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

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

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

(OP)
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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

I'm not following your terminology as "stagnation" and "static" I would normally interpret as the same thing? So I'm not sure how you got two different values?

Can you explain a bit what you mean?

But either value looks very low.

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi LittleInch.

Thanks for your response. Regarding the use of terminology, this terminology is from the software itself and this is what I am not clear about myself. Also, how are you implying 7.72 psig is low if minimum required is 7? I am only asking so I can further improve the system design if we need to have a certain margin above this minimum.

Thanks again.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Because nothing is that exact.

And from the definitions provided it looks like static pressure is your flowing pressure upstream of the sprinkler.

Stagnation pressure is pitot pressure or the pressure seen at flow =0 in a tube facing into the flow.

what sort of velocity do you have there? 5 psi difference between the two seems quite big to me.

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi LittleInch.

Thanks but I was thinking it is the opposite: Flowing pressure = stagnation pressure and vice versa.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

7 psig is the minimum pressure required by code WHEN the sprinkler is flowing at the required flow rate.

Some examples,

If your required flow rate is 15gpm and you have a K6 sprinkler the required pressure at the sprinkler when delivering 15gpm is (15/6)^2 = 6.25 psig The minimum required by code is 7psig. 6.25 < 7, so you must have at least 7psig at the sprinkler WHEN it is flowing 15gpm.

If your required flow rate is 25gpm and you have a K6 sprinkler the required pressure at the sprinkler when delivering 15gpm is (25/6)^2 = 17.4 psig The minimum required by code is 7psig. But 17.4 psig is very much higher than the code minimum of 7psig, so you must have at least 17.3 psig at that sprinkler WHEN it is flowing 25gpm.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Thanks 1503-44 for the great explanation. Will this required pressure be classified as required "stagnation" pressure or required "static" pressure? Fluidflow software is giving me an option to view both the available pressures at the sprinkler head when it is flowing at the required flowrate (in my case 38.9 gpm). I am not sure which one to look at for this compliance.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

I've told you that it's static pressure.

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

It is kind of both.

7 psig static pressure.

You must have a minimum of 7 psig to pop the sprinkler even when there is no flow at all going through the sprinkler. That is a static pressure, as there is no moving fluid.

7 psig stagnation pressure.

You must also have a minimum of at least 7 psig at the sprinkler when there is flow, so in that case it is a minimum stagnation pressure.

Minimum required stagnation pressure

Is the greater of
1) Pressure psig = (flow rate gpm / K )^2 When flowing. , or
2) 7 psig When flowing.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Thanks 1503-44 and LittleInch for the helpful responses. :)

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Fire sprinklers typically require only 7 pounds-per-square-inch (psi) to operate, which is less than the minimum required pressure for residential plumbing fixtures.

The average water pressure for most homes and businesses is between 30 psi and 50 psi; most sprinkler systems are designed to use pressures of around 30 psi. The minimum water pressure in a municipal water system is 20 psi. If the pressure is lower than that, the municipality will probably call an emergency.

The pressure at a point in a fluid is called the static pressure. The stagnation pressure is the pressure that the fluid would obtain if brought to rest without loss of mechanical energy. The difference between the two is the dynamic pressure. The total pressure is the sum of the static pressure, the dynamic pressure, and the gravitational potential energy per unit volume. It is therefore the sum of the mechanical energy per unit volume in a fluid.

The terminology that we normally see here is static pressure and residual pressure. Static pressure is the water pressure measured when the water is not running. Residual pressure is the pressure measured when the fire sprinklers are operating.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi bimr. Thanks for your detailed explanation. So from what I have understood so far, the stagnation pressure at the sprinkler inlet that the software is giving me is basically the residual pressure and this is what we need to maintain above 7 psi. Right?

Also, when you say that "The average water pressure for most homes and businesses is between 30 psi and 50 psi; most sprinkler systems are designed to use pressures of around 30 psi", you are saying this is the normal residual pressure available at the base of the sprinkler riser? Right?

Thanks for your support in clarifying the concepts.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

az5333,

I understand you're trying to make your system work, but it doesn't.

Look at this to really understand what the software is telling you https://www.engineeringtoolbox.com/pitot-tubes-d_6...

Now when you stick 5psi difference into a pitot tube calculator for water, it tells me that your flow velocity is >8m sec. That's very high and will be causing a lot of pressure drop.

Now I don't know what you put in the program, but my guess is you put in flowrates and asked it to calculate pressure.

Your sprinkler needs >7psi when it is flowing to work properly. Your system at the flowrate needed is only giving it 2.35 psi. Therefore it will dribble and not sprinkle.

Think old man at the urinal vs teenager after a few pints and I think you might understand the difference. Us old men do.

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Quote (az5333 (Mechanical)(OP))

Also, when you say that "The average water pressure for most homes and businesses is between 30 psi and 50 psi; most sprinkler systems are designed to use pressures of around 30 psi", you are saying this is the normal residual pressure available at the base of the sprinkler riser? Right?
Yes, that is correct.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Quote (LittleInch (Petroleum))

Think old man at the urinal vs teenager after a few pints and I think you might understand the difference. Us old men do.

If you want your old fire hose back, ask your doc for flomax.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Not quite there yet.....

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Thanks LittleInch and bimr. I have very high regards for both of your experiences and I am carefully considering your comments. Why do I get the feeling that the responses from both of you are conflicting?

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

I don't think we are. Your analysis program is confusing you by referring to the stagnation pressure for some reason, which is not often used. "Static pressure" is more commonly just called "pressure" as it is the one you can measure by tapping a hole in the pipe and installing a pressure gauge or transmitter. Stagnation pressure is only used really for pitot tubes.

Your analysis clearly shows that during operation the (Static)"Pressure" is 2.35 psi. This could also be called the residual or flowing pressure.

Check your inputs. Your figures (stagnation pressure minus (static) pressure when input into a pitot tube calculation) show that you have a flowing velocity at whatever point this is of >8m/sec. So you may have either have input a very large flowrate or a very small pipe. This is a very high velocity for any system.

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

I dont see any conflict here. Its all good stuff. Go back to the beginning and read all of them again.

7 psig is the minimum at any time, static or flowing, although you may need a lot more than 7 to deliver your flow at any specific sprinkler. That is all you really need to know. Check your output for numbers less than 7. If you see one, you have an error.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi LittleInch,

I changed the parameters slightly and increased the static pressures as suggested. I am attaching a screenshot of one of the sprinklers below. I have tried including all the parameters for your reference like pipe size, length, flow, velocity, static and stagnation pressures. Can you please have a look and let me know what you think of the results now? Many thanks for your support.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
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. :(

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi LittleInch / bimr,

Thanks for your responses. As per the maximum sprinkler coverage areas per sprinkler for our design (96.7 ft2) and the design discharge density of 0.38 gpm/ft2 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/ft2 over 3000 ft2) 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 ft2. 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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
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. sad

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
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).

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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

Wish I could read those numbers.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Thanks LittleInch and 1503-44 for all your help throughout this! I will definitely see what I can do to play with the hydraulics. I already have 4" branch lines gong through. smile

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Quote (az5333 (Mechanical)(OP)

The total number of sprinklers in design area (active sprinklers) are coming out to be 31.

Earlier you said that the number of sprinklers was 31 and now your drawing is showing over 50 sprinklers.

You also need to select a sprinkler head suitable for a clean room.

Are you sure that you are in the Extra Hazard (Group 2)?

Link



RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

You could try a more conventional layout. Here these layouts supply 2 sprinklers per branch off the cross main feeds, whereas yours has 4 branches and 12 sprinklers each.

The loop configuration is very much superior, but requires a bit more pipe

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi bimr,

31 is the number of activated sprinklers in the room (corresponding to the 2250 ft2 design area selected). The complete room is more than 3700 ft2 and not all sprinklers will be open at the same time for design purposes. For the EH2 category, I am following NFPA 13 Table 4.3.1.7.1 for Miscellaneous storage and the type of commodities that client has fall under EH2 category as per the table.

Hi 1503-44,

Thanks for this. I can definitely try this scheme. I actually found out just a while ago that I should NOT have been considering the reducer fitting attached directly to the sprinkler, as the loss for this is already accounted for in the sprinkler K-factor (please see NFPA-13 27.2.4.8.1 (9)). Also I found out that NFPA 13 allows us to use the total pressure (stagnation pressure) to calculate flow through the sprinkler (please see NFPA 13 27.2.4.10.1. This basically solves the issues for me completely as I am back to the original confusion I had about which pressure to consider. So I think I am looking good now as far as the design is concerned. Now only the water flow test is awaited to plot this on a hydraulic graph and see if system is "adequate" or not.

Please let me know in case you still think I might be interpreting these terminologies wrongly from above mentioned NFPA references I shared.

Thanks!!

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

This is why there are only 2 sprinklers per branch on the layouts above.
You can then use 1" diameter pipe for the branches.

Size of Pipe and Maximum Number of Sprinklers Allowed

1" <= 2 sprinklers
1 1/4" <= 3 sprinklers
1 1/2" <= 5 sprinklers
2" <= 10 sprinklers
2 1/2" <= 20 sprinklers

Your layout requires that 2.5" diameter pipe to be used for the branches.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Be careful. I think you're wrong and you're reading into that NFPA section what you want to see.

27.2.4.10.1 specifically refers to an orifice. An orifice is NOT a sprinkler. "When calculating flow from an orifice...." You're not calculating flow from an orifice, you're calculating flow in a branch network system

10.2 states that use of what it calls the "normal pressure" can be used, though doesn't say where.... and also refers simply to "pressure" everywhere else.

27.2.4.10.3 states that calculation of a sprinkler shall be calculated using the K factor. I agree with your exclusion of the pressure drop at the fitting immediately prior to the sprinkler (i.e. the 3/4" reducer)

Now it might be that your get out jail free card is if the k factor already allows for the 3/4" reduction.

You might want to read all of NFPA 13 and things like 27.5.2.1.1 lists a maximum of 8 sprinklers per branch line.... not the 13 you had earlier.

When you submit calculations, you will find people who know NFPA regs backwards and they will simply tie you in knots unless you've followed their unbelievably labyrinthian, rigid and prescriptive requirements. I speak from experience.

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

LI. I think if you use the pipe schedule design method, you are limited to 8 sprinklers, however if you do actual hydraulic calculations, I believe you could have more than 8. Personally I would stick to 8, because that should be enough in a well designed system.

I agree with ZLI's interpretation of 10.1

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi LittleInch,

Thanks for your note. Regarding your comment on 27.2.4.10.1: I thought the flow through a sprinkler is considered a flow through an orifice? I am concerned here about calculation of flow through sprinkler with the right pressure (stagnation or static). This was my original question. Flow through the branch network system is already being taken care of by the software and I am not concerned about that.

Regarding your comment on 27.5.2.1.1, this requirement is only for design using "Pipe schedule method". Here I have used a hydraulic design using area/density method. So this is not applicable to my case.

Please let me know if you still think it is otherwise?

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

In your pictorial, the sprinkler you highlighted is not the hydraulically most remote so will likely not be the worst case sprinkler for pressure.

If you really want to know better if you have enough pressure for what you want, why not solicit a fire suppression contractor who has a NICET certification to run preliminary calcs? My company has done this in the past (the solicitation of assistance) and it likely costs less than what is being spent doing it in house.

Also, there is a NFPA forum where you could get much more appropriate responses to questions (probably should have mentioned that earlier).

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi PEDARRIN2,

Thanks for your response. Yes I can post the same question there as well and see what responses I get.

You are right, the remote sprinkler is the furthest on the opposite end. This was highlighted just as a reference and I have been looking at the remote sprinkler.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Pedarrin2, Not that I object, but what would you say differently over there?

Az, Since double posts are disturbing site practices, please just post a link to this thread here. Then we can all get the benefit of any further developments.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi 1503-44,

I have posted a link to this thread over there as well. Is there a better way to do this? Sorry, I don't want to create any double posts or issues.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

No worries.
You did it right.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

1503-44

I would say nothing different. I commented because it seems the OP does not seem familiar with some of the industry specific issues because of the questions being posed (stagnation pressure).

I could be wrong. If so, I apologize.

Also, the NFPA forum is populated by those who are versed in sprinkler design, are NICET certified, and are even code inspectors. OP would likely get better, more contextual answers, as opposed to the helpful, but maybe not as knowledgeable responses given.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

az5333,

As I noted above, the NFPA standards are, IMHO, unbelievably prescriptive and complex and their numbering system truly sucks. But this is my point. Unless you're very very familiar with the NFPA codes, you can get caught by someone who is... Anyway as mr 44 says and provides the diagram, you're finding out that multiple sprinklers >10 on a single branch might not be the best plan. All distributed header flow systems come to the same conclusion usually.

However in their (NFPA) language, and indeed most other peoples, an orifice is a specific device intended to either measure flow or create a pressure difference when flowing.
A sprinkler may well have some form of orifice within it, but this is all caught up in the K factor. You have NO IDEA what the diameter of the orifice is within the sprinkler and therefore cannot do any calculation. So you can't use 10.1 as an excuse to use the wrong pressure figure in a sprinkler design.

I've now told you several times that whilst it might get you out of a hole at the moment "stagnation pressure" is not "pressure" as most people and systems understand it. I'm not going to say it a fifth time.

However looking back at your posts, the end pressure from the 1 1/2" stub to the sprinkler has a static pressure, or flowing pressure or just pressure of 12.22 psi and a stagnation pressure (normally just ignore this) of 12.53 psig. So you're well above the minimum 7 psi. It's only when you get large velocities such as exist in that 3/4" reducer at 8m/sec does the stagnation pressure differ markedly from static pressure.

But that's not a lot to spare if the flowing hydrant test doesn't give you the pressure you need.....

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

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Agree with PEDARRIN2's comments regarding certification. However, I believe NICET certification is only applicable in the United States.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Albeit the risk of being proved completely wrong by going against the tide versus all the way more knowledgeable colleagues here, I'll state that in my opinion the "right" pressure to use for the calculation using the "Sprinkler equation" that fire systems love to use is the stagnation pressure in lieu of the static pressure.

So, IMO, the friction loss from the reducer should be included in your calculation, but not the static pressure drop due to velocity being higher.

Sorry if I'm just adding entropy to the topic...

Daniel
Rio de Janeiro - Brazil

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
Hi danschwind,

Thanks for your input and greatly appreciate the feedback! After a careful review of expert comments from this thread (which is highly appreciated!) and thorough review on my end, I also came to the conclusion to stick with the Stagnation pressure for sprinkler flow calculation purposes. Having said that, I have also posted the same topic with this thread's link on the NFPA forum to also fetch some feedback from the NFPA experts over there. Thanks again.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Some interesting provisions of the San Jose Fire Code in addition to NFPA requirements concerning hydraulic design.

3.14 Hydraulic Calculations
3.14.1 The pressure cushion for hydraulic calculations shall be the greatest of 10% of the water supply data or 8
psi.

3.14.2 All hydraulic calculations shall include a copy of the letter from the Water Company that states the water-flow data verified within six months of the submittal date. Water-flow data may be obtained from the San Jose Water Company, San Jose Municipal Water Company or Great Oaks Water. If you wish, San Jose Fire Department can perform a water-flow test and provide the water-flow data at an hourly rate (3 hours minimum). However, this test will not take the place of the water company declaration.
3.14.3 The backflow prevention requirements for each water company are unique. San Jose Water Company and Great Oaks Water Company require an additional check valve after their meter. San Jose Municipal Water Company requires a “Lead Free Dual Check Valve Backflow Device (or equivalent)”. We will need verification that the correct devices have been represented in the calculations.
3.14.4 Backflow Preventer Retroactive Installation – When backflow prevention devices are to be
retroactively installed on existing fire sprinkler systems, a thorough hydraulic analysis, including revised hydraulic calculations, new fire flow data, and all necessary system modifications to accommodate the additional friction loss, shall be completed as a part of the installation. New or changes to existing backflow preventers shall not be installed without Fire Department approval.
3.14.5 The minimum operating pressure shall be as listed for the sprinkler head but in no case shall it be less than 7 psi, regardless of the provisions in NFPA 13 to allow use of sprinklers in accordance with their listing.
3.14.6 A maximum flow velocity of 20 feet/second shall not be exceeded when designing to the criteria as set forth herein as sections 3.2., and 3.3. Provide calculations based on the Hazen-Williams formula. See SJFD Handout Titled “20 FPS CHART”
3.14.7 The area of operation may not be reduced as allowed by NFPA 13, sections 11.2.3.1.4 & 11.2.3.2.3.1, where quick-response sprinklers are installed.
3.14.8 The area of operation shall be increased by 30% as required by NFPA 13, sections 11.2.3.2.4 and 11.2.3.2.5, for pitched roofs (> 2” in 12”) where SSU, SSP, HSW, and Large Drop sprinklers are used, and for dry pipe or gas charged pre-action systems.
3.14.9 Provide documentation for all pipe length equivalents used to develop your calculations. As an Example, Tyco CPVC fittings are “special” in that they get reduced equivalent lengths (for 90° elbow) compared to other manufacturers, you need to provide note on plans and in the calculations that only Tyco CPVC fittings will be used. We will check these in the field, so, the fittings must be readily identified as Tyco CPVC fittings. If not, then you will need to revise your calculations to reflect the “normal” equivalent lengths.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

(OP)
HI 1503-44,

Interesting points. Thanks for sharing.

3.14.1: I've used a pressure cushion of 3 psi. NFPA 13 doesn't recommend any pressure cushion but it is good to know some limit.
3.14.5: Noted and kept under consideration.
3.14.6 A: NFPA does not provide any limit on velocity. Still a good to know information for consideration.
3.14.7: In my case, we are using standard response sprinklers K11.2, so I have reduced area by 25% as per NFPA 13.

Thanks again for sharing very useful information.!smile

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Quote (danschwind (Mechanical))

Albeit the risk of being proved completely wrong by going against the tide versus all the way more knowledgeable colleagues here, I'll state that in my opinion the "right" pressure to use for the calculation using the "Sprinkler equation" that fire systems love to use is the stagnation pressure in lieu of the static pressure.

Stagnation pressure is usually what is implied when just the term “pressure” is used in discussions.

The difference between Stagnation and Static Pressure is the Dynamic Pressure, which represents the kinetic energy of the flowing fluid. Dynamic pressure is a function of the fluid velocity and its density.



In the example with the 4 inch pipe, 700 gpm results in a fluid velocity of 17.6 ft/sec, a dynamic pressure of 2.1 psi, and static pressure of 97.9 psig, and stagnation pressure of 100 psig.

The Stagnation Pressure is the sum of the Static Pressure and the Dynamic Pressure, also shown in the equation below:

Ptotal = Pstatic + Pdynamic

For many liquid applications, the pipelines are sized to ensure low fluid velocities to reduce the head loss and pressure drop for a given flow rate, resulting in a small value of dynamic pressure. Also, because of the accuracy and scale of the instrument used to measure the pressure, the distinction between total (stagnation) and static pressure may be neglected. In the example above, the difference between stagnation pressure and static pressure is just 2% when the fluid is flowing at 17 ft/sec which is a typical velocity in fire systems.

Link










RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Quote (az5333 (Mechanical)(OP))

Thanks for your responses. As per the maximum sprinkler coverage areas per sprinkler for our design (96.7 ft2) and the design discharge density of 0.38 gpm/ft2 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/ft2 over 3000 ft2) 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 ft2. 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.

The online calculator from Meyer Fire:



Using the 11.2 K-Factor, the calculator shows you need 10.8 psi at the sprinkler to obtain the desired flow.

If you have the 43 psi at the farthest sprinkler, your system should be acceptable.

Link

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Some fire protection design standards from axaxl:

Link 1

Link 2

Link 3

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Hi,
let you consider the link underneath where you can find a methodology and a software .
https://www.canutesoft.com/how-to-calculate-a-fire...

Note:
To me the most important is to get a registered contractor to perform the calculation , knowledgeable about the codes , the requirement of the administration (safety bureau, fire fighting brigade ,) . If you fail to do so , you could have a long delay to obtain your operation license. This is based on 20 years experience in Asia .

Good luck
Pierre

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

I have the same opinion as bimr. Stagnation pressure should be the pressure figure unless stated otherwise.

Daniel
Rio de Janeiro - Brazil

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Quote (az)

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.

Definitely not a good sign that there is only 43 static. Stagnation pressure is very likely to be lower than 38.

But if you designed a tree system with a proper cross main, you might do better than 38 required at the riser. Have you tried that yet? Every psi will count.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

I'm trying to duplicate your results, but I don't see some critical information.
How far apart are the sprinklers.
C factor of pipe.

So far I used 10ft between sprinklers,
C = 140
5 ft to the cross tee
50ft of 3" pipe
35ft 4" pipe
And get 43 psig, but at the beginning of the branch at the cross connection.
There is still more pipe to go.
12 ft down is another 5 psi
And I do not yet include fitting losses.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

1503-44

If C is the friction factor used, most of the ones I have seen for wet pipe have been 120 for black steel.

RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Thanks. Yes I know the C values. Where does az533 say he has used 120?
Where does he say he has used a material with a known C value at all?

I am trying to show az533 how close his design is to failing, so I used C=140 so my example would have a lot less pressure loss. But thanks, he might not have noticed that.

I have also worked out two example configurations.
One example uses a configuration similar to az5333's "bush" configuration.
The other example uses a tree configuration, similar to the one I posted above.
Both examples have the same K=11.2 sprinklers at 10ft C/C.
Both examples cover a grid area of 5600ft2. 40' x 140'
Ex 1 has 4 branches 10' apart =30' and are each 130ft long. 13 sprinklers at 10' c/c.
Ex 2 has 26 branches, 15' long extending either side of a central 130ft long cross main. 2 sprinklers ea. At 10' c/c.
I have ignored all fittings for now.
I have kept velocities <30 fps.

Presuming I did this correctly.
Example 2, the tree configuration is far more efficient.
It needs 20% less flow. Important if supplied from a tank get, or if water is pumped.
It needs 8% less pressure. Very important with low residual pressure.
It has 7% less max velocity. Probably not significant.
And uses 60% less weight of pipe than a bush configuration.
Very significant.

It would appear that using short branch strings with few sprinklers are more efficient.
Minimising use of lots of smaller bore pipe pays off by collecting flow and quickly getting it moving through larger more efficient diameters.

I have not found any errors in my configuration analysis.
I think it shows just how bad some configurations can be.


RE: Static or Stagnation Pressure for Evaluating Fire Sprinkler Minimum Operating Pressure?

Oh boy, I'm late to this party. I made an account for this.

My career is what you're doing right now; fire sprinkler design. I'm currently at NICET level II, and will be eligible for III this year.

Bluntly, you're in over your head here and you need to have someone else do this or prepare for weeks of reading NFPA 13 and probably use different software or break out some spreadsheets and graph paper.

You are not limited to 8 heads per branch, nor any size requirements except that 1 inch pipe is the minimum size except in rare edge cases, none of which you'll find in EHII. That is the old pipe schedule system that served for uncalculated systems, which are no longer allowed to be installed. If you see a sprinkler system following that sizing and it's 30+ years old, you may be looking at a schedule system.

The 7psi minimum is for any sprinkler, meaning if it's in a 24 ft^2 light hazard closet, required operating pressure is 7psi regardless of coverage area and density showing that 2.4gpm is required and therefore pressure will be almost negligible.

For such a case, flow is also adjusted accordingly, and the head would operate based on Q=K*sqrtP

Overall, hydraulic calculation is not going to be well served or accurate by your chosen method.

To start, the table with area/density requirements is not your only limitation. I haven't taken a thorough look but there are stringent requirements for how that area is to be determined, not just its size.

Secondly, there are two pressures a sprinkler system cares about: static and residual. That being pitot tube pressure at 0 flow and available pressure at flow demand condition.

(You can use normal pressure calculations based on the velocity pressure formula found in NFPA 13, but this is extremely rarely practiced.)

Additionally, calculations are near recursive processes due to how they are performed.

You will start your calcs backwards, at the end head of your remote area, using Hazen-Williams. It's not enough to say "x gpm per y sprinklers" and run.

I'll make the math easier for myself and call your end head 100ft^2 at .4gpm/ft^2. Its flow is 40gpm, and with 11.2K, pressure required is nearly 12.8psi while flowing. Calculate pressure loss through the pipe to the next flow device, size change, or C factor change, adding equivalent lengths.

Next head, add another 40gpm, calc a now 80gpm of flow to the next change, and so on.

13 heads is 520gpm at the main/branch line intersection of your end line, and if that pipe isn't huge I shudder to consider how much you're losing per foot. Your end head pressure plus the sum of all the friction/elevation losses up to this point is now your overall pressure demand at that 520gpm.

Calc flow demand through main back towards riser to next branch line.

Now, if the branch line is identical, you'd be fine to assume its calculated demand is 520gpm at the same pressure. However, your overall demand is slightly higher due to demand from the friction loss across the main, and NFPA requires you to balance the pressure demand at that intersection to within 0.5psi, so if there is a larger discrepancy, you must adjust the flows of the heads on that line until the pressure demand is within 0.5 psi of demand 1 (branch line 1 + cross main friction loss). Your new demand let's call an even 1100 because I'm not going to actually go through that process manually.

(The other variable you can control to inflate pressure demand at an intersection is pipe size downstream. You can try that but loss is drastically different on the smaller diameters at the flows you've got.)

Calc 1100gpm worth of friction loss down the main to the third branch line. Repeat the balancing procedure for branch line 3. Note that because your loss through the main is now 1100 you're losing more to friction loss from line 2 to 3 than 1 to 2, and balancing line 3 will have accordingly increased overflow.

Repeat until you reach either base of riser, or preferably a fire hydrant or other water supply connection where a flow test was performed. Add outside hose allowance here.

Bear in mind that elevation loss/gain is calculated from reference point to reference point, so while this isn't the case for your building, branch lines on a pitch with one side going up from the main and the other going down will induce massive overflow at intersections unless you adjust branch line sizes accordingly.

If this system is existing, my recommendation would be to attempt a calculation based on relocating the main to split the branch lines with 6/7 heads on each side, setting the main closer to the 7 head side than the 6 to reduce overflow. From there, if it didn't work you'd be looking at either increasing the main/riser size, gutting the branch lines to increase their sizes, or making a grid system probably. Either that or estimating if adding a fire pump and its corresponding code parsing is cheaper.

Last point: you can eliminate one fitting serving the sprinkler (is, the tee or reducer the head screws into) per head from equivalent length calculations. So if the head screws directly into the tee you can omit that tee, but that still won't do a whole lot for you.

Best of luck, although I assume that by now this problem has reached a conclusion.

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