design of cross pipeline

8/3/2019 Design of Cross Pipeline

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Design of Cross – Country Pipe Line

Introduction:

In the modern age of Industrial word, the Oil refineries, petroleum products, and petrochemicals

form the major part of the industrial set-up all over the world. It is often economical and practicalto carry the liquid and Gaseous products through pipe-lines rather than by Tankers over ling

distance. When a pipe-line has to carry such products like crude oil, refined oil, chemicals like

naphtha, ethylene, propylene etc. over long distance ranging from 10 km to even 1000 km,.Passing through land, rivers, sea, mountains, marshy areas, private and public land and land,

rivers, sea, mountains, marshy areas, private and public land and crossing other services like

roads, railways, transmission lines, underground Pipes/Cables etc, such a pipeline is called“Cross-Country Pipe-Line”. As the name suggests it transfers the liquid/Gas products from one

place to another at far distance.

Engineering and Installation of Cross-Country Pipe-Lines form a special branch of piping

design and engineering., as it involves many aspects and parameter which are normallyfaced with, in inplant piping system within the boundaries of refinery or a chemical /

petrochemical plant Special Techniques have to be adopted for design, laying

welding/jointing, corrosion protection, testing, commissioning etc. The most common line

familiar to all, is water-line from reservoirs to different consumption points, like, water-mainfrom vaiterna/ Tansa dam to city of Mumbai Unlike water line, the hazardous chemical

conveying Pipelines, involves many more stringent precautions in their design and installation.

This is mainly due to fire and explosion hazards associated with the oils and chemicals.

This write-up highlights the main features of the engineering and construction of Cross-

Country Pipe-Lines. The objective of this note is to make the reader familiar with the broad

perspective of the Cross-Country Pipe-Line Work and the pipe-lines which are already installedin India. These also include the submarine lines and en-land pipe-lines.

Advantages of Cross-Country, Pipe-Lines over transport by Roads / Railways / Waterways

The most common modes of transport known to all include Trucks running over Roads,

Railway goods train and Ships/Launches/Boats/Barges on waterways. The transport by Airway

be cargo Air-crafts is also another way of bulk-transport. These modes of Transport havefollowing limitations.

a) Availability of sufficient roads, rail-tracks and port-harbour facilities to take

up the traffic load.

b) Condition of the tracks.

c) Hurdles and conditions of the vehicles

d) Maintenance and conditions of the vehicles



e) Procedures and control involved in the Transport operation (permits/ licenses/octroi/toll/RTO

etc.)

f) Manpower to run and maintain the transport system

g) Availability of fuel and power required to run the system.

h) Effect of Nature on the system like rains, storms, earthquakes, thundering, mist etc.

i) Pollution generated by the transporting vehicles.

k) Safety, insurance and security of the transported goods and materials.

l) Time taken for transportation and delays

m) Overall efficiency of the system.

While transportation by roads, railways, water and air-ways is widely used all over the world, it

has its own limitations due to the features used all over the world, it has its own limitations dueto the features (a) to (m) mentioned above. These limitations especially restrict or forbid their use

when large quantities of Oil, petroleum, water, chemicals are to be continuously supplied from

the source to the consumption point at users’ end. Hence the most reliable and efficient systemcan be provided only be Cross-Country pipe-lines.

The advantages are as given below :

a) Continuous un-interrupted transport is ensured.

b) No dependence on availability of roads, railways, bridges etc.

c) Least manpower requirement to operate the transport system except for inspection andmaintenance of minimum required level.

d) No hindrances on way due to any reasons which are listed in problems (a) to (m) for surface

transport, air/water ways.

e) Possibility of crossing any odd areas like seas, oceans, rivers, mountains and underground

space.

f) Safety & purity of the product is ensured. The product reaches exactly in the same condition

from source to the supply point, with minimal loss of quality or quantity.

g) Once laid down, the system works automatically especially with the help of modern

instrumentation, safety devices, interlocks, communication system and remote control devices.

h) Minimum or no tampering on the way.



h) Cost of Transport per Unit of the product conveyed is far less than the transport by

Trucks/railways/ water/ Airways.

j) Fastest mode of transport even between two countries or continents.

k) Comparatively much less hazardous than surface transport & minimum dependence on humanfactors.

There are of course certain disadvantages but they are offset by the advantages, to a large extent,

so as to make them ignorable as far as safety & techno-economic aspects are concerned. They

are listed as given below :

a) Right of way Acquisition to run the pipeline, especially thru’ private & agricultural land

& habituated areas.

b) High fire & explosion Hazards potential.

c) Problem of corrosion & leakages & repair work involved.

d) Daily on-route inspection, testing & quick arrangements for attending to repairs andrectification work.

e) Possibility of laying other services in future (like other pipe-lines due to ignorance of

its existence, among other agencies) causing damage.

f) Special Techniques & Agencies are required to design, engineer, install & operate the pipe-

line system.

g) Expensive cathodic protection required for the protection of u/g lines running in close

proximity of overhead High Tension electrical Transmission lines which induce the currents in

the metallic pipelines, causing the corrosion by stray-currents.

The modern techniques are well developed to offset the effects of the above disadvantages. Evenif a line has to shut-off for a day or two, the storage facilities at the users end take care of such

stoppages even for 15 days to 1 month.

Preliminary work for A Cross-Country Pipe-Line Project :

The following necessary work on planning & collection of information/data is required to be for preproject activities, once it is decided to install a Cross-Country Pipe-Line.

Data on the Product to be carried :

- Name, Qty/day, properties of the product

- Source of supply & location details



- Names & location of consumers.

- Qty/day to be supplied to each consumer

- Storage facilities at suppliers’ end & consumer end

- Pumping facilities at Suppliers’ end

- Unloading facilities at receivers’ end

- Safety requirements for the product

- Risk& Hazards associated with product.

- Interruptions in supply at suppliers’ end & at receiving end.

Route Survey & Analysis

There may be many alternatives for routing the pipe line from supplier to the consumer. It is

necessary to study the techno-economic comparison of the alternative routes. This surveyincludes the following activities :

a) Spot-level survey at every 50 to 100 metres & at least over 10 m on either side of the probable

route.

b) Soil Conditions in the form of bore-logs, trial pits, chemical tests on subsoil & ground

water etc.

c) Alignment Map With lengths, bearings, angles etc. to know the exact route & the total

length of the pipe-line.

d) Details on the route and their locations dimensions etc sea, roads (crossing and along

side the route) rivers, Nallas, pipe-lines, bridges, rail-tracks, transmission lines, undergroundservices including cables/pipes etc, Hills and mountains, buildings, plantation, forests,

agricultural land etc.

e) Cadestral Survey –The route may be passing thru’ so many lands belonging to private

owners, farmers, govt. authorities, defence wings etc. En-route information and data has to be

collected for such land pieces. Such data will include :

- Type of land and the owner’s name

- Length of the route thru’ the land.

- Problems in acquiring Right of Way (R.O.W.)



- Authority which will permit/grant Row

- Survey maps for the land available from the local Land Authorities (such as collector,

Tahasildar, Gram-Panchayat etc.)

- Land records regarding the title and ownership of the land

- Approx compensation required for acquiring the R.O.W.

- Status of Habitation on the land.

- Similar information of the adjacent plots on 50 to 100 m on either side of the route.

- Plans for future installations by others on the proposed route and/ or in the vicinity such as

roads/ rail-tracks/ buildings/pipe-lines etc.

f) Availability of construction Materials, Labour & facilities

Since the pipe-line has to pass thru’ different areas and over a long distance, it is essentialto know the availability of construction Labour and Materials on the way. Such as excavation

labour, transport facilities, access roads, construction material like stones, aggregates, sand,

cement, steel, structurals, etc., workshop facilities. This information will be useful in workingout project schedule and cost estimates and assessing the problems in construction.

g) Soil Resistivity Survey – required for design of cathodic protection system.

Names and addresses of the statutory and public bodies required to be contacted for

acquiring ROW, construction permission, blasting licences, excavating the public facilities(Roads, rivers, rail-tracks etc.) and cathodic protection work, power supply/water supply etc.

Such authorities include the following but not limited to the listed ones.

• Local land authorities – distr. Collector, Municipal corporation, Tahsildars, D.I.L.R. etc.

Owners of the respective Land.

• P.W.D. authorities – Local Office• Irrigation Dept.

• Electricity supply Agencies/bodies/Boards.

• Water-supply and Public Health Dept.

•

Controller of Explosives and use of Hazardous chemicals. • Industrial Development corporations

• Railway Authorities

• Marine and Port Authorities

• Salt-commissioner and controller

• Competent Authorities for Land and Row acquisition.

• State and Central Govt. for necessary permissions, licences, clearances etc. • Import/export rules/ regulations authorities



• Controller of Quarrying and Mining

• Navy/Army/Air force (Defence Authorities)

• Plants for future installations.

• Forest authorities

Project Schedule

Base on various data collected as in 3.1 and the cost Estimates, over all project schedule

has to be prepared based on past experience, and specific problems unique to the projectunder consideration. This schedule should cover only broad activities to serve as a guide

line for preparation of detail activity schedule.

This should generally include :

a) Preliminary Survey / Data Collection

b) Finalising the route

c) Cost Estimates / budget sanctions

d) Acquisition of R.O.W. and land

e) Basic Engineering package

f) Detail Engineering work

d) Construction work (Civil/Mech./Piping/Electr, Marine crossing, river crossing etc / cathodic

protection)

h) Testing/Flushing/Pigging.

j) Commissioning and Hand over

This will establish the overall completion time for the entire project work.

Finalizing the most optimum route

This involves the comparison of alternative router surveyed as in 3.1. The analysis should

include various parameters which are tabulated in the following format :



Salient Steps in Detail Engineering

After deciding the final route, cost estimates, broad project schedule and engineering. The

detail engineering will involve following main steps.

Detail Design of each system

§ Civil works including trenching, sand filling, back filling, buildings, concreting, river-weights,

valve-chambers, Test points, markers and construction infrastructure like site office, construction

water, power, site godown/open yards etc.

§ Construction Equipment required for transport, laying, welding, erection testing etc.

§ Piping : Stringing/ Welding/ Laying/ Testing pipe support system

§ Catodic protection system design, diode stations, sacrificial anodes, UPSinstallations,

on-line test-points, insulation flanges

§ Specific designs for submarine portions and river-crossings

§ Designs of all crossings, pipe-bridges, supports

§ Preparing Detail Design and Fabrication Drawings for all Systems

§ Quantity calculation for materials and work items.

Implementation Planning and Organising

§ Selection and appointing Agencies/Contractors/Suppliers for various activities andmaterials

§ Division of work among the staff on the project.

§ Progress monitoring and reporting system

§ Mobilising planning (manpower deployment planning) (Resource-planning)



§ Implementation work packages

§ Payment to subcontractor system

§ Inventory-control-planning

§ Safety/Security Guidelines

Organising Revisions/Change/alternatives/improvements in system design/drawingduring the project-process.

Preparation of As-Built construction drawings and final costing.

Data-Bank for the executed project, useful for future project.

Salient Features of Construction

Trenching : See. fig. 1

Generally Cross-Country Pipe-Lines are laid underground in an excavated Trench whilecrossing the land-areas. Minimum depth of the Trench should be Trench while crossing the land-

areas. Minimum depth of the Trench should be (1 M + Pipe dia + 150 mm). 1 M – is the depth of

overburden i.e. back-filled soil, and 150mm is the thickness of sand cushion to be laid beforelowering the pipe in the trench. Width of the trench is general minimum 1 M or as required by

higher dia. Pipes. Thus width should be (Dia. Of Pipe + 0.4 M on either side) or 1 metre

whichever is higher.



Pipe-Preparation in Yard

- Inspection/Testing/Stacking of Pipes/Numbering

- Edge-Preparation for welding

- Wrapping/Coating (generally reinf. bitiminous) and its testing

- Testing/Stacking bends/elbows/Tees

- Pipe-Sleeves for road crossing

- Valve-testing/stacking/numbering

- Other accessories like blinds/spectacle blinds, gaskets, bolts, nuts, washers etc.

- Selecting/ Stacking welding machine/electrodes etc.

Stringing at Site and Welding

After trenching is ready over substantial length pipe-lines made ready in the yard as in 5.2

are transported to the site and lined up over sleepers placed across the trench for welding



and lined up over sleepers placed across the trench for welding the joints. The joints are welded

continuously in 2 or 3 shifts. They are subject to inspection by D.P. check and Radiography.

Wrapping/ coating is completed over portions about the weld.

Lowering

Once a fairly long length say 100 m to 150 m is welded/Tested, then if is lowered into trench

over sand-bed already laid-Necessary small/big cranes, lifting tackles are used for lowering the

line. Back filling with soft earth free from stones is done after lowering.

Hydrotesting

A long length after lowering a back filling is hydrotested for the test-pressure which is

generally 1.5 times the operation pressure or as stipulated for specific service.

Overall Total Welding

After each 100 to 150 m length is lowered, tested, then they have to be welded to form a

continuous pipeline.

Testing of entire line is then taken up by filling the whole or section of line with water &

pressuring. Any leaks found are repaired and tested.

Pigging

For flushing and cleaning the entire length of all muck, dirt, welding rod bits etc, a pig is

passed thru the line, from one end, and it is pushed by water pressure. The pig travelsthrough the pipe, scrapping the muck and pushing it forward. At intermediate points flanged

joints are left to pass-out the muck. If a pig gets stuckup, its location is detected by passing an‘ISOTOPE’ and detecting its location by external instruments which tracks the isotope as it is

travelling through the pipe. The pipe line is cut, pig removed, pipe cleaned and rewelded. The

pig is passed through from that point onwards to flush the remaining portion in the forwarddirection.

Commissioning

It is done as per the procedure laid down for the specific product to be carried through the

pipeline.

Cathodic Protection

This provides the protection to the underground pipe from the corrosion by electrolytic process in subsoil water, whenever in e-m-f is induced’ in it (when pipe material is a good

conductor e.g. carbon. Steel)



Basic Principal and Phenomena

H.T. overhead Transmission lines conduct A.C. current under very high voltages of the order of

11 kv to 33000 kv and more. Due to fluctuations in voltage, magnetic field around theconductors also changes continuously. Any conductor in the magnetic field, thus cuts the

magnetic flux and e-m-f is induced in it. (Ref. Fig). 2 below)

u/g pipeline of metal (viz. carbon steel) is a conductor of electricity. If it lies within the

magnetic field of electricity. If is lies within the magnetic field of the O.H. lines, then itdevelopes a potential higher than the ‘Ground’ potential. As we know, earth i.e. ground is at‘Zero’ potential. When the u/g pipe is subjected to an induced e.m.f. if is supposed to be higher

potential than the surrounding ground. The subsoil water always contains many

dissolved salts of sodium, potassium and other elements. This makes the subsoil also aconductor of electricity. Thus the current flows from the pipe at flow of current depends on the

resistivity of the subsoil. This phenomenon sets up an ‘Electrolytic’ process between the

pipe which acts as ‘ANODE’ and the ground which acts as ‘CATHODE’. Once this process

starts, pipe starts losing the positively charges ions say Fe++ or Fe++ into the subsoilaround the pipe. This is the corrosion process by which the pipe. This is the corrosion

process by which the pipe gives up its material & develops a hole or a reduction inthickness.

Prevention of Effects of Induced EMF

To prevent this phenomenon, it is necessary to prevent the current from ‘PIPE’ to

“GROUND’. This is not possible, but it is possible to reverse the flow i.e. from ‘GROUND’ to

‘PIPE’. In other words pipe must act as ‘CATHODE’ and ground should act as ‘ANODE’.



This type of system is called ‘CATHODIC PROTHECTION’. In this case (ref. Fig. 3), the

current flows from ‘ground’ to ‘pipe’ and the pipe is said to be at –ve potential since the

ground has zero potential. The fig.3 is self explanatory. Applying a voltage by means of a batteryset or D.C. current (rectified from A.C. supply or from D.G.) to the ‘anodes’ inserted in the

ground surrounding the pipe and at regular regular intervals along the pipe This system, is

maintained to see that the pipe always acts as ‘cathode’ then there are no chances of corrosion of pipe.

3 – [Current Flows From Ground to pipe]

Diode Station in the Vicinity of Rail-Tracks with Electic Traction

The system described in 5.10.2, is often disturbed due to presence of other sources of electric conductors in addition to the O. H. transmission lines, (e.g. Electric Traction). In this

case, as shown in fig.4 stray currents flow from rails (which act as the path of return current) to

the surrounding ground. This phenomenon, causes ground at-ve potential and quite often atlower potential than the pipe. This causes the flow of current from pipe to ground and the

corrosion can take place. By supplying the current to a diode introduced in the circuit joining the

rail to the pipe in such magnitude that the surrounding ground will conduct the current fromground to pipe,



but not in the reverse way as diode acts like a ‘NON-RETURN’ valve. The calculations of

diode capacity, voltage to be applied etc. have to be made based on the survey conducted onfluctuations in the Traction field voltage. This arrangement prevents the pipe working as anode,

but maintains it at cathodic level and thus prevents the corrosion.

Sacrificial Anode

In addition to the impressed D.C. current as in 5.10.2, sacrificial anodes are introduced into the

ground, which maintain the pipe at lower potential than the ground, in case the D.C. supply failsfor some reason. These anoces are made of Metals which are ‘NOBLET’ than the pipe materials.

By this, it is meant that the metal which electrons surrounding or adjacent metals in contact is

called a ‘NOBLE’ metal e.g. Mangessium (REF. FIG. 5)



This makes the GR. WATER at + ve potential with respect to the pipe. Therefore the current

flows from magnesium to the pipe. In this case magnesium Anodes slowly lose its own metal to

the surrounding and in course of time dis This is why it is called ‘SACRIFICIAL’ anode periodically they have to be replaced by new anodes, say every 3 to 4 years.

Normal Subsoil Corrosion

Ever of there is no presence of Electric O.H. lines, or ‘Traction’ lines, any conducting metal

buried in the ground gets corroded by the similar phenomena. The subsoil water which itself is a‘solution’ of so many salts, contains +ve and –ve ions. The presence of metal conductor such as

‘pipe causes movements of there ions and often the current from pipe to ground. This causes

corrosion of the pipes which get electrostatically charged due to friction between pipe and the

fluid flowing through it. Hence the sacrificial Anodes are required to be provided (Ref. fig. 6)

On-Line installations

a) Test Points: Once the system of applying impressed current into ground to keep the pipe at – ve potential, it is necessary to check the potential difference between the pipe and the ground at

regular intervals, say at every 500 m. ‘Test points’ are installed close to the pipe. (se fig. 7) T.P.

Box indicating voltage between ground and the pipe. Ground should be at least 1.5 v above pipe potential Volt-meter is carried by the inspector and the voltage between Terminals inside the T.P.

Box is checked and recorded.



b) Insulating Flanges : If any portion of the pipe is above ground, then the same has to be

‘Electrically Isolated’ from the under ground portion. This is required so as to prevent the flow

of any other currents from sources outside and also the path of least resistance which the currentmay find through above ground pipe resting on steel or metal supports. (see fig. 8 on next page)



All installations and systems described in section 5.10.1 to 5.10.6 from the Total System of

Cathodic Protection.

Additional Information on Cathodic Protection (C.P.)

1.Necessity of cathodic protection is established on following criteria

a) Type of soil with its constituents like PH value, contents of chlorides and sulphates. b) Soil resistivity which determines corrosion level.

c) Importance of line(s) to be protected.

d) Study of stray currents i.e. induction from EHV/HV lines, rail lines.e) Dis-similar metals structures in the vicinity

f) Life of object to be protected e.g. 30/50 years

2.Two methods of C.P.

a) Sacrificial

Sacrificial is adopted for less important object & remotely located objects where electric

supply is not available early. Zinc, Aluminium & Magnesium are used as anode material.

b) Impressed current

Impressed current system is used for important objects and is dependent on electric power

supply. Hi-silicon coast iron graphite are normally used as anode material for impressed currentsystem.

3. Impressed current system comprises the following major equipment and accessories

a) Transformer Rectifier Unit

b) Anodes with tail cablesc) Anode junction boxes

d) Reference cell/electrode

e) Backfill materialf) Cables.

4. Criteria of selecting anodes and electrical equipment for buried pipelines.

Generally pipe lines which are buried, are buried, are coated and wrapped. This brings down the

current level (and potential level) which is required to be provided. Generally 10 mA/sq. meter

current criteria is used. For Pipe lines which are in sea water higher current upto 110 mA/sq.meter are used.

5. Following Data is necessary for engineering of C.P.

a) Dia meter of Pipe-line(s)

b) Length of buried pipelinec) Material of construction



d) Type of coating.

6. Monitoring/maintenance of C.B. System

a) Potentials are measured on frequent basis (or daily)

b) Maintenance of electrical equip on periodic basis.

7. When construction period is long, during such period temporary cathodic protection has to be provided, until the permanent C.P. system is ready and commissioned.



Additional Features about Piping

Pipe Thickness : The thickness is calculated in accordance with the standard methods and codes

for different services and duty, including due corrosion allowance.

Anchor blocks at change of direction, made of concrete should be used to counteract the effects

of outward thrust due to change in direction of fluid velocity.

If more than one pipe lines are running in parallel, minimum, clearance between the adjacent

pipelines should be the largest of

(a) O.D. of the larger pipe dia over insulation if any

(b) 600 mm or

(c) as stipulated for specific requirement like working spacer for excavation/repairs, restrictionsdue to ROW space, adjacent features like road edge, building etc.

Surge Effect : Whenever the valve at or near the receiving end is shut-off, there may be

surge pressure effect on the pipeline as well as Pumps/Valves at the supply end. It is therefore

necessary to decide the time-period for valve closing with appropriate communication betweensupplier and receiver. At times it may be advisable to introduce a surge tank or vessel at both the

ends. This avoids the effects of ‘Fluid-Hammer in the

system.

Piping : When a multipurpose pipeline is used for carrying different products periodically, pigging has to be done in addition to flushing and making the line ready for new fluid.

WHILE DESIGNING THE PIPE THICKNESS, THE FOLOWING ADDITIONAL FACTORE

SHOULD BE CONSIDERED SPECIFIC TO CROSS COUNTRY PIPELINES

PIPE-LINE supported on Brackets attached to a Road or Railway Bridge :

When a line runs along-side a bridge, the vibrations of the bridge due to Traffic Movement, are

also transmitted to the Pipe-line. It is necessary to estimate the vibration-levels (frequency andAmplitudes) of the bridge. Generally these data will be available with the respective authorities

or designers of the bridge. We have to check and prevent the natural frequencies of the pipe-line,matching with the exciting frequencies of the bridge, to avoid resonance effects. It is advisable to

provide lateral spring-loaded supports at random intervals, to get damping effect and random

frequencies. In case of railway-bridges, regular patterns of vibrations are more probable whenthe train is passing.



LONG EXPANSIONS LOOPS

As the line is exposed to out-side atmosphere, whenever it runs along the bridge-side,

thermal expansion and contraction take place due to Temperature variations. Generally along and wide loop is provided under the bridge as shown in fig. 10 structural behaviour of

short and long arms of the Loop will depend on the deflections and the stiffness of the arms.

Due analysis should be made to calculate the stresses induced in the pipe. Also note the

supporting arrangement of the pipe as shown in fig. 10. which has following main features.

§ The rollers are provided to allow free longitudinal movement of the pipe due to

expansion and contraction

§ Loose clamps are provided over pipe-line at intervals with 25 to 30mm gap allaround,

to prevent possibility of the line slipping off the supports due to long-length. (Long pipe-line behaves like a flexible wire and when expanded, may tend to moveout from the supports.

§ Lateral spring supports are provided at random intervals to prevent possibility of

pipe-natural frequencies matching with Bridge-frequencies.

Erection Stresses

The handling of pipes may induce local and excessive stresses in following conditions.

a) When cranes are used for lowering long lengths in position, local deformation/bending may

take place.

b) When pipe is pulled along the trench or through the sleeve laid across the road.

c) When the sub-marine portion of the line is gradually lowered from water level to below the

sea or river bed, it undergoes deformations at local points.

d) When long un-supported (un-back filled Trenches) lengths are hydrotested, the flexibility of long lengths, sometimes causes vibrating movements on micro-scale and

are more predominant than in case of small in-plant piping. These have to be correctly assessed

or damped by intermediate Temporary and / or permenant supports, thrust blocks, anchors,

backfilled portions etc.

e) When the pipe-line crosses a Hillock, it goes up the inclined plane and from peak runs down

the slope. The up-going line is subjected to a sort of compression due its own weight due to

sliding tendency or tension due to pulling effect, down the plane. The stresses due to any of theseeffects should be estimated and provided for.



CORROSION ALLOWANCE

In normal in-Plant piping, standard corrosion allowances are specified for various duties in

different design codes. Cross-Country pipe-lines run over a long distance and the leaks onany account cannot be permitted. Hence extra corrosion allowance is specified for cross country

pipelines. In any case minimum of 3mm or as specified, whichever is greater, is provided ascorrosion allowance.

Design codes generally followed for cross-country piping (in addition to normal codes for all piping)

§ ASME B-31.4/ 31.8 for thickness Design

§ API – 1104 for welding and related tests specifically on cross-country Gas and Oil

lines.

§ API – 5L for material of construction

Generally, in non-hazardous fluid line, say water-lines. Breather valves (pressure andVacuums) are provided at he highest points, say on Bill-top, to prevent ‘Air-lock’ or to suck-in

Air in case vacuum or cavitation takes place. But in pipelines carrying Gas or Hydro-carbon

liquids like crude oil, refined oil, naptha, ethylene, propylene etc. No BRATHER VALVE isPERMITED any where on the line. This is because the hazardous liquid cannot be allowed to

come out into the atmosphere and Air (which contains oxygen) cannot be allowed to be sucked-

in as the fluid may combine with atmospheric oxygen and catch fire. Anytime the line is to becommissioned, the fluid to be carried is filled into the pipeline by first passing the pig from

supply end. There are no chances of Air-Lock. In this case.



SPECIFICATION FOR COATING AND WRAPPING OVER UNDERGROUND PIPESß

1 GENERAL SCOPE OF OPERATION

1.1 Cleaning / Scraping external pipe-surface

1.2 Priming with synthetic primer

1.3 First Coat of coal Tar Enamel

1.4 First layer of Inner Wrapping of Fibre-glass tissue fabric

1.5 Final Coat (2nd Coat) of Coal Tar Enamel

1.6 Outer wrap of coal-tar impregnated Fibre-glass tissue Fabric

1.7 White Wash

2 REGULATION

All materials conform to AWWA C – 203-86 or BS – 4164-1987 or ASTM Standards

3. LIMITATIONS

Coal-Tar enamel based coating-wrapping should withstand the liquids carried uptoTemperature of 60 deg C

4. INSPECTION AND TESTING

Applied coating/wrapping should be tested by SPARK TEST to be applied with HOLIDAY

DETECTOR Any sections found defective with pin-holes, cracks, internal hollows, pockets,wrinkles, airpockets, less thickness etc. should be removed redone and retested until they are

made defects-free

5. HANDLING AND PLACING

The pipes already coated/wrapped should be carefully using special strap-type lifting clamps to

prevent concentrated loads and forming dents or depressions. The straps shall be of flexible but

strong and soft rubber sheet wide-enough to distribute the self weight of lifted pipes within theintensity which coating/wrapping can withstand without getting damaged or depressed.

SELECTION OF PROTECTIVE COATINGS FOR UNDERGROUND PIPELINES

Mr T K Roy, Vice President—Technology, STP Limited

ABSTRACT



Like the selection of pipe materials, coating materials for• protection of pipes vary depending

upon various factors. The paper summarizes the basic needs for selection of

coating material for the long in service life of gas and liquid transmission system.

INTRODUCTION

It is most important to recognise that the coating material by itself will not result in

optimum corrosion protection of the pipeline. A total pipeline protection system includes

consideration of steel quality, coating application, surface condition and treatments, design of coating and Cathodic protection system.

Practical experience, as well as soil corrosion •studies has led to the conclusion that the

properties of soil are more important than the composition of metallic material in

determining the character and rate of corrosion. Soil corrosion tests are for this reasonconcerned largely with determining the nature and predominance’s of the corrosive and

protective factors of those environments.

The elements of soil may be classified roughly as corrosive or accelerative and protective

or repressive. Relative concentration and composition of these two types of elements arethe determining factors for selection of pipe coating materials. The physical texture and

drainage of soils affects the concentration and availability of oxygen. Contact of soil

particles with metal surfaces gives rise to oxygen concentration cells and it is mainly by meansof the operation of cells of this type that metal corrodes in soils. Pores and holidays and other

imperfections comprise an important source of corrosion cells.

PREREQUISITE CONDITIONS FOR COATINGS

Surface conditioning:

Abrasive cleaning of the pipe surfaces to a white or near white blast quality is notsufficient for a good coating operation as ill effects of chloride contamination is not

removed by this process. It has been established that most harm is done by the

presence of ferrous salts which is not removed by abrasive cleaning process or by

high pressure water’ blasting. The steel surface energy plays a critical role for accepting coating material A non contaminated steel surface has a surface energy

higher than 73 dynes/cm2.

In normal condition even after blast cleaning the surface energy of steel surfaces

varies from 45 to 50 dynes/cm2 . In order to have a good wet ability the coating material shouldhave a surface energy well below 45 dynes/cm2 (As per ASTM D

2578)., Another criteria for steel surface which is to be considered for selection of

coating material is its mixed surface potential (micro anodes arid cathode). In order to overcome the surface anomaly it is necessary to treat the blast cleaned surface.

A chemically cleaned surface by removing the contaminants help wetting of the

coating effectively. Treatments with chromates and silicates or by using adhesion promoter the surface potential of the steel surface can he made more uniform which reduces the



driving force between anode and cathode, increasing resistances to electron flow and passivating

the surface.

Adhesion:

Adhesion of the coating material is the most important factor while selecting a proper material for steel pipes.

The adhesion is based on three mechanism~ mechanical polar and chemical adhesion.

Mechanical adhesion is achieved through physical anchoring of coating material in

the peaks and valleys obtained by blasting. This is not very strong in nature.

Polar adhesion is most widely occurring mechanism of coatings. The bond strength depend onthe availability of polar sites on both the substrates and the coatings. Adhesion achieves the

highest value when polar groups are in close molecular proximity. A good wetting of the coating

satisfies this condition.

Though chemical adhesion gives the strongest bond, it is rarely used for protectivecoatings of Pipe Lines, Chemical bond is achieved by functional groups on the substrate and

coating interacting chemically; This is focused for Pipe Lines other than steel. Adhesion is

extremely important against resistance to Cathodic disbondment. It has been found that theeffects of electrolytes particularly, if sodium and potassium ions are present, can be very

destructive in the interfacial bond under the Cathodic protection influence. This can lead to

ineffective Cathodic Protection.

Type of Coatings:

Coatings can be classified as organic and inorganic and in many cases a combination of the twois used.

Coatings provide corrosion protection through passivation, barrier and sacrificial ways. Most

pipeline coatings are based on barrier concept. Three types of organic coatings are in use for giving barrier properties Thermoplasts, Thermosets and Elastomers.

Thermoplast coatings are generally applied by hot melt technique and solvent evaporation

technique. Examples are Polyethylene, Polypropylene, Nylon, PVC, Coal Tar Enamel and

Asphalts. Common characteristics of these polymers are good mechanical properties and

resistance to moisture but sensitive to exposure to high temperature.

Thermosets costing are formed through cross linking induced chemicals by hear, chemicals or

radiation. Examples are Epoxy, Polyester, Phenolics which have generally good heat but

resistance but relatively poor in mechanical properties.

Elastomers are classified as hybrid of the two. Examples are urethanes.



Selection of coating materials

In selecting a coating material the nature of the soil corrosion and the soil movement are of

prime importance. Many materials pass the basic properties for the control of corrosion but onlya limited number meet the overall needs of pipeline protection.

Correct selection of the products most suitable for the required duty from the range available is

equally important taking into account the size of the pipeline, operating temperature,

comprehensive site survey information relating to soil condition including resistivities, acidity,redox potential, presence of a sulphate reducing bacteria and the nature of the terrain through

which the pipeline, route passes.

In the design of corrosion protection system for a burned pipeline and its subsidiary components,

both coating and Cathodic protection system must be considered together.

As the coating for a pipeline is considered to be the primary method of corrosion control then it

is to be decided which coating should be used for most effective method of protection in theenvironmental conditions appertaining along the total pipeline route. Essentially the Sating

material must be stable for the required length of service for the pipeline under consideration anddue regard must be given at th! planning stages to the choice of coating that will meet all the

conditions of service.

In addition the coating must be totally compatible with the micro environment surrounding and

with the Cathodic protection system.

Whatever may be the nature of the coating material, the effectiveness is corelated

with a number of technical characteristics which the protective coating must possess to a

satisfactory degree. These may be classified as follows:

1. Be chemically inert to any corrosive agents present around the pipeline

2. Be resistant to the action of any micro-organism and bacterial degradation present in theenvironment in which the pipeline is laid, both aerobic and anaerobic.

3. Be resistant to marine organism for submarine pipelines, coating should not be easily

penetrable by marine life such boners, barnacles.

4. Posses a high degree of electrical resistively, sufficient to ensure the electrical

insulation of the metal pipe from the laying environment.

5. Be highly impermeable to water and water vapour and shows negligible water absorption.

6. Be closely bonded to the metal, in order both to prevent the spreading of corrosion

under the coating in the case of local faults and to oppose the forming of moisture containing

pockets at the metal coating interface, due to parting by mechanical actions or electrolytic effectof the cathodic protection.



7. Possess an adequate impact resistance, so as to allow the pipes to be transported

and handled without undue deterioration of the coating.

8. Be capable of withstanding the stresses induced by the soil in which the pipe is laidand due to its physical and chemical nature and . resistance to considerable stress from soil

movement such as contraction by clay during prolonged dry spell.

9. Have a service life atleast as long as that expected of the pipe to be protected, retaining

unchanged its chemical & physical characteristics.

10. Suffer no alteration under the condition created by Cathodic protection (highalkalinity, nascent hydrogen and nascent chlorine)

11. Be very easy to apply, avoiding the use of sophisticated technological processes,

complicated machinery and high cost, hard to replace skill labour.

12. Special technical characteristics may be required by particular environmentalconditions in the laying and operation of the pipeline to be protected.

13. Be flexible enough.

Moreover, in the choice of coating two economic conditions will have to be made:

ease of application and repair and an acceptable overall cost. The matter will include the cost of

the coating material, of its application and repairs of any damage incurred in transportation andhandling.

Among the recent development of coating materials the hybrid system of Coal Tar Epoxy, Coal

Tar Polyurethane and rigid Polyurethane are in consideration. The hybrid system is aimed toreduce the shortcoming of both the individual material without increasing cost. Coal Tar Epoxyand Coal Tar Urethane are specially suitable for lower dia steel pipes below 12” where costly

Fusion bonded Epoxy is mostly being used in some countries. Rigid PU system can go to coat

bigger dia pipes but it is costlier than Coal Tar Enamel which is normally preferred material for pipes above 12” dia.

In FRE system also instead of using straight epoxy hybrid epoxy polyester system is under

study.This hybrid system improves the short comings of straight epoxy system viz impact

strength and mechanical resistance.

Pigging in Pipeline Pre-commissioning

INTRODUCTION

After a pipeline is constructed and before it is put into service there are a number of key

activities required in order to ensure that the pipe meets the requirements of its owners or

operators. These will vary to some extent depending on the service for which the line is intended,

http://www.pipingguide.net/2011/01/pigging-in-pipeline-pre-commissioning.html

http://www.pipingguide.net/2011/01/pigging-in-pipeline-pre-commissioning.html



but as a minimum they will be looking for verification that the line has been laid without

significant defects and is in a condition suitable to be filled with the intended product.

Pipeline pigging has a significant role to play in meeting these conditions, and pigs are met with

in a number of guises during pre-commissioning operations. This paper is intended to provide an

overview of the uses of pigs in these operations, and provide some basic information on train

design and pig selection. Some examples are drawn from a range of types of construction and

pre-commissioning projects in order to give a feel for the practicalities of the operations

described.

PIPELINE PRE-COMMISSIONING ACTIVITIES

The principal activities involved in preparing a pipeline for operation are those of filling,

cleaning and gauging; hydro-testing; dewatering and drying. As in many other pipeline

operations, pigs are the tool of choice in achieving many of the goals in this area. Activities that

are thought of as pre-commissioning cover the entire range necessary to prepare a newly laid

pipeline for handover to its operator. Following the construction phase of the pipeline build, the

line may be physically complete, but will require significant preparatory works prior to being

ready for service.

Note: Each answer will appear to be wrong to some readers and right to others. Some questionswill have what seems to be an absolute right answer. Others will not. So if you have got any

good answer for below questions, leave us a comment. Find more details see at the end of article.

TYPICAL QUESTIONS FOR PIPING ENGINEER’s KNOWLEDGE TESTING (With Answers)

1. Can you explain in detail three or more major differences between code ANSI B31.1 and codeANSI B31.3?

Answer: There is only one major difference between the two, B31.1 is for Power Piping and

B31.3 is for Refinery/Chemical Plant Piping.

2. There is a power plant inside a Process refinery. Where exactly the ANSI B31.1 & ANSI

B31.3 scope break occurs?

Answer: Based on my experience there were two cases. Case #1, B31.1 stopped at the Power Plant Unit block valves. Thus all piping inside the Power Plant was B31.1. Case #2, B31.1

stopped at the equipment (Boiler) isolation block valves and then all other piping was B31.3.

This is normally the choice of the owner/operator/client.

3. Which of the following piping system is more health hazardous. A) Fuel oil piping b) Process

piping with Caustic c) process piping with HF acid d) Sulphuric acid piping.Answer: c) process piping with HF acid

4. There is a steam piping with low pocket but without steam trap. What will be worst

consequence of this layout?

http://www.pipingguide.net/2009/07/typical-questions-for-piping-engineers.html

http://www.pipingguide.net/2009/07/typical-questions-for-piping-engineers.html



Answer: There will be a build up of condensate to the point that a slug will be pushed by the

steam flow. This slug of condensate will cause “water hammer” and could rip the piping apart.

5. In what circumstance, the reducer of a pump suction piping will be in bottom flat position.

Explain why the reducer should be so.

Answer: Still Needs a Proper Explanation for the this answer.

6. A P&ID shows a spec break (at Flange) between carbon steel & stainless steel specification.What additional arrangements you have to make for that dissimilar material flange joint?

Answer: Use the Gasket and bolts from the SS spec.

7. A stainless steel piping specification mentions Galvanized carbons steel bolts. What is your

first reaction ti this and how do you rectify it?

Answer: If that is what the Spec call for then that is what I am supposed to use. But, I would ask the Piping Material Engineer (PME) why he/she specified galvanized bolts.

8. How many types of piping specialty items do you know? Why it is called a piping special?Why not we include them in standard piping specification.Answer: I could possibly count 50 or more depending on the PME and how the piping material

specs were developed. They are called them SP items because they are NOT written into the

normal Piping Material (Line Class) Specifications. They are not included because they arenormally of limited use, purchased from a limited product line vendor and are often after

thoughts.

9. Draw a typical steam trap station layout and explain why the existence of a by-pass line

around the trap is not a good idea, when the condensate is returning to a condensate header?Answer: (No drawing) It is not advisable to have a bypass around a steam trap because the block

valve could be left open and defeat the purpose of the trap.

10. Explain what is a “Double block & Bleed” valve? Why we need a bleed valve? When do we

use this?

Answer: The primary purpose of a “Double Block & Bleed” is Safety. However it is not fail safe.The next better “Safety” set-up would be Double Block Valve with a Spec Blind between the

valves. The higher level of safety would be double block valves with a removable spool for

absolute isolation.

11. In a typical tie-in where should the spectacle blind be inserted? a) after block valve andtowards existing plant b) before block valve and towards new plant. Explain why.

Answer: The Spec Blind shall be placed on the Unit side of the Unit Block valves. This

placement allows for the closing of the Unit isolation block valve, the unit side is depressuredand drained. Then the spec blind can be installed for isolation of the unit.

12. “Stress intensification factor (SIF)” Where do we use this? Explain this term. How manytypes of these SIF’s exist?



Answer: Stress Intensification Factor (SIF) is a multiplier on nominal stress for typically bend

and intersection components so that the effect of geometry and welding can be considered in a

beam analysis. Stress Intensification Factors form the basis of most stress analysis of pipingsystems. As for the quantity, ask a Stress Engineer.

13. When all design parameters are same, whose thermal expansion is higher among thefollowing? A) Carbon steel b) Stainless steel c) Duplex steel d) Cast Iron e) Galvanized Carbon

steel.Answer: b) Stainless steel

14. In a hose station the hose couplings used for water, air & steam should be different type. Doyou agree? Explain your view.

Answer: I agree. If they are all the same then the hoses can be connected to the wrong services

and could result in the injury of an operator (i.e.: thinking the hose is connected to water when itis connected to steam).

15. What is your view on the usage of Metallic expansion joints? When they become necessaryand when they could be avoided?

Answer: I do everything I can as a piping designer to avoid the use of all types of expansion joints. Expansion joints are always the weakest point in any system where they are used.

16. A water cooler heat exchanger, located on a 20 m high structural platform. Water header islocated u/g. What precaution do you take, in case of Pressure loss in cooling water header?

Answer: I do not understand this question it does not appear to be a piping issue. I would assume

that the cooling water system has a (loss of) pressure sensor and the plant shut-down alarms and

sequence would be activated.

17. In what order do you arrange the pipes in the Pipe rack and why? How much % of areashould be reserved for Future expansion? Specify a range.

Answer: The largest hottest lines on the outside edge of the pipe rack working in with cooler

lines in towards the middle of the rack. This allows the longer loop legs as you lay the loops back over the other lines to the other side of the rack and back. The lower temperature loops would be

“nested” inside the larger, hotter loops.

“Future rack space” is normally at the direction of the Client. It may be anything from 0% to asmuch as 25%.

18. When a utility line (like condensate or water etc) is connected permanently to a process piping what precaution we have to take to avoid cross contamination?

Answer: Option #1, double block valve with a drop-out spool.

Option #2, Double block valve with a spec blind.Option #3, double block valves with a bleed valve.

19. A air fin cooler (2 air coolers with each having 2 inlet nozzles) needs a Typical piping

arrangement. How many types of piping arrangement is possible.Answer: There are a number of ways to pipe a Fin-Fan cooler depending on what the P&ID call

for?

design of cross pipeline

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