A new firewater (FW) system is being designed for a Green Hydrogen Elements facility. The plant spans a large area of approx. 2 x 1.5 km and contains multiple critical areas. The piping material is Fiber Reinforced Plastic (FRP). The main firewater ring is of DN250 and DN300 piping. The piping material is supplied by Amiantit. The jetty lines are the only parts that are made of carbon steel.
The firewater system is operated by two jockey pumps and two main fire pumps. Only one jockey pump and fire pump are active at a time. The firewater pumps are supplied by water from a storage tank. All piping is underground and is made of fiberglass, except for the upper part of the (carbon steel) lines to the fire hydrants. Most of the underground piping is buried in the ground and fully covered with soil, with the exception of some parts that are installed in concrete sleeves. The client has requested Dynaflow Research Group to perform a surge analysis on the entire system to calculate minimum and maximum pressures and unbalanced loads. Should the calculated results surpass the maximum allowable values, mitigation measures will be determined during the project as well.
Model Setup
The system is modeled in ISOtracer 2.5 and analyzed with BOSfluids 7.2. The system is equipped with multiple extinguishing systems, ranging from fire hydrants, monitors, hoses, and sprinkler systems in buildings. All these elements operate at a different flow demand and pressure. The client provided the fire impairment plan which contained the K-values for each one of them. The exact details of the various buildings were unavailable, so to account for their flow demand these sprinkler systems have been reduced to a single deluge nozzle with an equivalent flow rate.
Firewater Network Layout
Steady-state and transient scenarios
Because of the size of the plant, four different fire scenarios have been thought out. It includes a fire in one of the two compressor buildings as well as a fire in unit-90 or at the jetty. These are then combined with one of the applicable transient surge scenarios involving a pump trip, pump switchover, pump start, or valve closure.
Among all the combinations considered, the highest transient pressure observed is 18.4 barg, which is significantly below the allowable limit of 22.4 barg for GRE piping as specified by the ISO-14692-3 (2017) code. Similarly, the maximum pressure in the steel piping is 15.8 barg, remaining within its defined limit of 16 barg.
The analysis showed that the most critical scenarios are:
- A fire in unit 90 during pump trip and switchover: as soon as the pump trips or switches off, the pressure slowly decreases. A new pressure peak is generated when the stand-by pump is switched on. During this event, the maximum pressure is 13.8 barg.
- A fire in the MP compressor building during pump trip and switchover: because of the much higher flow capacity of the spray system at the MP compressor building, the pressure will drop much faster than in the previous case. In unit-90 the pressure even falls below atmospheric pressure at -0.55 barg. Fully buried piping can be taken to be vacuum resistant due to soil support, but this does not hold for the pipe sections that run through a concrete sleeve. These sections do run the risk of buckling failure because of the low internal pressure. It is therefore recommended to increase the wall thickness for these pipe sections to make them vacuum-resistant.
- A fire in the MP compressor building during pump startup: the firewater pumps start when the pressure at the pumps falls below 10.5 barg. This is a result of the firewater clients being opened. Upon startup, a pressure wave is propagated through the firewater network.
- System close-off: Closing the water clients causes pressure waves to propagate through the firewater system downstream of the closed valve. This typical water hammer phenomenon scales with the closure time of the valve. In the current analysis it was found that a closing time faster than 5 seconds leads to pressure peaks that exceed the maximum pressure allowable of the steel piping.
Conclusions And Recommendations
The following conclusions and recommendations can be made as a result of the conducted surge analysis:
- The most critical surge event with respect to high pressure is valve closure. For this case, the highest surge pressure of 18.4 barg is observed. This is below the allowable pressure of 22.4 barg for the GRE.
- The highest surge pressure that is observed in the steel piping is 15.8 barg, which is below the allowable 16 barg to protect connected equipment.
- The pump trip case is most critical in terms of low pressure and cavitation. This does not pose a threat to the buried piping, but it can cause buckling failure in the pipe sections within a concrete sleeve. Therefore, it is recommended that the thickness of these pipe sections be increased.
- Closing the valves in 5 seconds or more is recommended to prevent any excessive surge pressure peaks.
