Whilst there is extensive industry experience of under pressure welding onto live natural gas and liquid pipelines, there is limited experience for Carbon Dioxide (CO2) pipelines, either in the gaseous or dense phases. National Grid has performed a detailed research programme to investigate if existing natural gas industry under pressure welding procedures are applicable to CO2 pipelines, or if new specific guidance is required. This paper reports the results from one part of a comprehensive trial programme, with the aim of determining the preheat decay times, defined by the cooling time from 250 °C to 150 °C (T250–150), in CO2 pipelines and comparing them to the decay times in natural gas pipelines. Although new build CO2 pipelines are likely to operate in the dense phase, if an existing natural gas pipeline is converted to transport CO2 it may operate in the gaseous phase and so both cases were considered. The aims of the work presented were to:
• Determine the correlations between the operating parameters of the pipeline, i.e. flow velocity, pressure etc. and the cooling rate after removal of the preheat, characterised by the (T250–150) cooling time.
• Compare the experimentally determined T250–150 cooling times with the values determined using a simple one dimensional heat transfer model.
• Define the implications of heat decay for practical under pressure welding on CO2 pipelines.
Small-scale trials were performed on a 150 mm (6″) diameter pressurised flow loop at Spadeadam in the UK. The trial matrix was determined using a one dimensional heat transfer model. Welding was performed on a carbon manganese (C-Mn) pipe that was machined to give three sections of 9.9 mm, 19.0 mm and 26.9 mm wall thickness. Trials were performed using natural gas, gaseous phase CO2 and dense phase CO2; across a range of flow velocities from 0.3 m/s to 1.4 m/s.
There was relatively good agreement between the T250–150 cooling times predicted by the thermal model and the measured T250–150 times.
For the same pipe wall thickness, flow velocity and pressure level, the preheat decay cooling times are longest for gaseous phase CO2, with the fastest cooling rate recorded for dense phase CO2.
Due to the fast cooling rate observed on dense phase CO2, the T250–150 times drop below the 40 second minimum requirement in the National Grid specification for under pressure welding, even at relatively low flow velocities. The practical limitation for under pressure welding of pipelines containing dense phase CO2 will be maintaining sufficient preheating during welding.
The results from this stage of the technical programme were used to develop the welding trials and qualification of a full encirclement split sleeve assembly discussed in an accompanying paper (1).