17. PLANTEAMIENTO Y PROPUESTA
18.8. NÚCLEO DE EMPRENDIMIENTO:
Rules for braced and stayed construction are in UG-47 through UG-50 and in UW-19 of VIII-1. Stays are used in pressure vessels to carry part or all of the pressure loading when it is desirable and possible to reduce the span and/or thickness of a tubesheet, sideplate, or other pressure component. Opposite stayed surfaces are “tied” together by staybolts, tubes, or baffles, which carry pressure loading as tension members. Depending on the number of ties, the thickness of braced and stayed surfaces may be less than when the sur- faces are not stayed, because the loading is now resisted by both bending moments and bending strength and by tensile strength of the stays.
6.2.1 Braced and Stayed Surfaces
For braced and stayed surfaces tied together with threaded-end or welded-in staybolts of uniform diameter and symmetrical spacing, the following formulas apply for determining the minimum required thickness or internal design pressure:
t = p(P/SC)1/2 (6.1)
P = t2SC/p2 (6.2)
where
t = minimum required thickness of the stayed plate, in. P = internal design pressure (or MAWP), psi
p = maximum pitch between staybolts, in.
C = constant, the value of which depends on details of the staybolt end design as follows:
C = 2.1 for welded-in stays or threaded-end stays screwed through plates ≤ 7/16 in. thickness
with the threaded ends riveted over;
C = 2.2 for welded-in stays or threaded-end stays screwed into or through plates > 7/16 in.
thickness with threaded-ends riveted over;
C = 2.5 for threaded-end stays screwed through plates and fitted with single nuts outside the
plate, or with inside and outside nuts without washers, and for stays screwed into plates not less than 1.5 times the diameter of the staybolt measured on the outside of the staybolt diameter. If washers are used, they shall be at least half as thick as the plate being stayed.
C = 2.8 for threaded-end stays with heads not less than 1.3 times the diameter of the stays
screwed through plates or made with a tapered fit and having the heads formed on the stays before installing them and with threaded-ends not riveted over;
C = 3.2 for threaded-end stays fitted with inside and outside nuts and outside washer, where the
diameter of washer is not less than 0.4p and the thickness of washer is not less than the thickness, t, of the surface being stayed.
6.2.1.1 Special Limitations for Threaded-End Stay Construction
(a) Minimum thickness of plate to which stays can be attached, other than outer cylindrical or spherical plates, is 5/16 in.
(b) When two plates are stayed together and only one requires staying, the C value is set by the plate requiring staying.
(c) Maximum pitch for threaded-end staybolts is 8-1/2 in.
(d) When the spacing is unsymmetrical due to interference, half of the spacing on each side of the stay being considered measured to the adjacent stay shall be used for loading.
6.2.1.2 Special Limitations for Welded-In Stay Construction
(a) Required thickness of the plate shall not exceed 1 1/2 in. When plate thickness is greater than 3/4 in., the pitch shall be 20 in. or less.
(b) The maximum pitch for welded-in stays is 15ds, where ds, is the diameter of the staybolt.
(c) Welded-in stay details shall conform to one of those shown in Fig. 6.1. (d) Welds do not require radiography.
(e) Welds may require postweld heat treatment; see US-40(f) of VIII-1.
6.2.1.3 Welded Stays for Jacketed Vessels. Welded stays, shown in Fig. 6.2, are permitted when the vessel meets the following criteria:
(a) Vessel design pressure is ≤ 300 psi.
(b) Required thickness of the plate does not exceed 1/2 in. (c) Minimum fillet weld size is not less than the plate thickness.
(d) Allowable fillet weld load is calculated according to UW-18(d), and inside welds are visu- ally examined before assembly.
(e) Maximum diameter or width of the hole in the plate is 1 1/4 in.
6.2.1.4 Welded Stays for Dimpled and Embossed Assemblies. Welded stays may be used in con- struction of a dimpled or embossed assembly where a dimpled or embossed plate is welded to another dim- pled or embossed plate or to a plain plate and the following rules from Appendix 17 of VIII-1 are met:
(a) A welded attachment is made by fillet weld around the edge of the opening, or when the plate
filled with weld metal. The allowable load for the weld shall be equal to the product of the thick- ness of the plate containing the opening, the perimeter of the opening, the allowable stress of the weaker of the plates being joined, and a fillet weld joint efficiency of 0.55.
(b) When MAWP is determined by a UG-101 proof test of the dimpled or embossed assembly, a representative panel may be used that is rectangular with at least 5 pitches in each direc- tion and not less than 24 in. in either direction.
FIG. 6.1
TYPICAL FORMS OF WELDED STAYBOLTS (ASME VIII-1)
FIG. 6.2
(c) For a plain plate welded with one of the methods listed below, the minimum required thick- ness or the maximum allowable working pressure shall be determined by Eq. (6.1) or (6.2) using a value of C = 3.0. The welding procedures are:
(1) Resistance seam welding
(2) gas tungsten arc seam welding without filler metal (3) plasma arc seam welding without filler metal (4) submerged-arc seam welding with filler metal
(d) For a plain plate with other methods of welding than those listed above, the minimum required thickness or maximum allowable working pressure is calculated by using Eqs. (6.1) and (6.2) with the appropriate C value.
6.2.2 Stays and Staybolts
6.2.2.1 Load Carried by a Stay
(a) The area supported by a stay is based on the full pitch dimensions with the area of the stay subtracted. The load carried by that stay is the product of the area supported by the stay times the internal design pressure (MAWP).
6.2.2.2 Minimum Required Area of a Stay
(a) The minimum required area of a stay at its least cross section, the smaller of the area at the root of the threads, or at any lesser cross section, is obtained by dividing the load carried by a stay (from the calculation in section 6.2.2.1) by the allowable stress of the stay material at design temperature and multiplying this result by 1.10.
(b) Stays made from two or more parts joined by welding shall have the minimum required area of cross section of a stay determined the same way as in (a), above, but using a butt-weld joint efficiency of 0.60.
6.2.2.3 Special Requirements for Threaded-End Stays
(a) Stays screwed through a plate shall extend two threads minimum and shall be riveted over or upset, or they shall extend through with enough threads to be fitted with a threaded nut. (b) If the stay end is upset for threading, it shall be fully annealed.
Example 6.1 Problem
Using the rules in UG-47 through UG-50 and Appendix 17 of VIII-1, determine the maximum allowable working pressure (MAWP) of a dimpled plate/plain plate assembly which is resistance seam welded on 5 in. centers. Both plates are 1/4 in. thick SA-285 Gr.A with a design temperature of 150°F and no corrosion allowance.
Solution
The allowable tensile stress for II-D for SA-285 Gr.A at 150°F is 12.9 ksi. Since the joint is resistance seam welded, according to section 6.2.1.4(c), C = 3.0.
Using Eq. (6.2),
Example 6.2 Problem
A flat plate is stayed by welded-in stays. The design pressure is 115 psi at a design temperature of 100°F using SA-516 Gr. 60. Stays are located on 8 in. centers. There is no corrosion allowance. What is the mini- mum required thickness of the stayed plate?
Solution
(1) The allowable tensile stress from II-D for SA-516 Gr. 60 at 100°F is 17.1 ksi.
(2) If the plate thickness is > 7/16 in., C = 2.2. If the plate thickness is ≤ 7/16 in., C = 2.1. Using C = 2.2 in Eq. (6.1), the minimum required thickness of the plate is
t = (8)[(115)/(17,100)(2.2)]1/2= 0.442 in.
The minimum required thickness of 0.442 in. is greater than 0.438 in. (7/16 in.) and C = 2.2 is the correct factor to use.