Erosion corrosion is acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfaces of a tube can result in rapidly increasing erosion rates. Erosion corrosion can also be aggravated by transitions from laminar fluid flow to localised turbulent flow, resulting in erosion corrosion. A combination of erosion and corrosive fluids can lead to extremely high pitting rates. There are several sources of relvant information regarding the calculation of maximum velocities:
Normal practise is to use equation of API 14E RP and evaluate the empirical factor “C”. This can be unreliable especially for clean production where the limiting value of C (125) can be too restrictive. In practice, values of 1000 for C have been recorded in pipes where no erosion has been detected. See RP for equation, very simple...
As part of the design process we can calculate the values of C along the completion length and assess whether for these C values erosion will occur or not depending upon the expected operating conditions. An interesting paper that deals with the subject and elaborates on this process is: "Criteria for Sizing Multiphase Flow-lines for Erosive/Corrosive Service" by S.J. Svedeman - SPE Paper 26569.
The debate regarding the erosional velocity still big concern, many investigators reviewed the velocity limits specified in the RP 14E and concluded that the limits are too conservative.
Some investigators define the use the C value as follows:
C=100 for continuous service
C=125 for intermittent service
C=150-200 increases where corrosion is controlled by inhibitors
In cases where sand is present even at very low velocities erosion will be a problem and there are no guidelines on the amount of reduction to the calculated velocity in these cases. Other investigators calculated that the velocity that would produce liquid impingement erosion corresponds to C=300. Other cases, for example the erosional velocity limits for water injection system correspond to values of C=450 without any solid and non-corrosive conditions but when corrosives are present this value can be reduced to C=250 depending on the corrosive condition.
Cavitation is another phenomena that can contribute to the erosive process. Cavitation can be very damaging to pipework and piping components, eg valves. When liquid passes through a restriction low pressure areas can be generated, for example downstream of a sudden step. If the pressure is reduced below the vapour pressure of the liquid, bubbles are formed. These bubbles then collapse generating shock waves. These shock waves can be of sufficient amplitude to damage pipework.
Cavitation is rare in oil and gas production systems as the operating pressure is generally much higher than liquid vapourisation pressures. Evidence for cavitation is sometimes found in chokes, control valves and pump impellers, but is unlikely to occur in other components.
As with droplet erosion, cavitation erosion is not well understood, and the most practical approach is to identify whether it is an issue or not and then act accordingly. Under normal operating conditions cavitation erosion is unlikely to occur in high pressure, naturally flowing wells unless it is immediately downstream of a severe flow restriction (eg a choke valve or nipple).
A Conoco paper (“An Alternative to API14E Erosional Velocity Limits for Sand Laden Fluids - ASME Transaction - Energy Resources Technology Journal”) Ref [2]. challenges API14E on the basis that it can be very conservative for clean service and is not applicable for conditions where corrosion or sand are present. It proposes a simple alternative approach that has been verified by a comparison with several multi-phase flow loop tests that cover a broad range of liquid-gas ratios and sand concentrations.
The following is an excellent source of information and is available on the net: “Erosion in elbows in hydrocarbon production systems: Review document” Mr N A Barton, TÜV NEL Limited, Scottish Enterprise Technology Park, East Kilbride, Glasgow, G75 0QU on Behalf of the Health and Safety Executive.
