Alexandria University

Faculty of Engeneering

Mechanical Dep.

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Table of Contents

Introduction ..................................................................................................... 3

Leakage Phenomena ....................................................................................... 4

1. Volume / Mass balance detection method .............................................. 5

2. Pressure drop detection method .............................................................. 6

2.1. Pressure point analysis .................................................................... 6

2.2. Pressure profile analysis .................................................................. 7

3. Pigs .......................................................................................................... 7

4. Computational Modeling of Pipeline Systems ....................................... 8

5. Photographic observations method ........................................................ 9

6. Ground penetrating radar ........................................................................ 9

7. Noise based leak detection method ....................................................... 10

Leak Noise Detection ................................................................................ 10

The Sounds of Water Leaks ...................................................................... 10

Factors Affect These Sounds .................................................................... 10

Water in the pipe ........................................................................... 10

Pipe material and pipe diameter .................................................... 11

Soil type and soil compaction ....................................................... 11

Depth of soil over the pipe ............................................................ 11

Surface cover ................................................................................. 12

How Do Leak Sounds Travel on Pipes ..................................................... 12

How Do Leak Sounds Travel Through Soil ............................................. 12

How to detect the exact leak location ....................................................... 14

Surveying ...................................................................................... 14

Pinpointing .................................................................................... 14

The acoustic method for detecting leaks in Gas Pipelines ....................... 15

8. Dogs ...................................................................................................... 16

References ..................................................................................................... 17

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Introduction

Our need of transporting fluid from one point to another led to an increase in

pipe line numbers, many of them carry toxic and hazardous products, often

close to the center of high population or through area of high environmental

sensitive.

The leakage or rupture of a crude oil or petroleum product pipeline is so

serious because it posses a threat to both the environment and public health

due to the pollution of surface water, groundwater, and soil.

Product pipelines such as those transporting gasoline are more dangerous

than crude oil pipelines due to the high volatility of the product and the

resultant danger of fire and explosion.

Leak detection systems range from simple visual line walking and checking

to complex arrangement of hard ware and software.

No one method is universally applicable and operating requirements dictate

which method is most suitable.

There are four main categories of pipe line failure:

Pipe line corrosion and wear.

Operation outside design limits.

Unintentional third party damage.

Intentional damage.

Many pipe line systems are operated for number of years with no regard to

any possible mechanical changes occurring in the line, Some of the products

may be corrosive, the pipe line may be left partially full for period of time or

atmospheric effect may cause external damage, these three reasons

responsible for pipe line corrosion and this may give rise to corrosion

developing along the line, this may cause material imbalance over period of

time.

Operation outside design guidelines is more common than is realized, as

operators seek to use the line for as many fluids as possible.

If the line is designed for a certain maximum pressure (and / or) temperature

then operation at higher pressure (and / or) temperature could lead to failure.

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Unintentionally third party damage can occur if an excavation or building

occur near buried lines and this kind of damage is mostly caused by the

machinery in the site.

Some other pipe line failure reasons is due to soil movement due to

earthquakes , wash out due to flood , landslides , frost , lightening , ice ,

snow , high winds , and operator error .

The cost of failure to detect leakage falls into four main areas:

Loss of life and property.

Direct cost of loss products and lie downtime.

Environmental cleanup cost.

Possible fines and legal suits.

These are all explanatory with the most costly of them being the last

however all of the Four areas being costly.

The cost may cost many millions of dollars so the cost of detection is small

compared to this cost.

Leakage Phenomena

When leakage occur in a pipe line measured pressure downstream of the

leak falls but the pressure at the same location is predicted to rise. The first

is not difficult to understand as the line is depressurized as mass leaves

through the leak the second effect can be explained as follow. The equation

predicted pressure based on measured flow based on measured pressure as

mass leaves the system through the leak hole and reduces the flow at the

downstream end is compared to the inlet flow. This may not have changed

and so to balance the system the equation predicts a downstream pressure

rise.

In physical terms the model thinks to line is ''packing'' and total system

inventory is increasing, there is therefore a divergence between

measurement and modeled pressure

The same is true of flow changes, but here the inlet flow could increase due

to lower pipe flow resistance between eaters the leak instead of passing

through the meter thus a real imbalance will result. The model however will

show an inconsistency since the pressure comparison will indicate line

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packing and the flow comparison a line unpacking. If selected pressure and

flow imbalance and the location from pressure profile imbalance and the

flow leak indicator. The impact of instrument accuracy is important from the

leak detection. It is vitally important to good leak sizing and location to have

the best pipe line instrumentation possible to minimize uncertainty.

1. Volume / Mass balance detection method

The mass-balance method, also referred to as the materials balance method,

is straightforward. It uses the continuity equation of one-dimensional flow

between an upstream point and a downstream point to calculate the amount

of flow due to leakage or rupture. For instance, if in a natural gas pipeline

the mass flow rate upstream is m1 and the mass flow rate downstream is m2,

and if there are no branches to divert the flow between the two points, then

the leakage flow rate is simply:

mL = m1 – m2

Use of this method requires accurate measurements of the mass flow rates

m1 and m2, for the leakage flow is determined from the difference between

the two quantities. Since even the newly calibrated flow-meters can have

more than 0.5% errors, it is difficult to detect leakage flow from this method

when the leakage rate m1 is much less than 1% of the flow in the pipe. This

limits the usefulness of this method to large leakage flow - 1% or more of

the flow rate in pipe.

Another problem is that since flow-meters are spaced at great distances apart

along a pipeline, sometimes more than 100 km apart, the method does not

indicate where the leak is located within such a long distance. Other

techniques are also needed to confirm and locate the leak.

Advantages:

- Simple to use.

- Simple to install.

- Low computational requirements. (If using simple steady state Inventory)

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Disadvantages:

- Susceptible to false alarms.

- No leak location.

- Limits the usefulness of this method to large leakage flow. (1% or more of the flow rate in pipe).

2. Pressure drop detection method

Leakage in a pipeline can also be detected by sudden pressure drop along the

pipe measured by pressure transducers. Again, the usefulness of this method

is limited by the accuracy of pressure transducers and by the spacing

between the transducers.

Because the pressure drop along a pipe having turbulent flow is proportional

to the square of the discharge Q, the relative pressure drop caused by

leakage is double that of the relative drop of the discharge due to the same

leakage.

For example, if a leak causes a 2% decrease in discharge Q or mass flow rate

m in the pipe, the corresponding pressure drop will be 4% and so forth. Due

to this amplification factor, and due to the fact that pressure transducers cost

less than flow-meters for large pipes, and can be tapped relatively easily

along the pipe at close intervals, the pressure-drop method is useful

especially when the spacing between transducers is small.

In spite of this, the method has its own shortcomings compared to the flow-

meter (mass-balance) method in that it needs more frequent calibration, it is

more susceptible to local disturbance such as caused by imperfect tapping,

and it requires more frequent maintenance.

2.1. Pressure point analysis

Simple analysis of the pressure points due to a leak devoloping in the

pipeline.

Figure 1: Pressure point analysis

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Advantages:

- Fast Response

- Simple to install

Disadvantages:

- Susceptible to False

- Alarms No Leak Location

2.2. Pressure profile analysis

Simple analysis of the pressure resulting from a leak developing in the

pipeline.

Figure 2: Pressure profile analysis

Advantages:

- Fast Response

- Simple to install

Disadvantages:

- Susceptible to False Alarms

3. Pigs

Pipe lines pigs are frequently used for pipe line commissioning, cleaning,

filling, dewaxing, batching, and more recently pipe line monitoring. This last

type of pig can be designed to carry a wide range of surveillance and

monitoring equipment and can be used at regular intervals to check internal

conditions rather than continuously monitoring the line.

Data, however, can be built up over a period of time to provide a history at

the line. This information can be used to predict or estimate when

maintenance, line cleaning, or repairs are required.

If a leak is detected, for example, by flow meter imbalance, the location can

be found by using a pig with acoustic equipment on board. This will alarm

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when the detection equipment output reaches a maximum and the precise

location of pig can be confirmed by radio transmitters also mounted on

board.

Pigs require tracking because they may become stuck, at a point of debris

build-up. Pigging should be carried out at a steady speed, but occasionally

the pig may stop and start, particularly in smaller lines.

Information on when and where the pig stop is therefore important in

interpreting the inspection records. Pig tracking is not new and may such

proprietary systems exist.

In the best systems, however, a picture of the line is often programmed in so

that outputs from junctions, valves, crossovers ad other geometries act as an

aid to location. Pig tracking can make use of the acoustic methods discussed

earlier. When the sealing cups at the front of the pig encounter a weld,

vibrational or acoustic signal are generated. Each pipe line therefore has its

characteristic sound pattern. When a crack occurs this pattern changes from

the no-leak case and the location can be found from direct comparison. The

technology has become so advanced that information on dents, buckles,

ovality, weld penetration, expansion and pipe line footage can be generated.

The equipment often simple, consisting of sensor, conditioning, and

amplifier circuits and suitable output and recording devices. Such a device

developed by British Gas. The range of detection is dependent on the pipe

line diameter and the type of pig.

4. Computational Modeling of Pipeline Systems

Both the mass-balance method and the pressure-drop method discussed

above have advantages and disadvantages, and have about the same

accuracy and reliability. Because of this, and because of the fact that most

pipelines already have both flow-meters and pressure transducers, it makes

sense to use both methods to enhance the reliability of leak detection to

confirm real leaks and to reduce false alarms. Both methods can and should

be incorporated into a common SCADA program that runs the pipeline.

This is usually done by setting up a system of equations on the computer,

based on fluid mechanics and input data pertaining to the pipeline system, to

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predict the velocity V, discharge Q, pressure p, temperature T, and density ρ

(for gas flow) at many locations along the pipeline. The predicted values are

compared with the measured values to determine if some abnormality exists

in the pipeline system.

Whenever a leak, rupture, or overheating condition exists, the measured

values at certain locations will differ noticeably from that of the computed

normal values. From the difference, a leak or rupture of the pipe can be

detected, and its approximate location can be determined.

Emergency measures, such as shutting a valve and stopping a pump, can

then be taken by the SCADA automatically, or through manual override.

This explains in a nutshell the principals involved in such computational

monitoring of pipeline integrity and safety.

5. Photographic observations method

Often, leaks have been discovered through visual observation of pipes or

their immediate surroundings. This can be done not only for exposed pipes

but also for underground and submarine pipelines.

For instance, one can patrol the right of way of a buried petroleum or natural

gas pipeline; withered vegetation in an area above the pipe may indicate

leakage in that area. In contrast, grass above a certain area of the water

pipeline growing more vigorously than elsewhere along the pipe may

indicate a water leak from the pipe.

Likewise, by sending a diver or special submarine to patrol an underwater

natural gas pipeline when bubbles are seen to rise from a certain spot on the

pipe, a leak can be pinpointed. Such visual observation or discovery by

divers or a submarine should be followed by taking photos of the scene for

further analysis.

6. Ground penetrating radar

Ground penetrating radar is capable of detecting the spilled natural gas and

petroleum that exists in the soil above the leak point of a buried pipeline. By

moving this radar along the right-of-way in an all-terrain vehicle, the

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pipeline can be surveyed for leaks, and the location of any leak can be

pinpointed.

7. Noise based leak detection method

Leak Noise Detection

As a type of leak detection, we can detect the leakage location by detecting

its sound. And we consider the water flow as an example.

The Sounds of Water Leaks

Water leaks in underground, pressurized pipes may make many different

sounds:

- "Hiss" or "Whoosh" from pipe vibration and orifice pressure reduction

- "Splashing" or "Babbling Brook" sounds from water flowing around

the pipe

- Rapid "beating/thumping" sounds from water spray striking the wall

of the soil cavity

- Small "clinking" sounds of stones and pebbles bouncing off the pipe

The "Hiss" or "Whoosh" sound, which often sounds like constant static

noise, is the only one which is always present for leaks in pipes with 30 psi

or higher water pressure. The other sounds may or may not be present, and

usually they are not as loud. So, we decide "Is there a leak?" by listening for

the "Hiss" or "Whoosh".

Factors Affect These Sounds

Water in the pipe

The loudness or intensity of the leak sound is directly proportional to the

water pressure inside the pipe (up to a limit)

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Figure 3: Sound Intensitty (loudness) vs. Water Pressure

Pipe material and pipe diameter

Metal pipes, such as iron mains, copper services, and steel pipes, transmit

water leak sounds that are louder and higher frequency than do PVC pipes or

asbestos-cement pipes. Thus, knowledge of the pipe material is important.

Large diameter pipes, whether they are PVC, concrete, steel, or iron,

transmit much less sound from water leaks than small diameter pipes. And,

large diameter pipes transmit lower frequency sounds than small diameter

pipes.

Soil type and soil compaction

Sandy soil and very loose soils, particularly over a freshly buried pipe line,

do not transmit the sounds of water leaks very well, nor do water saturated

soils such as bogs and swamps. Hard, compacted soil transmits the sounds of

water leaks best. Soil absorbs the sounds of water leaks very quickly.

Depth of soil over the pipe

Leaks in water lines that are only 3 or 4 feet deep are much easier to hear at

the ground’s surface than leaks in deeper lines. At 7 or 8 feet deep, only very

large leaks with good water pressure will produce enough noise to be heard

at the surface.

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Surface cover

The ground cover, whether it is an asphalt street, loose dirt, concrete slab, or

grass lawn, also makes an important difference. Hard street surfaces and

concrete slabs resonate with the sounds of the water leak, and the leak may

be heard for 5 to 10 feet or more on either side of the water pipe. Grass

lawns and loose dirt surfaces do not offer such a resonating plate-like

surface, and their surface variations make firm contact more difficult.

How Do Leak Sounds Travel on Pipes

Metal pipes, particularly iron mains between 6 inches and 12 inches, copper

services, and steel pipes transmit the sounds of water leaks for

hundreds of feet in every direction.

Asbestos-cement pipe and PVC pipe do

not transmit the sounds nearly as far.

Distances transmitted for the “Hiss” or “Whoosh” sounds of water leaks are a function of the pipe diameter as well as

the pipe material. Figure 4.

How Do Leak Sounds Travel Through Soil

Soil absorbs the high frequencies to a greater degree than the low

frequencies. For a leak in a pipe 6 ft deep, the “Hiss” or the “Whoosh” sound is weak and “muted,” i.e. only the lower frequencies are heard. For a leak in

a pipe 3 ft deep, the sound is louder and slightly higher in frequency. Figure

5.

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Figure 4: Distances transmitted for the sounds of water leaks are a function of the pipe

material.

Figure 5: only the lower frequencies of leak sounds are heard.

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How to detect the exact leak location

Surveying

“Surveying” is the term applied to listening for water leaks when there is no obvious evidence, like water flowing on the street. Every hydrant, valve, and

service line is a possible location to hear the sounds of water leaks.

Since the sounds travel on the pipe walls better than through the soil, always

listen at the hydrants, valves, and meters first. As you get closer to the leak,

the sound gets louder. Finally, decide which two of these locations are the

loudest. Now you are ready for “Water Leak Pinpointing.”

Pinpointing

“Water Leak Pinpointing” is the term applied to the process of pinpointing the exact leak location. For Acoustic Leak Detection, the exact leak location

is usually the spot where the leak sounds are the loudest.

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Figure 6: The exact leak location is usually the spot where the leak sounds are the loudest.

The loudness of a leak heard on an asphalt street or a concrete slab depends

upon the size of the leak, water pressure, and depth of the pipe. Hard, dry

materials like asphalt, concrete, rock, and compacted soil transmit sounds

better than wet clay, sand, or loose soil. The sounds travel further on iron

and steel pipes than on PVC pipes or Poly pipes.

The acoustic method for detecting leaks in Gas Pipelines

The methodology consisted of capturing the experimental data through a

microphone installed inside the pipeline and coupled to a data acquisition

card and a computer. The Fast Fourier Transform (FFT) was used as the

mathematical approach to the signal analysis from the microphone,

generating a frequency response (spectrum) which reveals the characteristic

frequencies for each operating situation.

This work aims to detect the characteristic frequencies (predominant) in case

of leakage and no leakage.

Figure 7 shows the change in the amplitude of the sound pressure generated

by gas leakage in the pipeline through orifices of 2 mm and 5 mm of

diameter.

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Figure 7: Microphone signal (in sound pressure) in the time domain.

a) Leak size with 2 mm b) Leak size with 5 mm

8. Dogs

The same Labrador retrievers used by law enforcement agencies to sniff out

illegal drugs and explosives can be trained to detect natural gas leaks,

provided that a special odorant is mixed with the gas.

The dogs have proven to be very reliable, with a success rate of detecting

leaks in excess of 90%. They can detect the scent at concentrations as low as

10-18 molar (1 part per billion billion).

The dogs are reliable, and demand no reward other than room and board.

They have proven to be successful not only under ordinary conditions, but

also in extremely cold weather when the pipeline was 12 ft underground

with an additional 3 ft of snow above.

The dogs are more reliable and cost less than many sophisticated high-tech

methods, but because they need training and care, their use in North

America has been curtailed or discontinued in recent years.

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References 1- Pipeline engineering, Henry Liu. 2- Pipeline leak detection techniques, Timur Chis. 3- Sub surface leak detection, Inc; manfactures and distributes water leak

detectors. http://www.subsurfaceleak.com/info_pages.html 4- Spectral Analysis for Detection of Leaks in Pipes Carrying

Compressed Air Rejane B. Santos*, Wellick S. de Almeida, Flávio V.

da Silva,, Sandra L. da Cruz, Ana M. F. Fileti. CHEMICAL

ENGINEERING TRANSACTIONS VOL. 32, 2013.