Calculations for restricted orifices to lower pressure below 100 PSI in a 1 1/2" Fire Hose. 3

[Biker Ben]

Hi everyone,

This is a fire protection question relating to NFPA 14 however, the fluid dynamics remain the same. There is code in NFPA 14 that limits the residual pressure at any 1 1/2" fire hose to 100 psi. In the old days, restricted orifices would be installed after the 1 1/2" hose valve at the hose inlet to restrict the pressure.

The restricted orifice inserts are no longer the recommended method. However, a friend of mine is the Fire Technician at a hospital with over 700 of these restricted orifices installed. During a recent inspection, it was discovered that some of them were missing. He asked me what calculations would have been used during the original system design to determine the size of the orifice.

My initial thoughts are that Bernoulli's Equation would be used because the increase in velocity caused by the restriction is resulting in lower pressure upstream of the restricted orifice (Conservation of Energy). Does anyone have experience designing systems that would have used this method? Again, the recommended scenario would be installing 1 1/2" Pressure Restricting Valves set as per the manufacturers' specifications. However, that is a very costly alternative compared to replacing the missing inserts from the original system design.

Any information is greatly appreciated. Thank you in advance.

Ben

 

[hacksaw]

restriction orifices do just that, limit the flow only.

your header line pressure is what needs attention and is set far up stream.

 

[Biker Ben]

The header pressure is set to meet the demand of the sprinkler system via a fire pump and jockey pump. The only issue is restricting the pressure directly as the 1 1/2" hose valve to a maximum of 100 PSI and a minimum of 65psi. Today this is done using a pressure restricting hose valve with variable settings. In older systems, restricted orifice inserts were used and yes they did reduce residual pressure at the hose inlet. I am thinking because the velocity would increase as water flowed through the restriction and therefore, pressure must temporarily decrease as per the conservation of energy.

I appreciate the comment but, I am looking for an intelligent response with a useable calculation. I did not design this system. I am helping a friend meet code requirements on an existing system.


Thank you,

Ben

 

[3DDave]

Pressure decreases due to friction energy losses - restriction orifices convert the excess pressure and flow into heat. At lower than design flow they will produce lower than design pressure drop, which is likely why they are no longer preferred.

http://www.emcocontrols.com/393/restriction-orifice-plates.

 

[LittleInch]

Forget bernoullis equations for this application. Yes increased velocity means lower pressure, but then velocity reduces and pressure increases. Bernoullis equation is frictionless, where as friction is what you need.

there are many equations for restriction orifices just search on this site and you'll get a lot of hits.

a key factor is whether you're in critical flow or not depending on your header pressure and what pressure you can see downstream - i.e. fow through each hose. If this changes then the size of the RO will also change.

Perhaps your friend should just measure the existing ROs and find if they are all the same size or vary and if so is there a pattern?

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

 

[casflo]

The orifice inserts you refer are restriction orífices that have high values of t/d (t, thickness and d hole diameter). As it is very difficult to find technical papers about the design of these orífices, I suggest you that try to design an orifice plate according to the Crane Technical Paper No. 410.

 

[Biker Ben]

Thank you to everyone who responded. I apologize, I am not in front of a computer during the day. I only see these responses at night.


I did search the internet and haven't found a calculation that I am confident with. In Sprinkler Design we use Hazen-Williams or occasionally Darcy-Weisbach.

Yes, the logical thing would be to copy the existing orifices, which should be similar at least per floor. I also suggested he do a practical test and find the residual pressure without and with a few different orifice sizes at the closest and most remote fire hose cabinets. The code is meant as a safety measure so that no one is hurt operating the fire hoses at high pressures.

I will also check out the Crane information.

That was my first time posting. Thank you all very kindly.



Ben

 

[georgeverghese]

The RO option would only restrict hose pressure when the hose is running at design rate. In all other cases, at low or no flow, the hose may be overpressured. Other solutions which would limit hose pressure at all times would be
a) Reduce pump delivery pressure to 100psi, or install a recycle PCV at pump discharge set at 100psi
b) Install a forward sensing PCV on a batch of say 4-5hoses, where the piping setup allows this
c) Install a forward sensing PCV at each hose connection

 

[katmar]

An old fashioned restriction orifice and a modern pressure restricting hose valve work on exactly the same principle. The difference is that the restriction orifice is a fixed size and has to be designed for average or worst case conditions, whereas the pressure restricting hose valve will adjust itself to the actual conditions. As various hoses are brought on or off line the header pressure will vary and the self adjusting valve will perform better.

Be careful when using the Crane equations for sizing orifices. Although the Crane TP410 manual is primarily about designing piping systems, its treatment of orifices concentrates on their use as flow metering devices. There are two pressure drops of interest across a thin orifice. For metering purposes the tapping points are usually installed 1 pipe diameter upstream and half a diameter downstream because these points give the maximum differential for a given flow and therefore allow the most accurate measurement of the flow. If the downstream tapping point is moved further downstream (>10 pipe diameters) significant pressure recovery is observed. This second type of pressure drop is known as the permanent pressure drop and is what is important for piping systems design. IMO it is not that clearly distinguished in Crane.

A good reference for the permanent pressure drop across thin and thick orifices is the article "Calculate head loss caused by change in pipe size" by WB Hooper (he of 2K fame) in Chem Eng, Nov 7, 1988, pages 89-92. These are the formulas I chose to use in AioFlo. You might find a copy of this paper floating around on the internet if you search carefully. Or, as LittleInch has observed, a search of Eng-Tips is sure to find many references for restriction orifice sizing.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

Try this one

http://www.blackmonk.co.uk/calculations/ROPLiqCalc.aspx

or

https://www.eng-tips.com/viewthread.cfm?qid=357490

as an example on this site

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