Points I couldnt understand in Aramco Shell Tube exchangers Standard 1

[Iomcube]

I am quoting from Aramco Standard...
When minimum wall tubes are specified, the tube-side pressure drop shall be based on 110% of the selected tube wall thickness. ??

Again...
Fixed tubesheet exchangers without the expansion joints are acceptable up to a maximum differential temperature of 28°C between tube mean metal temperature and average shell temperature.
What is meant by tube mean metal temperature Is it simply the skin / wall temperature?

 

[chicopee]

I'll take a stab at the quoted statement which does not appear to make sense since I do not know the inlet and outlet pressures to and from the tube ends, respectively. The wall thickness is to be calculated at a pressure equal to 110% of the pressure drop through the length of the tube. So for example if the pressure drop along the entire length of the tube is 100psi then the wall thickness is to be calculated for a pressure of 110 psi.

 

[IRstuff]

Is it simply the skin / wall temperature

Well, one of them... "Mean" means "Average"

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[EdStainless]

If you order average wall tubes the wall tolerance is usually +/-10%, but their rating is based on the nominal (specified) wall.
So if you order minimum wall tubing then the specified and nominal are not the same, the nominal is actually heavier than the specified wall.
So the ID of a 0.083"aw tube is larger than that of an 0.083"mw tube.
You calculate the pressure drop using nominal wall thickness, not specified. You don't just scale the pressure drop.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[marty007]

When minimum wall tubes are specified, the tube-side pressure drop shall be based on 110% of the selected tube wall thickness.

When designing a heat exchanger, you must calculate pressure loss for the fluid passing through the tubes. If you are buying min-wall tubes, then the actual tubes that will be received will be slightly thicker than the minimum. This will effectively reduce the flow area inside the tube. What Aramco is asking is that when you perform the pressure loss calculations for the fluid passing through the tubes, you must use a value for tube ID that reflects the tube being 10% thicker than the minimum specified. This will increase the pressure drop. They are doing this to ensure you don't build a piece of equipment with excessive pressure loss.

Fixed tubesheet exchangers without the expansion joints are acceptable up to a maximum differential temperature of 28°C between tube mean metal temperature and average shell temperature.

When performing a thermal design on a heat exchanger, you have heat transfer occurring on both the inside and outside of the tube. The actual metal temperature of the tube will be somewhere between the temperature of the shell side fluid and the tube side fluid, and will vary along the length of the tube as the temperature of the to streams increases/decreases. The mean metal temperature is the average temperature of the tube along the length of the tube.

 

[Christine74]

TEMA standards contain formulas for calculating the Mean Metal Temperature of the tubes.

I'd also caution that you should always use the nominal thickness of the tubes when checking for tube vibration, even if you assumed 10% thicker tubes while running your pressure drop calculations.

-Christine

 

[Iomcube]

marty007 & chicopee as well as others thanks for making me understand