centrifugal compressor start up purge
centrifugal compressor start up purge
(OP)
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.
Thanks
Thanks





RE: centrifugal compressor start up purge
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
www.muleshoe-eng.com
Please see FAQ731-376: Eng-Tips.com 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
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
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
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.
David
RE: centrifugal compressor start up purge
Thanks
Tarlan
RE: centrifugal compressor start up purge
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376: Eng-Tips.com 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
CAN I USE VOLUMETRIC FLOW OR NOT??
Thanks for your help David
RE: centrifugal compressor start up purge
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.
David
RE: centrifugal compressor start up purge
Now it is clear.
RE: centrifugal compressor start up purge
David
RE: centrifugal compressor start up purge
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
Tarlan
RE: centrifugal compressor start up purge
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.
regards
ASA-KUWAIT