Regulators for fire protection systems
12 March 2013
A fire protection system is one of the most critical components of an offshore facility. The system includes sprinklers, hoses and pumps. Pressure regulators are used to control the water pressure, helping to maximise safety, whilst minimising the number of devices required in the loop. These devices are self-actuated, which means they do not require an external power supply. Should a power supply fail, this can put the fire protection system into default mode.
The pumps run 24 hours a days, 7 days a week. They must remain running, because it takes them too long to restart and get the system up to full capacity. Flow is only needed in the main line in the eventuality of a fire. However, the pumps must continually discharge, because the main line must be kept under pressure.
Figure 1 shows pressure regulators installed on the pump discharge line of a fire protection system. The pump intake can either be connected to the public underground water supply or to a static water source such as a tank, reservoir or the sea. Depending on the design of the system, fire pumps or jockey pumps are used. A fire pump provides water at high pressure to the fire hydrants, sprinkler system risers and hose standpipes. A jockey pump is a smaller device that is designed to maintain pressure in the fire protection piping system so that the larger fire pump does not need to be run continuously. The fire pump is usually connected to an alarm and will raise an alert whenever it starts.
Pressure reducing regulators are used to keep the downstream line at a constant pressure. A relief or backpressure regulator allows excess flow to be by-passed back to the pump inlet or to a tank or drain, and prevents the pump from exceeding the set pressure of the system.
There are a number of challenges for this type of regulator. Firstly, to improve pump efficiency, they must provide less than 5% accuracy to maintain the pressure as close as possible to the head-loss of the pump. Using a pilot-operated back pressure regulator, such as the Fisher® 63EG-98HM from Emerson Process Management, helps to reduce the cost of operation and maintenance by protecting the system from pressure variation and bumping.
The second challenge is to use the correct materials of construction to cope with both corrosion and cavitation. The most commonly used materials for sea water applications are Copper-Nickel Alloys, Monel®, Hastelloy® C and Titanium. Within Emerson’s range of regulators, the Fisher 63EG-98HM Aluminium-Bronze and Monel regulators are recommended for use with sea water. These are available in sizes DN 15 to DN 300x150, for flow rates up to 2000 Nm3/h sea water. This regulator is equipped with a special cage to handle cavitation. If the effect of cavitation is too great, an anti-cavitation plate can be provided and attached downstream of the back pressure.
Emerson recently launched the Fisher LR and MR series of industrial regulators in order to provide end users with a complete range of back pressure and pressure reducing options. These are suitable for applications requiring excellent levels of accuracy even at high flow rates. www.FisherRegulators.com
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