Continued from Design of Cross – Country Pipe Line - 1
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, restrictions due 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 between supplier 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 and Amplitudes) 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 when the 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 a
long 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 these effects 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 on
any 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 as corrosion 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 and
Vacuums) 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 is PERMITED 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 be commissioned, 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 upto
Temperature 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 the intensity 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 reason
concerned 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 are
the 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 means of 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 not
sufficient 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 should have 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 on the 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 protective
coatings 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 the effects 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 two is 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 only a 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 the environmental 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 and due 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 the environment 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 effect of 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 laid
and 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 (high
alkalinity, 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 environmental
conditions 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 and handling.
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 to reduce the shortcoming of both the individual material without increasing cost. Coal Tar Epoxy and 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.
0 comments :
Post a Comment