There are two types of rupture disk- Forward Acting & Reverse Acting Rupture Disk. A rupture disk is used to protect the vessel from being over-pressurized. In an industry, air pressure acts as a very crucial parameter that needs to be maintained every time in many devices and equipment, including tanks, vessels, or pipelines. When the pressure increases, there are chances of the equipment getting burst. Hence, there must be some way to monitor and stop the process quickly.
The pressure transmitters can help but they are regarded as the best choice with a controller like PLC or DCS. If a direct hardwiring interlock is needed, the pressure switch can be suitable in that case. For solving this purpose of safety there are two other options also.
The relief valves and rupture disks are the two types of pressure relief devices found in an industry that is used to protect a pressurized vessel or piping system in the event of overpressure. The rupture disk is called a burst or bursting disk or pressure safety disk.
The rupture disk protects a vessel, equipment, or system from being over-pressurized or causing potential damage under vacuum conditions. A rupture disk is a device that saves other parts of the system from damage due to pressure. Once the rupture disk gets punctured, it cannot be used again. It is a safety instrument that immediately detects the pressure and cut off the circuit.
Before discussing types of rupture disks, we will first understand what is rupture disk and why it is required to install in pressure vessels.
Need of Rupture Disk
A closed vessel has differential pressure acting inside it. This differential pressure can occur at a higher side at any time in the vessel. If in case the working air pressure of a vessel exceeds a preset limit due to certain circumstances such as no way for air to pass, potentially damaging vacuum level, or increasing liquid level, the vessel would corrode after some time or explode immediately.
This could damage property, human life, and/or the environment. So, in order to provide immediate relief to the vessel, a rupture disk is needed. The rupture disk provides an instant response to an over-pressurization or under-pressurization in any process tank or piping system. But once the rupture disk ruptures, it cannot reused Hence, it is a one-time use device.
What is a rupture disk?
The rupture disk is a one-time use type pressure relief device that is actuated or burst by sensing a high pressure. The rupture disk works on the principle of pressure relief. The rupture disk is designed to function by the bursting of a pressure-containing disc and thereby releasing the pressure from one side.
The rupture disk is used to prevent the vessel from breaking or getting damaged due to leaks/cracks due to an increase or decrease in pressure in pressure above or below a given pressure. The rupture disks are mainly used to protect against over-pressurization.
The rupture disk has a thin metal diaphragm that explodes when the pressure in the vessel increases. So, it consists of a rupture disk and a disk holder. The rupture disk is designed such that it relieves or ruptures at 1.5 times the maximum allowable working pressure of the vessel.
The assembly of the disk has a thin, circular membrane which is usually made of metal, plastic, or graphite. These things are which is firmly clamped on the disk holder. The materials used in making a rupture disk can range from various alloy sheets of steel to higher quality materials like Hastelloy, Inconel, or Tantalum. It can also have coatings or plastic liners like PTFE or FEP.
Types of rupture disks
There are two types of rupture disks. They are:
- Forward-acting rupture disk
- Reverse-acting rupture disk
Forward Acting Rupture Disk
This type of rupture disk is faced in series with the process. When the pressure in the vessel starts to increase or go beyond the safety limit, the disc will start to grow towards the forward direction because of the acting pressure. After the safety limit is crossed, the rupture disc will burst. Therefore, it shows that the rupture disc will need a large amount of pressure or force to burst. This is because; it is acting in the same direction as the direction of the process.
Reverse Acting Rupture Disk
Reverse acting type of rupture disk is faced in opposite with the process. When the pressure in the vessel starts to increase or go beyond the safety limit, the disc will start to grow towards the forward direction (the forward direction for this disc will be reversed from the process direction) because of the acting pressure. After the safety limit is crossed, the rupture disc will burst. When compared to forward-acting, the reverse-acting disk needs less force to burst. This is because it is placed in the opposite direction of the process.
Functions of a Rupture Disk
The rupture disk is a type of protection that can be used as either a primary way or a secondary way of providing protection.
In the primary relief, they can be used directly on the tank. In this case, the disk has the sole responsibility to protect the vessel from pressure damage. In the event of increased pressure, it would burst and release air from the vent immediately.
In the secondary relief, the disc is mounted above the primary relief valve. Usually, the primary relief valve also performs the same function. But the rupture disk would protect the valve from process media which can corrode or plug it. It can also act as a seal, and prevent any leakage through the valve if the disk is intact. This type is recommended in applications where a pressure vessel needs to be protected as exothermic reactions may develop abnormally high and uncontrollable pressure conditions.
The disks also provide data to the PLC or any other controller through a digital input. So, the disc would have a relay contact that operates only when the disc bursts. In this case, the relay will change its state. Because of this, the PLC will know this and will immediately trip the entire process for safety reasons.
Combination of Rupture Disk and Pressure Relief Valve
The combination of rupture disk and pressure relief valves, a type of protection, is used in industries. This can be done in two ways: rupture disk upstream or downstream of the relief valve. Both types have their own pros and cons.
A rupture disk placed at the inlet of a relief valve
The process benefits of installing a rupture disk at the inlet of the pressure relief valve are:
- It ensures a positive seal of the system (in case of toxic fluid systems because of the danger of relief valve leak)
- It protects the relief valve from fluids that contain solids, which may plug the valve.
- It provides corrosion protection to the valve. Hence, reduces valve maintenance.
- It allows in-situ testing of the relief valve, for example, calibration.
A rupture disk placed at the outlet of a relief valve
In this case, the disks protect the valve from the downstream fluids. If the relief fluids are vented in the common header, media may result in either corrosion or polymerization. The rupture disk can isolate vented media from the relief valve.
Selecting a Type of rupture disk
The following factors need to be considered.
- Burst pressure
- Permissible overpressure or vacuum
- Working air pressure
- Working process and system to be protected.
- Burst temperature of the fluid.
- Vacuum resistance
Advantages of Rupture disk
- It is simple and reliable.
- The disk has no moving parts.
- It provides leak tightness.
- Low cost.
- Light in weight and can open easily.
- It can be manufactured in various sizes according to pressure variance.
Disadvantages of Rupture disk
- It cannot be tested.
- The disk can degrade with age/corrosion.
- It needs a shutdown to refit a new rupture disk in case of a burst.
Rupture Disk in LOPA
We all have seen the above image many times during our training sessions. Did you notice our Rupture Disk there? Yes, rupture disk falls in Prevention as well as Mitigation areas. So, when the process goes uncontrollable, the rupture disk will burst and release the pressure as per settings. This way rupture disk will help to decrease the severity of the emergency created or can also avoid it in total.