Hydraulic Demand Exceeds the Flow Test - Four-Story Office Retrofit
An 8,000 sq ft floor plate in a four-story office building (48,000 sq ft total) is being fitted with a new wet-pipe system. The space is light hazard: ordinary furniture, no storage over 6 ft, and the owner has confirmed in writing that the floor will not be used for storage. Standard-spray pendants, K-5.6, are laid out on a 12 ft x 12 ft grid under an unobstructed ceiling, with deflectors 9 ft above the floor. The design uses the minimum 1,500 sq ft remote area at 0.10 gpm/sq ft plus a 100 gpm hose allowance, and the completed calculation shows 68 psi required at the base of the riser at 250 gpm. The public water flow test at the hydrant nearest the service shows 55 psi static and 35 psi residual at 1,000 gpm. The backflow preventer is listed at 12 psi loss at design flow, and the elevation difference from the riser to the top of the remote area is 40 ft. The reviewing engineer has rejected the submittal as deficient.
1. Using the flow test data, verify whether the calculated demand can be met by the public supply. Show the arithmetic and state the conclusion.
Read the worked answer
Convert the flow test to a supply curve using the standard relation P = Pstatic - (Pstatic - Presidual)(Q/Qtest)^2, which holds because friction loss varies with roughly the square of flow over the range of a test.
- Pstatic = 55 psi, Presidual = 35 psi at Qtest = 1,000 gpm.
- At Q = 250 gpm: (250/1,000) = 0.25; 0.25^2 = 0.0625.
- Pressure drop = (55 - 35) x 0.0625 = 1.25 psi.
- Available pressure at 250 gpm = 55 - 1.25 = 53.75 psi.
Demand is 68 psi at 250 gpm. Shortfall = 68 - 53.75 = 14.25 psi. The system cannot be supplied by the public main as currently designed, so the rejection is correct.
Sanity checks worth stating in the response: the 68 psi is at the base of the riser, not at the top of the remote area, so the 40 ft of elevation (40 x 0.433 = 17.3 psi) and the 12 psi backflow loss must already be inside that number. If they are not, the demand is even higher. Also confirm the test was taken at the same elevation as the riser and that no other hydrant was flowing; a test taken at a lower elevation would make the shortfall larger, not smaller.
2. The contractor proposes switching to quick-response sprinklers so the remote area can be reduced 25 percent, claiming this will solve the problem. Evaluate that claim quantitatively.
Read the worked answer
A 25 percent remote-area reduction is only available where the code's conditions are all met (light hazard, wet system, listed quick-response sprinklers, and the applicable ceiling-height/slope limits), so eligibility must be confirmed first. Assuming eligibility:
- New remote area = 1,500 x 0.75 = 1,125 sq ft.
- Sprinkler flow = 1,125 x 0.10 = 112.5 gpm.
- Hose allowance is unchanged at 100 gpm (it is set by hazard, not by area).
- New total = 212.5 gpm, versus 250 gpm before, a flow ratio of 0.85.
Now separate the demand into components that scale with flow and those that do not. Elevation (about 17 psi), backflow (12 psi), and sprinkler operating pressure (about 7 psi at K-5.6 for 150 gpm over 15 sprinklers, roughly 7 to 10 psi depending on the layout) do not change. Only pipe friction scales, and it scales as roughly Q^1.85: 0.85^1.85 = 0.735.
If friction in the sprinkler piping is, say, 25 psi of the 68 psi total, the saving is 25 x (1 - 0.735) = about 6.6 psi. Demand falls to roughly 61 psi against 53.75 psi available - still about 7 psi short.
The claim fails. The remote-area reduction is a legitimate efficiency measure, but it does not deliver the 14 psi needed here, and a submittal that relies on it without rerunning the full calculation should not be approved.
3. List the compliant options for closing the gap, and state which you would recommend and what documentation you would require.
Read the worked answer
Realistic options, in order of cost and schedule impact:
- Recalculation audit. Verify the C-factors (120 for new black steel, higher for cement-lined), the actual internal diameters (Schedule 40 versus thin-wall changes friction), fitting equivalent lengths, and that the remote area is drawn on the most hydraulically distant sprinklers. Loose takeoffs routinely overstate demand by 5 to 10 psi and could close most of the gap at no material cost.
- Upsize the undersized piping between the riser and the remote area if the audit shows a restrictively small run. This is often cheaper than a pump in a retrofit.
- Fire pump. A 250 gpm rated booster sized to add about 20 psi is the reliable fix. Check the churn condition: NFPA 20 permits shutoff pressure up to 140 percent of rated, so 53.75 plus roughly 28 psi puts about 82 psi on the system - still well under the 175 psi working pressure of standard components, but confirm it.
- Supply-side improvement such as a larger service or a new tap, only if the purveyor allows it and the schedule permits.
Recommendation: audit first, then a pump if the shortfall survives the audit.
Documentation: the stamped flow test with date, time, elevation and hydrant used; the revised hydraulic calculation showing node-by-node pressure; the pump curve with the design point plotted; a backflow device loss curve rather than a single number; and written AHJ acceptance of any safety margin the jurisdiction imposes.