rectangular storage tank tmin calculation 1

[jmcghee]

I have an in service rectangular water storage tank and I'd like to calculate the minimum wall thickness that is required to keep the stress in the tank walls from exceeding the allowable stress for the material, so I can make a conclusion of whether replacement is required. I initially treated a 1 foot wide strip of the tank as a fixed-fixed beam with the water acting as a distributed load (62.4 lb/ft^3 * 8 feet (height of tank) * 1 foot wide section), but I can't seem to calculate realistic values.

If someone could assist me in my calculations, that would be much appreciated. Thanks!

 

[khardy]

Look for “Rouark & Young’s Formulas for Stress and Strain.” There is a section on stresses in flat plates for both uniform and uniformly decreasing loads. These formulas are also summarized in several mechanical engineering references. For example, my PE exam reference manual (Lindeburg) has them in the machine design section.

Your best bet, if you can find it, is Megyesy’s “Pressure Vessel Handbook” there is a chapter on rectangular tanks. (If you deal with tanks a lot you should by this book anyway.)

Also, are you sure there aren’t any stiffeners or tie-rods inside the tank?

Good luck.

 

[BarryEng]

Go to

http://www.usbr.gov/

Go to top LH corner & search for EM27
You will now have a copy of (6.2MB) Moments & Reactions for Rectangular Plates developed by the USBR (United States Bureau of Reclamation) in the 1960's I think.

This question has been answered in previous threads (in this web site) but I have forgotten where - it gave the direct link.

Found it - thread256-166662.
The direct link is

http://www.usbr.gov/pmts/hydraulics_lab/pubs/EM/EM27.pdf

 

[BarryEng]

I meant to mention - do a search (on this web site) for 'rectangular tanks' & you will get a lot of threads.

These will give you info on what other problems have been experienced by tank designers.

 

[metengr]

There really is no easy way to evaluate degraded tanks because local corrosion damage typically results in leak before break, provided the materials of construction have adequate fracture toughness. I presuming the tank was correctly designed to begin with.

Attempting to calculate a minimum wall thickness will not be realistic because in most cases with water storage tanks, corrosion damage only effects localized areas. Since this is an in-service water storage tank, you are probably concerned about internal corrosion damage from through-wall pitting (aka local thinned areas - LTA's) and eventual leaks.

I would suggest API 579 for evaluating LTA's in tanks. This is an excellent reference book for in-service component assessment but it is very expensive. Here is what I would do as another option, instead of attempting to over analyze this;

1. Remove the tank from service, and drain it.

2. Clean the inside surfaces and have a thickness survey performed by a nondestructive testing vendor using ultrasonic testing.

3. Based on the results of the UT thickness survey, you can decide on the extent of local repairs to the tank wall to avoid leaks. The repairs should be performed in locations where the LTA is below 50% of original wall for starters. If the damage is too extensive, you can decrease the repair scope threshold by using only 35% of orignal wall and monitor the remaining areas until you have some idea of a corrosion rate. At some point, the tank may have to be replaced or major repairs would be required depending on volume.

4. I would periodically inspect the tank to stay on top of degraded areas.

 

[MJCronin]

McGhee....

I suggest that you review the design requirements of a new design and then compare the methodology against the mess that your boss hass assigned you.

As I recall, the goal in rectangular metallic tank design is not only minimum wall thickness for checking membrane stress, but an evaluation of stiffener size and spacing.

Most rectanglar tanks I have seen have an angle stiffener at the top lip of the tank, at each corner and spaced across each face.

My opinion only

-MJC

 

[LSThill]

jmcghee (Mechanical)

You need to search for the following:

British Engine Technical Report 1981 Volume XIV

Non-Circular Pressure Vessels

-Some guidance notes for Designer
M. Straczewdki B.Sc. Eng.
Stress Engineer, Research Dept.
Page 62 - 88, total 26pages.

Leonard Stephen Thill

 

[innovation2]

App 13 of ASME Section VIII Div. 1

 

[LSThill]

jmcghee (Mechanical)

See attach;

British Engine Technical Report 1981 Volume XIV

Non-Circular Pressure Vessels

-Some guidance notes for Designer
M. Straczewdki B.Sc. Eng.
Stress Engineer, Research Dept.
Page 62 - 88, total 26pages.

Leonard Stephen Thill


L S THILL

 

[Raydel]

There is a very reliable method for evaluating rectangular tanks in the book: Pressure Vessel Handbook by Eugene Magyesy. I have used this method, and have found it to be simple an sound.

Ray Delaforce

 

[LSThill]

Raydel (Mechanical) (Ray Delaforce)coade support

Whitch Education of "Pressure Vessel Handbook by Eugene Magyesy" are you reference.

LSThill