piping flexibility – a detailed approach

7/28/2019 PIPING FLEXIBILITY – A DETAILED APPROACH

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PIPING FLEXIBILITY – A DETAILED APPROACH

(PIPE STRESS ANALYSIS)

INTRODUCTION:

Many prefer to do quick checks of the system with the older methods of analyzing pipe

systems. These methods vary extensively and, excluding the high computational time content, some

are quite accurate. It should be noted that the results of such systems can be quite varied. In the

1950s when the systems were all that was available, Heating, Piping, and Air Conditioning magazine

ran a series of articles where many of the then-competing systems were pitted against a common

piping system to compare the results. A brief discussion of the results follows, and for those more

interested, there is a more detailed discussion of these hand-type calculations in the Appendix.

There were 12 different methods employed. The average result was a 3062-lb force with a

standard deviation of 391 lbs and a range from the low resultant to the high of 1040. The percent

deviation represents a ± of 20 percent from the average. That is not all that impressive but it is about

what the state of the art was at that time. Given the many variables that any model might require in a

complex system, it might be quite good enough.

In an interesting book, Introduction to Pipe Stress Analysis, Author - Mr. Sam Knappan uses a

graph (see Figure 7.3) that shows the variety of results from the various pipe flexibility systems

available in the 1980s. While the principles remain the same, the actual code values, formulas, and

rules have had several opportunities to change in the last three decades. It is advisable to consult the

current codes for specifics.

It should be pointed out that there were no PC-type flexibility pro-grams available when

Knappan’s book was written and the computer analyses sited may no longer be extant. In fact, some

of the “HAND” methods cited may not be available either. The current crop of computer programs

gives comparable results.

One of the difficult elements in any engineering system is to determine the degree of

accuracy that precision gives as opposed to the reality that precision accomplishes by precise

calculations. Assumptions and variables often are the more critical elements.

As was pointed out, this is really a rather sophisticated ratio analysis. Therefore, it does not

give supposed absolute values, which would be required in any formal code check. We will use the

same configuration with a more accurate method known as the Spielvogel method. This method was

used in the early study, and it was well within a standard deviation of 0.6 of the average of the

methods used in the articles discussed earlier. I might add that the average is a little suspect as many

of the methods have been abandoned. This is not totally true of Spielvogel. It is known that some



testers use it to check new releases of current laptop programs. They also use it for simple

configurations.

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:

Range calculation stress because of displacement in every point of piping system

should not exceed the allowable stress.

The calculation of reaction force should not damage to support or connected

equipment in piping system.

The calculation of displacement should not exceed the limitation range in ASME Code

B31.3

CODE ASME B31.1 FOR PIPING STRESS ANALYSIS:

Piping stress analysis can use code ASME B31.1 as reference calculation and design. Code

ASME B31.1 for power piping analysis is belonging to the American society of mechanical engineers.

In code ASME B31.1 there are empirical formulas that apply to the sustain load, expansion load, and

the combination of sustain and expansion load (during operations), and also

occasional load are described as the following below:

SUSTAIN LOAD

The stresses (S) that occur due to sustain the load such as pressure, weight and other

mechanical loads can be expressed by the equation as follows:



(P Do / 4 tn) + 1000(0.75 i Ma / Z) ≤ 1.0 Sh

RANGE THERMAL EXPANSION LOAD

The stresses that occur due to thermal expansion can be expressed as the equation below:

SE = 1000(i Mc / Z) ≤ SA + f(Sh – SL)

COMBINATION LOAD OF SUSTAIN LOAD AND THERMAL EXPANSION LOAD

The stresses due to combination of sustained load and thermal expansion load (Sls + SE),

can be calculated with the equation:

Sls + SE = (P Do / 4 tn) + 1000(0.75 i Ma / Z) + 1000(i Mc / Z) ≤ (Sh + Sa)

OCCASIONAL LOAD

The stresses that occur due to pressure, weight, and other sustain load can be expressed asthe equation below:

(P Do / 4 tn) + 1000(0.75 i Ma / Z) + 1000(i Mc / Z) ≤ K Sh

Where:

P = internal design pressure (psi)

Do = outside diameter (in)

Ma = Moment due to sustain load (in-lbs)

Mb = Moment due to occasional load (in-lbs)

Mc = Range of the moment due to thermal expansion (in-lbs)Z = section modulus of the pipe (in3)

tn = nominal wall thickness of pipe (in)

i = stress intensification factor

K equal to 1.15 for the occasional load which work less than 1% of the operating period and is equal to

1.20 for the occasional load which work less than 10% of the operating period.

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:

Piping system is a duplicate of an existing piping system, which is in operation

showed a satisfactory performance.

Piping system can be judged easily that has sufficient flexibility when compared with

the flexibility of existing piping system which had been analyzed before.



Piping system with uniform size, which is supported by only two supports without

any restraint point between them and meet the empirical equation as following

below:

(Dy / (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: 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.

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.

The factors of stress intensity in piping component except straight pipe must be

taken into calculation in comprehensive analysis method.

In the flexibility analysis, all of piping components between two anchor points must

be analyzed as a complete system.

PIPE WALL THICKNESS:

Pipes are produced in various thicknesses that have been standardized. Each pipe wall

thickness is given a name pipe in the form of schedule number, not in the form of actual pipe wall

thickness.

Initially, the pipe wall thickness is classified into three groups as following below:

Double Extra Strong (XXS)

Extra Strong (XS) Standard

Currently the naming of pipe wall thickness has been replaced with schedule which providing certain

number, starts from 5 and 5S, then followed with 10 and 10S, so in multiples of 10 to Schedule 40 (20,

30, 40), and then have multiple of 20 such as pipe wall thickness schedule 60, 80 , 100, 120, 140, 160.



The pipe wall thickness which has schedule 40 is generally same with schedule STD pipe sizes

1/8 inch up to 10 inch. Likewise, schedule 80 is the same as the schedule XS for pipe sizes 1/8 inch up

to 10 inch.

One thing should be considered is the use of pipe that has Schedule 5 and 10 are more

widely used on stainless steel pipe. While the pipes which are classified as Small Bore, usually have

minimum pipe wall thickness of Schedule 80, although it may have thickness more than that, it's just

going to make a pipe becomes excessive strength than is needed.

Pipes are usually produced with having different length depending on the material, size

and schedule. But in general the pipes are produced with having average length of 20 feet or 6 feet

for pipe Carbon Steel. This length is also called as random length. Sometimes pipe which having

length twice than random length are also widely available and include preferred especially for use in

the pipe rack. This size is also called as Double Random Length, or equal to 12 meters.

piping flexibility – a detailed approach

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