Anti-Surge Control for Centrifugal Compressor

Compressor is an equipment for increasing pressure of gas stream. One type of compressor is centrifugal compressor. One thing that is interesting about this centrifugal compressor is the anti-surge control, where other types of compressor are not necessarily required. Why? Because centrifugal compressor is relatively a constant head-variable volume machine or we can say the flow entering centrifugal compressor fluctuates more than other type of compressor. So let's talk about this "surge".

What is surge? Surge is a condition involving volume and pressure fluctuation that happened when attempting to operate at higher pressure ratio than compressor design maximum. See the compressor chart below:

 At chart above, there is a surge limit line that can be reached from stable operating point by either reducing flow or decreasing suction pressure. Surge usually occurs below 50-70% rated flow. Anti-surge control system senses conditions approaching surge, and maintain the pressure ratio below the surge limit by recycling some flow to compressor suction. Recycle flow is taken from air cooler outlet after compressor to prevent overheat. Below is the simple diagram of anti-surge control:

1. Minimum volume

Flow element (FE) senses the flow and flow transmitter (FT) send it to surge control (SC) system. When suction flow to compressor is decreasing below minimum set point, surge control system order the surge control valve (SCV) to open and recycle discharge flow to suction flow until the surge condition is passed.

2. Pressure limiting

Pressure transmitter (PT) senses process pressure and send it to surge control (SC) system. When discharge pressure exceeds the set point pressure, surge control system order the surge control valve (SCV) to open and recycle discharge flow to suction flow until the surge condition is passed.

For example, I take the PHE-FOXTROT Compressor P&ID snapshot as seen below:

At P&ID above, there is two sensing that send information to surge control system, which are FT-100 (for sensing inlet flow) and PDT-100 (for sensing differential pressure between suction and discharge). The surge line is flowing through air cooler (HAL-100) outlet to gas scrubber (MBF-100) inlet via surge control valve (SCV-102).

If you want to read detail sharing knowledge about this anti-surge control, write on your email in comment and I will invite you to shared folder on Dropbox. Thank you :)

References:
GPSA Section 13 Compressors and Expanders

Why Some Fractionation Columns Having Different Diameter?

A fractionation is a unit operation utilize to separate mixture into individual products. The separation involves separating components by relative volatility. Below is the example of fractionation schematic diagram:

 

Heat is introduced to the reboiler to produce stripping vapors. The vapor rises through the column contacting the descending liquid. The vapor leaving the top of the column enters the condenser where heat is removed by some type of cooling medium. Liquid is returned to the column as reflux to limit the loss of heavy components overhead.

Some type of fractionators are demethanizer (top product is methane, C1), deethanizer (top product is ethane, C2), depropanizer (top product is propane, C3), debutanizer (top product is butane), distillation column (products are naphta, kerosene, etc) and many other type fractionators, depends on the products to be made and the feed composition.

The interesting thing is the diameter of fractionation column can be different between the top and bottom section. Why? Let's see the example below:

 

 Demethanizer

 Atmospheric Distillation Unit

On demethanizer figure above, it shows that the top section has larger diameter than bottom section, looks like a bottle upside down. While on atmospheric distillation unit, the bottom section has larger diameter than top section and looks like a bottle.

After having some discussions and reading some references, I try to conclude the answer of this question. It depends on the feed composition that enters the column. When light components is more than heavy components, the top section column might be having larger diameter than the bottom (like demethanizer). Vice versa, when heavy components is more than light components, the bottom section might be having larger diameter than the top (like distillation column). This answer is only a philosophical answer (I know this is still a shallow answer hehehe). Further calculation and sizing is needed to get more precise and detailed answer. Anyone would be adding or correcting my answer? I am very welcoming it. Let's discuss! :D 

Reference:

- GPSA Section 19 Fractionation & Absorption

- http://nptel.ac.in/courses/103102022/evaluation%20of%20crude%20oil/images/2.5.3.jpg

- Discussion with senior and friend: Abang Joni, Mas Andri, and Togel (thank you)