Electrical conductivity of 50% glycol with inhibitors 4

[hawkeyes]

Does anyone have any information on electrical conductivity of 50% propylene glycol with inhibitors? I have a system that is using heated glycol to heat make-up air. The glycol is heated in a shell and tube heat exchanger, and I need to detect any glycol leaks into the condensate by measuring conductivity.
Alternatively, is there something I can add (without deleterious effect) to increase the conductivity?

 

[1969grad]

Check with Union Carbide or Dow as they manufacture glycols. I understand they do have a web site.

 

[cme]

Glycol soutions are fed (topped-off) buy a pumped glycol feeder available by JL wingert, armstrong, etc

Heat exchanger tubes are .035" wall

I've never had to monitor glycol leakage as you propose

 

[walkes]

Further to cme's statement, the injection pump for the feed can be alarmed to allow the user know that there may be a system leak.

See the following link:

http://www.axiomind.com/home.htm

Under products see RIA10-1-SAA.

Another method would be to draw a line on the tank indicating the level when the system is topped up.

 

[tombmech]

Measuring glycol concentrations by electrical conductivity is not recommended, especially in process systems, heat exchangers, etc. The quality of the water will almost completely determine the conductivity, rather than the glycol.

The equipment referenced above is OK for small stuff, but large chilled water systems are often measured by specific gravity - the same method typically used to check the antifreeze in your car. However, this is not a reliable indicator of leaks, and it requires periodic sampling - not something that can be used in a control loop.

A second method in favor these days - one much more accurate - is measurement with a refractometer. Handheld refractometers are economical at only $200-$300 each. For process applications, an online digital model is preferred. Those can cost a thousand dollars or more. However, they can continuously monitor mixtures under a wide variety of conditions.

Misco has a bunch of them -

http://www.misco.com/index.php

AFAB Industries is another -

http://www.refractometer.com/

 

[hawkeyes]

In this particular system we are using an expansion tank with a bladder, so we don't have a make-up pump.

 

[cme]

I always eqip the system with a 2" hose connection with check valve so the contractor can use a pump to fill the system ...... those glycol feeders are not for system filling ..... i specify premixed / pretreated drums

 

[Drazen]

Forget about conductivity, it is directly related to temperature and is used in domestic/process water systems (water treatment) where you don't have such variations in temperature that can normally occur in thermo-systems.

Frankly, I don't see any need for continual monitoring of glycol concentracion! Maintenance people can do regular checks with gravity tester (the same as for cars) and if you heve leaks, mixture will leak, so concentration will not change.

If lack of water in system can endanger your primary process, I propose you use of flow switch, that is sufficiant.

As I understand, your make-up air is outside air entrance to HVAC system. In that case you can save money and just set an alarm on your pre-heater controller, normally that can give you warning on malfunctioning of pre-heating system (would it be leak or something else).

If you really need additional warning, then use flow switch!

 

[hawkeyes]

Drazen, you seem to have drifted from the original question. The problem is glycol in the condensate which could be fatal to the boiler. We need to be able to tell if any glycol is leaking into the condensate i.e. heat exchanger tube failure or leakage.

 

[Whoops]

How about a secondary heat exchanger, i.e primary steam/water heats intermediate water. Intemediate water is pumped to a secondary heat exchanger where it heats the glycol. No chance of glycol contamination. The secondary circuit would need a pump, expansion vessel & pressure relief valve,but no worries about the boiler..

 

[Drazen]

Yes, sorry, it seems I didn't understand it fully - is it that you have steam/hot water heat exchanger where hot water (with glycol) is heated side?

I still don't beleive that you can handle this problem with measuring conductivity.

Secondary exchanger for purpose of separation is allways the best, but also the most expensive solution.

I had problem similar to yours in food industry, where we had to prevent any possible leak in plate heat exchanger. Temporary solution was to maintain higer pressure on clean side (it was water-water exchanger so it was not a big problem) so if some leak occurs, it would run in non-critical direction.

But you have steam-water system and problem is that pressure drops on steam side when you shutdown the boiler. There are some design possibilities, to install check valve downstream the boiler, automatic on-off valve upstream the steam trap, which closes when boiler shuts down, and you additionaly need one safety valve between these two. That way you can achive contolled minimum pressure on steam side when boiler is off. This is not so simple, but maybe it is cheaper then additional heat exshanger (for which you will also need additional piping, control valve trim and so on).

In my mentioned situation we decided on permanent solution - to replace plate exchanger with monster-big shell and tube exchanger, beacuse they are much, much more resistant to any leaks! But you already have shell & tube?! It shouldn't leak so easily - when it leaks it means it is significantly damaged - has apparent crack on plate-tube weld or on tube itself (while with plate exchanger slightly imbalanced gasket tightening can cause big leak).

Interesting?!

 

[tombmech]

The refractometer is still a viable solution. The expense is trivial if you're comparing it to boiler damage. There are some conductivity curves vs. glycol concentration, but they are for industrial quality glycol, do not include inhibitors, and are for ethylene glycol, only.

DOW quotes refractive indexes for all of its glycol treatments, not conductivity. Conductivity is a reasonable solution to measure turbidity, not glycol concentration.

 

[hawkeyes]

This is what Hercules has to say. "I suggest you use an "inhibited" glycol - one with corrosion inhibitors already added. This will contribute conductivity that will enable us to use conductivity meters for detection in the event of leaks".

 

[tombmech]

Well, I suppose you can try testing for it yourself - put in a conductivity meter output to a DDC system. Check for your typical water conductivity (including chemical treatment), then mix in a little glycol. If you can measure it with the conductivity meter output, then make that threshold your alarm setpoint.

My concern is that you will not be able to reliably measure increasing concentrations to detect a leak, or reliably enough to prevent damage to your boiler - but maybe so.

The best that I could find several days ago was the graph below. This one is for Ethylene Glycol, not propylene. I found it at:

http://www.meglobal.biz/literature/product_guides/MEGlobal_MEG.pdf

and

http://www.meglobal.biz/literature/product_guides/MEGlobal_DEG.pdf

The second one is for "Diethylene Glycol", but I don't even know what that is. In any event, these are not with inhibitors. What you can deduce from the graph is similar to everything I've read about glycol. You may get a small spike in conductivity as glycol concentration is initiated, then it rapidly declines to very little. That would seem to be a contradictory mechanism to test for a leak.

 

[hawkeyes]

Dow came back with the following:
Properties for Dowfrost (Propylene Glycol)
Electrical conductivity for 50% at 25 deg C = 2,060 micromhos/cm
and 249 micromhos/cm for 50% at 0 deg C. Looks like the inhibitors increase the conductivity substantially. I'll have to check detection equipment sensitivity.

 

[tombmech]

So ... you'll have to constantly measure temperature as well, and program a PID loop to compare the data vs. the alarm threshold setpoint. Meanwhile, your conductivity probe will have to measure over an entire magnitude change from 25 deg. C to 0 deg. C.

Also, I thought we were talking about leaks of a Glycol aqueous solution into non-glycol water. So, the conductivity values will have to adjusted for the volume of the 50% glycol leak (small) divided by the volume of water solvent in the shell (very large). Seems like you would need conductivity values for glycol at 1%, 0.1%, or less, instead.

Unless I'm missing something, this may be too difficult.

 

[athers6]

Could you use a TOC monitor on the condensate to monitor gycol contamination?
The other way might be to trace the Glycol with a Nalco product, TRASAR, and have a Trasar unit monitoring the condensate stream.