Flexibility Analysis Based on ASME B31.3

Based on code ASME B31.3, flexibility analysis in the piping system needs formal analysis or does not need formal analysis. Piping system can be does not need formal analysis it meet the following requirement:

1. Piping system is a duplicate of an existing piping system, which is in operation showed a satisfactory performance.
2. Piping system can be judged easily that has sufficient flexibility when compared with the flexibility of existing piping system which had been analyzed before.
3. Piping system with uniform size, which is supported by only two support without any restraint point between them and meet the empirical equation as following below:

(D_y / (L-U)²) ≤ K₁

Where:

K₁        = 208000 SA/Ea (mm/m)²

U         = anchor distance, straight line between anchor, m

L          = developed length of pipe between anchor, m

Y         = resultant of total displacement strain to be absorbed by piping system, mm

D         = outside diameter of pipe, mm

Ea        = reference modulus of elasticity at 201 C (700 F), Mpa

SA       = allowable displacement stress range, Mpa

While a piping system can be said require for analysis of flexibility when meeting the following requirements:

1. Piping system that does not meet of the three requirements above must be analyzed by one of methods analysis of the following: simple analysis method, approximate analysis method or comprehensive analysis method.
2. Comprehensive analyses that are acceptable include analysis method and method that use charts which can calculate force, moment and stress which is caused by the displacement strain.
3. The factors of stress intensity in piping component except straight pipe must be taken into calculation in comprehensive analysis method.
4. In the flexibility analysis, all of piping components between two anchor points must be analyzed as a complete system.

Flexibility Analysis in Piping System

Flexibility analysis is an analysis of the ability of pipe to change its length and deform elastically. This condition occurs because of load which is affected by high temperature during operation in piping system. Piping system must be enough flexible so thermal expansion or movement of support or end point of pipe will not cause as follow:

-          Failure on pipe and support due to excessive stress.

-          Leakage in welded joint pipe.

-          High stress or distortion which cause damage to connected equipment such as pump, tank or valve because excessive force and moment in the pipe.

Furthermore, if the piping system has enough flexibility, so the pipe will experience changes in length due to thermal expansion or contraction and able to return to the initial length when the load due to expansion or contraction is eliminated.

In the Code ASME B31.3 flexibility analysis in the piping system is regulated in paragraph 319.4. Code ASME B31.3 specify special requirement of flexibility in the piping system as follow:

1. Range calculation stress because of displacement in every point of piping system should not exceed the allowable stress.
2. The calculation of reaction force should not damage to support or connected equipment in piping system.
3. The calculation of displacement should not exceed the limitation range in ASME Code B31.3