ASME B31.3 vs ASME B31:4: Usage and Calculations For Ore Concentrate (Slurry) Pipeline 2

[jackieray]

Dear all,

Greetings everyone. I have something I want to ask about pipeline construction code.

I have recently started working at a mineral processing company that produces ore concentrate. The concentrate slurry (water and ore mixture, 66% solid) is transported over 62 miles (100 km) through the slurry pipeline to a concentrate dewatering facility. I am now tasked with a project to determine the cause of accelerated wear to the pipeline which, according to the senior engineers here, cuts the service life of the slurry pipeline to almost half than what it used to be (15 years decreased to 7 years).

Before tackling the main bulk of the project, I discussed about the current method of wear calculation with the pipeline engineers. They said NDT is used to measure pipeline wall thickness wear for each mile (mile 1, mile 2, mile 4, ..., mile 62). However, due to the mountainous terrain of the pipeline location, the NDT measurement that was started only a little over 1 year ago cannot be done regularly (IMO, even quarterly NDT measurement for each mile is near impossible). Also, some part of the pipeline is buried underground, adding to the difficulty of the NDT.

The engineers have calculated the minimum allowable thickness (MAT) for each mile since there are differences in concentrate pressure (specified by pipeline constructor). Here are some questions I want to ask to the distinguished engineers here.

1) I noticed that they are using ASME B31.3 to calculate the MAT. As I have read previously on the threads here, the B31.3 is said to be used for process piping. Should the MAT calculation be redone using ASME B31.4 which is specified for pipeline transportation for liquids and slurries?

B31.3 MAT formula : t = PD/2(SEW+PY)
B31.4 MAT formula : t = PD/2S

2) I have tried calculating the MAT using both codes. I found that the MAT determined using B31.3 is well thicker than those of B31.4, which is in line with what the members of EngTips found. However, I also found that my calculation for MAT using B31.4 is also thicker when compared to the calculations of the pipeline engineers, which used ASME B31.3 as their code.

Can anyone point out my mistake in calculating the MAT value? Below is the data of the pipeline that I used (values determined using B31.4 unless otherwise noted).

- Carbon Steel API 5L X60, seamless, NPS 6", Sch 80, D 6.625 in, d 0.438 in (by most standards it should be 0.432 in but 0.438 is what the engineers here use)
- Slurry operating temperature (T) = maximum (approx.) 100 F (operational data)
- Quality factor for seamless pipe (E) = 1
- Weld joint strength factor for T<400 F (W) = 1
- Coefficient for ferritic steel & T<482 F (Y) = 0.4
- Specified minimum yield strength for API 5L X60 (Sy) = 60200 psi (from API 5L Specification)
- Design factor (F) = 0.72
- Basic allowable stress (S) = approx. 43300 psi (calculated using S = S x E x Sy from ASME B31.4)
- Pipeline measured internal slurry pressure (P) = up to 4500 psi (on steep hills, value determined by pipeline conctructor)

Using the B31.4 formula for MAT, I found that the MAT is around 0.344 in (8.73 mm) for 4500 psi of slurry pressure.
However, the pipeline engineers' calculation using B31.3 formula for MAT is 22% lower, around 0.270 in (6.87 mm).

Thank you in advance, everyone.

 

[LittleInch]

Velocity is Q/A.

Just use consistent units and ID. What did you get.

You can work out density if you know the dry density of the Ore.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jackieray]

@LittleInch

My bad. I was mistaken. Thank you for the corrections. I messed up the units. Below is the calculations, Google helped with the conversions (sorry for not doing them myself).

Q = max. 500 GPM
Q = 0.0315451 m^3/s

A = 3.14 x r^2

ID = OD - (2 x t)
ID = 6.625 in - (2 x 0.438 in)
ID = 5.749 in

r = 0.5 x ID
r= 2.8745 in

A = 3.14 x (2.8745^2)
A = 14.48 sq.in
A = 0.008374177 m^2

So,
V = Q/A
V = 0.0315 m^3/s ÷ 0.0083 m^2
V = ~0.94 m/s

I'm currently still away from my PC. Will post the elevation as soon as I get home.

EDIT: wrong formula for A. Ended up getting a different result compared to @LittleInch.

 

[LittleInch]

You get the right answer, but area is just Pi x r^2. You added another 2 for some reason?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jackieray]

@LittleInch[/b

Again, my bad. I have revised it. Still ended up with different result now.

 

[LittleInch]

You're going too fast.

2.87 x 2.87 x 3.14 is 25.86 in^2....

No idea how you got 14.48

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jackieray]

@LittleInch

I must be very tired yesterday... Thanks for the corrections, again.

For the update, I have now met with the pipeline engineers. Turns out the difference in our calculations was caused by 1) they used B31.3 2012 version to calculate (the formula is still the same as the 2022 version, however) and 2) they used 60200 psi as the S, not counting the design factor 0.72 and E = 1 as opposed to the S formula in B31.4-2022 (S = F x E x SMYS). They used 60200 psi as their S, and S used 43344 psi (F x E x SMYS) as my S.

Hence why their calculations for MAT are somewhat thinner compared to what I found.

Also I'm sorry but I cannot find the elevation profile that is acceptable to upload here, due to confidentiality reasons... I'm sorry, again. But if you don't mind me asking... what kind of calculations that we can do using elevation profile?

 

[dik]

LI... thanks for the articles.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[1503-44]

Pressures are primarily the result of flow rate, diameter, length, pipe roughness, viscosity and ELEVATIONS.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

jackie,

Lets start from the basics here:

ASME B 31.3 is a process piping design code and whilst it is sometimes used for pipelines it is not the most appropriate code, especially for buried pipelines.

It's wall thickness calculation uses the value of S as found in table A-1 of the B31.3 code. For X60 at ambient temperatures, this is 25,000 psi

ASME B 31.4 is a liquids including slurries pipeline code and appropriate for use for buried and above ground pipelines, but is more suited to buried lines.
The wall thickness calculation uses a value of S which is essentially equal to Sy x F x E. For liquid lines, the design factor F is a max of 0.72. E is 1. So for B31.4 S = 43,344

The MAT thickness using a design factor of 1 is a very basic assessment seeing if the remaining wall thickness would lead to yield and then rupture of the pipeline. This is a very crude assessment as it takes no account of the size and length of the defect or wall thinning. For this assessment you probably need to start looking at ASME B31G.

In terms of your OP - why is this happening? You need to consider any changes in the slurry, why the original designers do not apparently either include any allowance for this erosion (3mm, 5mm, whatever) or use internally lined pipe (rubber, PE). Your wear will be concentrated at bends which I hope are at least 5D "bends" and not "elbows". This is where you should concentrate on measurements, possibly building pits to allow surveys on a regular basis.

The calculations you do using a profile are about the required pumping pressure for the flowrate required, the maximum pressure at the low points and normally ensuring a positive pressure (>say 5 bar) at the highest point. In mountainous regions this often means that a substantial back pressure is required at the end point in order to prevent slack flow, which leads to high local velocities and increased fluid disturbance. Maintaining a back flow with slurry is difficult and the valve or device often wears out very quickly. But they do exist.

If you can even give us elevations at 5km or 1km intervals making sure you include the highest and lowest points then we can see what you are looking at. Oh and pumping pressure and pressure at the arrival point.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jackieray]

@LittleInch

I am sorry for not replying to your post sooner. I just got back from another jobsite at work, and got a packed schedule.

Thank you for the detailed answer. I already had a chance to discuss with the pipeline engineers here, and I will try to respond each of your points.

1) I have finally found the issue regarding the differences in my calculations to the engineers' calculations. It is not only because 1) they used B31.3-2012 which has different formula for MAT, but 2) they also used 60200 psi as their S. Considering that the S used in their calculations is much higher than 25100 or even 43344 as per B31.4- 2022. I am going to recommend the use of B31.4-2022 code to calculate MATs. The average difference of the thickness for my calculations and their calculations are around 0.830 mm. IMO, it's quite thick.

2) Thank you for the info regarding B31G, I'm gonna look into it further. At my company, the method to assess the thickness is using NDT, but the efforts needed to do NDT on even 1 section (1 mile) of the pipe are quite plenty. We have to dig the pipes underground (1 - 2.5 m deep), which tend to be quite long so that we can lift the pipe to finally measure the 6 o'clock area of the pipe. So this is why the NDT measurement cannot be done regularly, at least so far.

3) Yes, I have confirmed that the pipes we used for the pipelines aren't lined with anything. No rubber, no PE, no anything. We also don't have any corrosion allowance (CA). The only thing resembling any kind of "extra thickness" is the actual thickness of our pipe is somehow a biiiit thicker than the usual Sch 80 pipe (0.438 inch vs 0.432 inch). I don't think it has any significant impact in increasing the service life of the pipe. For elbows, I was told that we don't use them here. From what I've seen, it's just as you say, they're all bends.

4) May I send the elevation profile to you privately?

5) We do have a guidance for 1) minimum flowrate and 2) expected pressure at specific points in the pipeline. For number (1), I've been trying to calculate the minimum velocity needed to avoid solid deposition, but I haven't been able to since the data that we have is incomplete and there are loads of methods that you can use. I also found an Excel calculation by Mr. Sape A. Miedema to determine limit deposit velocity (LDV), but haven't been able to use it properly due similar reasons. For number (2), it's good that the designers have included expected pressure by sections of the pipelines but the pressure gauges & transmitters that we installed at several valve stations are not working properly. I'm gonna recommend to reactivate & recalibrate all the pressure gauges & transmitters so we can monitor the pressures so that they don't exceed the maximum design pressure.

By the way, the MAWP from the designers is much lower (4900 psi) compared to the MAWP determined using B31.4-2022 (5700 psi). In actual measurements as shown in the DCS, even at the high pressure sections the pressure rarely exceeds 4000 psi though. If the measurements are correct and well maintained, then it shouldn't be an immediate problem.

Once again, I am sorry for not replying to you guys sooner. I know that I've been helped tremendously by your replies, and I don't mean to insult anyone by not responding to them quicker.

Thank you!!!