How to Calculate Pipe Thickness for External Pressure

A Methodology to calculate allowable external pressure is given in paragraph UG-28 of Section VIII Division 1. An example of application would be a pipe within a pipe heat exchanger. It requires the assumption of a thickness, that can then be used to calculate allowable external pressure, using values determined from figures and tables (figure G, and applicable external pressure chart from ASME Section II part D, based on specific materials, modulus of elasticity, and temperature, using ratios of length/outside diameter, and outside diameter/thickness.

If assumed thickness doesnt provide sufficient stiffening/support assume greater t and repeat.

The piping standards (B31.1, 31.3, etc.) to the extent that they address the subject, send you back to the standard and paragraph referenced above (In ASME B31.1, see paragraph 104.1.3). Normally pipe is not exposed to external pressure, other than the possibility of it's being being under a full or partial vacume.

Hope this helps,

Dan W

Dear Mr. Dan W,

Thank you very much for your comment on my query.My problem has been solved.I am a young engineer working as a piping design and estimation engineer with C&J Engineering in UAE.Every day i face new problem about piping i want to solve of those problem ...as i want somebody else who can shear his knowledge and eprience with me ,then it wil be very easier to me to solve the problem.....Would you want to be my counter part for discussing about piping engineering ,if yes then let me know about you...

thanks

Soumen Banerjee

σ = P x [ R^2 - (R-t)^2 ] / [ R^2 - (R-t)^2 ] + αΔTE

σ = Stress

P = external pressure

R = external radius

t = thickness

α = coefficient of thermal expansion

ΔT = change in temp (°C)

E = modulus of elsticity

the rest is algebra.

N.B. Valid only from 0-100 °C

Dear Sir,

Thank you very much for your reply ,but it is not clear to me wheather change in temp. means what?is that temp. differance between external and internal temperature.

say ,i have the data of a pipe :pipe size,external presure,external temp,internal temp and inernal pr. is atm pr.so how can i calculate the thickness?can u pl give an example for this?

thanks

Soumen Banerjee

As you've written it (R^2 - (R-t)^2/R^2 - (R-t)^2) = 1, regardless of the value of R & t. It's the same above the line as below the line, ie. R^2 - (R-t)^2 divided by itself.

Have you copied the formula correctly? Where does this formula come from?

JohnnyG

I don't understand the problem because when the higher pressure is internal,the pipe wall is stretched may be until its limit.But,you just study possibility of collapse in a submarine?

I just want to know when any pipe is being subjected to externel pressure then how can we select the pipe schedule.I got the Mr. Dan's answer is correct.I had one example for that but didnt get it where it kept.

Thanks,

Soumen

For design of piping systems under external pressure, first start to calculate the pipe wall thickness under internal pressure. Second start to make a check for that thickness if the pipe subjected to external pressure.

Note: if the pipe is not subjected to internal pressure, you can assume a pipe wall thickness and proceed the external pressure calculation to see how much the pipe will resist the imposed external load, may be you need to increase the pipe wall thickness.

The pressure piping code ASME B31 guide us to use the ASME BPVC, Section VIII, Div. 1, UG-28 for checking the assumed/calculated thickness of pipe wall.

Quoted

UG-28 Thickness of Shells and Tubes Under External Pressure

Where, A = Factor determined from Fig. G of Subpart 3 of Section II, Part D.

B = Factor determined from the applicable material chart, psi.

D_o = OD of cylindrical shell course or tube, in.

E = Modulus of elasticity of material at design temp., psi

L = Total length, in. (see Fig. UG-28.1).

P = External design pressure, psi.

P_a = Calculated value of max. allowable external working pressure for the assumed value of t, psi.

R_o = Outside radius of spherical shell, in.

t = Min. required thickness of cylindrical shell or tube or spherical shell, in.

t_s = Nominal thickness of cylindrical shell or tube, in.

(c) Cylindrical Shells and Tubes :

(1) Cylinders having D_o / t ≥ 10)

Step 1. Assume a value for t and determine the value of L/ D_o and D_o /t.

Step 2. Enter Fig. G at the value of L/ D_o .

• For values of L/ D_o > 50, enter the chart at a value of L/ D_o = 50,

• For values of L/ D_o < 0.05, enter the chart at a value of L/ D_o = 0.05.

Step 3. Using the value of L/ D_o , move horizontally to the line for value of D_o /t.

From this point of intersection, move vertically downward to determine factor A.

Step 4. Using A, enter the applicable material chart, move vertically to an intersection with the material/temp. line for the design temp. (see UG-20).

• If A falls to the right of the end of the curve, assume an intersection with the horizontal projection of the upper end of the curve.

• For A falling to the left of the end of the curve, see Step 7.

Step 5. From intersection obtained in Step 4, move horizontally to the right and read the value of B.

Step 6. Calculate the max. allowable external working pressure, P_a = 4 B / [3(D_o/t)] .

Step 7. For values of A falling to the left of curve, P_a = 2AE / [3(D_o/t)] .

Step 8. Compare the calculated P_a with P. Increase t until P_a ≥ P.

(2) Cylinders having D_o / t < 10 :

Step 1. Using the same procedure as given in UG-28(c)(1), obtain the value of B.

• For values of D_o /t < 4, the value of A can be calculated using the following formula : A = 1.1 / (D_o / t)²

• For values of A > 0.10, use a value of 0.10.

Step 2. Using the value of B, calculate a value P_a1 using the following formula : P_a1 = [2.167/(D_o/t) - 0.0833] B

Step 3. Calculate a value of P_a2 using the following formula : P_a2 = [2S/(D_o / t)][1 - 1/(D_o/t)]

where, S = Lesser of 2 times the max. allowable stress in tension at design metal temp., from the applicable table referenced in UG-23, or 0.9 times yield strength of the material at design temp. Values of yield strength are obtained from the applicable external pressure chart as follows :

(a) For a given temp. curve, determine the B value that corresponds to the right hand side termination point of the curve.

(b) The yield strength is twice the B value obtained in (a) above.

Step 4. The smaller of P_a1 or P_a2 shall be used for the max. allowable external working pressure P_a .

Compare P_a with P, and change t until P_a ≥ P.

Unquoted

what is IDR of a pressure vessel?

what is the skirt of a pressure vessel?why and when it is used?if the vessel is made by stainless stell then it is necessary to consider the corrosion allowance if yes then what value can be considered? from where we can find out the data for this?is idr should be equel to OD,if yes then why?

I have to design one storage tank ....data goven by client are

Capacity-16000 gallons
Material-A 36(ss)
Dish type head and length of the vessel should be limited by 12 m long

how can i design this tank and which code i have to follow?

Pr.-Atm ,Temp-Ambiant

Pl. help me to design this storage tank

Thanks

Soumen

• Design of pressure vessel shall be in accordance with ASME BPVC, Section VIII, Division 1.

• Where Design of storage tank shall be in accordance with API 650 (or API 620).

I have to design one steel storage tank capacity is 16000 gallons tank length is limited to 11 m.material is A36.How can i design the tank.I know that i can refer storage tank code API 650 but how can i start?What is the design procedure?

Can anybody help me by approaching step by step design procedure...

Thank & Regards,

Soumen Banerjee

thanks for your good information regarding calculation of thickness for external pressure, but in case stiffener is required then how we design stiffener, how can we calculate the space between two stiffener & its size .

with regards

mkhan

In the case of determining external pressure as discussed previously in this thread, the value 'L' must be determined, which equates to the maximum unsupported length of the cylinder being considered. The greater the value of L, the more prone to failure because of the combined twisting and crushing moments. As an example, a shell and tube heat exchanger tube subject to external pressure, with no stiffening elements, 'L' would be the linear distance between the tube sheets.

A properly designed stiffening element provides a line of support however, and therefore, when stiffeners are used, the numeric value of 'L' can be reduced. In determining the value of 'L', it is taken as the sum of ½ the linear distance to the next line of support on one side (measured from the center of the stiffening element), plus ½ the linear distance to the next line of support on the opposite side. Each stiffener is evaluated independently, with the design being limited by the strength of the weakest component.

In evaluating the stiffening element itself, a comparison is made between the required moment of inertia I_s, and the actual moment I, provided by a given stiffening element. UG-29 of ASME Section VIII Division 1 provides the method, and formulas are given for evaluating the required moment of the stiffening element, and the combination of element and cylinder.

The actual moment must be equal to or greater than the required moment. I.e., what you actually have can't be less than what you are required to have.

Once the required moment (I_s) is determined by the formula in UG-29, the actual moment can be determined from the following formula, if the stiffener has a rectangular cross section, with T and H being the thickness and height of the ring, and 12 being a constant. For other geometries, the formula would have to be modified.

I = ( T_r x H_r³ ) / 12

Dan W

I have a large, straight pipe header that gradually reduces in size as after each branch, is it correct to assume 'L' to be the distance between the reducers or should I consider pipe supports and restraints.

Based on 140' of ASTM A-53 welded steel 48" pipe, I caculated that we would need XS (0.5" thickness) to withstand full vacuum (assumed P=15 psia) as STD won't withstand the pressure. Passes the sanity check but if I could take credit for the pipe supports (not sure of their type or distance between them yet) I may be able to use STD pipe and save 8,800 lbs of steel. Actual conditions in the pipe are water at 1.5 psia, 115°F and atmospheric pressure outside.

In order to qualify as a line of support, the pipe supports would have to provide sufficient stiffening to resist the twisting/crushing moments applied to the section(s) under consideration. I.e., the actual moment of inertia supplied by the properly designed stiffening element must meet or exceed the required moment of inertia for a given geometry.

I think in this case that you would have to look at the maximum diameter and length of each of the sections individually, and design each section as required for its particular geometry (Diameter, Length, Thickness) - Dan W

Dear Sir

How to calculate value of A by manual calculation.

A=Factor determined from Fig. G of Subpart 3 of Section II, Part D.

We are waiting for your reply.

Amit Sharma

From Fig. G Geometric Chart for Components Under External or Compressive Loadings (for All Materials), fellow step 3:

Step 3. Calculate the values of both (L / D_o) and (D_o / t), use the value of (L / D_o) and move horizontally to the line for value of (D_o / t).

From this point of intersection, move vertically downward to determine factor A.

Note: If thickness t is not yet known, you can assume t with an arbitrary figure to find (D_o / t), and then proceed the calculations that proves that assumed t is OK.

Hi

thnx for help. I have one more question about surge vessel

What are the step to design surge vessel?

I have data

pump flow rate- 15 q.m / hr.

pump head -15 m

pump should be start 18 times/ hr.

Pl. help me out to solve it.

Thanks

soumen

sir is it possible to have a free pdf copy of BPV code section 8?

dear sir

in step 2 when d/t<10

we need to do same as in step 1 upto d/t=4

but to use different formula as in step 1

am i interpreting rigth

When

When D_o / t is less than 10, that means a heavy wall in relation to a cylinders diameter. In that case, the second formula utilizes the modulus of elasticity of a given material at the appropriate temperature. The modulus of elasticity can be found on the specific external pressure chart applicable for a given material. The numbers given at the base of the temperature curves on each external pressure chart are the modulus of elasticity for that material, given for each of the temperature lines on the chart. As an example, for carbon steel with a yield strength over 30,000 psi, at 300 degrees, chart CS-2 provides an "E" value (modulus of elasticity) of 29 x 10 to the sixth, or 29,000,000.

Dear sir what is the value of " L " to be assume ?

The value of "L" should not have be an assumed value, it should be a known dimension. It equates to the linear distance between lines of support. In the case of a pipe in pipe heat exchanger, it would be the linear distance between the shell flanges (tube sheet in this case), or the total length of the pipe in pipe run, unless the internal pipe were to have properly designed stiffeners, which is not likely to be true. The calculations become considerably more complex if the length "L" is established as the distance between stiffening elements, rather than the overall linear length, as the design of the siffeners themselves has to be proven (required moment of inertia versus actual moment of inertia).

I think the question about the assumption of "L" pertains to when creating a Piping Class. At this point the materials and the thickness of the pipes are being specified and the lengths are not yet known since the pipeline has not been designed yet.

Hi Sir,

Thank you for the very useful post. I have few questions regarding to my work.

1) I have calculated wall thickness based on internal pressure. Add all the mill tolerance (12.5%) and corrosion allowance (6mm). Let say now I chose SCH XXS (22.23mm) for 8" pipe.

2) Now I want to calculate for the external pressure based on UG-28.

My questions are;

- What would be the t to determine Do/t ?

- I am using t as 22.23 - 12.5% mill tolerance = 19.45mm (as I assume that will be the least pipes may be received from mill)

- There is a theory that t supposed to be 22.23 - 12.5% mill tol - CA (6mm) = 13.45mm

As we didnt found any specific guide for defining t.

- As for material API 5l X65Q, used for temperature -18celc to 121celc, should I use Fig. CS-2 or CS-3 for defining factor B?

Thank you Sir, in advance. Apologize if you found the questions are not challenging.

● From ASME VIII-1, UG-28, t is defined as the min. required thickness of cylindrical shell or tube or spherical shell, measured in inches. So, in case of pipe, the thickness t must be used at corroded condition (deduct the corrosion allowance), in addition to deduction of mill tolerance (12.5%).

● Defining the Chart of external pressure d depends on the material properties, SMYS and design temp., and API 5L - X65 has a SMYS of 65 000 psi with a design temp. 121 ^oC (250 ^oF).

So, you have to use Chart Fig. CS-3, where you can easily read note (b) These curves shall not be used for design temp. above 300 ^oF (149 ^oC ). Above 300 ^oF, use the appropriate temp. curve shown in Fig. CS-2.

thank you for such clear and fast reply.

Hi Sir,

I have one question regarding my work.

I have 1/2" pipe length of 120mm attached to the header; function as hydrotest drain point. After hydrotest, it will be permanently weld.

Should it be tested for external pressure as well?

Hope you could advise. I used to ignore since in my opinion, it will not prone to collapse due to its short length. And also it is not function during operation.

Thank you in advance.

I believe that your assumption regarding the hydro test drain would be correct because of its short length and small diameter; it would likely calculate out to be acceptable. However, there is generally little provision in the code for making assumptions, so I would go ahead and calculate it for external and/or internal pressure as applicable, removing the potential for being second-guessed at some future point. There are cases in the code where even internal projection of a set-through nozzle / manway ring, etc. has to be calculated for both external pressure and internal pressure, even though the internal projection would not likely be much more than the 120 mm that you stated. - Dan W

As stated by Abdel, the t used in UG-28 needs to be the minimum thickness. If the pipe is ordered by its nominal (average) thickness (as appears to be the case as reference is made to a schedule (XXS/Extra Extra Strong), the 12.5% mill under-tolerance must be deducted from the pipe t. ex.., t_Nominal x 0.875 = t_minimum.

Additionally, the code states that all calculations used in the formulas in the code shall be based on dimensions in the corroded condition, as it is assumed that the material representing the corrosion allowance will disappear over time.

From UG-16 (e) of ASME Section VIII Division 1, quoted verbatim "Corrosion Allowance in Design Formulas: The dimensional symbols used in all design formulas throughout this Division represent dimensions in the corroded condition.

So; t _Minimum = t _Nominal x 0.875 and then;

t _{Minimum - Corrosion Allowance = t to be used to calculate allowable pressure, either internal or external.}

_{Given as a single expression: t = (}t _Nominal x 0.875) - Corrosion Allowance.

What should I do if the intersection of my L/Do and Do/t values falls off the lower right side of figure G? Do I use the maximum value for factor A?

i know this is an old thread Mr. Halim but im looking for some information regarding the same subject external pressure calculation.

During Hot/ Cold tapping we are using the same method to calculate the maximum allowable external pressure for hydrotest purposes. As the branch connection, weldolet or split tee is welded on to the existing pipe, and the pipe is hydrotested for the new weld. I was wondering whether this method is appropriate to calculate the maximum allowable external pressure. and one more query i have is during hot tapping when the line is hot and pressurized does that have any effect on the calculation, as i dont see the method taking that into account. lets say the internal pressure of the existing pipe is already at 100barg during hot tapping. Will this pressure not have a nullfying effect on the applied external(hydrotest) pressure, is there any allowance in the code to exceed the Max allowable external pressure calculated by adding the internal pressure? i did not find any such information in UG-28 please advise .

Thanks

hi

the thickness of a pipe subjected to external pressure can be found using the ASME secVIII & secII. procedure is clearly eloborated in secVIII div1 step wise. pl have a look.

rgds

krishna

Hi All,

I have a question;

Can any other code replace the ASME VIII for external pressure calculations for piping?

Thanks

Hi ,

I want to know about "L" - Total length to be considered for piping with many elbows and then connected to equipment.

Is the total length L= Total piping length ( all spool lengths added)

or Total length= Shortest distance between two equipment.