8 replies to this topic

[Babu Prasad]

Our Propane refrigeration system equipped with fin fan coolers to condense propane vapors. We are facing difficulties to remove moisture content from these exchangers after doing hydro testing during our major shutdown. This moisture content creates plugging in low temp chillers when unit started back.  Even we observed free water content in the low point area which takes lot of time to remove from the system. Each fin-fan exchanger purged with dried Inst air followed by nitrogen but it is difficult to remove moisture content with low volume of available air/nitrogen.  Purging with large amount of warm fuel gas reduced moisture content but it consume lot of man-hour to complete the process which delays startup process still, we could not reduce the moisture content to minimum requirement . Dry out method by vacuum pulling need to be confirmed as these exchangers not designed for negative pressure.

Please advise best alternative method to carryout pressure test for these exchangers and kindly suggest if any method to remove this moisture content effectively.

Note: Propane refrigeration system equipped with large number of bays and design pressure of bay 26 bar/temp 110 deg C

Edited by Babu Prasad, 14 September 2013 - 01:51 PM.

[ColinR33]

Typically hydrotesting is the preferred method for pressure testing, pneumatic testing is too dangerous.  Do you hydro every turnaround or were there modifications done to the unit requiring hydro? If you have to hydro the equipment you will just have to face the fact that you have to take the time to properly drain and dry the system before startup.  Blowing heated air through the equipment works to dry it out, this takes time but is very effective in my experience, and worth it for equipment that is in low temperature service.  Do you not have a refrigerant drier installed on your refrigeration system?  A properly designed system should have one of these, it is essentially a dessicant cartridge through which you circulate a portion of your refrigerant flow from the accumulator during startup to remove trace moisture.  Typically not used for bulk moisture removal although it may be possible to source a larger rental unit for startup work.

 

Cheers,

[Babu Prasad]

Mr.colin,

Thanks for your reply. We do hydro test as part of inspection during turnarounds. Due to large numbers of exchangers it has been performed during turnaround which is cost effective and also operation concern. We tried to use hot dry air with piston blowing but it is not sufficient to cover whole surface area of the exchangers to remove the moisture effectively. Our refrigerant system does not equipped with in built drier due to may be large size of unit or may be it decided not benefit as it is will be idle during normal running time. Rental unit may be other alternative but operation feasibility yet to be studied.  Thanks once again for sharing your views.  

[gegio1960]

I'd like to add my 2 cents to this item in accordance with my (design) experience.

1) you can't afford the risk to run the plant without eliminating the moisture.

2) vacuum before start-up is normally foreseen as an efficient method to dry-out the plant.

3) the design pressure of the system should be high enough to allow the vacuum in the equipment... but I agree that the suitability for vacuum conditions shall be verified. Half vacuum could be enough.

4) you're speaking of restart after a major shutdown. Did you verify what has been done at first start-up and/or other restarts?

5) the drier cartridge could be installed with a dedicacted revamp. It' usually sized for only a part (25% or 10%, too) of the total propane flow, installed on a bypass and utilized countinously.

6) if you don't have very big problems in replacing propane inventory (eg dry propane available at the site, existing facilities for off-spec propane storage, possibility to rerun the wet propane in other units...) you can consider this as a last resource.

Good luck!

[PingPong]

The problem could be caused by remaining waterlayer(s) on the pass partition baffle(s) below the tubes in the headers of the aircooler bundles.

 

These water layers can not be drained, so to remove them you need to remove a few plugs at both ends of each header and use a small diameter hose or tube to suck off the water layer. That is assuming the bundles have plug type headers.

 

To dry the aircoolers you do not really need to use dry instrument air. Simply use ambient air. Rent a blower and an electric heater and blow warm 90 ^oC air (20 ^oC lower than mech design temperature) through the bundles at maximum blower capacity. After that purge with nitrogen to remove oxygen.

 

I have never seen or designed refrig systems with driers in them. That does of course not mean that they don't exist, but I never saw it.

Edited by PingPong, 15 September 2013 - 05:56 AM.

[Babu Prasad]

Mr. Pingpong,

The water layers removed as suggested by you from each header plugs. We tried hot air blowing and it consumed lot of man hour as the quantity of the air flow to remove the accumulated water in the tube was not sufficient when compared to original process flow. Any have above method helped to some extent even though it consumed lot of day but we could not achieved to requirement level.

[marchem]

if, by design, you have some pressure in the system below 0 degrees equivalent saturation temperature you need to remove all the water to avoid problems, the usual way is by vacuum but that works for relatively little amounts of water and before water being trapped below oil or other fluids which may make extraction difficult if not impossible. In those cases also dry nitrogen doesn't work and you need to remove all fluids from the system before to vaporize and extract water with a vacuum pump or equivalent procedure, for example with dry air or nitrogen, note that at the end you will need to remove air with vacuum pump to avoid problems with incondensables.

[Babu Prasad]

I'd like to add my 2 cents to this item in accordance with my (design) experience.

2) vacuum before start-up is normally foreseen as an efficient method to dry-out the plant.

3) the design pressure of the system should be high enough to allow the vacuum in the equipment... but I agree that the suitability for vacuum conditions shall be verified. Half vacuum could be enough.

4) you're speaking of restart after a major shutdown. Did you verify what has been done at first start-up and/or other restarts?

5) the drier cartridge could be installed with a dedicacted revamp. It' usually sized for only a part (25% or 10%, too) of the total propane flow, installed on a bypass and utilized countinously.

6) if you don't have very big problems in replacing propane inventory (eg dry propane available at the site, existing facilities for off-spec propane storage, possibility to rerun the wet propane in other units...) you can consider this as a last resource.

Good luck!

 

We are suffering every turnaround to remove moisture from these exchangers which is goes for hydro test.  In my previous plant we used vacuum pump to remove the moisture from whole refrigeration loop prior startup but I was not sure about equipment design for vacuum!   

Is it allowed to use half vacuum as suggested by you in our system which design pressure is 26 bar?

Replacement of propane inventory is affordable and it was carried out as alternative but it will be better if it is replaced at one go otherwise bleeding out small stream of propane will take longtime to achieve the spec. thanks for sharing your views.

Edited by Babu Prasad, 15 September 2013 - 03:54 PM.

[gegio1960]

Babu,

you shall verify whether the vacuum conditions can be applied to your pieces of equipment.

This is a mechanical engineering activity and so, as a chemical engineer, I can't and mustn't directly answer to your question.

Anyway, I asked to do that several times, with quick and positive results.

You've to pass the equipment mechanical drawings to your mech dept for the verification or, in alternative, you can ask the original manufacturers.

The basic principle is that vacuum conditions have lower probability of damage with the following conditions:

- higher design pressures (ie higher thickness of materials);

- lower diameters;

- lower lenghts.

According to the above, I don't see big problems in air coolers designed for 26 barg... but devil is in the details.

 

Moreover, I'd say that the complete procedure is the one depicted by pigpong and marchem, ie vacuum after blowing with (hot & dry) air/nitrogen.

 

About the note of pingpong "I have never seen or designed refrig systems with driers in them", I'd say that Drier is the first piece of equipment that is deleted when the cost cutting run is started ;-)))

 

Regards