Overpressure in manufacturing processes may be the result of a wide variety of causes, from runaway reactions and failed regulators, to operator mistakes and faulty equipment. Overpressure can have serious consequences, not just damaging equipment and systems, but also potentially leading to personal injury and even loss of life.
So, pressure relief valves are widely seen as the primary safety solution. However, sometimes an issue that can be overlooked is that these valves themselves are also in need of protection to maintain the intended level of safety.
Most often pressure relief valves are spring loaded and are set to open at a specified pressure. When pressure rises to an unacceptable level, they are pushed open and pressure is released. Once the pressure level has normalised, the spring forces the valve closed, and the process is resealed.
So, what do the pressure relief valves need protection from? The media itself. The valves are used in almost every industry so there is a wide variety of process media that they can be exposed to. Even where the media seems relatively benign, corrosion and wear & tear are always a risk since we are dealing with mechanical moving parts. The risks are obviously much higher with more corrosive, sticky or viscous media.
The three main issues which can affect PRV’s are: A build-up of product media inside the pressure relief valves, corrosion and leakage.
A build-up of product media means there is a risk of blockage or reduction of the orifice. A complete blockage is not the only hazard. In many situations, the build-up of pressure is very rapid so the relief of pressure needs to be at a very high flow rate with a high volume and at high speed. In this scenario, even a small reduction of free-flowing orifice may mean the valve cannot relieve pressure effectively enough, leading to damage of the processing equipment.
Corrosion affecting the PRV needs to be considered in relation to specific media, especially where non-standard materials of construction for the pressure relief valve may be required. The corrosion can either lead to the valve leaking at an increasing rate, or it can cause it to fail to lift when required, so it cannot relieve pressure as intended.
Where leakage is concerned, there are two separate issues. Firstly, a small amount of process media that leaks through can lead to an unexpected build-up of product on the downstream side, creating the same risks as addressed for an upstream build up. Secondly, there are environmental concerns, depending on the media being processed. Emission regulations are becoming stricter across the globe, so leakage may have important consequences, both for the environment and for a business if it fails to meet the required limits.
To protect against all of these issues; build-up, corrosion, and leakage, a rupture disc can be placed upstream of the pressure relief valve.
Protection against Build-Up
By using a rupture disc, the pressure relief valve is sealed and is protected from contact with the media. The discs are designed to have no gaps or crevices where product could adhere or build-up, unlike the typical edges and angles found in a valve inlet design. If there is overpressure, the discs burst – without fragmenting – and allow the relief valve to work as intended. Once the pressure is relieved, the valve will reseal the system again and the discs can be replaced at a convenient time. This results in a reduced downtime when compared to one where you are required to regularly clean and maintain the valves, if there were no rupture discs to protect them.
Protection against Corrosion
Not only does a rupture disc prevent the media from corroding the valve internals, it also means that it’s cost-effective to achieve chemical compatibility when special materials of construction are required. By using an upstream rupture disc made from a resistant alloy the pressure relief valve can be made from lower cost material, meaning construction costs are reduced, without any loss of reliability from the pressure safety system.
Protection against Leakage
The rupture discs will prevent leakage entirely. This means the pressure relief valves require greatly reduced maintenance. Furthermore, when regular valve set pressure testing is required this can be done in-situ. The space between the valve inlet and the rupture disc can be pressurized in-situ until the relief valve opens, avoiding removal for bench testing and the resulting process downtime. The rupture disc offers a back pressure allowance so that it will not be affected by such testing.
The use of rupture discs at the inlet of pressure relief valves will improve the level of pressure safety as selected and reduced the need for cleaning, maintenance and repair. Corrosion issues – both upstream and downstream of the pressure relief valve – as well as outage costs can be substantially reduced leading to better yield of the production site. Ultimately by adding rupture discs to the inlet of your pressure relief valve, it will result in higher process safety in a facility with reduced costs of ownership.