Line Pipe Allowable velocity 2

[Mark62]

What is a typical allowable water velocity used for sizing a rigid flowline (API 5L Line pipe)? The flowline is to be used for water injection, the fluid is process water, which should have been cleaned of most of the nasties.

Second question, I have a typical absolute roughness of 0.15mm for new line pipe and have seen references that this might deteriorate to about 1.5mm due to pipe corrosion. Is this about right?

I know that there are a lot of unknows, such as the exact nature of the water, how much gas, is there any sand etc. but at this stage I just need an indication as to what diameter pipe I might need.

Many thanks

 

[nosaMgiarC]

If your talking about water injection then you'll have a very low (nominal) solids content, so straight forward erosion should be minimal. However in a carbon steel system you could get synergistic corrosion erosion which is related to velocity. You could also get straighforward erosion if you water oxygen content is too high, ie greater than about 20 ppb (yes billion). Assuming you don't get into the synergistic corosion erosion regime then you can subject the system to high velocities and typically you will be limited by the pressure drop in the system well before you hit an erosion issue. Running a clean system you could easily push up to 15 m/s, but if you have significant solids then this number comes down rapidly!

Yes you could hit absolute roughness of 1.5mm, however this will typically be in a system that has poorly badly designed. With an erosion allowance of 0.1 mm/yr it would still take 15 yrs to reach this stage.

Hope this helps

C.

 

[geoffreydavies]

I would also pay attention to the hydraulics of the system. Take care with surge, and don't put any fast closing valves in the line. Especially if you have a few miles of large diameter pipe with water at high velocity.

 

[Hookem]

Mark62:

Typical high end water velocity limit is 15 feet per second. Typical low end water velocity is 3 feet per second. Most economic choices center around 7 - 8 feet per second. However, crude oil, diesels, HVGO, asphatines, etc. require separate considerations due to viscosity, temperature, particulates, etc.

 

[samlukeben]

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.