Safety Analysis and Evaluation of Hydrogen Cylinder Periodic Inspection Station

Abstract

With the rapid development of the hydrogen energy industry, the number of hydrogen cylinders has reached a very large scale. At present, both domestic and international experiences of hydrogen cylinder inspection are zero, which makes the inspection work more challenging and unpredictable. In recent years, more and more attention has been paid to the responsibility of safety in production, but the research on the risk and safety of cylinder inspection institutions is very limited. In this paper, the Job safety analysis (JSA) method is used to carry out systematic risk identification, risk assessment, risk prevention, and other research work of the cylinder inspection station. After the JSA method and experience accumulation, the management strategy is continuously perfecting, and the security risk level is absolutely decreasing.

1. Introduction

Driven by the current round of technological revolution and industrial transformation, and based on a consensus among countries around the world, the world’s energy landscape is undergoing profound adjustments to promote a transition to clean, low-carbon energy in order to cope with global climate change. As a clean and carbon-free secondary energy source, hydrogen energy is an important component of the energy strategy in various countries. In recent years, China’s hydrogen energy industry has developed rapidly [1]. By the end of 2021, China had more than 10,000 fuel cell vehicles and over 100 hydrogen refueling stations, making it the world’s largest producer of fuel cell commercial vehicles [2].

Although multiple ways have been proven to store hydrogen energy, the Hydrogen actually is stored as a compressed gas. In mobility applications, compressed hydrogen is the most popular fuel, owing to its high energy density. For similar reasons, compression is an appropriate choice as a method for storing hydrogen energy. To use compressed hydrogen in drones, light-weight cylinder technology is necessary [3,4].

As the most commonly used fuel storage system for fuel cell vehicles at present, there are more than 10,000 cylinders in Foshan that are overdue and uninspected. In order to solve the current problems, we pay special attention to the safety performance of in-service hydrogen cylinders and introduced the first domestic standard for periodic inspection of Type-Ⅲ hydrogen cylinders, and built a regular inspection line with an annual inspection capacity of 24,000 gas cylinders.

The risk research of gas cylinder inspection and testing industry has not been developed in China. Managing hazards in place (MHP) [5] is a policy tool in environmental health that allows for incomplete elimination, mitigation, or remediation of environmental hazards. It is widely accepted that the practice of minimizing exposure to hazards rather than eliminating them is part of the toolbox for protecting the environment for human and ecosystem health. There is neither systematic risk analysis and prevention measures, nor a relatively accurate evaluation system. As the first hydrogen cylinder inspection station in the world, Guangdong Quality Supervision Inspection Institute of Hydrogen Storage and Transportation Equipment (Foshan) bears great social responsibility and demand. In this paper, the Job safety analysis (JSA) method is used to carry out risk identification and systematic research on the cause of accidents for the hydrogen cylinder inspection institutions, and then the appropriate index evaluation method is selected to carry out a systematic risk assessment for the cylinder inspection institutions, which also provides effective technical support for the basic risk value of the inspection.

2. Job Safety Analysis

JSA is a procedure that helps integrate accepted safety and health principles and practices into a particular task or job operation. In a JSA, each basic step of the job is to identify potential hazards and recommend the safest way to do the job.

The significance of JSA mainly includes the following aspects:

(1)

Make risk management refined to each specific operation.

(2)

The operator shall manage the risks in his own operation.

(3)

By participating in the preparation, discussion, communication, and implementation revision of the JSA, etc., to improve the understanding of implementation and control methods in the operation.

JSA is conducted in preparation for a defined work task so that actions to eliminate and control the identified hazards can be implemented before the task is executed. A key principle in construction projects is that most risks should be mitigated as early as possible in a project’s life span. Figure 1 illustrates how risk is ideally reduced during the client, designer and contractor are planning and design processes [6].

Figure 1. JSA’s position in a construction project’s overall risk management.

JSA is one method of dealing with this residual risk. The method should ideally be applied in situations where safety is not ensured by adherence to procedures or plans or by established barriers [7]. Scientific regional risk assessment [5] of urban public security can provide important support for risk early warning and management system, and introduce a comprehensive evaluation and management method of risk factors in urban public safety areas. The traditional method of JSA does not consider time series constraints, and it needs to be improved to solve the typical problem [8].

3. Hydrogen Cylinders Periodic Inspection Process JSA

The purpose of periodic inspection is to test and evaluate the safety of hydrogen cylinders after a certain period of use, and test the performance and risk level of hydrogen cylinders after actual use, to ensure that the gas cylinders can be safely used in the next inspection period. The periodic inspection process for hydrogen cylinders can be divided into the following steps as Figure 2.

Figure 2. Hydrogen cylinders periodic inspection workshop process flow chart.

3.1. Risk Identification

Risk identification includes determining the source of risk, determining the conditions for risk generation, and describing the characteristics of risk. Risk identification is the basis and premise of underlying risk management in underground coal mines [9]. As we know, the interactive relationships of 4M1E are complex. Man, machine, material, method, and environment are important in every step to influence the whole process’s safety.

Since the operation of each inspection project is different in the implementation process, the number and size of risk factors in the project operation process are different and each risk factor is identified according to the implementation of the inspection process. We identified the risk factors in the inspection preparation stage, inspection implementation stage, and follow-up stage.

3.1.1. Inspection Preparation Stage Risk Factors

The main work contents of the inspection preparation stage are as follows: onboard hydrogen system removal, cylinder registration, recycling, replacement, cylinder removal, cylinder removal, valve removal, and automatic cleaning. Table 1. lists main potential risk factors.

Table 1. Risk factors on inspection preparation stage.

Risk Factors	Potential Accident
The leakage cylinders submitted for inspection in advance are not processed in time and stored separately, resulting in leakage	Fire
When the exhaust operator error caused by the cylinder hurt personnel or injured by airflow	Struck
When discharging gases, grease or static electricity or sparks exist in the operating environment	Fire/Burn
The gas cylinder treated by the device was not analyzed for combustible gas concentration, resulting in subsequent operation with media	Explosion
Valve loader, cylinder fixing frame, turnover mechanism, fixture malfunction, or human misoperation injury	Mechanical damage

3.1.2. Inspection Implementation Stage Risk Factors

The implementation of a cylinder periodic inspection project is the core step of cylinder inspection, each result determines whether the cylinder can continue to use. Moreover, for hydrogen cylinder products with high pressure grade, complex inspection process and lack of experience, especially for pressure test (reach 105 MPa, water) and leak test (reach 70 MPa, nitrogen gas), which need to be more focused on. For the consideration of energy saving and equipment stability, the leak test’s pressurization mode selects early-stage liquid pressurization + 2nd-stage gas pressurization with nitrogen as the test medium is selected.

The equipment supplies the high pressure test gas through the high pressure air supply system, and at the same time carries out the gas cylinder leak test through the 6 × 6 test station (each row is separately set up in the explosion-proof isolation room), which can realize the independent and complete work of 35 MPa to 70 MPa gas cylinder leak test. Table 2. lists potential risk factors.

Table 2. Risk factors on inspection implementation stage.

Risk Factors	Potential Accident
Injury caused by falling cylinder during inspection process	Struck
During internal inspection, the residual flammable gas in the cylinder was not cleaned	Fire/Burn
The pressure test and leak test increased pressure too fast, resulting in the explosion of the cylinder body	Explosion
Device failure causes injury, such as rupture of high pressure pipe inflated by leak test or flying joint	Struck
In the process of liquid nitrogen vaporization, the carburetor and pipeline are cooled rapidly	Frostbite
Mass vaporization of liquid nitrogen or accumulation of nitrogen caused by leak test exhaust	Choking
Equipment connection risks caused by improper use or installation of sealing materials or wear of joint threads	Explosion
Gas pressure source—Liquid nitrogen vessel leakage or vacuum failure	Choking

3.1.3. Follow-Up Stage Risk Factors

The follow-up work mainly includes inspection mark spraying, gas cylinder scrapping, and vehicle hydrogen system assembly. Nowadays, scrapped gas cylinders are repaired and used again in society, so inspection institutions should be strengthened to supervise the disposal of unqualified gas cylinders, and flattening and other methods of total destruction should be adopted to eliminate the possibility of the use of gas cylinders and repeated circulation. Table 3 lists potential risk factors.

Table 3. Risk factors on follow-up stage.

Risk Factors	Potential Accident
Leakage of hazardous materials due to spray curing process	Poisoning
Damage caused by flattening or handling of cylinders	Struck
Subsequent use risks may be caused by errors in the installation process of the vehicle hydrogen storage system or the tightening of the valve	Explosion
Liquid nitrogen vessel broken or safety releasing failure	Explosion
Aging circuit and poor insulation performance of equipment	Electric shock
Noise of all kinds of running equipment and inspection process	Hearing loss/Secondary accident

3.2. Risk Assessment

3.2.1. LEC Method Danger Degree Assessment

In view of the actual situation of the project, the actual inspection process is selected as the evaluation object, and the risk evaluation of the working conditions is carried out on various dangerous and harmful factors in the production process and the risk degree of accidents that may be caused. According to the scoring rules of the LEC method [10], the scores of L (likelihood), E (exposure), and C (consequence) are obtained through investigation and statistics, and the calculated risk score (D: danger) is obtained to evaluate the risk level of operation. For specific evaluation, see Table 4.

Table 4. Results of operational risk assessment.

Potential Accident	Risk Score: D = L × E × C				Danger Degree
	L	E	C	D
Explosion	1	3	15	45	General risk, attention should be paid
Fire/Burn	1	6	3	18	Slight risk, acceptable
Struck	0.2	6	7	8.4	Slight risk, acceptable
Mechanical damage	0.5	3	3	4.5	Slight risk, acceptable
Poisoning	0.5	3	3	4.5	Slight risk, acceptable
Frostbite	1	6	1	6	Slight risk, acceptable
Choking	1	6	1	6	Slight risk, acceptable
Electric shock	1	1	3	3	Slight risk, acceptable
Hearing loss/Secondary accident	1	1	3	3	Slight risk, acceptable

Through the risk assessment and analysis of the operation conditions of the actual inspection process, it can be seen that after the project is put into use, the risk degree of cylinder explosion and vessel explosion is “General risk, attention should be paid”, and other dangerous and harmful factors are “Slight risk, acceptable”.

3.2.2. Accident Consequence Simulation

In order to evaluate the consequence and reduce the bad influence, we choose those general risks to do the accident consequence simulation. Pressure vessel explosion has the following characteristics [11,12,13]: Pressure vessel in operation due to overpressure, overheating, or corrosion, wear, and the compression of the original difficult to bear; The explosion of pressure vessel not only causes equipment damage but also affects the surrounding equipment and buildings. The fragments of its explosion can fly hundreds of meters and can produce a huge shock wave, its destructive power and lethality is huge; the explosion of pressure vessels can also make toxic or flammable substances leak out, causing major fires or secondary accidents. In this case, there are only physical explosions in consideration because of nitrogen characteristics. Analysis of influence range of damage radius of physical explosion as follows:

• Principle of calculation

The energy released by the low temperature liquid vessel blasting is the energy of gas and saturated liquid because the former is small, often negligible, because the burst boiling low temperature liquid explosion is completed instantly, so it is an adiabatic process; once the gas leak test, the gas pressure will quickly drop to the atmospheric pressure, so we usually think no heat exchange between the gas and the atmosphere. The physical blasting energy can be calculated in the following formula:

$$\mathrm{W}=\frac{{\mathrm{P}}_1\mathrm{V}}{\mathrm{k}-1}\left[1-{\left(\frac{{\mathrm{P}}_2}{{\mathrm{P}}_1}\right)}^{\frac{\mathrm{k}-1}{\mathrm{k}}}\right]$$

W: Physical explosion energy of vessel/cylinder (J);

P₁: Pressure of vessel/cylinder explosion;

P₂: Atmospheric pressure, take 101,325 Pa;

V: Volume of vessel/cylinder (volume of the highest liquid level, when storing low temperature liquid, the filling rate shall not be greater than 0.95 and filling rate of 0.95).

The k: the adiabatic index of the gas, (diatom 1.4; multiatom 1.29).

➢

Explosion energy calculation of vessel/cylinder

Liquid nitrogen: P₁ = 1.6 MPa, V = 30 m³, k = 1.4

The physical explosion of the obtained liquid nitrogen vessel/cylinder can be caculated as:

$$\begin{gathered} {\mathrm{W}}_{\mathrm{c}}=\frac{70\times 30}{1.4-1}\times (1-(\frac{101,325}{70\times {10}^6}{)}^{\frac{1.4-1}{1.4}})=4410 \mathrm{MJ} \\ {\mathrm{W}}_{\mathrm{v}}=\frac{1.6\times 30\times 0.95}{1.4-1}\times (1-(\frac{101,325}{1.6\times {10}^6}{)}^{\frac{1.4-1}{1.4}})=62.18 \mathrm{MJ} \end{gathered}$$

➢

TNT equivalent calculation of the physical explosion of the vessel/gas cylinder:

$${\mathrm{W}}_{\mathrm{TNT}}=\frac{\mathrm{W}}{{\mathrm{Q}}_{\mathrm{TNT}}}$$

$${\mathrm{W}}_{\mathrm{TNT}}=\frac{62.18}{4520}=13.76\mathrm{kg}=4520 \mathrm{kJ}/\mathrm{kg}$$

The TNT equivalent for the physical explosion of the resulting liquid nitrogen vessel/cylinder is:

$${\mathrm{W}}_{\mathrm{TNT}-\mathrm{v}}=\frac{62.18}{4520}=13.76\mathrm{kg}$$

$${\mathrm{W}}_{\mathrm{TNT}-\mathrm{c}}=\frac{4410}{4520}=9757\mathrm{kg}$$

• Explosion impact and damage damage

(1)

Simulation ratio of the explosion

The explosion simulation ratio to the benchmark dynamite quantity (1000 kg of TNT) is:

$${\mathsf{\alpha }}_{\mathrm{v}}=\sqrt[3]{\frac{{\mathrm{W}}_{\mathrm{TNT}-\mathrm{v}}}{1000}}=\sqrt[3]{\frac{13.76}{1000}}=0.24$$

$${\mathsf{\alpha }}_{\mathrm{c}}=\sqrt[3]{\frac{{\mathrm{W}}_{\mathrm{TNT}-\mathrm{c}}}{1000}}=\sqrt[3]{\frac{9757}{1000}}=2.14$$

(2)

Quite a distance in the benchmark explosive amount (1000 kg of TNT) explosion experiment R₀ = R/α

According to the contents of Table 5, three thresholds of 0.02, 0.03, 0.05 were selected, and three considerable distances were calculated as 56 m, 42.5 m, and 32.5 m based on the contents of Table 6.

Table 5. Injury effect of shock wave over-pressure on human body.

Over-Pressure ΔP/MPa	Injury Effect
0.02–0.03	Minor damage
0.03–0.05	Audauditory organ injury or fracture
0.05–0.10	Severe visceral injury or death
>0.10	Most of the people have died

Table 6. Impact wave over-pressure of 1000 kg TNT explosion.

R0/m	5	6	7	8	9	10	12	14
ΔP/MPa	2.94	2.06	1.67	1.27	0.95	0.76	1.50	0.33
R0/m	16	18	20	25	30	35	40	45
ΔP/MPa	0.235	0.17	0.126	0.079	0.057	0.043	0.033	0.027
R0/m	50	55	60	65	70	75
ΔP/MPa	0.0235	0.0205	0.018	0.016	0.0143	0.013

• Calculation of the explosion damage radius

According to formula R₀ = R/α

In this project, once the physical explosion occurs, its different degrees of damage radius is shown in Table 7.

Table 7. Radius and impact of explosion damage.

	Vessel Explosion	Cylinder Explosion
Radius of death	7.8 m	69.55 m
Radius of serious injury	10.2 m	90.95 m
Radius of minor injury	13.44 m	119.84 m

It can be seen that when the explosion isolation and protection measures are not taken, the explosion risk is great, and once an accident occurs, it will have a huge impact on the surrounding equipment, buildings, and personnel.

3.3. Risk Prevention

According to the results of the risk assessment in this paper, the factors of moderate risk in the periodic cylinder inspection process are “cylinder explosion and vessel explosion”. The prevention and control measures for the above risk factors are as follows:

• For the cryogenic vessel, we choose the safety relief valve and bursting disc two-stage safety release device. It is recommended that a remote monitoring and fault diagnosis system be configured, and a security manager be equipped to perform grading and device maintenance. Reasonably set up safe distance and explosion-proof area around the storage vessel can reduce the accidental influence. And we remained enough distance for the LN₂ vessel as Figure 3.
  

  Figure 3. LN₂ vessel position diagram.
• For cylinders in pressure test and leak test, we choose proof wall and strict explosion-proof area. Especially for the leak test area, the whole area adopts auto camera monitoring and inspection, and the explosion-proof door cannot be opened when there is pressure in the cylinder. At the same time, sealing accessories and pressurization equipment should be repaired and replaced regularly. It is recommended to configure a multi-stage inspection mechanism to prevent loosening or failure of fasteners or connecting pipes. The 50 × 50 channel steel is used as the supporting column, the wall is reinforced with 30 × 50 channel steel, and the two sides of the wall are welded with a 5 mm steel plate.
• After the project is put into use, the management should be strengthened, the implementation of strict rules and regulations to strengthen the maintenance of machinery and personnel safety management, completely eliminate potential accidents, to minimize the possibility of all kinds of accidents.

4. Conclusions

Although JSA is a risk assessment method intended to contribute to risk-informed decisions and ensure safe operations, it also has several other benefits in terms of both safe and efficient operations. Good project management leads to good safety management and thus good safety performance. The article built the whole JSA chain for cylinder inspection station with risk identification, risk assessment, and risk prevention.

With the rapid development of hydrogen energy, both at home and abroad, the experience for hydrogen storage cylinder inspection is lacking, we drafted the first domestic hydrogen cylinder periodic inspection standard, and as China’s first automatic periodical lines with high efficiency, reasonable safety assessment by applying the method of JSA. We aim to improve testing quality, reduce the accident risk, and ensure the safety of the follow-up work with a remarkable practical significance.