Well, I'm don't fundamentally disagree, but there is definitely such a thing as too much overflow. From NFPA 20, 2007 ed:
A.5.8 The performance of the pump when applied at capacities over 140 percent of rated capacity can be adversely affected by the suction conditions. Application of the pump at capacities less than 90 percent of the rated capacity is not recommended. The selection and application of the fire pump should not be confused with pump operating conditions. With proper suction conditions, the pump can operate at any point on its characteristic curve from shutoff to 150 percent of its rated capacity. If the pump is rated 400 GPM and the demand is 222, that only represents 55% - well below the recommended 90% threshold, which would be 360 GPM. Steve Leyton Protection Design & Consulting San Diego, CA -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Chris Cahill Sent: Thursday, November 06, 2008 12:59 PM To: [email protected] Subject: RE: Fire Pump Suction If I understand your concern you are worried the tank will run out before 30 minutes because of overflow due to a supply calc vs. a demand calc. You are worried about calc'ed world vs. real world. Fine but in what real world will the actual design area operate. Odds are you won't be in the remote area which increases overflow meaning less than 33 minutes. Odd are less than the calc'ed number of heads will flow decreasing the demand meaning more than 33 minutes. Odds are 2 heads (or less) will flow and you won't have anywhere near 222 gpm coming out. This probability is the driving force showing in reality the tank will last more than 33 minutes. Overflow is good from a fire suppression standpoint anyway. More water per time equals higher probability of suppression and quicker time. It's also not linear, you get more bang for the buck with increase in density. I don't know of specific figures, just going from experience. If you put the fire out in 10 minutes and the tank runs dry in 20 is there a problem? IMHO the system failed if the full remote area operates in the first place. Or at least a serious investigation is warranted to determine why. If you still need the sprinklers after about 10-15 minutes there is a problem. The extra heads and 30 minutes already have a deal of safety in them. The actual produced is always a balance between flow and available pressure. So yes if the entire remote area opens you will in reality drain the tank faster than 33 minutes. Could be much quicker depending on the shape of the pump curve and the overflow. But assuming you are light hazard based on the very little information provided I don't see a problem. If you are really curios start running system supply calc's for different areas and you should see the tank will never last 33 minutes with a full sized calc'ed area and much longer when less than the full area. Now maybe you are 45 minutes from the closest FD and this becomes an engineering question to consider. Some day the code may differentiate between available levels of fire service but not yet. But hey maybe I missed the point of your question. Chris Cahill, P.E. Fire Protection Engineer Sentry Fire Protection, Inc. 763-658-4483 763-658-4921 fax Email: [EMAIL PROTECTED] Mail: P.O. Box 69 Waverly, MN 55390 Location: 4439 Hwy 12 SW Waverly, MN 55390 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of A.P.Silva Sent: Thursday, November 06, 2008 2:11 PM To: [email protected] Subject: RE: Fire Pump Suction The maximum sprinkler demand as per hydraulic calcs. is 222 gpm. The water supply is by means of storage tanks having a total volume of 7448 gallons. Just barely enough for approx. 33 minimute at maximum demand. The contractor has installed a 400 gpm fire pump. NFPA 20 requires gauge pressure at the fire pump suction flange to be 0 psi or higher when pump is operating at 150% of rated flow, which in this case is 600 gpm. Is this realistic? I know, if the suction pressure is 0 psi or higher for 600 gpm flow, it will work for all cases. However, I would expect a flow between the supply and demand with all design sprinklers flowing. Anyone with experience with pumps, care to comment? Tony -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Joe Hankins Sent: November 4, 2008 8:56 AM To: [email protected] Subject: Re: Fire Pump Suction Boiling (cavitation) starts starts when the water pressure at the impeller is reduced to the vapor pressure of water at the water temperature. You see this addressed in the pump literature as "NPSH" The term net positive suction pressure (NPSH) is defined as the difference between the suction pressure and the vapor pressure of water and must be a positive number to avoid cavitation. At 68 degrees F water temperature, the vapor pressure is 0.35 psi. So, as long as the inlet pressure is greater than 0.35 psi absolute or -14.3 gauge, no cavitation. In practical terms, so long as the suction pressure is above -12 to -13 psi, you should be fine. When a typical centrifugal fire pump starts to cavitate, you'll know it. (sounds like the pump is full of rocks), and you're not going to damage the pump unless you let it cavitate for an extended period of time. The problem, of course, is that the pump performance deteriorates significantly. 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