6 replies to this topic

[A mukherjee]

Hello everyone,
I am facing problem in sizing header at compressor discharge..
Suppose there are 2 compressors of 8000Nm3/hr of air flow at 9Kg/cm2(g)...
So total 16000 Nm3/hr of air will be there at the common header...If the discharge lines are of 6",then what will be the size of header?
Thnxx in advance.

[Zauberberg]

It all depends on the maximum air demand in your plant, and also on the required pressure level of Instrument Air system and Nitrogen system (if it is produced from the Instrument Air), the presence of Air buffer vessel etc. Obviously, there is a limit in pressure drop you can afford.

Below you will see the link for design procedure of Compressed Air systems, published by Iranian Petroleum. They recommend maximum 10% pressure drop of the initial value (compressor discharge) but this figure is only a general guideline - you need to consider those issues I mentioned in my first paragraph.

http://217.174.18.60...pr/e-pr-330.pdf

Some useful information you can find in the article attached below - particularly those related to branch connections, tube supports, air delivery, and some other subjects.

Attached Files

•  Instrument Air Piping System Design.pdf   65.75KB   334 downloads

[A mukherjee]

Thnxx a lot zauberberg...Actually I want to know suppose the discharge lines of each compressor is 6"(specified by the compressor vendor),then what will be the size of header?Compressors are of the same capacity and pressure mentioned in the very first post...The air from the compressor header is going to nitrogen generator system....In the system,i have .8kg/cm2 pressure drop.

[Zauberberg]

I believe there must be an Air Dryer unit first, upstream of the Nitrogen Generator package. Normally the air is dried first, before being introduced to the Nitrogen package.

As said in my previous post, you will size the header based on (at least) two constraints:

- 10% pressure drop, assuming maximum anticipated air delivery, and/or
- Minimum pressure required for the Instrument Air Dryers, and Nitrogen Generators - BUT please be aware of the fact that this will vary depending on whether you have Plant Air, Instrument Air, and Nitrogen buffer vessels that can provide reasonable surge volume - I would strongly recommend having those.

Also, we don't know if the compressors work in lead/lag mode (based on estimated flow demand), or you actually have both of them operating 100% of time?

Now, looking at your flow rate (80,000 Nm3/hr @ ~9barg and let's say 50 degC), a 100m 6" header with a couple of 90 degrees elbows, and several 90 degrees tee connections will result in 0.7bar pressure drop, and the velocity of 27 m/sec at the header outlet (lowest pressure). I would say this is quite reasonable figure. You can be a little bit more conservative and go for 8", particularly if you ever consider future tie-ins for another air compressor.

[A mukherjee]

Thnxx a lot Zauberberg...
Actually The capacity of each compressor is 8000Nm3/hr..
Is there any velocity limitation of air,Gaseous Nitrogen?
I can find velocity limitation only for Oxygen.

[Zauberberg]

There is no such thing as velocity limitation dependent on chemical specie (Nitrogen, Oxygen, or Air). There are some guidelines depending on whether we handle steam (saturated/superheated), dry or wet gas, air, etc.

Find attached the page from C.Branan's "Rules of thumb" and see recommended velocities for different kind of fluids. Notice that the limit for air is 100 ft/sec, but you must not forget the pressure drop limitations you have. In addition, gas velocities above 60 ft/sec usually create substantial noise.

Attached Files

•  C.Branan - Recommended Velocities.pdf   51.98KB   200 downloads

[A mukherjee]

Thnxx a ton...