# Fire protection -- Water in a cylinder pressurized with Nitrogen

• chrismof
In summary, the conversation discusses the pressure inside a pressurized cylinder used in the fire protection industry. The cylinder contains 200 litres of water and 40 litres of nitrogen gas at 10.0 bar. The remaining pressure after the water is expelled depends on the amount of gas that has left the tank, which is affected by factors such as the design of the tank and whether the valve is left open. The gas volume will expand to 240 litres and the pressure will be 1.833 bar(abs) or 0.833 bar(gauge). The nitrogen gas remains at the top of the cylinder until activated and the release of water occurs at a rate of 14.16 litres per minute for approximately 13 minutes. The
chrismof
I wondered if anyone could help, I work in the fire protection industry. We currently have a project using a pressurised cylinder. The cylinder hold 200 lites of water plus 40 litres of nitrogen gas as a propellant at 10.0 bar. I’m trying to work out once the 200 litre water volume has been expelled what would be the remains pressure. I believe the formula to use is Boyles law but I’m struggling to understand. Would appreciate it if anyone could help.

When it's empty doesn't that mean it's at atmospheric pressure? So 1 bar?

The answer will depend on the amount of gas that has left the tank, which in turn depends on two things you haven't specified: 1) Is the design such that gas sprays out with water? 2) Is the valve left open after the last of the water has been forced out of the tank?

Last edited:
The volume of the reservoir is 200 litres of water plus 40 litres of gas = 240 litre.
40 litres at 10 bar will expand to 240 litres at ? bar, to expel the 200 litres of water.

P1·V1 = P2·V2; holds for absolute pressures.

If the 10 bar is not absolute, but is gauge pressure, then the initial pressure will be 11 bar(abs).

P2 = P1 * V1 / V2 = 11 * 40 / 240 = 1.833 bar(abs) = 0.833 bar(gauge).

Thanks to all your prompt replies. The nitrogen gas is not mixed with the water but remains at the top of the cylinder until activated. Further to my question the release of the water would be at 14.16 litres / minute for approximately 13 minutes. What I am trying to check is at what point the nitrogen gas pressure would drop below 7.0 bar.

chrismof said:
What I am trying to check is at what point the nitrogen gas pressure would drop below 7.0 bar.
Can you confirm that the pressure you are specifying is the gauge pressure, above atmospheric pressure. Do you understand why the absolute pressure is one bar higher than the gauge pressure?

P1 = 10 bar(gauge) = 11 bar(abs);
V1 = 40 litre;

P2 = 7.0 bar(gauge) = 8.0 bar(abs);
V2 = V1 * P1 / P2;
V2 = 40 * 11 / 8 = 55 litres;

The change in gas volume is V2 - V1 = 55 - 40 = 15 litre of water.

For a constant flow rate of 14.16 litres / minute, the pressure would fall to 7.0 bar(gauge) in;
15 / 14.16 = 1.06 minutes = 63.6 seconds.

Hi,
Thank you for your reply the pressure would be the gauge pressure.

chrismof said:
once the 200 litre water volume has been expelled
chrismof said:
the release of the water would be at 14.16 litres / minute for approximately 13 minutes.
chrismof said:
the nitrogen gas pressure would drop below 7.0 bar.

These are not all the same point.

## 1. What is the purpose of using nitrogen in a cylinder pressurized with water for fire protection?

Nitrogen is used in a cylinder pressurized with water for fire protection because it helps to maintain the pressure in the cylinder and prevent the water from evaporating. This ensures that the water is readily available to be used in case of a fire emergency.

## 2. How does nitrogen help to suppress fires?

Nitrogen helps to suppress fires by displacing the oxygen in the air, which is necessary for combustion. This creates an inert environment that makes it difficult for the fire to continue burning.

## 3. Can nitrogen be used in all types of fire extinguishing systems?

No, nitrogen is typically used in fire extinguishing systems that utilize water as the extinguishing agent. It cannot be used in systems that use other types of extinguishing agents such as foam or dry chemical.

## 4. Is there a risk of explosion when using nitrogen in a pressurized water cylinder for fire protection?

No, there is no risk of explosion when using nitrogen in a pressurized water cylinder for fire protection. Nitrogen is a non-flammable gas and is commonly used in fire suppression systems.

## 5. How often should the nitrogen in a pressurized water cylinder be checked?

The nitrogen in a pressurized water cylinder should be checked regularly, typically every 6 months, to ensure that the pressure is maintained and there are no leaks in the system. This helps to ensure that the fire protection system is functioning properly in case of an emergency.

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