REQUIRMENTS OF ASME B31.3 (PROCESS PIPING CODE): 

This code governs all piping within the property limits of facilities engaged in the processing or handling
of chemical, petroleum or related products. Examples are a chemical plant, petroleum refinery, loading
terminal, natural gas processing plant, bulk plant, compounding plant and tank farm.
The loadings required to be considered are pressure, weight (live and dead loads), impact, wind,
earthquake-induced horizontal forces, vibration discharge reactions, thermal expansion and contraction,
temperature gradients, anchor movements.

The governing equations are as follows:


1.Stresses due to sustained loads.

SE < SA

WHERE: SE = (Sb^2+ 4St^2)^1/2

Sb = resultant bending stress, psi

= [(IiMi)2 + (IoMo)2] / Z

Mi = in-plane bending moment, in.lb
Mo = out-plane bending moment, in.lb
Ii = in- plane stress intensification factor obtained from appendix of B31.3
Io = out- plane stress intensification factor obtained from appendix of B31.3
St = Torsional stress ,psi
= Mt / (2Z)
Mt = Torsional moment, in.lb

SA = Allowable displacement stress range:

(Allowable stress) cold = Sc = (2 / 3) Syc ⇒ Syc = (3/2)Sc
(Allowable stress) hot = Sh = (2 / 3) Syh ⇒ Syh = (3/2) Sh

Syc = yield point stress at cold temperature
Syh = yield point stress at hot temperature

Allowable stress =Syc + Syh
=3/2 (Sc + Sh )
= 1.5 (Sc + Sh )
= 1.25(Sc + Sh )---- after dividing with F.O.S
Final allowable stress = [(1.25(Sc + Sh) – SL

SA = f [(1.25(Sc + Sh) – SL

Sc = basic allowable stress at minimum metal temperature
f = stress range reduction factor from table 302.2.5 of B31.3

STRESS CATEGORIES
The major stress categories are primary, Secondary and peak.

PRIMARY STRESSES:
These are developed by the imposed loading and are necessary to satisfy the equilibrium between external
and internal forces and moments of the piping system. Primary stresses are not self-limiting.

SECONDARY STRESSES:
These are developed by the constraint of displacements of a structure. These displacements can be caused
either by thermal expansion or by outwardly imposed restraint and anchor point movements. Secondary
stresses are self-limiting.

PEAK STRESSES:
Unlike loading condition of secondary stress which cause distortion, peak stresses cause no significant
distortion. Peak stresses are the highest stresses in the region under consideration and are responsible for
causing fatigue failure.

  • CLASSCIFICATION OF LOADS
    Primary loads:
    These can be divided into two categories based on the duration of loading.
  • Sustained loads

These loads are expected to be present through out the plant operation. e,g. pressure and weight.

  • Occasional loads.
    These loads are present at infrequent intervals during plant operation. e,g. earthquake, wind, etc.
  • Expansion loads:
    These are loads due to displacements of piping. e,g .thermal expansion, seismic anchor movements, and
    building settlement.
  • HOW PIPING AND COMPONENTS FAIL (MODES OF FAILURES)
    There are various failure modes, which could affect a piping system. The piping engineers can provide protection against some
    of these failure modes by performing stress analysis according to piping codes.
  • FAILURE BY GERNRAL YIELDING: Failure is due to excessive plastic deformation.
  • Yielding at Sub Elevated temperature: Body undergoes plastic deformation under slip action
    of grains.
  • Yielding at Elevated temperature: After slippage, material re-crystallizes and hence yielding
    continues without increasing load. This phenomenon is known as creep.
  • FAILURE BY FRACTURE: Body fails without undergoing yielding.
  • Brittle fracture: Occurs in brittle materials.
  • Fatigue: Due to cyclic loading initially a small crack is developed which grows after each cycle and
    results in sudden failure.WHEN PIPING AND COMPONENTS FAIL (THEORIES OF FAILURE)
    Various theories of failure have been proposed, their purpose being to establish the point at
    which failure will occur under any type of combined loading.
    The failure theories most commonly used in describing the strength of piping systems are:
  • Maximum principal stress theory
    This theory states that yielding in a piping component occurs when the magnitude of any of the
    three mutually perpendicular principle stresses exceeds the yield point strength of the material.
  • Maximum shear stress theory
    This theory states that failure of a piping component occurs when the maximum shear stress
    exceeds the shear stress at the yield point in a tensile test.
    In the tensile test, at yield, S1=Sy (yield stress), S2=S3=0.So yielding in the components occurs
    when
    Maximum Shear stress =τmax=S1-S2 / 2=Sy / 2

5 STRESS ANALYSIS

5.1 General

  • Stress analysis shall be performed according to ASME B31.3 para. 319.4.
    5.2 Selection criteria for lines subject to comprehensive stress analysis:
    As a general guidance, a line shall be subject to comprehensive stress analysis if it falls into any of
    the following categories:
  • All lines at design temperature above 180°C.
  • 4" NPS and larger at design temperature above 130°C.
  • 16" NPS and larger at design temperature above 105°C.
  • All lines which have a design temperature below -30°C provided that the difference between the maximum and minimum design temperature is above:
    190°C for all piping
    140°C for piping 4" NPS and larger
    115°C for piping 16" NPS and larger
  • Note: These temperatures above are based on a design temperature 30°C above maximum operating temperature. Where this is not the case, 30°C must be subtracted from values above.
  • Lines 3" NPS and larger with wall thickness in excess of 10% of outside diameter. Thin walled piping of 20" NPS and larger with wall thickness less than 1% of the outside diameter.
  • All lines 3" NPS and larger connected to sensitive equipment such as rotating equipment.
    However, lubrication oil lines, cooling medium lines etc. for such equipment shall not be selected due to this item.
  • All piping subject to vibration due to internal forces such as flow pulsation and/or slugging or external mechanical forces.
  • All relief lines connected to pressure relief valves and rupture discs.
  • All blowdown lines 2" NPS and larger excluding drains.
  • All piping along the derrick and the flare tower.
  • All lines above 3" NPS likely to be affected by movement of connecting equipment or by structural deflection.
  • GRE piping 3" NPS and larger.
  • All lines 3" NPS and larger subject to steam out.
  • Long vertical lines (typical 20 meter and higher).
  • Other lines as requested by the stress engineer.
  • All production and injection manifolds with connecting piping.
  • Lines subject to external movements, such as abnormal platform deflections, bridge movements,
    platform settlements etc.

5.3 Design temperature:
The design temperature for the selection of lines subject to stress analysis shall be as stated on the P&ID's/line lists.
Calculation of expansion stress shall be based on the algebraic difference between the minimum and maximum design temperature. The maximum design temperature shall not be lower than the maximum ambient temperature.
Reaction forces on supports and connected equipment may be based on the maximum algebraic difference between the installation temperature and the maximum or minimum design temperature.

For uninsulated lines subject to heat sun radiation, 60°C shall be used in the calculations, where this is higher than the relevant maximum design temperature.

5.4 Environmental temperature:
The minimum/maximum environmental temperature shall be as specified by the project. Unless otherwise specified, the following environmental temperatures shall apply:
• Installation temperature: 0°C
• Min. ambient temperature: -40°C
• Max. ambient temperature: 36°C
5.5 Design pressure
The design pressure for the piping system shall be as stated on the P&ID's/line lists. Where internal pressure below atmospheric pressure can exist, full vacuum shall be assumed for stress calculations.
5.6 Vibration
The effects of vibration imposed on piping systems shall be evaluated and vibration sources which can be realistically determined shall be accounted for. This also includes acoustic induced vibration.

5.7 Loads:
Environmental loads such as snow, ice and wind acting on exposed piping shall be evaluated. When affecting the integrity of the piping system, the imposed deflections or movements from the main structure shall be accounted for.

Process conditions which may result in impulse loadings, such as surge, slugging, water hammering, reaction forces from safety valves and two phase flow, shall be included in the calculations.

The effect of blast loads shall be evaluated, for piping which is required to maintain the integrity in
an explosion event.