Basic design methodology for using fixed monitors nozzles as the primary fixed protection for a process unit has been use for over 60 years. The overall basic approach is to install enough fix monitors around a process unit to allow coverage from two monitors for each piece of equipment (i.e., vessels, heat exchangers, pumps, towers, etc.) in the unit.
Because the fire protection water supply piping loops around the unit the two monitor protecting an item could normally be feed water from the different sides of the unit. This would allow at least one monitor to reach the equipment even if a section of the supply piping was damaged or shut down for repair.
The most common failures involving the placement of fixed monitor nozzles is when the two basic implied conditions in this standard are overlooked, i.e., each monitor nozzle must have a clear path to the protected object and that the monitor is placed close enough for the discharging water to reach the object with enough authority to protect it. These two errors are becoming more frequent as newer units are getting much larger in capacity and more congested.
Originally the fixed monitors used small 250 or 500 gallon per minute nozzles that had limited reach. With nozzle advancements such as combination Straight Stream/Fog nozzles:
much larger flow rates of as much as 5,000 gpm increase the effective reach,
the greatest change that has happen in the last 20 years is a nozzle that has the ability to mix foam into the discharging water stream without a complicated proportioning devise.
These two changes have empowered the unit responder on duty to not only cool much more flame impinged equipment because of the higher gpm but to also limit the spread of a pool fire by actually extinguishing some or all of it using firefighting foam before the fire brigade arrives with mobile foam apparatus.
With the history of previous losses, and the fact that process equipment is larger, thus much more expensive, the use of these fixed monitors has been further improved to provide more specific protection for the higher loss frequency objects like process pump rows.
Today, the enhanced fix monitor protection of process pumps often includes specially located monitors at the end of the pump row, some of which are placed under the pipe rack if necessary. These specially located monitors usually have higher flow rates, from 1,250 to 2,000 gpm with a foam educting nozzle fed from a foam. The water supply piping can be in the pipe rack. If necessary the entire monitor may be bolted together so it can be removed if needed during a turnaround.
The priority in installing these high flow foam fixed monitors is based on the hazard presented by the pump: 1. Hot pumps located within 15 feet of a tower, 2. Other hot pumps operating above the ignition temperature of the product, 3. Any pump within 15 feet of a tower, 4. Pumps located under pipe racks with cable trays, major pipe racks or vessels.
Fixed water spray systems above pumps are designed for cooling the pump from a small seal fire on the pump or a small fire on the adjacent pump. They do not have a sufficient volume of water to cool a deep seated pump fire.
Other areas that need these high flow/foam fixed monitors are elevated vessels with pumps under them, large compressors and pipe racks that have critical DCS cabling.
Currently the older smaller fixed monitors and additional new fixed monitors located around process units that have flammable and combustible liquids are being changed to a minimum of 1,000 to 1,500 gpm foam educting nozzles with foam totes at each monitor. This change will allow the first responder from the unit to dramatically change the first few minutes of a fire for the better.