After completed the Part 2: Static Head, we will continue to try on the Open/Close questions on Module ASME VIII – Internal/External Pressure as a part of in-the-exam API 510 Body of knowledge (BOK)
If you don’t know how API 510 Certification scheme, See API 510 Certification Instructions
ASME VIII – INTERNAL PRESSURE
The inspector should be able to determine:
a) The required thickness of a cylindrical or spherical shell based on circumferential stress given a pressure UG-27(c)(1) and UG-27(d);
c) The vessel part MAWP for a cylindrical shell based on circumferential stress given a metal thickness (UG-27(c)(1));
d) The required thickness of a head (ellipsoidal, and hemispherical) given a pressure. (UG-32 (c), and (e))
e) The vessel part MAWP for a head (ellipsoidal, and hemispherical) given a metal thickness. (UG-32 (c), and (e)).
f) Whether a head (ellipsoidal or hemispherical) meets Code requirements given both pressure and metal thickness. (UG 32(c) and (e)).
The inspector should also be able to compensate for the corrosion allowance: add (+) or subtract (-) based on requirements of the examination problem. The Section VIII, Appendix 1 formula for cylinders, which is based on outside diameter, can be used. The Appendix 1 formulas for non-standard heads will not be required.
ASME VIII – EXTERNAL PRESSURE
The inspector should be knowledgeable of the rules for design of shells and tubes under external pressure (UG-28). The inspector will not be required to perform external pressure calculations.
Question 1: The stress trying to split a vessel shell longitudinal weld open is called?
(a) Hoop stress
(b) Circumferential stress
(c) Longitudinal stress
(d) (a) and/or (b) above
Explanation: See What is Hoop Stress in pressure vessel?
Question 2: The main cylindrical or spherical shells dimension used in the UG-27 cylinder formulae is?
(a) Internal diameter
(b) External diameter
(c) Internal radius
(d) External radius
Explanation: See UG-27(c)(1) and UG-27(d)
Question 3: A vessel with a longitudinally seamed shell circumferentially welded to hemispherical heads is pressurized internally until it fails. Which of these formulae would you use to calculate the pressure at which the split would occur?
(a) P = 2SEt/(R + 0.2t)
(b) P = 2SEt/(R – 0.4t)
(c) P = SEt/(R + 0.6t)
(d) Either (b) or (c) above
Explanation: The Hoop stress always trying to split the vessel open along its length. Confusingly, this acts on the longitudinal weld seam. For the purpose of the API 510 exam this is the governing stress in a shell cylinder.
Question 4: Which of these vessels could you not use the UG-27 formulae for (using a given joint efficiency of E =1) if the material has an allowable stress at its design temperature of 20000 psig?
(a) P = 50 psi
(b) P = 100 psi
(c) P = 2000 psi
(d) P = 8000 psi
Explanation: Looking at the formula for the cylindrical shell in UG-27 (c) (1) for circumferential stress, there are two limitations applied to it:
– The thickness must not exceed one-half of the inside radius, i.e. it is not a thick cylinder.
– The pressure must not exceed 0.385SE, i.e. not be high pressure. In practice this is more than about 4000 psi for most carbon steel vessels. (ASME VIII Div.1 is only classified for Pressure not exceed 3000psi or 20Mpa (U-1 Scope))
Question 5: A vessel has the following given parameters:
Outside Diameter – OD = 36 in, Wall thickness – t = 1 in, Joint efficiency – E = 1, Allowable stress at design temperature – S = 20000 psig. What is the maximum allowable working pressure (MAWP) at its design temperature?
(a) 546 psig
(b) 895 psig
(c) 1136 psig
(d) 2409 psig
Explanation: Use formula UG-27(c)(1) and note that R = ID/2 = (OD-t)/2.
Question 6: What is the retirement thickness of a 2:1 ellipsoidal head of t/L > 0.002 with the following parameters:
Inside Diameter – ID = 40 in, Design pressure (included static head) – P = 300 psig, Joint efficiency – E = 0.85, Allowable stress at design temperature – S = 15800 psig.
(a) 0.4 in
(b) 0.45 in
(c) 0.55 in
(d) None of the above
Explanation: Use formula UG-32(c)
Question 7: A VERTICAL vessel has the following given parameters:
Inside Diameter – ID = 8 feet, Nameplate indicating MAWP = 456 psig, Vessel height is 43 feet, both heads are 2:1 Ellipsoidal heads, Joint efficiency – E = 1 for all welds, Allowable stress at design temperature – S = 17500 psig. What is the minimum required thickness for each heads and shell?
Explanation: Do NOT forget the Static Head applied on each part, or 1 feet = 12 in
Question 8: A HORIZONTAL vessel has the following given parameters:
Outside Diameter – OD = 96 in, Actual nominal thickness of shell is 0.5 in and heads is 0.325 in and Corrosion Allowance is 0.123 in , Vessel height is 43 feet, both heads are 2:1 Ellipsoidal heads Joint efficiency – E = 1 for all welds, Allowable stress at design temperature – S = 15000 psig. What is the maximum allowable working pressure (MAWP) at its design temperature?
(a) 125.4 psig
(b) 117.6 psig
(c) 114.1 psig
(d) 121.9 psig
Question 9**: A cylindrical shell has been found to have a minimum thickness – t = 0.353 inch. Its original thickness was 0.375 inch with an original inside radius – IR = 12.0 inch and following parameters:
Original Design Pressure – P = 300 psig, all Joint Efficiency – E = 0.85, Allowable stress at design temperature – S = 13800 psig. What is its present MAWP?
(a) 300 psig
(b) 275.32 psig
(c) 338.46 psig
(d) 294.64 psig
Explanation: In API 510 examination, some redundancy parameter will not be used, read it carefully and slowly list every essential parameter.
Now we will try to be knowledgeable on External Pressure:
Question 10: What is the relationship between the maximum internal pressure (P) a vessel can resist and the maximum external pressure (Pa) it can resist?
(a) Pa = 2/3 P
(b) Pa = P x Factor-A
(c) P = 2/3 Pa
(d) There is no straightforward relationship
Explanation: Use formula UG-28, Pa = 4B/3(Do/t)
Question 11: Where is the ‘line of support’ assumed to be in a vertical vessel head with head depth – h (where h excludes the straight flange part)?
(a) On the tan line (which is h/2 from the head-to-shell circ-weld)
(b) On the head-to-shell circ-weld
(c) h/3 into the head from the tan line
(d) h/3 into the head from the head-to-shell circ-weld
Explanation: See Figure UG-28-1 (a) and A line of support is: a circumferential line on a head (excluding conical heads) at one‐third the depth of the head from the head tangent line as shown on Figure UG-28.
Question 12: Which formula should you use to determine the maximum allowable external pressure (Pa) for a cylinder with a wall thickness of 20 % of its external diameter Do (or Do/t = 5)?
(a) Pa = 4B/3(Do/t)
(b) Pa = 2AE/3(Do/t)
(c) Pa = 1.1/(D/t)2
(d) None of the above
Explanation: Use formula UG-28 (c) (2) when Cylinder having Do/t < 10.