What is overpressure relief?

By Sam Gocher
Wednesday 24th August, 20223 minute to read

With the popularity of pressurisation systems increasing, it’s important for you to understand how they work in order to decide whether they’re the right fit for your building. There’s a lot to know about these systems, and a hugely important aspect to be aware of is overpressure relief.

What is the definition of Overpressure Relief?

When air is pumped into a zone in order to protect it, there is a risk of raising the air pressure so high that the stair or lobby door is forced shut against the jamb, which in turn prevents occupants from escaping. That’s why we have overpressure relief, it’s there to stop the air pressure in the protected zone from getting too high. The door must be able to be opened with no more than 100N of force at the door handle - the overpressure requirements have been set with this in mind.

How is Overpressure Relief achieved?

There are three options to choose from when it comes to achieving overpressure relief:

1. Counter-balanced flap valve

One option is the use of a counter-balanced flap valve, otherwise known as a barometric damper. The vent would remain shut until the pressure exceeds the design pressure, resulting in the pressure overcoming the force of the counterbalance weight and the flap opens to allow the air to escape until the pressure reduces, resulting in the weight forcing the vent closed again.

2. Variable relief damper

Another option is using variable relief dampers. To use this solution there must be the presence of pressure sensors in the pressurised zone. Once the overpressure limit is reached, or close to being reached, the relief dampers will open to provide relief. The greater the amount of overpressure, the more the damper opens.

3. Variable supply fans

The third and final option is variable supply fans. Much like variable relief dampers, variable supply fans use pressure sensors to detect the pressure. The fans then slow down when the maximum pressure is reached, or close to being reached. This is the option we use on our pressurisation systems here at Sertus. Our system uses machine learning to predict when overpressure may occur and prevents it from happening by acting in advance.

Due to variable relief dampers and variable supply fans both using pressure sensors, it’s important to note the special requirements when using these. As stated in BS EN 12101-6:

Variable supply fans or dampers controlled by pressure sensors shall not be used unless the system can achieve over 90% of the new air supply requirements within three seconds of a door being opened or closed.

Three seconds isn’t a substantial amount of time to affect a change in air supply and therefore is something to bear in mind if you were to use pressure sensors. However, most modern systems are capable of achieving the three second requirement.

You may be wondering what sort of pressure would be considered overpressure. BS EN 12101-6 explains that in the staircase a pressure differential of 50 pascals with a tolerance of 10% at the test stage is to be achieved. It also says that the maximum overpressure allowed is 60 pascals. Therefore, the expectation would be for overpressure relief to apply for anything above 55 pascals.

There are some very helpful sources on our blog and YouTube where you can find out more information about pressurisation systems. We have videos: ‘What is a Pressurisation System?’, ‘What is the Stack Effect?’ and ‘What is Protected Space?’ which may be of some interest. If you have any questions about pressurisation and/or overpressure relief, please don’t hesitate to contact us.

What is Overpressure Relief?

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