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

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