4 replies to this topic

[ankur2061]

Gentlemen,

Can anybody help in providing "C" values for the erosional velocity other than recommended by API RP 14E, for various kinds of fluids such as follows:

a) gas/liquid i.e. two-phase

gas/liquid/solid (soild content <1%)

c) gas/liquid/water/solid (solid content <1%)

The gas, liquid and solid mentioned in above examples are associated gas, crude and sand/mud transported from typical oil and gas production platforms.

Also request to provide reference for these values.

Regards,
ankur2061

[Art Montemayor]

ankur2061:

The “c” used in API 14E is an empirical constant, intended to represent the corrosive and erosive effects of sand and other solids in a fluid. The definition of its use and the assigned values of 100, 125, 150, and 200 are clearly explained in API 14E.

The “c” has nothing to do with whether a fluid is 2-phase or not or the proportion of gas-to-liquid. The 2-phase degree is taken into consideration by the density of the mixed fluid in the equation:

V_e = c / (rho_m)^1/2

Where,
V_e = the fluid erosional velocity, ft/sec
rho_m = gas/liquid mixture density at flowing pressure and temperature, lb/ft³.

As API 14E clearly states, “Different values of “c” may be used where specific application studies have shown them to be appropriate.” You would be wise not to rely on what other people simple pass on to you without running your own study or trials. Normally, no one does this; they simply employ 100 or 125 for “c” values.

Why do you ask these questions and what do you intend to do with the responses? Never forget that API 14E is an RP (Recommended Practice) and is NOT a guarantee for anything.

[ankur2061]

Dear Art,

Thanks for the insight. I have gone through API RP 14E and the equation and "C" values are all clear to me.

However, latest studies show that the API RP 14E values for the constants are far too conservative and impractical. In fact studies done by Shell and BP show that using API RP 14E values would require replacement of major pipelines at many of their installations which have been running sucessfully at higher velocities than as calculated by the API equation without any problems.

In fact I will quote verbatim from the abstract of the paper titled "An Alternative to API 14E Erosional Limits for Sand-Laden Fluids" submitted by Mamdouh M. Salamah to ASME for further clarity:

"The current practice for eliminating erosional problems in piping systems is to limit the flow velocity (Ve) to that established by the recommended practice API RP 14E based on an empirical constant (C-factor) and the fluid mixture density (m) as follows: Ve = C/. The 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. The validity of the equation has been challenged on the basis that the API RP 14E limits on the C-factor can be very conservative for clean service and is not applicable for conditions when corrosion or sand are present."

Since I am involved in preparing some pipeline design guidelines for my client, who is aware of the limitations of the API formula, the client requested me to find out more appropriate "C" values than given by API. That is how the question came in the forum.

Sometimes we don't have answers to all our questions and hence we resort to help from others. I was trying to find out something new on the subject, and not a re-run of API RP 14E.

[Art Montemayor]

Ankur:

Once again, I stress the importance of noting that API 14E is a RECOMMENDATION. It is not a design basis and much less a standard, rule, or mandate. If you (or others – such as Mr. Salamah) can come up with the correct “c” value that will yield an acceptable, cheap, durable, and safe pipe size than it should be published and followed. However, as you and most of us know, that will never be the case since no one in his/her right mind is going to undertake the time and expense to field test every conceivable combination of oil, water, gas, sand, and dirt in order to generate the “correct” answer and donate it to humankind. It just isn’t going to happen.

In fact, in all of my professional years designing and operating equipment I have never heard or seen a design that ensures that the assumed quantity, flowrate, and type of sand mixed with the crude oil will always be constant. What happens in reality is that sand travels as a semi-slug! It gushes through sometimes; at other times, it settles as it forms deposits. As the deposit increases, the pipe cross-section decreases and the resulting velocity INCREASES – entraining and slugging through the sand that had previously settled. And this goes on, and on, and on ……. So what is going to be the design figure or criteria for the sand proportion in the equation to be used? The worst case, of course. And this is what, unfortunately, the API has had to confront. That is why the result is a conservative answer – and thank God for that. No one to date has come up with a more logical manner of establishing a design procedure for handling this type of fluid flow. And I don’t see anyone forming lines to establish a design equation that they will guarantee the results as being “accurate”.

I respect your client’s desire to obtain a more accurate answer (in order to save capital piping money), but if there are “more appropriate "C" values” out there in process design land, I wish they were identified and labeled as such (with guarantees) so that we could all use them. I don’t think that is the case, unfortunately. Therefore, that is why we all seem to be forced to follow API 14E recommendations. I really wish I could generate a more optimistic and hopeful answer on this subject.

[djack77494]

Ultimately, this like many engineering decisions comes down to economics (assuming that safe and environmentally sound operations are achieved). Do the economics favor a lower capital cost for shorter life, smaller diameter pipe or more up-front cost for a longer life. Though we rarely discuss pipe sizing in those terms, this is ultimately the basis for selecting the line size. Let me note that it's actually quite a bit more complicated that this. Smaller pipe = higher hydraulic losses = greater pumping costs. Smaller pipe = higher velocities = more reactionary forces. Higher velocities = more erosion but less deposition. We could go on and on. On top of this, the true analysis of these decisions is very much local and varies with time as energy and commodity costs move up and down. It's normally not worth the effort to analyze each local situation, so we rely on previous work based on outdated assumptions. BTW, I agree that API14E is quite conservative.
Doug