Why is back pressure important when specifying a Rupture Disk Device?
Back pressure affects the performance of Rupture Disk Devices since they respond to differential pressure. When the Device is selected and manufactured with the correct process parameters in mind (such as burst pressure, operating pressure, back pressure, coincident temperature), it will activate at the required burst pressure.
Through correct disk selection & specification, any back pressure conditions can be allowed for, resulting in the appropriate activation of the disk. An omission or error in consideration of back pressure could increase the observed burst pressure of a Rupture Disk Device, placing the pressure system at risk.
This article explains what back pressure is, how it affects the performance of a Rupture Disk Device and what methods can be used to mitigate the effects of back pressure.
What is back pressure?
Back pressure is the pressure on the outlet of the Rupture Disk Device. There are two types of back pressure:
- Superimposed back pressure: Superimposed back pressure is the pressure in the discharge header before the Rupture Disk opens. Depending on the system, superimposed back pressure can be constant or variable.
- Built-up back pressure: Built-up back pressure is the pressure that develops as a result of flow after the Rupture Disk opens.
Rupture Disk Devices are differential pressure sensitive devices. If a burst pressure of 9 bar is specified and the application introduces a 1 bar back pressure on the downstream side, the actual pressure on the upstream side required to activate the device will be 10 bar.
How to manage applications with back pressure:
Rupture Disk Devices are often installed in series with a Pressure Relief Valve (PRV). In these types of applications, the devices are considered ‘close coupled’.
As Rupture Disk Devices are differential pressure sensitive, International Codes & Standards require attention to the pressure that might arise in the volume between the Rupture Disk Device and the valve.
A means of venting and/or monitoring pressure build up between the Rupture Disk Device and PRV is required. This is often accomplished using an excess flow valve, and/or a suitable pressure monitoring device such as a pressure gauge, pressure transmitter, or pressure switch.
When multiple Rupture Disk Devices discharge into a common header, the resulting discharge of one device could subject the remaining Rupture Disk Device installations to a transient elevated back pressure condition.
In such an application it is recommended to use a double disk assembly to isolate the upstream or “primary” Rupture Disk Device from sources of variable back pressure. The downstream or “secondary” Rupture Disk Device is selected to withstand back pressure events leaving the primary disk to burst within its specified rating.
Double disk rupture disk assemblies consist of three safety head components: an inlet, a mid-flange and an outlet with a primary rupture disk installed between the inlet and mid-flange. A secondary rupture disk is installed between the mid-flange and outlet flange.
Such configurations can be equipped with forward-acting, reverse-acting, or a combination of forward and reverse-acting disks as required for the process application conditions.
BS&B Innovation: SkiTM Rupture Disk
In certain circumstances, the Ski Rupture Disk Device can be used in place of a double disk assembly to provide back pressure independence in a single device, thus reducing weight, material usage, and capital cost.
The SKI is the first reverse buckling rupture disk designed to be backpressure independent and offers the unique ability to have variable back pressure without impacting burst pressure.
Design Principle: Two reverse buckling rupture elements are placed together. The inlet (upstream) side rupture disk element is calibrated to the required burst pressure based upon process pressure conditions.
The outlet (downstream) side of the rupture element holds back the variable back pressure conditions from reaching the upstream element. The small volume between the two rupture elements is factory set at atmospheric pressure. When the upstream burst pressure is reached, the SKi’s two rupture elements open together. The SKi rupture disk is US and International patent pending.
How do you compensate for back pressure?
The key to handling back pressure is to take it into consideration when sizing and selecting Rupture Disk Devices at the project engineering phase.
If there is back pressure in your system, then selecting the right type of Rupture Disk Device which can withstand back pressure is important. BS&B can help you select the product suited to your application, for instance, Reverse Buckling Technology, Double Disk Assemblies, or back pressure independent Ski Rupture Disks Devices that can mitigate the back pressure acting on to a rupture disk. Back pressure monitoring products can also be recommended.
Avoid damage risk to your facility by including back pressure in Rupture Disk Device selection.