There are several steps involved in designing firewater systems. Not only calculating firewater demand and ensuring proper coverage but also evaluating implications that changes and modifications may cause. A well-designed firewater system not only helps prevent the escalation of fires but also ensures the safety of personnel, property, and the surrounding environment when a fire occurs. In this article, the essentials of designing a firewater system are discussed.
Design considerations
The design process has several important elements which are important to evaluate and quantify during the different project phases.
Risk analysis
Firewater demand
Firewater hydraulic calculations
Firewater supply
Installation and mechanical completion
Commissioning and testing of the firewater system
Relevant standards and guidelines normally used or referred to in connection with the design of firewater systems generally (water spray fixed systems/fire deluge systems, fire sprinkler systems, and fire pumps) among others ISO 13702, National Fire Protection Association (NFPA) and for the Norwegian Continental Shelf (NCS) NORSOK S-001.
Risk analysis
Start the process by evaluating and quantifying the risk sources the different areas are exposed to. There are several useful tools for performing this task, for example, doing hazard identification studies (HAZIDs) or Quantitative Risk Analysis (QRA's). A HAZID focuses on identifying potential hazards and is normally the starting point for all other assessments of inherent risks.
As a result of the risk assessment, it is easier to form the basis for evaluating which fire suppression/extinguishing method to use in each of the considered areas.
For more information related to how to perform a HAZID or QRA, follow this link for more insights on the topics.
Firewater demand
The firewater system shall provide adequate coverage for the relevant fire and explosion scenarios indicated in the risk analysis.
After the design for relevant process equipment in the various areas on an installation is done, the calculation of firewater demand can be done. Requirements and regulations related to firewater density can vary depending on country and industry.
For the Norwegian oil & gas sector, the NORSOK S-001 requirement states:
10 (l/min)m2 for process areas and equipment surfaces
20 (l/min)m2 for wellhead (including riser balconies, manifolds located on FPSO turrets, etc.)
At this stage, it is beneficial to indicate the necessary foam density to ensure that the capacity for foam is included in the planning process ahead, e.g. regarding the size and location of foam storage tanks.
The different area demands are then calculated and summed up for the total asset, both for the general area coverage and for dedicated equipment protection. This gives the minimum amount of fire water supply required in total for the installation.
Selecting and placing nozzles
After the demand in the designated areas has been mapped, the engineer will then select and place the firewater spray nozzles. Each relevant fire zone will have a number of nozzles where the area required water flow will be divided. Based on the manufacturers information from the approval testing, each nozzle needs to meet the lowest required pressure to secure the required water density in the area.
Wind can have a massive impact on the system's performance and this needs to be evaluated when selecting high-velocity (HV) or medium-velocity (MV) nozzles. There may also be local regulations stating which nozzles to use outside and inside.
There is a recommendation in NORSOK on using high-velocity nozzles for area coverage in a fully open process and drilling areas as well as in areas where deluge is used for explosion suppression.
The nozzles shall be placed such that they give sufficient coverage and account for all obstructions and ensure that each nozzle provides overlap horizontally. Item protection would be included at this stage. Here, the safety factors discussed earlier help reduce this uncertainty.
After the firewater nozzles have been placed and the firewater piping is routed, the entire firewater ring main system including all deluge valves shall then be calculated by an approved computer software, for instance Sunrise Pipenet. This ensures that the systems can be optimized with regard to pressure loss and pressure and flow requirements for the nozzles. These calculations would look to reduce the imbalance in each system. The imbalance should not exceed 15% of the required minimum flow. It would therefore be wise to do these calculations as early as possible, to avoid costly relocations of equipment and re-routing of piping.
It is important that if there are any future modifications on the distribution networks, this can affect the imbalance, thus needs to be calculated for each case, with no assumptions!
Firewater Hydraulic Calculations
Based on the demand needed for each area, the deluge distribution system needs to be calculated with a hydraulic modelling program. The aim at this point is to balance all included systems so it gives the optimal system flow and system pressure, thus reducing the needed fire pump size. The required flow and pressure for each deluge valve in the fire protection system are based on the required flow at the hydraulically most remote nozzle (MRN), the number of nozzles, pipe sizes, system pipework configuration, etc. This is based on isometric drawings from the asset's 3D model, to ensure that the hydraulic model matches the piping model. At this stage, there has to be close collaboration between the fire protection engineers and the piping engineers to find the optimal routing and design with regard to weight, cost, and available space.
It is important to place all nozzles before sizing the firewater pumps due to the risk of imbalance in the firewater system. As the water distribution network with piping nozzle heads is not uniformly formed and that often different nozzle types are used, imbalance is expected in the network. The most remote nozzle (MRN) in terms of pressure, will define the rest of the system. All other nozzles will receive water at a higher pressure than the MRN. If the water distribution system has a high imbalance, there is unnecessary high consumption of firewater and sub-optimal utilization of the firewater pumps. Here, it is necessary to evaluate the results of each system to eliminate any errors and possibly improve the design.
As a general guideline, the flow velocity should not exceed 10 m/s upstream the deluge valve. The regulating deluge valve is in most of the systems of an elastomeric type, which would most likely be damaged if it experiences high flow velocity over time.
It is beneficial to do firewater transient calculations at this stage, to highlight if there are any pressure surge problems in the network and also to ensure that the priming time of the distribution systems fulfills the requirements. Normally, deluge valves are given an opening signal at the same time as the fire pumps get a start signal. The valves use a shorter time to open than the fire pumps use to provide pressure and flow to the fire water system. A fire pump can use 20-30 seconds before contributing water and pressure to the distribution systems and the deluge valves start to open after a couple of seconds. The water from the elevated parts of the ring main is being drained in the opening sequence, which can cause high surge pressures when the fire pumps fill those parts of the ring main again. These surge pressures can cause damage to the piping network, nozzles, and/or firewater monitors.
After all the deluge distribution networks are calculated, the next step is to calculate the ring main system for each of the defined scenarios to verify that the fire pumps are able to deliver sufficient flow and sufficient pressure.
Fire water supply system
The size and capacity of firewater pumps are determined based on the specific requirements of the fire protection system and the outcome of the calculations stated above.
The pumps are designed to deliver a minimum flow rate and pressure to the system and are typically controlled by sophisticated controllers that monitor and adjust the flow and pressure based on the needs of the system.
On assets based offshore, the pumps are pumping seawater from below the asset and use it for firefighting purposes. Usually, one or several jockey pumps or the seawater utility system are keeping the pressure in the ring main stable and also keep it water-filled. The jockey pump will also be capable to supply small consumers like a firewater hydrant, to avoid the larger firewater pumps starting too often.
Securing the integrity of the fire protection systems
Installation and mechanical completion
Mechanical completion is the final step in the construction and installation phase and involves verifying that all components of the fire deluge system or fire sprinkler system have been installed and connected correctly. It is essential to verify the design against the physical installation to ensure that the system will function as intended in the event of a fire.
Key areas to verify during the construction and installation phase of the firewater system are the correct location of fire fighting equipment and fire water nozzles and sprinklers, possible obstructions, and shadow spots. Shadow spots are areas where the firewater system cannot effectively reach, which can result in either a fire not being extinguished properly or reduced cooling water of critical structures and supports. These spots can occur due to structural obstacles or equipment that can block the flow of water, e.g. pipes, cable trays, and structural elements.
Furthermore, verification is critical to ensure that the correct piping sizes and nozzles are used. Incorrect piping sizes and wrong nozzle types can lead to insufficient water and pressures, which can cause the firewater system to be ineffective in extinguishing fires. This is of special importance if several different firewater nozzle types are used in the same area, as it can be easily mixed up by the responsible construction operator.
Commissioning and testing
Commissioning and testing of firewater systems are critical steps in ensuring the safety and reliability of industrial facilities. Firewater systems are expected to provide an immediate and adequate supply of water to control and extinguish fires in the event of an emergency. As such, these systems must be designed, installed, and tested in accordance with rigorous standards and guidelines to ensure they are capable of performing their intended function.
The commissioning process involves a series of tests to verify that the firewater system has been installed correctly and is functioning as intended. The deluge systems are tested separately and also, perhaps most importantly, scenario tested with several systems in operation at the same time. This includes testing the firewater pump capacity, time to most remote nozzle, verification of flow and pressure from the fire pumps, water density and clogged nozzles.
In seawater systems, there are risks for some physical contamination, debris, or organisms, which can lead to clogged-up nozzles, thus a compromised distribution network. This is typically solved by installing a strainer/filter, directly downstream the firewater pumps, upstream the deluge skid or in each nozzle.
It is worth mentioning that installing nozzles with an integrated strainer is not recommended due to the work required to clean them regularly. Another measure used to control the risk for blocked or clogged deluge or sprinkler nozzles is to inject chemicals in the ring main.
Further, commissioning and testing also involve verifying that the firefighting system is integrated with other fire protection and life-saving safety systems, such as fire alarms and control systems, to provide a comprehensive fire protection solution.
Future modifications and pump degradation
Adding nozzles & piping in the existing distribution systems may cause imbalance and lead to higher consumption of firewater than what the system was originally designed for. It is therefore essential to verify the hydraulic limitations in the system prior to doing any modification work, to verify that the system is still fulfilling its design intent after the planned modifications. It is important to not assume at this stage but verify through hydraulic calculations.
The firewater pumps experience fatigue and degradation over time, this is also a factor that has an effect on the firewater systems performance. Regular testing and re-calculations should be done to verify the system's performance and operability.
How we can help? If you are interested in knowing more about the services ORS Consulting can provide within the firewater system design, please do not hesitate to be in touch.
