Low Pressure at Pump Inlet

[Oremus]

I'm working on this project with a booster pump on city supply. I know municipalities like to keep at least 20 psi in their systems - our system is fine there at the point of connection but at our pump inlet calculations are showing 2 psi available after friction losses. there's about 30 psi at the city tap location. I've never came across this and was wondering if this acceptable?

 

[stookeyfpe]

2 PSI at what flow? Is this at 140% NPSH or 100% of the pump's rated flow rate at "x" PSIG?

Also, is this a fire pump for a fire protection system or a pump used as part of municipal water distribution system? If its the later, you may want to post this question in Civil/Environmental Engineers forum.

 

[Oremus]

This is a fire booster pump flowing 1800 gpm. There's roughly 30 residual psi available at the city tap while flowing this quantity of water. After friction losses of flowing through about 1400 feet of 8" c-900 calculations are showing only 2 psi available at the pump inlet before being boosted.

Sorry for the confusion - hope this clarifies.

I'm thinking the existing 8" should be upsized to at least 10" but I'm wondering how much of a problem this really is as long as there is positive pressure at the pump intake.

 

[SprinklerDesigner2]

I'll go out on a big limb playing engineer when I am not one but for what it is worth.

I'm in Georgia and here is what the Georgia Environmental Protection Division says about fire pumps taking suction from city water supply.

http://img34.imageshack.us/img34/1833/lowsuctionfirepumps.jpg

I know from experience Ohio requires the same and if you dig I believe you will find every state requires a minimum of 20 psi suction pressure at 150% of pump rated capacity because federal clean water standards require it.

If the system requires 1800 gpm the smallest pump you should be using is 1,500 gpm. 1,500 gpm @ 150% = 2,250 gpm. Your suction supply must be large enough so suction pressure does not drop below 20 psi anywhere in the suction pipe with the pump opeating at 150% of capcity.

I'll check if you want but off my head I don't think 10" will do it.... you'll probably have to go 12".

Any elevation difference between the pump suctin and tap or connection to city water main? Be careful, 10' over 1,400 feet doesn't look like much but it means a lot.

Using C-900 exactly what are you using? DR-14? DR-18????

Ok, you got at least a tee, gate and two elbows plus fittings to the pump suction flange anything else (like a backflow preventor perhaps) on the suction side of the pump??? This is a real big deal because remember, the 20 psi minimum requirement is anywhere in the pump suction like and that goes all the way to your pump suction flange.

I'll look at it some more when I get done with an errand.

Any of you PE's out there see anything where I can be wrong?

 

[SprinklerDesigner2]

Clarification.

I am not sure all states require a minimum 20 psi suction pressure with the pump operating at 150% but I do know all of them that I have been licensed in do.

Ohio does and so does Georgia.

As an example the Georgia Fire Marshal at the top of page 27 of their rules and regulations

http://www.gainsurance.org/External...e Safety Standards - effective 02-01-2007.pdf

has this:

(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 know the state fire marshal will never allow the exception. He might allow 19 psi at 150% but there you are pushing for it.

It's been my experience something similar to this is buried in somewhere in nearly every state code or regulation. You might have to dig but I'll bet it is somewhere.

Flowing 2,250 gpm the friction loss through 1,400 feet of DR-14 (ID-7.98" C=150) your friction loss is 40.02 psi and this doesn't include equivalent fitting lengths or a backflow preventor on the suction side should you have one.

Using 8" you're pulling a negative suction of about 10 psi.

We need to remember the 20 psi reequirement isn't confined to city lines it includes the private suction lines too... if a rubber gasket in your line collapse it's going to pollute the city line just the same.

Using 10" DR-14 (ID=9.78") the friction loss will be 14.92 psi which allows approximately 5.0 psi at the pump suction flange with the pump operating at 150%.

Using 12" DR-14 (ID-11.64" C=150) the loss is down to 6.41 psi which leaves a suction pressure of approximately 23.5 psi at the pump suction flange with the pump operating at 150%. You are OK here as long as you don't have a lot in the way of equivalent fitting loss or have a backflow preventor on the suction side of the pump.

Also remember I used a C-Value of 150. You, we, shouldn't.

I saw somewhere once where a series of tests was conducted on newly installed PVC pipe and the actual measured C-Values were closer to 140 and 145. In this case, 1,400 feet of 8", the difference between a C-Value of 150 and 145 is a whopping 2.6 psi. Tell you what, if the calculations were mine to do I would use a C-Value of 140 just to be sure.

And by all means watch that backflow preventor if you have one. I've measured friction losses through lots of these over the years, using extremely accurate test gauges, and so far I've never had one where the head loss was what the literature says it would be. It has always been higher and sometimes higher by 3 or 4 psi.

I've made a lot of mistakes over the last 35 years. I've run into beams, missed ductwork and I'll have more mistakes as time goes on. Big deal, I can deal with these, these kinds of mistakes can be fixed esily enough but the one mistake I never want to deal with is having a fire pump with not enough water. That can get nasty and expensive to fix and sometimes there might not be a way out.

If you want to get cheap get cheap on the pipe downstream the fire pump... not the suction side.

That's my two cents.

 

[SprinklerDesigner2]

And one last thing having to do with the tapping valve and sleeve.

Tapping sleeves are not tees.

When doing an 8" tap when they pull the coupon you're going to find it is considerably less than 8" in diameter and it wouldn't surprise me in the least to discover the actual equivalent fitting length of one of these puppies is as high as 300 or 400 feet.

 

[Oremus]

SDesigner2,

Thanks for your input.

One thing I noticed is that what you were saying about the entire suction line not being allowed to fall below 20 psi was at odds with the 'Designer's Guide to Automatic Sprinkler Systems', Robert M Gagnone which stated the 20 psi was a typical number of most authorities but that number was only required at the city main location.

Upon further investigating I found in NFPA #20 ('07 ed) the following:


5.14.3 Suction Size.

5.14.3.1 Unless the requirements of 5.14.3.2 are met, the size of the suction pipe for a single pump or of the suction header pipe for multiple pumps (operating together) shall be such that, with all pumps operating at 150 percent of rated capacity, the gauge pressure at the pump suction flanges shall be 0 psi (0 bar) or higher.


I guess I kind of answered my own question. Although your approach is a more cautious one and I appreciate that.

 

[SprinklerDesigner2]

"One thing I noticed is that what you were saying about the entire suction line not being allowed to fall below 20 psi was at odds with the 'Designer's Guide to Automatic Sprinkler Systems', Robert M Gagnone which stated the 20 psi was a typical number of most authorities but that number was only required at the city main location."

Be careful.

Whatever state or locality this installation is in I would attempt an exhaustive search on whatever water distribution rules they may have.

Just two examples.

Found this for Tennessee.

http://www.state.tn.us/commerce/sfm/documents/firepumpshopdrawings.pdf

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]

The Montana EPA

http://www.deq.state.mt.us/wqinfo/Circulars/DEQ3.PDF

on page 22

8.1 WATER MAIN DESIGN
8.1.1 Pressure
All water mains, including those not designed to provide fire protection, must be sized after a hydraulic analysis based on flow demands and pressure requirements. The system must be designed to maintain a minimum normal working pressure of 35 psi. Maximum normal working pressure should not exceed 60 psi. Minimum pressure under all conditions of flow (e.g. fire flows) must be 20 psi. Minimum required pressures must be based on those occurring at ground level at the highest building sites or fire hydrant served by the proposed water mains excluding service line head losses."

"Fire flow"? Lots of discussion here on fire flows.

City of Boise, Idaho.

http://www.cityofboise.org/Departments/PDS/PDF/Applications/FireApps/FIREApp_Sprinkler-Policies.pdf

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.

Another thing to consider is a possible adverse impact a smaller 8" line may have to the total project cost.

If a small building probably no impact but for a large installation, say 100,000 sq. ft. or multiple buildings, that 30 or 40 psi you are losing on the suction line could more than be made up by having the addiitonal 40 psi available for the systems.

8" PVC will cost somewhere somewhere around $4.50 a foot while 10" might run $8.00 I'm guessing on the 10") which means 1,400 feet will cost an additional $5,000.

$5,000 isn't much money. The trench will still be a 24" wide trench so I don't see an increase in excavation costs and an extra $1,000 for fittings should more than make up the difference in fitting costs.

In a worst case scenario the added cost for the 10" suction line might be $7,000 or $8,000?

Assume a 400'x500' warehouse with heads on 10'x10' spacing. We're going to have 20,000 feet of branch line and if by having the extra 40 psi you can reduce the branch lines from 2 1/2" to 2" sch. 10 pipe the savings in the pipe ($1.80 vs $1.35) alone would be $0.45 per foot or $9,000 just for the pipe.

On couplings, assume 20' average pipe length with an extra 2 couplings per line for 300 couplings ($4.53 vs $3.95) would be another $175 and on top of this add the cost of branch line elbows etc.

Added to this would be the real added possibility of downsizing mains and risers. Of course if the facility is 10,000 sq. ft. this really isn't a factor but if it is any size at all I can easily see the cost savings achieved downstream the pump more than offset the added cost of a larger suction pipe.

Whatever happens with 40 psi more to work with the overhead pipe in the facility will be smaller and for something this size it isn't hard to envision a total overhead cost savings of $18,000 to $30,000 once you consider the additional fabrication cost and installation labor associated with larger pipe.

The larger the facility the more cost savings I envision.

Something like this would be a win-win all the way around for everyone. A superior system for less cost. Can't beat that.

In any event I would be very concerned about whatever suction pressures may be required by other authorities outside the NFPA standards.

Now that's my three cents worth.