8/11/23, 4:12 PM
external pressure design thickness calculation for pipe with an example – The piping talk
THE PIPING TALK
Sharing information about process piping
PIPING CALCULATIONS
external pressure design thickness calculation for pipe with an
example
By thepipingtalk on 10/2019
When we say internal pressure thickness design we have to deal with the tensile properties of the material, where the pressure is trying
to expand the material from inside and trying to tear it into two. With external pressure the opposite happens. The pressure is trying to
compress the material to squeeze it together.
internal pressure vs external pressure
Steel has very similar properties in tension and compression. So you may think external pressure on pipe will never be as much as
internal pressure, and as we are calculating thickness of pipe for internal pressure, this will take care of the external pressure as well.
This is wrong because in compression the failure can occur well below the yield point.
“This phenomenon is called buckling where a column/pipe loaded in compression with critical load, the column/pipe will fail before the load
that would actually required failing in tension.”
https://thepipingtalk.com/external-pressure-design-thickness-calculation-for-pipe-with-an-example/
1/6
8/11/23, 4:12 PM
external pressure design thickness calculation for pipe with an example – The piping talk
Jacketed pipes, pipe deep inside sea, vacuum piping, internal piping inside vessel are some of the instances where pipe experience this
phenomenon of external pressure. External pressure thickness of pipe/tubes/cylindrical shells are calculated according to- ASME
SECTION VIII DIV-1 UG-28. There are two separate procedure specified for calculating minimum required thickness, for Do/t ≤ 10 and
Do/t >10. In this article we will discuss about the first one which is commonly applicable for pipes with an example.
We will calculate the external pressure design thickness for 6” NB pipe having external pressure 250 psig at 750°F , Material of
construction is A312 TP 304L , corrosion allowance – nill.
There are 4 steps:
STEP-1
Find Do/t where Do = outside diameter = 6.625 in , t = thickness in corroded condition.
For 6” ss pipe the minimum commercially available thickness is sch-5s = 0.109”. mill tolerance is ±12.5%, so in calculation we have to use
the least thickness we will get considering negative 12.5% mill tolerance. So t = 87.5% of 0.109” = 0.095 in.
STEP-2
Calculate L/Do where Do = outside diameter = 6.625 in
L = total length between two end support of pipe i.e maximum unsupported span. Consider length of straight pipe with a flange
on one end of the spool and a valve some distance away. That distance between the flange and valve would be the length under
consideration. In the absence of any stiffener similar to the valve or flange, stiffener ring can be added around pipe.
For the sake of calculation we will use length as 330 in (8300 mm approx).
For L/Do greater than 50 enter the chart at a value of L/Do = 50 and for values less than 0.05 enter the chart at a value of L/Do = 50.
STEP-3
Determine factor A form Fig. G sub part 3 of section ii, part D (geometric chart for component under external or compressive loading).
Find out factor A in the graph by moving horizontally to the line for value of Do/t. interpolation may be done for intermediate values of
Do/t. from this point move vertically downward to determine value for Factor A (see fig. 1). There is also a tabular chart available in with
values from the graph which you can use.
https://thepipingtalk.com/external-pressure-design-thickness-calculation-for-pipe-with-an-example/
2/6
8/11/23, 4:12 PM
external pressure design thickness calculation for pipe with an example – The piping talk
Fig.1 – geometric chart for component under external pressure
In our case factor A = 0.00025 from fig. G.
STEP-4
Using the values of A calculated in step 3, enter the applicable material chart in subpart 3 of section ii part D. as our material of
construction is TP 304L we have to see FIG. HA-1- which is chart for determining shell thickness of component under external pressure
when constructed from austenitic steel.
Move vertically to an intersection with the material/temperature line for the design temperature. Interpolation may be done for
intermediate temperature. and then move horizontally to right to read the value of Factor B.
As shown below Fig. 2 in our case we get value of Factor B =2750
Fig.2 graph for determining external pressure thickness
In case the values of A falls to the right of the end of material temperature line, assume an intersection with the horizontal projection of
the upper end of the material temperature line and then move horizontal to right end to read value of Factor B. see refer next step
When Factor A falls left hand side refer STEP-6
STEP-5
https://thepipingtalk.com/external-pressure-design-thickness-calculation-for-pipe-with-an-example/
3/6
8/11/23, 4:12 PM
external pressure design thickness calculation for pipe with an example – The piping talk
Calculate maximum allowable external working pressure (Pa)
This shows the allowable external pressure on pipe is only 52.6 psig where as we have to design the pipe to sustain 250 psig. So repeat
again from step 1 with next available thickness.
With thickness 6” sch 40 = 0.28 thickness you will get Do/t=27.04, L/Do=50, Factor A = .0018, Factor B= 6000
(Pa) =(4*6000/3*27.04) = 295.8 psig which is > 250 psig.
So our calculated external pressure design thickness is 6” sch 40=0.28 inch thick pipe, which will be safe for an external pressure of 250
psig.
STEP-6
When Factor A falls left hand side of the curve Calculate maximum allowable external working pressure (Pa) by below formulae
For example if A=0.00006, E from graph up to 750°F=23.8 x 10^6 PSI = 164.0 X 10ˆ3 mpa.
ASME SECTION VIII DIV 1
BUCKLING OF PIPE
EXTERNAL PRESSURE DESIGN THICKNESS
MAXIMUM ALLOWABLE EXTERNAL WORKING PRESSURE
PIPE THICKNESS CALCULATION
EXTERNAL VS INTERNAL PRESSURE PIPE
UG-28
VACUUM PIPE
thepipingtalk
READ NEXT →
PIPING CALCULATIONS
PIPING CALCULATIONS
Pipe Branch reinforcement pad strength
calculation- with an example
Piping thickness calculation with an example
 2 COMMENTS
https://thepipingtalk.com/external-pressure-design-thickness-calculation-for-pipe-with-an-example/
4/6