Freeze Protection Temperature Setpoint Value?

[majesus]

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.

 

[CRG]

The problem is not just about the margin for minimum temperature and also about the quality/uncertainty of the heat transfer coefficient.
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"

 

[BigInch]

They may use 4[°]C because, water being radically different than most elements, begins to expand at 4[°]C, even though the temperature is falling. If a blockage was to form, due to closed valves, ice plugs in poorly traced regions, or for whatever other reason, pressure could start to increase at 4[°]C.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[IRstuff]

Note that the viscosity of water increases by 50% going from around 21°C to 4.4°C, so even if it doesn't freeze outright, the increased viscosity increases the loading on pumps, etc.

TTFN

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[BigInch]

If its reaching near freezing temp, the pumps are most likely stopped.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[majesus]

Thanks for the input. I understand there is a lot of parameters involved. Just to keep my question simple and focused: this heat trace design is strictly intended for freeze protection only when the process is off. I.e. there is no flow in the pipeline (viscosity is not an issue for pump loading).

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?

 

[BigInch]

Your safety factor acts much differently than a temperature set point safety. Your factor won't even exist until the trace powers up. The temperature safety factor is there to be sure that you power up before the freeze.

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

 

[77JQX]

I think you're looking at this wrong, if what you're trying to do is save money on pipe heating. In the end, what you pay for is heat leaking from the pipe into the environment, and the rate of heat loss isn't going to be much different at 2°C versus 4°C. Also, I wonder if a 2°C setpoint will give you enough of a thermal gradient to transfer heat into the interior of the pipe at a fast enough rate to prevent freezing.

 

[cvg]

this is mostly a thermodynamics problem, not electrical. Get your mechanical engineer to run the calcs. And don't push it on the setpoint. The kWh may be expensive, but not nearly so expensive as shutting down the mill, stopping all production for a week, installing a new pipeline and re-starting everything - just because it froze.

 

[StoneCold]

We turn our heat tracing on when the Ambient temperature reaches 40F or 4.4C. Regardless of whether or not the pipe is flowing or the temperature of the heat traced pipe. If you want to save money on the heat tracing operating cost, increase the insulation thickness. Ultimately it is the insulation that impacts how many watts you pull per hour, assuming you are using self limiting heat tracing.

Regards
StoneCold

 

[majesus]

Thanks for the help. This became a curiosity of why we use a setup point above 0C (say 4C) and I think BigInch has the simple answer that I was looking for:

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.

 

[CRG]

Why would the thermal expansion of water dropping from 4 degC to just above 0 degC be of any more concern than the thermal expansion of water rising in temperature from 4 degC to 8 degC. I think that someone just pulled the 4 degC number out of their a**. It is just as important to consider the thermal expansion of water when you switch on the heat trace and the line heats up as it is to be concerned with the expansion of the water as it approaches the freezing temp. The 4 degC was most likely used because of the lack of confidence in the calculation set.

 

[Compositepro]

"IEEE recommends a 10% "safety factor" as a minimum."

This statement seems completely meaningless to me in the context. Could you clarify?

 

[BigInch]

CRG,
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