PD Blower Driven Backwards

[wrd2032]

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.

 

[bimr]

Don't think your scenario occurred as you described.

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.

 

[wrd2032]

Thanks for the reply.

What explains the water leaking out of the blower?

 

[bimr]

Perhaps, some of the water drained back by gravity. Do you have an exhaust silencer?

 

[wrd2032]

We don't have an exhaust silencer, but there is an inlet silencer.

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.

 

[bimr]

The bottom of the pipe in the tank has 30 feet of water head on it. My point is that once the water started moving up the pipe, it would not stop at the water level, but continue up until the energy is dissipated. If enough water went over the wall, it could start a siphon. I don't think that happened, it just splashed some water over. The splashed water drained to the blower. Probably more likely with a small diameter pipe.

 

[georgeverghese]

Agreed a check valve would stop the reverse flow into this Rootes blower.

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.

 

[1gibson]

I'm with bimr. Take a drinking straw, put your finger on the top, then submerge it in a glass. When you remove your finger, the liquid in the straw will "bounce" well above the liquid level in the glass.

 

[bimr]

Tried the drinking straw test. The cup had 3-Inches of water depth. When the air was released, the water shot up 3/4-Inch over the water surface.

 

[LittleInch]

The point is that the blower must still have been leaking a lot of air by going backwards which is not a great idea normally.

Add the check valve right on the outlet of the blower.

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