Hydrant Flow Test 3

[Rookie2]

Hi,
We are designing a distribution system for a subdivision. We have had two hydrant flow tests done at this point and can't provide calculations that show that there is sufficient pressure for fire flow. There are 3 pump stations that pressurize the system we are tying in to. There was only 1 pump running at one station at the time of our last flow test. We feel that there is plenty of water pressure there (and please correct me if I'm wrong) due to the fact that the adjoining subdivision has water meters blowing off of the yokes due to the high pressure. We have spoken to the municipality about this and their response was to double check the calculations, which we have of course done several times.

How can we come up with a more accurate flow test? The line we are tying in to is a dead end 12" line with 2 hydrants upstream of where we are tying in. What result could be expected if we opened both hydrants and tested the hydrant on the subdivision frontage? Could that possibly cause additional pumps to kick on?


Thanks

 

[RWF7437]

http://www.firehydrant.org/info/ftest2.html

 

[BigInch]

Having high pressure in the area is quite possible without having sufficient flow. When flow is low or nil, pumps are near shutoff head, so the pressure can be quite high then. Which is exactly why you do a hydrant test, to prove there is sufficient pressure, but more importantly ... flow at the same time.

http://virtualpipeline.spaces.msn.com

 

[CivEnvEng98]

Find a way to loop your system. I don't mean back to the 12" dead end, but where the 12" branched off of the main system.

 

[RWF7437]

Rookie2,

You don't mention tanks or reservoirs in your post. Are there any ?

You don't describe elevations of anything. What are the elevations of the pumps, hydrants, your project, the services which are being blown off their yokes, tanks ( if any), etc.

What fire flow do you need ? 500 gpm, 3000 gpm ?

What were the hydrant test results ?

Do you have EPANet or any similar modeling software ?

 

[Rookie2]

The required fire flow is 1000 gpm @ 20 psi. The test results were: 920 gpm with residual pressure=56 psi. The static pressure was 150 psi. The most hydraulically remote hydrant that we are modeling is 77' higher than the existing hydrant. The meters are roughly at the same elevation of the proposed hydrant (+/-10). We are modeling in Watercad and modeling the connection point as a reservoir and pump.

 

[Rookie2]

Thanks for the responses. By the way, what triggers the pumps to come on at the pump stations? Is it timed based on typical high demand hours, or triggered by high demand, or low pressure in the system?

 

[cvg]

it sounds like the distribution system is a bit sub-par. you are drawing down the system pressure by nearly 100 psi with only 920 gpm of flow at your test hydrant. a looped system onsite will help reduce head loss to your remote hydrants. But it seems you will be borderline unless the city upgrades the system.

 

[Rookie2]

Yes, according to my calculations, it doesn't work at all just taking the static head that it needs to overcome into consideration. Nevermind the friction loss. We do have plans to install an offsite line that will loop the 12" line, however this is planned at a later date and we are only tieing in to a 6" line. With these results, we are not sure if that will correct the problem. What effects does the fact only one of the available pumps were running at the time of the flow test have on the issue. Is it wrong to think that when more pumps are running there will be high pressure in the line?

 

[cvg]

You can't count on those pumps running during your fire, so I don't think you can press that issue with the city. The extra pumps may not do that much to help anyway (but it is difficult to know without seeing a schematic of the entire system). your big problem seems to be the small 6" line you are tieing in to. Recommend you install the offsite loop line to bring in more water to your hydrant.

 

[RWF7437]

Rookie2,

Thanks for the information.

First, the system already has higher than desirable pressures in some places. 150 psi is probably too high and can cause leaks, water hammer, and similar problems. Most systems I've worked on try to maintain system pressures between 40 psi and 85 or 90 psi.

Second, cvg is on track. Looping the system would help but it also sounds like the Water Utility should be seeking to enlarge some of the existing lines too. They should also be seeking to write a Master Plan and to include some storage reservoirs to provide fire flows and peak demands. The Master Plan must also address how high pressures can be reduced. Pressure reducing valves work but they waste energy so strategically placed storage is the preferred solution. ( maybe you can get them to hire you to write that plan ?)

Third, you may have to build that looped line earlier than you'd planned.

Fourth, you must ask the Water Utility to tell you how their pumps are controlled. Without that information you can't model the system and only they really know it.

good luck

 

[cvg]

rookie2 - RWF elaborated on many things that probably should be done. If you are within city limits / service area than the city does need to provide adequate water pressure and capacity to your property. However, simply turning on the pumps is probably not the solution. Upgrades to the backbone water system including the supply, transmission mains and distribution system may be necessary to provide adequate fire flow to your property. A water network analysis of your pressure zone would be the typical next step to discover where the problem lies.

 

[Rookie2]

Thanks for the great advice.

 

[BigInch]

just guessing, but the pump configuration is "probably" parallel, so with 2 pumps running keep same discharge pressure at the pump station & double the flow. That may give you more flow at the hydrant, however it would also tend to reduce your hydrant pressure due to greater flows yielding greater head losses in the pipes along the route.

http://virtualpipeline.spaces.msn.com

 

[chadl]

I apologize if I missed it, but what size pumps does the city utilize? That can easily set an upper limit to the water supply. Most systems I've seen like this utilize parrell pumps and pressure transducers that activate pumps as pressure drops, either at the outlet of the pump station, or at a remote point on the system downstream of the pumping station. Often there is a delay in pump activation, ie if the pressure dropped below the minimum outlet pressure, a second (or third pump) may turn on, but there maybe a built in delay from a few seconds to a few minutes to prevent pumps from activating on a brief pressure drop. A steep supply curve like you described could occure if the flow test was performed quickly prior to the second or third pump to turn on. If the pumps are located at seperate areas you may not be drawing the pressure low enough at the other pumping staitons to cause them to activate, and even if they were turned on, they may not contribute significantly to your flow if they are running close to there shutoff head. It maybe worth performing another flow test, and flowing for a longer duration, but I would want to talk to the city and see if they provided you with some information on the size and operation of the pumps first.

Generally, more pumps will provided better pressure/flow, however it is very dependent on the arrangement of the system, and how it is opperated.

 

[Rookie2]

Very helpful, Thanks

 

[UtahWater]

How do I model the flow at hydrant without forcing flow or pressure? I typically determine the available flow at a hydrant at a given pressure (usually 20psi) or the pressure at a given flow (usually 1000 gpm or 1500 gpm). I have a reviewer that wants to know the flow and pressure if I just open the hydrant and let it flow...without forcing the pressure to 20 psi or the flow to 1000 gpm. The system is all still on paper, so a field test isn't going to happen...any help?

Thank you!