AVOIDING OVERLOAD IN FIRE EXTINGUISHING SYSTEMS WITH EMPTY PIPEWORK

If dry pipes of a fire extinguishing system are filled with premix in the event of an alarm, an overload situation may occur at the proportioner. In this article, we explain which factors lead to this and how to counteract the situation preventively.

First, let us look at the technical data of a foam proportioner. When do we speak of overload? The operating range of a proportioner determines the minimum and maximum water flow rate that the device can handle. The maximum water flow rate is also called the nominal volumetric flow rate. An overload situation exists when the water flow rate is higher than the nominal volumetric flow rate.

Example: FireDos proportioner FD10000/3-S:

• Operating range: 450 – 10,000 l/min
• Nominal volumetric flow rate: 10,000 l/min
• Overload would exist at a water flow rate of 12,000 l/min.

Various factors can cause overload, also possibly potentiating each other:

• Incorrectly dimensioned proportioner: The selected proportioner is too small, so that the water flow rate is higher than its nominal volumetric flow rate.
• System expansion without adaptation of the proportioner: The area to be protected against fires increases, causing the extinguishing water demand also to increase. If the proportioner is not retrofitted accordingly, i.e., replaced or re-dimensioned, an overload will occur.
• Abrupt opening of dry alarm valves or deluge valves on initial activation: This is the most common cause of overload.
• Activation of further dry alarm valves or deluge valves in case of fire spreading.
• Pipe burst in a wet system.
• A combination of the above, increasing the overload.

The effects of overload on the proportioner and the entire fire extinguishing system should not be underestimated. Here are the most common consequences:

• Damage to the water motor due to high mechanical loads on the components. This can lead to reduced or no proportioning of the foam agent at all.
  
  
• Cavitation in the pump: The foam agent pump rotates faster due to the higher speed of the water motor.

• Vibrations: Caused by the higher speed of the water motor and foam agent pump, they are transferred throughout the entire fire extinguishing system.
  
  
• At very high loads, a 'water hammer', i.e., a strong pressure surge, can occur.

What happens when a dry pipework is opened and filled abruptly? Here we see three characteristic curves of a fire extinguishing system:

• the throttling curve, i.e., the pump characteristic curve,
• the system characteristic curve 'empty pipeline'
• and the system characteristic curve full pipeline’.

High water flow rate due to lack of backpressure at alarm activation

The pump characteristic curve and the system characteristic curve 'full pipework' intersect at a water flow rate of approx. 8,500 l/min. If a fire alarm is activated, the alarm valve opens abruptly. At pump start/activation, the pump is not yet running at the operating point. The complete cross-section of the dry pipework is released so that the fire pump reacts to the pressure drop and starts immediately. Due to the lack of backpressure from empty pipework, the intersection of the pump characteristic curve and the 'empty pipework' system characteristic curve shifts to the right. The curves meet at a water flow rate of approx. 13,000 l/min.

Backpressure increases as pipework gets filled

Thus, for a short time after opening the alarm valves, a very high water flow rate is established at very low pressure. When the pipelines are filled, the back pressure slowly increases again. The operating point of the pump characteristic curve slowly moves back towards the designated operating point for the filled pipework.

Avoiding overload – Considering high volumetric flow rates during filling

Theoretically, an FD10000 proportioner can be used with a maximum water flow rate of 10,000 l/min. But when filling a dry pipework, it would be loaded with 13,000 l/min and thus overloaded. This can be avoided by taking appropriate preventive measures.

The actual volumetric water flow rates are measured at the FireDos test stand. Here is an example:

• After opening the alarm valve, the water flow rate increases steeply, up to 12,000 l/min in this example.
• As the empty pipes are filled, the backpressure builds up again. The volumetric flow rate gradually adjusts to the previously hydraulically designed, i.e., planned, water flow rate, to 8,000 l/min in this case.
• This means that the tested FD8000 proportioner is overloaded with a water flow rate of 12,000 l/min in some cases; i.e., by up to 50%. 

• Cascade connection of the fire pumps
  If there are several extinguishing water pumps, it may be worthwhile to switch them in a cascade, i.e., one after the other with a time delay. This prevents the full extinguishing water flow of all pumps from acting on all system components right from the start.

• Selection of a larger FireDos proportioner
  By selecting a larger FireDos proportioner, overload due to incorrect sizing can be prevented.

• Opening motor valves in stages
  Due to the gradual opening of the motor valves, only a small cross-section of the pipelines is released when the alarm valve is activated, and only then the complete cross-section is released after a time delay and complete filling.

• Deluge valves with integrated flow control, e.g., by BERMAD

• Installation of a BERMAD/INBAL control valve
  With the help of a bypass, these control valves allow only a limited water flow through the pipelines.

• When the alarm valve opens, the pressure initially drops abruptly. This is registered by the pilot valve, which prompts the Inbal or Bermad valve to close completely. Here in the example, the control valve is already closed.
• The extinguishing water can no longer flow to the alarm valve via the control valve. It must take the route via the bypass through the orifice. The orifice is designed for the maximum permissible water flow rate of the proportioner. An overload is therefore no longer possible.
• If the level in the empty pipework rises and thus also the backpressure, this is registered by the pilot valve. When the backpressure exceeds a defined value, the control valve opens again and releases the regular cross-section of the pipeline.

INBAL and BERMAD control valves can only open and close, intermediate positions are not possible. They develop their protective function exclusively in combination with a bypass.

Here we are looking into the virtual showroom of FireDos. The exemplary extinguishing system with proportioning system has been installed perfectly. It has a control valve and a bypass so that overload is prevented. The extinguishing water flows from left to right.

The main components are:

1 = Proportioner
2 = Control valve (from Inbal)
3 = Bypass with integrated orifice
4 = Alarm valve
5 = Line for inlet pressure measurement 
6 = Line for pressure drop measurement

The Bermad control valve 426 allows overload protection without a bypass. This works as follows: The Bermad 426 measures the pressure drop upstream of the FireDos proportioner and of the actual control valve. Since the pressure loss increases proportionally with the water flow rate, it is suitable as a control variable. The opening speed controller installed in the Bermad module then opens the respective cross-section in the valve according to this controlled variable. Unlike standard control valves that can only open and close, intermediate positions are also possible.

We are often asked whether, for example, an FD10000 can be loaded with an extinguishing water flow of 10,500 or 12,000 l/min or more, as such values are partly tested and achieved during approval tests. The answer is: No. Results of approval tests should not be used for system design because of the risk of overload. Here is an excerpt from the approval procedure at FM and VdS:

These tests are used exclusively for product approval. When it comes to system design, you should not refer to the limit values of the test procedure. Operation with a water flow rate higher than the nominal flow rate is improper use, so that the approval can be withdrawn.