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p_bihari@yahoo.com
science forum beginner

Joined: 03 May 2006
Posts: 13

Posted: Wed May 03, 2006 6:27 am    Post subject: Compressor Suction throttling

We have a centrifugal compressor in MEG plant which is recycling gas
flow thru the system. Makeup is added at suction & product is removed
from the loop.

Compressor is a constant speed machine. Which is giving higher pressure
at discharge say 18 Kg abs. We dont need 18 Kg in the system so there
is a pressure killing device which is reducing it to 17 Kg abs. Further
system drop is 3 Kg. So at compressor suction it is 14 Kg.

Now somebody proposed to install IGV i.e. instead of dischrge throttle
it should be suction throttle.

I am not able to understand how this is going to reduce the power
consumtion in the compressor.

Because,

1. Will the preesure ratio change in this case, I suppose so bcoz
suction pressure is different now.

2. If it changes, it will increase or decrease. My opinion - it will
increase mathematically.

3. IGV control will reduce flow or not, bcoz we dont want to reduce
flow. (My plant capacity will go down.)

4. How to calculate suction pressure and power consumption in this
case.

Thanx
beav
science forum addict

Joined: 18 Oct 2005
Posts: 63

Posted: Wed May 03, 2006 3:07 pm    Post subject: Re: Compressor Suction throttling

On 2 May 2006 23:27:06 -0700, p_bihari@yahoo.com wrote:

i'm looking at this as a thought question. i can't give you a
supporting calculation...

 Quote: We have a centrifugal compressor in MEG plant which is recycling gas flow thru the system. Makeup is added at suction & product is removed from the loop.

OK

 Quote: Compressor is a constant speed machine. Which is giving higher pressure at discharge say 18 Kg abs. We dont need 18 Kg in the system so there is a pressure killing device which is reducing it to 17 Kg abs. Further system drop is 3 Kg. So at compressor suction it is 14 Kg.

OK

 Quote: Now somebody proposed to install IGV i.e. instead of dischrge throttle it should be suction throttle.

i'm presuming an IGV is an Inlet Guide Vane throttling device - a
multivane "carburetor" that chokes inlet flow down... am i right?

 Quote: I am not able to understand how this is going to reduce the power consumtion in the compressor.

i figure it won't.

 Quote: Because, 1. Will the preesure ratio change in this case, I suppose so bcoz suction pressure is different now. 2. If it changes, it will increase or decrease. My opinion - it will increase mathematically.

its just acting as a damper. your 1 KG abs pressure killer is doing
the same thing...

 Quote: 3. IGV control will reduce flow or not, bcoz we dont want to reduce flow. (My plant capacity will go down.)

you are already throttled by 4 KG abs by the pressure drop and
pressure killer...

 Quote: 4. How to calculate suction pressure and power consumption in this case.

what about just putting some kind of a Variable Power controller on
the compressor itself? take off the pressure killer and then figure
out ways to reduce the system loss.

your compressor would probably run slower with less power draw and
you'd actually be doing a little debottlenecking...

of course, i could just be nuts...

 Quote: Thanx
mrdarrett@gmail.com
science forum addict

Joined: 28 Dec 2005
Posts: 55

Posted: Wed May 03, 2006 5:36 pm    Post subject: Re: Compressor Suction throttling

beav wrote:
 Quote: On 2 May 2006 23:27:06 -0700, p_bihari@yahoo.com wrote: i'm looking at this as a thought question. i can't give you a supporting calculation... We have a centrifugal compressor in MEG plant which is recycling gas flow thru the system. Makeup is added at suction & product is removed from the loop. OK Compressor is a constant speed machine. Which is giving higher pressure at discharge say 18 Kg abs. We dont need 18 Kg in the system so there is a pressure killing device which is reducing it to 17 Kg abs. Further system drop is 3 Kg. So at compressor suction it is 14 Kg. OK Now somebody proposed to install IGV i.e. instead of dischrge throttle it should be suction throttle. i'm presuming an IGV is an Inlet Guide Vane throttling device - a multivane "carburetor" that chokes inlet flow down... am i right? I am not able to understand how this is going to reduce the power consumtion in the compressor. i figure it won't. Because, 1. Will the preesure ratio change in this case, I suppose so bcoz suction pressure is different now. 2. If it changes, it will increase or decrease. My opinion - it will increase mathematically. its just acting as a damper. your 1 KG abs pressure killer is doing the same thing... 3. IGV control will reduce flow or not, bcoz we dont want to reduce flow. (My plant capacity will go down.) you are already throttled by 4 KG abs by the pressure drop and pressure killer... 4. How to calculate suction pressure and power consumption in this case. what about just putting some kind of a Variable Power controller on the compressor itself? take off the pressure killer and then figure out ways to reduce the system loss. your compressor would probably run slower with less power draw and you'd actually be doing a little debottlenecking... of course, i could just be nuts... Thanx

I vaguely recall from one of my process classes that you're supposed to
throttle liquid flow downstream of the pump, to prevent cavitation, but
you're supposed to throttle upstream of a gas compressor. I do not
remember why - possibly for efficiency...?

Then again, it's been nearly a decade since I took the class, and I'm
operating only on memory here. (And I must confess I have no
experience working with gas compressors.)

A variable speed control on the compressor sounds like the more
efficient approach, but my intuition tells me it is also the more
expensive approach. A cost-analysis must be performed to determine if
this is a wise idea (for example, if the additional cost to add the
variable speed drive is paid for by what would be saved in electric
bills within say two years, go for it).
p_bihari@yahoo.com
science forum beginner

Joined: 03 May 2006
Posts: 13

Posted: Thu May 04, 2006 7:40 am    Post subject: Re: Compressor Suction throttling

VFD installation is not practical as well as economical since its a big
machine of 3500 kW. Gas Volume flow is large with ~400,000 Kg/Hr.
Kelvin Hales
science forum beginner

Joined: 03 May 2005
Posts: 45

Posted: Thu May 04, 2006 8:28 am    Post subject: Re: Compressor Suction throttling

In article <1146637626.129869.189310@i39g2000cwa.googlegroups.com>, wrote:
 Quote: We have a centrifugal compressor in MEG plant which is recycling gas flow thru the system. Makeup is added at suction & product is removed from the loop. Compressor is a constant speed machine. Which is giving higher pressure at discharge say 18 Kg abs. We dont need 18 Kg in the system so there is a pressure killing device which is reducing it to 17 Kg abs. Further system drop is 3 Kg. So at compressor suction it is 14 Kg. Now somebody proposed to install IGV i.e. instead of dischrge throttle it should be suction throttle. I am not able to understand how this is going to reduce the power consumtion in the compressor. Because, 1. Will the preesure ratio change in this case, I suppose so bcoz suction pressure is different now. 2. If it changes, it will increase or decrease. My opinion - it will increase mathematically. 3. IGV control will reduce flow or not, bcoz we dont want to reduce flow. (My plant capacity will go down.) 4. How to calculate suction pressure and power consumption in this case.

Here at KHACE we do dynamic simulation of compressors and their associated processes and control systems. If
you can find the characteristic curves for your machine (you should have them in records, or perhaps get them
from the manufacturer if you can trace the purchase order number), you will have curves of volume-flow against
head at various speeds, or for various Inlet Guide Vane (IGV) settings, as well as curves of efficiency and
power consumption (these are the standard sets of curves for compressors, and typical of what is required to
do a simulation or design calculation).

Suggest you have a look also at some text books on control of centrifugal compressors. (The ISA used to offer
several.) For example: <http://www.fetchbook.info/Centrifugal_Compressors.html>

Inlet Guide Vane (IGV) adjustment is an efficient way of modulating flow through a compressor. it is not the
same as throttling the inlet flow by means of, say, a control valve - which will reduce pressure and density,
rather IGV change the way that flow impinges on the compressor intake stage; by providing a pre-rotation. It
is, therefore, more akin to having a variable 1st-stage compressor characteristic. You will save overall
energy by not increasing head (energy imparted to the flow) more than is necessary, and then having to destroy
the unwanted portion. OTOH, it is not necessarily straightforward to just add IGVs to an existing machine.
This is really one to discuss with your compressor vendor/manufacturer.

Kelvin B. Hales
Kelvin Hales Associates Limited
Consulting Process Control Engineers
Web: www.khace.com
beav
science forum addict

Joined: 18 Oct 2005
Posts: 63

Posted: Fri May 05, 2006 5:21 pm    Post subject: Re: Compressor Suction throttling

On 4 May 2006 00:40:20 -0700, "technologist" <p_bihari@yahoo.com>
wrote:

 Quote: VFD installation is not practical as well as economical since its a big machine of 3500 kW. Gas Volume flow is large with ~400,000 Kg/Hr.

WOW! BIG is an understatement!

Kelvin (in his follow on post) clearly has the most on-point response.
You would need an IGV designed for the gas path, rather than
throttling it down.

FWIW, if the manufacturer could use IGV control for added efficiency,
would they have designed and installed it a the time of manufacture?
perhaps they have a refit available using the known pump curves?
p_bihari@yahoo.com
science forum beginner

Joined: 03 May 2006
Posts: 13

 Posted: Sat May 06, 2006 3:56 am    Post subject: Re: Compressor Suction throttling Thanks to all for your time. However, probably I was not able to explain my question clearly. Its not related to IGV only. It is discharge throttling vs IGV. My question says, 1. IGV is going to reduce the flow (whatever efficient design is supplied by the Manufacturer), whereas we dont want to reduce the flow as it is going to reduce the plant capacity. 2. Why a power saving in case of IGV vs discharge throttling if flow is same. (Mind it, this is a constant speed machine). 3. I dont have any doubt that IGV will save power in an stand alone system vs no flow control. SO no ISSUE no Question. Thanks if u can refocus on this question.
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