Why Pressure Loss Between Street and Backflow?

1/8/2021

We are inspecting an existing fire sprinkler building for an upgrade to an Extra Large Orifice (ELO) system.

We performed a hydrant flow test and noticed a 14 psi static loss between the hydrant, 185-ft of 8-inch ductile, 2 elbows and 8 feet of elevation rise to the supply side of the RPZ backflow preventer. We recorded 73 psi at hydrant and 59 psi at the number 1 test cock on the RPZ while static.

I spoke with the water authority and they confirmed no meter or check valve on the 8" fire line; it is straight into the valve room plus 2 elbows. The building is 20 years old and the pump test provided 120% of rated capacity.

Any idea how the static pressure inside the building is 14 psi less?

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17 Comments

Dan Wilder

1/8/2021 08:27:30 am

Check your gauges for calibration and try to use the same type for both. Using a glycerin filled 100# and an Air/Water 300# will not be equal when comparing them.

Do both gauges move equally with changes in the static pressure (up and down)?

I'm assuming the RPZ is doing it's job and there is an additional loss across the valve? Not that this makes a difference on the static, but just making sure the valve is doing its job.

Mike

1/8/2021 09:06:09 am

Check the gauges.

Andrew K

1/8/2021 09:14:46 am

I can only agree with previous responses. In general, we should consider the the pressure gauges as a possible source of psi discrepancy. I would offer that if you have the opportunity to use one certified pressure gauge for both locations, it may eliminate any discrepancies between different gauges.

Jesse

1/8/2021 09:30:58 am

First of all check the gauges. We've all seen way too many mis-calibrated gauges. If the gauges are good, it's likely the UG pipe is fouled. I ran a basic HW manual calc presuming a flow of 1000-gpm. Your friction loss is in the area 1.38 psi. Your 8-ft rise in elevation accounts for a loss of 3.46 psi for a total loss of 4.84 psi. You've lost 14 psi, so we're at the point where the culprit may be scale buildup on your UG pipe. If this is the case, you'll need to replace the UG.

Jesse

1/8/2021 09:37:41 am

Sorry - I just re-read the original post. Earlier my brain registered the pressure differences while flowing, not static.

This one's weird. Thinking its the gauges.

Casey Milhorn

1/8/2021 09:38:51 am

Completely agree with checking the gauges. Also make sure you are doing them at the same exact time. Sounds like you are in an industrial area and water usage can spike up and down dramatically affecting the static pressure available, especially if the city water main is a dead end line and/or undersized. Other than that, physics is physics.

Franck

1/8/2021 09:56:56 am

For the same elevation, static should be the same everywhere in your system.
If you have a difference, it may come from a pressure gauge reading (as indicated above), a possible change in elevation, or the provision somewhere of a pressure reducing system.
By elimination, you can guess where is the answer...

But first thought is of course to check the calibration of the pressure gauges.

If difference is coming while flowing the system, then it comes to friction losses, as indicated by Jesse. If more difference than expected, then it may come from a smaller pipe size than expected (smaller diameter installed, or smaller inside diameter because of scale build up, MIC, improperly opne valves...

Glenn Berger

1/8/2021 10:13:24 am

After checking the gauges (easy answer), you might have underground piping that has a problem (internal or external). Do you know if there are soil corrosive issues?

Shane

1/8/2021 10:29:52 am

my best guess there is a small leak, due to corrosion, in the underground piping or at a fitting connection after the backflow.

C.T. Carbonari

1/8/2021 10:34:50 am

I always check testing equipment & methodology (i.e. gauges, connections, locations,piping configuration) whenever unexpected hydraulic anomalies occur.

Assuming the hydrant is public and on a dead end line, there is typically a key valve near the property line at point of connection with the city water supply.
I would check to ensure it is fully stroked open.

C.T. Carbonari

1/8/2021 11:06:58 am

Also, this sounds like a Fire Service Main Flow Test (as opposed to a traditional hydrant test)

Knowing how much flow (gpm) you achieved during your test would help in assessing your pressure drops.

@ Q=1500 gpm for 8" DI ; c=100 (existing) w/205 ft equiv. pipe
your pressure drop should be in the 4.5 psi range.

A 14 psi residual drop is an indicator of a restricted orifice.
(Heavy Corrosion, Valve partial open)

Louie Lincoln link

1/8/2021 02:42:32 pm

I doubt that all of the gauges are that far off. With the pump only flowing at 120% if the original was 150%, there is a flow restriction.
The flow loss would account the 30% difference. It would be helpful if you could get your hands on the original pump start report.

Felipe Pedraza

1/8/2021 07:09:22 pm

If the guages have been checked.and are of the same make and kind, proceed to checking all roadway valves near the source and make sure that they are fully open and in working condition. I have found some of them close and even broken in the past. Also check with utilities for any watermain repair near the area, or waterline taps to the main and the cut out line donut went into the main and is either blocking a watermain roadway valve ir is blocking the water at the BF.

Tony Brown

1/9/2021 08:22:30 am

I have come across this situation three times. Once was due to a throttled down curb valve, the other two was because a neighbor had tapped into my clients incoming service.
My gages are always checked before leaving the shop and I have them checked and calibrated on a regular basis, even brand new ones so if you are confident that your gages are okay then check all valves between the hydrant and the backflow. 14 psi loss on 185 + of 8" ductile is a huge loss so I would check for a mechanical cause, i.e., valves, broken pipe,

FRED WALKER

1/11/2021 10:08:11 am

1. Check gauges, ensure calibration, easy first check swap gauges.
2. If gauges check and you have 0 flow through the backflow preventer; then you have a leak, an undocumented tap, or some other condition creating flow between the hydrant and the backflow preventer.

If the hydrant is located on a distribution main and there is a Tee-off to the backflow preventer you may need to take static at hydrants on either side of the Tee as well as the backflow preventer to account for flow in the distribution main.

C.T. Carbonari

1/14/2021 10:58:07 pm

I hope the OP responds with clarification when this issue is resolved.
Knowing the actual field solutions to these more empirical types of questions benefits all of us!

Jesse

1/15/2021 07:40:17 am

Agree with C.T. Would be beneficial to all of us if the OP circled back with the resolution. Differential diagnosis are great, and this created some great discourse. We're all better at what we do because of it.

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