What is the fire pump cavitation?
Cavitation refers to the liquid generating bubbles when the liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at that temperature. The cavitation of the fire pump is generally after the start of the feed pump, the outlet valve has not been opened or the external load is greatly reduced, and the flow of the unit is low or zero. The flow of the feed water is very small or zero. At this time, there is only a small amount of water in the pump or no When the water passes, the friction heat generated by the impeller cannot be taken away by the water supply, so that the temperature inside the pump rises. When the temperature inside the pump exceeds the saturation temperature under the pressure of the pump, the feed water will vaporize and form cavitation.
What are the hazards of fire pump cavitation?
As we all know, when the pump is in operation, a large amount of steam is generated to form bubbles. When a liquid containing a large amount of bubbles passes forward through the high pressure region in the impeller, the high pressure liquid around the bubbles causes the bubbles to sharply shrink and rupture. At the same time that the bubble condenses and ruptures, the liquid particle fills the cavity at a high speed, and at this moment, a strong water hammer is generated, and the metal surface is struck at a high impact frequency, and the impact stress can reach several hundred to several thousand. At atmospheric pressure, the impact frequency can reach tens of thousands of times per second, and in severe cases, the wall thickness will be broken down.
How to properly handle the fire pump cavitation?
First of all, from the perspective of prevention, it is recommended to open the recirculation pipe when the feed water flow rate is reduced to a certain extent, so that a part of the feed water flow is returned to the deaerator, so that there is enough water in the pump to pass the pump. The heat generated by the friction is carried away, and the feed water is vaporized by keeping a certain temperature from rising.
Second, choose a better factory. In general, experienced fire pump manufacturers consider such problems before manufacturing fire pumps. Manufacturers generally specify a minimum allowable flow rate for feed pump operation (eg, 25% to 30% of rated flow). This approach can effectively prevent the water from vaporizing due to too little water. Modern high-speed feed pumps generally adopt variable speed regulation, which is low speed at low flow rate, and cavitation is not easy to occur at low speed, so the minimum flow allowed is much smaller than that of fixed speed feed pumps.