Strainer Allowed in 10-Diameter Suction of Pump?

2/24/2023

For a horizontal split case fire pump, 10 pipe diameters of straight pipe is to be provided for the suction inlet.

My design includes a gate valve and strainer within the 10 pipe diameters.

Is the strainer allowable in this range?

I'm unable to find in NFPA 20 any input on this. Thanks in advance.

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11 Comments

Dan Wilder

2/24/2023 07:04:28 am

While not specifically called out, the intent is to provide laminar flow to the pump via the 10 diameters of straight pipe with the correct orientation of elbows/tees when installed to the centerline of the pump.

If items like a check valve or a butterfly valve are prohibited due to the nature of water disruption across the valve, that same disruption of water flow through a strainer would also be anticipated and should be avoided even if not specifically called out.

NFPA 20-22' Ed - 4.29.3 Devices in Suction Piping.
Where located in the suction pipe of the pump, check valves and backflow prevention devices or assemblies shall be located a minimum of 10 pipe diameters from the pump suction flange.

Glenn Berger

2/24/2023 08:13:27 am

The 10 pipe diameters needs to be maximized as much as possible without the presence of any device which can cause a non-laminar flow condition.

IMO, the strainer shall not be placed within the 10 pipe diameter zone prior to the pump suction.

Casey Milhorn

2/24/2023 08:31:14 am

I agree with Dan and Glenn, and just to be clear, there is NOT an issue with a vertical change of direction for elbows/tees within 10 pipe diameters of the suction. This only applies to elbows/tees that create a change of direction in a horizontal fashion.

Koray link

2/24/2023 08:39:38 am

10 pipe diameters straight suction pipe rule that is specifically for horizontal split case fire pumps is not because of turbulent flow concern, it is because of pressure difference on both sides of the impeller that may ruin the pump.
Since there is no clapper in strainers, like in butterfly valves, check valves or backflow preventers that can cause turbulent flow, i think it is ok to use it on a suction line

Casey Milhorn

2/24/2023 09:00:01 am

I think turbulence is too broad of a term here. It would be more in line with air entrainment or cavitation. You could probably argue that the strainer "could" cause one or both of these conditions.

Alex

2/24/2023 09:32:37 am

I agree with others this morning, I would not instal the strainer within the 10’.

Jess Lutz

2/24/2023 09:40:48 am

My advice would be to tread carefully, as it could be a high-risk decision $$$$.
So many questions immediately come to mind:

Why can't the strainer be moved to allow the 10ft?
What happens if the AHJ doesn't allow it?
Have you consulted with the AHJ?
Is this scenario approved by the PUMP manufacturer??
What are the implications if you need to relocate it at the 11th hour?

The persons responsible for the design (Engineer or Contractor) must work to get these answers.

SCHULMAN

2/24/2023 09:56:32 am

If the strainer does its job and traps something within 10' of the suction side of the pump wouldn't that be as bad as a valve clapper? .. create pressure differential on both sides of the impeller?

Derrick

2/24/2023 11:12:07 am

My personal opinion is that it would be ok, but I would try to to put it furthest away from the pump suction as possible. I definitely would check to see if it’s ok with the AHJ.
I’ve added a duplex strainer inside 10 pipe diameters before due to space restraints and there have been no issues.
I would also check with the pump manufacturer, strainer manufacturer, and the hydraulic institute standard for rotodynamic pump…I think it’s ANSI/HI 9.6 maybe. It has a lot of suction pump piping requirements…up to AHJ though. Make sure you have the justification and data ready when you present to the AHJ

Franck

2/25/2023 02:00:24 am

I would be very cautious with anything located on the suction side susceptible to create unbalanced flow, turbulences, caviation, blockage...

If 10 x diameter is fine for horizontal elbows or check valves and backflow preventers. A butterfly valve is not allowed at less than 50 ft (whatever the size of the piping) !

This being said, how should we characterize a strainer ?

I quite agree to the fact that 10 x diameter would be fine, as for backflow preventer, all the more since strainers are quite often provided at the suction inlet (screen), with no specific requirement.

Some comments state that they were installed at less distance with no issue.
This is also true for butterfly valves, upside down eccentric reduction, horizontal elbows very close to suction side of the pump...
It works well, until it fails !
These requirements are for reliability purposes and to limit early aging of the pump impeller.
I have seen pumps with many unfavourable factors on the suction side and performing well for years, and others with strong degradations within a 10 year period. Situation is different from one facility to the other (and depends on water quality - raw water or clean water - materiual quality for the impeller, size of the equipment...).

At the end, the most important thing is that the pump is able to deliver the flow and pressure that are required.
Regular testing of fire pump characteristics can help to ensure that the duty is done (it has to be done at least annually up to 150% of the nominal flow of the pump).
And the regular testing can also help to make preventive maintenance on due time and replace the impeller, if needed, before the situation is not good anymore (don't forget to correct the obtained value to the rotational speed for diesel pumps, as a pump operating at a higher speed than nominal can look "good", while it is only "good" because it rotates quicker...).

If there is no other solution than to provide an odd arrangement, this has to be validated by the AHJ, but it might not be a "real issue" for the purpose of the pump (as long as minimum requirements are fullfilled).
It will be anyway a deviation from NFPA 20 standard and has to be be mentionned for later reference.

But again, this is my practical point of view, and in no way an argument against the standard requirements and its application.
I often make recommendations when I find these odd situations on existing installations.
In Europe, for example, it is quite common to have a butterfly valve instead of a gate valve on the suction side, or elbow n the horizontal plane very close to the pump suction side.
While it is possible to change the valve, it is somehow impossible to correct the issue with the elbow. But this is documented in my report and I warn them about a possible early aging and necessity to regularly check the pump characteristics.

Phil Watkins

2/26/2023 03:08:35 am

I'd say the intent from NFPA 20 is not to have strainers on suction pipes for centrifugal pumps whether in a 10x NB location or otherwise. If you do fit them, I think you would have to make assumptions in your calculations for the strainer to be clogged.

Relevant sections below.

4.16.9.1 – No device or assembly unless identified in 4.16.9.2 that will stop, restrict the starting of, or restrict the discharge of a fire pump or pump driver shall be installed in the suction piping.

4.16.9.2 (4) – Suction strainers shall be permitted to be installed in the suction piping where required by other sections of this standard.

Chapter 8 describes requirements for positive displacement pumps; paragraph 8.5.5 requires a suction strainer (so not applicable if you have a centrifugal pump)

Chapter 7 describes requirement for vertical shaft turbine pumps; paragraph 7.3.4 requires a suction strainer (not applicable if you have a centrifugal pump)

A strainer in the suction line should not be required because the water supply to the fire water tank should be clean filtered and in any case flushed before it enters the fire water tank.

Strainer Allowed in 10-Diameter Suction of Pump?

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