Liquids can occur in a natural gas transmission network e.g. as a byproduct of gas processing or compression (NGL, oils, glycols). At the numerous T-junctions that are found in a typical transmission network, such liquids are generally not equally divided with the gas. This behaviour, sometimes called route preference, causes some locations to be more bothered by liquids than others. Gasunie has introduced the Double Stream Model for the prediction of route preference as an operational tool. A multi year case history presently exists in which this model has been applied to a full scale natural gas transmission network. Some physical backgrounds and real cases are presented.
Liquids can occur in a natural gas transmission network (120..1200 Psig or 8..80 Bar) e.g. as a by-product of gas processing or compression. Some examples: Glycols sometimes enter the network as entrainment of cold separators used to process natural gas to desired water specifications. Compressors can be a source of seal or lubrication oil. Natural gases are subject to form condensates (natural gas liquids or NGL's), with a maximum likelihood in the pressure range of 300..600 Psig (20..40 Bar), due to the retrograde type of condensation behaviour. Early observations in the 600 Psig (40 Bar) regional transmission network of Gasunie by Oranje [l], showed that liquids - once present - would only arrive in some places, while most locations remained dry. These phenomena were obviously related to a typical form of twophase flow behaviour in the large number of T-junctions occurring in the network. At T-junctions liquids are generally not equally divided with the gas, a behaviour called route preference. From a statistical analyses of his observations Oranje concluded that: if <20% of the gas stream enters the branch of a T-junction (see Figure l), all liquid will flow to the run; if *35X of the gas stream enters the branch, all of the liquid will flow to the branch. Somewhere between 20 and 35% gas in the branch, there is a cross-over point, were the liquid flow sharply changes direction. This so-called 20/35-rule of thumb holds for reduced T-junctions (D,tD,=D,), while for regular T-junctions (D3=D,=D2> a similar cross-over point was found somewhere below 20% gas to the branch. In the late 80's and early 90's Gasunie has been closely involved in the further perfection of the early rules of thumb. The latest development in this field in which Gasunie has been involved, is the Double Stream Model designed by the University of Amsterdam [2, 31. A stand-alone PC-version for high pressures (up to 1600 Psig or 110 Bar) and large pipe diameters (up to 48" or 1.2 m) has been introduced at Gasunie as an operational tool [4]. There is now a multi-year case history in which this model has shown to be generally accurate and reliable, also for a full scale network, although the model has originally been developed for laboratory experiments on air/water, small pipe diameters (2..4" or 5..10 cm) and atmospheric pressure.
