Proportioning systems for high-viscous (fluorine-free) foam agents: Things to consider.

The success in firefighting depends, to a great extent, on the correct proportioning and handling of foam agents which are used in fire extinguishing systems. Special attention must be paid to the foam agents' physical properties – most prominently, to their viscosity. Highly viscous foam agents have some special requirements to proportioning systems in the course of this.

The higher the viscosity, the less fluid is a liquid. The lower the viscosity, the more fluid it is. With the viscosity of foam agents, it is distinguished between

• Newtonian fluids, e.g. the low-viscous extinguishing agents AFFF, Hi-Ex, Class A and multi-purpose foam agents and
• non-Newtonian fluids, i.e. pseudoplastic foam agents such as AFFF-AR and FF (fluorine-free foam agents).

The foam agent to be selected depends on the type of intended use or the risk to be protected from. Is it about flammable solids or about fluids? Polar fluids require other foam agents for extinguishing than non-polar fluids do. Also, the depth of the fluid is one decision criterion.

The viscosity of foam agents has a major influence on the correctness of proportioning. Highly viscous foam agents have a significantly higher pressure loss while flowing than low-viscous foam agents do. This rules many proportioning technologies out which are based upon pressure ratios. In turn, such proportioning technologies are suitable where the high-viscous foam agent is delivered by pumps. As air trapping is the biggest problem when handling high-viscous foam agents, displacement pumps are the only suitable pump type. Off-the-shelf barrel pumps do not qualify as too much air would be trapped during delivery. The high viscosity would prevent this air from escaping, making the foam agent useless.

The foam agent's viscosity influences the pressure loss of a pump significantly while flowing through the suction line. The correct dimensioning of the suction line is therefore crucial for correct proportioning of the foam agent. When considering the most important operating parameters of a pump, the correlation between foam agent viscosity, pump suction capacity and the suction line diameter, becomes clear:

NPSH value

Every pump has a specific NPSH value, also referred to as suction capacity. The value can be taken from the manufacturer's datasheet. It depends on the pump type and the number of revolutions. Putting it simply, the NPSH value is the negative pressure or the pressure drop which the pump generates inside the suction nozzle.

Volumetric efficiency

Flow rate actually delivered in relation to flow rate theoretically delivered.

Two limitations must be considered when dimensioning the suction line: Firstly, the maximum permissible flow velocities; and secondly, the pressure loss in the suction line.

Flow velocity

Depending on their viscosity, foam agents have different maximum permissible flow velocities. For foam agents with a viscosity similar to water, which is independent of the state of movement (Newtonian fluid, e.g., AFFF foam agent), a flow velocity of 1.0 to 1.2 meters per second in the suction line should not be exceeded.

For pseudoplastic foam agents with a viscosity depending on the state of movement (non-Newtonian fluid, e.g., AFFF-AR foam), a flow velocity of 0.6 to 0.8 meters per second in the suction line must not be exceeded.

Exceeding the permissible flow velocities may lead to evaporation of foam agent components and the hazard of explosion-like increase in volume, showing as a pressure surge.

Dimensioning of the foam agent suction line

The suction line of a foam agent pump must be dimensioned in such a way to take pressure losses under different operating conditions into account:

• minimum / maximum expected water flow rate
• minimum / maximum expected foam agent temperature in the suction line

When calculating the dimensioning of the foam agent suction line, several factors must be considered. This includes the height difference between the foam agent tank and the foam agent pump, the foam agent density, acceleration of gravity, the length of the suction line as well as the pressure loss coefficient as a function of the flow velocity and the viscosity of the foam agent, and finally the flow velocity.

 

The following factors for the suction line layout can be influenced:

• h, the height difference between foam agent tank outlet and foam agent pump inlet
• L_total, the length of the suction line
• d_i, the diameter of the suction line

No pressure loss calculation can be made without indication of viscosity since the pressure loss coefficient is calculated as follows:

Viscosity is the most important physical property of a foam agent with regard to proportioning, and is decisive for its possible application in the case of a fire. Highly viscous foam agents can be applied efficiently only if the proper proportioning technology, based upon pumps, is used. The correct proportioning of a foam agent depends on the dimensioning of the foam agent suction line. Apart from the correct calculation of the dimensioning, many more parameters must be observed. In practice, it must be ensured e.g. by short and straight lines that nothing can compromise the delivery.