Shell evaluation and inspection interval calculations

Published 18 June, 2015 by admin

By Carlos F Molina

TANK SHELL EVALUATION ACCORDING TO API 653

What we are going to do is to dissect the diagram in FIG. 4.1 of API653 until we truly understand it. and there will be no way we fail any of the questions related to this in the exam. First of all we are going to see the following diagram.

average-hoop-stress

Look at the vertical sections drawn in the picture. Remember that hoop stresses (stresses that are tangential to the shell of the tank) are higher in vertical planes than in horizontal planes.

Corrosion can affect tank shells in many ways, as the standard says. In time, uncontrolled corrosion can weaken or destroy the tank´s shell, resulting in holes or possible structural failure, and release of stored products into the environment. But the most common seen form of damage is a “generally uniform loss of metal over a large area or in localized areas”. In an scheduled external inspection scenary, where you have to evaluate a tank shell, you will have to follow the procedure of 4.3.2.1. of API 653 and figure 4.1.

THE PROCEDURE

4.3.2.1 For determining the controlling thicknesses in each shell course when there are corroded areas of considerable size, measured thicknesses shall be averaged in accordance with the following procedure.

1. Calculate the minimum thickness t2 in the corroded area

2. Calculate the critical lenght L in which thicknesses “average out”, with one of the following formulas

equation-1.15

3. Locate L, in several vertical planes that you think are more affected by corrosion, and measure at least 5 points to get t avg in each one of those vertical planes. One of those planes will have the lowest average thickness, that you should compare to tmin acording to the formulas in 4.3.3.1

minimum-acceptable-thickness

Esentially, this procedure limits the size of the zone you are evaluating so it is not too big. This way the “averaging out” is a more realistic way of evaluating metal loss than the arithmetical average.

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It is like they say in my homeland about the problem with statistics: “If I eat 2 chickens and you don´t eat any, then we both ate 1 chicken each”. Given that metal loss profiles can have very differente thicknesses, there is a need for a method to average the corrosion. That is what the standard is trying to avoid: a situation where the shell evaluation is too simplisitic. What I try to say is that smaller the thickness in any point, the smaller the zone were thicknesses average out, with 40inches limit as a maximum. After you have calculated the critical lenght, then you will have to take five measures in it and alculate the arithmetical media and compare it to the minimum allowable.

UT MEASUREMENT

Usually, 4, 6 or 8 measurements of shell thickness are taken in several vertical lines comprising 360° in each course of the tank shell to fullfill the UT requirements of external inspection, although this has to be an agreement with the owner/operator. When using a tank crawler, usually you measure every foot on eight lines in the eight wind directions in the tank shell. This can be modified depending on the configuratio of the tank.

Remember that this procedure for shell evaluation is for localizaed corroded areas. For a complete shell plate, you will have to take UT measurements in the best agreement with the owner.

EXAMPLE

CUI was detected after the insulation of a tank was removed for inspection. It generated a corroded area away from vertical welds, in the bottom of the third course of the tank, which is in operation. The inspector took 5 UT measures along 3 vertical planes each, in the positions where he thought there was more corrosion, following the instructions in API 653, 4.3.2.1.c The authorized inspector shall visually or otherwise decide which vertical plane(s) in the area is likely to be the most affected by corrosion. His findings are illustrated in Fig 1. The product stored is crude oil with a specific gravity of 0.978. Corrosion rate is 0,5mm/year. Having in mind the average corrosion measured, is the tank safe to operate until next inspection due in 5 years? D= 15,24m. The third course is originally 12mm thick A36M steel. Maximum liquid level is 11,88m and courses are 8ft wide.

tank-shell-evaluation

Figure 1. UT Measurements of a localized thinned area showing planes 1, 2 and 3

 

 SOLUTION:

1) Find the controlling thickness

minimum-thickness

2) Calculate the critical lenght

critical-lenght

3) Calculate Tavg along several vertical planes, with at least 5 thickness measures in each plane, and compare against T min

average-thickness

average-thickness

average-thickness

The lowest average thickness is 7,94mm, in plane #2. Then we calculate Tmin in USC

variables

minimum-thickness-plane

As API 653 says, the criteria for continued operation is as follows:

i) the value t1 shall be greater than or equal to Tmin (see 4.3.3 or 4.3.4), subject to verification of all other loadings listed in 4.3.3.5; t1 is Tavg

criteria-for-continued-operation-1

The tank is safe to operate.

ii) the value t2 shall be greater than or equal to 60 % of Tmin; and

criteria-for-continued-operation-2

The tank is safe to operate.

iii) any corrosion allowance required for service until the time of the next inspection shall be added to Tmin and 60 % of Tmin

criteria-for-continued-operation-3

The tank is safe to operate until next inspection

criteria-for-continued-operation-4

The tank is safe to operate until next inspection
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This analisis shall be made for as many thinned areas there are in the tank shell. If the corroded area still is larger than 40 inches. If the corroded region is larger than L in the vertical direction, the region must be divided into multiple sections such that no individual section is larger than L. Each section must then be evaluated separately.

See you next week with more articles on tank inspection based in API 653.

Latest comments

Hi. Check the video. https://apiexam.com/calculating-intervals-for-tank-inspection/

- Carlos F. Molina

Can you do the same calculation for tank bottom

- Frank Doe

Hi. I am not quite sure of your question, H is a data given in the example´s text.

- Carlos F. Molina

Dear sir I can't understand that how to get H value . please sir explain

- Mohammad ishraq

Dear sir I didn't understand that how to get H maximum liquid level. please explain it sir

- Mohammad ishraq

Thanks for explaining!

- Sam

Hi, Thanks for your explanation. it helps me a lot. please keep going.

- Erkan

El operador/dueño debe tener registros de construcción, de inspección y de alteración. En caso de faltar alguno, remitirse a los otros. Ahora bien, aunque no es obligatorio por el API 653 que el operador tenga los registros de operación, se pueden solicitar para ver el nivel de operación y asegurar lo continuidad del servicio. Si únicamente falta el dato del nivel de diseño (ya que si no hay registros constructivos no se conoce el material, su soldabilidad (11.1.2) y resistencia al esfuerzo), el inspector debe calcular el nivel máximo de operación y de prueba a partir de las mediciones de UT e inspección de las zonas con pitting y corrosión localizada de acuerdo al procedimiento en este post, y con el criterio de un ingeniero de tanques de almacenamiento experimentado. El objetivo del inspector es asegurar la continuidad del servicio.

- imageniero@gmail.com

Carlos, Tu post esta bastante completo. Me surge la duda de que hacer cuando se desconoce la historia constructiva del tanque que impide conocer el Nivel Máximo de Liquido. En este caso como se calcularía el tmin?

- Miguel Perruolo

Hi Carlos, I just started preparation for API-653, my exam will be on November-15.I found helpful info in your blog, sure it's gonna help me lot.I hope I can post if I have any doubts.Keep posting new articles.Good day.

- Vinoth kannan

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