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Transcript of Pipeline Installations
SECTION 1: INTRODUCTIONThe overall design of a pipe line will be determined by its location, the type of fluid being transported and its operating pressure and temperature. A typical pipeline has many installations which aids its functions as a medium for fluid transport. The following installations will be described, regarding the factors a plant designer would take into consideration.
Pig traps Block valves Offtakes Compressor stations.
SECTION 2.1: PIG TRAPPig traps are installations used for inserting pigs into a pipeline, then launching, and finally receiving the pigs at the receiver end .A pig trap is similar to an air lock or torpedo launch tube. They are isolated from the pipeline by a valve so it can be depressurized to load the pig. A pig which stands for pipe intelligence gauge can range from air-filled plastics to magnetic-flux leakage inspection tools. Once loaded, the trap door is closed and the trap is pressurized. With the main trap valve open, fluid can be directed behind the pig to push it into the pipeline. The reverse process applies at the receiver end which is connected to the end of a pipeline in a safe manner. The pig trap when in use is essentially part of the pipeline, and thus , it is important that it is capable of withstanding the pipeline conditions such as pressure, temperature and the effects of the service medium , i.e. Corrosion.
2.1.1 DESIGN Pig traps are pressure vessels, and like other pressure vessels, should be capable of retaining pressure whilst allowing tools to be launched and received. This determines the overall trap dimensions. Tools such as pigs are long and heavy and must be considered, along with the internal pressure requirements. Typical basic design parameters to be considered include design code, design pressure, design temperature, materials, external loadings and cyclic requirements. DESIGN CODE This should be of national or international standards such as ASME B31.4, B31.3, B31.8, PD5500, EN13445, and AS2885. Since they are attached directly to the end of a pipeline as an isolatable extension, traps can often be designed to a vessel code rather than a pipeline code. This provides a significant challenge in meeting the pipeline internal diameter with a thicker calculated vessel wall. In this case a specification break between the pipeline and trap. In other cases, the trap can be designed to the same code as the connecting pipeline. Whichever approach is selected must ensure all appropriate loadings and conditions are addressed to produce a safe working design. DESIGN PRESSURE The design pressure must be known in order to determine the wall thickness of the vessel. The design pressure should never be less than the pipeline pressure. The design pressure can be clientspecific or based on the ASME/ANSI pressure/temperature class tables. Upon completion, each vessel should be subjected to a hydro- static test pressure at least 1.25 times the design pressure. The actual test pressure is generally dictated within the chosen code. Some codes allow for the vessel to be pneumatically tested in lieu of hydrostatic testing. However, due to the risk associated with the use of air gas, special precautions must be taken. TEMPERATURE Temperature would take account of the maximum des temperature, but also the minimum ign design temperature for material selection.
MATERIALS Often specified by the clients, however where possible it is better to allow the trap manufacturer to select materials that are compatible with the pipeline materials and meet the design specification requirements. This approach ensures that the most suitable, cost effective and readily available materials that fully meet the design requirements are selected. It is important that line products are specified (whether it is sour, toxic, or corrosive). This will influence not only the metallic elements, but also the elastomeric materials, which typically constitute the closure-sealing element. Finally any material selected must be compatible with its mating material in respect of weld ability, wall thickness, chemical and mechanical properties.
EXTERNAL LOADINGS These loads consist of those imposed by attaching pipe work acting upon nozzles. In addition, external pressure due to water depth in subsea application could fall into this category. CYCLICAL REQUIREMENTS If the unit is subjected to frequent pigging operations, this may in turn create sufficient cyclical loading warrant fatigue analysis. In addition, support loads, wind blast and seismic loadings and loads induced during transportation of units should be considered. 2.1.2 LAYOUTS In addition to mechanical and process considerations, the trap designer must also consider practical issues such as location, environment and logistics. For subsea applications, the trap should include additional protection for the valves and pipe work in form of protection frame to reduce the risk of damage clashing with fishnets or ship anchors. For unmanned, hazardous, or frequent pigging operations, multiple launching capabilities should be considered. Pig traps must be located away from ignition sources and must also have sufficient space to open the door without infringing the space of the operator and allow the tools to be loaded and unloaded safely. The layout must ensure that all valves are accessible from either the ground level or via permanent platforms or ladder and that operation of any valve does not require the operator to enter the closure door opening envelop. The operation of the valve must where possible be from the side opposite the door hinge. Units should be located so that they are oriented with their end closures pointing away from personnel areas and critical items of equipment to minimize the risk of damage which might occur in the unlikely event of a pig being ejected from the trap under pressure. Dip trays and bunding must be installed to prevent contamination due to service medium.
Access must be available for lifting equipment to facilitate maintenance and operational requirements. On offshore applications, lifting facilities must be available for loading and unloading of tools. 2.1.3 OPERATION Pigging of a pipeline is generally undertaken in a manner that avoids any adverse effect on the environment and incompliance with applicable health and safety regu lations and procedures. Manufacturer s instructions should be followed for operation of pig traps. Major operations include: preparation (valve arrangements), sending pigs, receiving pigs, and tracking(including pig speed calculations).
2.1.4 MAINTENANCE Routine and scheduled maintenance is essential for optimum performance of the pig trap. Inspection should form part of the maintenance regime relating the conditions of components to inspection and maintenance in the future.
2.2 BLOCK VALVESControlling the fluid travelling through the pipe and equipment, and accessibility to the controlling apparatus are one the two major concerns in any piping facility. Block valves are used for interrupting the flow or to shut in a section of the pipeline. A block valve is normally either fully open or fully closed. This is why gate valves are preferable. The block valve is a major component in the control valve manifold. With these valves the operator can isolate any segment of the line for maintenance work or isolate any rupture or leak.
Fig 2. Gate valve
2.2.1 DESIGN The principal feature of gate valve is that while closing, they require no force to overcome the pressure of the working medium because the stopping element moves crosswise to the flow and require to only overcome the friction. This is due to the small area of the sealing surfaces of the gates (two narrow rings across the passage), which are reliable and leak proof. The advantages of gate valves over other stop valves arey y y
Negligible hydraulic resistance when the passage is open fully No reversal of the flow of working medium Possibility of use for highly viscous medium.
The block valve design are classified in terms of various features linked with operating conditions; chemical compositions, temperature and other parameters of working medium, level of operating pressure, type of drive and material from which the body is made of. The mechanical considerations for design of block valves arey y y y
Resistance to weathering Leak-tightness of seat and packing Resistance to bending between supports Should meet seismic standards
2.2.2 OPERATION The block valve should be able to block flow in both directions. Three methods of controlling block valves can be considered; locally, remotely, and automatically. The appropriate method will be determined from likely effects of a leak and acceptable release volumes, based on the total time in which a leak can be detected, located and isolated. The closure time of the valve should not create unacceptably high surge pressures. 2.2.3 LAYOUT Locations of block valves to control pipeline spills can be optimized for controllability, maintenance, and spill control. For onshore pipelines, the spacing for sectionalizing block valves should consider limiting the pipeline liquid contents between adjacent valves. The different types of block valves include; Manual gate valves which are locally operated valves and are placed in the check valve sections from time to time in other to ensure positive isolation, Remote gate valves which are used to protect sections of oil and gas pipelines in the event of a serious failure or break in the pipeline and Station block valve located both in the inlet or fluid suction side and outlet or fluid discharge side to separate the pipeline from the pump station in case of an emergency. 2.2.4 MAINTENANCE In order to maintain the block valve properly, the operator will need to implement a scheduled routine of sealant system t