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AJA
06-28-2010, 06:50 AM
I have replaced a 500 gal tank from a commercial vacuum system with a 48 gal tank. The vacuum pump was rebuilt before the smaller tank was installed. The new system is powered by a 30 hp engine and develops good vacuum at the 2 inch input of the pump.

The line from the pump enters at the top of the spherical 48 gal tank very similar to the way it was plumbed to the 500 gal tank. The 2 inch suction hose to the spherical tank enters about 6 inches below the pump line at the top of this tank. The 500 gal tank had its 4 inch hose entering at its bottom. The commercial system would fill the 500 gal tank with liquid in about 25 minutes. My system will draw the liquid to within 6 inches of the opening of the tank and stop, never filling the tank.
What am I missing that would cause my system not to work?
AJ

mayfieldtm
06-28-2010, 09:13 AM
What is the total height your raising the liquid?
(total height above the source of the liquid)
If I remember correctly about 30' (for water) is max. Then you just suck a vacuum.
(also depends on the specific gravity of the liquid)

Tom M.

winchman
06-28-2010, 09:46 AM
It seems like filling the tank completely would be really bad if there's no sure-fire way to keep the liquid from getting into the pump. Anyway, you're probably getting it over 90% full as it is.

There's probably part of the hose making a short up-then-down arc before it enters the tank. That would be more likely to cause problems than a line that connects to the bottom of the tank. It's also more likely to run into the vertical lift limit Tom mentioned.

millwrong
06-28-2010, 10:19 AM
Sounds like you have a vacuum leak somewhere. A 30hp vacuum pump for 50 gal.? Should hear the air flow through the leak. Can you put a guage on the suction side and see what sort of vacuum you're pulling?

Black_Moons
06-28-2010, 11:48 AM
Id wonder about seperation.

Water/liquid through a long vacuum tube will often turn into a mist.

Id wager the old tank generated somewhat of a cyclone inside to seperate the water from the air. If your tank does not do that, it might just bypass the tank entirely and go right into the motor, especialy when it starts to fill up

AJA
06-28-2010, 06:34 PM
The total height needed to lift the water is 15 ft. The hose is stiff enough that it makes a nice sweeping bend as it connects to the tank. The water never enters the tank it just sets in the hose 6 inches from the opening of the tank. The tank never fills.

I have checked for leaks by applying water on the connections and have seen no leaks.
Is there a better way to find a vacuum leak?

I will check the amount of vacuum at the suction side of the tank tomorrow.
Thanks for your replies
AJ

macona
06-28-2010, 09:52 PM
A perfect vacuum would support about 10 meters of water. You vacuum just does not suck enough for an inlet that high, move it down towards the bottom. the lower the better. The higher up it is the lower flow rate you will have.

winchman
06-29-2010, 04:36 AM
A mechanic's stethoscope would be good for locating a leak in a vacuum system.

http://www.harborfreight.com/media/catalog/product/cache/1/image/370x/9df78eab33525d08d6e5fb8d27136e95/i/m/image_934.jpg

Just be careful you don't get it too close.

Wow, the more I think about it, that's just plain scary. Does a mechanic's stethoscope have some protection built in to keep you from rupturing your ear drums if you put the tip too close to a vacuum leak?

Duffy
06-29-2010, 10:48 AM
The total USEABLE lift is only about 19 feet-for everything. Beyond that, you will get, in a centrifugal water pump, cavitation which is really water boiling. When that happens the pump no longer lifts.This applies equally to a vacuum pump. Total losses in the system reach about 19 feet and the system stops, It works a bit better when the watew is really cold.

Rich Carlstedt
06-29-2010, 12:54 PM
I don't understand all of what you said, so let me repeat it in my terms
1. You replaced a larger tank with a smaller one.
2. I assume that the top of both tanks is still at the same height off the ground
3. Your pump is piped into the new tank, the same as the old tank.
4. The feed pipe to the tank NOW enters near the top of the tank instead of the bottom.
5. The feed pipe has liquid in it that is just short of reaching the tank inlet
6 The lift is about 15 feet to the tank top

Conclusions:
You are assuming the rebuilt pump has higher capacity (1) and higher vacuum pressure (2)
People confuse these two functions !
Without measurements, you have no way of knowing this !
It could be less than the old pump.
A sweeping turn has no effect on a vacuum system when it comes to "lift"
The temperature of your liquid can have a dramatic effect on the system, as the vapor pressure of the fluid prevents reaching the maximum capacity of the pump under certain conditions.
Lets make some assumptions:
Maximum Vacuum is 30 ~ inches of mercury or 390 inches ~ of water (14.6 PSI) normally but the temperature lowers the efficiency by 2 inches (M) or 26 inches water. (FYI) if the water is at 100 degrees (F) for example

When you said the lift you have in the 2 inch hose is about 15 feet (180 inches) it means you have about 15 " Mercury or negative 7 PSI .
This is a bit weak for a pump. 20 inches vacuum (M) (or -10 PSI) is more normal for a mechanical pump.
The old tank filled, because the feed line was at the bottom.
Your new pump sounds to be about the same vacuum pressure as the old tank as you said it also stopped 6 inches from the top

Making the system smaller as you did makes it faster in reaction time.
25 minute fills are terrible times for a tank, even for 15 foot lift.
I think your pump has a problem.
You need to put a manometer or gauge on the pump inlet pipe
Having a spherical tank is a terrific idea.
Moving the feed to the bottom of the tank will get it to fill, but not to capacity

Rich

PS
To give you an idea, I have filled a 55 gallon drum with water in about 5 seconds, using a 5 HP vacuum system with a 5 foot head and a 2 inch line

AJA
07-02-2010, 10:46 AM
Sorry it took so long in following up, but the unit is 100 miles from my home base.

I want to thank everyone for their input

Rich your understanding is correct except for the pump. It is the same pump that was on the larger tank only being rebuild.

To update:
1. Measured the vacuum at pump inlet to be 8 inches of mercury.
2. Pump has been disassembled , finding two of the three veins stuck in place.

I still need a good way to check for vacuum leaks.

Thanks again for all the input.
AJ

jimmstruk
07-02-2010, 11:03 AM
One way to check for vacuum leaks is to plug inlet and outlets, fill with liquid and apply some pressure to system. This is what I do to diesel engine fuel system when there is a problem of sucking air. It does not take much pressure to find a leak. JIM

AJA
07-08-2010, 07:26 AM
Just to update everyone.

I freed up the two stuck vanes in the pump and reassembled the pump. I checked the vacuum at just over 20 inches of mercury. The pump is back in the system and working good. I filled the tank in less than 2 minutes.

I wish to thank everyone for their help.
AJ