Eductor flow back pressure

[chief]

I have a slurry mixing package which has been installed such that the EDUCTOR discharge has to overcome a back pressure head of 25 ft before entering the tank destination.
The EDUCTOR has a 2" nozzle and the supply is from a 6" feed line with a flow rate from a pump of 1000gpm, capability of 100ft head. The discharge from the Eductor goes to an 8" manifold but has a T-junction and several 90 degree elbows.
Manufacturer recommendation is for a supply of 50 psi to the Eductor and minimum back pressure for efficient operation.
I have to mix a slurry with a weight of up to 16 lb/gal (sg approx 2.0)via powder into a hopper of the Eductor.
My question is , will the Mixer work effectively against this back pressure .
I already have had problems with back flow into the Hopper when cutting off the supply because of the pipe and tank elevation. Fitting a check valve in the discharge will only add to my back pressure problems.
Normal practice is to fit the Eductor package on top of the tank to prevent back pressure. But I cannot change the location of this unit.
Appreciate any feedback from people who have come across this problem before.

 

[zdas04]

The design of an eductor for backpressure is very different from the design of an atmospheric discharge. If the one you are using is currently discharging to near atmospheric pressure then the chance that it will work with 25 ft of backpressure is about zero.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[MikeHalloran]

At the very least, the eductor discharge should terminate about the level in the destination tank, because an eductor with the supply shut off is just a tee with one leg shut off.





Mike Halloran
Pembroke Pines, FL, USA

 

[chief]

The flow back, is from the pipework not from the tank. The eductor discharges into the top of the tank which gives another 8' foot head above the tank. As already stated , the total back pressure caused by THE PIPEWORK is 25 feet.

 

[chief]

I should add that I have not been able to test this unit with a powder to the Mix hopper as I am in a very remote location and the powder has not arrived. Also when I first supplied water to the Eductor I discovered the pumps and motors were wrongly sized and were delivering over 100psi against a recommendation of max 50 psi, with the motor exceeding itS FLA. The flowrate was much higher than recommended also, but I did not have a flowmeter to verify this, and could only Guesstimate.
I did not design this mess but am trying to find a way to make it work. I can sort out the pump/motor and supply pressure problem , but I have my doubts about the effectiveness of the Mix against the head back pressure.

 

[MikeHalloran]

The presence of manifold and elbows and extra plumbing in the eductor discharge seems odd at first, but I wonder if it's intentional.

I.e., just using an eductor with open discharge to introduce powder into a stream of water will tend to produce a stream of powder surrounded by a stream of water. Maybe the manifold and elbows and pipe are intended to force mixing.

Once the slurry enters the receiving tank, what keeps it in suspension?



Mike Halloran
Pembroke Pines, FL, USA

 

[chief]

I can assure you that the T-junction and 90 degree elbows are not intended to assist with the mix . They were designed by someone who never considered or read the requirements of the Mix eductor vendor.
I should also add that there is a shear mixing arrangement at the powder inlet to the Eductor to assist with the mix effect.
I have calculated the back pressure caused by the pipework head will be in the region of 20psi for a sg mud of 2.0 zero.

 

[thancrus]

Hi chief,

reasons for hopper back-flow can be restrictions in discharge line, partially closed valves on feed end, pump pressure, fluid viscocity too high, partially plugged nozzle.

"I already have had problems with back flow into the Hopper when cutting off the supply because of the pipe and tank elevation"

could you be a litte more specific about when exactly backflow occurs?

 

[chief]

As previously stated,the pipework is elevated AFTER the Eductor, without a check valve. The contents of the elevated pipework flowback to the Eductor and overflow the Hopper when the supply pump is stopped or the supply valve is closed.
I can fix the flowback problem by installing a check valve, but of course this will add further to my back pressure problem.
My question has always been , has any one had experience of an Eductor operating efficiently against a head of 25 ft with slurry up to 2.0 specific gravity.

 

[MikeHalloran]

So far as I know, the first eductors to enter commercial production were used to feed water into a steam locomotive's boiler. The boiler's steam output was the motive force. The Sg was around 1.0 of course, but the head was way more than 25 feet. You've already determined that you have an excess of motive flow available, so the eductor should work okay while feeding powder.

The problem you're trying to solve has nothing to do with dynamic head loss in the discharge pipe. It has to do with the discharge pipe being full of motive fluid when you shut off the pump, and not having anyplace to go, except into the powder hopper.

So, give it a place to go. Put a dump valve in the eductor discharge line. Open it before you shut off the motive fluid supply. If it's big enough, the discharge line will drain through it instead of filling the powder hopper.

Does there happen to be such a valve already in the manifold you mentioned?





Mike Halloran
Pembroke Pines, FL, USA

 

[chief]

I appreciate your input MIKE. I already have a rats nest of a piping system. I guess I will have to add to it by including drains after the Eductor. This is complicated, by the fact, that there are 3 different SLURRY mediums in 9 different tanks to drain back, without contaminating each other.
A check valve would help to prevent the overflow mess, but I have to be careful and prevent the slurry solidfying in the elevated piping, hence the dump lines. I will also add a purge air quik - connect to maintain these lines clear after each mix.
As mentioned previously , the design lay-out has made it difficult to operate the system effectively, without a multitude of manual operations.
I await the arrival of the first batch of powder and will then have a test of the mixing quality.

 

[englady]

try a solids shut-off valve to preven backflow

 

[HEC]

Chief,

Have had similar experience with mixing milk powder and returning to a silo, much higher static head. For that application I used a booster pump post the eductor to ensure the throat pressure did not become positive and end up with wet powder in the mixer. This is not your problem, just for interest sake. The other feature that will solve your problem is installation of a valve on the powder feed above the eductor and below the hopper. Thsi valve is closed before the system is stopped. I used a buterfly valve for cleaning reasons, a ball valve would be appropriate for your application.

Good Luck.

Mark Hutton