This paper calculates minimum flow velocities required to move solid particles in oil and gas process piping and pipelines based on a hydrodynamic model. The results can be applied to sand erosion in oil and gas wells and black powder movement in gas pipelines.
In oil and gas production and pipelines, solids can be a major problem, causing erosion if the velocity is too high, and partial blockage and fouling of pipe if it is too low. The results are that pipelines cannot be inspected by In Line Inspection (ILI) pigs, have great difficulties in pigging, and erosional damage erosion damage is experienced by downstream equipment such as gas compressors on pipelines and gas turbines driving equipment such as compressors and generators. The velocity to move solids in water, oil, and gas is calculated for a number of operating conditions in this paper. The calculation is based on a hydraulic model to determine thevelocity required to lift a particle from a bed of loose particles and transport it down a pipe. This velocity is a function of fluid properties, particle size, shape, and density, and pipe diameter. When the fluid velocity is less than required to move particles, they accumulate in a bed for which the maximum bed height can be calculated. If pipe ocity is increased at a later time, it can result in movement of large quantities of solids, which can plug filters and damage downstream equipment. Pigging to clean a pipeline may also loosen solids adhering to a pipe wall, freeing them up to travel great distances down a pipeline. One important result of these calculations is that once a particle begins moving in a gas pipeline, it will continue to move until it reaches a compressor station or the flow rate in the line is reduced.
A technique to calculate the velocity required to sweep solids through horizontal pipelines by fluid drag has been developed by Wicks (1). A similar analysis has also been presented by Wicks and Fraser (2) to predict the entrainment velocity for water in flowing oil, one of the most important and fundamental methods for analysis of internal pipeline corrosion. The following analysis was developed to predict at what fluid velocity particulates can be dislodged from a bed of loose particles in the bottom of a pipe, and then carried through the pipeline. Higher velocities are required when the particles are wetted by compressor oil or glycol or are “glued” together by paraffins, asphaltenes, or corrosion inhibitors. The following section reviews the development of Wicks' analysis A schematic of the forces acting on a particle in a bed of particles is shown below in Figure 1. Gravity and buoyancy are constant, while lift and drag vary with flow conditions, depending on inertial and viscous forces in the fluid. At low flow rates, gravity and buoyancy predominate, and particles settle to the bottom and form a layer of sediment.
