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Flexible Coupling in Pump Suction

Flexible Coupling in Pump Suction

(OP)
Is it acceptable to use a flexible coupling in a pump suction line that runs from a fire water storage tank to a pump house which sits on a separate foundation? We expect the tank to settle .65" initially and another 1.17" over 5 years and the pump house foundation is on piles so it will not settle. I seem to get conflicting answers when doing research. NFPA 20 shows using flexible couplings for strain relief, while other sites say to route my flexibility in the line. My opinion is that if NFPA 20 is ok with it, then I am ok with it. Unfortunately it is too late to make changes to the suction line, but will 2 flexible couplings in the straight run out of the tank be sufficient to handle the settlement? Any opinions would be greatly appreciated.

RE: Flexible Coupling in Pump Suction

1.17"is not strain relief, it severe misalignment,I would be looking for A better option.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Flexible Coupling in Pump Suction

(OP)
Would an expansion joint be more sufficient?

Here is a picture of the line

RE: Flexible Coupling in Pump Suction

If the round purple circles under the pipes are the piles or are supports which won't settle like the tank will then a couple of flexible joints in that single leg or just a single length of Braided hose should be enough.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flexible Coupling in Pump Suction

(OP)
Yes, the purple is concrete piles for supports. Off to the right where the pipe changes color is the pump house tie points.

RE: Flexible Coupling in Pump Suction

I would put two flex joint in the common horizontal run before the elbow. But,I would use ball-type joints rather than rubber or metal bellows joints. We use these on crude booster pumps connected to pipelines that have a lot of movement.

Johnny Pellin

RE: Flexible Coupling in Pump Suction

Remember that flexible couplings are not to be used under negative pressure situations.

I assume the yard piping is not settling. It is the foundation of the water tank that is settling.

What JJP is suggesting is correct. This peice of pipe should be in a accessible concrete vault to allow the joint to work as the storage tank settles. You need to consider a detail to prevent shearing of the pipes between the vault and the spigot cast into the foundation wall of the sotrage tank.

RE: Flexible Coupling in Pump Suction

(OP)
This is what our Mechanical Engineer is recommending..

RE: Flexible Coupling in Pump Suction

Flexible couplings can be, and are used on pump suctions if rated for vacuum. If rubber, they will have steel wire embedded to hold the form under vacuum. I always just cheat and call a flexible coupling company and get them to select it (and quote it) based on my specifications. They will want to know max vacuum, axial travel, radial travel, angular travel, flange rating and bolting spec. They will also want to know if you want tie rods.

I used to count sand. Now I don't count at all.

RE: Flexible Coupling in Pump Suction

I would go for something a little longer (2-3D)given the amount of vertical movement, but otherwise looks good to me.

Come on guys, this is a fire water inlet pipe connection dropping just over an inch over what looks like several feet of pipe.

No need for anything fancy here...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flexible Coupling in Pump Suction

Bellows, metal or not, don't do well under external pressure; they buckle and fail.

I'd go with Dresser couplings, which are not strictly ball type joints but do allow a few degrees of misalignment, at either end of the longish leg between the tank valve and the tee.


Mike Halloran
Pembroke Pines, FL, USA

RE: Flexible Coupling in Pump Suction

Hi All:

The differential movement is all at the wall of the storage tank...not spread out along the length of the connecting pipe (which has bedding underneath supporting the pipe). All the pipe shear and movement is at the wall. JJP's suggestion or DGrayDDp's suggestion is the way to go if you can achieve that type of movement. If this water is potable in any way I would go with the latter.

RE: Flexible Coupling in Pump Suction

If you go with some bellows device such as the one shown in your picture, I would specify it to be lined to minimize pressure drop.

RE: Flexible Coupling in Pump Suction

QT, if the pipe is fully supported by some sort of bed, why does it need the piles?

Mike Halloran
Pembroke Pines, FL, USA

RE: Flexible Coupling in Pump Suction

QT, if the pipe is fully supported by some sort of bed, why does it need the piles?

Good point. smile

If you take a look at the drawing everything is hard flanged or welded. If there was a diferential settlement of 0.6"-1.1" at the storage tank you are putting one hell of a twist on the piping if the soil is truly that liquid. The pipe would bend and would crack somewhere. I would think that is one hell of a lot of stress on that pipe. I would think that you would want to prevent that

There should be a pile pipe support close to the gate valve

RE: Flexible Coupling in Pump Suction

... and a pipe support next to the gate valve would stop the tank from settling, for how long?

Better, IMHO, to put a flex coupling of some kind next to the gate valve, and another one next to the tee.

I stand by my advice about the bellows.

Mike Halloran
Pembroke Pines, FL, USA

RE: Flexible Coupling in Pump Suction

The pipe support is to prevent the pipe from settling...not the tank LOL. The flex coupling takess the differential movement

RE: Flexible Coupling in Pump Suction

Given a little sleep, I think I misremembered the limitations of bellows, i.e., they do okay with external pressure, less so with internal pressure. Mea culpa.

Mike Halloran
Pembroke Pines, FL, USA

RE: Flexible Coupling in Pump Suction

In the system as described, I don't see internal or external pressure being a problem.

RE: Flexible Coupling in Pump Suction

(OP)
Sorry, my terminology is incorrect. I meant to say, "to compensate for the misalignment," not "to give flexibility." Didn't want to confuse anybody.

RE: Flexible Coupling in Pump Suction

That is along the lines of what I was trying to suggest.

Mike Halloran
Pembroke Pines, FL, USA

RE: Flexible Coupling in Pump Suction

Why are you not using bell and spigot ductile iron pipe or ductile iron with mechanical joints

RE: Flexible Coupling in Pump Suction

(OP)
Client specs call for carbon steel

RE: Flexible Coupling in Pump Suction

I don't think carbon steel will last

RE: Flexible Coupling in Pump Suction

your flexible joint of 2 Feb post should be good enough to remove 1" of movement, but could be with being a little longer.

No need to complicate things here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Flexible Coupling in Pump Suction

Be aware that hoses are not expansion joints. They can handle bending and axial alignment but cannot handle axial movement- they do not stretch and when compressed, the overbraid slacks off and no longer supports the corrugations against internal pressure.

The best way to use a hose as an alignment tool is as a replacement for an elbow.

RE: Flexible Coupling in Pump Suction

To get back to the original query-

Quote:

...but will 2 flexible couplings in the straight run out of the tank be sufficient to handle the settlement?
Yes, I believe such an arrangement would be suitable, with the joints you originally proposed. You would need to support the straight run of pipe adjacent to each flexible joint, with supports incorporating integral height adjustment so you can follow the tank down.

I'm guessing that the pipe from the tank is about 12", but the joint datasheet should identify the lateral or angular displacement it can handle.

The Victaulic Style 233 joints look all right, but my concern would be the (perhaps long term) ability to maintain a proper seal on the outside surface of the CS pipework.

I'm not familiar with the ball-type joint proposed by Johnny Pellin, but feel it may be a little over the top for this application.

RE: Flexible Coupling in Pump Suction

(OP)
Thank you again to everyone for your helpful responses.

Only reason I proposed the victaulic couplings are because the stress engineer is having second thoughts about the flexible hose. What you do not see in the original picture is that there will be a catwalk crossover (width will be more than likely 3'-0") close to the valve for accessibility reasons. The valve is 14" and the stem is projected to be quite tall. Due to the catwalk, the thought was that placing the flexible hose underneath it could create issues if for some reason the hose ever needed to be replaced and maintence could not get to it comfortably. However, placing it outside the catwalk would create too much load on the 14" tank nozzle. So the victaulic couplings were suggested as an alternative with one presumably close to the tee and another close to the edge of the proposed catwalk.

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