Pressure Safety Relief Valve

Pressure Safety Relief Valve & Rupture Disk Combination When selecting safety relief equipment, it is important to consider process-specific conditions. Sudden pressure increases that may occur in the process, as well as conditions such as whether the process environment is corrosive, toxic or polymerizing, should be taken into account when selecting the most suitable and reliable relief equipment. Rupture disks and safety relief valves can be combined in two different ways: in parallel or in series combination below the safety valve. The selection should be made according to the equipment and process. [caption id="attachment_128277" align="aligncenter"] Figure 1: Safety relief valve and rupture disk combination[/caption]

1.1. Pressure and Pressure Relief

Industrial facilities commonly use pressure-containing equipment specific to the nature of the process. Examples include: tanks, reactors, pipelines, pump discharge lines, boilers, turbines, heat exchangers, etc. Pressure protection in equipment operating at particularly high pressure values is of paramount importance for process safety. Excessive pressure accumulating inside such equipment and rupturing it creates a major explosion effect at the facility and can lead to a catastrophe with loss of life and property. To protect against the destructive effects of excessive pressure, equipment must have multiple layers of protection. Following redundant protection logic, if a malfunction occurs in one protection device, the other protection equipment must continue to reliably protect the system.

1.2. Rupture Disks

A rupture disk is a pressure relief device that controls pressure in a pressurized system (pressurized tanks, pipes, etc.) by preventing potentially dangerous overpressure and vacuum. The disk is designed to burst open at a predetermined pressure. Rupture disks are commonly used in production processes as an alternative to safety relief valves or together with safety relief valves. Some of the reasons for using rupture disks in pressurized equipment are: legal requirements, process safety, negligible leakage, rapid full-flow relief, excellent corrosion resistance and maintenance costs. Due to legal requirements, the "Pressure Equipment Regulation (2014-68-EU)" in effect in our country contains information about pressurized equipment, accessories and installations. According to this, protection of the environment and workers against excessive pressure is required. Insurance requirements also emphasize the importance of protection. Providing reliable pressure control is very important from a process safety perspective. It is possible to provide a negligible leakage rate with rupture disks to reduce product losses and ensure tightness, especially in toxic products. Rupture disks provide rapid and effective full-flow relief with operating pressures reaching up to 98%. When working with corrosive, aggressive and polymerizing products, superior corrosion protection is provided through material selection specific to the product. Rupture disks have no maintenance cost; visual inspection is sufficient. This provides the user with a significant advantage over safety relief valves in terms of maintenance costs. Rupture disks have three types of applications in processes: as primary relief equipment, as secondary relief equipment (in parallel) and together with a safety relief valve (below it). If a rupture disk is to be used together with a safety relief valve, it can be placed below the safety valve or installed in parallel as two side-by-side pressure relief systems. If the systems are installed side by side, the opening pressure of the rupture disk will be higher than the safety relief valve. When pressure increase is very large and sudden, if the safety relief valve cannot respond quickly enough or does not open, the rupture disk serves as a reliable backup system to relieve the pressure. If the rupture disk is placed below the safety relief valve, it performs roles such as superior tightness, protection against forced environment and on-site testing. Rupture disks are manufactured in different structures and characteristics depending on where they will be used in the process. For example, in low-pressure systems, flat-moving disks are mostly used to provide protection against pressure and vacuum. At high pressure values, advanced-moving and reverse-moving disks should be used. Disks are equipped with sealing elements from different materials, in single and double-layer configurations. Rupture points are determined using laser technology and full opening is achieved at the adjusted pressure from these points.

1.3. Basic Rules and Standards

There are international rules and standards followed in the applications of safety relief valves and rupture disks, and these standards must be adhered to when applied to processes. For example, EN ISO 4126-1, one of the defined standards for safety relief valves, defines a safety relief valve as a spring-loaded, mechanical pressure relief device that recesses. In EN ISO 4126-2, rupture disks are defined as non-reclosing, mechanical pressure relief devices. Sections 2-3-6 of the EN ISO 4126 standard contain information about the application, selection, installation of rupture disks and their use in combination with safety relief valves. Information about rupture disks is also contained in standards such as API 520 and ASME VIII. For example, for practical and applicable design, the installation of a safety relief valve and rupture disk should be at the closest distance. Figure 3 shows an illustration of incorrect and correct application of pressure relief equipment located at the tank outlet. Again, rules such as the relationship between pipe diameter and the distance between the safety relief valve and rupture disk, the set pressure ratios of the rupture disk and safety relief valve, etc. can be found in standards, and applications should be carried out in accordance with these rules. [caption id="attachment_128279" align="aligncenter"] Figure 3: Example of incorrect (left) and correct (right) application[/caption]

1.4. Design and Application

Designs made in safety relief valve and rupture disk combinations should be applicable and engineering calculations should be performed specific to the process. The EN ISO 4126-3 standard (Combined Safety Relief Valve and Rupture Disk Safety Devices) contains matters relating to combination and design. Figures 4 and 5 contain images related to the application. [caption id="attachment_128280" align="aligncenter"] Figure 4: Application in safety relief valve and rupture disk combination (1)[/caption] Particularly in polymerizing and adhesive products, malfunctions such as failure to open at set pressure can occur due to clogging in valve springs and body. If the valve seat is sticky, it is no longer possible to guarantee the defined set pressure. This creates a hazard even before the safety relief valve is first operated. An example of this is shown in Figure 6 below. In such processes, protecting the safety relief valve with a rupture disk is the most reliable solution and greatly reduces valve maintenance costs. [caption id="attachment_128282" align="aligncenter"] Figure 6: Safety relief valve in polymerizing and adhesive products[/caption]

1.5. Advantages of the Combination

Safety relief valves and rupture disks have certain advantages and disadvantages within themselves. For example, while a safety relief valve cannot provide complete and rapid opening at rapid, sudden pressures, a rupture disk opens at its set value, providing the advantage of full-flow and rapid relief. While a rupture disk cannot be reused once opened, a safety relief valve has the advantage of being reusable. In summary, the advantages of the combination of a safety relief valve and rupture disk are as follows: Protection: Protects the safety relief valve in aggressive, corrosive, or polymerizing environments. Safety: Provides low leakage rate and high tightness (e.g.: toxic environment). Cost: Reduces cost (prevention of high-quality sealing material cost in the valve). Testing: On-site testing (if required) – Safety relief valve lift-testing and maintenance are performed on-site (See Figure 7). Life: Extends the service life of the safety relief valve. [caption id="attachment_128283" align="aligncenter"] Figure 7: On-site testing of the safety relief valve[/caption] With the combination, the advantages of the safety relief valve and rupture disk are merged and strong protection is provided. Cleaning and testing are necessary to ensure that the safety relief valve responds to the specified pressure in an emergency situation. A REMBE® rupture disk mounted on the inlet side of the safety relief valve protects the valve against caking and sticking. Reverse-type rupture disks such as KUB® have a smooth metal surface on the process-facing side, which prevents residue formation. Fracture points are positioned away from the process. This ensures that the specified opening pressure is always maintained and eliminates the possibility of delayed response.