Ubicación y descripción de los hitos
Hito 9: Playa y Chiflón 4
10.1.2 Ámbito 2: Lota. Corazón de la epopeya minera
Design Pressure = Full Vacuum Design Temperature = 500°F Shell and Head Material is
SA-285 Gr. B, Yield Stress = 27 ksi Corrosion Allowance = 0.0625" Cylinder Dimension Shown is Inside Diameter
A. Is the 7/16 in. thickness acceptable for external pressure? If it is not acceptable, what minimum thickness is required? Round your answer upward to the nearest 1/16 in.
Calculate design length, L, in.
L = Tangent/Tangent length + 2 × 1/3 × (Head Depth) Head Depth = 48/4 = 12
L = 150 × 12 + 12 × 2/3 L = 1 808 in.
Calculate outside diameter Do, in.
Do = 48 + 2 × (7/16) = 48.875 Do t = 48. 875 (0. 4375−0.0625)= 130. 33 L Do = 1808 48.875 = 37
Determine the value of A using Work Aid 3B and the calculated Do/t and L/Do.
A = 0.000065 (See Figure 11)
For the specified material, which figure in Section II of the Code should be used? Figure CS-1.
From the value of A and the appropriate temperature curve for the material, what is the maximum permissible external design pressure Pa? Note that A falls to the left of the temperature line (See Figure 12).
2 3 4 5 6 7 8 9 2 3
50.0 40.0 35.0 30.0 25.0 20.0 18.0 16.0 14.0 12.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.5 3.0 2.5 2.0 1.8 1.6 1.4 1.2
.00001 .0001
Length + Outside Diameter = L/Do
D o/t = 400 Do/t = 100 Do/t = 125 Do/t = 150 Do/t = 200 Do/t = 250 Do/t = 300 D o/t = 500D o/t = 600 D o/t = 800D o/t = 1,000 20203.f11 Do/t = 130 A = 0.000065 L/Do = 37 Figure 11: Factor A 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 20,000 18,000 16,000 14,000 12,000 10,000 9,000 8,000 7,000 6,000 5,000 4,000 3,500 3,000 2,500 2,000 .00001 .0001 .001 .01 .1 FACTOR A FACTOR B
GENERAL NOTE: See Table CS-1 for tabular values
up to 300˚F 900˚F 500˚F 700˚F 800˚F E-29.0 = 106 E-27.0 = 106 E-24.5 = 106 E-22.8 = 106 E-20.8 = 106 20203.V61 A=0.000065 Figure 12: Figure CS-1
Pa = 2AE 3(Do / t)
E = 27 × 106 psi from Figure CS-1 (Figure 12) at T = 500°F
Pa = 2× 0.000065 × 27 × 106 3× 130. 33
Pa = 9 psi
Since the calculated Pa is less than 15 psi, the proposed
7/16 in. shell thickness is not sufficient.
Now determine how thick the shell must be in order to have Pa ≥ 15 psi. This is a trial-and-error process, by which the
thickness is increased until an acceptable value is found. The intent is to use the thinnest shell that will meet the requirement. Without going through all the iterations, we will assume a new shell thickness of 9/16 in. and thus a corroded thickness of 1/2 in.
Do t = 48.875 0. 5 = 97.75 L Do = 37 (as before) A = 0.000114 Pa = 2× 0.000114 × 27 × 106 3× 130. 33 Pa = 15.7 psi
B. An alternative to increasing the wall thickness is to add stiffener rings in order to reduce the unstiffened length of shell. This alternative will probably be less expensive than to increase the thickness of the whole shell to 9/16 in.
To determine the number of stiffener rings that are required, you must calculate the maximum allowable design length, L. This calculation is done by working the procedure backwards.
Do
t = 130.33 from before. This is based on the originally
specified shell thickness of 7/16 in.
Calculate the required value of B using Pa = 15 psi.
Pa = 4B 3(Do/ t) B= 3Pa(Do/ t ) 4 = 3× 15 × 130. 33 4 B = 1 466
Locate the calculated B in Figure CS-1 (Figure 12). Since B is below the bottom of the chart (i.e., to the left of the temperature lines), the alternative calculation procedure must be used.
Calculate A using the following equation.
Pa = 2AE 3(Do/ t) A = 3Pa(Do/ t) 2E = 3× 15 × 130. 33 2 × 27 × 106
A = 0.0001086
Using the value of A calculated above, and the value of Do/t, go to Figure G in Section II of the Code and
determine L/Do.
L/Do = 8
L = 8 × 48.875 = 391 in.
This value is the maximum acceptable length between stiffeners. It is also the maximum permitted length to the first stiffener from the top or bottom, considering 1/3 of the head depth as part of the length.
Total length to stiffen = tangent/tangent length + 2 × 1/3 × (head depth)
= 1 808 in.
Maximum number of spaces between stiffeners =
Total length L =
1808 391
= 4.62
Rounding up to the nearest whole number = 5 spaces Number of stiffeners, N = (Spaces - 1)
Flat Covers
The unstayed, circular flat head or cover is another type of head. Unstayed means that the cover is merely a flat plate that does not have any reinforcing bars to strengthen it. The ASME Code contains design rules for flat covers that include items such as minimum required thickness, bolting, and welding requirements. Flat covers are not often used for pressure vessels in refineries and petrochemical plants. However, the flat cover is commonly used as a bolted cover on the channel end of shell and tube heat exchangers. Flat covers will not be discussed further in this course due to their limited applicability to pressure vessels. Flat covers are discussed in MEX 210.
Quick-Opening Closures
Some pressure vessel applications require that the vessel be opened frequently for maintenance or operational reasons and not just during normal T & I’s. The following are examples of such applications:
• Large strainers or filters that are installed in piping systems that must be frequently opened for cleaning.
• Pressure vessels in batch process operations that must be frequently entered for cleaning.
• Mixing vessels where additional material is manually added during the process after the vessel has been depressurized.
In these situations where frequent pressure vessel entry is required, it is preferable to have a faster means to open and close the vessel than is provided by a standard bolted flanged connection. A quick-opening closure provides this faster internal access.
A quick-opening closure consists of the following main parts (Refer to several standard figures that are contained in Course Handout 3):
• Hub. The hub has an integrally-forged ring that extends beyond its outside diameter at one end. There is a gasketed joint in the face of this ring. The other end of the hub is welded to the nozzle neck where the quick-opening closure will be installed.
• Dished head. The dished head closes the opening of the pressure vessel nozzle.
• Gasketed joint. A gasketed joint between the hub ring and dished head provides a pressure-tight seal at the closure. • Clamping ring or yoke. This component is split into two
pieces across its diameter, clamps the head against the hub ring, and forces the head against the gasketed joint to seal the opening. Loosening the yoke allows the closure to be opened.
• Bolts. The bolts locate the yoke with respect to the head and hub ring. When the bolts are tightened, the yoke pieces are brought together and tighten the head against the hub ring. When the bolts are loosened, the yoke pieces are moved apart and the head can be moved away from the hub ring. Two bolts are used whatever the closure diameter for most pressure vessel applications. • Hinge. The hinge is attached to both the dished head and
the hub so that the head can remain supported and be easily swung open and shut.
The closure can be opened and closed quickly because there are usually only two bolts. Opening and closing may be done either manually or by hydraulic or electric operators depending on the application and the size of the closure.
It is much simpler to open a quick-opening closure than a standard flanged joint. Consequently, it is also much easier to
The detailed design of quick-opening closures must meet the normal ASME Code requirements with respect to material selection and dimensions (e.g., head thickness). Paragraph UG-35(b) of the ASME Code contains additional design requirements that focus specifically on quick-opening closures. The ASME Code has these additional requirements because of the potential danger if the closure is not properly operated. Several of these requirements are highlighted below. Refer to Paragraph UG-35(b) for details.
• The closure must have a locking mechanism. The locking mechanism must be designed such that a failure of any one component of the mechanism cannot result in failure of all other locking elements and subsequent release of the closure. One design approach that meets this requirement is to have separate plates bolted between the head and the two halves of the yoke. These plates must be unbolted first before the yoke bolts are used to separate the yoke halves and unbolt the closure.
• It must be possible to see from external visual observation that the holding elements are in good condition and that their locking elements are in full engagement when the closure is in the closed position.
• The closure and its holding elements must be fully engaged in their intended operating position before pressure can be built up inside the vessel. This feature ensures that all components of the closure are in the position that they were designed to be in before they are exposed to the pressure loads.
• If internal pressure would force the closure away from the vessel, the closure must be designed such that the pressure must be fully released before the closure can be fully opened for access. This feature ensures that the closure head will not be rapidly blown back due to a high internal pressure and cause damage or injury.
• Warning devices may be required that will warn the operator if pressure is applied to the vessel before the closure and its holding elements are fully engaged in their intended position. These warning devices also signal if an attempt is made to operate the locking mechanism before releasing the pressure inside the vessel.
Several companies manufacture standard quick-opening closures that meet all ASME Code requirements.
Nozzle Reinforcement for Design Pressure
The wall thickness of a nozzle under pressure loading is determined by the same procedures used for cylindrical shell sections. External loadings that are transmitted through a nozzle connection to a pressure vessel, such as by connected piping, may also affect the required nozzle thickness. These external loadings are discussed in a later section of this module. However, pressure loading is normally the main factor and is discussed in detail in this section.
Material thickness tolerance is an additional factor that must be considered in nozzle thickness calculations and that is not considered in vessel shell calculations. Pressure vessel nozzles are frequently fabricated using pipe material specifications when the nozzle is a standard pipe size. Pipe material specifications permit the wall thickness that is supplied to be less than the nominal thickness that is ordered by an undertolerance. The permissible undertolerance is stated in the pipe material specification and can be as much as 12.5% of the nominal thickness. Therefore, the undertolerance must be considered when the required wall thickness for a nozzle is calculated.
Reinforcement of
Pressure Vessel Openings
Calculation of the required wall thickness for a nozzle is one step in the design of openings in pressure vessels. However, there is more to the design of openings than calculating the nozzle thickness, cutting a hole in the vessel, and welding the nozzle in. The ASME Code specifies design rules that must be
The design of pressure vessel openings must address two types of stress conditions. First, the membrane stresses must be kept within allowable limits. Second, localized peak stresses that are caused by abrupt geometric changes at the nozzle-to-shell corner must be kept within allowable limits. Evaluation of these peak stresses is important for the evaluation of cyclic loads that may cause a fatigue failure but is beyond the scope of this course.
The ASME Code uses simplified rules to ensure that the membrane stresses are kept within acceptable limits when an opening is made in a vessel shell or head.
;;;
DP Rn
d
d or Rn + tn + t Use larger value 2.5t or 2.5 tn
Use smaller value 2.5t or 2.5 tn + te Use smaller value
c h tn trn tr t te d or Rn + tn + t Use larger value For nozzle wall abutting the vessel wall
For nozzle wall inserted through the vessel wall
When the opening is made, a specific volume of material is removed from the pressure vessel shell or head. Therefore, this metal is no longer available to absorb the applied loads. The ASME Code simplifies the design calculations by viewing the nozzle-to-vessel junction area in cross section, as shown in Figure 13. The use of this simplification permits the nozzle reinforcement calculations to be made in terms of metal cross- sectional area rather than metal volume. The ASME Code design rules state that the metal area that is removed for the opening must be replaced by an equivalent metal area in order for the opening to be adequately reinforced. The replacement metal must be located adjacent to the opening and be must contained within defined geometric limits in order to provide adequate reinforcement. The replacement metal area may take two forms:
• Excess metal that is available in the shell or nozzle that is not required for pressure or to absorb other loads; or
• Reinforcement that is added to the shell or nozzle. Figure 14 shows several typical nozzle design configurations. Please note 32-AMSS-004 requires that all nozzles in hydrogen, hydrocarbon, caustic amine, wet sour and steam services shall be attached by welding through the total thickness of the vessel shell or head, including reinforcement.
Full Penetration Weld With Integral Reinforcement
(a-1)
Separate Reinforcement Plate Added
(b) (c) (d) (e)
Full Penetration Welds to Which Separate Reinforcement Plates May be Added
(f - 1)
(f - 2)
(f - 3)
(f - 4)
(g)
Self - Reinforced Nozzles (a)
Figure 14 provides examples of inserted versus abutted nozzles, pad reinforcement versus no reinforcement, and self- reinforced nozzles. Note that self-reinforced nozzles are forged fittings that are designed with extra thickness in the nozzle-to- vessel junction area in order to provide adequate reinforcement. Work Aid 3C summarizes the ASME Code procedure used to calculate the required nozzle reinforcement.
Additional
Nozzle Reinforcement
Additional nozzle reinforcement must be provided if the vessel shell and nozzle do not have available sufficient excess thickness that is not required for pressure loads or for other transmitted loads. Additional reinforcement can be in one of the following forms:
• A reinforcement pad.
• Additional thickness in the vessel shell or head around the opening.
• Additional thickness in the lower part of the nozzle near its attachment to the vessel.
In all cases, the reinforcement must be located within the reinforcement zone boundaries in order for the reinforcement to be considered effective.
If a reinforcement pad is used, its material should have an allowable stress that is at least equal to that of the pressure vessel shell or head material that the reinforcement pad is attached to. No credit can be taken for the additional strength of any reinforcement that has a higher allowable stress than that of the vessel shell or head material to which it is attached. If a material with a lower allowable stress than the vessel material is used for reinforcement, the reinforcement area must be increased in inverse proportion to the ratio of allowable stress values for the two materials. This increase compensates for the lower allowable stress of the reinforcement material. The reinforcement pad material is normally selected to be the same as the vessel material in order to avoid the need to make this compensation.
When a reinforcing pad is used, its thickness is normally set equal to the vessel shell or head nominal thickness, and its required diameter is calculated based on the amount of additional reinforcement area that it must provide. However, any portion of the reinforcing pad that extends outside the boundaries of the reinforcement zone cannot be considered effective. Additional reinforcement must be provided in another manner should it be necessary to extend the reinforcing pad diameter outside the boundaries of the reinforcement zone. It should also be noted that the Pressure Vessel Design Data Sheet requires that nozzle reinforcement not be the factor that limits the maximum allowable working pressure of a pressure vessel. Therefore, a nozzle cannot be the weakest component of a pressure vessel.
The ASME Code specifies circumstances under which no nozzle reinforcement evaluations are needed. It also provides rules to evaluate the reinforcement of openings that are located near each other. These situations are not discussed in this course, and Participants are referred to the ASME Code for details.
The nozzle reinforcement that is determined using the ASME Code procedure considers pressure design only. Vessel nozzle-to-shell intersections must also be adequate for the loads that are imposed by any attached piping or equipment. The ASME Code does not contain specific procedures for the evaluation of these external nozzle loads. The external nozzle loads must be checked by the pressure vessel designer by the use of generally accepted procedures.
Small Connections
SAES-D-001 specifies that connections less than 2 in. size shall be used in utility services only.
Sample Problem 5 - Nozzle Reinforcement
You are reviewing the nozzle design details that are proposed by a vendor for a new drum and have selected an 8 in. nominal pipe size nozzle into the shell for detailed evaluation. The vendor has not provided any reinforcement for this nozzle, and he has not provided any calculations to verify that use of the nozzle without reinforcement is acceptable.
Using Work Aid 3C, determine if this nozzle requires additional reinforcement. If it does, assume that a 0.5 in.-thick reinforcement pad of SA-516, Gr. 60 material is used. What must the minimum pad diameter be? Neglect any contribution of weld areas in these calculations. The information that is needed to perform your evaluation is in Figure 15.
8" Nozzle (8.625" OD) 0.5" Thick
0.5625" Thick Shell, 48" Inside Diameter DESIGN INFORMATION
Design Pressure = 300 PSIG Design Temperature = 200°F Shell Material - SA-516 Gr. 60
Nozzle Material - SA-53 Gr. B, Seamless Corrosion Allowance = 0.0625"
Vessel is 100% Radiographed
Nozzle does not pass through Vessel Weld Seam
20203.FIG15
tr = Pr SE1− 0. 6P = 300× (24 + 0. 0625) 15 000× 1− 0.6 × 300 = 0.487in. trn = 300(3. 8125+0.0625) 15 000× 1− 0.6 × 300 = 0.0784 A = dtrF A = (8.625 - 1.0 + 0.125) × 0.487 × 1 A = 3.775 in.2 required area
A11 = (Elt - Ftr)d = (0.5625 - 0.0625 - 0.487) × 7.75 = 0.1 in.2 A12 = 2(Elt - Ftr) (t + tn) = 2(0.5625 - 0.0625 - 0.487) (0.5625 - 0.0625 + 0.5 - 0.0625) = 0.0243 in.2
Therefore, A1= 0.1 in.2 available in shell
A21 (tn - trn)5t = (0.5 - 0.0625 - 0.0784) × 5(0.5625 - 0.0625)
A21 0.898 in.2
A22 = 2(tn - trn) (2.5 tn + te)
= 2(0.5 - 0.0625 - 0.0784) [2.5 × (0.5 - 0.0625) + 0] = 0.786 in.2
Therefore, A2 = 0.786 in.2 available in nozzle
A1 + A2 = 0.1 + 0.786 = 0.886 in.2
Since this value is less than A, the nozzle is not adequately reinforced, and a reinforcement pad is required.
te = 0.5625 in.
A5 = [Dp - (d + 2 tn)] te
2.889 = [Dp - (7.75 + 2(0.5 - 0.0625)] 0.5625
5.136 = [Dp - 8.625]
Dp = 13.761 in.
Therefore, the minimum required reinforcement pad diameter is 13.761 in.
Confirm that this diameter does not extend beyond the permitted reinforcement limit.
2d = 2 × 7.75 = 15.5 in.
Therefore, Dp = 13.761 in. is acceptable
Nozzle Flange Rating
ASME B16.5, Pipe Flanges and Flanged Fittings, provides steel flange dimensional details for standard pipe sizes through 600 mm (24 in.). Standard ASME B16.5 flanges are acceptable for most pressure vessel nozzle flanges and for shell flanges when the vessel diameter corresponds to a standard pipe size. Specification of an ASME B16.5 flange involves selection of the correct material and flange "Class." The paragraphs that follow discuss this process in general terms. Work Aid 3D provides the specific procedure to follow.
Flange material specifications are listed in Table 1A in ASME B16.5, a portion of which is excerpted as Figure 16. A copy of ASME B16.5 is contained in Course Handout 1. The material specifications are grouped within specific Material Group Numbers. The process for determining the Material Group Number is contained in Work Aid 3D. For example, if the pressure vessel is fabricated from carbon steel, ASTM A105 is an appropriate flange material specification in most applications. ASTM A105 material is in Material Group No. 1.1.