Hello!
This is my first time posting, I’ll try to keep it short.
We have a project with a new customer where the water authority is requiring a
backflow preventer on the suction side of an existing fire pump. We want to
confirm the proper approach due to potential hydraulic concerns.
* Building height: 130 ft
* System type: Wet pipe sprinkler and standpipe
* Stair Tower 1: Class III combined automatic standpipe
* Stair Tower 2: Automatic Class III standpipe
* Standpipe flow requirements - 65 psi at 750 gpm (System installed 1975)
* Fire pump: 70 psi at 750 GPM, city main supply
* Hydrant flow test not performed, assuming static pressure at street
around 75 psi.
* No standpipe flow information. I recommended a standpipe flow test to be
performed for documentation purposes.
Existing Suction Piping
* 4” × 6” concentric reducer at pump suction flange, recommended
replacement with 4” × 6” eccentric reducer
* 1975 water meter to be removed during backflow installation. Couldn’t
find data sheets on this water meter to determine friction loss at system
demand.
Pump Test Data
Flow
Suction Pressure
Discharge Pressure
Churn
75 psi
155 psi
100% (750 GPM)
45 psi
110 psi
150% (1125 GPM)
5 psi
50 psi
Sprinkler system demand: 268 GPM at 68 psi
* Light Hazard
* Apartments
Concern
* At 150% flow, suction is already 5 psi without a backflow preventer.
* Adding a backflow preventer with additional fittings will introduce
friction loss and will pull city main below zero during high demand.
* Preliminary calculations indicate the standpipe may not achieve 65 psi
residual at 750 GPM; (System installed 1975), verification pending a standpipe
flow test.
Questions
1. If testing confirms the standpipe cannot achieve 65 psi at 750 GPM, what
is the recommended course of action?
2. How should we address the requirement for a backflow preventer when pump
suction is already low at 150% flow?
3. Are there alternative design approaches to meet the water authority’s
requirement while maintaining proper pump operation?
4. Water authority wants the backflow detector assembly on the suction side
of the pump.
Thank you for any guidance.
Thank You,
Brett Barrall
Fire Sprinkler Designer
[email protected]<mailto:[email protected]>
[image004.jpg@01D445D6]
50 S. Museum Road, Reading, PA 19607<https://goo.gl/maps/A3D5Y6oTJUT2>
Office: 610-775-1200 ● Toll Free: 1-800-344-4012
www.bsgi.com<https://www.berkshiresgi.com/>
Satellite Offices in York & State College
● PA State Contract & Costars Participating Vendor ●
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