Low flow rate measurement of H2O2

[twr]

We are adding hydrogen peroxide ~35% concentration, and need to find an appropriate flowmeter and/or flow switch. The flow is 50-60 mL /min, and it will be coming directly from a positive diaphragm pump, pulsing once a second for less than half a second.

I think that we may want to use a differential pressure flowmeter. We have been looking at the coriolis, but the start-up costs are pretty high and we will need quite a few meters.

 

[dcasto]

Why not use a metering pump. google metering pumps for vendors

 

[kenvlach]

"a positive diaphragm pump, pulsing once a second for less than half a second."

Sounds like you may already be using a metering pump. Just calibrate -- measure flows at different settings. Pump into a graduated container at different pump speeds (& strokes, if electric-powered), & prepare a chart or curve of the flow rate. If air-powered, measure flow as function of air pressure.

 

[jmw]

There is a new ultrasonic from Flow Technology Inc. for low flow rates that is intended to be cost competitive with coriolis meters at small sizes.

JMW

http://www.ViscoAnalyser.com

 

[StoneCold]

I would suggest using an ISCO syringe pump if you are starting from scratch. They work great for small flows.

Regards
Stonecold

 

[moltenmetal]

The OP is probably feeding peroxide into a water treatment system.

If you need a flow switch, look at the Efector thermal dispersion switches. They have adjustable switching points and if the lower housing is made with small enough flow passages, they work with flows this low.

Any flowmeter or transmitter you use will need to deal properly with the pulsile output of your pump, and most low-end flowmeter products produce significant errors due to their inability to generate an accurate mean flow out of all those pulses. Use a pulsation dampener to smooth things out so that whatever flowmeter you use will have a better chance of working.

You can forget about the calibration curve method- it won't work with peroxide. The oxygen you need to pump WITH the peroxide will cause you no end of trouble without a flow switch at minimum.

If it's basic indication with switching that you want, once you've got the flow properly pulse-dampened a rotameter will work fine. You can get an optical emitter/detector pair switch which will do a decent job of low flow sensing. But you need to make sure that you vent the oxygen produced by the slow decomposition of the peroxide or it will wreak havoc with your pump's ability to maintain prime- and with the flowmeter's readings. That can be done with a solenoid valve and timer interlocked to the flow switch, or by other means. Some of the pump vendors have their own little tricks to deal with peroxide.

If you actually need accurate flow measurement and transmission at these rates, you're pretty much stuck with either coriolis, thermal (Brooks, Porter, Horiba etc.)or positive displacement flowmeters. If you could find a magmeter with a tiny flowtube, that would work too- 35% is plenty conductive enough- but it's tough to get velocities high enough at these low flows to make the mag principle work properly. None of these options are what you'd call inexpensive, but coriolis is the most expensive by far- and although not totally insensitive to errors induced by any stray 2nd phase (oxygen) in there, it's the method of these which is least sensitive to this problem. Stay away from the laminar flow differential pressure units- they're junk even with water, and will be disastrous with peroxide.

 

[kenvlach]

The OP is probably feeding peroxide into a water treatment system.

If so, an ORP sensor & controller might be appropriate.

 

[Luising]

Well, the best way to measure your flow will depend on your flow patern, it is most likely that you have a turbulent flow, (just in case you dont you have to pay atention to this) anyway magnetic flowmeters work great on small flows, if the pump itself is not enough to measure your flow try adding a pulsation dampener after the pump (slightly oversized is better) and then add the magnetic flowmeter (any other flow meter should work nice but I have used magnetics, optiflux, for small flows ande they work superb).

 

[dcasto]

I still believe you can get a metering pump that will measure cc/hour, If we can feeed moraphine to a patient and not kill them, the same pump can pump a chemical into water at an exact dose. The sytem can have a feed back control based on other measurements like flow, GC results, or other instruments.

 

[danw2]

Although I'd consider the pulse counting technique already mentioned, I have used the Siemens low flow FC mass flow meter in several situations with great success at
ranges of 6kg/hr. Its turndown ratio is high, a claimed 500:1, so your ~3.6kg/h should be within its range.

general description:

https://pia.khe.siemens.com/index.asp?Nr=13786

spec sheet:

https://pia.khe.siemens.com/efiles/...en_extract/sitransf_fcmass2100di15_fi01en.pdf

Dan

 

[moltenmetal]

kenvlach: I was thinking this was related to UV oxidation etc. rather than to simply "sweetening" water by adding H2O2. If it's the former, the low concentrations of H2O2 required in the feedwater render ORP control very unreliable.

 

[camabee]

At the risk of mentioning a specific product, there are stepper motor driven diaphragm dosing pumps on the market with integrated flow measurement (automated drawdown) and 4-20mA or Profibus feedback of the current delivery rate
see

http://www.alldos.com/pdf/209M-EN-15.820019.pdf

 

[twr]

camabee,
We were using diaphragm dosing pumps - the challenge we faced was a single source peroxide charging a steam line going to 14 different flowpaths at 2 second intervals. The steam backpressure varied as much as 5 psi from one path to the next, so that is what caused the problem with the dosing. The diaphragm was pneumatic, as well, so there were some paths that would get a double pulse with low backpressure, and other paths might not get a pulse at all.

The peroxide flow would not be turbulent, pulsing over a 20 m line would send a wave over, but I do hear the feedback about the pulsation dampener.

Also, the bigger issue became what is the actual flow through the line? We are using a dosing pump but did not have a representative continuous flow to be used for legal record. We were not catching each pulse of the pump on the data acquisition system - that may be a hardware problem.

Thanks for the feedback!

 

[dcasto]

Daniels makes a micro turbine that is used to dose ethylmercaptain into propane. They pack a complete system. Here is how we get rid of the pulseing. We put the mercaptain in a pressure vessel rated at 250 psi. The propane is at 200 psi. There is no pump, just N2 to keep the mercaptain at 250 psi. 250 psi tanks are very very common, they are household propane tanks, sizes to 30,000 are available.