Reliable pressure relief on flare systems

Paul Boughton

James Hay dispels the myths surrounding rupture discs

Over the past decade the use of flares to burn off hydrocarbons has been significantly reduced and in some countries all but eliminated, except for unusual or emergency cases. To achieve a safe system to recover the hydrocarbons, end users have modified their processes and used recovery systems to channel the once waste gas back into the plant or use it directly for secondary purposes such as injection or utility fuel.

To ensure that the flare is still available under certain conditions that require the reliable opening of the flare line, various valves and controllers are employed.  The problem can be that these are not failsafe and may not respond and open fast enough, or at all, in an emergency. How can a reliable and safe installation be ensured? Enter the Rembe bursting disc, a reliable and failsafe opening device.

There are a lot of myths surrounding bursting discs (also known as rupture discs), since most design engineers have no training or experience in this field. It is a subject that is not covered in any university undergraduate engineering degree. Many different types of valves and piping control systems are covered, but not an item that is often the most important safety device in the plant!

The usual way of specifying rupture discs is to apply the same data and knowledge used for safety valves. This isn’t really an appropriate method. A bursting disc needs the right design criteria if it is going to operate to suit the process conditions that it will help control.

Myth number 1 is that ‘discs always fail’. Here we need to define the word fail. Does this mean premature bursting, leaking discs or that the disc is bursting often and needs to be replaced?  

Premature burst is sometimes prevalent with older designs of rupture discs. These discs are mechanically scored to set the burst pressure. Incorrect fitting techniques, mechanically induced stresses and the operating pressure exceeding the operating ratio can all damage the disc and cause the disc to open at a lower set pressure than that specified.

Leakage of mechanically scored rupture discs can occur due to pin-holing or cracking of the score line. These mechanically scored discs are susceptible to damage due to mechanically induced stresses, corrosion, vibration and incorrect fitting techniques, which can all contribute to leakage along the score-line.

Rembe bursting discs are not mechanically scored. The KUB, for example, is a two-layer disc where the burst pressure is controlled by a laser-cut element on the downstream side of the process. No scores or other marks are facing the system process, thereby increasing the durability and increasing the service life of the disc.

Frequent bursting is a common complaint. History shows that upon investigation, repeated opening of the discs is usually due to the operating envelope being exceeded. In many cases the operator has increased or changed the operating pressure but did not apply the changes to the design of the existing disc.

In this circumstance, the disc is doing its job correctly, the process has exceeded the set pressure and/or temperature. The Rembe KUB has a 98% operating ratio and has a wide range of pressures, which means the set pressure of the disc can be increased without the need to replace the existing holder for a different style of disc.

The second myth is that it takes time to change out the disc. This is true, but it is not necessarily a bad thing. If you have had an excess pressure event then the worst thing you can do is rapidly replace the disc and start the process again; what about the root cause of the event? Having a fast reset valve is of no use if it takes hours or days to investigate or resolve the overpressure cause, or if you need to wait for HQ or a certifying 
authority approval to resume the process. By the time the investigation is completed and the repair or rectification deployed the disc will have been replaced.

Proper location and ease of access built in at the design stage can also take hours off of the time needed to replace a disc; another Rembe advantage is that there are no special tools, skills or multiple pages of technical information to read to change the disc, just common sense.

Rupture discs are also the fastest acting solution. In a recent case study, an offshore operator in the field of oil & gas conducted its own analysis on a fast opening valve and found that even for a stated opening time of 300ms, system pressure will reach a maximum pressure surge by a factor of five times the maximum allowable operating pressure of the system. By implementing the same simulation using Rembe rupture discs, the customer proved that the maximum pressure seen is well below the design limits. This is based on the fact that the discs respond in 2 to 3ms, ensuring instantaneous pressure relief. The choice of the operator to use a rupture disc rather than a high-speed valve was simple and straightforward.

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James Hay is with Rembe in Germany.

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