2.0 Design of Tubing and Tubing Systems


2.1 CLASS I INSTRUMENTATION TUBING DESIGN

In ASME Section III‐Division‐I sub‐section NB (Class I components), the design criterion/design requirements for instrument tubing has not been covered separately. Thus design guidelines given for small size of piping is being followed for Class I instrument tubing also. Also as the outside diameter of instrument tubing is being limited to 1” (25 mm); so any design concession permitted for lower size piping (<1”) will also be applicable to instrument tubing.

As per NB 3630 (Piping design and analysis criteria) the piping of 1” NB or less, which have been classified as class I in design specification, may be designed and analyzed as per subsection NC.

Thus for instrument tubing, the material & testing requirements shall be as per subsection NB whereas the design and analysis will be as per subsection NC.


2.2 REQUIREMENTS OF MATERIAL FOR INSTRUMENT TUBING/PIPING AS PER NB2000

a. Pressure retaining material should confirm to the requirements of one of the specifications for material given in NB‐2121.

b. Impact testing for austenitic stainless steel is not required. Also impact testing is not required for a pipe/tube with a nominal pipe size less than 6”, irrespective of wall thickness.

c. Seamless pipes, tubes and fittings need not be examined by the rule of NB‐2510(examination of  pressure retaining material).

d. Wrought seamless and welded (without filler metal) pipes and tubes shall be examined and may be repaired in accordance with the requirements of class‐I seamless and welded (without filler metal) piping and tubing of SA‐655 (specification for special requirements for pipe and tubing for nuclear and other applications).


2.3 DESIGN REQUIREMENTS OF INSTRUMENT PIPING/TUBING AS PER SUBSECTION NC (NC 3600)

i. MAXIMUM ALLOWABLE STRESS
For design/calculating minimum wall thickness of instrument tubing/piping, the maximum allowable stress for the material at design temperature shall be used as given in ANSI/ASME B36.19.

ii. PRESSURE AND TEMPERATURE RATINGS
The pressure ratings at the corresponding temperature given in ANSI/ASME B36.19 shall not be exceeded and piping/tubing product shall not be used at temperature in excess of those given in ANSI/ASME B36.19 for all the materials of which the tubing is made.

iii. ALLOWANCES
Increased wall thickness of tubing shall be taken for providing allowances for corrosion or erosion, mechanical strength & bending etc.

iv. DYNAMIC EFFECTS
Impact forces caused by either external or internal loads shall be considered in the piping/tubing design. Also the effect of earthquake and non‐seismic vibration shall be considered in the tubing design.


2.4 PRESSURE DESIGN (INTERNAL PRESSURE) OF INSTRUMENT TUBING/ PIPNG (Ref. NC3640)
a) Minimum Wall Thickness of straight tube/pipe:
The minimum wall thickness of straight tube/pipe shall not be less than that determined by eq. (I) as follows:


tm = minimum required wall thickness, mm
P = Internal design pressure, kPag
DO = Outside diameter of tube/pipe, mm
S = Maximum allowable stress in the material due to internal pressure and joint efficiency at design temperature, kPa
A = Additional thickness, to provide for material removed in threading, corrosion and erosion allowances and allowance for structural strength needed during erection.

Y = a coefficient having a value of 0.4. For pipe/tube with a  ratio less than 6, the
value of ‘Y’ for ferritic and austenitic steels designed for temperature of 480 oC and below should be taken as per eq. (2) below


Where
d = Inside diameter of tube/pipe.

b) Wherever bending of tubing/piping is likely to be involved in installations, the minimum wall thickness after bending shall not be less than the minimum wall thickness calculated as per eq. (1) for straight tube/pipe. To meet this requirement, actual wall thickness of tubing/piping is to be increased as per following Table –2‐1 (This is based on NC 3000):


c) Also, unless otherwise justified by the design calculation the ovality of tubing/piping after bending should not exceed 8% as determined by following eq. (3).

Where
Do = Nominal outside diameter of tube/pipe
Dmax = the maximum outside diameter after bending or forming
Dmin = the minimum outside diameter after bending or forming


2.5 ANALYSIS CRITERION OF TUBING/PIPING SYSTEM
Analysis requirements for tubing/piping systems as per NC‐3650 are given below. “The design of complete piping system shall be analyzed between anchors for the effects of thermal expansion, weight and other sustained and oCcasional loads.” The detail requirements/analysis criteria are given in following sub‐sections.

a. CONSIDERATION OF DESIGN CONDITIONS (STRESS DUE TO SUSTAINED LOADS)(Refer NC 3652)

The effects of pressure, weight and other sustained mechanical loads must meet the requirements of following eq. (4).


Ssl = Stress due to sustained loads, kPa
P = Internal design pressure, mm
Do = Outside diameter of tube/pipe, mm
B1, B2 = Primary stress indices for the pipe/tube (As per Figure below) NC 3673.2 (b)1
MA = Resultant moment loading on cross section due to weight and other sustained loads, kN‐m. NC 3653.3
Z = Sectional modulus of pipe/tube, mm3
Sh = Basic material allowable stress at design temperature consistent with loading under consideration.
tn = Nominal wall thickness, mm

b. CONSIDERATION OF LEVEL A AND B SERVICE LIMITS (REF. NC3653)
i. STRESS DUE TO SUSTAINED PLUS OCCASIONAL LOADS
The effect of pressure, weight, other sustained loads and oCcasional loads including earthquake, for which level B service limits are designated, must meat the requirements of following eq. (5).


But not greater than 1.5 Sy

Where
Mb = resultant moment loading on cross section due to non reversing dynamic loads e.g. oCcasional loads such as thrust from relief and safety valves loads from pressure and flow transients and earthquake.
Sy = material yield strength at temperature consistent with the loading under consideration, kPa.
Sol = stress due to oCcasional loads, kPa.
Pmax = Peak pressure, kPa

ii. SUSTAINED PLUS THERMAL EXPANSION STRESSES
The effects of pressure, weight, other sustained loads and thermal expansion for which level A and B service limits are designated, shall meet the requirements of following eq. (6).

0.75 i shall not be less than 1.0

Where
Ste = Sustained plus thermal expansion stresses.
MC = range of resultant moments due to thermal expansion
SA = Allowable stress range for expansion stresses.
i = Stress intensification factor (refer NC‐3673.2)

= ratio of bending moment producing fatigue in a given number of cycles in a straight pipe/tube with girth butt weld to that producing failure in the same number of cycles in the fitting or joint under consideration.

Other terms are same as of eq. (4)

Allowable stress range for expansion stresses (SA) can be calculated using following equation
SA =  ƒ(1.25 SC + 0.25 Sh) ……. (7)
SC = Basic material allowable stress at minimum (cold) temperature.
Sh = Basic material allowable stress at maximum (hot) temperature.
f = stress range reduction factor for cyclic conditions for total number N of full
temperature cycles over total number of years during which system is
expected to be in service from table‐2‐1A below NC 3611.2 (e)‐1


Stress intensification factor ‘i’ can be calculated using following equation (8)


Where
C2 and K2 are stress indices for class‐1 piping products or joints from NB 3681 (a)‐1. For straight pipe/tube the value of C2 and k2 are 1.
For curved pipe/tube or welded elbows ‘I’ can be computed as per equation (9) below (refer NB 3681)




where




tn = nominal wall thickness of tube/pipe
R = bend radius
r = mean radius of tube/pipe

iii. CONSIDERATION OF LEVEL C SERVICE LIMITS
In section II in calculating the resultant moment MB, moment due to SEE conditions is proposed to be used which is more conservative, thus separate analysis for level C service limits is not required.

iv. TESTING REQUIREMENTS AS PER SUBSECTION – NB
Requirements of material testing as per subsection NB is briefly mentioned above. In addition to examination/testing requirements as per SA‐655, tubing should be hydrostatically tested at not less than 1.25 times the design pressure with minimum holding time of 10 min.


2.6 ANALYSIS OF SS TUBES USED IN NPCIL
2.6.1 WALL THICKNESS AND PRESSURE RATING OF DIFFERENT SIZES OF INSTRUMENT TUBING
The maximum design pressure and temperature are taken as 195 kg/cm2 and 310oC respectively. Though the above pressure and temperature may not exist simultaneously in any system, still to be on conservative side, all the sizes of tubing will be designed for above ratings.

Using eq. (1) in the analysis criteria above, the minimum wall thickness of straight tubing can be calculated.

Thus following equation can be used



We can make following assumptions
• There will be no threading on the tubes
• Corrosion, erosion is negligible (hence allowance for corrosion and erosion may be neglected)
• Bend radius is not less than 3Do. The actual wall thickness is to be increased as per Table‐2‐1 above.

Following data may be used
P = design pressure (= 195 kg/cm2)
S = maximum allowable stress of S.S. 304L material at 310oC temp. (= 986 kg/cm2)
Y = 0.4

By putting the above variables, the minimum wall thickness for different sizes (Do) of straight tubing is tabulated in following Table‐2‐2.


Note: It can be seen from Tables – 22 & 23 that specified wall thickness of all sizes of tubing as per PBM17 is more than required wall thickness as per ASME Section III except for 16 mm size. As maximum pressure and temperature may not be simultaneous so 1.8 mm wall thickness instead for 1.83 mm of 16 mm size will be adequate from pressure rating considerations.

“For example, the maximum pressure & temperature in PHT system will be 125 kg/cm2 and 310oC respectively. For this application, the required minimum wall thickness for 16mm OD tube, including the bending allowance, should be 1.3 mm, which is less than specified wall thickness of 1.8 mm. Similarly, in some applications like F/M supply circuit, the maximum pressure and temperature may be 195 kg/cm2 and 40oC respectively. For this service also, the minimum required wall thickness including the bending allowance for 16mm OD tube should be 1.62mm which is less than specified wall thickness of 1.8 mm”.

2.6.2 STRESS ANALYSIS OF TUBING SYSTEMS (TUBING CONFORMING TO PBM17)

2.6.2.1 ANALYSIS FOR SUSTAINED MECHANICAL LOADS

When the tubing is installed in the field, the effects of pressure, weights and other sustained mechanical loads must meet the requirements of eq. (4) i.e.


The above equation may be verified for different sizes of tubing having wall thickness as given in Table‐2‐2 and other constants to be calculated/taken as below:

B1 = 0.5 (as per NB – 3680)




and





Where
tn = nominal wall thickness of tube

R = Bend radius
r = (Do – t)/2 = mean radius of tubing



Thus for different sizes of tubing systems Ssl value is tabulated in Table‐2‐4
2.6.2.2 ANALYSIS FOR OCCASIONAL LOADS (LEVEL A&B SERVICE LIMITS)
As per requirement of ASME – Section III installed tubing system should satisfy the equation (5) of Section 4.2.1 as given below:


Based on the seismic analysis carried out for different tubing layouts, the recommended conservative value of Mb is 200 kg mm for all sizes of tubing systems for SSE level of earthquake. Thus for different sizes of tubing systems Sol value is tabulated in Table‐2‐4. This can be seen that Sol is less than 1.8 Sh for all the sizes of tubing thus satisfying the above equation.

2.6.2.3 ANALYSIS FOR STRESS DUE TO THERMAL EXPANSION AND OTHER SUSTAINED LOADS

As per requirement of ASME Section III installed tubing system should satisfy the following equation


The maximum value of stress (iMc/Z) due to thermal loading (temperature variation from 25oC to 310oC) for different tubing systems comes out to be 1600 kg/cm2 provided that tubing system is supported as per recommended practices. Based on the above data and other parameters/constants, Ste has been calculated & tabulated in TABLE‐2‐3 for different sizes of tubing.




This may be seen from the table that Ste value for different sizes of tubing is less than the value of Sh + SA (viz. 2615 kg/cm2).


Note:
1. The values of MA, Z, P, Sh used for calculation of STE are same as given in Table24.
2. The value of �� used is based on requirement such that 0.75 �� should not be less than 1.0
3. SA = f (1.25 Sc + 0.25 Sh) where f = 1 & Sc = 1106 (kg/cm2)



2.7 Consideration for various forces
The design of tubing/piping systems for sensing lines should take account of all the forces and moments resulting from thermal expansion and contraction and from the effects of expansion joints if any.


2.8 Tube Bending Considerations
Bend radius in instrument tubing/piping should be subject to following limitations;

i) Minimum wall thickness at any point in the completed bends should not be less than required minimum wall thickness for the design pressure.
ii) The ovality of instrument tubing/piping after bending should not exceed

8% as calculated below:

 Where –
Do = Nominal O.D. of tube/pipe
Dmin = The min. outside diameter of tube/pipe after bending
Dmax = The max. outside diameter after bending
The above requirements are met if bend radius is more than 3Do.
2.9 Special design aspects to meet the requirements of class-I tubing and tubing systems

In addition to the general requirements of impulse connections as mentioned above, the following requirements should also be met for impulse connections for pressure/differential pressure measurement in safety and safety related systems. For safety and safety-related systems the safety classification of instrument sensing lines including the first accessible isolating valves should at least remain the same as that of process systems, and from the valves up to instruments they should meet at least the requirements of ANSI-B-31.1.

SS tubes should meet the design intent of ASME Section III sub-section NB/NC.

For seismic classification the instrument sensing lines should be of SSE Category for safety and safety-related instrumentation systems.

A single instrument sensing line should not be used to perform both a safety-related function and a non safety-related function unless the following can be shown:

a. The failure of the common sensing line would not simultaneously

1. cause an action in a non-safety-related system that results in a plant condition requiring protective action and
2. also prevent proper action of a protection system channel designed to protect against the condition.
Tubing system should be such that the failure of non safety impulse line/tubing should not affect the reading of safety system.


2.10 CONCLUSION

1) MATERIAL SELECTION
    a. Based on the requirements of corrosion resistance, tensile strength, hardness and weldability, austenitic stainless steel grade SS-304L material as per ASTM A-213/SA655 has been selected and specified for instrument tubing. Also the instrument SS tubing should be seamless, cold finished and
full annealed. From welding consideration the tubing should have delta ferrite of 5 to 10%.
    b. Based on the requirements of different applications the tubing in different sizes have been specified i.e. OD of 6mm, 10mm, 12mm, 16mm, 20mm and 25mm.

2) NON-DESTRUCTIVE INSPECTION
All finished tubing should be inspected by ultrasonic or eddy current methods or any combination of these methods in accordance with the requirements of NB-2550.

3) Based on the analysis of tubing systems carried out above for our installations the stress values for different loading (service limits) are well within the required limits.

4) Thus, if SS 304L instrument tubing are supplied as per specification above and installation of tubing systems is done as per recommended practices(see section-10) then instrument impulse tubing systems will be meeting the intent of ASME Section III-Sub-Section NB-Class I components.

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