Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure? 6

[az5333]

Hi fellow members.

I am working on designing a wet pipe fire sprinkler system for a small room and I am using "Fluidflow software" to perform the hydraulic calculations. I am stuck at a very simple concept but I need your help in clarifying that. Sprinkler manufacturers recommend that the sprinkler operate at or above a minimum residual (flowing) pressure of 7 psi. Now my question is, when we look at the pressure available at the sprinkler head, do I look at the stagnation pressure or the static pressure? For example, for one of the most remote sprinklers, the stagnation pressure is coming out to be 7.72 psig and static pressure is 2.35 psig. Should I be comparing the stagnation pressure (7.72 psig) to the min. requires pressure (7 psig)? Appreciate the help and support.

 

[1503-44]

I see a number less than 7. Dont you see that?
That sprinkler
WILL NOT OPEN!

 

[az5333]

Hi 1503-44,

Yes I see that. I think we need a fire pump then. The residual pressure is coming to be too high to the available residual.

 

[LittleInch]

That 1 1/2" to 3/4" restriction isn't do you much good. you lose 5 psi over 1" of fitting.

But you don't really have a huge amount of pressure spare.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

I thought the sprinkler had to connect to 3/4"
That 3/4" piece is only 1" long.
Otherwise I have no idea what its doing there.

 

[az5333]

Hi LittleInch,

You are right. That reducer is killing me. We are already cramped on available pressure. In your experience, is there a better way to connect a 3/4" NPT sprinkler head to a 1 1/2" feedpipe coming downwards? I am out of ideas. This system is part of a very large existing sprinkler system and I want to avoid asking to add a pump just for this extension of the sprinkler system. Thanks.

 

[az5333]

Hi 1503-44,

Yes that 1 inch pipe is just a fictitious one to denote that the 3/4" sprinkler head is being connected to the reducer of that size.

 

[1503-44]

az,
If you cannot lower the sprinkler by 1 foot to get 0.43psi more static pressure, then unfortunately you must have a pump. The building inspector will flag that one way or another and it will not pass, so resolve it now.

 

[LittleInch]

What does the data sheet for the sprinkler say?

Flow and pressure are interlinked.

Do you have a set flow rate you need to achieve or how is the analysis being set up?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Agree with LittleInch, you need to input the flow data for the sprinkler heads as well as the total flow that is available.

he most commonly used residential sprinklers are 4.9 k-factor pendant sprinklers. The minimum pressure and flow for these sprinklers is 7 psi at 13 gpm for spacing up to 16 feet by 16 feet. Hence, the most common demand for two sprinklers flowing is 26 gpm. Of course, this can vary depending on system requirements and the designer, and increasing sprinkler spacing or using sidewall sprinklers can increase the flow demand significantly beyond 26 gpm.

Link

 

[pierreick]

Hi,
You may download PSIM2 pump system improvement modeling tool to support your work to validate the calculations.
Pierre

 

[PEDARRIN2]

Is this for a residential or commercial/industrial application?

Most sprinkler drawings I have seen do not have the tee turned down toward the sprinkler. Typically, it is turned up and an inverted u bend is formed. This allows the sprinkler location to be other than right below the pipe feeding it.

Would also recommend referring to NFPA 13, or whatever standard is relevant, for appropriate methods for performing hydraulic calculations.

 

[az5333]

Hi LittleInch / bimr,

Thanks for your responses. As per the maximum sprinkler coverage areas per sprinkler for our design (96.7 ft[sup]2[/sup]) and the design discharge density of 0.38 gpm/ft[sup]2[/sup] per sprinkler, the minimum flowrate that is needed per sprinkler is 36.7 gpm. Therefore, in the software, I am setting the residual pressure (at the base of the riser) such that this minimum flowrate is achieved per sprinkler. During this process, I also am checking if the minimum operating pressure condition is met (i.e. 7 psig). I have used K11.2 SC/SR sprinklers for the application. I have used "Extra Hazard Group 2" design curve from NFPA 13 (0.38 gpm/ft[sup]2[/sup] over 3000 ft[sup]2[/sup]) and then reduced the area by 25% considering the provision given in NFPA 13 section 19.3.3.2.7 with a final selected design area of 2250 ft[sup]2[/sup]. The total number of sprinklers in design area (active sprinklers) are coming out to be 31. Hopefully this clarifies the overall design scenario I am considering for the application. Currently, my residual pressure at the base of riser is coming out to be 38 psig with these calculations as per the software. We are scheduled for a hydrant flow test with the city in a couple of weeks, but all we have so far is the static pressure at the base of the riser and that is 43 psig. So things are definitely not looking good. That is why I was trying to clarify if it is the stagnation or the static pressure I have to consider, as stagnation pressure values gave me quite a relief in terms of required residual pressure. Please let me know if you think I need to change my approach or re-visit the design parameters.

Hi pierreick, Thank you for referring me to that software. I will definitely give it a go.

Hi PEDARRIN2, this is for industrial application. This is an extension of the existing sprinkler system for their new clean room that they have built inside the same area. The U-bend you are referring to is not required for K11.2 or greater sprinklers as per NFPA 13 section 16.3.11.5 (I guess you are referring to what is termed as "return bends"?).

Hopefully this clarifies things for all of you and hoping to get more insight from your experiences. Highly appreciate the excellent discussion so far.

 

[LittleInch]

OK, now we're getting somewhere.

I'm no expert on NFPA and sprinklers, but from what I can figure out, you have chosen sprinklers with a k factor of 11.2 and you need 36.7 gpm.

To achieve that you need apparently 11 psi, not 7 at the base of the sprinkler. To get 36.7 gpm at 7 psi you need sprinkler with a k factor of 13.9 or higher. Or a flow of 30.1 gpm at 7 psi /K of 11.2 so more sprinklers.

So as you can see there is a trade off and balance between sprinkler flow and volume and number versus pressure and k factor.

But I agree it doesn't look good if your non flow pressure from the mains is only 43 psi.

Bigger pipes? Do you have a big static difference from base of riser to sprinkler?

But we can't see your system so don't know where the big pressure losses or height differences are.
But can you get 1 " connection sprinklers? or have more sprinklers at lower flow rate? It's that last reducer down to 3/4" which is killing your pressure.

I've now learnt more about sprinklers that I ever knew before, but this is simple juggling options and designs.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Looks like static loss is 43-6.62 psi = 36.4 psi
The sprinkler is 84ft above the riser pressure indicator.

Since there is not enough static pressure, the problem is not pipe diameters, its elevation.

You can't invent static pressure.

If there is not even enough static pressure, there is no way he will get 11psi when flowing, unless he installs a pump, or elevated tank. A larger pipe will not help static pressure. It would help in reasonable flowing pressure.

Since there is insufficient static pressure, that must be increased first, using an elevated tank, or a fire pump and a jockey pump. The jockey pump must keep the line pressured all the time to at least 7 psi at the sprinkler. If the static pressure drops below 7, the fire pump must kick in. Hopefully the fire pump will be sized to give the proper flow at 11 psi, or whatever it is that's needed there for whatever flow needs to be delivered to put the fire out under the sprinkler.

 

[az5333]

Hi LittleInch,

You are right I would now need 10.6 psi instead of 7 psi with K11.2 sprinklers at a required flowrate of 36.7 gpm. The only major pressure difference is across the reducer and that is where the static pressure drops half to the stagnation pressure value. Had I not have a reducer, things would have been much smoother. And no there are no 1" sprinklers available for K11.2 Standard response / standard coverage sprinklers. Looks like I am out of options. :-(

 

[1503-44]

You ran out of options when you saw the 6.62 psig static pressure.

 

[az5333]

Hi 1503-44,

The sprinklers are only 12 ft. high. The pressure loss is being taken up by all the activated sprinklers (which is 31 in my case). The available pressure at the cross mains is 26 psi and then it distributes into 4 branch lines (on one side of the cross mains only). Schematic of the entire process is somewhat like shown below and I am looking at the 2nd to last sprinkler right now (circled in red - ignore the last sprinkler as it is in another room).

 

[1503-44]

OK. Your last diagram only showed 1 sprinkler. I didnt know there were 12 on the string.

Wish I could read those numbers.

 

[LittleInch]

I'll confess I'm no expert at sprinkler systems which I guess have issues in terms of how many sprinklers you need to allow for, their water coverage etc, but a distributed network problem like this always runs into issues of variable pressure and how you control flow to make it as even as possible.

If you have the same sprinkler and the same set up for each one, then at the moment you will get more water flow out of the ones nearest the main branch compared to the ones at the end.

With 26 psi at the main junction, you should be able to get 7psi at the sprinkler, but you may need to have more smaller sprinklers or a larger branch pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Yes. Do try a larger branch diameter, but so close to minimums, I am not hopeful. That's a long string.