How to calculate HVAC Pressure Relief Outlets Sizing and Door Air Loss in Excel Sheet?

A pressurized vessel or system is protected by a pressure relief valve (PRV) in the event of an overpressure occurrence. Any circumstance that might raise pressure in a vessel or system over the designated design pressure or maximum permissible operating pressure is referred to as an overpressure event (MAWP). Relief valves should be independent and extremely dependable because they are the final line of defense.

When overpressure is not monitored, it can harm a system and even result in an explosion. Safety valves and pressure relief valves guard vessels, piping systems, and equipment against this danger. It is imperative that the appropriate valve is always utilized because these valves are so crucial.

At a certain set pressure, the relief valve is intended to open. The pressure inside the vessel will cease increasing and start to decline as fluid is redirected. The valve will shut once it reaches the reseating pressure.

Similar to this, equipment occasionally has to be shielded from an internal vacuum. Vacuum relief valves open under these circumstances at a predefined low pressure limit, allowing air or an inert gas into the apparatus to prevent the pressure from dropping too low.

Types of pressure relief devices:

The two primary kinds of pressure relief devices are closing and non-reclosing. The two additional categories for the closing type are

  1. Spring-loaded and
  2. Pilot-operated

When choosing PRVs, a number of application-specific criteria are taken into account. Backpressure, service, discharge capacity, and intake pressure losses are a few of them. The sort of relief valve to use is determined in part by back pressure.

When the total of the maximum variable superimposed backpressure and the built-up backpressure is less than 10% of the fixed pressure, conventional relief valves are employed. When the total of the maximum variable superimposed backpressure and built-up backpressure exceeds 10% of the set pressure and up to the manufacturer’s suggested limit, bellows-type relief valves are utilized. In actual usage, this is capped at 50% of the preset pressure. A pilot-operated relief valve should be utilized if the backpressure rises by more than 50%.

In reality, the backpressure should not rise over 94% of the set pressure for reliable operation, despite the fact that pilot relief valves can theoretically be utilized for up to 100% of the backpressure.

When compared to traditional relief valves of the same size, pilot-operated relief valves offer a significantly larger discharge capacity. Pilot valves can also be employed for high intake pressure loss situations since remote sensing line choices are readily available.

Considerations to take into account while choosing and sizing safety or pressure relief valves.

1. Size and type of connection

The intake and discharge pipe sizes must match the size of the valve. According to the National Board, the size of the inlet and discharge pipework that are linked to the valve must be at least as large as the openings on the valve itself.

2. Set pressure

A safety or pressure relief valve opens when the predetermined pressure, which is expressed in pounds per square inch (PSIG), is reached. The maximum allowed working pressure (MAWP) of the boiler or other vessel cannot be exceeded by the set pressure of the valve. The MAWP of the apparatus must be at least 10% higher than the maximum operating pressure anticipated under typical conditions.

3. Temperature

The volume and viscosity of the gas or liquid moving through the system are impacted by temperature. The appropriate construction material for the valve is also influenced by temperature.

4. Back pressure

Back pressure is the pressure created by the pressure in the discharge system on the outlet side of the pressure relief valve, which may be constant or changing. It may alter the upstream valve’s set pressure, causing it to continually pop open, which might harm the valve. Valves should be used for installations with variable back pressure such that the back pressure doesn’t go over 10% of the valve set pressure. A bellows-sealed valve or a pilot-operated valve can be necessary for installations with large amounts of steady back pressure.


Different valves are required for various services (steam, air, gas, etc.). Additionally, the valve’s building material must be suitable for the service.

6.Capacity needed

Both safety valves and relief valves must have a specific capacity for pressure release. The shape of the valve, the media’s temperature, and the relief discharge area are only a handful of the variables that affect the needed capacity.

What will happen if I adjust the velocity and the pressure?

As the velocity of the door opening increases, the pressure differential across the door also increases. This can cause air to be forced through gaps in the door, resulting in air loss. Higher pressures can also cause doors to warp and seal poorly, resulting in air loss.

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