Fluid transport by pressurized gas 3

[nbog]

I saw somewhere interesting way of liquid pumping: instead of a pump, guy used simple pressurized vessel (gas from top) to push liquid through a pipe. Never thought about advantages of doing it that way. Although it was in a lab/pilot scale system.
Any comments?

 

[MikeHalloran]

I've worked on small scale systems where vessels were kept pressurized (~0.5 barg) with air to move and dispense salt water without pumps.

Disadvantage; the gas goes into solution in the liquid, so when the liquid's pressure is reduced, you get gas bubbles. To minimize that, you might vent the vessel when you are not pumping. Costs a valve, releases some vapor.

Disadvantage; the flow rate changes as the liquid level goes down. It can be offset by introducing the gas near the bottom of the vessel, but then you can't use a self- relieving air regulator, and the gassing gets worse.

Disadvantage; you have to vent the vessel to replenish the liquid. If you don't put a manual vent valve in the cap, the cap and pickup tube will get launched by the stored air pressure, and customers will complain about the liquid being splashed on their neckties.

Disadvantage; the particular system was a medical instrument, and had no other need for an air compressor. Small air compressors are not remarkably reliable, and small quiet ones, e.g. for office use, are expensive.

Disadvantage; when you cycle the pressure to minimize gassing, you will eventually fatigue a polypropylene bottle and crack it, so you need a catch basin.

Aside from that ... well, it was better than peristaltic pumps for our purposes, but it did provide some interesting problems.



Mike Halloran
Pembroke Pines, FL, USA

 

[MortenA]

Big advantages i CIP system: No pumps that are diffucult to clean in a CIP system.

Best regards

Morten

 

[EdStainless]

There are two ways to look at this.
One is to use overhead pressure in a tank/vessel to push liquid through a system. This is OK for batch work, but would require multiple tanks for continious feed, or a clever pressure control/recharge system.
The other is to use air lift to pump fluids. This works fine, in fact there are air lift pumps built (special eductors) and used in water and brine wells.
It isn't very efficent, and there is a lot of gas to handle. If you needed to lift fluid a specific height and the head and flow never changed you could taylor a system just for that. In a pharma application this would need to be clean (sterile?) gas. That is a major cost.

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[zdas04]

If you have intermittent service, then "blowcases" are used all over the world to empty a low pressure vessel into an higher pressure system.

These devices have a line from the bottom of the vessel that is extracting liquids (call it a "separator") into the blowcase with a check valve on it. At the top of the blowcase there is a 3-way valve that is normally set to vent the blowcase back to the separator. When the blowcase is full, the 3-way valve repositions to bring high-pressure gas into the blowcase, a dump valve on the blowcase opens, the check valve back to the separator closes, and the liquid is expelled. When the blowcase gets to the low-level setpoint the dump shuts and the 3-way valve repositions to send the high-pressure gas into the separator for recycling.

The big issues with a blowcase are: (1) finding a compatible gas source at adequate pressure; and (2) sizing the blowcase to be compatible with the volume requirements. I've installed blowcases for up to 1,000 bbl/day, but those were pretty special and had to dump really fast to be able to cycle that often.

David Simpson, PE
MuleShoe Engineering

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

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[djack77494]

We used blowcases for transferring aluminum alkyls from portable receivers into our storage tanks. Since no exposure to air or moisture is permissible, and since it's very difficult to get a pump to survive such an environment, the idea of just adding dry nitrogen to the top of the vessel to effect the transfer seems very good (albeit a bit expensive regarding nitrogen consumption).
Doug

 

[nbog]

But more interesting about the system I saw is that the guy used it to feed a pneumatic atomizer. I didn't see anything on the schematics about pressure control, that's why I was puzzled. This would require good pressure control loop as requirements downstream are probably not constant i.e. change in liquid flow rate.

 

[JLSeagull]

As David and Doog suggest the blowcases described are common for offshore applications as well as corrosive chemical handling. Two common offshore examples include the closed drain system with blowcases below the main deck. This includes pneumatic level switches, etc. Also the flare drums including the complex batch logic executed by the DCS to move condensate back to the platform using fuel gas, etc. The complexity exists in evaluating the flaring events, etc. to determine when to and not to block in the liquid drum to blow back to the platform. Besides alkyls like TEAL, other really nasty inorganic stuff is moved with nitrogen.

 

[unclesyd]

We use N2 to move out of an autoclave as a normal operating procedure. The 2500 # batch of polymer is at 285C and 250 psig and is forced out through a die plate into a ribbon using only N2. The N2 come a header kept at 300 psig and is heated prior to introduction to the autoclave. These autoclaves originally had "acid eggs" attached to charge additives while at operating pressure and temperature. This system has been replaced with PD pumps on all but 3 claves were the acid egg is still used to make special polymers. The system feeding the autoclaves is also emptied by N2 but at a lower temperature and pressure.
The reason for using N2 is that to pump the polymer it would take a gear pump to move the highly viscous material.
We also have a system on a highly reactive process where N2 is used to assist emptying the reactors in case of what we call a "running dump". We need to get the reactants into a water filled dump tank as quickly as possible. This system has some tricky controls as pressure is a controlling factor in the reaction rate.

As noted above these systems normally require some sophisticated controls to prevent operational excursions.

 

[Montemayor]

nbog:

This "pumping" method is old hat. It was being used at the beginning of the last century in such tough applications as transfer of Sulfuric Acid in Sulfuric Acid plants. It was called an "acid egg".

Think about it. How did our grandfathers manage to handle, transfer and ship such fluids as sulfuric acid, hydrochloric acid, caustic soda, and Phenol when they didn't have stainless steel, Teflon, and exotic alloys? All they had was riveted pipe, cast iron, and a lot of packing material. They often used Bell & Spigot joints on cast iron pipe.

They had to resort to basics and ingenuity. Acid eggs was one of them. However, as UncleSyd states, you have to be ultra-careful in mitigating the hazards involved. I have transfered liquid Phenol from rail tankcars into storage tanks on a daily basis and I had to fully instrument and safeguard the operation. It just looks simple, but it ain't.

 

[psafety]

When using pressurized systems like this consider the liquid filled tanker, rail-car, etc that is being pressurized; as the liquid empties the intial vessel is now a pressurized volume (receiver) of motive gas. This is now a case of pressure "equalization" to the receiving vessel. The rate of compressible flow is restricted by pressure loss of interconnected piping.

We unload many chemicals using compressed air, I've found many cases where engineers consider the second vessel's vent only needs to relieve the intial regulated air flow rate and overlook the first vessel becoming a receiver.

 

[BenThayer]

there are also significant environmental impact to consider. what liquid were you moving and where do you vent the pressurized gas you brought into the system.

the site i am now at used this method relatively frequently but had to stop years ago due to environmental regulations. this is not really an issue with the "acid egg" but if you are moving solvents, you have to be able to remove the nitrogen somehow.

and then on the safety side of it, someone that did not have enough nitrogen pressure available decided to use air instead. bad things can happen really fast.

 

[schnipp]

I've seen it used in practical use transferring fermentation processes from germenation vessel to fermentation vessel.

MortenA hit the nail on the head: no difficult to clean pumps that lead to contamination. Especially if in a neutral pH situation.

 

[pumpking]

Interesting comments coming out. Im from UK and it is becoming hugely 'discouraged' to use air pad discharge against pumping out due to a number of reasons:

1) containers "can" be classed as a pressure vessel and so a whole string of specifications can become relevant.
2) If you use air pad dscharge, then there is a need to 'clean' the air discharged from the receiving vessel by a fume scrubber - pump discharge can have an air return line to supply tank.
3) Safety issues as have been mentioned above.

It is quite a hot topic over here, and more and more people are moving away from air pad discharge - we have developed a mobile pumping station for this use, and a great market is appearing in front of us (

http://www.cdrpumps.co.uk

- look in news section !!)

 

[SteveWag]

30 years ago and earlier I specified and maintained quit a few "ejector stations" for low flow, high head applications for municipal sewage service.
Sewage pumps have quite large passage-ways and don't do high heads well. Sewage discharge piping needs to be 4-inches or better and requires 2 FPS in order to carry solids, meaning 70-80 GPM.

For low flow situations it would take a long time to fill a wet-well with enough stuff to turn on a conventional pump, resulting in smells.

The standard station had two tanks about 200 gallons each, with flow entering a true wye, passing thru one or both check valves and entering one or both tanks. When one tank filled to a level high enough to reach a tank mounted probe, a solenoid was opened and air from a receiver entered the tank. At the same time another solenoid closed the tank vent. The air forced the contents out the bottom connection, thru a check, and into a discharge main. While this was going on, the "other" tank was receiving sewage flow. Because the tanks filled from the top, the flow split more or less equally, without regard for how full the tank was. The bottom exit insured all the settled solids exited. The air that was vented as the tank filled was the air that was used to expel the contents, not gasses generated by decomposing sewage.

I still use this system for pumping scum (very hard to do with a centrifugal) in treatment plants, but people look at me strangely when I suggest it for sewerage collection/transporting systems .

Steve Wagner