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.

 

[az5333]

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.

 

[bimr]

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

 

[1503-44]

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

 

[az5333]

Hi bimr,

31 is the number of activated sprinklers in the room (corresponding to the 2250 ft[sup]2[/sup] design area selected). The complete room is more than 3700 ft[sup]2[/sup] 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!!

 

[1503-44]

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.

 

[LittleInch]

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.

 

[1503-44]

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

 

[az5333]

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?

 

[PEDARRIN2]

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).

 

[az5333]

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.

 

[1503-44]

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.

 

[az5333]

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.

 

[1503-44]

No worries.
You did it right.

 

[PEDARRIN2]

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.

 

[LittleInch]

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.

 

[bimr]

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

 

[danschwind]

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

 

[az5333]

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.

 

[1503-44]

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.

 

[az5333]

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.!