PD Blower Driven Backwards
PD Blower Driven Backwards
(OP)
We've got a 30 foot tall tank, full of water. We have a positive displacement, dual axle lobe blower pumping air into the tank. The air supply pipe runs from the blower up over the tank wall, then to an air header at the bottom of the tank. The check valve supplied by the blower manufacturer was _not_ installed.
On startup, the blower ran fine, and blew air out the air header inside the full tank. The blower motor has no VFD. When we de-energized the blower motor, I'm told you could hear "fluid motion" in the pipes, and then water started leaking out around the blower! There was a manual valve in the line, which someone then closed, and the water stopped. I'm told the blower is fine.
I'm trying to work out what exactly must have happened. The only thing I can come up with is that the pressurized air in the supply line flowed backwards through the lobes of the blower, which must spin very freely and have enough weight to build up some inertia. Once the line was back to atmospheric, the inertia of the blower lobes was enough to pull a vacuum in the line, drawing the water from the tank back through the air supply pipe. The air supply line tops out probably 4 to 6 feet above the tank's water line.
Does this make sense? We're installing the provided check valve now in the air supply line. Anything else needed to prevent this issue from recurring?
Thanks in advance.
On startup, the blower ran fine, and blew air out the air header inside the full tank. The blower motor has no VFD. When we de-energized the blower motor, I'm told you could hear "fluid motion" in the pipes, and then water started leaking out around the blower! There was a manual valve in the line, which someone then closed, and the water stopped. I'm told the blower is fine.
I'm trying to work out what exactly must have happened. The only thing I can come up with is that the pressurized air in the supply line flowed backwards through the lobes of the blower, which must spin very freely and have enough weight to build up some inertia. Once the line was back to atmospheric, the inertia of the blower lobes was enough to pull a vacuum in the line, drawing the water from the tank back through the air supply pipe. The air supply line tops out probably 4 to 6 feet above the tank's water line.
Does this make sense? We're installing the provided check valve now in the air supply line. Anything else needed to prevent this issue from recurring?
Thanks in advance.





RE: PD Blower Driven Backwards
You have a 30 feet deep pipe filled with air. If you shut off the air, then you will have a reverse flow from the water pressure at the tank bottom, where the pressure is approximately 12 psi. The water just shot back up the pipe and stopped.
The check valve should prevent the rapid backflow.
RE: PD Blower Driven Backwards
What explains the water leaking out of the blower?
RE: PD Blower Driven Backwards
RE: PD Blower Driven Backwards
I'm not sure how gravity could explain it. The blower and tank sit on the same floor level. The tank has a 30 foot tall sidewall. The air pipe runs from the blower discharge all the way over the tank sidewall, peaking about 34 feet high, then comes back down into the tank, all the way down to a header a couple feet off the floor of the tank. If, for instance, a big valve in that air line were suddenly opened while the blower was running, I would expect the water level in the air header to rise up to 28 feet or so to match the water level of the tank. I don't have any theory, besides the one proposed above, of how the water inside the air header makes it the other six feet up to the top of the header, then back down the air header on the outside of the tank, finally making its way into the blower.
RE: PD Blower Driven Backwards
RE: PD Blower Driven Backwards
Once the check valve is installed, any minor leakage / reverse flow of pressurised air through the check valve wont be enough to reverse rotate the blower, hopefully.
Depending on how well this check valve holds, the rootes blower may then see full discharge pressure on startup on the next startup, unless a manual startup bypass valve (use a manual globe if possible) is installed on blower discharge (upstream of the check valve)to slowly crank up the blower discharge pressure.
RE: PD Blower Driven Backwards
RE: PD Blower Driven Backwards
RE: PD Blower Driven Backwards
Add the check valve right on the outlet of the blower.
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