Supplying a 6"-Pipe with a 3"-Water Supply

What is indicated on the flowmeter if all eight hydrants are run simultaneously?

• While network modelling is always possible it is iterative and tedious. Network software for this task can be found and purchased to speed up and simplify it, however the basics should be looked at first. If the eight hydrants supply the required flow, then bingo.
• 0.025m³/sec down a 3" Ø pipe gives a velocity of 5.4m/sec, which is on the high side generally (rule-of-thumb: if liquid is doing more than 3m/s then the pipe is undersized, However this does not take into account the possibility of the pipe being a ring).
• The 1.5bar delivery pressure will be to a mobile fire pump inlet: 48ft of head isn't enough to do much to a fire that is higher up than that.
• What are the prospects of boosting the supply pressure during a fire?

PWSlack,

Thank you for your swift response. I haven't tested all eight hydrant points simultaneously.

The 3" supply pipe isn't a designated supply for the hydrant system, it also supplies a large section of the refinery. I am looking to tee off this pipe. I would have to look into the possibility of boosting the pressure during a fire. I am not sure if this is feasible.

I am sure you can appreciate my concerns on this project due to the serious implications if water is unavailable.

The pressure drop (as stated by Duane, many thanks by the way) is a concern. As the supply feed starts at 6" then reduces to 3" to possibly supply a 6" ring main.

I will do some more research. Thank you all for your thoughts. Much Appreciated.

Each hydrant with 1500 LPM?

Then it is 8*1500/60 = 200 LPS = 0.2 m³/s

and that is approx 44 m/s

As per ASHRAE Fundamentals Handbook

Max water velocity to minimise erosion
operation Velocity
hours per year m/s
1500 4.6
2000 4.4
3000 4.0
4000 3.7
6000 3.0
All this is assuming that your 3" pipe is feeding only the hydrant else the velocity is going to be shooting up further.

As per this link you should be only about 1.15 m/s recommended or 1.6 m/s maximum

I am not sure you are supposed to

Off the top of my head (I am not prepared to do the calculations) I would suggest that your pressure drop in the 3" pipe would be so high that the required flow rates would not be achievable.

Okay, so I used the Hazen Williams formula at the flow rate of 0.2 m3/sec and come up with a head loss of 2130 m/ 100 m of 3" pipe. It is likely that at those velocities the turbulence will be so high that the equation no longer applies and losses will be even higher.

Just return this question to the authority having jurisdiction: they will have to agree on the final solution and when they don't trust, forget it.

It all comes down to the fire loading of the local situation, if they think that 3" can supply the water you need, you're lucky.

else: find an investor for a water tank and pump. (good for the global economy)

Depending on your Fire departments strategy in a fire at this facility, they will ask for a certain flow at a minimum pressure (to feed a "pumper unit" without cavitation).

From your details they would assume each hydrant is providing 1500 litres simultaneously and the ring main pressure is still above 1-2 bar. It's a worse case end point, not a "design to" point

Equally there is good reason that the ring main is a ring or it would likely require a 8" single and 8 " mains supply if the fire was multi-point on the line.

I seriously doubt that a 3" main at 4-6 bar will deliver 8 x 1500 lpm and only halve in pressure. To work it out, go back to your fire people and ask how many nozzles of what capacity they'd likely use.

These nozzles have operating pressure and litres used numbers. E.g. our common Akron uses 400 l/m at 700 kPa. Add it all up to find total P x V delivered on the fire. "Delivery on the fire" should be equal or less than that delivered by the main. My rule of thumb says 3" main at 6 bar is capable of supplying 1 pumper running 4 Akron's. Fine for a medium house fire. Nothing like what I'd want/need at a refinery fire.

A few further thoughts

The ASHRAE numbers are generally conservative but I would hope you aren't using your fire main for > 1500 hrs a yr so 4.6m/s is ok

Some rough numbers for the existing 6" main assuming its a ring so you supply 4 hydrants clockwise and 4 anticlockwise the supply flow is 0.1m3/s each way down a 6" line gives 5.6m/s so this gives a fair supply rate and not far from the ASHRAE numbers.

Given the 5.6m/s above it does look as though the existing system is designed to supply all eight simultaneously which was going to be my next question. If you can reduce the number of hydrants to be supplied in one incident then you might have a chance.

I would also go back to the 6" part of the header and make my tie in there and run a 6" to connection to the fire main. It will be longer and bigger ergo more expensive but cheaper than building a new refinery if the old one burns down. Ideally to meet the 0.2m3/s supply an 8" (6.4m/s) would just do. These are very high velocities but this is a line almost never used and so its normal to go with higher velocities (provided your pressure can take it). Even though your 3" section is short the 40m/s velocity is going to give ~2bar per m!!!!! so its going to chew your pressure instantly.

One issue I would have with the booster pump is how you control and operate it. My experience on firewater systems is to have a small jockey pump keeping the pressure up allowing essentially for leaks but not man enough for a user. As the pressure falls when the first user is operated the big (often diesel) fire pump kicks in.

I think the min of 1.5 bar which gives the 48ft limit noted above acknowledges that fires are usually at grade and its most important to focus on the grade level fires to protect structures and fires at height are left to burn. Although I am no expert in fire fighting strategy and naturally the fire i was closest to at Texaco-Pembroke started in oil soaked insulation at the top of the vac column well over 48ft up.

As a rule of thumb based on squares I would expect to be able to supply two off 2.5" devices through a 3" supply.

As a fire fighter I have serious concerns with you supplying a hydrant with a 3 inch supply line. I doubt that meets building code requirements and will be a safety issue.

Working for a water utility, we rarely see the domestic supply combined with the fire supply. In the situation you have described, usually a seperate tap is made from the main in the street for the fire loop. This tap would typically be a full size (6" or 8") main. Over the last 20 years, we have gone exclusivly to 8" or larger to keep velocities to an acceptable level. If there is concerns from the water purveyor, a backflow assembly is often installed between the private lines on the factory and the public mains. Large meters are also available to be placed on fire lines, but we often do not install those.

Capacity charges rarely apply to fire services as they are not constant use and a demand on the system.

Certainly, on top of the diameter concerns, the diameter changes will turn the flow from laminar to turbulent even earlier. If you install tapered adapters and preserve some laminar flow, the laminar flow for any head is proportional to the diameter cubed, so a 3 inch line ~= 27, and each 2.5 inch line ~= 15.625, so even with just 2 in use, the main becomes a bottleneck. A 6 inch line ~= 216, is actually 8 times larger and matches 13-14 2.5 inch lines, which is good since these connectors and minifolds do increase turbulence.

Thumb rule for flow in pipe is 10 D^2 where D is pipe dia. in Inches.

If you have same flow with two different size of networking then there is no problem. Otherwise when more points are in use flow become lees at fire points and also pressure.

I suppose turbulent flow is the second power, cross-secton, and laminar, the best case outcome when you can get and preserve it, is third power. The rule of thumb is conservative, the laminar case is a limit. (The laminar velocity profile in a round pipe is a cone, and the volume of the cone vares to the third power.) Manifolds and simple valves have turns and cross-sectional area and shape changes that induce turbulence, so the second power case is, unfortunately, all too likely to be closer.