By : S.H.DHAKATE
05/01/13
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STRESS IN PIPING SYSTEMS
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What What is a pip pipin ingg sy system stem?? Type of of stresses. Deta Detail ilss ab about out str stres essses. es. Allowable st stresses. es. Flex Flexib ibil ilit ityy in in pip pipin ingg sy system stem.. Methods Methods to check check flex flexibi ibility lity of of piping piping sys system tem / stres stresss analy analysis sis.. Proj Projec ectt spec specif ific ic wor workk inst instru ruct ctio ionn
What is piping system?
To transfer the fluid from one place to another place with the help of pipe is so called piping system. Simplest piping system seen by you is the water system in your house where the water is transferred from underground tank to the overhead tank through pipes with the help of a pump. We call it plumbing. Similarly in chemical / petrochemical, fertilizer plants, piping is used to transfer the different fluids from one equipment to another equipment and from one plant to another plant. Type of stresses.
The stresses will be developed because of internal forces as well as external forces. There are two type of stresses developed in the piping system. -
Primary stresses Secondary stresses
Primary stresses
They are NOT SELF LIMITING in nature. i.e. deformation will not bring relaxation on stress.. -
Pressure st stresses Dead weight st stress esses
It is also called SUSTAINED LOADING, other loading is OCCASIONAL LOADING i.e. earthquake, wind etc Secondary stresses
By : S.H.DHAKATE
05/01/13
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They are SELF LIMITING in nature i.e. deformation will result in redistribution of stress. -
Thermal stresses
This is also called THERMAL LOADING. Other loading in this category is seismic anchor movement, equipment settlement and thermal anchor movement. Hence primary factors for stresses to develop in piping system is internal pressure and thermal strain.
Details about stresses Primary stresses
Because of internal pressure This depends on three factors 1. Fluid handled 2. Pressure and temperature of fluid 3. Size of the pipe and material of construction of pipe Thus as per thin wall cylinder formula Hoops stress / circumferential stress Load = p d Area = 2 t Stress = load / area =p d/2 t
p – Internal Pressure d – diameter of pipe t – thickness of pipe
Longitudinal stress Load = p π d 2 / 4 Area = π d t Stress = load / area =pd/4t As can be seen from above, hoop stress is twice of the longitudinal stress for the same pressure and diameter of pipe. Hence Hoop stress is maximum stress.
Hence to take care of the pressure stress, the thickness calculations is done and specification is prepared. As per ANSI B 31.3 Code for process piping tm = t + c
tm – minimum thick of pipe t - calculated thickness
Piping
By : S.H.DHAKATE
05/01/13
t = P D / 2 (S E +2PY)
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P – internal pressure E – quality factor S – Allowable stress as given in code books (tabulated)
Y – Coefficient c – corrosion / erosion allowance Manufacturing tolerance of 12.5% is to be consider over tm.
Example : OD = 356 mm, Pr = 10 kg/cm2, Matl = API 5L Gr B, S = 20 ksi = 1380 kg/cm2, t = 10*356 / 2 (1380*1 + 10*1) = 1.28 mm
c = 1.5 mm
tm = t +c = 1.28 + 1.5 = 2.78 mm considering manufacturing tolerance of 12.5% t = 2.78 / 0.875 = 3.177 Adopted thickness of pipe by piping engineer is 9.5 mm considering the ready availability and general mechanical strength. Dead weight stresses We have seen that temperature and pressure are significant factors governing the stresses in the piping system.
Permitted Piping Stresses
Basic Allowable Stresses in material. Tensile strength is the highest stress which the specimen can accommodate without failure. Allowable stresses are defined in the various codes. -
Time independent stress related to initial Yield stress or tensile strength as measured in tensile test. Time dependent : is related to the “creep rupture strength” at the high temperature. At temperature above 1/3 of the melting point metals will exhibit creep in a standard tensile test.
As per ANSI B 31.1 allowable stress is ¼ of TS or 5/8 of YS As per ANSI B 31.3 allowable stress is 1/3 of TS or 2/3 of YS Nomenclature: Sc – Allowable stresses at room temperature
By : S.H.DHAKATE
05/01/13
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Sh – Allowable stresses at operating temperature As per code, the sum of longitudinal stresses with the pipe in corroded condition and due to all sustained loads (i.e. internal pressure, weight, wind etc.) shall not exceed tabulated allowable stress at design temperature Sh. As we have seen, if the pipe is designed to have the hoop stress equal to Allowable stress Sh, then associated axial / longitudinal stress will be Sh/2 due to internal pressure. Hence the remaining value of this allowable stress i.e. Sh/2 is to be used for other sustained loading other than internal pressure, i.e. dead weight, wind etc. After internal pressure, dead weight is the most significant effect producing a sustained longitudinal stress, but it is conventional to limit it to ½ of Sh/2 i.e. Sh/4. SUPPORT SPAN
The pipe is to be properly supported to avoid undue stresses in the piping system because of dead wt of pipe and its content. Basis - Pipe with standard schedule - Lowest grade of Steel for safer side - Temperature – ambient lines - lines at 100 deg - lines at 300 deg - Empty / water filled - Pipes as pin ended simply supported Maximum bending moment occurs at centre of span for simply supported beam. Mmax = wL2 / 8 kg-m
w – weight in kg / m
If f is the stress allowable for weight purpose i.e. = Sh / 4 Then f = M / Z = wL2 / 8 Z L2 = f 8 Z / w L = 4.7 √ Z/w Example : same example as taken for thickness calculation D = 356mm, Matl = APE 5L GR B, Stress = 20ksi = 1380 kg/cm2 Thk = 9.5 mm, Z = 873 cm3, w = 170 kg /m.(weight filled with water)
By : S.H.DHAKATE
Therefore
05/01/13
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f = stress of weight purpose = 1380 /4 = 345 L = sq root (345*8*873/170*100) = 11.9 mtr
SAGGING IN THE PIPING SYSTEM Sagging or the deflection at the centre of pipe span is to be restricted to avoid the pocket formation in liquid lines when the lines are emptied. We have derived the allowable pipe span based on the maximum stress equal to Sh/4. The equation used has no indication of deflection at the centre of span. It is better to limit this span to 6 mm. On the basis of elementary bending theory Deflection delta = 5 w L4 / 384 E I
With units as kg/ m and cm4
L = span in mm w = weight in n/mm E = Young’s modulus, n/mm 2 I = Moment of inertia mm 4
L = 0.1486 * 4th√ (EI/w) With this E = 19244 n / mm2 L = 3.11 * 4th√ (I / w) For 14” dia pipe I = 15525 cm 4, and w =170 kg/m L = 3.11 * 4th√ (15525 / 1700) = 3.11 * 3.091 = 9.61 mtr. Hence for dead weight supporting, both the aspects are to be considered and optimise the span requirement for piping system. The span requirement for continuous run and for change of direction of pipes has to be studied properly and then the supporting should be done to avoid developing stress in the piping systems.
By : S.H.DHAKATE
05/01/13
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SECONDARY STRESSES
These are because of thermal expansion of the pipe. This is because of the fluid temperature in the pipe. What we have seen till now is that, to take care of internal pressure we adopt the suitable thickness of the pipe and to take care of dead weight, we decide the supporting span. Now we will see the effect of temperature of fluid on the piping system. This is mainly because of the thermal growth of pipe, expansion or contraction. In both cases it is called thermal expansion. This will cause development of thermal stress in the piping system. will develop if the piping system is allowed to move freely, without any restriction. No thermal stresses
There will be restrictions to the movement of piping during thermal growth because the piping is connected to equipments which will act as anchors or fix points. Hence this will cause the stresses to develop in the piping system, reactions forces will be there on equipment nozzles. Therefore, we have to see that the piping system is flexible enough so that the thermal stresses are within allowable limits and also the reactions forces on equipment are within acceptable limits. : the layout and design of the piping , including supporting elements shall be made with due regard to preventing the followings. As per the code ANSI B 31.3 Clause 321.1.1 Objectives
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Piping stresses in excess of those permitted in the code Leakage at joints. Excess thrusts and moments on connected equipments Excessive stresses in supporting (restraining) elements Excessive interference with thermal expansion and contraction in piping which is otherwise adequately flexile. Unintentional disengagement of piping from its supports Excessive piping sag in piping requiring drainage slope
FLEXIBILITY IN PIPING SYSTEM Piping routing is to be made as short as possible from one point to another to have lesser capital expenditure. The longer pipe may also generate excessive pressure drops.
By : S.H.DHAKATE
05/01/13
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What is modulus of elasticity? E = Stress / strain Figure But if straight pipe is directly connected from one anchor to another, then it has to absorb the expansion equal to L e. that will create the stresses of the magnitude S = E (∆ / L) = E e ∆ e L S E A F
= thermal expansion, in = expansion rate, in/in = pipe length, in = axial stress, psi = modulus of elasticity, psi = pipe cross section area, in2 = axial force, lb
Force required to squeeze this amount is F=AS=AEe Take a 6” standard wall carbon steel pipe for instance an increase of temperature from 70 deg F ambient to 300 deg F operating creates an axial stress of 42300 psi and an axial force of 236000 lbs in the pipe. E = 28.3 x 1000000 psi A = 3,14 x 6 x 7.11 / 25.4 = 5.2737 in 2 e = 0.001517 in / in (1.82 in / 100 ft ) F = 5.2737 x 28.3 x 1000000 x 0.001517 = 226405 lbs Therefore straight connection anchor to anchor is the most inflexible piping routing. Hence flexibility has to be provided to avoid excessive forces on anchors i.e. equipment nozzles by means of change of direction in the piping routing. Figure L shape, Z shape, U shape, expansion loop are used to add flexibility to the piping routing / layout. Expansion bellow is the flexible element which is added to layout to have the flexibility in the piping system. But its use is very limited because it is a sophisticated
By : S.H.DHAKATE
05/01/13
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than expansion loop and requires to install sufficient restraints for maintaining the stability. VISUAL CHECK / SIMPLIFIED FLEXIBILITY CHECK Visual check is the first important examination on anything we do. If the design looks strange, then most likely something is wrong with it. By now, we at least know that we can not run a piping straight from one point to another. This also applies to a situation when there are two or more line stops are installed at a straight header. Line stop or axial acts directly against the expansion of pipe hence no place to relieve. The length of the leg is direct measure of the flexibility. Therefore, the key is to locate the availability of the perpendicular leg and to determine if the length of the leg is sufficient. Simple check is done by the following methods -
Guided cantilever method As given in ANSI B 31.3 Piping Code.
1. Guided Cantilever Method This simplified analysis is based on the guided cantilever method. Guided cantilever beam restrained in such a way that its free end will not rotate when it is deflected in the direction perpendicular to longitudinal axis of the beam.
For piping system under thermal expansion loads, the behaviour of the piping appropriates that of the guided cantilever. The thermal growth forces the pipe legs to translate while pipe rotations are restricted by piping continuity. Therefore, this method can be used to check flexibility of the piping system. For a guided cantilever, from the basic beam theory, the moment induced by imposed deflection is
By : S.H.DHAKATE
05/01/13
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M = 6 E I ∆ / L2 And displacement reaction of guided cantilever is F=2M/L = 2* 6 E I ∆ / L 2*L = 12 E I ∆ / L 3 or = E I ∆ / 144 l3 For thin wall pipes, equation can be further reduced by using I = π r 3 t and S = M / (π r 2 t ) S = M /Z = 6 E r ∆ / L2 or = E D ∆ / 48 l2 S = thermal expansion stresses, psi M = Induced Moment, lb E = Modulus of elasticity, psi r = Mean radius of pipe, in I = Moment of Inertia, in4 ∆ = Deflection, in L = length of pipe leg perpendicular to deflection, in l = length in feet unit, ft D = outside dia of pipe, in Z = Section modulus of pipe, in 3 Hence, it is observed here is that stress S in inversely proportional to the square of length l and force F is inversely proportional to the cube of length l. Solving the equation for pipe length l gives l = √ E D ∆ / 48 S if E =29*106 psi, and S = 20000 psi then
l = 5.5 √ D ∆
Example : 6” pipe, 80 ft long at 600 deg F, expands 4 in, l = 5.5 √ 6*4 = 27 ft Hence we need 27 ft leg of pipe to absorb the thermal expansion of 80ft long pipe. By determining the proper allowable stress and taking into account the appropriate stress intensification factor, above equation gives an estimate of the minimum
By : S.H.DHAKATE
05/01/13
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allowable offset pipe span L required to sustain a piping thermal movement delta normal to the piping. Second method is as per ANSI B 31.3 Piping code. D y / ( L – U ) 2 < 0.03 Where D = Outside diameter of pipe, in y = resultant of total displacement to be absorbed, in L = developed length of piping between anchors, ft U = straight line distance between anchors, ft. FLEXIBILITY Thermal Stresses The self limiting stresses in the piping system are essentially cyclic and initial hot stresses if they are of sufficient magnitude will decrease with time because of plastic strains (local yielding) and will appear as a stress of reversed direction when the pipe cools. This phenomenon forms the basic difference between the self limiting stresses and the sustained stresses (WT + PR). What do you mean by cyclic? Initially the pipe is at room temperature. When the plant is started, the pipe becomes hot because of the hot fluid flowing through it. Pipe will cool down again when the plant is shut down for annual maintenance. So this cycle repeats again when the plant starts. This phenomenon is called the self springing of the pipe is similar to the effect of cold springing of the pipe.(Pre stressing of pipe). The sum of hot and cold stress will remain same, although with time hot stresses will gradually reduce. This sum is called Expansion Stress Range. This is the criteria used for design of piping for flexibility. This concept of constant expansion stress range is now used to define the stress range
As per B 31.3 limits the stress range to 78% of YS. SA = 1.25 ( Sc + Sh ) Reduce 0.5 Sh for pressure stress and 0.5 for dead wt stress SA = 1.25 Sc + 0.25 Sh
allowable
By : S.H.DHAKATE
05/01/13
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Expansion Stress Range / Displacement Stress Range as per clause 319.2.3 of ANSI B 31.3 is as follows. SA = f ( 1.25 Sc + 0.25 Sh ) f – Stress range reduction factor. SIF –Stress Intensification Factor is a ration of maximum stress intensity to a normal stress. It is used as a factor of safety to account for the effect of localised stresses on the piping under repetitive loading. This factor is applied to welds, fittings, branch connections and other piping components where stress concentration and possible fatigue failure might accure. Usually experimental methods are used to determine these factors.
By : S.H.DHAKATE
05/01/13
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SORF FLANGES – SLIP ON FLANGES 1. B 31.1 - Power Piping Slip on flanges exceeding 4” in nominal pipe size shall be limited to the 300 lbs or lower service ratings 2. B 31.3 – Process Piping Slip on flanges shall be double welded as shown. When used for any of the followings 1. Services subject to severe erosion, corrosion or cyclic loading. 2. Services which are flammable , toxic or damaging to human tissue. 3. severe cyclic conditions. If slip on flanges re single welded weld shall be at the hub. The use of slip on flanges shall be avoided where many large temperature cycles are expected, particularly if flange is not insulted. 4. ASME Sec VIII Division I As per applicable ANSI / API standards. 5. B 16.5 – Steel Pipe Flanges. Slip on, Socket Welded and Threaded flanges are not recommended for services for temperature above 260 deg. C. or below -50 deg C. if the severe thermal gradients or thermal cycling are involved. 6. IBR Slip on flanges for all pressure conditions up to and including 17.6 Kg/cm2 and temperature not exceeding 399deg C. except that Bossed flanges shall not be used on pipes 6” in nominal bore. THK AS PER IBR TABLE J Page 531. NB ½” 1 ½” 2” 3” 4”
THK 5/8” 16 mm 7/8” 22.2 mm 1” 25.4 mm 1 ¼” 32 mm 1 3/8” 35 mm
ANSI thk 14.5 mm 21 mm 22.5 mm 29 mm 32 mm
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05/01/13
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Des-similar Material welding Alloy Steel to stainless steel Electrodes – special welding 1 ¼ Cr ½ Mo – SS 18 Cr 8 Ni - P4 electrode 13 D Carbon Moly SS 18 Cr 8 Ni – P4 Electrode 13 B 13 – ENiCrFE-2 electrode ERNiCr-3 bare wire B, D heat treatment SS to SS – E 308 P11 to P11 – E 8018 . B2 P1 to P1 - E 7018 . A1
STRESS RANGE Stress due to thermal expansion / contraction in piping.- diminishes with time as a result of local yielding or creep. This reduction of stress will appear as stress of opposite sign in cold condition. This phenomenon is known as self springing of line and is similar in effect to cold springing. Thus, although the hot stress tends to diminish with time, the sum of hot and cold stresses for any one cycle will remain practically constant. This sum is called stress range. SA = f ( 1.25 Sc +0.25 Sh) f – stress range reduction factor based on cycles. VALVES 1. Process division group – basic material 2. Project division group – type of valve 3. Design division group – valve specification Valve Classification List 1. 2. 3. 4. 5. 6.
Type of valve, sizes, manufacture model no. Pressure and temp. rating of valves Material of construction Packing material valve trim material including stem, seating ring and disc Manufacturer recommended service
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05/01/13
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Basic Purpose of Valves 1. Shut off - Block valves 2. Throttling services – to regulate flow/ pressure – throttling valves Cv – Valve’s flow co-efficient Q = Cv . root( delta P/G) Q - flow rate Cv – flow co-efficient Delta P – Pressure Drop across valve G – Specific Gravity of fluid PLANT COST (Ref CPI- Chemical Process Industries) Piping and valves = 22% Pumps = 4% Compressors = 4.5% Drivers = 6.5% Furnaces = 4% Vessels = 15.4% IBR Pipe thickness calculation Maximum Working Pressure WP = 2 fe (t-c) / D-t-c) t - minimum thickness WP – maximum working pressure f – allowable stress as specified in table 3 of reg. 343 D – OD of pipe e – efficiency factor = 1 for seamless pipes c – o.1 cm Maximum permissible working stress up to 260 deg C is 865 Kg/cm2 The lower value is obtained on the following basis. CS up to 454 deg C : TS / 3.5 or Et / 1.6 AS up to 454 deg C : Sr / 1.6 or Se TS – Minimum Tensile Strength at 20 deg C. Et – Yield Point (0.2% proof stress) at temp. t Sr – The average stress to produce repture in 100,000 hrs. and in no case more than 33 times the lowest stress to produce repture.
By : S.H.DHAKATE
05/01/13
Sc – Average stress to produce an elongation of 1%(creep0 in 100,000 hrs. The adopted thickness shall be greater than the calculated thickness .
Example : material – ASTM A 106 Gr. B seamless. Max working pr = 25 Kg / cm2 Max working temp 21 deg C TS = 60000 psi Max working stress = 60000/3.5 = 17142 psi =1207 kg/cm2 Et = 35000 psi Max working stress = 35000 / 1.6 = 21875 psi = 1540 kg/cm2 lowest allowable stress = 865 kg/cm2 WP = 2 Se (t – c ) / D – t – c =2 * 865*1*(t – c ) / D – t – c t = D / 70.2 + c
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By : S.H.DHAKATE
05/01/13
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MAIN ACTIVITIES OF PIPING 1. 2. 3. 4. 5.
General layout (updating) Equipment layout (updating) Key plan P&I (updating in according with piping routings) Line list (updating) 6. 1st MTO 7. Piping Study 8. Nozzle orientation 9. Auxiliary Structures 10. Technical Data for foundation 11. Stress analysis 12. Secondary structures 13. Underground piping routing 14. 2nd MTO 15. Comments on Civil Drawing 16. Comments on electrical and instrumental drgs. 17. Comments on Vendor drgs. 18. Piping arrangements 19. Specifications for special items 20. Steam tracing / jacketing 21. Piping Supports 22. Isometrics and final MTO Piping clearance : 1. at grade, in operating areas and undersides of pipe ways for maintenance : 3000 mm 2. Clear areas under pipeways for services aisle : 3000 mm 3. over main roads : 6000 mm 4. over secondary roads : 4000 mm 5. over platforms walkways passagesways stairwells and working areas : 2100 mm 6. operating aisle ways : 1800 Foundation heights 1. 2. 3. 4. 5.
columns and vertical vessel : 300 mm Tanks : 1000 mm Pump shaft center line between 600 to 900 mm Standard foundation :300 mm pipe support : 100 mm
Piping layout 2. The pipe rack or sleepers shall have total 20% of free space for future piping
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05/01/13
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3. Except where unavoidable the minimum distance between the bottom of pipes and the grade shall be 500 mm 4. Minimum depth from grade to top of pipe for underground lines shall be 300 mm 5. Adjustable types of supports are preferred for piping connected to pumps / compressors. 6. Access to exchanger space is needed for tube-bundle removal, for cleaning and around the exchanger’s bolted ends. 7. Provide drop out spool in the piping that must be removed frequently for maintenance of for installation or removal of temporary suction strainers and blanks. 8. On the end of pipes is preferable install blind flanges instead of welded caps. 9. Provide chain wheel or extension stems for valves, where the bottom of the hand-wheel is more than 2 M above the operating level. 10. Relief valves piping that discharges into the atmosphere shall be carried 3000 mm above any platform or ladder within a radius of 8000 mm of the valve. Provide a 10 mm weep hole to prevent an accumulation of liquid in the vertical relief valve stacks open to atmosphere. 11. Individual relief valve discharge piping grater than 2” shall enter the header at an angle of 45 deg in the direction of flow of the header. In all cases, the discharge piping shall enter in the top of the side of the header. 12. Piping systems shall be designed with sufficient flexibility to permit installation of blanks for isolation of equipment. 13. All column block valves are located directly against the column nozzles. 14. Arrange pumps suction lines to be as short and direct as possible. 15. Each high and low point in a run of piping shall have a vent or drain valve. 16. Personal protection, it takes places in proximity of stairs, platform, or working areas. In such cases, the following areas are those to be taken into consideration. – upto 2200mm from the plane of working area. – upto 600 mm beyond the platform or walkway. 17. Allow room for loops and other pipe arrangements to cope with expansion. 18. The nearest welding shall be not less than 50 mm NOZZLE ORIENTATION 1. The elevation of manhole from service platform shall be set between 450 mm to 1050 mm 2. The davit for flange manhole shall be oriented in order to have a proper opening. 3. Foreseen pads for possible pipes supports, 4. Make a one line sketch for possible auxiliary structure. 5. The level gauge shall be oriented in order to have accessibility to the interception valves as example from the ladder of auxiliary structure.
By : S.H.DHAKATE
05/01/13
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