We would not pick a 750 gpm pump if the demand indicated a required pump capacity of 1100 gpm. It would not be uncommon to select a 1000 gpm pump for your 1100 gpm demand. That way you still have the additional capacity of up to 1500 gpm.
When you calculate a system at for example .30 gpm per square foot over a 3,000 sq. ft hydraulically remote area, the numbers come out to a demand of .30 x 3000 or 900 gpm. In reality the flow from sprinklers in that 3,000 sq. ft remote area will be greater than that. So for example I would take that 900 gpm add my hose stream allowance of 500 gpm and come up with a total "theoretical" demand of 1400 gpm. If I pick a 1500 gpm pump then I have the ability to provide up to 2250 gpm. Many designers will take that "theoretical" demand and add a safety factor of 10%-15% to that. If we did that we would be more realistically looking at 1400 gpm + 10% -15% = 1540-1610 gpm of flow within that 3,000 sq. ft design area. With the 1500 gpm pump selection we are still in a an acceptable range of below that 150% of 2250 gpm. It's just not good to make a selection that tight. Things change including the condition of underground piping, building layouts, building uses and such. It's best to allow some flexibility when selecting the pump so that a small change to one of these doesn't force an owner to have to buy a new pump. But I'm sure this is one of several theories and methods and ideas when it comes to selecting pumps. DISCLAIMER: This is not intended to be a complete and detailed analysis or approach to pump selection. There are other factors involved which the fire protection technician/designer needs to consider. Craig L. Prahl, CET Fire Protection Group Mechanical Department CH2MHILL Lockwood Greene 1500 International Drive PO Box 491, Spartanburg, SC 29304-0491 Direct - 864.599.4102 Fax - 864.599.8439 [EMAIL PROTECTED] http://www.lg.com -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Reza Esmaeili Sent: Wednesday, October 31, 2007 9:43 AM To: [email protected] Subject: Fire pump question/ Why 150% at not less than 65% ? Hi! I know that: 1- As per NFPA-20 listed fire pumps shall furnish not less than 150 percent of rated capacity at a total head of not less than 65 percent of the total rated head to provide some safety factor in case of greater than expected demand at the time of fire. 2- On the other hand, NFPA allows to choose a fire pump working at 150% of rated capacity if it delivers the required water flow at required pressure to fire protection system. I think these two issues are against each other, because if I choose a pump which is working at 150% of rated capacity, then I have already overloaded the pump, so in case of greater than REQUIRED flow, it might not be able to supply this extra demand at not less than 65 percent of the total REQUIRED head. for Example if he required demand is 1100GPM at 100Psi, I can do as followings: * 1100gpm/1.5 = 733Gpm, so I will choose a 750GPM Pump. * 100psi/0.65 = 154Psi, so I will choose a 154psi Pump. I know it is not usuall to choose a pump working at 150% of rated capacity, and 135% is more common, this is just a scenario to ask my question. So the pump is rated at 750Gpm, 154psi and will work at 150% of rated capacity to supply my required demand of 1100Gpm at 100Psi. My question is that If in case of greater than expected demand at the time of fire the demand increases to 150% of required flow (1100 * 150% = 1650Gpm), then can I be sure than the total head is not less than 65 percent of the total REQUIRED head? Should I check it from the pump curve? Thanks for your attention, Reza __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum To Unsubscribe, send an email to:[EMAIL PROTECTED] (Put the word unsubscribe in the subject field) _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum To Unsubscribe, send an email to:[EMAIL PROTECTED] (Put the word unsubscribe in the subject field)
