Thermal Relief for HDPE line

[js2k9]

is it normal to use thermal relief for HDPE potable water line considering its low thermal conductivity?

 

[europipe]

Give more info;
min and max. design temp, volume, lenght, size of system?
Is it in the sun?
greetings

 

[js2k9]

design press = 350 kPag
design temp = 83degC
yes this is exposed to the sun.
this is just a 50mm line

 

[C2it]

Given the very low Young's modulus I would think that thermal relief is not necessary. Isn't 83 deg C a bit hot for HDPE ?

 

[ione]

Thermal annealing for polyethylene items is not unusual. Some production cycle (i.e. extrusion) after the forming phase foresee the use of a rapid cooling procedure during which the molecules of plastic are “frozen” in their position and residual tension can arise. Annealing process relaxes residual tensions through heat application (around 100 °C) improving mechanical properties. Annealing also contributes to minimize shrinkage phenomena during service.

 

[BigInch]

That depends on its pressure at minimum temperature, the fluid's maximum temperature and the bulk modulus of the fluid within. If the line (assume longitudinally restrained) is closed off near its maximum pressure rating and the fluid heats up for any reason, pressure will increase, regardless of how low its Young's modulus may be. A low E value would simply mean that the pressure increase caused by fluid volume expansion due to temperature change would be LESS than if it were steel, but it would still increase. If the increase in pressure is great enough to put it over its maximum pressure rating, you would need to have thermal relief.

95C is a common maximum temperature rating for HDPE. Melt is 210 C.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

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[ione]

BigInch,

Melting point should be 130 °C for HDPE.

 

[BigInch]

That may be the initial softening point.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[ione]

If I may add, a temperature of 83 °C is practically the max continuous service temperature for HDPE.

 

[BigInch]

83 is pretty close to the 95C rating value, but not yet too hot for HDPE.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[C2it]

I'm just doing some work with HDPE. Looking at the Georg Fischer catalogue, allowable pressures for PE80 and PE100 drop off to practically nothing at 60 deg C. Melting point is 130 and 131 deg C respectively. My concern about 83 deg. C is that is is probably well into the creep range.

A very low E mod. at 900 MPa means that fluid thermal expansion is absorbed by the pipe for very little force. Remember that the HDPE has a high expansion rate too.

 

[ione]

ASTM D 3417 identifies 260 °F (126.67 °C) as the melting point for HDPE.

 

[dcasto]

At that temperature I'd suspect the OD would grown monthly and the line would sag unless continously supported.

the HDB at 60C is 1/2 its 23C rating.

 

[ione]

Agree both with C2it and dcasto. Mechanical properties of HDPE decay as service temperature increases. Take a look at the attached paper (especially page 5).