. Are there shoes?
2. Are there areas that may not be as well insulated as others?
3. Is the line above grade where there could be different wind speeds?
4. Is the insulation subject to getting wet?
5. Will there be extended times where the slurry line will not be flowing? If so, design for the worst case/area of the line.
6. If the line is flowing, then things are a lot more forgiving for areas of poor insulation. (a) the areas of poor insulation typically average out due to the flow. (b) the line will slowly pinch off if it starts to freeze thus slightly increasing the insulation value in the problem area.
The problem with computer programs is that they tend to simplify the inputs for a general average answer. I would recommend that you get some help from someone with heat transfer experience that knows more than how to use the computer program.
Otherwise, instrument the line and control the current by RTD input controlled relays. Try Google and search "heat trace controller"
RE: Freeze Protection Temperature Setpoint Value?
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RE: Freeze Protection Temperature Setpoint Value?
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RE: Freeze Protection Temperature Setpoint Value?
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Freeze Protection Temperature Setpoint Value?
What is confusing is the 4C setpoint. My question is why not lower to 2C? From the answers in this post, it seems that the 4C setpoint is a margin of safety is added for conservative of design. However there is already a "safety factor" value that is included in the calculations. IEEE recommends a 10% "safety factor" as a minimum. This percentages accounts for the quality/uncertainty in the heat trace calculations:
- uncertainty in the heat transfer coefficient
- environment variables (wind effects, extreme ambient temp)
- weak spots in the insulation
- heat trace cold spots, etc...
Maybe there is no "strict value" used for minimum temperature setpoint. I am just being curious and questioning why would 4C be used and not a lower value?
RE: Freeze Protection Temperature Setpoint Value?
The key idea is to realize that actually there is no temperature safety factor. Your object is to prevent overpressurization, not freezing. Pressurization begins to occur at 3.99°C Have you calculated at what temperature your pipe bursts. [Hint: It may be higher than 0°]
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Freeze Protection Temperature Setpoint Value?
RE: Freeze Protection Temperature Setpoint Value?
RE: Freeze Protection Temperature Setpoint Value?
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StoneCold
RE: Freeze Protection Temperature Setpoint Value?
The temperature safety factor ( set point of 4C) is there to be sure that you power up before the freeze. Such is the case to prevent the situation when you have a cold spot along the line. I've been talking to some heat trace vendors and they even use 10C for their side of caution. Our design shall be at 4C.
The other comments were extremely helpful as well. The tailings piping design is HDPE with 2" of polyurethane insulation. The design has accounted for pressurization with additional factors to deal with the slurry. The motor pumps have also been sized to account for the cold water as the environment in this application is in very low sub-freezing conditions.
RE: Freeze Protection Temperature Setpoint Value?
RE: Freeze Protection Temperature Setpoint Value?
This statement seems completely meaningless to me in the context. Could you clarify?
RE: Freeze Protection Temperature Setpoint Value?
No it is not just as important to consider the thermal expansion of water when you switch on the heat trace. When the temperature is rising, you are not imminently poised to breaking a pipe due to mechanical pressure generated by freeze ice, which can be a thousand times more than simple fluid pressure due to fluid expansion from temperature increase. True, pipes should be and are protected from fluid pressure due to thermal expansion of contained fluid, however high pressure water about to freeze may not be able to pass by already frozen plugs to get to a relief valve.
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso