Research Papers: Petroleum Engineering

Interfacial Contamination Between Batches of Crude Oil Due to Dead-Legs in Pump Station Piping

[+] Author and Article Information
K. K. Botros

NOVA Chemicals,
Centre for Applied Research,
Calgary, AB T2E 7K7, Canada
e-mail: kamal.botros@novachem.com

E. J. Clavelle

NOVA Chemicals,
Centre for Applied Research,
Calgary, AB T2E 7K7, Canada

G. M. Vogt

TransCanada Pipelines Ltd.,
Calgary, AB T2P 5H1, Canada

Contributed by the Petroleum Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received October 26, 2015; final manuscript received March 31, 2016; published online May 12, 2016. Assoc. Editor: Christopher J. Wajnikonis.

J. Energy Resour. Technol 138(5), 052908 (May 12, 2016) (8 pages) Paper No: JERT-15-1404; doi: 10.1115/1.4033401 History: Received October 26, 2015; Revised March 31, 2016

Some oil pump station design layouts may contain multiple dead-legs. During the transportation of heavy crude through the pump station, these dead-legs will be filled with this crude. When a light crude batch is introduced next into the pipeline, following the heavy crude ahead, two phenomena will occur. First, contamination between batches at the interface of the two crudes will occur due to axial turbulent diffusion along the length of the pipeline itself. Second, as the light crude flows through the pump station and passes by each dead-leg containing still heavy crude from the preceding batch, the heavy crude trapped in these dead-legs will start to drain out into the passing light crude in the main run. This causes further contamination and spreading of the mixing zone between the two batches. These two different sources of contamination are addressed in this paper with the objective of accurately quantifying the extent of the contamination, with particular emphasis on the second phenomenon which could cause appreciable contamination particularly for large size and number of these dead-legs. A computational fluid dynamics (CFD) model has been developed to quantify the drainage rate of the contaminating crude into the main stream and its impact on widening the mixed zone (contamination spread) between the two batches. Two drainage mechanisms of the heavy crude in the dead-legs into the main stream of the light crude have been identified and quantified. The initial phase is a gravity-current-induced outflow of the initially stagnant fluid in the dead-leg, followed by a subsequent draining mechanism primarily induced by turbulent mixing and diffusion at the mouth of the dead-leg penetrating slightly into the dead-leg. It was found that the second mechanism takes a much longer time to drain the first, and that the break point in time where drainage switches from a predominantly gravity current to a turbulent diffusion appears to be at a specific time normalized with respect to the length of the dead-leg and the gravity current speed. The results show a consistent trend with actual interface contamination data obtained from the Keystone 2982 km pipeline from Hardisty (Canada) to the Patoka Terminal (U.S.A.).

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Fig. 1

Example of a dead-leg in a pump station design layout caused by a by-pass line

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Fig. 2

Schematic of interface development due to axial turbulent mixing between heavy and light crude batches (L is the distance along the pipeline from the start to the location of the mid concentration point within the mixed zone)

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Fig. 3

General correlations for axial diffusion coefficient [12]

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Fig. 4

Concentration profile of interfacial contamination between heavy and light crudes of Table 1 (KMP)

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Fig. 5

CFD computational domain

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Fig. 6

CFD generated top-view of the state of the heavy and light crudes at time = 50 s when the first gravity wave arrives at the closed end of the dead-leg

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Fig. 7

CFD images of a vertical cross section passing through the centerline of the dead-leg at different times, showing various stages of heavy crude draining out at the mouth end (top image: 100% light crude, bottom image: almost 100% heavy crude)

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Fig. 8

Results of CFD simulation showing the volume of heavy crude remaining in the dead-leg following the passing of a sharp interface with light crude

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Fig. 9

Resulting concentration profile of heavy crude in light crude at the outlet of the pump station due to presence of four dead-legs in the station piping layout

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Fig. 10

Normalized volume of heavy crude remaining in the dead-leg versus normalized time following the passing of a sharp interface with light crude

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Fig. 11

Concentration profile at pump station inlet at 90 KMP, and results from CFD simulation of the heavy crude volume remaining in the dead-leg

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Fig. 12

Concentration profiles at pump station (at 90 KMP) inlet and outlet and further diffusion profile to the next pump station at 180 KMP

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Fig. 13

Concentration profile at pump station inlet at 180 KMP, and results from CFD simulation of the heavy crude volume remaining in the dead-leg

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Fig. 14

Concentration profiles at pump station (at 180 KMP) inlet and outlet

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Fig. 15

Data from Keystone pipeline of an interface between two batches of crude oil arriving at Patoka Terminal [24]

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Fig. 16

Comparison between field data from Keystone pipeline and strictly axial diffusion along the pipeline based on the condition in Table 2




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