The costs associated with installing and maintaining pressure-relief valves (PRVs) can often be reduced by equipping systems with upstream rupture discs
Reclosing pressure-relief devices, commonly referred to as pressure relief valves (PRVs), are designed to allow for the release of overpressure and to reclose when the pressure returns back to an acceptable level.
PRVs are commonly used throughout facilities in the chemical process industries (CPI) for safety functions, including the following:
- Protect equipment and personnel by safely releasing pressure before equipment rupture or an explosion can occur
- Prolong equipment life and prevent damage to pipes, reactors and storage tanks by keeping pressures at desired design limits
- Ensure the production process runs efficiently and pressure-related downtime is minimized
While pressure relief is a required element of chemical manufacturing, PRVs are not the only device capable of protecting a process from pressure-related hazards.
Rupture discs are non-closing pressure-relief devices that are intended to burst once the intended burst pressure is reached. After activation, the membrane remains open, resulting in a complete discharge of the pressure in the installation.
Both rupture discs and PRVs may be used independently as the primary source of pressure relief. However, using rupture discs in combination with PRVs often offers the most appropriate and most cost-effective solution.
Several arrangements exist in which rupture discs and PRVs may be used in tandem, but perhaps the most common method is installing a rupture disc upstream of a PRV, which offers several significant advantages, which are outlined throughout this article.
Lower-cost PRV materials
Depending on the process media, PRVs are often required to be constructed of certain special — and often expensive — materials to protect against corrosion. These materials are not only necessary for the body of the PRV but also for its internal components, including the spring, poppet, seat, screws, O-rings, gaskets and more.
Therefore, PRVs constructed of such non-standard materials can be extremely expensive and may have long lead times. And if any of the internal components become corroded, costly replacement parts are needed and repair could result in downtime.
This can all be avoided by installing a media-compatible rupture disc upstream of the PRV. Then, the valve is physically isolated from the process and protected from the potentially corrosive media. In other words, the PRV’s exposure to the media is limited to an overpressure situation when the rupture disc bursts and opens. In short, this combined assembly allows for the use of PRVs and related spare parts that are constructed of “standard” materials, resulting in a substantial reduction in maintenance and replacement costs.
FIGURE 1. The use of upstream rupture discs can enhance PRV operation while also reducing costs
Allow for in-situ testing
Periodic testing of PRVs is required by industry standards organizations, including the American Society of Mechanical Engineers (ASME; www.asme.org), American Petroleum Institute (API; www.api.org), EN (European Standards) and various other regulating bodies to ensure proper calibration and to meet regional standards.
This process usually requires the uninstallation of each PRV, which depending on the scale of the process, can be a time-consuming and costly endeavor because of the associated downtime. However, using rupture discs upstream of PRVs may allow for testing without the removal of the devices. Below are the required steps to carry out such a testing procedure:
- Pressure is applied in the space via compressed air between the rupture disc and PRV. The pressure is manually or automatically monitored
- The pressure between the rupture disc and PRV rises to the opening pressure of both the rupture disc and PRV. The rupture disc should not be damaged during the test cycle
- As the pressure reaches the opening pressure of the PRV, it will open. The measured opening pressure can now be compared to the rated nominal opening pressure of the PRV to determine if it is functioning within its design parameters — all without removing the PRV
Prevent leakage and emissions
In order to achieve leak tightness, most spring-operated PRVs rely on special metal-to-metal sealing surfaces, which inevitably results in some leakage that increases as the operating pressure approaches the valve’s set pressure. PRV leakage rates are addressed in industry standards, and acceptable leakage rates are defined in API 576, which covers the inspection of pressure-relieving devices. API 527, the standard covering seal tightness in PRVs, calls for a maximum daily leakage rate of 1.5 std. ft3 for metal-seated PRVs.
Furthermore, it is common for PRVs to partially open when pressure builds but doesn’t quite meet the PRV’s set pressure. PRVs may leak or “chatter,” resulting in unwanted emissions and lost product.
Where such leak rates are unacceptable for environmental or safety reasons, rupture discs positioned upstream of the PRV eliminate emissions by up to 100 times in a simple and cost-effective manner by providing a leak-tight seal and avoiding PRV chattering and leaking.
Tandem installation design
When installing a rupture disc upstream of a PRV, there are several important considerations that must be understood to ensure proper operation.
According to ASME UG-132(a)(4)(a), the marked burst pressure of the rupture disc should be between 90 and 100% of the marked set pressure of the PRV. However, EN ISO4126-3 paragraph 7.2 says, “ The maximum limit of bursting pressure … shall not exceed 110% of the … set pressure or a gauge pressure of 0.1 bar, whichever is greater …” and “ The minimum limit…should not be less than 90% of the…set pressure.” While slightly different, the basic guidance between the two standards is the same. In short, keeping the rupture-disc specified burst pressure and PRV set pressure at the same nominal value, ignoring tolerances, meets the intent of each of the standards and is relatively easy to implement.
It is also critical to note that no fragmentation of the rupture disc is allowed, because loose parts may obstruct the valve orifice or restrict the valve from reclosing. It should also be noted that not all rupture discs are created equal, and many can experience fragmentation. Non-fragmenting discs are engineered with an “opening feature,” which may be created by a score line, laser or other means, that guides the rupture disc’s petals to open in a pre-determined manner without fragmentation. Without this opening functionality, the metal stretches until the weakest part of the disc can no longer withstand the pressure and breaks into pieces.
Additionally, sufficient distance needs to be available for the rupture disc to open without blocking the PRV nozzle. For example, a single-petal rupture disc may extend beyond the height of the holder and reach into the inlet section of the PRV.
The rupture disc should also be “close-coupled” with the PRV, thereby assuring that the pressure drop during flow at the inlet of the relief valve does not exceed 3%, as required in industry standards. Longer distances between the rupture disc and PRV may result in the creation of reflective pressure waves upon opening of the rupture disc and may result in undesired re-closing of the rupture disc or even fragmentation.
Since the rupture disc, like the PRV, is a device that reacts to differential pressure between the upstream and downstream side, measures need to be taken to avoid that any unnoticed pressure increase occurs in the closed cavity between the rupture disc and PRV inlet. This pressure may increase due to a number of factors, including temperature within or around the process. Any unnoticed pressure changes are commonly identified through the use of a pressure gage or indicator.
Industry code requires that the capacity of the PRV must be derated by 10%, but combination capacity testing will decrease that requirement.
Finally, as one can deduce from the technical nature of these assemblies, extreme care must be taken when selecting the correct rupture disc for the job, and therefore, when selecting the ideal rupture disc manufacturing partner.
A reliable rupture disc is required to ensure it bursts precisely at the intended moment (burst tolerance), that it can handle the full pressure of the system without degrading (operational ratio), and that it can endure the repeating on-and-off nature of the chemical production process (cycle life).
Further Reading
1. The Next Generation of Overpressure Protection, Chem. Eng., Oct. 2023, pp. 22–23.
2. Safety Relief Valves: Installation and Maintenance, Chem. Eng., Oct. 2022, pp. 40–42.
3. Rupture Discs: Effectively Minimize Leaks and Emissions, Chem. Eng., July 2017, pp. 42–46.
Author
Daniel Willis is the national sales manager for industrial protection products at Fike Corp. (Email: [email protected]). He is responsible for sales and business development for all pressure-relief products within the U.S., Latin American and Caribbean markets. He has over 40 years of experience in the overpressure protection industry, including nearly 24 years as a sales manager for Fike and has tremendous knowledge of the changing industry codes and regulations impacting industrial pressure relief.