Moisture measurement in instrument air

What levels are you talking about?

Well we have had instances where an operator would open a drain valve at the bottom of the Fisher pressuregulators which regulates the pressure from 100 psi down to 40 psi and get maybe a 1/4 of a small coke cup.

After that the air has water droplets in the air spray.

We have had valve positioners malfunction as a rsult of the excess water.

It is my intention to keep all the instrument air dryers well maintained, change the reactivated alumina dessicant on a regular basis, but just in case we do get abnormal amounts of water, a 4-20 ma generated alrm on the DCs would alert the operator.

That's what I thought. I am only familiar with equip that test in the ppm range.

Although, Meeco or Ametek might make something for you. I'll do a little looking too.

Have you checked with GlobalSpec - CR4's parent?

AMETEK has some good equipment for this.

Panametrics has aluminum oxide based sensors that are down and dirty but work well enough in this application.

One reason I recommend the chilled mirror type of hygrometer is that this measuring technology is a standard based on first principles. This means that it requires no calibration. Sensor based instruments such as Panametrics require periodic calibration to insure accuracy, and they do drift. Advantages the Panametrics sensor has over a chilled mirror system include faster response time and the fact that they are water specific. In this application response time is usually not an important consideration and as long as water is the highest condensable gas in your sample it will be the only gas to condense on the mirror. We have evaluated and used all types of hygrometers and make our recommendation based on many years of experience.

Many thanks to all you folks for your assistance and guidance.

It is really refreshing to know that technical folks in the field all over the world have this cooperative spirit.

I probably should have given a bit more information on my problem:

I am the I&C engineer on a large power plant in the Caribbean.Tropical weather ( or to tease a bit , golf weather all year round) so no chance of freezing anything.

We have dual chamber Pall Trinity ( I think they may have been bought out by someone else) reactivated alumina dessicant dryers.

We do have problems with the spool valves which do the absorbing/reactivating cycles on the dryer, as a result we can have water or a heavy concentration of moisture in the instrument air system before the operator can realize he has a problem.

We even had situations where machines tripped off-line as a result of water in the air system causing pneumatic fuel valve problems.

I observed that many folks are using the refregerated-dryers now .

Has anyone out there switched from dessicant to refregerated drying and if so what has been your experience.

I know I need to fix the dryer problem but installing hygrometers will al least hel me to manage the problem until final solution

I am still looking forward to your comments and assistance.

Thanks again.

CR4 is a great help.

Refrigeration dryers work very well however, they have similar limitations as desiccant dryers. If you put too much air through them (exceed their capicity) the air will come out wet. You should probably start by knowing approximately how dry your air needs to be. Room air at 50% RH is about 16,000 parts per million of water by volume (ppmv) at atmospheric pressure. Air dried to -40 degrees has about 130 ppmv. Air used for critical applications can be dried to about 5.0 ppmv. As you can see, there is a substantial range.

He's talking about significant amounts of H2o. I agree with the dessicant statement. However it seems his app should allow for inexpensive monitoring of the dryer performance downstream where the negligible drift factor is easily overcome.

A factory in north-west London uses instrument air that is dried to -40degC dewpoint. If there is water at any point downstream of the dryer then there is something seriously amiss with it, and it is overdue for attention.

Fitting an additional measuring device will simply illustrate the problem, not solve it.

Fitting an additional measuring device will simply illustrate the problem, not solve it

Definitely the case. Putting in a different measuring instrument wont repair the problem. Did the problem have a known start date; if so what happened about that time? Otherwise, I would call the manufacturer of the driers (or a competitor thereof).

PWSlack is correct. You need to look at your system, and then install a monitor. I was researching some of the same products and have some downloaded product files that may be of intrest to you. Here's a link to Bosch Rexroth product website http://www.boschrexroth.com/pneumatics-catalog/Vornavigation/Vornavi.cfm?Language=EN&PageID=p55166 . I'll send you a couple of the files I downloaded during my research on dryer and hygrometer products.

many thanks

Normally, instrument air is de-hydradrated using a compressed air refrigeration machine which dries the air to 35 dF dew point. The accumalated condensate is drained through an automatic dump valve. I would think the -40 dC would be too low, the moisture would freeze in the moisture storage tank. The air dryer refrigeration system, these are packaged units, is sized according to the capacity of the air compressor.

Hope this helps.

g scott

_______________________________________________________________________

When I ponder, Isits and thinke, sometimes I just sits.