Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations IDS on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Problem with a project and in need of input from a FPE or knowledgeable PE.

Status
Not open for further replies.

SprinklerDesigner2

Mechanical
Nov 30, 2006
1,251
Several years ago Stookey provided me a contact but it appears their contact info is no longer valid. That said I understand the need to engage the services of a Georgia FPE or or knowledgeable PE to help me address this problem and I am looking for one.

There are two separate issues I am dealing with; one involving amendments to NFPA #13 and the other amendments to NFPA #20.

CHAPTER 120-3-3 RULES AND REGULATIONS FOR THE STATE MINIMUM FIRE SAFETY STANDARDS

First having to do with modifications to NFPA #13 beginning on page 27 with the meat of the issue on page 29:

(8) NFPA 13, 2002 Edition,
Standard for the Installation of Sprinkler Systems
Modifications:

snip

3. Add a new subsection 14.4.4.8.3 to read as follows:
“14.4.4.8.3 There shall be a minimum 10 psi (0.69 bar) cushion between the hydraulically
calculated sprinkler system demand and supply when there is a backflow prevention device present.
“Exception: 10 psi (0.69 bar) cushion may be lowered with permission of the authority having
jurisdiction.”

NFPA #20 beginning on page 33

(18) NFPA 20, 2003 Edition,
Standard for the Installation of Stationary Pumps for Fire
Protection
Modifications:
(a) Modification to Chapter 2:
1. Add a new paragraph 2-1.1.1 to read as follows:
“2-1.1.1
At 150% rated capacity or below, the pump suction supply shall not drop below 20 psi
(1.38 bar).
Exception: Suction supply pressure may be lowered upon approval of the authority having jurisdiction.”

I didn't see any problems coming on this job. I can appreciate the water supply is marginal but that is what fire pumps are made for with the understanding fire pumps don't make water but boost pressure.

Flow test results 45 psi static, 27 psi residual with a flow of 628 gpm.

Addressing the minimum 20 psi suction with pump operating at 150% know I got it as demonstrated here:

Calculation from source to pump suction flange at 150 percent of pump rated flow

Calculations showing a minimum 10 psi safety cushion

Calculations demonstrating a minimum 10 psi cushion is available

I talked with the local authority having jurisdiction about the exceptions and even though I do not believe I need either of the exceptions available it would help a lot with local official granting the exceptions if they can be convinced the system is fully adequate.
 
Replies continue below

Recommended for you

SK, you have alot going on here, but i will give you input on a few things to think about.

First of all, why are you using these older versions of 13 and 20? Is that what the ahj is using? I only work in CA, so perhaps i am in a bubble, but we are on the 2010 versions, and i would assume the 2013 versions will be adopted in the next year or so.

I have not looked up the 13 reference lately, but all of the spinkler contractors here, use the 10% reduction of the water supply. In general, I have seen water supplies get worse over time, not better, so I would be reluctant to ask for a deviation on that one. Just my personal opinion.

Re the pump you noted: "At 150% rated capacity or below, the pump suction supply shall not drop below 20 psi". I did a quick check of the 2010 version and could not find it, but i am fairly sure you can now go below 20 psi in the more recent standards. You may want to check that. Dropping suction mains to less than 20 psi at full flow (150% Q) is not that big of a deal. The mains can handle it. When i worked for FM in my previous life, i have done plenty of fire pump tests where we dropped the pressure below 20 and absolutely nothing happened. Collapsing mains, etc are old wifes tales. It may cause some interference to heavy water users on the main near you, say a dead end main, and the plant next to you is a heavy water user and you drop the pressure low. But, i have never seen, or heard of this actually occurring. I agree, its not the best situation, but sometimes it will occurr on boosters when you dont have a good water supply.

BTW, nice drawings. I appreciate your attention to detail.

This is not meant to answer your questions in full, but just some things to get the discussion started.
 
Sdpaddler50,

The state of Georgia has not changed editions yet and we still follow them.

Both the 10 psi "cushion" and 20 psi pump minimum suction pressure @ 150% of rate modifications to the standards adopted by the state fire marshal to which I included a link CHAPTER 120-3-3 RULES AND REGULATIONS FOR THE STATE MINIMUM FIRE SAFETY STANDARDS.

Personally I think it is overkill and this project would fly right through without question in any of the other dozen states I have done business in and I believe there has to be a reason the state included the ability for local authorities having jurisdiction to accept 1)less than 20 psi at pump suction @ 150% and 2)less than a 10 psi cushion.

That said I don't believe I need either exception because 1)at 150% my pump suction does not fall below 20 psi anywhere in the suction line and 2)I have more than a 10 psi cushion, almost double that actually, on the overhead sprinkler design.

If I do need the exception I need to make the local authority having jurisdiction feel comfortable about granting the exceptions.

If it fails I will never bid another fire pump in Georgia where the residual pressure available is less than 35 psi with the pump running at 150%.
 
The issue of the 20 psi is to prevent the municipal water supply piping to the fire pump from collapsing.
 
This very issue was discussed at some length back in 2009 where Tennessee accepts zero at the suction:

Fire Pump Shop Drawings - 2006 Codes – NFPA 20, IBC, and IFC
FIRE PUMP SHOP DRAWINGS

The fire pump suction piping size must be sufficient to operate at 150 percent of rated capacity with zero psi minimum gauge pressure unless the supply is a suction tank with its base at or above the pump. [NFPA 20 5.14.3]

and the City of Boise, Idaho
Safety Factors
a. All hydraulic design areas shall have a safety margin of at least 5% (cushion) on the available residual pressure at the calculated fl ow, per Idaho State Fire Marshal's policy.

b. Where fire booster pumps are installed, the maximum design flow shall be available at not less than 5 psi at the pump suction, at 150% of rated pump capacity including any inside and outside hose or other required system demands.

require less than 20 psi.

If anyone has any additional links addressing minimum suction pressures allowed across the country I would appreciate seeing them.

The whole point is that I do have more than 20 psi at 150% of the fire pump rating throughout the suction line and I do meet the minimum 10 psi "cushion" in my calculations and whoever reviewed the calculations misinterpreted what they were saying.

I think what happened is whoever interpreted the modifications combined the 20 psi minimum pump suction pressure and 10 psi cushion together so that we need a minimum of 30 psi at the pump suction which I don't believe was the intention of the modifications.

In this case the local fire official recognizes he is not an engineer

A tank is out of the question due to financial and the room just isn't there.

The thing is the building is a 42,000 sq ft 100 year old wood storage building obviously without sprinklers that the owner wants to turn into a mercantile.

yz5eu.jpg


It's going to be an antique mart so plastics is not an issue. I love old buildings like this.

Sprinklers came about with the occupancy change from storage to mercantile and the thing is if this project doesn't fly I will live the rest of my life with egg on my face and the building reverts back to being a non-sprinkled storage building in a congested downtown area.

What I need from a qualified engineer is a letter stating based on the review of the water supply and calculations it is his opinion I do meet both the requirement for the minimum 20 psi suction pressure and the minimum 10 psi cushion required by the amendments. I don't see why this wouldn't be possible but if not then a statement that in his professional opinion lowing the minimum 20 psi suction pressure to 10 psi would not adversely affect the operation of the system.

 
Chicopee - that is not correct. Mains do not collapse when the pressure drops below 20 psi.

SprinklerDesigner - I did not read your post in detail, and now i can see what you are asking. You can google Fire Prot Engr in the state your project is based in to find one. You may also want to google "Society Fire Protection Engineers - and the state" and that can give you some good leads on finding an FPE. I am registered in CA as an FPE, so i would prefer not do it. If you cant find an FPE in your state, and still need a technical opinion however, you can email me: sdpaddler50@yahoo.com.
 
Sdpaddler, I should have expounded on the 20 psi. The fire engines require that minimum pressure for their operation and when that connection is made and in the event water is still flowing to the sprinkler systemm than we are nearing a partial vacuum in the municipal water line which is not advisable since there are old water lines probably some still made out of wood; then water main collapse.
 
SD2:

If your looking for Tom Gardner, he left his former client and started his own firm: Protection Engineering Group, which is based in Hot-lanta. If you talk to Tom, tell him I said hello.
 
I'm going to play devils advocate here, because while I don't know a thing myself, I just read something from a FPE employed by the NFPA itself that is singing a different tune about this 20 psi business.



Written by Matt Klaus (NFPA Author/Senior FPE):
"The minimum [residual] pressure required by many water authorities is 20 psi (1.4 bar). Pressures less than 20 psi (1.4 bar) may create vacuum conditions in the water system piping in areas of the water system that are located at higher elevations relative to the location of the flow. This can cause possible collapse of the piping or introduce groundwater contamination through leak points in the piping. Low pressures such as 20 psi (1.4 bar) can also cause a backflow of nonpotable water through check valves, which can contaminate a potable source. While fire apparatus can operate at such low pressures, the minimum net positive suction head (NPSH) of the pump needs to be maintained, or the pump will not perform well, and damage due to cavitation can occur."

Source: NFPA 291A 2013



"What problems can the presence of a vacuum cause in pipe
systems?

When one says vacuum, one is actually referring to sub-atmospheric pressures. In case a pipe is being drained from water (whether this is done on a scheduled and controlled basis, or a rapid event such as in the case of a pipe burst), and air is not admitted to take its place, negative pressures that can cause pipe-collapse can occur. This phenomenon is especially prevalent in plastic pipes, particularly those of lower classes (thin walls) and to large diameter, thin wall pipelines."

Source:



According to this information - and I'm only going off of what I'm reading, not claiming any empirical knowledge, it seems plausible that large diameter thin wall plastic piping in particular is susceptible to collapse due to negative pressure in certain situations and that if you got in the habit of using close to or below 0 psi when your pump was at 150% rated capacity even with AHJ approval, you may damage or decrease the effectiveness of the pump as well as contaminate potable sources of water elsewhere down the line, as well as cause more inefficiency when a fire truck pumper is using a hydrant on the same network. I know that this is the unpopular side of the discussion, but I thought both sides deserved a voice..
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor