12 replies to this topic

[Leinster]

Dear All,

I am new to designing fire fighting systems and I would like to know the relationship between Kv Flow Coefficient and the K-Factor?

I am trying to determine the pressure and flow which can pass through the sprinklers however cannot find any information on Kv and everywhere just references K-Factor.

 

Any assistance would be highly appreciated.

[breizh]

K= 2.1487e-3 *D (mm) /CV^2         K=894*D(in)^4/CV^2  

 

and 

 

• C_v = 1.16 K_v         (1)
• K_v = 0.862 C_v         (2)

Hope this helps

 

Breizh

[Leinster]

Thank you Breizh. 

I presume you take the bore of the sprinkler as the D? The Kv seems very low which drastically increases the pressure loss in the system. 

If I take a K25 sprinkler with a 4mm bore and I rearrange the same formula the Cv comes out as per below;

 

Cv = SQRT(2.1487e-3*D(mm)/K

= SQRT(2.1487e-3*4/25

=.018542

 

Kv = 0.015983

 

Please advise as I am sure I must be missing something.

[breizh]

http://kb.eng-softwa... from Cv or Kv

 

I guess your math is correct , additional reference.

Breizh

[katmar]

The fire fighting and sprinkler industry uses some slight variations on the usual definitions, and this can lead to confusion. 

 

In normal piping work we use the K-value terminology to represent the resistance coefficient of a fitting.  This is the multiplier that is used with the velocity head to calculate the pressure drop.  A high K-value represents a high resistance.  For Valves we use the Cv terminology.  Cv is a flow coefficient and a high Cv represents a low resistance.  Cv is the American flow coefficient for valves, and the Europeans use Kv which is a similar concept but using different units.  I have written a detailed description of all of this at

http://www.katmarsof...essure-drop.htm

if you want the full details.

 

In the sprinkler industry they also talk of a K-factor, but unlike normal piping where K is assumed to be a resistance factor, in the fire industry K is a flow coefficient.  In the USA the definition of K is exactly the same as for a valve Cv, except that in the fire industry the fluid is always water so they leave out the SG from the definition (since it is 1 for water).  In the USA you will find people using the formula Q = K x √P where Q is in USgpm and P is in psi.

 

Just to make it doubly confusing, in Europe (and South Africa and Australasia and probably others) a "metricated" K value is used by the fire industry.  They use the same K-factor terminology as the Americans and the flow formula is exactly the same but the units are litre/minute for Q and bar for P.

 

If you get a spray nozzle K-value from a US catalog you must multiply it by 14.4 to use it with "metric" units.

 

[Leinster]

Katmar,

So if the CV is the same as K-Factor in the US then surely to calculate Cv from a European K-Factor you just divide by 14.4?

[katmar]

Yes, the inverse of multiplying is dividing.  Just be sure that your European K-factor is a nozzle K-factor and not a resistance coefficient.  I have not seen nozzle capacity vs pressure drop characteristics expressed in terms of resistance coefficients, but just be careful.

[Leinster]

Hi Katmar,

 

I would just like to revisit this seemingly simple equation because the pressure drop seems to be far too high if I divide the -European- K-Factor by 14.4. For example if I want to model using K-Factor 25 sprinklers this gives a Cv 1.6667 which results in a massive pressure drop.

 

Any assistance would be appreciated.

[katmar]

A sprinkler with a European K-factor of 25 would be very small.  It is likely that in a European catalog from a USA based manufacturer they would still call a sprinkler with an American K-factor of 25 a K25 unit.  That is just its model number.  Can you give a web address for the nozzles you are using because it is very hard to comment without knowing the details of your nozzles.

[Leinster]

Katmar,

 

Please see below link to the sprinkler data sheet.

 

http://www.angussprinkler.com/utcfs/ws-3198/Assets/WS020%201001%20A.pdf  

[katmar]

From the data sheet it certainly seems that the K values listed are metric ones.  You would probably not get a 1/2" nozzle with an American K of 25.  I think the best thing would be to send your calculations to the Angus company for verification.  The sort of nozzle you have shown is not what I would call a fire-fighting nozzle.  I have seen that type used where the aim is simply to keep a tank cool when neighboring tanks are burning.  The name of the nozzle seems to confirm that.

[Leinster]

Many thanks.... I have tried contacting Angus but they have been most unhelpful hence why I have tried to post here (you have been considerably more helpful)! They just claim there is no pressure drop which is clearly not true!!!

 

You are correct and my apologies for the misinformation, we are using them on a cooling ring for a storage tank.

This is a European K-Factor however you had earlier said to calculate the Cv from a European K-Factor I must divide by 14.4?

[katmar]

I understand your frustration with the supplier.  All too often the reps are untrained and uninterested. Last month they were selling biscuits and next month they will be selling second hand cars.  Every now and then you find an exception to the rule, but they are few and far between.  In similar instances I have found more knowledge and interest from the insurance people.  The supplier just wants your order and your money.

 

I still maintain that to get the Cv (or American fire nozzle K-factor) from a European K-factor you must divide by 14.4. See  http://www.canutesoft.com/basic-hydraulics-calcs/k-factor-formula 

If you need more flow (or less pressure drop) then maybe you should get the K80 version of this nozzle.