Design of Emergency Block Valves in Petrochemical Engineering

This paper integrates domestic and international design standards to examine the principles for positioning emergency block valves in petrochemical engineering. It analyzes the specific fire protection requirements for the valve body and actuator, offering recommendations for design selection. From a fire safety perspective, this paper elucidates the crucial role of emergency block valves in mitigating both direct losses and indirect impacts during fire incidents. The petrochemical industry processes numerous flammable and explosive substances, leading to a relatively high risk of fire and explosion. The Emergency Block Valve (EBV) differs from a standard emergency shut-off valve in its comprehensive fire protection design, which effectively controls the spread of incidents. In recent years, advancements in international standards and domestic specifications have led to an increased implementation of Emergency Block Valves (EBVs) in engineering applications.

1. Setting Principles of Emergency Block Valves

The placement principles for emergency shut-off valves should adhere to international, national, and industry standards. Chapter 8 of the American Petroleum Institute's API RP 553, "Refinery Control Valves" (2012 Edition), outlines fundamental guidelines for installing emergency shut-off valves on equipment such as compressors, pumps, vessels, and heaters. According to Article 7.2.15 of GB 50160-2022 "Design Fire Protection Standards for Petrochemical Enterprises," when the distance between liquefied hydrocarbon equipment with a capacity exceeding 50 m³ and its extraction pump is less than 15 m, the inlet pipeline should be equipped with a shut-off valve near the base of the equipment. Furthermore, the "Basic Requirements for Emergency Shut-off Systems of Oil and Gas Storage Enterprises (Trial)", issued by the Ministry of Emergency Management of the People's Republic of China on February 24, 2022, stipulate that emergency shut-off valves must be installed on all process material inlet and outlet pipelines directly connected to atmospheric pressure storage tanks with a nominal diameter of at least 30 meters or a nominal volume of at least 10,000 cubic meters.

In alignment with these regulatory requirements and the design practices of domestic and international engineering firms, emergency shut-off valves should be installed on the outlet (or inlet) process pipelines of high fire-hazard equipment to facilitate the rapid isolation of flammable or toxic materials during emergencies. Specific installation principles include, but are not limited to:

Between liquefied hydrocarbon processing equipment with a volume exceeding 50 m³ and the pump inlet.​

Between flammable liquid processing equipment with a volume exceeding 8 m³ and an operating temperature above 316°C and the pump inlet.​

Between process equipment with a volume exceeding 16 m³ that contains flammable liquids with a flash point below 28°C and the pump inlet.​

Between process equipment containing flammable gases and flammable liquids, with the liquid phase comprising more than 40% by mass when released into the atmosphere, and the pump inlet.

For equipment containing extremely hazardous and highly hazardous materials that utilize sealed pumps, emergency shut-off valves should be installed at both the inlet and outlet of the pump.​

For compressors transporting flammable gases at a normal flow rate exceeding 30 t/h, as well as extremely hazardous and highly hazardous gases, emergency shut-off valves should be installed at both the inlet and outlet of the compressor.

Store flammable liquids at the inlets and outlets of pressure tanks with a storage capacity exceeding 50 tons.​

Store extremely hazardous and highly hazardous liquids at their respective inlets and outlets.

Guided by the aforementioned installation principles, the use of emergency shut-off valves (ESDVs) in new or modified domestic equipment is becoming increasingly prevalent. However, failures such as the inability of ESDVs to close during emergencies—due to malfunctions or design flaws—can pose significant safety risks, potentially disrupting normal equipment operation. Therefore, meticulous attention to the design, selection, and maintenance of these valves is crucial to ensure their reliable performance in safeguarding personnel, equipment, and the environment.

2. Design and Selection of Emergency Shut-Off Valves

According to API RP 553 "Refinery Control Valves," emergency shut-off valves are classified into four types: A, B, C, and D. Class A refers to on-site manual globe valves; Class B includes power-operated globe valves; Class C encompasses motor-operated globe valves; and Class D pertains to remotely operated globe valves. This section focuses exclusively on the design and selection of Class D remotely operated globe valves.

2.1 Types and Structures of Emergency Shut-Off Valves

Emergency shut-off valves are typically categorized into three types: ball valves, gate valves, and butterfly valves.

In medium and low-pressure applications:​

• For valves with a nominal diameter (DN) of 200 mm or less, ball valves or gate valves are generally selected.​
• For valves with a nominal diameter greater than 200 mm, gate valves, triple-offset butterfly valves, or double-offset high-performance butterfly valves are preferred.​

In high-pressure applications:​

• For valves with a nominal diameter (DN) of 200 mm or less, ball valves or gate valves are typically selected.​
• For valves with a nominal diameter greater than 200 mm, gate valves are generally preferred.​

In low-pressure applications：

ball valves are primarily utilized for nominal diameters (DN) of 200 mm or less. For nominal diameters exceeding 200 mm, gate valves are often preferred due to their cost-effectiveness in larger sizes. Emergency shut-off valves do not necessitate flow capacity or noise calculations; their nominal diameter should match that of the process pipeline.

Internal leakage in a valve refers to the leakage between the valve plug and the valve seat when the valve is in the closed position. The internal leakage rate of an emergency shut-off valve should comply with the leakage standards specified in API 598 or GB/T 13927. API 598 is utilized for the sealing performance testing of various valve types, including gate, ball, and butterfly valves. It mandates zero leakage for valves with resilient (soft) seats, while allowable leakage rates for metal-seated valves are specified based on the valve's size. External leakage in a valve refers to the unintended escape of the process medium from the valve body to the environment, primarily occurring at the valve stem packing and the valve body-bonnet seal; this type of leakage is also known as fugitive emissions. The external leakage rate of the emergency shut-off valve must comply with the standards specified in ISO 15848 or GB/T 26481. Additionally, the valve's packing and stuffing box should be designed to be fire-resistant and capable of withstanding high temperatures to minimize external leakage during a fire.

When an emergency shut-off valve serves as an emergency cut-off on both the inlet and outlet main pipelines, a bi-directional sealing valve trim is required. The selection of components such as the valve body, seat, trim, and gaskets should consider the operating conditions and properties of the process medium. The corrosion resistance requirements for emergency shut-off valves are identical to those for standard shut-off valves, as outlined in SH/T 3005-2016. Whenever feasible, the valve seat of an emergency shut-off valve should utilize a metal-to-metal hard seal. If a soft-seated valve is chosen, it must incorporate an anti-static design to prevent electrostatic discharge.

2.2 Fire Protection Design of Emergency Shut-off Valves

In the event of a fire, the emergency shut-off valve must effectively isolate high-risk equipment for an extended period. Currently, large-scale oil refining and chemical plants face spatial constraints due to site area limitations, facility layout, and process pipeline configurations. Emergency shut-off valves are typically installed in areas prone to fire hazards. These valves are designed to remain securely closed in high-temperature environments for at least 30 minutes without compromising their functionality. Valve fire protection standards employ test methods that assess leakage in valves subjected to flames during and after exposure, evaluating the fire safety performance of their design and materials. The commonly adopted fire test standards are the American standards API 607 and API 6FA. API 607 was initially developed for fire testing quarter-turn valves with soft-seated designs, including ball and butterfly valves, to certify their fire-safe performance. API 6FA is a fire protection certification standard that encompasses a broader range of valve types compared to API 607. Typically, API 607 is applied for fire safety testing of ball and butterfly valves, whereas API 6FA is used for gate valves. The fifth edition of API 607, published in June 2005, significantly broadened its scope by incorporating straight-stroke valves and hard-sealed valves, thereby enhancing its versatility.

Currently, the most widely used fire protection standards are API 607 and API 6FA, while ISO 10497 aligns closely with the fifth edition of API 607. In engineering applications, API 607 and API 6FA differ primarily in test pressure and maximum allowable leakage, as shown in Table 1 below. Therefore, any of these three certifications are generally recognized.

Table 1: Analysis of Differences Between API 6FA/API 607 Third Edition/BS 6755 Part 2 and API 607 Fifth Edition (ISO 10497)

2.3 Actuator of Emergency Shut-off Valves

Emergency shut-off valves typically utilize either pneumatic or electric actuators. The petrochemical industry relies on a dependable instrument air system in the plant area, making pneumatic actuators the preferred choice. It not only ensures safety in emergency situations but also offers rapid closure and a short response time.

2.3.1 Fire Safety Type

This type of actuator typically uses a failure-closing (FC) mechanism with instrument air and employs a spring-return, single-acting cylinder actuator. The spring surface is treated for corrosion resistance. The primary method involves cutting off the gas source using a fusible mechanism to close the valve. There are two common approaches: one involves using a fusible air supply pipeline to disconnect the gas source. When a fire occurs, the gas source pipeline leading to the valve melts, cutting off the gas supply and causing the valve to close. The second method involves installing a fusible plug on the actuator. When a fire occurs, the fusible plug melts, releasing the pressure in the cylinder and allowing the spring to reset, thereby closing the valve. For this fire safety emergency shut-off valve, whether using the fusible air duct method or the fusible plug method on the actuator, factors such as temperature and sunlight must be considered.

2.3.2 Fire-Resistant Type

This type of actuator typically employs two methods. One is to use fire-resistant materials or apply fire-resistant paint to the shell. As the surrounding temperature rises rapidly, the volume of the refractory material expands quickly, forming a protective heat-insulating carbon layer. This carbon layer not only provides thermal insulation but also absorbs a significant amount of heat during combustion, offering fireproof protection. Another method is to cover the actuator's outer shell with a fireproof cover. The fireproof cover comes in two types: flexible and rigid, as shown in Figures 1 and 2. The flexible fireproof cover consists of a rainproof layer, stainless steel wire mesh, plastic braid, aluminum foil, and heat-insulating fiber, all sewn together with stainless steel wire. Asbestos materials are prohibited. Its advantages include easy installation and maintenance, and it does not cause corrosion. The rigid fireproof cover is made of a stainless steel insulating board filled with heat-insulating mineral fibers. The heat-insulating material within the stainless steel sandwich helps reduce the heat transfer rate to the actuator inside the enclosure. Additionally, a window can be incorporated as per user requirements for convenient observation, operation, and maintenance. This is why fireproof covers are the preferred choice for emergency shut-off valves in the petrochemical industry. Refractory materials or fireproof covers must also be windproof and resistant to sunlight. For more details, refer to API RP 2218, "Fire Protection Application Measures for Petroleum and Petrochemical Plants.

Figure 1: Image of the flexible fireproof cover

Figure 2: Image of the rigid fireproof cover

According to the UL 1709 rapid temperature rise test standard for steel structure fireproof materials, regardless of the type of fire-resistant actuator used, it must meet the requirement of withstanding a hydrocarbon fire for more than 30 minutes at 1,093°C. Additionally, it is advisable to use stainless steel pipes with good fire resistance for gas source pipelines. In areas where pneumatic actuators cannot be used, such as certain tank areas, electric actuators can be selected instead. According to API RP 553, electric actuators should be the preferred choice for gate valves. In large-diameter pipelines, the pneumatic actuators for straight-stroke valves require significant thrust and occupy a relatively large volume. This makes them not only expensive but also difficult to install in limited spaces. For emergency shut-off valves, the most critical requirement is to close the valve in the event of a fire, so the electric actuator must be fire-resistant. The approach is similar to the fire-resistant type used for pneumatic actuators in section 2.3.2. A fire-resistant coating can be applied, or the fireproof cover shown in Figures 1 and 2 can be used. Additionally, the power and control cables should be fire-resistant to meet the overall fire protection design requirements.

2.4 Accessories of Emergency Shut-off Valves

The accessories of an emergency shut-off valve include the handwheel, gas storage tank, solenoid valve, and feedback switch. The design and installation of all accessories should consider fire resistance and high-temperature conditions, ensuring they do not compromise the overall fire protection requirements of the valve. The handwheel can be configured based on the operating conditions of different installation sites. The gas storage tank must be made of at least carbon steel and have a valid pressure vessel certification. Additionally, the minimum volume must meet the gas storage capacity required to open and close the valve once at full stroke under instrument air pressure. The solenoid valve should be direct-acting, with a 316 stainless steel valve body and low power consumption. The solenoid valve used for spring-return, single-acting actuators should be a 2-position, 3-way type, either a general type or a power-off exhaust type. The solenoid valve for double-acting actuators should be a 2-position, 4-way type or a 2-position, 5-way type, with a coil made of high-temperature insulated material.

3. System Design

The emergency shut-off valve can be shut down by the SIS or DCS system for safety interlocking. However, the specific system control should be determined based on the results of the project’s safety integrity level (SIL) analysis. The emergency shut-off valve located in front of the pump suction port should be linked with the pump motor. When the emergency shut-off valve closes, the pump motor should be immediately shut down to prevent damage to the pump caused by the absence of flow at the inlet. Moreover, if the valve is not open, the pump cannot be started. This can be implemented in the control system by monitoring the feedback from the emergency valve position. When the emergency shut-off valve is detected to be fully open, the pump is allowed to start. Conversely, when the valve is detected to be fully closed, the pump is interlocked and stopped.

4. Conclusion

The installation of the emergency shut-off valve is intended to meet the increasingly stringent safety requirements of petroleum refining units. While it may increase initial investment, it plays a crucial role in ensuring the long-term stable and safe operation of the facility. The emergency shut-off valve not only helps prevent the spread of fire but also significantly reduces the likelihood of related safety accidents, effectively minimizing potential losses. In the long term, the installation of emergency shut-off valves is essential and will become a key component of the safety design in modern petrochemical engineering. As an instrumentation professional in the petrochemical industry, only by correctly and reasonably selecting and applying emergency shut-off valves can a solid foundation be established for the safe operation of petrochemical equipment.