Design & Construction of Piping Systems: Prepared by Eng. Mostafa A. El-Sayed

Design & Construction of Piping Systems

Prepared by Eng. Mostafa A. El-Sayed

Aim of Seminar To know piping design basics by going through the following points:       

Design of pressure components. Pipe Span calculations. Design of pipe supports & hangers. Stiffness & flexibility. Expansion & stresses. Line expansion & flexibility. Supports & anchorage of piping.

Design of pressure piping 

Many decisions need be made in the design phase to achieve this successful operation, including: -

Required process fluid quantity. Optimum pressure-temperature. Piping material selection. Insulation selection (tracing). Stress & nozzle load determination. Pipe support standard.

The codes provide minimal assistance with any of these decisions as the codes are not design manuals.

Design of pressure components 

Pipe Structure “static” design, not Layout design.

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Limitations: Code, Pressure, Temperature, How long is the plant lifetime, What is the plant reliability, etc..

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Piping designed according to B31.3 has less lifetime than B31.1 due to lower F.S.

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Reliability of piping under B31.1 should be higher than B31.3

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Given that the code is a product of pressure technology, one of the concerns is the pressure-temperature ratings of the components.


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Each system be it vessel or piping has some base pressuretemperature rating. This is essentially the pressure temperature rating of the weakest member of the system. This can be translated that no minor component (valve, flange, etc) shall be the weakest link. The key components of the design conditions are the design pressure and the design temperature. Design pressure is defined as the most severe sustained pressure which results in the greatest component thickness and the highest component pressure rating. Design temperature is defined as the sustained pipe metal temperature representing the most severe conditions of coincident pressure and temperature.


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Thus we can try to simplify our stresses into two main categories; Pressure stress is the circumferential stress (primary stress) or hoop stress, which is known to be not self limiting. Temperature stress is the shear or bending stress (Secondary stress), known to be self limiting. In addition VIBRATION, has to be addressed as low cycle high stress named as “thermal expansion cycles”, represented by f=1 for 7000 cycles, otherwise detailed design has to be performed to prove that the pipe will withstand high cycle, low stress loads.

Design of pressure components Wall Thickness Calculation 

The code assists the designer in determining adequate pipe wall thickness for a given material and design conditions as follows: - Calculate the pressure design thickness “t” - Add the mechanical corrosion and erosion allowances “c” to obtain the thickness tm=t+c - Add mill tolerance (MT) to tm, then select the next commercially available wall thickness. - Provided t

Code Equation: tm = PDo/2(SEq + PY) + A = t + A

where: P = Internal design pressure Do = pipe outside diameter S = the pipe material allowable stress, S is for the listed pipe material at hot temperature E = quality factor Y = stress-temperature compensating factor.

Design of pressure components The E FACTOR is a “allowable stress penalty” based on the method of manufacture of the pipe. It is based on the quality of the weld in seam-welded pipe and will have a value ranging from E = 0.6 for FURNACE BUTT WELDED (FBW) to E = 1.0 for SEAMLESS PIPE, (SMLS). This FACTOR is a carry-over from the old days where pipe was manufactured using rivets. The E FACTOR for seam-welded pipe can be improved: increasing this factor from 0.8 to 1.0. The Y FACTOR is included to account for effects of creep considering the non-linear reduction in ALLOWABLE STRESS at design temperatures above 482º C (900º F).

Design of pressure components Wall thickness problem: P = 4135 KPa (41.35Bar) D = 219.1 mm (8.625 in) S = 130 MPa at 260ºC (18,900 psi at 500º F), from TABLE A-i E = 0.85 (TABLE A-lB for A53 pipe) Y = 0.4 (TABLE 304.1.1) Convert MPa allowable stress to kPa for consistency of units, then: t = 4135*219.1/2(13000*0.85+0.4*4135) Metric units: t = 4.0 mm then tm plus mill under run tolerance is: tm = 4.0 + 1.6 + 1.0 (The 1.0 mm value is 12.5% mill under run tolerance of 8.2 mm nominal wall pipe expected to be purchase). tm = 6.6 mm

Design of pressure components U.S. customary units: t =8.625*600/2(18900*0.85+0.4*600) t =0.159 inch tm = 0.159 + 0.063 + 0.040 tm = 0.262 in. (The 0.04 inch value is 12.5% mill under run tolerance of 0.322 inch nominal wall expected to be purchase). The next commercially available pipe wall is SCHEDULE 40, with a nominal wall of 8.2 mm (0.322 in.). This is the wall to use for these stated conditions.

Design of pressure components Test Pressure The hydrostatic test pressure at any point in the system should be not less than 1.5 times the design pressure. For Temp. above 650F (343C), the min. test pressure PT is given by; PT = 1.5(ST/S)(Design Pressure) ST = allowable stress at 650F, S = allowable stress at design temperature

Questions .

Design of pressure components Miter Bends .

Design of pressure components Miter Bends Miter Bends have a pressure limitation, as calculated by equations (4a), (4b), or (4c) of paragraph 304.2.3 of B31 .3 which could derate a piping system. A miter is defined when the angle α is greater than 3 degrees at a weld as shown in Fig. 12.0.. Multiple miters are, whose angle of miter cut is less than 22.5 degrees, limited to a pressure that will generate HOOP STRESSES not to exceed 50% of the yield strength of the material at temperature. This is done by restricting the maximum pressure to the lesser value as calculated by equations (4a) or (4b) in the code. Single miters, or miters whose bend angle is greater than 22.5 degrees is limited to HOOP STRESSES of 20 % of the yield of the material at temperature by equation (4c).

Design of pressure components Designers wishing to use miters in a system but do not wish to pay this pressure penalty can simply increase the wall thickness of the miter, thus reducing the HOOP STRESS to values less than the Code limit. This technique seems straight forward, but one question remains, where does the miter start? The code provides a method to determine the distance the miter extends into the straight pipe. This distance is defined as M [ 304.2.3(c)], where: M = the larger of 2.5 x (r x T)0.5 or tan (θ) x (R1- r2) as shown in Figure 12.0. The T used in this equation is T less mill tolerance.

Design of pressure components

Design of pressure components An example of Miter Bend maximum allowable internal pressure calculations per paragraph 304.2.3 for ө =22.5° and ө = 30° is as follows: Calculate the maximum allowable internal pressure in a DN900, 9.5mm nominal wall (NPS 36, 0.375in. nominal wall) miter bend constructed of A515 Gr 60 plate mat, Temperature=260°C (500°F), c=2.5mm (0.10in.), E=1.0 (fully radiographed), R1=1 .5D, r2=0.5(Do-T). Mill tolerance=1.2 mm (0.047 in., 12½ %). A. For θ = 22.5º equation 4(a): Pm= SE(T-c)/r2 * T-c/[(T-c)+0.643 tanθ√(r2(T-c)]

Design of pressure components Metric units: S = 119266 kPa, E = 1.0, T = 9.5 -1.2 = 8.3 mm, r2 = 0.5(914.4-9.5) = 452.5 mm, then: Pm =(119266*5.8/452.5) * 5.8/[5.8+0.643tan(22.5)*√(452.5*5.8) Pm = 455 kPa Equation 4(a) using U.S. customary units: S = 17300 psi, E 1.0, T = .375 - .047 = .328, r2 = 0.5(36 - .375) = 17.813, then: Pm =(17300*0.228/17.813) * 0.228/[0.228+0.643tan(22.5)*√(17.813*0.228) Pm = 66 psig

Design of pressure components Next test equation 4(b) with θ = 22.5° Pm=SE(T-c)/r2* R1-r2/(R1-0.5r2) Metric units: Pm=119266x5.8/452.5 * (1373 - 452.5)/(1373 - 0.5 x 452.5) Pm=1225 kPa U.S. customary units: Pm=17300 x 0.228/17.813 * 54-17.813/ (54-0.5*17.813) Pm= 178 psig As we usually consider the lesser value of the above calculations, the multiple miter elbow with θ=22.5°, results in maximum allowable pressure to be 455 kPa (66 psig).

Design of pressure components B. For a miter fabricated using θ= 30° test using equation (4c): Pm= SE(T-c)/r2 * T-c/[(T-c)+1.25 tanθ √(r2(t-c))] Metric: Pm= 119266*5.8/452.5 *5.8/[5.8+1.25*0.577*√(452.5x5.8)] P = 205 kPa U.S. customary units: Pm= 17300*0.228/17.813*0. Pm = 30 psig The maximum pressure for this piping system containing a miter with θ= 30° is 205 kPa (30 psig). If the maximum pressure of this system were greater than 205 kPa, (30 psig), then the designer would have to either change θ to a lesser angle or increase the wall thickness of the miter and recalculate Pm.

Design of pressure components Branch Connection

Design of pressure components Branch Connection Branch Connections Branch connections are made in piping systems by any one of several methods. These could be tees, pad reinforced or unreinforced intersections, crosses, integrally reinforced weld-on or weld-in contoured insert fittings, or extrusions. [ 304.3.1]. The philosophy of the code for intersections is centered around the available pressure reinforcement offered by the geometry of the intersection. The process of making an intersection weakens the run pipe by the opening that must be made in the run pipe. Unless the wall thickness of the run pipe is sufficiently in excess of that required to sustain pressure at an intersection that is NOT manufactured in accordance with a LISTED STANDARD, it is necessary to provide added reinforcement. This reinforcement is added metal, local to the intersection, that is integral with the run and branch pipes

Design of pressure components Branch Connection The amount of required pressure reinforcement is determined by performing AREA REPLACEMENT CALCULATIONS using the design conditions established for the intersection. Area replacement calculations are not required for intersections using LISTED-RATED or LISTED-UNRATED TEE INTERSECTIONS provided the intersection is used within the pressure-temperature bounds stated in the LISTING STANDARD. Area Replacement calculations are not required for UNLISTED TEE INTERSECTIONS provided the tee component has successfully completed the requirements of paragraph 304.7.2, which are: 1) duplicating a successful operating system, 2) experimental stress analysis, 3) proof test.


Design of pressure components Branch Connection 

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The branch & run angle between 45 and 90 deg. And the axes intersect. The principle is that the area removed by the opening is added or accounted for as added reinforcement or excess area due to thickness above the pressure requirements. d1 = effective length removed from the run at the branch, d1 = Db or d1 = [Db - (Tb-c) ] / sin β d2 = 1/2 the width of reinforcement zone d2 = greater of d1 or [ (Tb – c) +(Th – c) +d1/2 ], but less than Dh


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L4= height of reinforcement zone = smaller of 2.5(Tb – c)+Tr and 2.5 (Th – c) Dh: Outside diameter of header Db: Outside diameter of branch th: header pressure design thickness tb: branch pressure design thickness Th: header thickness minimum per purchase or minus mill tolerance Th: nominal wall thickness of header Tb: branch thickness minimum per purchase or minus mill tolerance Tb: nominal wall thickness of branch


Tr: thickness of reinforcement pad

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c: sum of mechanical (thread & groove), corrosion and erosion allowances

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β: angle between the header and the branch axes

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Required area A1 = th.d1.(2 - sin β)

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A2: excess area in run =(2 d2 - d1) (Th – th - c)

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A3: excess area in branch = 2.L4 (Tb –tb – c) / sinβ

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A4: area provided by weld & area of reinforcement pad

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A2+ A3 + A4 ≥ A1

Design of pressure components Branch Connection Area replacement rules of B3 1.3 are valid for branch connections meeting the following conditions: 1) the run pipe diameter to thickness ratio (Dh/Th) is less than 100 and the branch to run diameter ratio (Db/Dh) is not greater than 1.0. 2) for run pipe with (Dh/Th) ≥100 the branch diameter Db is less that one- half the run diameter Dh. 3) angle is between 45 and 90 degree. 4) the centerline axis of the branch intersects the centerline axis of the run. Branch intersections that do not meet these conditions may be qualified by proof testing or other means specified in paragraph 304.7.2 of the code.


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The required percent replaced area within the prescribed reinforcing boundaries depends on the angle of the intersection. This percent required area will range from 100% of the area removed, th.d1.(2 - sin β), for a 90 intersection to about 130% required for 45 intersections. The strength of an intersection grows increasingly weaker as the branch angle β departs from 90. This increasing weakness in strength with decreasing 1 is accounted for by the term (2 - sin β) in the required area equation. The change in required area for decreasing β, expressed in percent is illustrated in Figure 13.


Design of pressure components Branch Connection An example of the area replacement rules, consider the following two 900 intersections, the first is an UNREINFORCED FABRICATED TEE, the second is a PAD REINFORCED FABRICATED TEE, (see Figure 14.0). Both intersections are the same pipe sizes and have the same design conditions. Find the replaced area in the UNREINFORCED FABRICATED TEE for the conditions: Run pipe: DN 200 Nom. Wall 8.2 mm (NPS 8 Sch 40) ASTM A53 GrB. Branch pipe: DN 100 Nom. Wall 6.0 mm (NPS 4 Sch 40 ASTM A53 Gr B SMLS P = 4135 kPa (600 psig), T = 204º C (400º F), c = 2.5mm (0.10 in).


Design of pressure components Branch Connection Example A, metric area replacement calculation for an intersection: DN 200, P = 8.2 mm x DN 100, T = 6.0 mm, UNREINFORCED FABRICATED TEE. I. Nomenclature. (Reference FIG. 304.3.3) T=204ºC, P=4135kPa, c=2.5mm Dh = 219.1 mm Th = 8.2 mm Header Material: A53 Gr B ERW E=0.85 Db = 114.3 mm Tb = 6.0 mm Branch Material: A53 Gr B SMLS E=1.0 Material SE, Header: 117 MPa, Branch: 138 MPa Th = 7.2 Tb = 5.2 (T - 12½ % mill tolerance) d = Db – 2(Tb - c) = 114.3 - 2(5.2 - 2.5) = 108.9 mm d2 = the greater of d or (Tb - c) + (Th - c) + d = 108.9 mm L4 = the lesser of 2.5(Th - c) or 2(Tb - c) + Tr L4 = 2.5(5.2 - 2.5) + 0 = 6.7 mm

Design of pressure components Branch Connection The pressure design thickness for the header and branch pipes, using equation (3a): t =(PxD)/2(SE+PxY); th=3.8 mm tb=7 mm. II. Required Area A1 = (thxdl)x(2-Sin(β)) =413.8mm2 III. Area Contributing to Reinforcement A2 = (2xd2-d1)*(Th-th- c) = 98 mm A3 = 2*L4(Tb-tb- c) = 13.4mm A = (area of additional metal, including weld metal, within the reinforcing zone, tc = 4 mm) = 32 mm2 A5 = A2 + A3 + A4 =143.4 mm IV. PERCENT AREA REPLACED = (A ) x 100 = 34%

Design of pressure components Branch Connection Example A intersection is not suitable for pressure design. Considering the percent replaced area is only about 33%, a reinforcing pad must be added to the intersection and area replacement calculations are tested again as follows in example B. Had the above example percent replaced area been very near the 100% minimum, possibly more weld metal could be added to obtain the 100% mark. The weld metal tested was the minimum as required by Para. 328.5.4 of the code. The retest with the pad in example B yields about 200% replaced area, the code requirements for pressure design of the intersection are satisfied. The pad was made from excess run pipe. The pad OD selected for this intersection is 203.2 mm (8 inches).

Design of pressure components Branch Connection Example B, metric, intersection: DN 200, th=8.2 Nom. wall x DN 100, tb=6.0 mm Nom. wall, 900 PAD REINFORCED INTERSECTION, Pad dimensions: Tr = 8.2mm, dia = 203.2 mm. (Mill tol. 12.5%) I. Nomenclature. (Reference FIG. 304.3.3) T = 204º C P = 4135 kPa c = 2.5 mm Tr = (8.2 - 1.0) = 7.2 mm Dh=219.1mm, Th =8.2mm, Header Material: A53 Gr B ERW E=0.85 Db =114.3mm, Tb =6.0mm, Branch Material: A53 Gr B SMLS E=1.0 Material SE, Header: 117 MPa, Branch: 138 MPa Th = 7.2 mm Tb = 5.2 mm (T-12.5% mill tolerance) d1 = Db – 2(Tb - c) = 114.3 - 2(5.2 - 2.5) = 108.9 mm d2 = the greater of d or (Tb - c) + (Th - c) + d = 108.9 mm L4 = the lesser of 2.5( Tb - c ) or 2.5( Tb - c) + Tr L4 = 2.5(7.2-2.5)= 11.7 mm

Design of pressure components Branch Connection The pressure design thickness for header and branch pipes, calculated by equation (3a): t = (P x D)/2(SE + P x Y); th = 3.8 mm, tb = 7 mm II. Required Area A1 = (th x d x (2 - Sin (β)) = 413.8 mm2 III. Area Contributing to Reinforcement A2 =(2*d2-d1)*(Th-th-c) = 98mm2 A3 =2*L4(Tb - th – c)=23.4 mm2 A = (area of PAD: 7.2(203.2 - 114.3) = 640 mm2 (pad OD = 203.2 mm), plus weld metal, (2tc2 + 0.51Tr2 = 68.9 mm2 within the reinforcing zone, tc = 4.2 mm) = 708.9 mm2 A5 = A2 + A3 + A4 = 830 mm IV. PERCENT AREA REPLACED = (A x 100 = 200%

Questions

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Pipe Supporting

Pipe Supporting 

The objective during the pipe supports design phase is to prevent the following: • • • • • • •

overstress of piping leakage at joints overstress of supports excessive forces on equipment excessive interference with thermal expansion excessive pipe sag (especially for piping requiring drain) excessive heat flow, exposing support to temperature outside their limits • Etc..

Pipe Supporting 

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The purpose of pipe supports is to control the weight effects of the piping system, as well as loads caused by pressure thrust, vibration, wind, earthquake, shock, and thermal displacement. The weight effects to be considered shall be the greater of operating or hydrotest loads. The B3 1.3 guidance for pipe support types and materials of construction is presented in the B31 .3 TABLE 326.1 LISTED STANDARD, MSS SP-58. The material selection for clamps and bolts, for example, is of particular importance in elevated temperature service. SP-58 assistance would be in the selection of a clamp material for example in 750F (400C) service. A review of the tables in SP-58 reveal that Carbon Steel clamp material would not be suitable, nor would the common type bolting, ASTM A307 used in clamps. The designer would be guided to use an alloy steel for the clamp such as ASTM A240 and ASTM A193-Grade B7 bolts.

Pipe Supporting - Span Pipe Support Span, based on deflection Pipe support span is a decision that faces the designer in most pipe supporting jobs. As a guide to the selection of support spacing, the following equation based on permissible mid span deflection is offered. The permissible mid-span deflection, y, concept is one technique commonly selected for support spacing. This technique is based on a specified midspan, y deflection of the supported pipe considering the pipe, contents, and insulation weights. The equation is: L= [y.E.I / 22.5.W]¼ where: L = pipe support spacing, feet, y = permissible mid-span deflection, inches E = modulus of elasticity at design temperature, lb/in (TABLE C-6) I = moment of inertia of pipe. W = weight of supported pipe, including pipe, contents, insulation, lb/ft.

Pipe Supporting - Span Pipe Support Span, based on stress As a guide to the selection of support spacing, the following equation based on permissible stress is offered. The permissible mid-span deflection, y, concept is one technique commonly selected for support spacing. This technique is based on stress of supported pipe material considering the pipe, contents, and insulation weights. The equation is: L= [0.33.Z.Sh / W]1/2 where: L = pipe support spacing, feet, Z = section modulus, in3 Sh = Allowable tensile stress for pipe materialat design temp., psi W = weight of supported pipe, including pipe, contents, insulation, lb/ft.

Pipe Supporting - Span

Pipe Supporting - Span An example of the deflection pipe span approach is: What is the span of a seamless ASTM A106 Grade B, 6.625 inch OD, 0.28 inch wall thick, water filled pipe with 3 inch of insulation with a design temperature of 400 F? The specifications limit the mid-span deflection to 0.25 inch. Solution: Determine the uniform load, pounds per foot. Pipe = 19.0 lbs per ft Water = 12.5 lbs per ft Insulation = 7.6 lbs per ft ( 85 % Magnesia Calcium Silicate) then ,W = 39.1 lb per ft I = ( π /64)(Do4 – Di4), Do = 6.625, Di = 6.065 I = 28.14 in4 E = 27.7 x 106 psi, Table C-6, C ≤ 0.3 at 400°F. finally, L = [ 0.25x27.7x106 x28.14/(17. 1x39. 1)]1/4 L = 23 feet span The pipe support spacing would be 23 feet with a mid span deflection of 1/4 inch.

Pipe Supporting - Drainage Drainage Piping systems should be installed to drain by gravity, in direction of normal flow. Each span must be pitched so that the outlet will be lower than the maximum sag of the pipe. The pitch of pipe spans is the ratio between the drop in elevation and the length of span. It is called the average gradient and is expressed in inches per foot or mm per meter run. Gradient check for drainage; G = drop in elevation / span (in/ft.) While, condition for good drainage; G ≤ 4(maximum deflection) / span

Questions

Pipe Supports & Hangers 

Support Selection & Design

• Selection and design of pipe hangers is an important part of the engineering study. High temperature, high pressure pipes are critical to a point that early in the basic design phase supports locations and loads have to be decided upon. Concentrated hanger loads on structures, buildings and their effect on equipment have to be well known from the very beginning of the project.

• Basic information has to collected before proceeding with calculations and detailing of pipe supports, as follows; - A complete set of piping drawings - A complete set of steel and structural drawings/ data. - A complete set of drawings showing locations of ventilating ducts, electrical cable trays, equipment locations (pumps, tanks, etc) - A complete set of piping specifications and data. - Insulation specification. - Movement of all critical equipment connections such as boiler headers, steam drums, turbine connections, etc.. - The results of stress, flexibility, and movement calculations performed for critical systems.

Pipe Supports & Hangers Applying the previously mentioned basic info shall be in the following steps; - The determination of hanger locations. - Determination of the thermal movement of the piping at each hanger location. - The calculation of hanger loads. - The selection of hanger types, spring assembly, either constant support type, variable support or rigid support type. - Checking of clearances between the hanger components and nearby piping, electrical cable trays, conduits, ventilating ducts, equipment, etc. Recognizing that each new piping design presents an abundance of new problems to the engineer, no attempt is made to state fixed rules and limits which would be applicable to every hanger design, only guidance to ideas to solve simple practical support problems.

Pipe Supports & Hangers 

Support Design • Restraints (anchors and guides) are provided to direct thermal expansion to areas designed to absorb it and to ensure that expansion joint movements occur in the directions for which the joint is designed. Expansion joint design shall conform to the requirements of Appendix X, which provides guidelines for the design, fabrication and installation of bellow type expansion joints. • Supports’ elements shall be designed for all loads applied including weight, pressure, wind, earthquakes, friction …etc

Pipe Supports & Hangers • Spring supports are designed to carry the weight loads and prevent misalignment, buckling, eccentric loading and unintentional disengagement of the load. Spring supports should be provided with position indicators. • Constant supports of the counterweight and hydraulic types should be provided with safety devices and stops.

Pipe Supports & Hangers • Integral attachments such as plugs, ears, shoes, plates,…etc, are designed to minimize localized stresses, stress concentration in cyclic service and any harmful temperature gradient. The material should be of good quality and all requirements of the Code for welding, preheating and post-weld heat treatment should apply. Reinforcement by pad and complete encirclement reinforcement shall be used to distribute stresses and reduce heat effect in alloy piping. • Non-integral attachments include clamps, U-bolts, cradles, saddles, straps, clevises. For vertical pipe weight support, the clamp should be located below a flange or fitting or a welded lug.

Hangers Example 

Example Problem:

1-Problem Description. 2-Thermal movement calculations. 3-Hanger Load Calculations. 4-Selection of proper hangers.

Hangers Example

Hangers Example, Movement

Hangers Example

Hangers Example, Movement

Hanger Supports

Rod Hanger Assembly

Rod Hanger Assembly

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The pipe attachment and the structural beam attachment of a rod hanger assembly should allow the hanger to swing to allow for lateral movement of the pipe where there is horizontal pipe expansion. It should be noted that horizontal movement of the hanger will result in a vertical movement as shown previous slide. The subsequent horizontal forces should be checked.

Variable Hanger Assembly

Variable Hanger Supports 

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Variable spring hangers are recommended for general use in non critical piping and where vertical movement is small on critical piping. Acceptable practice is limit amount of supporting force variation (difference between hot load and as installed-cold load) to 25% for critical piping systems on horizontal piping. The amount of variation can be calculated by multiplying the spring scale in lb/inch (Kg/mm) by the amount of vertical expansion in inches (mm). The main problem with variable spring hangers is that this variation in load must go somewhere, it is transferred to the nearest restraint or equipment which may cause damage both to the equipment and/ or piping system.

Variable Hanger Supports 

Calculating the variability in accordance with MSS SP-58: Var. = (Hot load – Cold load)/Hot load x 100

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The load margin between the maximum load, either hot or cold, and the load at the maximum limit of the operating range must also be considered. This load margin should be greater than the weight of the hanger hardware that is supported by the spring, ex. Clamps and hanger rods used to connect the piping to the spring. If the total piping loads plus the load of the supported hanger hardware cannot be accommodated within the spring hangers operating range an alternate spring hanger design should be considered.

Constant Load Support Constant support hangers provide constant supporting force for piping throughout its full range of vertical expansion and contraction. 

Constant Load Support 

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This is accomplished by the use of a helical coil spring working in conjunction with a bell crank lever in such a way that the spring force times its distance to the lever pivot is always equal to the pipe load times its distance to the lever pivot. For use when the variation in a variable spring hanger is above 25%. The variation is transferred to the closest restraint or equipment and, in the case of equipment,. This increase in the load and/ or moment on the nozzle may cause structural damage. In such cases a constant load hanger would be selected. Because of it’s constant load effect the constant support hanger is used where it is desirable to prevent pipe weight load or expansion loads being transferred to connected equipment or adjacent supports or hangers. Therefore they are used for the support of critical piping systems.

Constant Load Hanger

Spring Hangers

Spring Hangers Example 

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Returning back to our Example: Difference in effect in using a variable spring as compared to a constant spring support hanger, as per Fig. H-1, page 157. Load for Hanger H-1 was calculated as 5363lb. Vertical movement at H-1 was 2.41 inches up, from the cold to the hot position of the pipe. Amount of variation is 1500lb/in x 2.41in.=3615lb, while the hot load was 5363lb, so as the direction of movement from cold to hot is upward, the cold load is 5363lb + 3615lb, or 8978lb. Pipe weight does not change throughout it’s cold to hot cycle, while the supporting force varies. Thus the hanger would exert an unbalanced force on the pipe equal to the amount of variation, or 3615lb. Most of this force would be imposed directly on connection A., where limits are established for the force which maybe applied. Changing the spring scale to lower the variability still imposes a high force on A. Appropriate hanger support type for H-1 is a constant support. The hanger will be calibrated to the calculated pipe weight, so it’s supporting force would be 5363 lb at cold position, and 5363lb also at hot position.

Selection of Pipe supporting Devices Piping Systems: Temperature classification. Piping systems, are divided into the three main temperature categories in order to provide a basis for the selection of hangers, anchors, or supports. 1. Hot systems a. The temperature range is from 120F (50C) to 450F (230C). Typical examples are low-pressure steam, hot water and certain process piping. b. The temperature range is from 450F (230C) to 650F (340C). Typical examples are boiler plant and industrial steam and hotwater piping systems. c. The temperature ranges from 750F (400C) and higher. A typical example is a high-pressure steam power-plant piping system d. In the temperature range 650F and higher, there is the possibility of metallurgical change if unalloyed carbon steel is used. It is suggested that hangers, anchors, and supports for piping which operates at above 650 F be of materials at least equal to those of the piping system itself.

Selection of Pipe supporting Devices 2. Ambient systems in which the contents of the pipe are not heated or cooled by mechanical means. Temperatures would range up to 120 F. Plant air and service water would be typical systems 3. Cold systems a. The temperatures range upward from 32 F. A typical example would be chilled water piping b. The temperature ranges downward from 32 to minus 20F, as in brine systems c. Below minus 20 F, as in cryogenic systems

Selection of Pipe supporting Devices .

Selection of Pipe supporting Devices Pipe Attachments. Hangers for the various systems described above may be selected from fig.11 in accordance with the following recommendations: For Type 1a systems, hangers Types 1 and 3 through 12 are used. Rollers should be types 41 through 47 with appropriate saddles of Type 39, items 1 and 2. Supports would be Types 35 through 38. For Type 1b systems, hangers Types 1, 3, 4 and 8 are used. Rollers should be types 41 through 47 with appropriate saddles of Type 39, items 1 and 2. For Type 1c systems, alloy hangers are used as required by the line temperature. Hangers should be of Types 2, 3, or 8 with saddles of Type 39, items 1 or 2, and the rollers of Types 41 through 47

Selection of Pipe supporting Devices For Type 2 systems, hangers can be of Types 1 and 3 through 12 with supports of Types 24, 26, and 35 through 38 For Type 3 systems, the hanger of support must be outside the insulation and the vapor barrier must be left undisturbed. A Type 40 insulation protection shield must be used to distribute the loading on the insulation. Hangers sized for the outside diameter of the insulation can be of Type 1, 4, 6, 7, 9, 10, or 11. For the Type 3c systems, special consideration must be given to the type and nature of the piping and its layout. Consideration may be given to the use of the welded lug attachments. Where used on Types 1c and 3c, the welded attachment must be of an alloy material which is compatible with the material of the piping system itself.

Selection of Pipe supporting Devices Spring supports;  For systems which operate at temperatures below 750F, a good rule is that the variation in supporting force be limited to 25% of the load. When the suggestions are followed for stress limits set forth in MSS SP-58, para 11, and ASTM specification A125, springs to suit specific conditions may be designed. 

However, the price of a specially designed spring includes engineering and setup charges, and unless a large quantity of a particular size is to used, it is not economical to design special individual springs, and a more prudent approach is to select spring devices which are available commercially.

Selection of Pipe supporting Devices 

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Vibration arising from pump pulse, compressor and similar conditions is a problem in piping systems. Such conditions can be avoided by use of commercially available spring supports. Systems that respond to exciting vibrations can be controlled satisfactorily by the use of dampening device. There are two general types to consider; the coiled spring and the hydraulic vibration dampener. There are two types of coiled-spring vibration dampeners; the opposed-spring type and the double acting spring type (type50). These types should be arranged so that the springs are in the neutral position during normal operating conditions of the system. The hydraulic vibration control is a unit which operates by means of a controlled flow of fluid through an orifice. Resistance to movement increases with the speed of displacement. One distinct advantage of the hydraulic device is that there is a min. of resistance to thermal movement of the piping. Both spring and hydraulic cylinder devices may be used to control sway and absorb shocks.

Selection of Pipe supporting Devices Hanger Rod;  Rod used for pipe support purposes is usually hot rolled steel with cut threads conforming to National Bureau of Standards Handbook H-2 Class 2A, for the coarse thread series. Rolled threads to the same standard may be used. It must be pointed out that the length of a rolled thread cannot be increased by running a die over it, since the basic diameter of the rod is less than the size of the threaded portion.  Safe load capacities of rods are based on the area at the root of the thread. A generally accepted standard for such capacities is given in table 5, taken from MSS SP-58.

Selection of Pipe supporting Devices .


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In addition to supporting gravitaton loads, the designer must also be concerned with the provision of a suitable system of anchors, guides, restraints, stops, and braces to control intended movement, maintain piping position, and protect equipment from possible excessive loading shock forces. The layout of each system section of piping should be reviewed, taking note of such factors as configuration branches, expansion joints and loops, pipe sizes, terminal connections, relation stiffness of each leg in all planes, and system operating conditions. The digestion of all these factors, coupled with visualization of the normal thermal movement of the system under consideration, enables an evaluation of the specific requirements necessary to assure positive control during all phases of operation.


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Anchors and restraints may be required to establish definite movement patterns, counteract thrust forces, or, as in the case of vibration-imposing equipment used to prevent transmittal and possible build-up of vibration throughout the entire system. Specific examples are the need for properly located anchors in a steam distribution system to prevent overloading of the smaller branches, anchors and guides, to actuate and align expansion joints and loops properly, and restraints of fixed points in the vicinity of compressor equipment or quick-closing control valves. Long straight runs or sections of piping that are obviously weak in some plane may require additional guiding or bracing to provide lateral structure stability. As in the case of all applications of anchors and guides, the overall installation must provide sufficient flexibility to accommodate thermal growth. For sections where the movement does not permit the use of rigid struts, guides with sufficient clearance to accommodate the normal movement may suffice by limiting the displacement. Positive strut action can be obtained at points subject to movement through the use of special devices such as hydraulic snubbers.

Selection of Pipe supporting Devices Risers are equivalent to concentrated loads; however, in the support of the load, several important points must be considered. These are: 1. 2.

Is the support to take the entire riser weight, or is this weight to be distributed among several supports? Are the hydrostatic-test conditions more severe than service conditions; that is; will the cold-water-filled condition impose stresses on the support higher than allowable (in cold condition) as compared with hot operating condition and the imposed stresses? When this decision is made, the system erection sequence should be considered and a determination made whether other supports are effective or ineffective during hydrostatic testing.

Selection of Pipe supporting Devices 3. Is the support to be located at a point of zero vertical movement and hence to be considered a rigid support? If this is the case, then the horizontal and flexural movements must be analyzed. Pure horizontal movement can be provided for long support rods which are allowed to swing. However, if flexural movement exists, it may cause tipping, and then must be assumed that the entire load can transfer to one support rod. In this case, the riser support must be designed for double the calculated load.

Guide Supports

Guide Supports

Limit stop Supports

Supports Friction

Restraint Supports

Limit stop Supports

Pipe Rack Supports

BREAK

QUESTIONS ŏŏ ∆ º

Stiffness & Flexibility 

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Prismatic member, straight members of uniform cross sectional area. They are the building blocks of structural engineering and also piping software packages. Assuming that the displacements are small, so that shortening of the beam due to bending, may be ignored. Each member has it’s own “local” axis which do not coincide with the axis for other members of the structure. It is thus assumed that a force applied in any one principal plane causes displacements in that plane only and that the shear centre coincides with the centroid of the member. There is a possibility of three linear displacements and three rotations at each end of the member. There are thus 12 possible displacement components for each member, or 12 degrees of freedom. Associated with each displacement there is a corresponding force or moment.


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The result of the derivation section can be summarized in a single matrix equation for member stiffness as follows; [F] = [K] [X] This is the member stiffness equation, F & X are 12-term vectors of member force and displacement respectively, and k is a 12x12 member stiffness matrix. This is the stiffness matrix for the most general case of a prismatic member in space neglecting shear and with the implicit condition that the deformations are so small as to leave the basic geometry unchanged. Not all structural members require the full 12 degrees of freedom to express their deformations. Since a member in space can have no moments transmitted to it through it’s hinged ends, it’s deformation depends only on the three linear displacements at each end, giving it a total of six degrees of freedom. It is important to note the symmetry of the member stiffness matrix k.

Stiffness & Flexibility Transformation of axis; The system of axis for a prismatic member is a local axis system. The x-axis is defined as coinciding with the centroidal line of the member. In a structure with many members there would thus be as many systems of axes. Before the internal actions in the members of the structure can be related, all forces and deflections can be stated in terms of one single system of axes common to all the – structure “global” axes. 

The directional cosines matrix can therefore be thought of as the 3x3 rotation matrix Ro. Thus any quantity can be redefined in terms of global axes by pre-multiplying by the rotation matrix. When used to redefine member forces and deflections in structure axes, this process is conventionally referred to as transformation of axes, and the symbol T is used for the transformation matrix. T = [Ro 0 ] [0 Ro]


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Basic requirements: Piping systems shall have sufficient flexibility to prevent thermal expansion or contraction or movements of piping supports at terminals from causing; Failure of piping or supports from overstress or fatigue Leakage at joints. Detrimental stresses or distortion in piping and valves or in connected equipment (pumps and turbines for example), resulting from excessive thrusts and moments in the piping.


Specific requirements:

In brief they are, -

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The computed stress range at any point due to displacements in the system shall not exceed the allowable stress range. Reaction forces computed shall not be detrimental to supports or connected equipment. Computed movement of the piping shall be within any prescribed limits, and properly accounted for in the flexibility calculations.

Stiffness & Flexibility Concepts: Displacement strains; Thermal displacements, Piping system will undergo dimensional changes with any change in temperature. If constrained from free expansion or contraction by connected equipment and restraints such as guides and anchors, it will be displaced from its unrestrained position. Restraint flexibility, where restraints are not considered rigid, their flexibility may be considered in determining displacement stress range and reactions. Externally imposed displacements, externally caused movement of restraints will impose displacements on the piping in addition to those related to thermal effects. Movements may result from tidal changes (dock piping), wind sway (eg. Piping supported from a tall slender tower), or temperature changes in connected equipment. Total Displacement strains, Thermal displacements, reaction displacements, and externally imposed displacements all have equivalent effects on the piping system, and shall be considered together in determining the total displacement strains. 

Stiffness & Flexibility Concepts, cont.: Displacement stresses; Elastic behavior, stresses may be considered proportional to the total displacement strains in a piping system in which the strains are well distributed and not excessive at any point (a balanced system). Layout of systems should aim for such a condition. Overstrained behavior, stresses can not be considered proportional to displacement strains throughout a piping system in which an excessive amount of strain may occur in localized portions of the system (an unbalanced system), unbalance may result from one or more of the following; highly stressed small size pipe runs in series with large or relatively stiff pipe runs. a local reduction in size or wall thickness, or local use of material having reduced yield strength. a line configuration in a system of uniform size in which the expansion or contraction must be absorbed largely in a short offset from the major portion of the run. 

Questions

Expansion & Stresses Effect of expansion and stresses within a piping system need to be determined by knowing the following;      

Which code that applies to system. Design Temperature and Pressure. Material Specification. Pipe Size & wall thickness of each of the piping components. The layout of the system including dimensions and thermal movement of terminal points. Limitations of end reactions on terminal points as given by equipment manufacturers.

Expansion & Stresses 

The requirements for formal analysis are identical to those of B31.1. The Code gives the following equation (same as B31.1) to check if formal (simplified or comprehensive) analysis is required: D y / ( L – U )2 ≤ 0.03 • • • •

D: outside pipe diameter, mm y : resultant of displacement strain, mm L: developed length, m U: anchor straight distance, length of straight line joining anchors, m


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Applicable code only will determine the minimum safety requirements for the material at the design conditions of pressure and temperature. Some code specify the modulii of elasticity for commonly used piping materials as well as formulae to determine stress intensification factors and flexibility factors. Codes state that, the piping system shall be treated as whole, in calculating the flexibility of a piping system between anchor points and that the significance of all parts shall be recognized. In addition, calculations shall take into account stress intensification factors which apply to components other than sections of straight pipe.

Expansion & Stresses

Expansion & Stresses The analysis of piping loaded by pressure, weight and thermal expansion so the analyst needs to understand the application of the Principal Axis system, to simplify. Consider a cube removed from a stressed section of pipe. Calculate the stress in the cube and compare it to some allowable stress limit. STRESS is ratio of FORCE to AREA or MOMENTS DIVIDED BY PIPE SECTION MODULUS. Each force acting on the cube, can be trigonometrically reduced to force components, represented by vectors, acting along each of the principal axis. The resultant of the component of each force acting on the face of the cube, divided by the area of the cube face is called the PRINCIPAL STRESS. The principal stress that act along the centerline of the pipe is called a LONGITUDINAL PRINCIPAL STRESS. This stress is caused by longitudinal bending, axial force loading or by pressure.

Expansion & Stresses RADIAL PRINCIPAL STRESS, acts on a line from the center of pipe radially through the pipe wall. This stress is a compressive stress acting on the pipe ID caused by internal pressure, or a tensile stress caused by external or vacuum pressure. CIRCUMFERENTIAL PRINCIPAL STRESS, sometimes called HOOP or TANGENTIAL STRESS acts on a line perpendicular to the LONGITUDINAL and the RADIAL STRESS. This stress attempts to separate the pipe wall in the circumferential direction. This stress is caused by internal pressure. When two or more PRINCIPAL STRESSES act at a point on a pipe, a SHEAR STRESS will be generated. One example of a SHEAR STRESS would be at a pipe support where a RADIAL STRESS caused by the supporting member acts in combination with the LONGITUDINAL BENDING caused by the pipe overhang.

Expansion & Stresses Allowable Stress Range B3 1.3 establishes a maximum allowable stress range that can be safely accommodated by a piping system before failure will commence for two separate stress loading conditions. These limits are for stress levels that can, 1.) cause failure from a single loading, and, 2.) cause failure from repeated cyclic loadings. The ALLOWABLE STRESS RANGE, SA [ 302.3.5(d)] is the stress limit for the 2nd stress level, those stresses that are repeated and cyclic in nature, or simply, it is the allowable for the SECONDARY STRESS, the DISPLACEMENT STRESS RANGE. B31.3 presents two equations for the calculation of SA.

Expansion & Stresses Equation 1a is as follows; SA =f(1.25 Sc+0.25Sh) and equation 1b is as follows, SA =f[1.25(Sc+Sh)-SL] Sc and Sh are the basic allowable stresses for the cold and hot conditions as defined in Section 1.3.4. Sc and Sh values are found in B31.3 Appendix A TABLE A-1. f is the STRESS-RANGE REDUCTION FACTOR; this factor can be selected from the table shown below or can be calculated by equation of B31.3 as: f=6.0(N-0.2)≤1.0

Expansion & Stresses STRESS-RANGE REDUCTION FACTORS f Cycles N Factor f 7000 and less 1.0 Over 7,000 to 14,000 0.9 Over 14,000 to 22,000 0.8 Over 22,000 to 45,000 0.7 Over 45,000 to 100,000 0.6 Over 100,000 to 200,000 0.5

Expansion & Stresses •The SL term is the LONGITUDINAL STRESSES to be discussed later. •Although equations are both the allowable stress, SA, for the calculated thermal displacement stress range, SE, each equation has a specific application. •Equation 1a is a system allowable stress of the entire piping system of the same material, thermal cycles, and temperature; •while equation 1b is a component allowable stress, SA, for each single component in the piping system where SL has been calculated for that component under analysis.

Expansion & Stresses Cold Springing 

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Cold springing is the intentional deformation of piping during assembly to lower the initial displacement strains in the operating condition. It is used to lower the forces transmitted to connected equipment and to lower the deviation from as-installed dimensions, such as inclination of hangers. However, cold springing does not change the magnitude of stress range. The amount of cold spring ”C.S.” is usually expressed as a percentage or fraction of the total expansion ∆


Expansion & Stresses The B31 .3 Code offers several methods to increase the flexibility [319.7] of a piping system. Added flexibility may be necessary to lower the pipe loads on load sensitive equipment such as pumps, turbines, or compressors. The traditional method to increase flexibility is to add expansion loops or off-sets in the piping layout. The key objective in adding loops or off-sets is to move the CENTER OF GRAVITY of the system away from the LINE OF THRUST. Consider a simple two anchor piping layout and construct a line drawn connecting the two anchors. Estimate the center of gravity. Flexibility is increased when the added pipe moves the center of gravity away from this line of thrust.


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Layout of piping system should provide inherent flexibility, however, for the cases where the system lacks flexibility the designer should consider increasing flexibility by means of bends, loops, offsets, swivel joints, bellow or slip type expansion joints.



This center-of-gravity/line-of-thrust concept is further illustrated by the following two computer analysis of the above pipe layouts.

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Both piping layouts are the same pipe size, temperature, and the anchors are the same distance apart.

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The L shape layout has a maximum expansion stress of 24,455 psi.

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The Z shaped has 42,594 psi.

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The L shape moved the center of gravity, cg away from the line of thrust which produced a lower stress, greater flexibility even though the Z shape had one more elbow.

Questions, Comments

Design & Construction of Piping Systems: Prepared by Eng. Mostafa A. El-Sayed

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