Introduction
A deluge fire fighting system is installed on three monopiles in the North Sea. For these systems, a water hammer calculation was performed.
The system consisted of a two-ring fire water system with four sprinkler lines connected to the second, lowest, ring. For future scenarios, four additional sprinkler lines might be connected to the first (highest) ring. This is taken into consideration in the system.
Analysis
Two types of analysis were performed:
- A slug analysis. During the opening of the deluge valve, firewater was rushing into the empty system. The waterfront would run like a slug through the system. When the waterfront hit an elbow or tee, the momentum change would generate an unbalanced reaction force. The slug load was largest at the first few elbows. The friction build-up during the filling of the system would reduce the velocity and thereby the slug loads.
- A surge analysis, as the result of a pump trip due to a mechanical failure.
“At what time do we open the valve?”
Results
The maximum unbalanced forces found as a result of the pump trip were much higher compared to the results found in the slug analysis. As predicted, the slug effect and the slug velocity were substantially reduced after the slug had passed three elbows. The load from the momentum flux was mostly dependent on the velocity of the water. The slug gradually diminishes in velocity over time and the distance traveled in the piping.
From the results, it could be concluded that the maximum load for an instant opening deluge valve is 2.6 kN at the first bend and diminishes at each bend thereafter. Slower opening times of the deluge valve resulted in a slower slug velocity and smaller loads. It was recommended to open the valve in the first 4 seconds to 5% and in the next 4 seconds to 100%.
