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.

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).

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.

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

D

_{O}= 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.

value of ‘Y’
for ferritic and austenitic steels designed for temperature of 480 oC and below
should be taken as per eq. (2) below

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).

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

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, mm

^{3}
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).

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).

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)

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.

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”.

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)

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

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.

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 25

^{o}C to 310

^{o}C) 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)

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.

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:

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.

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.

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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