Moving Liquid With an Industrial Vacuum

A common question we face is what rate a user might expect from an industrial vacuum system. This white paper addresses that problem for liquids – water, slurries, slurries – which rely on a degree of vacuum to pull the “solid column” through the hose. Solid products – sand, abrasive media, grains – do not rely on a vacuum, but rather on creating a high airflow that carries the solids with them. These products will be discussed in a separate whitepaper. Here we are talking about fluids.

A vacuum machine is typically defined by its suction (as expressed most commonly as inches of mercury, or “Hg) and its airflow (as expressed in the US as cubic feet per minute, or cfm). When a “solid column” of liquid is pulled through the hose, the airflow is not important….the system is under vacuum….while the “Hg becomes the dominant factor in determining flow rates for a given system.

The size of the hose used, the smoothness of its bore, the straightness of its runs, and the type of product all affect rate. Today’s whitepaper addresses what can be expected from a defined setup, using water.

The flow properties of water are well understood. Firefighters have made a science of its flow to help them determine the best layout of hose. To make things simple, we are going to look at a table that shows, for a given suction in “Hg, what rate water will flow through 100′ feet of a straight 4” diameter hose, or alternatively, how high that water can be lifted under vacuum. (Because of the laws of physics, a solid column of water cannot be lifted much more than about 30 feet….the atmospheric pressure which is “pushing” the water through the line is balanced by the weight of the water column, leaving no driving force upward. However, it is possible to make lifts up to ~100 feet through the introduction of air – meaning you no longer have a “solid column” of water. Triton can provide more information on how to accomplish that.)

5″ Hg – 324 gpm or 5 feet vertical lift

9″ Hg – 445 gpm or 10 feet vertical lift

15″ Hg – 587 gpm or 17 feet vertical lift

18″ Hg – 648 gpm or 20 feet vertical lift

26″ Hg – 790 gpm or 29.4 feet vertical lift

Let’s look at a vacuum that puts up 26″ Hg. The Triton T1500 and our other liquid ring vacuum systems put up 26″ Hg or more. At 26″, the machine can pull 790 gallons per minute of water through a straight 4″ hose, or alternatively it can lift a solid column of water 29.4 feet….or some combination therein.

Now let’s assume that you have to lift that solid column of water 10 feet. Looking at the table, you can see that it requires 9″ of vacuum to make that lift. Subtracting that 9″ from the 26″ that you have available leaves 17″ of vacuum to use for flow. We don’t show 17″ on the table, but you can estimate that its going to be in the range of 625 gpm. So, if you have 5000 gallons of water that you need to lift 10′ and flow through 100′ of a 4″ hose, it will flow at ~625 gallons per minute, and thus require ~8 minutes to do it.

This table assumes a 100′ hose. If it is a shorter hose, there will be less friction, and the rates will be higher. If it is a longer hose, there will be more friction, and the rates will be lower. Another 100′ section might reduce rates on the order of 30%.

Also, this table assumes a 4″ hose. A smaller diameter hose has more friction than a larger hose.

Finally, this table assumes water. The product you are trying to use may be heavier than water, which means it takes more force to lift the column, and there will be higher friction losses. To give an idea, here are the lifts that can be expected at 26″ Hg for products of various densities. Note that the value for water is the same as given in the previous table.

Water @ 8.3 lbs/gal can be lifted 29.4 feet

Slurry @ 9 lbs/gal can be lifted 27.4 feet

Sludge @ 11 lbs/gal can be lifted 22.4 feet

Sludge @ 14 lbs/gal can be lifted 15.6 feet

You can see that a heavier product can make a slight difference in terms of lift. Vertical lifting is the most challenging part of the job, and can be helped by introducing air into the inlet.