This topic however deals with low velocities. The adjective I have used is deleterious for low velocities. The reason being that, while high velocities can have an immediate impact in terms of noise or vibrations in piping systems, the undesirable effect of low velocities in piping is more subtle and long-term. The adjective deleterious signifies this subtle and long-term effect of low velocities.
Let us come to those deleterious effects of low velocities in piping and pipelines. I will categorize the flow in pipes and pipelines based on the type of fluid and the fluid phase.
Flow of Slurries (liquid-solid homogeneous phase)
At very low flowing velocities, phase separation will occur of the solid particles, with the higher density solid particles tending to settle down at the pipe bottom of a horizontal pipe run. The settling will be even more at bends (direction change), or where there is a reduction in the pipe diameter. The deleterious effect is that over a period of time a layer of solid builds in the pipe and pipe fittings, reducing the pipe diameter leading to a) excessive pressure drop b ) partial or full disruption of flow c) pump operating point moving towards shut-off leading to reduced pump efficiency and accelerated mechanical wear and tear of the pump and pump sealing system.
Flow of Liquids (comprising of entrained heavier liquid with a lighter liquid)
A typical example would be a hydrocarbon liquid with entrained water, where the hydrocarbon liquid is the continuous phase while the water is the discontinuous phase. At low flowing velocities the entrained discontinuous phase water will drop out from the continuous phase hydrocarbon liquid to the bottom of the pipe. Water accumulation will occur in low points of the piping over a long term. If the hydrocarbon liquid has even trace amounts of dissolved carbon dioxide or hydrogen sulfide, pipe / pipeline corrosion can occur. The mechanism of corrosion in simplistic terms is that carbon dioxide and / or hydrogen sulfide will react with the accumulated water in the pipeline leading to acid corrosion by formation of Carbonic Acid (H2CO3) and / or Sulfuric Acid (H2SO4). Such corrosion can lead to failure of carbon steel piping / pipeline over a long term.
Single Phase Gas Flow (with entrained liquids)
A typical example would be natural gas containing entrained liquid droplets of water and heavier hydrocarbons. At low flowing velocities the flow would be stratified in the horizontal pipe, where the gas travels at the top of the pipe and the liquid travels at the bottom of the pipe and liquid accumulation occurs at low points and direction changes in pipe over a long period of time. The same problem of corrosion can occur as discussed for flow of liquids. Additionally, the gas transport could see high pressure drops and reduction in flow, putting excessive loads on gas compressors. Liquid accumulation over a long term in pipelines due to low velocities can also lead to intermittent slug flow in pipelines. The high momentum of liquid slugs can lead to structural damage of piping / pipelines and their supports.
Often natural gas pipelines have been found to have black powdery material (solids) in small amounts. The black powder could be because of corrosion products, trace amounts of solids carried over from gas treatment plants, mill scale etc. Low flowing velocities in gas transmission pipelines can lead to accumulation and deposition on pipeline walls of the black powder over a long-term and lead to excessive pressure drop and reduced flow. Flow velocities need to be kept above a threshold velocity also known as minimum entrainment velocity to prevent black powder deposits.
3-phase flow (Gas-Liquid-Liquid with Oil as continuous phase)
A typical example would be crude oil from reservoirs with associated gas (dissolved or free) and free water. Low flowing velocities will cause similar problems as discussed above related to corrosion. Additionally, if the crude oil is heavy crude oil containing asphaltenes, then reduced flow velocities (reduced flow) for a given pipeline will drastically increase the asphaltene deposition rate on the pipe walls. The long-term effect could be partial or total flow stoppage requiring costly cleaning operations for full restoration of pipeline operations.
Quantification of Minimum Velocities
I had prepared a standard “Specification for Process Design Basis” for a middle-east oil & gas operating company wherein I had mentioned also about minimum velocities in pipelines. For unlined carbon steel pipelines transporting liquid hydrocarbons (light crude oil or condensate) containing free or entrained water even in a small quantity (e.g. 1% water cut), the velocities should not be allowed to fall below 1.5 m/s to prevent water dropout. For gas pipelines I had mentioned that the normal range of flow velocities should be 5 to 10 m/s. For bone dry gas, velocities up to 20 m/s may be allowed, subject to design considerations for noise and vibration prevention during all operational scenarios.
As mentioned earlier, black powder in natural gas pipelines will not be entrained at low gas velocities. This may lead to accumulation at some portion of the gas pipeline over a long duration. The entrainment velocity for these very fine particles is a function of their micron size. Generally, for a particle size of 1 micron to remain entrained, the gas velocity should be in the range of 2.5 to 4.5 m/s, depending on the pipe size.
Measures to prevent low velocities
In intermittent or batch transfer operations, when demand is low at the receiving station, continue pumping at the same high rate as required for peak demand but for a shorter duration. There is no need to reduce the flow rate for low demand in batch transfer operations. Standard Operating Procedures (SOPs) should address such turndown or low demand scenario without reducing the flow and thus the velocity.
Measures to mitigate low velocities
In continuous operations and prolonged turndown scenarios, low flow velocities are unavoidable. To prevent the aforementioned problems associated with low flow velocities, some measures that could be implemented are described below:
1. Injection of anti-corrosion and anti-scale additives in the piping / pipeline system.
2. Addition of emulsifying agents in liquid hydrocarbon-water systems to prevent phase separation occurring at low velocities.
Note: The addition of additives to prevent corrosion, deposits and phase separation should be carefully evaluated from the viewpoint of chemical compatibility with the process fluid and any adverse effects in downstream applications.
To conclude, low flowing velocities in pipes and pipelines can create problems such as corrosion, scaling and deposit formation and design and operational measures should be considered to prevent operations at low velocities.