Pump discharge pipe

[ChasBean1]

Folks, I'm wondering about the threaded rods (usually ~5/8 or 3/4 inch) that I often see on pumps' discharge pipes on vertical rises. The rods are connected to flanges on either side of a rubber boot/stop check assembly. There's usually one on each side of the pipe, external and parallel to the pipe. A nut at each end of the rod, if tightened, looks like it will compress the rubber vibration boot at the outlet.

I've heard the rods called "alignment" rods, but I thought they were to compress the rubber boot. Can any of you precisely explain the purpose of these threaded rods? I've seen these left loose on some projects, but instinct tells me they should be snug (possibly to prevent expansion of the rubber/flex boot).

Your thoughts are appreciated...

 

[butelja]

They are there to resist the pressure thrust acting upon the bellows expansion joint, and are known as ties. An unrestrained expansion joint will elongate (possibly to failure) when it is pressurized. If the piping is restrained but a joint with no ties or loose ties is pressurized, then this entire pressure thrust must be resisted by the pump flange. This is generally not a good thing, and can lead to bearing and seal failures.

 

[KernOily]

As butelja said, you need to make sure the nuts on the tie rods are snug. The short explanation is - the 'rubber boot' is an expansion joint that is used to isolate the movement of the pipe from the pump when the system is in operation. If you look in the plant data books for that pump you should be able to find the datasheet on that rubber expansion joint. The datasheet will show the operating length of the joint. The nuts should be tightened to compress the joint to the length shown on the datasheet. It shouldn't take very much. I've found that lots of times, well-meaning but uninformed plant personnel will loosen the nuts for one reason or another. Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com

 

[ChasBean1]

Thanks to both of you. And I found web info - the maker of the particular unit is FlexiCraft. From what I understand, the 'control arms' should be set so that the flex spool piece won't exceed its design maximum elongation. Although I haven't had a reply from their rep yet, I would expect to see the rods loose during initial system fill, and snug at the maximum design pressure (or near max stretch length).

Thanks again, -Chas.