Considerations of cross country pipelines
For cross country transport of natural gas, crude oil, water, petrochemical and petroleum products, Steel Pipelines are used. It can withstand high pressures over long distances. Special Techniques have to be adopted for design, laying welding / jointing, corrosion protection, testing, commissioning etc. of cross country pipelines.
Advantages of Cross-Country, Pipe-Lines over transportation by Roads/ Railways/ Waterways are:
1) Continuous un-interrupted transport is ensured.
2) 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.
3) No dependence on availability of roads, railways, bridges etc. 4) Least manpower requirement to operate the transport system except for inspection and maintenance of minimum required level. 5) No hindrances on way as for surface transport, air water ways. 6) Possibility of crossing any odd areas like seas, oceans, rivers, mountains and underground space. 7) Once laid down, the system works automatically especially with the help of modern instrumentation, safety devices, interlocks, communication system and remote control devices. 8) Minimum or no tampering on the way. 9) Cost of Transport per Unit of the product conveyed is far less than the transport by Trucks/railways/ water/ Airways. 10) Fastest mode of transport even between two countries or continents. 11) Comparatively much less hazardous than surface transport & minimum dependence on human factors.
High fire & explosion Hazards potential and problem of corrosion & leakages & repair work involved are some disadvantages of pipelines.
Points to be considered while selecting the pipelines:
Pipe Thickness : The thickness of pipeline is calculated according to the standards 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 (c) as stipulated for specific requirement like working spacer for excavation / repairs, restrictions due to ROW space, adjacent features like road edge, building etc.
A surge pressure is induced whenever the valve at or near the receiving end is shut-off. This surge pressure is induced on pipeline and pump/valve at the supply end. It is therefore necessary to decide the time period for valve closing with appropriate communication between supplier and receiver. So a surge tank or vessel should be considered at both ends to avoid the effects of Fluid-Hammer in the system. Piging:
Pigging has to be done when a pipeline is used for carrying different products periodically, in addition to flushing and making the line ready for new fluid.
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 outside atmosphere, whenever it runs along the bridge side, thermal expansion and contraction takes place due to Temperature variations. Generally a long and wide loop is provided under the bridge 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 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 30 mm gap all around, 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 move out from the supports. Lateral spring supports are provided at random intervals to prevent possibility of natural frequencies of pipe matching with Bridge frequencies.
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 hydro tested, 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 permanent 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 3 mm or as specified, whichever is greater, is provided as corrosion allowance.
Following are the standards for cross country 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 Vacuum) 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 Breather Valve is permitted 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.
**The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.