5 replies to this topic

[J_Leo]

Hello,

 

I have some a question regarding the dew point of instrument air in LNG plant.

 

I saw specifications in past project: -40°F @ atm. But per a client standard, it is -40°F under normal operating conditions. Can I use simulator such as Hysis to get the moisture requirement? There is a moisture analyzer on the air drying package with high alarm. I just want to figure out what are the normal and alarm values for the analyzer.

 

Thank you very much.

 

Happy Holidays

 

Leo

 

 

[breizh]

http://www.uigi.com/WebPsycH.html

 

Consider these  resources to support your study .

 

Hope this helps

Breizh

Edited by breizh, 07 December 2013 - 06:29 AM.

[Art Montemayor]

 

Leo:

 

You don’t tell us what type of dryer you are using, but I have to assume you are operating an adsorption dryer – using either silica gel or activated alumina as the adsorbent.  If so, you have little or no control on the specified dew point of the product instrument air.  What an adsorption dryer does (and does well) is that it dries well below the stated -40 ^oF dew point (@ atmos. pressure).  The product air will be produced well below the specified value – and when for some reason or other the dew point is exceeded, the alarm will go off.  You should set the alarm in the analyzer to activate at a dew point that is below the lowest temperature the product air will be at, considering your local ambient conditions in order to avoid forming water ice in the instrument air system and causing a blockage.

 

As I have stated, the product air dew point should be well below the -40 ^oF specification.  What it will be depends on the design of the dryer, the condition of the adsorbent and the efficiency of your regeneration cycle.  You will find this out when you operate the dryer.

 

It is extremely difficult – if not impossible – to control the product air dew point at a specific, constant value using an adsorption dryer.  There are just too many variables involved in the design and  its operation.  What is done in the design is that you calculate for a conservative amount of adsorbent as well as a superficial velocity, allow for good and thorough bed regeneration, and you compensate for aging, attrition, and dusting by using more adsorbent than theoretical.

[J_Leo]

Breizh,

Thank you very much for the reference you provided. I will use it to calculate the water content in the IA at dew point.

Best regards,
Leo
 

[J_Leo]

Art,

It could be either silica or activated alumina as the adsorbent. The vendor hasn't be selected. This will be done during EPC.

I agree with you the moisture from the dryer cannot be controlled. What we do is to specify the moisture at required dew point and set this as the alarm point. I think I can calculate the moisture value using the reference document Breizh provided. If moisture greater than the value then alarm goes off.

Thank you very much for your expertise.

Best regards,

Leo
  

[legend_revolve]

Hello,

I'm also facing this problem right now since I need to determine the set point for dew point alarm at high level as per client requirement.

However, I'm still a bit confused regarding this matter.

 

You don’t tell us what type of dryer you are using, but I have to assume you are operating an adsorption dryer – using either silica gel or activated alumina as the adsorbent.  If so, you have little or no control on the specified dew point of the product instrument air.  What an adsorption dryer does (and does well) is that it dries well below the stated -40 ^oF dew point (@ atmos. pressure).  The product air will be produced well below the specified value – and when for some reason or other the dew point is exceeded, the alarm will go off.  You should set the alarm in the analyzer to activate at a dew point that is below the lowest temperature the product air will be at, considering your local ambient conditions in order to avoid forming water ice in the instrument air system and causing a blockage.

 

Art,

 

I don't understand the justification of setting dew point alarm at below the lowest temperature the product air will be at.

 

From my understanding, dew point is proportional to the amount of water vapor which means that an increasing dew point means there is a greater concentration of water vapor molecules present and vice versa. 'Below lowest temperature of the product air will be at' doesn't mean that it will be lower than the dew point of the air. If we're setting the analyzer to set off alarm at that temperature doesn't that mean we're having off-spec air running in the system? Isn't this an unwanted situation?

 

Shouldn't we set the alarm at a lower dew point or at least the same? So that when the alarm go off, it actually tell us that the air dryer is close to producing an off-spec air?

Please enlighten me on this matter.

 

best regard,

legend_revolve

Edited by legend_revolve, 15 May 2014 - 03:28 PM.