Sizing Of Pipelines

SIZING OF PIPELINES

Oil Pipelines
Pumping a specified quantity of a given oil over a given distance may be achieved by using a large diameter pipe with a small pressure drop, or small diameter pipe with a greater pressure drop.

The first alternative will tend to a higher capital cost with lower running costs. It is necessary to strike an economic balance between these two.

There are no hard and fast rules, which can be laid down for achieving this balance. For instance, a pumping station in a populated area may consist of a simple building, involving the provision of electrically driven pumps, taking power from outside sources and little else. To obtain the same pumping power in remote or undeveloped countries would involve a considerably more complicated and expensive installation. Obviously in this latter case, it is desirable to reduce the number of pumping stations at the cost of using larger diameter piping.

Similarly, the cost of the pipeline will vary considerably, depending upon circumstances. It will be costly in highly industrialised areas, environmentally sensitive areas, offshore or in hostile, mountainous or swamp areas; cheaper in flat, soft but firm, undeveloped terrain.

Gas Pipelines
Sizing problems encountered in gas lines differ considerably from those of oil lines. A simplification results from the negligible weight of the gas as the pressure in the line is virtually independent of the ground elevation on the other hand, the compressibility of gas introduces the complication of the density decreasing and consequently the volume rate of flow increasing in the direction of flow. In an oil line of constant diameter laid on level ground, the pressure decreases uniformly with distance and the velocity stays constant whereas, in a gas line, the velocity increases as the pressure gradient decreases with an exponential, which becomes progressively steeper.

The characteristics of pumps and compressors also determine the site of any pipeline booster stations as well as the initial pipeline conditions which have to be met Pumps need to be sited in positions where they are receiving the crude oil at a pressure greater than the vapour pressure of the crude oil, whereas compressors have to be sited at a location where both the pressure and velocity of the gas are at optimum conditions.