Storage tanks are widely used storage facilities in petrochemical production. Atmospheric pressure storage tanks usually have connecting pipes that directly communicate with the atmosphere, meaning that the working pressure of the tank is atmospheric pressure, which corresponds to a pressure storage tank (pressure vessel).

With the increasing requirements of the country for the safety management of hazardous chemical atmospheric storage tanks, how can hazardous chemical enterprises effectively prevent and control the safety risks of atmospheric storage tanks, ensure the integrity of atmospheric storage tanks, and standardize the inspection (testing) work of atmospheric storage tanks? The author has analyzed and organized the qualification and inspection (testing) requirements that many enterprises are concerned about, and now shares them with everyone.

　　one

　　Standard specifications for integrity management of atmospheric storage tanks

There are many standard specifications involved in the integrity management of atmospheric storage tanks, and the most common ones closely related to the maintenance and inspection (testing) of atmospheric storage tanks are as follows:

1. "Integrity Management of Atmospheric Storage Tanks" (GB/T 37327-2019)

2. Risk based Inspection and Evaluation of Atmospheric Storage Tanks (GB 30578-2014)

3. Safety Technical Regulations for Vertical Cylindrical Steel Welded Storage Tanks (AQ 3053-2015)

4. Design Specification for Vertical Cylindrical Steel Welded Oil Tanks (GB 50341-2014)

5. Steel Welded Atmospheric Pressure Vessels (NB/T 47003.1-2022)

6. Technical Specification for Inspection and Applicability Evaluation of In Service Atmospheric Storage Tanks (T/CCSAS 042-2023)

7. Maintenance and Inspection Regulations for Atmospheric Pressure Vertical Cylindrical Steel Welded Storage Tanks (SHS 01012-2019)

Currently, there is a phenomenon of scattered standards in the integrity management of atmospheric storage tanks, and some standards are outdated in revision; There may even be inconsistencies in specific terms and conditions between standards. The above-mentioned standards and specifications for the inspection (testing) of atmospheric pressure storage tanks should focus on the "Integrity Management of Atmospheric Pressure Storage Tanks" (GB/T 37327-2019) from the perspective of legal effectiveness and applicability.

two

　　The concept of atmospheric pressure storage tank

When it comes to "atmospheric pressure storage tanks", employees of hazardous chemical companies are not unfamiliar, but a considerable number of them may not be able to accurately define the concept of atmospheric pressure storage tanks or the scope of control for atmospheric pressure storage tanks. One important reason is that,The range of atmospheric storage tanks determined by different standards varies.

For example, in the standards mentioned earlier, some standards define the scope of "atmospheric pressure vertical cylindrical steel welded storage tanks" as "vertical cylindrical steel welded storage tanks with an internal pressure not exceeding 18kPa". In addition, different standards have different regulations on the range of medium and volume for atmospheric storage tanks.

Based on the properties and risks of atmospheric pressure storage tanks, the author supports the following concepts, which are also the foundation of this article's discussion.

1. Category, volume, and medium range refer to the applicable scope of GB/T 37327-2019, which states: "This standard is applicable to vertical cylindrical steel welded atmospheric pressure storage tanks, and the integrity management of other atmospheric or low-pressure (working pressure less than 0.1 MPa) storage tanks can refer to this standard. ”

2. Pressure range: GB/T 37327-2019 does not provide the pressure range for atmospheric pressure storage tanks. Currently, the mainstream definition is GB 50160, which states: "Storage tanks with a design pressure less than or equal to 6.9 kPa (tank top gauge pressure). ”

According to the above scope, personnel engaged in special equipment management will find that there is still a gap between the pressure range of the atmospheric storage tank and the pressure range of the pressure vessel. GB 50160 defines low-pressure storage tanks as tanks with a design pressure greater than 6.9 kPa and less than 0.1 MPa (tank top gauge pressure). ”This is also the scope of application mentioned in GB/T 37327-2019, which can be referred to as "low-pressure storage tanks".

It should be further clarified that there are also regulations that classify "storage tanks with a design pressure greater than 6.9 kPa and less than 0.1 MPa (tank top gauge pressure)" into two categories: "micro internal pressure storage tanks" and "low-pressure storage tanks", with a limit of 18 kPa.

　　three

　　Risk Analysis of Atmospheric Pressure Storage Tanks

This article focuses on how to prevent and control the risks of in-service atmospheric storage tanks from the perspective of equipment integrity. The risks associated with atmospheric pressure storage tank operations (such as hot work) and process operation safety risks (such as liquid spillage inside the tank) are not within the scope of this article.

The equipment risks that atmospheric pressure storage tanks should focus on preventing are firstly due to corrosion and thinning of the tank body, or local sinking of the tank foundation, cracking of the tank bottom lap weld, resulting in medium leakage inside the tank, causing accidents such as fire, explosion, poisoning, and burning. Maintaining the integrity of tank related accessories and instruments is also an important part of preventing and controlling tank safety risks.

From the perspective of normal operation and risk prevention of atmospheric storage tanks, GB/T 37327-2019 stipulates that the integrity evaluation of various components of atmospheric storage tanks includes the tank body and foundation, sealing system, cathodic protection, anti-corrosion coating, breathing valve, instrument electrical system, lightning protection and anti-static facilities, and other related accessories.

Given that the scope of atmospheric pressure storage tanks discussed in this article includes not only flammable liquid storage tanks, but also acid-base storage tanks, the user unit should pay close attention to the risk of the tank roof (tank top), especially for some enterprises where there is a lack of personnel activity platform on the tank roof. Special attention should be paid to preventing the consequences of corrosion and thinning of the tank roof, which may cause operators (inspectors) to fall into the tank while walking on it. When the author was providing consulting services at a company in Shandong, the equipment management personnel of the company mentioned that there was a serious hidden danger of thinning the top of an alkali tank in the company. Fortunately, it was discovered in a timely manner.

　　four

　　Maintenance and inspection (testing) of atmospheric pressure storage tanks

There are certain differences in the requirements for daily inspection (monthly inspection), annual inspection, maintenance, and inspection (testing) of atmospheric pressure storage tanks compared to the standards listed in Part 2 of this article. Considering that daily inspections (monthly inspections) and annual inspections are conducted without the need to open the tank, which facilitates implementation, this article will not delve into them specifically. Taking three representative standards as examples, we will focus on discussing the requirements for maintenance and inspection (testing).

1. Safety Technical Regulations for Vertical Cylindrical Steel Welded Storage Tanks (AQ 3053-2015)

　　11.3 Determination of Inspection Cycle

　　The determination of tank inspection cycle should meet the following requirements:

　　c) The periodic inspection period should be determined based on the measured corrosion rate and the minimum allowable thickness of the tank body. The actual inspection period should be determined based on the principle of ensuring that the thickness of the tank body during the next inspection is not less than the minimum thickness required by the standard.

　　d) When the corrosion rate is unknown, it can be determined based on the corrosion rate predicted by the operating experience of storage tanks under similar working conditions; When there is no operating experience or data similar to storage tanks, the periodic inspection period shall not exceed 6 years, and the periodic inspection period for large storage tanks shall not exceed 4 years.

　　e) For tanks with severe corrosion, the user should shorten the regular inspection cycle reasonably according to the actual situation.

　　11.4 Risk Based Inspection (RBI)

　　The unit using the storage tank can also use the RBI program to determine the periodic inspection period. RBI comprehensively evaluates the possibility and consequences of tank leakage or accidents to determine the acceptable risk of the minimum thickness of the tank before the next periodic inspection, and accordingly increases or decreases the periodic inspection period determined in 11.3.

2. Maintenance and Inspection Regulations for Atmospheric Pressure Vertical Cylindrical Steel Welded Storage Tanks (SHS 01012-2019)

　　3.1 Maintenance cycle

　　The maintenance cycle of storage tanks should be determined based on comprehensive inspection and maintenance (regular inspection), and it should be ensured that the thickness of the tank body is not less than the minimum thickness required by the standard during the next maintenance. Usually 6 years.

　　3.1.2 For storage media with a corrosion rate of ≤ 0.1mm/a and reliable anti-corrosion measures, if the technical condition of the storage tank is confirmed to be good during the last comprehensive inspection and maintenance and can ensure safe operation, the maintenance (comprehensive inspection and maintenance) cycle can be extended with the approval of the enterprise's professional technical leader, but generally not exceeding 9 years.

　　3.1.3 For stainless steel storage tanks with a nominal capacity of no more than 5000m ³ and clean storage media, leakage prevention and operation monitoring measures shall be in place. After approval by the professional technical leader of the enterprise, the maintenance (comprehensive inspection and maintenance) time can be determined as needed.

　　For tanks with severe corrosion, the maintenance cycle (comprehensive inspection and maintenance) should be reasonably shortened according to the actual situation.

　　3.1.5 The maintenance cycle of storage tanks (comprehensive inspection and maintenance) should be relatively fixed, and a corporate system should be established to clarify it; When making changes, the corresponding procedures should be followed.

　　3.2 Maintenance Strategy

　　Risk based inspection and evaluation results can be used to determine the inspection strategy for storage tanks.

3. Integrity Management of Atmospheric Storage Tanks (GB/T 37327-2019)

　　8.1.1 Atmospheric pressure storage tanks are generally evaluated for integrity 3-6 years after they are put into use, and the subsequent evaluation time is determined by the inspection agency based on the evaluation results.

　　8.1.2 The integrity evaluation of various components of atmospheric pressure storage tanks includes inspection and evaluation of the tank body and foundation, sealing system, cathodic protection, anti-corrosion coating breathing valve, instrument electrical system, lightning protection and anti-static facilities, and other related accessories. If necessary, a suitability evaluation shall be conducted.

Chapter 8 of this standard also provides specific provisions for the inspection and evaluation of various components (including related accessories) of atmospheric storage tanks.

five

　　Regarding the qualifications of maintenance, inspection (testing) institutions

The issue of inconsistent standard requirements and the qualification of maintenance and inspection (testing) institutions are common concerns for hazardous chemical enterprises. And if the latter is solved, the former will also be easily resolved.

In order to implement the requirements of the "Notice of the General Office of the State Council on Comprehensive Administrative Law Enforcement of Emergency Management" (State Council Letter [2023] No. 51), on July 17, 2023, the Ministry of Emergency Management issued the "Guidance Catalogue for Comprehensive Administrative Law Enforcement of Emergency Management (2023 Edition)". In the interpretation of the document, it is emphasized that "the names of law enforcement matters are generally summarized and refined based on the content of the laws, administrative regulations, and departmental rules that establish the matter

Based on this, it can be considered that,From the perspective of inspection (testing) institution qualifications, the "regular inspection" and maintenance of atmospheric pressure storage tanks are different from the "mandatory" regular inspection qualification requirements for pressure vessels under laws and regulations.

Everything has two sides. From the perspective of risk management, the reality is that some enterprises do not attach importance to the maintenance, inspection (testing) of atmospheric storage tanks. Recently, the author has personally experienced that a certain refining enterprise has not conducted open tank maintenance on gasoline storage tanks for more than 10 years, nor has it conducted integrity evaluations of storage tanks; Although a certain refining enterprise provided a third-party inspection report for atmospheric storage tanks, the inspection content was incomplete and there was no next inspection (integrity evaluation) date, making the risk uncontrollable.

In summary, it is necessary to establish regulations on the maintenance and inspection (testing) content of atmospheric pressure storage tanks, as well as the corresponding capabilities of maintenance and inspection (testing) institutions, to guide users in preventing and controlling safety risks and maintaining equipment integrity.

　　six

　　Summary

The unit using atmospheric storage tanks is the main responsible unit for the safety management of atmospheric storage tanks. It should formulate an atmospheric storage tank management system (maintenance regulations) in accordance with relevant standards and specifications, specify the maintenance and inspection (integrity evaluation) cycle and content, and effectively prevent and control the risks of atmospheric storage tanks.

In the selection of maintenance and inspection (testing) institutions for atmospheric pressure storage tanks, the focus is on the capabilities of the institutions, that is, the institutions responsible for maintenance and inspection (testing) should have the corresponding capabilities for the projects they undertake, including non-destructive testing projects.