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residual pressure/k-factor 5

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twd22285

twd22285

Specifier/Regulator
Jul 31, 2007
73
I am new in the water industry i just started work for an overhead fire sprinkler company in the design department. For the majority its coming fairly easy, but some of the "simpler" terms are confusing to me i still have not found a good explanation for k-factor and residual pressure. Any help would be much appeciated. Thank You
 
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I will take a stab at it:

The k-Factor is simply a constant. It is related to the size of the orifice in the sprinkler. I assume it was derived from empirical data. If you are familiar with the ideal gas law, PV=nRT. R is simply the gas constant that was derived from empirical data.

Residual pressure is simply the pressure remaining in the piping system while water is flowing out of the system, whereas static pressure is the pressure in a piping system when no water is flowing.

I don't know if this helps or makes you more confused. Some of the more intellectual guys on here can probably give a better explanation. This is basically how it was explained to me many years ago when I started in fire sprinklers.
 
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K-factor = coefficent of discharge. From a fluid dynamics it based on Bournelli's law and the vena contracta of an orifice.

The larger the diameter of the discharge orifice, the larger the k-factor. The smaller the diameter of the discharge orifice, the smaller the k-factor.

K-factor is important because it used as the basis for calculating the flow of the sprinkler using equation:

Flow [Q] = (k) (square root of minimum discharge pressure)

K-factor is established by the manufacturer of the sprinkler and confirmed by the testing laboratory. The k-factor can be modified by the designer in cases where the diameter and length of pipe nipple is used as the basis for the calculations.

As to residual pressure, see the following answer:

 
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THANKS TRAVIS MACK AND STOOKEYFP THAT HELPS A LOT I REALLY APPRECIATE YOUR INPUT. I UNDERSTAND THEM BOTH A WHOLE LOT MORE THAN I DID YESTERDAY AT THIS TIME. IT WAS FRUSTRATING DOING THE HYDRAULIC CALCULATIONS AND DEALING WITH TWO TERMS (K-FACTOR, RESIDUAL PRESSURE) SO FREQUENTLY AND NOT UNDERSTANDING THEM. THANKS AGAIN
 
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Oh be still my heart!

Are your mentors having you perform hydraulic calculations by hand before they give access to a computer program?

For your sake I hope they are.

I first started hydraulic calculations back in the early 1970's before personal computers were around and, like many of my contemporaries, I got pretty good at trees (easy), loops (not all that hard) and even simple ladder grids (it is doable).

I know it is frustrating at first but down the road you will thank them many times over. Being able to calc a system using a $20 pocket calculator, a piece of ^1.85 graph paper and worksheets will benefit you in ways you can't imagine. The biggest benefit will be when you have a problem on a computer program you'll know right what to look for in finding a solution instead of haphazardly increasing pipe sizes until "something" works.

A while back I started a project and someday I am going to finish.
 
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From an engineering point of view the residual pressure term is not very neccesary, but the term is more related to fire figthing operations hydraulics, in which they feel the neccesity to make a difference when the water is flowing and when it is static. When performing hydrant flow test the different terms "static" and "residual" are useful too.
Related to sprinklers, if you continue on this field, don´t worry, you will soon be using this term as an expert, even if you don´t feel a real need to use it.
 
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"Are your mentors having you perform hydraulic calculations by hand before they give access to a computer program?" ANSWER: Unfortunatly no. but i'll ask to do so on my next calc project, instead of using haas on computer do it for me. thanks for your input all of you much appreciated.

tony
 
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The statement previously made by DAVIDCR "From an engineering point of view the residual pressure term is not very neccesary, but the term is more related to fire figthing operations hydraulics, in which they feel the neccesity to make a difference when the water is flowing and when it is static." is INCORRECT.

I am a FPE and I just wanted to make sure everyone knows this statement can cause confusion. Anyone who knows even a little bit about water-based fire protection system hydraulics is aware that the available residual pressure at various flow rates is critical when engineering and/or designing the systems. An incorrect water flow test (which indicates the residual pressure is higher than the actual residual pressure for a given flow rate) can lead to SERIOUS issues down the road!
 
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FFP1 is correct.

I've done a lot of jobs over the past 32 years and, like everyone with time in the trade, I've had a few problem jobs.

I've had to reroute main because of conflicts with duct work, I've hit steel beams I didn't pick up on the drawings and I have suffered sprinkler fitters who just didn't care about craftsmanship.

But the most challenging problems I've had to deal with is when the residual pressure I thought was there wasn't.

With a good residual water pressure you can overcome about anything but it takes what seems to be an unlimited supply of money to overcome a lower then expected residual pressure.

 
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I am working with a 21 story high rise and I would like to get an idea what should my system pressure be at the pump while sitting in the off position. I have been told many conflicting answers that confused me to say the least. If my setting PSI is ? what should the standpie PSI be on the 21st floor? There are no drawings to indicate a true design PSI. The building is 25 years old and engineered drawings are scarce. Is there an equation that can help me determin this and is there some rule of thumb guidelines?
Thanks
 
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Pressure = 0.433 (H) where H is the height of the building in feet.
 
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If u are asking what pressure the pump should read at rest the answer is it depends on what the city pressure is and what the fire pump churn pressure is. For example if the city pressure is 40psi and the fire pump is rated at a churn pressure of 120 psi, then when the fire pump operates the discharge gauge will read about 160 psi ( 40psi + 120 psi). The jockey pump would be set to maintain the system pressure at about 160 psi. If u look inside of the fire pump controller the mercoid pressure switch should be set to start the pump at about 150psi.

The jockey pump controller should be set to come on at 155 psi and go off at 160 psi. The 21st floor pressure gauge will be about 69 psi(21 floors if 10' high per floor is 210' times .433 = 90.93 call it 91 psi - 160 psi = 69 psi.
 
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Thank you folks for your input. I can certainly understand why people use this posting option.
CMurry
 
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Hello every body I have been studing all UK British Standards related to the fire sprinkler design but I couldnt get the following answer (for light hazard like residential building):
-residual pressure of the system (2.2bar? @ 60L/min in the most unfavourable sprinkler head if it stands alone? or 2.2bar? @ 40L/min x 2 =80L/min when two sprinkler heads are open in the most unfavourable branch of the system)
-2.2bar?


 
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Start a new thread. We in the US are adjusting to SI units. Your post makes something ugly even uglier.

And give us a link for the UK standard. We are all from America and most are from a time zone that includes the phrase "you want me to do what?"
 
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Helpful Member!FFP1 (Mechanical)


he is right!!!

the water flow information ( static, residual, and flow) are used to size the pipe

the k-factor is a ratio that is based on volume and pressure
 
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