阅读说明：本技术 获取管路中环烷酸临界流速的方法 (Method for obtaining critical flow rate of naphthenic acid in pipeline ) 是由 叶成龙 柴永新 黄贤滨 屈定荣 谢守明 于 2019-11-07 设计创作，主要内容包括：本发明实施方式提供一种获取管路中环烷酸临界流速的方法,所述管路的内管壁包覆有腐蚀产物膜,所述方法包括：当所述环烷酸在所述管路中流动时,在预定区域检测所述环烷酸的当前流速,并获取所述环烷酸在当前流速下对管壁产生的剪应力；当所述环烷酸在第一流速下产生的剪应力达到所述腐蚀产物膜的剪应力强度时,将所述第一流速作为所述环烷酸在所述管路中的临界流速。本发明上述技术方案考虑到了环烷酸腐蚀形态与流速的关系,实现环烷酸腐蚀预测,以便弄清楚流速对炼油装置环烷酸腐蚀的影响规律,对炼厂加工含酸、高酸原油时的设备防腐工作具有重要的指导意义。(The embodiment of the invention provides a method for acquiring critical flow rate of naphthenic acid in a pipeline, wherein the inner pipe wall of the pipeline is coated with a corrosion product film, and the method comprises the following steps: when the naphthenic acid flows in the pipeline, detecting the current flow rate of the naphthenic acid in a preset area, and acquiring the shear stress generated by the naphthenic acid on the pipe wall at the current flow rate; when the shear stress generated by the naphthenic acid at a first flow rate reaches the shear stress intensity of the corrosion product film, the first flow rate is taken as the critical flow rate of the naphthenic acid in the pipeline. The technical scheme of the invention considers the relationship between the naphthenic acid corrosion form and the flow velocity, realizes the prediction of the naphthenic acid corrosion, so as to clarify the influence rule of the flow velocity on the naphthenic acid corrosion of an oil refining device, and has important guiding significance on the equipment corrosion prevention work when a refinery processes crude oil containing acid and high acid.)

1. A method of obtaining a critical flow rate of naphthenic acid in a pipeline having an inner pipe wall coated with a corrosion product film, the method comprising:

when the naphthenic acid flows in the pipeline, detecting the current flow rate of the naphthenic acid in a preset area, and acquiring the shear stress generated by the naphthenic acid on the pipe wall at the current flow rate;

when the shear stress generated by the naphthenic acid at a first flow rate reaches the shear stress intensity of the corrosion product film, the first flow rate is taken as the critical flow rate of the naphthenic acid in the pipeline.

2. A method of obtaining a critical flow rate of naphthenic acid in a pipeline having an inner pipe wall coated with a corrosion product film, the method comprising:

when the naphthenic acid flows in the pipeline, detecting the current flow rate of the naphthenic acid in a preset area, and acquiring the shear stress generated by the naphthenic acid on the pipe wall at the current flow rate;

in the case that the shear stress intensity of the corrosion product film is smaller than the surface hardness of the corrosion product film, when the shear stress generated by the naphthenic acid at a first flow rate reaches the shear stress intensity of the corrosion product film, taking the first flow rate as the critical flow rate of the naphthenic acid in the pipeline;

and under the condition that the surface hardness of the corrosion product membrane is smaller than the shear stress strength of the corrosion product membrane, when the shear stress generated by the naphthenic acid at a second flow rate reaches the surface hardness of the corrosion product membrane, taking the second flow rate as the critical flow rate of the naphthenic acid in the pipeline.

3. The method of claim 1 or 2, wherein the obtaining the naphthenic acids creates shear stress to a pipe wall at a current flow rate comprises:

1) obtaining Reynolds number Re:

wherein D is the pipe diameter of the pipeline, ρ is the density of the naphthenic acid, V is the current flow velocity of the naphthenic acid, and μ is the kinetic viscosity;

2) finding out a friction coefficient f through a Moody graph according to the Reynolds number Re;

3) calculating the corresponding shear stress tau of the naphthenic acid at the current flow rate:

4. a method according to claim 1 or 2, wherein the predetermined region is a high flow velocity region in the pipeline.

5. The method of claim 1 or 2, wherein the predetermined region is a high temperature heavy oil region in the pipeline where the flow regime is abrupt.

6. The method of claim 1 or 2, wherein the tubing is a carbon steel material, a Cr5Mo material, or a 304 stainless steel material.

7. The method of claim 1 or 2, wherein the corrosion product film is a ferrous sulfide product film.

8. The method of claim 1 or 2, wherein the naphthenic acid has a temperature greater than 220 ℃.

9. The method of claim 8, wherein the naphthenic acid has a temperature of 350 ℃ to 400 ℃.

10. The method of claim 1 or 2, wherein the naphthenic acid has an acid number greater than 0.5 mgKOH/g.

Technical Field

The invention relates to the field of corrosion risk evaluation of oil refining devices, in particular to a method for acquiring critical flow rate of naphthenic acid in a pipeline.

Background

Naphthenic acids are a generic name for a very complex mixture of carboxylic acids having a wide boiling range, with the general formula R-COOH. In the actual production process of the oil refining device, because two corrosive mediums of naphthenic acid and sulfide always coexist, the temperature of naphthenic acid corrosion and high-temperature sulfide corrosion is also approximately similar, so the naphthenic acid corrosion always follows and interacts with the sulfide corrosion. Naphthenic acid corrosion is mainly related to factors such as material quality, temperature, sulfur content, acid value, flow rate and the like. An empirical data table for naphthenic acid corrosion prediction is given in API RP 581Risk-Based Inspection method, but the influence of four parameters of material, temperature, sulfur content and acid value is only considered, the influence of flow rate is not considered, the application is limited, and the predicted result has large error under the condition of high flow rate.

In fact, the naphthenic acid corrosion form and the flow rate are closely related, in a low flow rate area, the naphthenic acid corrosion is generally expressed as uniform corrosion, and in a high flow rate area, the naphthenic acid corrosion is mostly expressed as groove-shaped local corrosion in a downstream direction due to obvious erosion effect. The portion of the refinery in which naphthenic acid corrosion is most severe is often a high-temperature heavy oil portion where the flow rate is high or the flow regime is mutated. Such as furnace outlets of atmospheric furnaces and vacuum furnaces, elbows, tees, valves, hot oil pumps, thermocouple inserts, transfer lines, atmospheric towers, vacuum tower feed inlets, tower internals and the like, the prediction of naphthenic acid corrosion by API RP 581Risk-Based Inspection method has large empirical data and actual deviation. The flow rate regime is an important factor affecting naphthenic acid corrosion. Numerous scholars have conducted extensive studies on the effect of flow rate on naphthenic acid corrosion, but no established method has been developed so far for industry to predict the effect of flow rate.

The rule of influence of the flow velocity on naphthenic acid corrosion of an oil refining plant is clarified, and the method has important guiding significance on equipment corrosion prevention work when a refinery processes crude oil containing acid and high acid.

Disclosure of Invention

The invention aims to provide a method for accurately acquiring critical flow rate of naphthenic acid in a pipeline.

In order to achieve the above object, in a first aspect of the present invention, there is provided a method of obtaining a critical flow rate of naphthenic acid in a pipeline, an inner pipe wall of the pipeline being coated with a corrosion product film, the method comprising:

when the naphthenic acid flows in the pipeline, detecting the current flow rate of the naphthenic acid in a preset area, and acquiring the shear stress generated by the naphthenic acid on the pipe wall at the current flow rate;

when the shear stress generated by the naphthenic acid at a first flow rate reaches the shear stress intensity of the corrosion product film, the first flow rate is taken as the critical flow rate of the naphthenic acid in the pipeline.

In a second aspect of the present invention, there is also provided a method of obtaining a critical flow rate of naphthenic acid in a pipeline having an inner pipe wall coated with a corrosion product film, the method comprising:

when the naphthenic acid flows in the pipeline, detecting the current flow rate of the naphthenic acid in a preset area, and acquiring the shear stress generated by the naphthenic acid on the pipe wall at the current flow rate;

in the case that the shear stress intensity of the corrosion product film is smaller than the surface hardness of the corrosion product film, when the shear stress generated by the naphthenic acid at a first flow rate reaches the shear stress intensity of the corrosion product film, taking the first flow rate as the critical flow rate of the naphthenic acid in the pipeline;

and under the condition that the surface hardness of the corrosion product membrane is smaller than the shear stress strength of the corrosion product membrane, when the shear stress generated by the naphthenic acid at a second flow rate reaches the surface hardness of the corrosion product membrane, taking the second flow rate as the critical flow rate of the naphthenic acid in the pipeline.

Preferably, the obtaining of the shear stress of the naphthenic acid on the pipe wall at the current flow rate comprises:

1) obtaining Reynolds number Re:

wherein D is the pipe diameter of the pipeline, ρ is the density of the naphthenic acid, V is the current flow velocity of the naphthenic acid, and μ is the kinetic viscosity;

2) finding out a friction coefficient f through a Moody graph according to the Reynolds number Re;

3) calculating the corresponding shear stress tau of the naphthenic acid at the current flow rate:

optionally, the predetermined region is a high flow rate region in the pipeline.

Optionally, the predetermined region is a high-temperature heavy oil region in which a flow state in the pipeline is suddenly changed.

Optionally, the pipeline is made of carbon steel material, Cr5Mo material or 304 stainless steel material.

Optionally, the corrosion product film is a ferrous sulfide product film.

Optionally, the naphthenic acid has a temperature greater than 220 ℃.

Optionally, the temperature of the naphthenic acid is 350-400 ℃.

Optionally, the naphthenic acid has an acid value of greater than 0.5 mgKOH/g.

According to the technical scheme, the relationship between the naphthenic acid corrosion form and the flow rate is considered, the naphthenic acid is compared with the shearing stress generated by the pipe wall and the shearing stress intensity of a corrosion product film under different flow rates to obtain the critical flow rate of the naphthenic acid, and the naphthenic acid corrosion prediction is realized, so that the influence rule of the flow rate on the naphthenic acid corrosion of the oil refining device is clarified.

Additional features and advantages of embodiments of the invention will be set forth in the detailed description which follows.

Drawings

The accompanying drawings, which are included to provide a further understanding of the embodiments of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the embodiments of the invention without limiting the embodiments of the invention. In the drawings:

FIG. 1 is a flow chart of a method for obtaining critical flow rate of naphthenic acid in a pipeline according to one embodiment of the present invention;

fig. 2 is a flow chart for obtaining the corresponding shear stress of the naphthenic acid at the current flow rate according to one embodiment of the present invention.

Detailed Description

The following detailed description of embodiments of the invention refers to the accompanying drawings. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

In the embodiments of the present invention, unless otherwise specified, the use of directional terms such as "upper, lower, top, and bottom" is generally used with respect to the orientation shown in the drawings or the positional relationship of the components with respect to each other in the vertical, or gravitational direction.

The first embodiment is as follows:

as shown in fig. 1, in a first aspect of the present invention, there is provided a method for obtaining a critical flow rate of naphthenic acid in a pipeline, an inner pipe wall of the pipeline being coated with a corrosion product film, the method comprising:

s1) detecting a current flow rate of the naphthenic acid in a predetermined area while the naphthenic acid flows in the pipeline, and acquiring a shear stress generated by the naphthenic acid to a pipe wall at the current flow rate; the method is also applicable to equipment, not limited to pipelines.

S2) when the shear stress generated by the naphthenic acid at the first flow rate reaches the shear stress intensity of the corrosion product film, taking the first flow rate as the critical flow rate of the naphthenic acid in the pipeline.

When naphthenic acid flows in a pipeline, wall shear stress, namely shear stress, to the wall surface of the pipeline can be generated, a corrosion product film is coated on the surface of the inner pipeline wall of the pipeline and has certain bearable shear stress strength, and when the shear stress generated by the flowing naphthenic acid exceeds the shear stress strength which can be borne by the corrosion product film, the corrosion product film can be peeled off the pipeline wall by the naphthenic acid. Therefore, the flow rate at which the shear stress generated when the naphthenic acid flows reaches the shear stress intensity of the corrosion product film is used as the critical flow rate of naphthenic acid in the pipeline.

Flow rate is a critical factor in naphthenic acid corrosion. At a certain temperature, the corrosion rate of a certain material in crude oil is related to the flow rate, and a critical flow rate exists, and once the critical flow rate is exceeded, the corrosion rate generates jump mutation. Theoretically, the critical flow rate is a fluid velocity that removes the corrosion product film from the corrosion surface or prevents the formation of the film, and may also be a flow rate that strips the protective corrosion product film from the corrosion interface. Thus, for naphthenic acid corrosion, the shear stress generated by the critical flow rate should be equal to the shear stress intensity of the corrosion product film.

Further, since the corrosion product film is not perfectly flat, the fluid shear stress can also generate vertical impact at some portions of the corrosion product film, and the surface hardness of the corrosion product film needs to be considered. Surface hardness refers to the ability of an object's surface to resist deformation or damage. In this embodiment, since the tube wall is coated with the corrosion product film, and the corrosion product film also has a certain surface hardness, when the naphthenic acid flows in the tube wall coated with the corrosion product film, and the shear stress generated reaches the surface hardness of the corrosion product film, the naphthenic acid may destroy the corrosion product film.

The corrosion product films have different surface hardnesses, some of which are greater than their own shear stress strength, and some of which are less than their own shear stress strength. In this example, when the surface hardness of the corrosion product film is greater than its own shear stress strength, the critical flow rate of naphthenic acid is: flow rate when the shear stress generated under naphthenic acid flow equals the shear stress intensity of the corrosion product film. When the surface hardness of the corrosion product film is less than the self-shear stress intensity, the critical flow rate of the naphthenic acid is as follows: the flow rate at which the shear stress generated under naphthenic acid flow equals the surface hardness of the corrosion product film.

Example two:

accordingly, in a second aspect of the present invention, there is also provided a method of obtaining a critical flow rate of naphthenic acid in a pipeline, the inner pipe wall of the pipeline being coated with a corrosion product film, the method comprising:

when the naphthenic acid flows in the pipeline, detecting the current flow rate of the naphthenic acid in a preset area, and acquiring the shear stress generated by the naphthenic acid on the pipe wall at the current flow rate;

in the case that the shear stress intensity of the corrosion product film is smaller than the surface hardness of the corrosion product film, when the shear stress generated by the naphthenic acid at a first flow rate reaches the shear stress intensity of the corrosion product film, taking the first flow rate as the critical flow rate of the naphthenic acid in the pipeline;

and under the condition that the surface hardness of the corrosion product membrane is smaller than the shear stress strength of the corrosion product membrane, when the shear stress generated by the naphthenic acid at a second flow rate reaches the surface hardness of the corrosion product membrane, taking the second flow rate as the critical flow rate of the naphthenic acid in the pipeline.

As shown in fig. 2, the obtaining of the shear stress of the naphthenic acid on the pipe wall at the current flow rate comprises:

s11) obtaining the reynolds number Re:

wherein D is the pipe diameter of the pipeline, ρ is the density of the naphthenic acid, V is the current flow velocity of the naphthenic acid, and μ is the kinetic viscosity;

s12) finding the friction coefficient f through a Moody graph according to the Reynolds number Re;

s13) calculating the corresponding shear stress τ of the naphthenic acid at the current flow rate:

the shear stress generated by the naphthenic acid flowing to the pipe wall and the shear stress suffered by the corrosion product film are a pair of forces with equal magnitude and opposite directions. The naphthenic acid produces a shear stress on the pipe wall, i.e., the shear stress to which the corrosion product film is subjected.

Optionally, the predetermined region is a high flow rate region in the pipeline. Naphthenic acid corrosion generally appears as uniform corrosion in low flow velocity regions, and as erosion is significant, it appears as a local corrosion of a groove shape in the downstream direction in high flow velocity regions. In the low flow rate region, the influence of the shear stress generated by the naphthenic acid on the corrosion product film coated on the inner pipe wall is not enough to strip the corrosion product film, so the selected predetermined region in the technical scheme of the invention is the high flow rate region in the pipeline.

Optionally, the predetermined region is a high-temperature heavy oil region in which a flow state in the pipeline is suddenly changed. As can be appreciated from physical knowledge, the region of high flow rate is generally the region of the pipeline where the flow state changes abruptly. In the present embodiment, naphthenic acid flows through the pipeline, and therefore this region is also a high-temperature heavy oil region. The portion of the refinery in which naphthenic acid corrosion is most severe is often a high-temperature heavy oil portion where the flow rate is high or the flow regime is mutated. Such as furnace outlets of atmospheric furnaces and vacuum furnaces, elbows, tee joints, valves, hot oil pumps, thermocouple insertion parts, oil transfer lines, atmospheric towers, vacuum tower feed inlets, tower internal components and the like.

Optionally, the pipeline is made of carbon steel material, Cr5Mo material or 304 stainless steel material.

Optionally, the corrosion product film is a ferrous sulfide product film.

Optionally, the naphthenic acid has a temperature greater than 220 ℃. The corrosion effect of naphthenic acid is greatly influenced by temperature; there is little corrosion below 220 c and the corrosion gradually increases with increasing temperature.

Optionally, the temperature of the naphthenic acid is 350-400 ℃. Naphthenic acid has two significant corrosion stages in terms of temperature. The first stage is at 270-280 deg.c, and naphthenic acid is gasified to start corrosion. When the temperature rises again, the corrosion action is weakened on the contrary, and when the temperature rises to 350-400 ℃, because sulfides in the crude oil are decomposed into sulfur elements, the sulfur elements have severe corrosion action on metal equipment, and the corrosion of naphthenic acid is intensified under the interaction of the naphthenic acid, the sulfur elements and H2S. After 400 deg.C, naphthenic acid is gasified completely, and its corrosion action is retarded.

Optionally, the naphthenic acid has an acid value of greater than 0.5 mgKOH/g.

Naphthenic acid is the most important acidic oxygen-containing compound in crude oil, has similar chemical properties with fatty acid, is typical monocarboxylic acid, and accounts for about 90 percent of the total acidity. The naphthenic acid content of the crude oil is generally 0.02-2.0%. The content of naphthenic acid in crude oil is generally judged by the acid value, and when the acid value is more than 0.5mgKOH/g, corrosion of equipment is caused. Naphthenic Acid Corrosion (NAC) of refinery units has been one of the challenges that refiners have been demanding to solve. With the increase of the use of crude oil with high acid value, the problem becomes more prominent, especially at local high-temperature and high-flow-rate parts, so that accidents frequently occur, and the safe production of refineries is seriously threatened.

Example three:

at flow rates below the critical flow rate, the iron sulfide protective film is not sufficiently destroyed, but the naphthenic acid corrosion is also affected. The reason is that naphthenic acid corrosion also follows, from a kinetic point of view, four steps of transport of the corrosive medium to the metal surface, adsorption on the metal surface, reaction with surface active centers and desorption of the corrosion products. The slowest of which is the control step of the corrosion reaction. The control steps of the corrosion reaction are affected by the nature of the crude oil, the material of the equipment, the temperature of the reaction and the difference of the flow rate and the flow state. Naphthenic acid corrosion is affected by other parameters such as the content of corrosive media, temperature, equipment materials and the like besides flow rate. To avoid interference from other factors, focusing on the flow rate effect on naphthenic acid corrosion can be done by constructing a flow rate influencing factor f (v), which is defined as follows:

f (V) -flow rate influencing factor;

Corrate_V-corrosion rate at flow velocity V;

CorrateV_V0corrosion rate at flow rate zero.

The corrosion rate irrespective of the effect of the flow rate can be obtained experimentally or by reviewing relevant literature, and API RP 581 has given a relatively complete set of empirical data on corrosion rates at naphthenic acid and sulphur interactions in refinery units. Therefore, only the flow rate influence factor is calculated, and the theoretical corrosion rate after finally considering the flow rate influence can be obtained.

The effect of flow rate on erosion corrosion can be measured by a system comprising three dimensionless numbers: shedward number Sh, stewart number Sc, Reynolds number Re, and formula (2-1) describing the degree of turbulence development x/d:

Sh＝c·Scⁿ¹·Reⁿ²·(x/d)ⁿ³ (2-1)

where Sh (schwarund number) is the ratio of total mass transfer to mass transfer by molecular diffusion:

re (reynolds number) is the ratio of inertial force to frictional force:

sc (schmitt number) is the ratio between momentum transfer and mass transfer by molecular diffusion:

in the above formulae, K is a mass transfer coefficient (m/s), L is a characteristic dimension (m), C is a kinematic viscosity (m2/s), D is a molecular diffusion coefficient, V is an average flow velocity, x is a distance from a fluid-state transition, D is a pipe diameter, and C, n1, n2 and n3 are constants.

Sh is another expression form of the mass transfer coefficient, Sc characterizes the relationship between the thickness (Dh) of the viscous underlayer on the material surface and the thickness (Dd) of the diffusion boundary layer, and if Dh and Dd exist at the same time, the method comprises the following steps:

the practical significance of equation (2-1) is that Sh can be converted to corrosion rate, while Re is a function of flow rate V. Thus, the general relation between the erosion corrosion speed and the flow velocity can be obtained:

EC＝Const·V^a (2-6)

wherein EC is the erosion corrosion rate and a is a constant.

By definition of the flow rate influencing factor, it is clear that f (v) can be expressed as:

F(V)＝1+Const·V^a (2-7)

the expression of f (v) includes two unknown parameters. Although the exact values of the two unknown parameters in the expression cannot be obtained by theoretical derivation, the complete curve can be obtained by analyzing any two points F (V1) and F (V2) on the curve for a specific material by experiments or literature data. Because the obtained experimental data have errors, the curve error obtained by calculating only two points is very large, especially for an exponential equation. Therefore, it is more scientific and reasonable to obtain enough experimental data F (V1), F (V2), F (V3) … … F (vn), and then to fit these experimental data by least square method to obtain the analytic formula, so as to minimize the error.

According to the technical scheme, the relationship between the naphthenic acid corrosion form and the flow rate is considered, the naphthenic acid is compared with the shearing stress generated by the pipe wall and the shearing stress intensity of a corrosion product film under different flow rates to obtain the critical flow rate of the naphthenic acid, and the naphthenic acid corrosion prediction is realized, so that the influence rule of the flow rate on the naphthenic acid corrosion of the oil refining device is clarified.

While the embodiments of the present invention have been described in detail with reference to the accompanying drawings, the embodiments of the present invention are not limited to the details of the above embodiments, and various simple modifications can be made to the technical solution of the embodiments of the present invention within the technical idea of the embodiments of the present invention, and the simple modifications are within the scope of the embodiments of the present invention.

It should be noted that the various features described in the above embodiments may be combined in any suitable manner without departing from the scope of the invention. In order to avoid unnecessary repetition, the embodiments of the present invention will not be described separately for the various possible combinations.

Those skilled in the art will appreciate that all or part of the steps in the method for implementing the above embodiments may be implemented by a program, which is stored in a storage medium and includes several instructions to enable a single chip, a chip, or a processor (processor) to execute all or part of the steps in the method according to the embodiments of the present invention. And the aforementioned storage medium includes: a U-disk, a removable hard disk, a Read-Only Memory (ROM), a Random Access Memory (RAM), a magnetic disk or an optical disk, and other various media capable of storing program codes.

In addition, any combination of the various embodiments of the present invention is also possible, and the same should be considered as disclosed in the embodiments of the present invention as long as it does not depart from the spirit of the embodiments of the present invention.