PASS/EQUIP → F.A.Q.

6. Frequently Asked Questions (F.A.Q.)

We set some pressure in the components, and in the calculation the pressure turned out to be higher. Why?
If in the general data, an option "Vessel contains operating fluid" is specified, then when calculating, a hydrostatic pressure from the liquid column height in this component is automatically added to the specified pressure.

Why, when selecting austenitic steels as per GOST 34233.1, allowable stresses obtained in calculation are not the same as in GOST tables?
allowable stresses should not contradict item 8.1 (formula 1). It seems that, when compiling tables of allowable stresses, GOST authors simply forgot about their own note to table 1 (for austenitic chromium-nickel steels n_B = 3.0) and divided them into 2.4, and it turned out that all is determined by yield strength. To avoid trouble, the program checks both values and selects the minimum one.

We set St3 steel shell, performed calculation without allowance for hydrosulphuric corrosion medium, and the strength condition was not met. Then we set corrosion medium, and everything was ok. Why?
It seems that, when calculating the allowable stresses, the developers of GOST 34233.1 mixed up the lines, and the allowable stresses do not correspond to the mechanical properties. In the routine calculation, the program takes the minimum value, and in conditions of the hydrosulphuric medium is focused on the strength and yield limits. That’s the reason of inconsistency.

You have all errors in the calculations! We checked the calculations manually, and the intermediate calculations do not match the results.
Sometimes errors take place. But in this case, most likely, it is a matter of dimensions. For example, diameters are given in mm and moment in Nm. Or, as per the formula, the result should be in radians, and the output is configured in degrees. The program performs calculations in the internal consistent system, and displays all intermediate values and results in the dimensions specified by User.

We have calculated a standard flange, and its strength is not reached even at a zero pressure. Why?
In accordance with GOST 33259-2015, i. 7.5 "…… Sizes of flanges of nominal diameters DN ≤ 600 take into account the effect of medium internal pressure in connections, when using gaskets as per GOST 15180 (Elastic flat gaskets) without external loads, bending moments and corrosion effect. Working efficiency of flange connection of all sizes when using all types of gaskets, taking into consideration specific operating conditions of the connection (including external loads, bending moments, corrosion effects of the working medium and environment, etc.), as well as flanges DN > 600 from the action of the medium internal pressure, shall be confirmed by calculation, operating data or tests. Calculations to be performed as per the approved procedure..." When using inelastic gaskets, for example, SWG, to provide the increased specific compression pressure, a high design load on the bolts is required, at which the strength of some "standard" flanges is not ensured.

We have calculated a standard flange, and its strength is not reached even at a zero pressure. Why?

In accordance with GOST 33259-2015, i. 7.5 "…… Sizes of flanges of nominal diameters DN ≤ 600 take into account the effect of medium internal pressure in connections, when using gaskets as per GOST 15180 (Elastic flat gaskets) without external loads, bending moments and corrosion effect. Working efficiency of flange connection of all sizes when using all types of gaskets, taking into consideration specific operating conditions of the connection (including external loads, bending moments, corrosion effects of the working medium and environment, etc.), as well as flanges DN > 600 from the action of the medium internal pressure, shall be confirmed by calculation, operating data or tests. Calculations to be performed as per the approved procedure..." When using inelastic gaskets, for example, SWG, to provide the increased specific compression pressure, a high design load on the bolts is required, at which the strength of some "standard" flanges is not ensured.

Why in GOST calculation of flanges there is a checkbox "Consideration for allowances in stiffness calculation"? Indeed, Russian GOST 34233.4 i. 4.12 says in black and white: "When determining flexibility of flanges and the forces acting on the flange connection components, nominal dimensions of the flange should be used without deducting the corrosion allowance".
Properly speaking, yes. If you follow the letter of GOST, then no allowance should be incorporated into the formulas of flexibility coefficients. However, besides the letter, there is also a physical meaning. De facto, new and corroded flanges will have different flexibility. Moreover, ASME VIII, from which this GOST was rewritten and which historically appeared earlier, says that, in a proper way, one should carry out two calculations — with and without allowances. According to our experience, the deduction of allowances in calculation of coefficients can both reduce the margin of safety and increase it (but in calculations of stresses, the strength always suffers). Realizing that both points of view are quite reasonable, we introduced this option.

Why in GOST calculation of flanges there is a checkbox "Consideration for allowances in stiffness calculation"? Indeed, Russian GOST 34233.4 i. 4.12 says in black and white: "When determining flexibility of flanges and the forces acting on the flange connection components, nominal dimensions of the flange should be used without deducting the corrosion allowance".

Properly speaking, yes. If you follow the letter of GOST, then no allowance should be incorporated into the formulas of flexibility coefficients. However, besides the letter, there is also a physical meaning. De facto, new and corroded flanges will have different flexibility. Moreover, ASME VIII, from which this GOST was rewritten and which historically appeared earlier, says that, in a proper way, one should carry out two calculations — with and without allowances. According to our experience, the deduction of allowances in calculation of coefficients can both reduce the margin of safety and increase it (but in calculations of stresses, the strength always suffers). Realizing that both points of view are quite reasonable, we introduced this option.

We selected a standard flange from the database, and the program says that the dimension ratios are out of the valid range. What to do?
This happens, especially with small diameter flanges (DN15, DN20). The restrictions on the ratios are now spelled out in a "hidden" form, in clause 4.2.2 of GOST 34233.4. The standard developers answers that small-diameter flanges, based on experience, are not a source of problems... To fulfill the application conditions somehow, you can try to reduce the outer diameter of the flange in the model and (or) increase the inner diameter, as well as calculate according to ASME VIII div.1(2), which actually recommends GOST 34233.4 (clause 4.2.2 "special calculation methods").

We selected a standard flange from the database, and the program says that the dimension ratios are out of the valid range. What to do?

This happens, especially with small diameter flanges (DN15, DN20). The restrictions on the ratios are now spelled out in a "hidden" form, in clause 4.2.2 of GOST 34233.4. The standard developers answers that small-diameter flanges, based on experience, are not a source of problems... To fulfill the application conditions somehow, you can try to reduce the outer diameter of the flange in the model and (or) increase the inner diameter, as well as calculate according to ASME VIII div.1(2), which actually recommends GOST 34233.4 (clause 4.2.2 "special calculation methods").

By what formula loads on nozzles and supports were calculated? Where can we see the course of calculation?
It is impossible to give all the course of calculation of these loads, as they are obtained numerically, by the finite element (FEM) method from the beam model. In general, this process is described by the matrix equation [K]*{δ} = {F}, which expands into a system of linear algebraic equations consisting of several hundred or even thousand of unknowns, i.e. the results are obtained in the form of large numerical arrays in the computer memory. This method can be verified on the simple models, for which an analytical solution is known, or to estimate the likelihood from physical considerations. Or perform a similar calculation in another program (ANSYS or similar).

By what formula loads on nozzles and supports were calculated? Where can we see the course of calculation?

It is impossible to give all the course of calculation of these loads, as they are obtained numerically, by the finite element (FEM) method from the beam model. In general, this process is described by the matrix equation [K]*{δ} = {F}, which expands into a system of linear algebraic equations consisting of several hundred or even thousand of unknowns, i.e. the results are obtained in the form of large numerical arrays in the computer memory. This method can be verified on the simple models, for which an analytical solution is known, or to estimate the likelihood from physical considerations. Or perform a similar calculation in another program (ANSYS or similar).

We checked the "Automatic determination of loads" box in the nozzle, and the report generated zero loads. Why?
Most likely, the supports were not set in the model. Without fixing conditions, no load can be obtained in the components. This is not a program trouble; just methods of structural mechanics work in this way. Or nothing more is attached to the nozzles. The program will not "invent" the loads itself, it can only recalculate them from one point to another.

We checked the "Wind resonance calculation" box in the column, and obtained loads considerably exceeding the wind ones. When should we select this checkbox?
Wind resonance calculation is, in a sense, some sort of a "reverse calculation". It calculates the wind speed, at which resonance may occur. At this speed, wind loads are calculated. It is reasonable to select this option for long and smooth vessels, such as stacks.

In column, in the "Oscillation period" section we obtained incredible movements of several tens of meters. What is it?
These are the movements obtained by the Rayleigh method for calculation of column oscillation period. The column is located horizontally, fixed at the foundation, and then the movements of its components under the gravity action are calculated. De facto, these movements never occur — it is just a mathematical fiction to calculate the ratio of the mass and stiffness characteristics of the column to determine the lowest natural frequency.

In column, in the "Oscillation period" section we obtained incredible movements of several tens of meters. What is it?

These are the movements obtained by the Rayleigh method for calculation of column oscillation period. The column is located horizontally, fixed at the foundation, and then the movements of its components under the gravity action are calculated. De facto, these movements never occur — it is just a mathematical fiction to calculate the ratio of the mass and stiffness characteristics of the column to determine the lowest natural frequency.

When calculating hydrotesting pressure of as per GOST 34347-2017, a height of the liquid column is subtracted, why?
As a matter of fact, there is no clear understanding, what shall be considered as hydrotesting pressure. Is it pressure at each specific point of the vessel, or is it pressure on the pressure gauge at the top? We think that this is a gauge pressure. In the course of testing, a liquid column will be added to this pressure, and the final pressure in each component of the vessel will be exactly equal to the calculated one. And, if not to subtract a liquid column, it turns out that it was taken into account twice.

When calculating hydrotesting pressure of as per GOST 34347-2017, a height of the liquid column is subtracted, why?

As a matter of fact, there is no clear understanding, what shall be considered as hydrotesting pressure. Is it pressure at each specific point of the vessel, or is it pressure on the pressure gauge at the top? We think that this is a gauge pressure. In the course of testing, a liquid column will be added to this pressure, and the final pressure in each component of the vessel will be exactly equal to the calculated one. And, if not to subtract a liquid column, it turns out that it was taken into account twice.

When updating, the program displays an "error http" message, what to do?
Perhaps, an antivirus blocks access to the Internet or in your organization Internet access is organized via proxy server, and you need to register its settings in Parameters-> Settings-> Program Update.

When determining vessel category as per TR CU 032/2013, the program decided that the vessel is filled with gas, but we have selected "Vessel contains working fluid" option; what's the matter?
There is a Rostekhnadzor’s letter, which states that in the presence of a gas phase in any quantity, the vessel shall be classified as the vessel containing gas. That’s why the program calculates filling of the vessel, and if it is less than 99%, then when determining the category, it is considered as "containing gas".

Why is it impossible for the heat exchanger to set tubes made of brass?
because in accordance with GOST 34233.1, brass properties are guaranteed for a thickness range from 3 to 10 mm. Tubes are often made thinner, and for less thickness, the brass is not included in the list of available materials. On our own initiative, we are afraid to expand this range. You can override it through the user material.

Why should the condition s₁ > c_s+c_s1 be met for the nozzle, when the corrosion c_s1 has been already taken into account in c_s?
Because the passing tube corrodes on both sides at the same time. If we assume that the internal part is completely corroded and there’s nothing terrible, then we should set the initial configuration without the internal part.

Why does the tube state that, according to RD 26-18-8-85, its thickness is insufficient? Is it bad?
Because for nozzles, according to the standard, full penetration is required, and with a thick shell and a thin nozzle, welding is difficult and high residual stresses can occur. This is just a notification, doesn't matter.

What is "Controlled stud stretching", when tightening the flange?
This is a progressive method of tightening, during which high-pressure oil is simultaneously supplied to the axial holes of the studs. The stud is slightly stretched, upon which the nut is screwed by hand, and then the pressure is released.

How to calculate a double-walled vessel in the ground?
You should create a separate model of the inner and outer casing. The inner casing should be calculated for 2 modes: internal pressure of the product and external pressure from the jacket side. And the outer casing should also be calculated 2 times: On the internal pressure of jacket and external pressure of soil. The problem is that there are no methods for calculating vessels in the ground, so we recommend setting the ground pressure as p = ρgh, where ρ – soil density, and h – backfilling height. This is rather conservative calculation, it provides an additional safety and does not consider many parameters (variable pressure plot, supporting effect of soil, pressure arch height, etc.), but this is better than nothing.

How to calculate a double-walled vessel in the ground?

You should create a separate model of the inner and outer casing. The inner casing should be calculated for 2 modes: internal pressure of the product and external pressure from the jacket side. And the outer casing should also be calculated 2 times: On the internal pressure of jacket and external pressure of soil. The problem is that there are no methods for calculating vessels in the ground, so we recommend setting the ground pressure as p = ρ*g*h, where ρ – soil density, and h – backfilling height. This is rather conservative calculation, it provides an additional safety and does not consider many parameters (variable pressure plot, supporting effect of soil, pressure arch height, etc.), but this is better than nothing.

Why don't you make a selection of wind and seismic area from database? Or more visually - click the mouse on the map, and that's it!
Because these parameters are defined by customer, this information should be included in the technical design or calculation assignment. The calculator is not authorized to independently assign wind and seismic areas. That’s why we consider it methodically wrong to introduce such a feature.

Can you add German/Ukrainian/Chinese languages to the program?
Technically, it’s not a problem to add any languages, but we don’t have our own translators, and considering the scope of information to be translated (interface, reports, help system, database), giving this work for outsourcing will lead to a very high, really very high price for the customer. Or we can prepare and transfer source texts for self-translation and then attach them to the program. Cost of this work will be much lower, but still considerable.

Can you add German/Ukrainian/Chinese languages to the program?

Technically, it’s not a problem to add any languages, but we don’t have our own translators, and considering the scope of information to be translated (interface, reports, help system, database), giving this work for outsourcing will lead to a very high, really very high price for the customer. Or we can prepare and transfer source texts for self-translation and then attach them to the program. Cost of this work will be much lower, but still considerable.

How does specific load G_a and specific weight of metal G_s correlate in the service platforms of column vessels?
Specific load G_a includes the dead weight of platform metal (G_s) plus all possible loads (snow, equipment, materials, people, etc.). This value is used to determine the load on the column in working and testing conditions (for these conditions the worst case is the maximum load). And metal weight G_s is used to form the table of materials and to determine the loads on the column in construction conditions, because for construction the worst case is the minimum load (the column is calculated for tipping).

How does specific load G_a and specific weight of metal G_s correlate in the service platforms of column vessels?

Specific load G_a includes the dead weight of platform metal (G_s) plus all possible loads (snow, equipment, materials, people, etc.). This value is used to determine the load on the column in working and testing conditions (for these conditions the worst case is the maximum load). And metal weight G_s is used to form the table of materials and to determine the loads on the column in construction conditions, because for construction the worst case is the minimum load (the column is calculated for tipping).

Calculation shows that for a vertical vessel four supports are worse than three ones. How come? And why we cannot set more supports? In fact, 10 supporting legs can carry a larger load in total.
No, they cannot, because, due to construction errors, it is impossible to load all the supports evenly (by analogy with a stool, which has 10 legs - they will never be equally loaded). For three supports, this is still true, but four in terms of calculation are considered as two. There is an option "Precise construction", which implies levelling and concrete grouting — in this case, four supports are considered as four.

Calculation shows that for a vertical vessel four supports are worse than three ones. How come? And why we cannot set more supports? In fact, 10 supporting legs can carry a larger load in total.

No, they cannot, because, due to construction errors, it is impossible to load all the supports evenly (by analogy with a stool, which has 10 legs - they will never be equally loaded). For three supports, this is still true, but four in terms of calculation are considered as two. There is an option "Precise construction", which implies levelling and concrete grouting — in this case, four supports are considered as four.

What is the purpose of an option "Fr includes the component of pressure" in the loads on the nozzle? What is the component and when we should use this checkbox?
In short, the normal state of this checkbox is turned off. It should be turned on in some special cases; if they happen to you - you will learn about it. In more detail: In the presence of internal pressure in the nozzle, an axial thrust force F_p = pπd²/4 appears. This checkbox indicates whether this thrust force is taken into consideration in the specified axial load F_R. If the checkbox is on, then in calculations F_R is reduced by F_p, since in GOST methods the thrust force from pressure is already taken into account.

What is the purpose of an option "Fr includes the component of pressure" in the loads on the nozzle? What is the component and when we should use this checkbox?

In short, the normal state of this checkbox is turned off. It should be turned on in some special cases; if they happen to you - you will learn about it. In more detail: In the presence of internal pressure in the nozzle, an axial thrust force F_p = p*π*d²/4 appears. This checkbox indicates whether this thrust force is taken into consideration in the specified axial load F_R. If the checkbox is on, then in calculations F_R is reduced by F_p, since in GOST methods the thrust force from pressure is already taken into account.

How can I speed up the work with the program? It is very boringly to wait every time for the model rebuilding. And the calculation takes a very long time.
When editing "heavy" models, you should use the yellow or red mode of the light signal. If you need to change several components of the model at once, you can use F4 (the model is not rebuilt between editing the components), or a gray light signal. Before calculation, it is worthwhile checking the program settings (Parameters-Settings): Multithreading option slightly speeds up the calculation of mass characteristics and model generation; Accuracy control parameter accelerates the calculation of masses and centres of gravity (the more, the faster); Managing the accuracy of filling volume calculation can speed up the filling calculation for highly rugged cavities (such as shell side).

How can I speed up the work with the program? It is very boringly to wait every time for the model rebuilding. And the calculation takes a very long time.

When editing "heavy" models, you should use the yellow or red mode of the light signal. If you need to change several components of the model at once, you can use F4 (the model is not rebuilt between editing the components), or a gray light signal. Before calculation, it is worthwhile checking the program settings (Parameters-Settings):

Multithreading option slightly speeds up the calculation of mass characteristics and model generation;
Accuracy control parameter accelerates the calculation of masses and centres of gravity (the more, the faster);
Managing the accuracy of filling volume calculation can speed up the filling calculation for highly rugged cavities (such as shell side).

The program is running normally, but it crashes at any model changing. What’s the matter?
Most likely, the program was updated through Help-> Check for updates. As of today, this method does not work properly on Windows 7 anymore. We have no idea, what’s the reason. Perhaps, this is due to the termination of support for Windows 7 by Microsoft. In this case, you can only delete the program through the control panel, clean up its former folder, and reinstall it manually. You also need to check the multithreading settings (Parameters-Settings-Geometric kernel mode) and select "Standard multithreading" mode or "Multi-threading disabled». Another possible reason is a rare type of video card integrated with the motherboard and not fully compatible with VBO technology. In this case, installing an additional video card can help.

The program is running normally, but it crashes at any model changing. What’s the matter?

Most likely, the program was updated through Help-> Check for updates. As of today, this method does not work properly on Windows 7 anymore. We have no idea, what’s the reason. Perhaps, this is due to the termination of support for Windows 7 by Microsoft. In this case, you can only delete the program through the control panel, clean up its former folder, and reinstall it manually. You also need to check the multithreading settings (Parameters-Settings-Geometric kernel mode) and select "Standard multithreading" mode or "Multi-threading disabled». Another possible reason is a rare type of video card integrated with the motherboard and not fully compatible with VBO technology. In this case, installing an additional video card can help.

For the material, it is necessary to input strength and yield limits. Why it is impossible to input the ready values of allowable stresses?
Allowable stresses are calculation values. All appropriate calculations are rather complicated and depend on many factors. In each regulatory document and standard (GOST, ASME, EN) the calculation algorithms are quite different. Even if we input the appropriate values manually, they cannot be used, when switching the calculation standard, for example, from ASME to EN. Even within one calculation, it will be impossible to use them, for example, in calculation for hydrotesting. But mechanical characteristics are objective — it is always possible to calculate allowable stresses individually for each case.

For the material, it is necessary to input strength and yield limits. Why it is impossible to input the ready values of allowable stresses?

Allowable stresses are calculation values. All appropriate calculations are rather complicated and depend on many factors. In each regulatory document and standard (GOST, ASME, EN) the calculation algorithms are quite different. Even if we input the appropriate values manually, they cannot be used, when switching the calculation standard, for example, from ASME to EN. Even within one calculation, it will be impossible to use them, for example, in calculation for hydrotesting. But mechanical characteristics are objective — it is always possible to calculate allowable stresses individually for each case.

Why is it impossible to calculate an element made of steel 09G2S KP245 GOST 8479 (forging) taking into account the hydrogen sulfide corrosive environment?
Because when calculating taking into account the hydrogen sulfide corrosive environment, the allowable stresses are determined not according to the table, but on the basis of mechanical characteristics. In GOST 8479, these characteristics are given only at 20 degrees :(

Can the program calculate the framed roof of the tank?
No, the framed roof itself is not calculated. This should be a numerical calculation for buckling by the finite element method, taking into account the geometric nonlinearity (loads are significantly redistributed for small deformations of the shallow shell; this must be taken into account). For such calculations, special software systems have been developed for the calculation of building structures (for example, SCAD, Lira, Ansys). We do not undertake to implement such functionality in our program yet. It is possible to define the roof frame through the "Designer" tool and export the beam and shell model along with the loads to an APDL file to study the strength and buckling of the tank, for example, in the Ansys program. Or you can specify the roof frame and support unit prepared in a third-party CAD system (Compas3D, Solidworks). The picture will be plausible, but the program can only calculate the load due the weight of the frame and the consumption of materials.

Can the program calculate the framed roof of the tank?

No, the framed roof itself is not calculated. This should be a numerical calculation for buckling by the finite element method, taking into account the geometric nonlinearity (loads are significantly redistributed for small deformations of the shallow shell; this must be taken into account). For such calculations, special software systems have been developed for the calculation of building structures (for example, SCAD, Lira, Ansys). We do not undertake to implement such functionality in our program yet. It is possible to define the roof frame through the "Designer" tool and export the beam and shell model along with the loads to an APDL file to study the strength and buckling of the tank, for example, in the Ansys program. Or you can specify the roof frame and support unit prepared in a third-party CAD system (Compas3D, Solidworks). The picture will be plausible, but the program can only calculate the load due the weight of the frame and the consumption of materials.