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If the water is taken from a municipal main pipe, Net Positive Suction Head Required (NPSHR) must be considered. This means that at "worst case conditions" (often considered as 150% of the rated pump capacity), there must be sufficient residual pressure at the inlet of the pump for the continued function of the municipal supply. This is especially critical for fire protection because the municipal piping might go on to supply other fire protection systems, even in the same building. As outlined by the NFPA 20 standard, "It should be noted that the use of residual pressures of less than 20 psig (1.4 barg) are not permitted by many state health departments.”
Where the Authority Having Jurisdiction or AHJ has no specific requirements regarding the NPSHR, there still remains a primary concern as to the ability to maintain sufficient residual pressure to prevent the development of negative pressure at any point in the street mains. Negative pressures could result in the collapse of the mains or other water system components, or back-siphonage of polluted water from some other interconnected source.
Avoiding cavitation in low pressure scenarios
Another problem emerging from a low suction pressure is the possibility of cavitation damage to the pump. Cavitation is a phenomenon that occurs when the water pressure falls to vacuum state and as a result, vapor bubbles are created. When the pressure is very low (or even negative) at the inlet suction of a pump, vapor bubbles form and pass into the pump. Under these conditions they do little damage. However, when the vapor bubbles reach the higher pressure created by the pump towards the outlet side of the impeller, they implode. This implosion of vapor bubbles creates very high pressure jets/waves that can rapidly weaken and erode the metallic parts of the pump, pit the metal and eventually drastically reduce pump functionality.
If the possibility of insufficient suction pressure does exist, then the most common and efficient solution is the installation of a pump suction control valve. This valve should be situated at the outlet side of the pump, with a sensing line to the suction or inlet side of the pump. When necessary, or when the suction pressure falls below the allowable or set minimum, the valve will regulate the inlet pressure to stay above a preset minimum.
For more info contact booster pump supplier in uae or call us at +971 4 252 2966
Where the Authority Having Jurisdiction or AHJ has no specific requirements regarding the NPSHR, there still remains a primary concern as to the ability to maintain sufficient residual pressure to prevent the development of negative pressure at any point in the street mains. Negative pressures could result in the collapse of the mains or other water system components, or back-siphonage of polluted water from some other interconnected source.
Avoiding cavitation in low pressure scenarios
Another problem emerging from a low suction pressure is the possibility of cavitation damage to the pump. Cavitation is a phenomenon that occurs when the water pressure falls to vacuum state and as a result, vapor bubbles are created. When the pressure is very low (or even negative) at the inlet suction of a pump, vapor bubbles form and pass into the pump. Under these conditions they do little damage. However, when the vapor bubbles reach the higher pressure created by the pump towards the outlet side of the impeller, they implode. This implosion of vapor bubbles creates very high pressure jets/waves that can rapidly weaken and erode the metallic parts of the pump, pit the metal and eventually drastically reduce pump functionality.
If the possibility of insufficient suction pressure does exist, then the most common and efficient solution is the installation of a pump suction control valve. This valve should be situated at the outlet side of the pump, with a sensing line to the suction or inlet side of the pump. When necessary, or when the suction pressure falls below the allowable or set minimum, the valve will regulate the inlet pressure to stay above a preset minimum.
For more info contact booster pump supplier in uae or call us at +971 4 252 2966
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