Minimum suction pressure 1

[Falcon03]

In our FCC unit we have a variable speed centrifugal wet gas compressor, the suction pressure is in the range of (0.7 to 0.9) barg (1.7 to 1.9 barA). My question is what will be the effect if we try to run at lower suction pressure? Below 0.65 barg.

In other word, is there is a minimum suction pressure which should be avoided? Tks

 

[zdas04]

Centrifugul compressors are pretty interesting beasts. They don't cavitate like a centrifugul pump, but they do have a maximum compression ratio per stage before they begin to stall. This max tends to be around 2 ratios. So you can figure your minimum suction pressure by converting the fixed discharge pressure to absolute and use:

P(s) = P(d)/R^N

Where R=2 ratios and N=number of stages.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[25362]

Lieberman and Lieberman in their A Working Guide to Process Equipment (McGraw-Hill) dedicated Chapter 28 to: "Centrigugal Compressors and Surge" with direct reference to situations such as that mentioned by Falcon03.

 

[rlewis]

MInimum suction pressure really isn't a problem. You do however need to supply sufficient volumetric flow since this is what a centrifugal "sees". Dropping the suction pressure decreases gas density, and so you either handle less mass flow to keep at the same point on the curve, or move the operating point to the right. The compressor curve has limits to the left (surge) as well as to the right (choke). You just need to stay "on the curve". You also mention this is variable speed. Hopefully the manufacturer gave you a curve with various speed lines. If not, you can use "fan" laws" to estimate performance at the higher speed. Assuming discharge pressure is what you want to maintain, a higher speed will be required to produce more head at the lower pressure. Head is proportional to the log of pressure ratio. It is hard to give a tutorial in this little space, but I hope this helps !

 

[Cootey]

We had a similar problem with a recycle gas compressor. The approach we took was:

1. Established that the compressor could operate at the head and flow at the lower suction pressure. Note that the curves are typically shown in 'actual' versus 'standard' units.

2. In our case the desired operating point could be accomodated, though very close to the surge line on the curves.

3. Given the compressor only had rudimentary surge control facilities, we discussed with suppliers of surge control equipment.

As it turned out, the cost of upgrading the surge control systems wiped out the economic benefit of the lower suction pressure operation.