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centrifugal compressor start up purge

centrifugal compressor start up purge

I'm now working with centrifugal compressor and trying to reduce start up time. During start up compressor is being purged with process gas by sequence control. Purging timer is set so that three volume changes in compressor and corresponding lines is being acheived. Can somebody help me with one question. Do anybody knows any procedure or practise where i can found how many volume changes should be applied during start up. Compressor is handling CH4 gas.


RE: centrifugal compressor start up purge

There was a study done in the 1920's that tested several thousand piping configurations and found that a clearing purge with 2 bara upstream of the exhaust valve would clear the air in 2.64 pipe volumes with 99.98% confidence.  Most people round that up to 3 pipe volumes.  I found that factoid in a 1960's vintage magazine article while I was in grad school.  

The article that I found referenced the original, but I was unable to find it either on the Internet or through the inter-library loan program.  I had to go with the reference to the reference in the paper I was writing.  I looked for that paper today and it must be in a box somewhere that I can't put my fingers on.  

I've been using the 3X number for 15 years without ever tripping an oxygen sensor after a purge so I'm pretty comfortable that it is plenty, but I don't know how much overkill it really is--Oxygen sensors in the 20's were pretty primitive.

David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376: Forum Policies for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat"  The Master on Dr. Who

RE: centrifugal compressor start up purge


Thanks for replay. This explanation is quite valuable. I was looking for this matter almost three weeks and having this information is better than nothing.

RE: centrifugal compressor start up purge

If you remember from your college days:

a.) A perfect plug flow would require one volume purge.

b) A well mix batch flow follows the "first order transfer function". [1-(1/exp(Vol-purge-gas/V-equip))]
Generally one (vol-purge-gas/V-equip)= 63.2% air removal , two = 86.5%, three = 95%, four = 98.2% etc.  

If three purge volumes work, the physical equipment is closer to well mix batch than perfect plug flow.

RE: centrifugal compressor start up purge

Dear zdas04

I have another questions. Purge time is calculated in a way described below:
1)Calculated volume of compressor and piping within purging envelope,
2)Calculated purging pressure,
3)Calculated mass of gas required for purge,
4)Calculated mass flow through the orifice on pressurization line at the purging pressure,
5)calculated required purge time.

According to this calculation it takes 28 min to purge compressor,but when i calculate the same in volumetric flow it makes difference of 8 minutes. Should the purging time calculation done through mass flow or volumetric flow? Can I use volumetric flows to reduce time for purging.

RE: centrifugal compressor start up purge

If your exhaust pressure (atmospheric) is below the critical pressure for the upstream pressure (usually around half) in absolute terms then your purge rate is only a function of upstream pressure and orifice size.  So what you want to do is:

1.  Calculate the empty volume being purged
2.  Calculate the amount of gas to purge 3 volumes (assume your sytem is at 2 atm absolute) so it is

   3 X Empty Vol * (2 atm / 1 atm)

3.  Calculate sonic velocity of your purge gas at 2 atm
4.  Multiply sonic velocity times orifice area
5.  Divide #2 by #4 to get the purge time

I have a feeling that this arithmetic will result in a value less than 28 minutes.


RE: centrifugal compressor start up purge

That is exactly what I think. But calculation is done in another way.Compressor volume is 51.21 m3. It is being purged from purging/pressurization line on inlet with orifice plate in it and purged through blow down valve on discharge. Blowdown valve has orifice plate as well. Pressure inside compressor during the purge is 9 bar(132 psi). mass of 51.21m3 of gas is 476 kg. times 3 is 1428kg. Mass flow rate at purge time is 3060kg/h. And from here time calculated is 28 minutes. I would like to reduce time for purging. Flow through compressor during purge 580m3/h(mach figure is far less than 0.7 and no sonic velocity concern). 3 volumes of compressor is 153m3. And it makes only 15 minutes. Is it right to use volumetric flow? Why vendor used mass flow? I would really apresiate if some body can explain it to me



RE: centrifugal compressor start up purge

Here is a quote from a class I'm teaching this month "higher pressures require more time and use substantially more gas for the purge with no improvement in purge efficiency"  Purging at 132 psi is a really wasteful process that was written by someone without a clue.  Sorry, but that is the way it is.  I've been teaching this stuff since 1994 and I run into that more-pressure nonsense in every single class I teach.  Just because so many people believe it don't make it so.  I'll be teaching the class in London in April if you can get someone to open their mind enough to learn something.

David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376: Forum Policies for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat"  The Master on Dr. Who

RE: centrifugal compressor start up purge

Thanks David, that is another useful information.But I cant use bigger size orifice in blow down system and cant reduce the pessure in compressor. Blowdown orifice size is calculated to let depressurization of compressor from initial pressure to pressure of 6 bar within 15 minutes. Pressurization/purging orifice is sized for maximum flow through inlet line with mach figure below 0.7. Honestly I wanted to increase  pressurization/purge orifice size and reduce the time for compressor pressurization, but it will increase pressure in compressor and reduce purging effitiency as you said. But first I need to find answer to question.


Thanks for your help David

RE: centrifugal compressor start up purge

If you are purging to atmosphere, calculate the sonic velocity through your purge valve (calculated at upstream pressure) divided by the area of the purge valve.  That gives you the volume flow rate of the purge.  Then purge 3 * Volume * (8 bar / 1 bar) or 24 times the compressor volume at that volume flow rate.

The rate of flow into the compressor is somewhat irrelevant as long as the inlet piping is bigger than the exhaust piping (otherwise the pressure will tend to decrease with time and the calculation becomes impossibly complex).  

Introducing gas above 0.6 Mach into an air filled system is a very good way to blow the system up.  At some value around 0.6 Mach, density increases rapidly and mixing stops.  The consequence of these two factoids is that you can create a "pseudo piston" that will compress the gas in front of it until the inlet gas does enough work to slow below 0.6 Mach.  About a dozen people die each year re-discovering this fact.  Every single compressor explosion that I've ever reviewed has been caused by this phenomenon (most of the time the investigator refers to it as "dieseling", or using the heat of compression to raise the temperature of an explosive mixture above the auto-ignition temperature).

The more I learn about your process the worse it sounds, but yes, you can use volumetric flow.


RE: centrifugal compressor start up purge

Thanks David

Now it is clear.

RE: centrifugal compressor start up purge

It would be clearer if I had typed it right.  Velocity TIMES area is volume flow rate.  Velocity divided by Area is a unit of 1/(time X Length) which doesn't mean anything that I know of.  Glad you got the point in spite of my error.


RE: centrifugal compressor start up purge

I am currently reviewing a similar concern for the purging of a compressor and associated piping prior to start-ups.    My client would like to reduce the purge time from the original design.  Is anyone aware of a relevant standard in usage for the purge requirements of compressors?
I found NFPA 69, Annex E has some information on this subject.   There is a helpful graph of the number of purge volumes verses the fraction of original concentration for well mixed systems.
Are there other industry standards that are being used?


RE: centrifugal compressor start up purge


I was said that ASME B19.3 standard which is Safety standart for compressors. I never ssen this standart because it is withdrawn currently. Im trying to get it now and if i get it i will let you know.  I can desciber how it was calculated in my case by vendor. But generally it done as described below:
1. calculated volume of compressor and pipiwork
2. calculated pressure of gas inside the compressor during purging.
3. Calculated mass of gas for compressor and pipework at purge pressure
4. times 3
5 calculate time required to purge compressor

I didnt go to volumetric flow. see the note about mach figure above. It was confirmed by several people and i must thank  zdas for this note. Dont let mach figures be above 0.6.

Good luck


RE: centrifugal compressor start up purge

I think no need for any calculation you have to do it by practice:

first check the gas if heavier than methane then you have to use lower purging via blow down. other wise to flare or fuel gas net work.

second check the line temp if its get cold then end of the purge procedure and start the compressor

finally, record the timing and build your own practice.




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