Wind Load on Occasional Load

Wind load is one of occasional load which is caused against the pipe; the force that occurs can be calculated using equation below. Magnitude of the wind load is the result of momentum wind on pipe. Wind load is modeled as uniform force in the direction of wind along pipe. Wind force that occurs can be calculated by using Bernoulli equation as follow:

F = C_d D q / 1000

Where:

F          = win load (N/m)

C_d        = drag coefficient

D         = outside diameter of pipe include insulation (mm)

q          = dynamic pressure (N/m²)

Occasional load that occurs in piping system in certain cases will have similar properties with sustain load. Therefore stress analysis of support will be important step to get distribution of stress along pipe. However, the optimum support position to withstand occasional load is not always same for sustain load. In the design process, compromise design is required so support can hold both load type, sustain load and occasional load. For example, the rigid support can be used to hold dynamic load but will reduce flexibility of pipe, so need other type of support. The snubber support type is suitable support and generally used to hold both type of loading.

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.