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

Piping System Definition

Pipe is used to flow fluid either in liquid or gas from one point to other point. Piping system consist of combination pipes which have relatively short total length and used to flow fluid from an equipment to other equipment which operates on a plant. Piping system is also equipped with components such as valves, flanges, bends, branches, nozzles, reducers, support and so on.

Piping has differences with the pipeline among others of their function, location of use and the total length. In the explanation above, piping is used to flow fluid between equipment which operate in a plant while pipeline has more function as a transmission and distribute fluid from one region to other region.

Figure 1: Piping System in a Plant

Stress in Piping System

The theory of stress on the piping system is development of the theory of stress that has developed in mechanics. Definitions that are used as force, moment, stress, strain, and others are same with the definitions used in mechanics. Stress that occurs in piping system can be caused by internal pressure, external pressure, weight of pipe itself, and thermal expansion load in the piping system because of temperature differences. At both pipe stress analysis and mechanics, some terms of stress had been known based on direction as follow:

1. Longitudinal stress, the stress along the axis parallel to the direction of the pipe.
2. Circumferential stress or hoop stress, the stress with circular direction and direct with circular wall pipe.
3. Radial stress,   the stress with a direction parallel to the straight line that runs to the axis of pipe and out through the wall pipe.

Stresses that occur in piping system will be totaled and analyzed to take the largest and dominant value of stress to be used as input for stress analysis on piping system. Based on stress equation, from the three type stress above, radial stress has relatively small value, so sometimes that value can be ignored. Longitudinal stress has the largest and dominant value of stress than others so it can be included as an input for the stress analysis. Stress analysis is performed at all points along the pipeline to generate distribution of stress value along the pipeline.

Thermal Expansion Load in Piping System

Thermal expansion load is the load incurred as a result of thermal expansion in piping system. Thermal expansion load can be divided as follow:

-          Thermal expansion load which is caused movement is restricted by support during expansion.

-          Thermal expansion load because of high difference of temperature and working very fast in the wall pipe so it can impact to stress on the wall pipe.

-          Thermal expansion load because different coefficient of expansion between different material.

Support of pipes is installed along piping system to withstand both occasional load and sustain load. However if the increase of temperature in the piping system occur during operation, the pipe will experience expansion so produce high stress either in the fitting are or rigid support area. In this case, location and type of support must be well designed to get optimum analysis stress during operation. The use of loop for expansion is an alternative solution to overcome the large thermal expansion.

Analysis of thermal load calculation on the support can use guide cantilever method in which pipe is modeled as a beam which is held rigidly at one of edge and at some point it is given support, so the amount of stress can be calculated on that support. Force and moment that occur due to thermal expansion load can be seen the equation below:

P = 12 E I Δ / L³

M = 6 E I Δ / L²

Where:

P          = Force at support (N)

M         = Moment at support (N/m)

E          = Modulus elasticity (Pa)

I           = Moment area inertia (m⁴)

Δ          = Additional length because thermal expansion (m)

L          = Length of pipe (m)

Occasional Load in Piping System

Occasional load is the load that occurs “sometimes” in the piping system during normal operation. Occasional load can be interpreted also as a load on the piping system that occurred in the period a part of the total period piing system operation, for example 1 up to 10% of the operating period of piping system. There are several things that can cause occasional loads, namely:

1. Natural phenomena such as hurricanes and earthquakes will cause excitation force against the pipe that is dynamic. Dynamic analysis on piping system is needed to get the stress distribution caused by dynamic loads which occur in piping system.
2. Snow, occur in the piping system located on the earth which experience winter. Concentration buildup of snow is very thick in certain parts along the piping system will lead to the imposition of excessive weight that must be retained by the piping system.
3. Postulate plant accident, an accident occurred on the piping system that arise by certain causes whether from the operator or from a third party (third party damage).
4. Unusual plant operation, an error occurs in the operating conditions are made possible by negligence of the operator or procedural error in operating the piping system.

Sustain Load in Piping System

Sustain load is the load experienced by the installation of piping system continuously. This load is combination of the load caused by internal pressure and weight load. In the all of piping system, pipe is designed to be able to withstand weight loads of fluid, insulation, components, and structure of the pipe itself. All of the weight loads are then distributed to the pedestal component or support, so it must be done also on support design of piping system that can support the weight load. Simple method to calculate the stress and the load on the support (pedestal) is by modeling the pipe as a beam with uniform load distribution along the dimension of the pipe.

Modeling support type to withstand weight loads are part of the pipe stress analysis on the piping system. In general there are two type of support which is used in the piping system, they are simple support and fixed support. But in reality, support condition generally is a simple support that fixed on the one part, so that the maximum stress that occurs can be calculated using equation as follow:

σ = W L² / 8Z

Where:

σ          = stress (Pa)

W        = weight of the piping system (Newton)

L          = length of pipe (m)

Z          = inertia of the pipe (m⁴)

Weight load experienced by the piping system can be classified into two types as follow:

-          Live Load

Live load includes weight of fluid flowing through the piping system or other fluid used for testing the piping system

-          Dead Load

Dead load includes the weight of the components of the piping system, insulation, and permanent weight acting on the piping system.

Loads on Piping System

Operation of piping system receive vary much and complex loads which includes sustain load, occasional load, and expansion load. Calculate load is the important action to perform pipe stress analysis. Each occurred load in the piping system is caused by input load which has different value in each point and also caused by environment load such as wind load, snow load, seismic load and so on. Each component of load either due to operating condition or environment must be considered when doing pipe stress analysis to get safely piping system design.

Pipe stress analysis is intended to ensure that piping system is safe for operation and verify structure support can resist existing load in the piping system. In addition, pipe stress analysis is also performed to determine the pedestal pipe load so the piping system can be properly supported. This condition can be carried out by doing calculation and comparison to allowable parameters value as follow:

-          Stress on the pipe wall

-          Displacement due to pipe expansion

-          Loads on the nozzle

-          Personal frequency system

Loads on the piping system can be classified based on type of load that occur as follow: sustain load, occasional load and thermal expansion load.

Code Standard for Design Piping System

Piping system must pay attention to the good of plan qualification both in technical term and economical terms. Qualification plan of piping system in term of mechanical can be found by doing some analysis, such as determination of pipe wall thickness, hydraulic analysis, pipe stress analysis, and analysis flexible pipe. While in terms of economical, plan of qualification piping system depends on the financial policy of the company or industry but keeping in terms of eligibility based on the mechanics that have been set in the Code Standards. This condition is to ensure the security and use current piping system plan which is operated safely for the environment.

Planning piping system can use rules code and standard. Code and standard that are applicable in the planning piping system, namely:

a. ASME B31.9 Building Services Piping
b. ASME B31.8 Gas Transmission & Distribution Piping
c. ASME B31.7 Nuclear Power Piping
d. ASME B31.5 Refrigeration Piping
e.ASME B31.4 Liquid-petroleum transportation piping system
f. ASME B31.3 Process Piping on Petroleum Refineries, Chemical, Pharmaceutical, Textile, Papper, Semiconductor, and Crycogenic Plant.
g. ASME B31.2 Fuel Gas Piping
h. ASME B31.1 Power Piping
i. API 5L Spesification of Line Pipe Material
j. API 576 Pipeline Coating
k. DnV 1981 Rules For Submarine Pipe Systems
l. DnV RP F105 Free Spanning Pipelines
m. DnV RP E305 On Bottom Stability Design Of Submarine Pipeline
n. ANSI B16.5 Pipe Flange and Flange Fitting

Piping System History before Century

Piping system is a transportation system that humans use to flow the fluid either in the form of a liquid phase or gas phase from one place to other place to their need. Use of the piping system is started since 2700 years BC to flow water from springs to urban areas. At that time the material used comes from non metallic material which joins between two non metallic materials with asphalt. At 2400 BC in Egypt introduced the use of pipes with metal material, it is copper.

The well-known history of the use of piping systems in ancient times is the Romans time. They flow water that is often referred to as the aquaduct. Length of piping system which is used is approximately 250 miles and had used valve and stopcock
to regulate water flow. Most pipes are made from manufacturing processes such as rolling and welding. Material of Valve and other pipe material is made from bronze. Julius Frontinus was a Roman made standard dimensions and materials to make pipe for use at that time as the table below.

Table 1: Pipe Name in Roman before Century