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.

 

[LittleInch]

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.

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[az5333]

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.

 

[LittleInch]

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.

 

[az5333]

Hi LittleInch.

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

 

[1503-44]

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.

 

[az5333]

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.

 

[LittleInch]

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.

 

[1503-44]

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.

 

[az5333]

Thanks 1503-44 and LittleInch for the helpful responses.

 

[bimr]

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.

 

[az5333]

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.

 

[LittleInch]

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_612.html

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.

 

[bimr]

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.

 

[bimr]

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.

 

[LittleInch]

Not quite there yet.....

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

 

[az5333]

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?

 

[LittleInch]

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.

 

[1503-44]

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.

 

[az5333]

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.