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Fire Water Flow Detection

offshore-fire-alarm

Application Overview

Offshore oil platforms, as well as many ships and barges, commonly use automatic deluge type fire suppression systems. Seawater is usually the preferred suppressant for large systems. The use of seawater poses significant challenges for the header and branch flow detecting devices. Conventional “flow switches” cannot meet the application requirements for a variety of reasons.

Conventional Electronic Flow Switches/Meters

There are many electronic instrument techniques for measuring liquid flow rates. Electronic flow devices are complicated when compared to a simple mechanical target type flow switch. The level of long-term reliability of a mechanical switch is significantly greater than an electronic flow device. The following points illustrate why this is true:

  • Electronic flow switches are inferential devices. That is, they measure something that happens when flow occurs. Examples include thermal loss, changes in Doppler frequencies, etc. Inferential type electronic flow switches have many more parts, are inherently more complicated, and therefore less reliable than a simple target type mechanical flow switch with very few parts. A target type switch is directly actuated by seawater flow pushing on the target; is simple but reliable.
  • An electronic flow switch can fail in hundreds of different ways. A properly designed mechanical switch can only fail in few ways, all of which are predictable.

  • The multiple transistors in an electronic microchip commonly age and fail in time. Such a failure might leave the output relay in either the “normal” or the “alarm” condition. Unfortunately, this unknown condition makes it nearly impossible to predict or force a failure position of the output relay contacts. They might fail in the “normal” or, just as easily, fail in the “alarm” condition. An electronic unit must be tested every few seconds to verify that it is operating properly and is safely monitoring the firewater flow rate. Unfortunately, the “test” device must also be electronic. This requires that something be done to “test” the “test device”. The only practical alternative solution (insurable risk) is to install three separate flow devices and configured in a “voting mode”. The assumption is made that the output answer that two out of the three provide is correct. This solution is complicated and expensive. The three devices and their monitor also require considerable maintenance and frequent calibrations.

  • Electronic devices are sensitive to lightning strikes and voltage surges, which can cause failures in a multitude of ways. They can also fail to operate when the supply voltage is low.

  • Electronic flow devices cannot monitor firewater flow when facility supply power is off. Generally, they are set up for “alarm” mode on loss of power. Most platform facilities do not require additional false alarms in the event of power interruption. Also, the possibility of fire must be continuously monitored, especially during the event of power loss.

  • Electronic flow switch circuitry tends to be susceptible to corrosion failure in wet and salty environments found on offshore facilities

  • There is no known history of satisfactory service being obtained through the use of electronic flow devices to monitor offshore firewater flow.  

Conventional Inserted or In-Line Mechanical Type Target flow Switches

Conventional target type detectors rely on an obstruction (“target”) placed in the pipeline. The obstruction produces a force due to the resulting pressure drop when the flow rate starts. This force is used to actuate an output switch. The size of this obstruction must be large in firewater systems so that the low flow volume, due a single nozzle, can be reliably detected in 2, 3, 4, or 6 inch size firewater flow piping. The seawater flow increases substantially as more nozzles begin to operate. The resulting high flow volume results in an unacceptably high-pressure drop across the obstructing “target”. This causes either too little seawater to flow or the “target” to be torn off and carried down the line, possibly plugging off flow to some nozzles. Other reasons that common target type flow switches fail include:

  • A rod or arm, extending upwards into an annular cavity, holds the “target” in place. This cavity recess collects deposits over time and can prevent the “target” from moving and detector from operating.

  • Many designs use thin metal bellows or corrugated diaphragms as a pressure containing seal between the electrical switch housing and the flow line containing the target. Corrosion of these thin members is commonly the cause of early failure on these devices.

  • Some designs use “O” ring seals and sliding parts to operate the output switch. Deposits and surface corrosion work together to prevent operation of the switch contacts.

  • High seawater flow rates occur during periodic testing of the fire system as well as during fires. “Targets” which remain in the flow path are essentially bluff bodies and generate very strong Von Karman vortices during high flow velocity events. The load due to the high flow rate, added to the twisting action caused by the vortices, can easily break the target/arm assembly off and send it downstream. Most damaged conventional designs then fail to alarm because they revert to the “safe” position when the target/arm detaches. Their switch contacts deactivate, usually resulting in a resetting of the alarm system to the normal “safe” position. This causes the central alarm system to potentially fail to respond to the fire.

Delta Controls Model 626 Non-obstructing Low Flow Rate Detector

The Delta Controls Model 626 flow detector specifically engineered for the detection of  flow in seawater deluge fire protection systems. The Model 626 incorporates all of the attributes essential to correctly detecting flow in such seawater systems. The design and construction techniques used were developed to suit the needs of this demanding and critical service.

  • A target engineered for the inside dimensions of the pipeline hangs down into the pipeline bore. The relatively low flow volumes due to a single discharging nozzle are enough to cause the target to actuate the output alarm switch and then rotate on its hinge.

  • The target is hinged at the top which allows rotation upwards until  flush against the inside pipe wall. The pipe bore is left fully open and unrestricted until the seawater flow stops. Gravity then pulls the target back into the down position and resets the output switch contacts to the normal position. The Model 626 again quiescently monitors the seawater flow rate; awaiting the next event.

  • High flow velocities do not damage the Model 626. Only the thin edge of the target is exposed to the seawater flow. The hinge pin and target easily withstand the force caused by the designed high volume flow rate.

  • The flow detector is constructed of non-corroding materials such as Monel 400, Alloy 20, and Hastelloy C.

  • The cupped target is fitted and does not rub on the inside body or pipeline. The offset top mounted target hinge cannot prevent the target from rotating. The torque available from full line pressure (foot-pounds) is available to force the target to rotate. Buildup, deposits, and other contaminants will be pushed out of the way and the switch contact will change to the “alarm” position.

  • Most of the Model 626 exposed exterior surfaces are made from the same materials as the internal parts and are suitable for a wet and salty environment. The output switch housing is constructed of stainless steel. This allows the exterior to easily withstand saltwater spray and waves. Gold switch contacts and plated terminal blocks are available for added protection when needed for extreme conditions.

  • The force of gravity and force stored in the field of a permanent magnet provides a “set point” force. The force due to the flow of seawater provides all the power needed to operate the flow detector switching contacts. There is also a load spring inside the switch housing which pushes the output switch. It acts as a bias force, ensuring that the output switch contacts revert to a known position.

  • Testing is easy and reliable. A simple testing procedure is opening a downstream discharge flow orifice equivalent to one nozzle (or other selected flow volume) and witnessing operation of the Model 626 flow detector. Maximum available system flow can be tested without damage to the flow detector or its target.

  • 25+ years of reliable field service have verified the quality of design, attention to detail, and fabrication techniques used on the Model 626.

fire-water-flow-detection-piping-app-page-schematic

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