SALTWATER INJECTION SYSTEMS CAN TOLERATE HIGHER VELOCITIES

Mamdouh M. Salama
Conoco Inc.
Ponca City, Okla.

The API erosional velocity limit is overly conservative for saltwater injection systems constructed from corrosion-resistant material and containing no solids in the water.

Under such conditions and providing that pressure drop is not of concern, a flow velocity of 50 fps can be safely used without concern for erosion.

SELECTING FLOW VELOCITY

Factors influencing the selected flow velocity include:

Pressure drop
Noise level
Vibration
Erosion
Corrosion.

Pressure drop, noise, and vibration are well understood and are part of the design of most flow systems. Not well understood are erosion and corrosion.

While erosion is defined as the removal of material from a solid surface by the repeated application of mechanical forces, corrosion involves the removal of material by an electrochemical reaction.

To avoid erosion, the oil industry's current practice for sizing process piping, flow lines, pipelines, and tubing is to limit flow velocity to the maximum erosional velocity as calculated by Equation 1 (see Equation and Nomenclature boxes)."

The empirical constant C in Equation 1 is 1.00 for continuous service and 125 for intermittent service. Consideration should be given to reducing these values if solids production (sand) is anticipated.

In the latest API RP 14E 2 higher C-values of 150-200 may be used when corrosion is controlled by inhibition or by corrosion resistant alloys.

Based on the API RP14 E equation, most seawater injection systems limit the flow velocity to 8-10 fps. In many cases, this restriction has negative cost implication. New recommendations are proposed that show that much higher velocities can be tolerated safely.

ASSESSMENT

The original API criterion is specified for clean service (noncorrosive and sand free), and it is noted that the C-factor should be reduced if sand or corrosive conditions are present. No guidelines, however, are provided for these reductions.

It has been argued by several investigators that under these conditions the API RP 14E relation is extremely conservative. 3 4 Their work led to the changes in the 1991 edition. Also, because of its conservatism, the API RP 14E criterion is not followed by most oil companies. 4 5

Mobil does not limit flow velocities, and Arco uses a C-factor of 200 for continuous service. 4 When corrosion is controlled and if sand can be avoided, Arco changes the C-factor to 250 for intermittent service. 4

Data developed by Arco on velocity effect of inhibited systems (with and without solids) on carbon steel and 316 stainless steel for pipes, elbows, and chokes, showed that for a straight pipe section no erosion/corrosion was observed for C-factors up to 500. 4 For other components, no erosion/corrosion was reported for C-factors up to 300, even with sand. 4

Phillips does not use RP 14E to determine production rates. 5

One North Sea operator produced from a condensate well at a velocity of 286 fps (C-factor of 726) for 1,050 days without failure. 6 Another North Sea operator has used a C-factor of 300 as the upper limit for several subsea water injectors with L80-13 Cr tubing. 6

One should not, however, be surprised if corrosion failure occurs in this system at the joints because of the susceptibility of 13 Cr to crevice corrosion and pitting.

For N-80 steel, tests have shown no erosion damage after repeated impact by liquid slugs at a velocity of 100 fps, which corresponds to a C-factor of 800. 7 When erosion damage was observed, it was attributed to the presence of microscopic solid particles in the liquid.

In a seawater flow loop containing fiber glass pipe and pipe bends of CuNi and stainless steel, 3-month tests were conducted at a velocity corresponding to C-factors between 220 and 260 in. 8 The tests showed no erosion damage in the fiber glass, CuNi, or stainless steel.

Also, recent single (distilled water) and two-phase (water and nitrogen) flow-loop test results on simulated tubular joints 9 showed that, providing corrosion can be suppressed, a C-factor of 450 can be used without any concern for erosion.

Based on these experiences, it is clear that for non-corrosive water injection systems where solids are not present, providing that pressure drop is not of a concern, a C-factor of over 400 and a flow velocity of 50 fps is safe.

VELOCITY LIMITS

In carbon steel water-injection systems, the velocity limit appears to be controlled more by corrosion than erosion. This is because the oxygen corrosion rate, even at very low levels of oxygen (about 20 ppb), is finite and not zero.

The oxygen corrosion rate is influenced by oxygen level, mass flow rate of oxygen to the surface, and temperature. Using the estimate for diffusional flux to surface and both field and laboratory oxygen corrosion rate measurements, Equations 2 and 3 establish allowable C-factors for carbon steel injection water systems. 10

These two equations are derived based on corrosion rates due to oxygen diffusion and, therefore, will predict very high allowable C-values for very low oxygen concentrations.

At very low oxygen levels, an activation rather then diffusion controlled corrosion mechanism may take effect and corrosion will occur. To incorporate this effect, Equations 1 and 2 should be used for predicting allowable C-factors up to 250.

These equations do not incorporate the reduction in corrosion rate at high levels of oxygen and chlorine due to scale formation and, therefore, the predicted C-factors will be conservative.

Equations 2 and 3 were derived by assuming that the tolerable corrosion rate corresponds to a reduction of 20% of the wall thickness of the pipe (t, in.) within its life time (Y, years). This assumption is justified based on ANSI/ASME B31G-1984 failure prediction criterion for corroded pipes.

Because most design engineers prefer to base the selection of the allowable velocity on the corrosion allowance (delta, in.) criterion rather then the 20% wall thickness criterion, the above equations can be written as Equations 4 and 5 by replacing t by 5 delta.

In terms of flow velocity (V, fps), Equations 4 and 5 can be rewritten as Equations 6 and 7.

EXAMPLE

For the design conditions of 25-year life, 20 ppb oxygen concentration, no chlorine, and 30 C., the allowable flow velocity depends on the corrosion allowance of the carbon steel component corrosion allowance as given in Equation 8 and Table 1.

Assuming that the corrosion allowance for the pipe is the normal 0.16 in. (4 mm), the allowable flow velocity is 18 fps.

REFERENCES

"API RP 14E Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems," Third Edition, 1981, P. 22.
"AP[ RP 14E Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems," Fifth Edition, 1991, P. 23.
Salama, M.M., and Venkatesh, E.S., "Evaluation of API RP 14E Erosional Velocity Limitations for Offshore Gas Wells," Paper No. OTC 4485, 15th Offshore Technology Conference, Houston, 1983.
Deffenbaugh, D.M., and Buckingham, J.C.,"A Study of the Erosional/Corrosional Velocity Criterion for Sizing Multi-Phase Flow Lines," Southwest Research Institute Final Report, Project No. 04-2433, prepared for the Minerals and Management Service, U.S. Department of the Interior, 1989.
Heidersbach, R.,"Velocity Limits for Erosion-Corrosion," Paper No. OTC 4974, 17th Offshore Technology Conference, 1985.
Erichsen, H.,"Nipple, Lock and Flow Coupling Recommendations and Subassembiy Description for North Sea Wells," Private communications, Conoco Norway, Sept. 1, 1988.
Camach, R.A.,"The Design, Construction, and Testing of a Liquid Impingement Apparatus and a Study of Metal Surfaces Eroded by Liquid Impingement," Master of Science Thesis, University of Tulsa, 1988.
Saetre, O.,"Testing of Composite Pipes in High Velocity Seawater," 10th International Conference on Offshore Mechanics and Arctic Engineering, 1991, pp. 577-583.
Salama, M.M.,"Erosional Velocity in Noncorrosive Two-Phase Flow Systems," Unpublished Report, 1992.
Salama, M.M.,"Erosional Velocity Limits for Water Injection Systems," Paper No. 62, NACE Annual Conference, New Orleans, 1993.