Use of Orifice Plates for Pressure Reduction

[DTR]

Hi all,

Your considered advice would be welcome on the following:

We have Power Plant with a huge Gas Oil Storage Area with Cooling Systems giving a demand of around 25000 USGPM, this demand is supplied via 5 No. 20% Duty Fire Pumps.

Supplied from the same Underground Ring Main we have smaller Deluge Systems with only a fraction of the demand, we are therefore getting a lot of overpressure at the smaller systems, my question is can we use Orifice Plates upstream of the Deluge Valves within the Headers to absorb the excess pressure experienced on the smaller systems?

NFPA 13 & 15 advise against using Orifice Plates for balancing, but we are not balancing systems we are reducing excess pressure, or from your experience are they both the same in accordance with NFPA definitions.

We have around 60 Systems supplied from around 25 Headers so if we have to put Listed Pressure Reducing Valves on each header it will be very costly, however if we need to do this to be code compliant then so be it!

Your advice would be appreciated.

Thanks

Dave

 

[SprinklerDesigner2]

The short answer is no.

What are you calling overpressure, you getting static pressures above 175 psi?

 

[MGXFP]

At what pressures does your UG operate? What sort of pressures are you experiencing on your deluge systems?

Just out of curiosity, what size main are you flowing 25K GPM through? It must be quite large to maintain a reasonable velocity.

 

[cdafd]

lloks like you would have to use it before the dleuge, unless it has a rating of over 175 psi.


can you use something like this??

http://www.cla-val.com/brochure/B-Fire_Products_Brochure.pdf

 

[cdafd]

Other suggestion is hire a great fire sprinkler company to advise you, or a fire protection engineer that has delt with a facility similar to yours.

 

[TravisMack]

I think by over-pressure, he is referring to what is typically referred to as over-discharge in a sprinkler system. This is commonly seen with racks and some deluge systems.

This is not so uncommon in high piled storage areas. In the past, we may have a 4" main going to feed in-rack sprinklers, but to cut down on the over discharge, we would put in a short piece (maybe 24") of say 2" pipe. This would eat up a lot of the pressure and then reduce down on the over discharge of the in-rack sprinkler heads. This was more prevalent when you only had K8 or K11.2 sprinklers. Now, with heads going up to K25.2, you don't see this as much any more.

I hope this makes sense. I don't think you can use an "orifice plate", but I don't see why you can't stick a relatively short piece of a smaller diameter pipe in the supply line to help eat up some pressure. As long as it calcs out, why not?

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[DTR]

Hi all,

Thanks for your replies, thanks for the short answer Sprinklerdesigner, but I still think there may be some clarification in the definition of balancing and the use of orifice plates, we are not hydraulically balancing systems that operate simultaneously, we are reducing pressure on stand alone systems, but it looks like the correct way is PRV's, which is not good for our pocket!

To answer some of the questions raised as follows:

1.Sprinklerdesigner - Yes we are potentially looking at static pressures above 175psi,but we are at an early design stage so this will be addressed.

2.NewtonFP - We are getting around 7 bar at the Water Spray Nozzles, way to high! The Underground Main is 20",16" & 14" with 6" & 8" branches to Deluge Valve Headers.

3.cdafd - The Cla-Val PRV is a good valve I have used it previously as is the Tyco & Singer PRV, but I have so many headers it is a cost issue.We are already a great Sprinkler company (at least I think so!) who specialise in Fire Systems on Power Gen sites We are just stuck with our customers poor infrastructure design with the Underground Main, had we had the chance to have a say earlier in the design of the Project we would have supplied the Gas Oil Storage Area with a 20" diameter Main and the rest of the site with a smaller Underground Main to suit the smaller systems on the power islands.

Thanks again for your replies,much appreciated.

Dave

 

[DTR]

Travis,

Thanks for your reply, we used to call the short pieces of smaller diameter pipes "choker pipes" but it has been so long since I used them I never gave them a thought for this job!!...in essence they work in the same way as an orifice plate,as you state they eat up the excess pressure,even if the Calcs work... is the use of these code compliant?

Thanks

Dave

 

[TravisMack]

A quick google search yield a definition of orifice plate:
"An orifice plate is basically a thin plate with a hole in the middle."

I don't think that a length of pipe would be the same as a thin plate with a hole in it. We would go from say 4" to 2" and back up using concentric grooved reducers. However, the length when a pipe becomes a plate would be open for debate.

I will be interested in hearing other's opinions on this as well.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[cdafd]

brek out the sprinkler underground and reduce it off a tap with some type of pressure redcuing valve, so you are feeding multiple systems off one tap.

you may need to do this a few times, but for the price of some ug pipe and labor, might be a thought.


so what is the sprinkler company suggesting?????

 

[Cidona]

DIR,

I have come accross orifice plates being used on LPC projects. The reference in NFPA to orifice plates seems to only be addressing the orifice of sprinklers.
When I have used orifice plates in the past (on LPC project), it was to balance then ceiling protection with the racks. The pressure in the racks was forcing more water out of ceiling protection than what was needed. Adding the orifice plate reduced the flow to the ceiling with the rack flowing at demand.
One of the things that will help this discussion is proper terminolgy. Installing the orifice will reduce flow, not pressure. Your static will be the same.
BTW, as far as I would be concerned if you are playing with pipe sizes to make the the calc work, you are trying to balnce the system/calce.
I don't think the choker will reduce pressure. If you are counting on the friction loss through the 'choker piece'(AKA orifce plate)to actually reduce the pressure via friction loss, are you figuring on the full demand? If so then what is antisapated when there is less the the full remote are flowing? More pressure than your calc.
As I understand it, the orifice plate is only of use to reduce flow. Which would usually only be desirable to help balance the calc when you are flowing 2 different design area similtaniously. e.g. ceiling and rack. You mentioned that you are not looking to flow areas similtanioulsy so I don't think they will help you out.
If you need to reduce pressure I think you need a PRV, like the CLA-VAL cdafd suggested.
If you need to balance 2 different areas and you are at the prelim plan stage, I'd sat make all the main smaller rather than putting in a choker.

 

[DTR]

Hi all,

Thanks again for your input,Cidona I too have experience with LPC Sprinkler Projects , in particular the use of orifice plates for balancing the roof and rack systems as you state, however these are Deluge Systems with a fixed area and density therefore the demand will not vary, my thoughts on the orifice plates were purely in a flowing condition not static, the reductioon in pressure I am looking for is at the Spray Nozzles as it is way above what is needed as the Pumps are sized for a demand 20 times bigger in the Gas Oil Storage Area!

I think the correct way forward is PRV's in the Headers or feed mains to the Headers to the smaller Deluge Systems and Sprinkler Systems.....now the fun starts...who will pay!?

Thanks again for your thoughts.

Cheers

Dave

 

[Cidona]

Dir.
I would be trying to avoid the PRVs (as I understand you also are doing by posting this topic). Due to Cost, Maintenance and system reliabiity factors.
There are a couple of items that I am unclear about in regards to your situation.
1. What is the problem with the that you are trying to prevent with the deluge systems? Is it that you are trying to reduce the FLOW so that your tank size works; or are you trying to reduce PRESSURE for component pressure listing requirements? Or something else?
2. Are the deluge systems and bulk mains existing?

 

[Cidona]

BTW, had a brain fart in my post ealier about less heads going off than you calc'd (since you're dealing with a deluge, they're all flowing), so you would be able to deal with a known friction loss(predictable range). So that would seem to increase the posibility of the choker pipe working.
Also, I think I'm changing my mind on my vague statement about what constitues 'balancing'. I think (that doesn't mean much BTW), that if the oricie plate is upstream of all flowing devises then it would not be considered balancing.