acceptable moisture level in IA

[MarkkraM]

I was wondering what a typical dew point people use for their insturment air? Do most plants go to the trouble (as we have) to use desicant dryers or do most simply use condensing type dryers?

My plant had dryers to bring the air down to -60degC.
We managed to stuff them up during our shutdown and I'm questioning whether we need such an expensive system.

I'm looking at three options.
1) Replace like for like our broken down desicant dryer which has a heated regen
(gives dew point of about -60degC): $50K
2) Replace broken down desicant dryer with one that has non heated regen cycle (gives dew point of about -30degC): $30K
3) Rely on our air supplier's condensing dryer (gives dew point of about 2degC) - no cost

 

[Assumptions]

Depends.

If you work to ISA Standards then it would be around that temperature(-60 to -70C Don't have the document handy.) ISA 7.0.01-1996.

If one had to justify a new relevant standard then one would need to look at the effect of a lower dew point on the field device on the coldest day. Assume 100psig at your moisture content expanding to atmospheric from your min environmetal temperature.
What would happen to the plant if the temperature went to a new low?

If the instrument can accept a high humidity and there are no further contaminants in the air then it might work.

I for one would want to gain a consensus knowing the downside from departing from a standard.


I have been in plants where the local controllers were half filled with ice because of poor IA on a very cold day.

Good luck.

Hope this has given you some ideas.

 

[MarkkraM]

Thanks Assumptions,
Did some more research. Found a guidline recommending the dew point for IA should be 10degC lower than the minimum temp reached on the site. -70degC seems a little excessive if you are in a climate that never goes gets below 0degC.

Is there any other driver for low dew points besides avoiding condensation of moisture in your instrument air?

 

[Assumptions]

Some other additions.

Check your field devices, they usually give a max humidity for the spec.
Don't forget that your IA will become 100PSIG at ambient in the supply lines. Then it will drop to 1 atm as it reaches the instrument. There may be some cooling.

If you have H2S in the IA then you may have additional corrision problems.
You may have corrision in your supply lines if they are not galvanised.

I am sure there are more points and I would want to examine, such as the considerations used in established in the ISA document were met before I went and changed an existing installation. Sort of like putting in a larger valve without examing the relief requirements.
Good luck in your research.

 

[Montemayor]

MarkkraM:

Contrary to what some may think, no pneumatic instrument "can accept a high humidity".

The difference between the $50M adsorption dryers and the cheaper non-regenerative dryers is the traditional trade-off: Less captital cost, higher operating costs and problems.

Don't be misled by thinking you're going to get something equal or better for less capital investment. The re-generative adsorption dryer is the equipment of choice because it is trouble-free (regardless of what "stuff them up" means). There are no moving parts in an adsorption dryer; you have to go pretty far and remote to abuse and ruin them from doing their job.

The dryer and cleaner your IA, the longer and better your pneumatic instruments will operate.

Art Montemayor
Spring, TX

 

[saxon]

Markkram, When specifing dewpoint requirements, beware of the differant types of dewpoints, Atmospehric vs. Pressure. There is a large difference. Make sure that the spec. is for pressure dew point, not atmospheric. For example; at 100 psi, -20f atm.dewpoint is equal to 20f pressure dewpoint. If the overnight temp. drops below 20f your instruments will freeze up or starve due to transmission system ice blockage. Plant control will be lost and in the morning you will return to smoldering pile of scrap (I won't mention the bodies strewn all over the plant site). All because someone didn't understand and tried to cheat the laws of physics by buying something cheap.

Before you buy a dryer for instrument air service, make sure you know the difference between the different dewpoints,how they interact and exactly how the dryer was spec'd by the manu'f.

Hope you're not one of those bodies.
saxon

 

[Assumptions]

Dug out a copy of ANSI/ISA 7.0.01-1996 "Quality Standard for Instrument Air".
Could not find a reference to my previous statement that dew poit should be -60 to -70C for this standard. I have used that temperature but that is for colder climates than yours.

What is stated "The pressure dew point as measured at the dryer outlet shall be at least 10C(18F) below the minimum temperature to which any part of the instrument air system is exposed. The pressure dew point shall not exceede 4C (39F) at line pressure..."

Futher : Definition-pressure dew point "The dew point value at line pressure of the compressed air system (usually measured at the outlet of the dryer system or at any instrument air supply source prior to pressure reduction. When presenting or referencing dew point, the value shall be given in terms of the line pressure.."

In addition to what has been stated in previous replies, I recall years ago that warmer climates would have problems with mold. That has aparently diminished some now I suppose with fungal sprays to the electronics.

In summary one would have to look at the coldest IA user in the plant and insure that the dew point meets the criteria.

Hope this helps a bit.

 

[Montemayor]

Since all of us on this thread are Chem E's, I'm sure that we all understand or are aware of the fact the Adsorption unit operation (which is the topic when discussing a dew point of -60 oF) does not accept exact, variable, design dew points. You get the end dew point that corresponds to the adsorbent and the conditions it finds itself in.

Normally, silica gel or activated alumina yield -50 to -75 oF atm. dew points - and that is what you should get. You can regenerate with a variety of methods and that fixes the actual performance dew point you get.

It doesn't matter what ANSI/ISA or anybody else fixes as the "standard" for water content in instrument air. An Adsorption system will give you a product that is normally superior than the ANSI/ISA specifications - and that is more than sufficient. You can't design, from a practical standpoint, to a obtain a higher dew point when using an adsorption dryer. That is one of the great advantages of adsorption drying (besides no moving parts) - the fact that the product is well below the maximum moisture content acceptable.

Art Montemayor
Spring, TX

 

[Assumptions]

Fully agree with you Montemayor. And I expect that when all is done the requirement will be to replace or repair the damaged dryers.
Not working in an actual plant, I can't say how we do things. However, being involved in many different plants I can say how it is done in general, as to how I have been involved and that is to -60C area. Since ISA is basically a users spec most large firms have accepted it as a standard. Then one has bean counters that want you to justify the standard. How does one answer that question? If you are not the head cheese, then saying in my experience it should be so and so won't necessarily get one a dependable system. If one can show why a certain spec is required then someone else, for instance a bean counter then has to take the responsibility.
It looks like the question will come down to whether it is a heated regeneration or not.
No data is given on min ambient temperature, min temperature that an IA supply is exposed to, normal humidity, what happens at max humidity conditions, what are the cycle times for drying, etc. What is the lowest IA supply line temperature on a cold clear night.
If it was my plant the small extra investment would assure reduced instrument maintenance and possible shutdowns, never mind the possible safety aspects.

I would be interested in hearing how things finally turn out.

 

[MarkkraM]

Thanks for all the replies.
Yes I was aware of the difference between atmospheric dew point and pressure dew point and the principles of the absorption process. Our instrument air supplier have recently install a refrigeration type dryer so the quality of the air should be reasonably consistent.

Agree that $20K is normally not a lot of money, but a combination of being over budget in an environment of financial strain means the question was asked. Also we've had on going issues with the heaters and would be nice if there was one less thing to breakdown. This specific incident was cause by a huge slug of water sent through to us from our supplier.

 

[tickle]

A knock out pot before the dryers might prevent future slugs reaching the dryers - but again this adds to the cost. But would be relatively easy to justify as a separate capital project.

 

[ce51ha]

Hi..

Interesting! I just about to install one instrument air dryer in my plant. We are about to use dessicant type...since you're saying it's headance, its that possible if i wanna know what exactly is your problem?

don't you think the regenerated type is the most reliable one?

 

[MarkkraM]

ce51ha,
I think other plants have had less issue than me. As Montemayor said, the re-generative adsorption dryer is the equipment of choice and I tend to agree. My dryers are quite old and did not have such a good maintenance thing going. One big problem we had once (when running only one dryer at a time) was the solenoid valves failing shut and cutting off the plants IA supply completely and forcing us into a shutdown. Depending on how critical your process is you may want to think about redundancy. What would happen if you IA failed and would you have the opportunity to perform maintenance on just one dryer?

My recent problem was due to a one off major shut down kind of incident. Because of the extreme financial strain we're in at the moment I was forced to ask how critical good drying was. From the responses so far and what I've read I'm not convinced a plant needs to achieve any more than a pressure dew point 10degC lower than the lowest ambient temp. There may be other optinos out there if your local climate permits.