4 replies to this topic

[blooma]

Hello all,

 

I would love to hear your opinions regarding redundancy Vs economy in compressed air systems.

I'm planing a compressed air system designed to supply air for instruments and pneumatic actuators.

My initial intention was to place two compressors, one reciever, one set of filters (1 miron and 0.01 micron before and after the driers) and two (dessicant type) dries.

The client is asking me to drop one drier.

Based on your good experience, will that lead to a catastrophe or is using two dries was just "overshooting"?

 

Thanks 

[Erwin APRIANDI]

Hi blooma,

 

How could we know it will lead to catastrophe or not if you don't give a background regarding to the Plant.

like what is the plant, Offshore Oil and Gas/Onshore Petrochemical/etc, and how big you size the receiver, and how long it is required to regenerate one dryers.

As what I know from instrument guy, air for instrument and pneumatic actuators can be out of specification for limited time

but it is not for continous condition (for more detail you should discuss with your instrument engineer)

 

That is why in some of my project, the instrument air back up are coming from a service air system.

 

Instrument Air System for facilities which handling a hazardous material is crutial, since the loss of supply for Instrument Air can lead to Major Shutdown since the plant is not having the capability to control the plant.

 

But I have worked in a confectionary plant before where the plant work in batch, they only have one dryers and very small receiver for 2 hour consumption. So the crutiality is determine based on many factors.

[blooma]

Hi Erwin,

 

Sorry fot the lack of information. I tried to make it as short as possible and I probably went too far...

The plant is a water DeIoinization plant.

Size of compressors: 120 Nm³/hr (each)

Size of dries: 150 Nm³/hr @ 35 deg C

Size of reciever: 400 - 1,000 lit

 

regarding your question about regeneration time - I realy don't know.

 

Blooma

[Art Montemayor]

Blooma:

 

I know you think you are giving all the basic data – but you are not.  For example, the topic here is an air dryer unit; but we don’t know the TYPE of unit.  “Dessicant” type is not a proper description.  Calcium Chloride is a desiccant; is that what you mean?  I think you mean to state you are selecting an adsorbent type of dryer.  That makes it a regenerative type of dryer.  If it were Calcium Chloride, that would make it a consumable type of desiccant.  A consumable type of dryer needs no spare capacity.

 

Indeed, as Erwin has pointed out, stand-by spare units (not “redundant”) are not commonly employed.  I have designed adsorbent dryers, fabricated some, and operated them; but I’ve never seen anyone use a stand-by, spare unit when drying instrument air – even on an offshore platform.  I personally don’t see the need for that type of “belt-and-suspenders” type of design.  Normally, if the adsorbent dryers are down due to maintenance or adsorbent change-out (usually every 2+ years), nitrogen or even service air is used provisionally as the instrument gas.  In the case of service air, a small vessel filled with old adsorbent can be used in-line to ensure no liquid water droplets get by.  In fact, a calcium chloride-filled vessel can be used provisionally to produce dry air while the adsorber unit is off stream.

 

To make my point clear, I designed and built an adsorber unit (using Mol Sieves) to dry compressed air at 3,000 psig for an air separation plant that I operated.  The unit was regenerated every 16 hours using dry nitrogen and an electric heater.  The air separation plant supplied critical oxygen supply for hospitals and yet I never designed nor had a stand-by, spare dryer unit.  The adsorber worked very well and never caused any down time.  It produced -100 ^oF dew point air – much, much more demanding than your instrument air requirement, and yet was very dependable.  That should tell you how hard it is to justify a spare unit.

[blooma]

Thank you both.

This was very helpful.