Why Suction of Fire Pump Needs 10 Diameters?

6/29/2020

We have a fire pump room which is very constrained.

NFPA 20 (Edition 2016), A.4.21.1.2 (2) For horizontal split-case fire pumps, there should be a distance of not less than 10 diameters of suction pipe for side connection (not recommended) to the fire pump suction flange.

The main pumps have a capacity of 3000 gpm (two are electrically-driven and two are diesel-driven) with two jockey pumps at 300 gpm capacity. All are in one room. Pumps suction and discharge pipe size is 12" as per Table 4.27 (a).

In this scenario, what is the meaning of 10 diameters - is it 10 times of suction size diameter of pump (means 10x12= 120") or something else?

Why is this requirement only limited to suction and why not for discharge side as there is tapping for pressure monitoring? Any thought would be much appreciated.

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29 Comments

Wayne Ammons

6/29/2020 08:03:44 am

The purpose for the 10x distance is to prevent turbulent flow from entering the fire pump and impeller which can cause wear and cavitation over time. If your suction size is 12 inch, you will need 120 inches from the fitting flange to the pump suction flange or suction fitting flange, depending on the configuration. Figure A.4.15.6 of NFPA 20, 2016 edition shows example diagrams to better explain this.

Jim Mernickle link

6/29/2020 10:11:07 am

Does the same hold true for all suction pipe sizing's ? So that a 4" feed pipe would require 40" of straight pipe before the pump and no fittings (90's, 45's etc) in that 40" space ?

Casey Milhorn

6/29/2020 10:28:28 am

So the 10 pipe diameters, from memory, only applies to backflow preventers and fittings that are at a right angle in a horizontal plane to the suction of the pump. If your underground lead-in is lined up with the fire pump and your only change of direction is in a vertical orientation, you can have less than 10 pipe diameters. Vertical changes in direction don't cause nearly the same type of issue as a horizontal change in direction due to the way gravity affects the fluid moving through the piping. Horizontal changes of direction cause the water to rotate and have the possibility for turbulence and air pockets.
If you absolutely have to make a horizontal change in direction, you can always add some vertical piping up and back down to meet the 10 pipe diameter rule.

Louie Lincoln link

6/29/2020 10:46:20 am

10 x the pipe diameter related to the length of the proposed pipe the length has to be at least 1X the diameter of the proposed pipe. I have seen a pump that was piped directly in from the side.
The pump shaved 1/2 of the impeller off. An impeller would cost tn the range of $5K-$20k.

Thomas Jones

6/29/2020 11:27:19 am

As mentioned, it is for turbulence. One thing I have done in tight pump rooms is create a "n" shape. So from your incoming line go up than change direction down for 10x the pipe diameter.

Shaikh

6/29/2020 11:43:18 am

If there is case where o & Y gate valve has to be included within this 10 times pipe dia of suction, Do we need to exclude this length of O & Y gate valve.

Frank link

6/29/2020 04:02:10 pm

SHAIKH, the 10 diameters are measure from the flagelo of the OS&Y Valve to the suction flange.

Nguyen Thong Nhat

6/29/2020 09:06:43 pm

Disagree, 10 x Diameter requirement apply only for tee and elbow, or any fitting that cause water turbulent in suction pipe, OS&Y valve isn't one of them.

Franck

6/30/2020 02:08:42 pm

If OS&Y valve is provided, no need to maintain this 10 x diameter as there is no obstruction, i.e. no turbulence, within the pipe when the valve is open.
If you have a gate valve (which is normally not permitted per NFPA on suction side... but can still be seen from time to time) or a backflow preventer or any other equipment, then you need to maintain a safe distance of 50 ft, whatever the size of your suction piping.
The 10 x diameter requirement is only for tee or elbow that are horizontally perpendicular to the plane of the fire pump. No need for vertically perpendicular (NFPA 20 has good drawings explaining what is correct and what is wrong).

Frank link

6/29/2020 04:03:36 pm

the Jockey puma shall be 30 GPM not 300 GPM, please check this capacity.

Sunil kumar

6/30/2020 11:54:42 am

Jockey pumps requirements are based on local client preference, so it is 300 gpm in this case.

Nguyen Thong Nhat

6/29/2020 08:52:06 pm

...there "should" be... It's mean you can follow or not follow it. Because Fire pump often run in a short time, so, if your pump room is small, you can pass this requirement. You also can find the explain that why it's should be 10x Diameter in ANSI/HI 9.6.6 figure 9.6.6.3.3b

Nimal Tissa Wijetunga

6/30/2020 07:41:47 am

Disagree. 10 pipe diameter is a requirement as per NFPA and FM Global.

Besides, there cannot be a small pump room and tight space and all these because take the pump orientation and the space requirement for other functions, you never have tight space in the pump room.

Nguyen Thong Nhat

6/30/2020 09:40:33 pm

Yes, I think there is an error of word in that requirement. it must be "shall" instead of "should", as shown in 4.16.6.3.1 and 4.16.6.3.2 (NFPA 20-2019 ed.). By the way, there are many commercial building having a small area for pump room that seldom seen in industry building, and they also needn't the FM approval.

Franck

6/30/2020 02:10:54 pm

This is a reliability issue.
Many AHJ or Insurance Companies will anyway not accept this wrong arrangement.
I will not for a new installation !
You will have early ageing of your impeller and pump characteristics degradation over a short-medium term.

koray uluç

6/30/2020 03:01:11 am

The concern for 10 pipe diameter straight line requirement is not turbulence nor cavitation. it is for the differential pressure occurrence on each side of the pump impeller. That is why it is not required for end suction pump but required for horizontal or vertical split case pump only.
Additionally on split case pump installations, elbows that are close to impeller but vertical to impeller shaft are not need to be far away from the suction flange as described on NFPA 20.
While working, continuous pressure difference on sides of the impeller cause damage on impeller bearings.

Franck

6/30/2020 02:14:11 pm

end suction pumps are normally not allowed by NFPA, only horizontal split case pumps (more robust and more reliable).

End suction pumps are anyway often used in Europe.
I don't think that any of them is UL listed or FM approved (but maybe I am wrong).

koray uluç

7/1/2020 01:36:32 am

Sorry, but this is a wrong info

Franck

7/1/2020 01:50:43 am

As often, I was too much self confident and the information I put was wrong (end suction pumps can be FM approved and UL listed). Mostly based on my European experience where such pumps are very often provided, and almost never UL listed or FM approved.
Thanks for the correction Koray.

Nithin Babu

6/30/2020 06:58:22 am

If space is constrain in pump room, then the requirement of 10 D can be satisfied by introducing diffuser, i remember victualic offer such diffuser

Franck

6/30/2020 02:19:28 pm

In addition to all the above comments, your question was also "why we don't have the same requirement on the discharge side ?".
The reason is that on the suction side you create turbulence and differential pressures on the impeller shaft that could deteriorate the pump characteristics over the time.
On the discharge side, you can't damage the fire pump. This is why there is no such requirement for a minimum distance between the discharge side of the fire pump and the backflow preventer, for example.

Drew Lange link

6/30/2020 04:31:32 pm

The Minnesota State Fire Marshal used to publish a great one-page newsletter named "Quick Response." The June 2008 issue covered Fire Pump Suction Piping Arrangement. It can be found at the link below. Keep in mind the NFPA 20 references were from a previous version of NFPA 20.

https://dps.mn.gov/divisions/sfm/programs-services/Documents/Quick%20Response%20Newleter/QR0608FirePumpsSuctPipe.pdf

Nimal Tissa Wijetunga

6/30/2020 10:46:29 pm

Within that 10 Pipe diameter distance, only OS&Y Gate Valve is allowed and this 10 pipe diameter is to get laminar flow to the impeller from the suction side and it is definitely a requirement.

JUHWAN KANG

7/1/2020 01:50:19 am

Could you inform where can I find any clause for that OS&Y Gate Valve is allowed within 10D?

Franck

7/1/2020 04:31:05 am

NFPA 20 (Last Edition)
§ 4.16.5.1 OS&Y should be provided on the suction side
§A.4.16.5 OS&Y should be located as far as feasible from the flange when suction supply is from public fire main, but there is no strict distance limitation. And no limitation if from a private reservoir.
§ 4.16.5.4 No other valve type or other approved device should be located at less than 50 ft
§ 4.29.3 check valve and backflow preventer are allowed at 10 D (or 50 ft if butterfly valves are isolating the backflow preventer)

The 10 D limitation in § 4.16.6 is for tee and elbows only.

Nimal Tissa Wijetunga

7/1/2020 10:27:15 am

Dear Franck,

Your information above is totally wrong.

Joe Meyer

7/1/2020 11:21:36 am

Nimal, you need to reply in the correct string and also comment specifically about what your concerns are. This forum is for fostering discussion.

Joe

Franck

7/1/2020 11:38:56 am

Dear Nimal
I always like to learn from my mistakes (and I make some from time to time)
Could you please give more details on what is wrong on the information?
Many thanks in advance

7/1/2020 11:19:04 am

You can review the section for suction diffuser, which according to the NFPA 20 ed 2019 is equvalent to the 10 pipe diameter of straight pipe. Bue review the equivalent pressure loss for this device and check with the manufacturer the installation requirements

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