Pressure and thermal stressors in a tube

[Enviro21]

I am working on an engineering problem as part of a private research project. The problem is out of my field of expertise and need some help.

The purpose of the question:

To determine if a metallic pipe (6061 Aluminium) of hexagonal shape 160mm long and 40mm OD measured across the flats with a 9.9mm dia thru hole and a wall thickness of 15mm at the thinnest point of the hexagonal wall will be strong enough to safely contain the peak water pressure of 2850 psi when heat is externally applied. The water completely fills the tube with no gas pockets. The heating is by two PCT ceramic heating elements of 30W each and rated at 240 deg C. (in free air), with insulation wool of approximately 25mm surrounding the pipe. The pulsed flow rate of the water is 97mm3 per pulse or 6.25 pulses/sec. and is dumped into a vessel of air with a pressure of 18 psi absolute.

What would the pressure change be due to thermal expansion of liquid in the tube in psi ?

What would the safety factor be for the containing tube compared to its maximum mechanical strength?

Can you please show me the calculations and formulas so I can learn something new.

 

[LittleInch]

Got a drawing or sketch of this?

Pressure rise is able to be calculated, but a good start point is about 3 to 4 bar per degree C from its neutral temperature. This might be a bit more because the tube is pretty thick.

Pulsed flow??

I don't understand what's going on here. How will this heating up occur and where does 2850 psi come from?

I would simply model this as a pipe with 15mm wall thickness and an OD of 40mm

If you design this to ASME B 31.3 then your FoS is 1.5 for yield stress or 3 for UTS whichever is the most onerous (usually UTS)

 

[Enviro21]

Hello LittleInch,
Thank you for your reply! Sorry for the apparent late reply but there is anywhere between 12 to 17 hours time difference depending where you are located in the US.
I have attached a drawing with explanations to help clarify things, but please refer to my original post for full details.

Cheers!

 

[Stick.]

What's your steady state temperature for this thing, as well as the highest temperature it would see over it's lifespan (like if the flow stops and the heater stays on or the heater is left on with an empty tube)? If the aluminum is allowed to reach 240°C, then the mechanical properties of the aluminum will drop significantly (even after subsequent cooling), which would somewhat change your analysis.

 

[SnTMan]

I'd start simple, assuming steady state. 2850 psi at 464 F. (OP, thx for the mixed units)

Thick wall tube > Lame's equation, highest stress at ID.

ro = 0.787 in, , ri = 0.297 in

Calculate hoop strress, compare to allowable stress of (unkown) 6061 aluminum.

If moderate stress results, call it a day, if not the analysis can be complicated refined as needed.

Assumptions: No thermal expansion of fluid: Entering temp not known, but how much can it heat up at 60 W in 160 ms?

Pulsed flow ignored: If a signicacant number of cycles is anticipated fatigue analysis may be warranted. Aluminum has no endurance limit.

Yield stress and temperature effects on properties may govern.

OT: its a shame the attachment cannot be (apparently) be rotated on screen. What a mess to view

OP. best of luck.

Regards,

MIke

 

[Enviro21]

What's your steady state temperature for this thing, as well as the highest temperature it would see over it's lifespan (like if the flow stops and the heater stays on or the heater is left on with an empty tube)? If the aluminum is allowed to reach 240°C, then the mechanical properties of the aluminum will drop significantly (even after subsequent cooling), which would somewhat change your analysis.

Regarding your temperature question Stick, I quote a portion of the manufacturer's product description.
"The role of Positive Temperature Coefficient materials are (PTC) materials that demonstrate a positive resistance change as temperatures increase.

Manufacturers can create PTC ceramics to feature different fixed temperatures that must occur before the significant resistance change happens. No matter where you set it, this point is called "The Curie Point." It is the heater's overarching Temperature Control point.

As the degrees grow higher, the materials' electrical resistance strengthens as well, limiting the flow of the current. Conversely, it allows the current to pass more quickly as temperatures drop. "

 

[Enviro21]

I'd start simple, assuming steady state. 2850 psi at 464 F. (OP, thx for the mixed units)

Thick wall tube > Lame's equation, highest stress at ID.

ro = 0.787 in, , ri = 0.297 in

Calculate hoop strress, compare to allowable stress of (unkown) 6061 aluminum.

If moderate stress results, call it a day, if not the analysis can be complicated refined as needed.

Assumptions: No thermal expansion of fluid: Entering temp not known, but how much can it heat up at 60 W in 160 ms?

Pulsed flow ignored: If a signicacant number of cycles is anticipated fatigue analysis may be warranted. Aluminum has no endurance limit.

Yield stress and temperature effects on properties may govern.

OT: its a shame the attachment cannot be (apparently) be rotated on screen. What a mess to view

OP. best of luck.

Regards,

MIke

Thanks Mike, that was really helpful. One more question. If the water at 2850 psia was heated, at what temperature would it flash to steam if the receiver was at 18-19 psia?

 

[SnTMan]

I don't know offhand but you should be able to look it up in the steam tables I'd think.

Thx for reworking the attachment. My remark was not a poke at you, rather at this site. You may not know but it was recently re-formatted specifically to make it hard to read