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Operating Pressure Vs Residual Pressure 2
- Thread starter mechfp
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- #3
Thanks cdafd, But one thing is again disturbing me. When we see any Design criteria for Rack storage in NFPA 13 for Example Table 16.2.2.1 "CMSA Sprinkler Design Criteria for....", there is something called as "Minimum operating pressure" given in that table. What does this mean? Is it the minimum operating pressure of Last (most remote) Sprinkler Head OR Is it the minimum operating pressure of the system OR is it the minimum operating pressure of Design Area (2000 sq.ft). What I know is minimum pressure for last sprinkler head must not be less than 7 PSI. If I design my system considering 25psi or 50psi to be the minimum pressure at the last sprinkler head with K-11.2 or K-19.6 sprinklers then the pressure for the system is going too high. Please help.
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- #4
not an engineer, designer or anywhere near one of those, but
I take it that is the minimum pressure any head in the system is required to have to work properly
cannot address the high pressure, as long as you are not over 175 on the system, there is nothing extra needed
what they said again:::
can I ask what you do for a living???
I take it that is the minimum pressure any head in the system is required to have to work properly
cannot address the high pressure, as long as you are not over 175 on the system, there is nothing extra needed
what they said again:::
can I ask what you do for a living???
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TravisMack
Mechanical
Using CMSA as your example, you have x number of sprinklers at y pressure. This is the minimum pressure that must be available at any sprinkler. If you have a gridded system, the pressures will be pretty similar and flows fairly close between all heads. If you have a tree system, the most hydraulically remote sprinkler will have y pressure. All other sprinklers will have >y pressure.
You system demand at the base of the riser will be >y in almost all cases, unless your base of riser is far above your sprinklers.
Example: K17 ESFR at 44' elevation. The minimum sprinkler demand pressure is 52 psi. 44' of elevation is going to add 19 psi. You will likely have somewhere around 30 psi of friction losses, depending on pipe sizes. So, the base of riser pressure would be 101 psi. There would be extra pressure requirements if you have a backflow preventer or other devices in the riser assembly.
Hopefully this makes sense.
Travis Mack
MFP Design, LLC
You system demand at the base of the riser will be >y in almost all cases, unless your base of riser is far above your sprinklers.
Example: K17 ESFR at 44' elevation. The minimum sprinkler demand pressure is 52 psi. 44' of elevation is going to add 19 psi. You will likely have somewhere around 30 psi of friction losses, depending on pipe sizes. So, the base of riser pressure would be 101 psi. There would be extra pressure requirements if you have a backflow preventer or other devices in the riser assembly.
Hopefully this makes sense.
Travis Mack
MFP Design, LLC
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- #6
Thanks Cdafd & Travis. You guys always give brilliant response & Explanation. It really helped a lot. I am designing a Fire sprinkler system for Storage facility wherein I am using CMSA Sprinklers. Considering K-11.2 with 50Psi operating pressure for the remote sprinkler head, I am getting total inlet pressure to be 800psi(55bar approx.). Design area = 2000 sq.ft & total no. of sprinklers 16. Pressure seems to be too high but M going to discuss this with AHJ.
Cdafd, M mechanical Engineer working in Middle East.
Travis, Your example raises one more question. How you have minimum sprinkler demand pressure as 52 psi. It depends on total no. of sprinkler. right? If you consider 20 sprinklers then the pressure will go much higher than what you described.
Cdafd, M mechanical Engineer working in Middle East.
Travis, Your example raises one more question. How you have minimum sprinkler demand pressure as 52 psi. It depends on total no. of sprinkler. right? If you consider 20 sprinklers then the pressure will go much higher than what you described.
stookeyfpe
Specifier/Regulator
This sounds like an FM design for plastics. With a K factor of 11.2 and discharge pressure of 50 PSI, the total flow/sprinkler is 80 GPM. I have no idea of how you are getting a 800 PSI inlet pressure and no sprinkler pipe or fittings are designed for these pressures.
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- #8
Yes Stookeyfpe You are right. I know am wrong somewhere. I am not getting exactly what I am looking for. My question is : The discharge pressure (50psi) for K-11.2 sprinklers as mentioned in NFPA 13, Is it for all 20 sprinklers in design area Or only for the most remote sprinkler?
If it is for the most remote sprinkler then the pressure at the inlet riser will be too high which is surely wrong.
If it is for all the 20 sprinklers ( 50psi for 20 sprinklers) then the last sprinkler will not get minimum operating pressure. Travis already gave one example but number of sprinklers there are not clear.
I hope I am clear with my question this time and am waiting for your responses.
If it is for the most remote sprinkler then the pressure at the inlet riser will be too high which is surely wrong.
If it is for all the 20 sprinklers ( 50psi for 20 sprinklers) then the last sprinkler will not get minimum operating pressure. Travis already gave one example but number of sprinklers there are not clear.
I hope I am clear with my question this time and am waiting for your responses.
Fire Protection Analysis Procedure
1. Determine Density / Area requirements
2. Evaluate Possible Hydraulically Remote Areas
3. Measure out the area of operation
4. Measure out each sprinkler area
5. Calculate flow at remote sprinkler Q = density x area
then use Outlet formula to determine Pressure at that point
6. Follow a path from the remote head to the supply point for each pipe along that path:
a) Use pressure at pipe start point to calculate flow from any sprinkler
b) Add sprinkler flow to previous pipe flow
c) Use dia, length, accumulated flow and roughness to calculate friction loss
d) Use elevation differences between pipe ends to calculate elevation pressure
e) Calculate pressure at other end of pipe: P 2 = P 1 + P e + P f
1. Determine Density / Area requirements
2. Evaluate Possible Hydraulically Remote Areas
3. Measure out the area of operation
4. Measure out each sprinkler area
5. Calculate flow at remote sprinkler Q = density x area
then use Outlet formula to determine Pressure at that point
6. Follow a path from the remote head to the supply point for each pipe along that path:
a) Use pressure at pipe start point to calculate flow from any sprinkler
b) Add sprinkler flow to previous pipe flow
c) Use dia, length, accumulated flow and roughness to calculate friction loss
d) Use elevation differences between pipe ends to calculate elevation pressure
e) Calculate pressure at other end of pipe: P 2 = P 1 + P e + P f
stookeyfpe
Specifier/Regulator
This is a CMSA design so it does not use the conventional discharge density/design area method. Also, it is inappropriate to reference some dribble out of an Autocad manual when the governing criteria is NFPA 13.
In this case the designer determines the 20 most hydraulically most remote sprinklers and calculates the design so that each of the 20 sprinklers has a minimum discharge pressure of 50 PSIG. This is a more demanding design because it is based on a minimum pressure rather than flow rate.
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- #12
This is on example of a cmsa head
Maybe looking at the manufactures design criteria might help answer your questions
Maybe looking at the manufactures design criteria might help answer your questions
TravisMack
Mechanical
As I stated above, if you are on a gridded system, each head will be very close to your 50 psi for all 20 heads. If you are on a tree type or looped system, then only your most hydraulically remote head will be at 50 psi. The others will increase based on friction losses of the piping.
Travis Mack
MFP Design, LLC
Travis Mack
MFP Design, LLC
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