Air blow through to clean transfer pipe 1

[Gregas]

Greetings,

We are looking at using compressed air to remove liquid from a transfer line. I'm having some problems working out how much air we will require for this (and hence whether it will work).

The line in question runs between a delivery tanker truck and a storage tank. After delivery, there is the requirement to drain the hose and transfer pipe so that the hose can be disconnected by the operator without the risk of spills or contact with the liquid. As stated, the idea is to connect air to the hose near the truck and blow the liquid through the hose and pipe into the storage tank.

Details:
Fluid: hydrofluosilicic acid (SG ~1.2, viscosity ~6.5 Cp)
Transfer line: consists of 4m hose connected to 10m of pipe.
Hose: DN50, length 4m
Pipe: DN50, lenth 10m, uPVC
The transfer line (hose/pipe) will be full of hydrofluosilicic acid after truck delivery.
The tank inlet is 3m above the truck delivery point.

The issue: How much air is required/velocity/pressure to empty the hose/pipe?

The fundamental issue: Is there a better way to solve the problem of draining the hose - manual handling is the issue (acid ph ~1) - currently hose is drained manually into a drain, but they don't particularly like this method...

I've started by assuming a liquid droplet size of 5mm, and performing a mass balance on it (force from air pressure - force of gravity - force of drag). In this case air at 700 kPa looks like it will lift the liquid the required 3m up into the tank. I'm still not confident though.

It would be great to hear from anyone who has advice or comments on how to go about this - whether it be some fluid mechanics or practical advise.

Cheers

 

[BigInch]

I think you will have to look at some flow cases using 2-phase flow correlations. The air to liquid ratio makes a difference in flow pattern. Too little air and the air will flow as bubbles at the top of the line, more air will tend to flow at the top of the line separately from the liquid at the bottom, even more air might develop a foam flow, still more air can create mist flows. When cleaning a liquid line with air, you may experience all the above at one time or another.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Zapster]

Try to google some of the following words/phrases:

transport velocity
annular mist
moving bed
Baker flow patterns map

Also check out Wolverine Tube Inc.'s Engineering Data Book III

http://www.wlv.com/products/databook/db3/DataBookIII.pdf

 

[Zapster]

I forgot to say. There is a good discussion in Chapter 12 in the databook linked above.

 

[BigInch]

Zapster, What a good reference. The only thing wrong with it is that it took some effort to lift all those chapters.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JStephen]

Make sure that the vent on the tank is adequate for the proposed air flow. This is not a normal condition anticipated in sizing vents.

 

[Gregas]

BigInch: I will look into some 2-phase flow calcs. Your description is helpful also - I guess at worst the air flow rate can be adjusted by the operator to get the best air to liquid ratio.

Zapster: That will keep me busy, and I was having troubles finding relevant search terms, ta.

JStephen: Good point, the tanks in question haven't been ordered yet so I'll go up a couple of sizes on the vent.

Cheers for the comments guys

 

[RossABQ]

I am not familiar with hydrofluosilicic acid so I looked it up. It is some pretty nasty stuff! I would not consider using air for this purpose; a water flush would be more appropriate, if that doesn't mess up the solution strength or cause an adverse (exothermic) reaction.

A better solution still might be to have a check valve or at least a block valve at the truck end of the transfer line (hard pipe), and a separate means of draining the relatively short hose section.

 

[hydtools]

I had to look it up, too. I agree with RossABQ that a water flush may be best. The acid is denser than the water which would float on top in the tank and may not seriously dilute the tank contents.

I'd be concerned about the mist going out the vent if you use an air flush.

Ted

 

[Gregas]

Ross & Hyd: Yes, hydofluosilicic acid is nasty with a pH <1. Commonly used to fluoridate potable water.

Good points - the current method of operation is to manually drain the hose/line with a drain valve/check valve arrangement into a waste tank.

Air was suggested to improve safety re the handling of the stuff - but as you've stated, putting high pressure air onto this is a risk in itself if it lets go.

The mist out the vent is a good one also, hadn't thought of that and certainly an issue (tanks vented out the building roof - a corroding roof is not what we want).

 

[pumpking]

Interesting post, in UK and for thet matter Europe, there is a lot of pressure to move away from air pad discharge for emptying tankers/containers for all the methods mentioned above, as well as commercial reasons such as needing to scrub the air before discharge to atmosphere.

We supply lots of pumps for tanker unloading applications and basically, you need to get the pump as low as possible (ideally 0.5 metre below tanker level, pipework to the tanker, sight glass after the pump discharge, NRV and then pipework to your tank. Use a digital load monitor to protect and switch off the pump when thanker is empty and driver is busy talking/drinking coffee !!

All you then have to handle is small amount of acid after tanker is empty, but not much. The discharge tank can also vent off to the road tanker if emissions are of concern - making it a balanced system, avoiding the expense of fume scrubbing !!

go and see

http://www.cdrpumps.co.uk

for more pump details and load monitor details