Q & A > Question Details
I am working in DHDS unit. Recently our unit tripped because of some strange problem. I request all to suggest a reason for the problem explained below.
We have one centrifugal Recycle gas compressor (RGC) and two reciprocating make up gas compressors (MGC) one running and the other stand-by. As per the regular change over of MGC we tried to take the other one in line and spare the running one. The discharge of MGC (40 KSC) goes to suction of RGC (39.4 KSC). After starting the spare compressor and once it got 50% loaded, the make up gas rose from 25 Tons per day to 35 tons per days, simultaneously RGC amperage came down from 210 to 176 amps and discharge pressure of RGC came down from 61 KSC to 53 KSC and this dropping of amps and discharge pressure continued and unit tripped on low hydrogen pass flows. As the discharge pressure of RGC reduced the discharge flow also reduced. I didn't understand why the discharge pressure of RGC came down.

Additional Information: Separator pressure is constant and when the RGC tripped, it started raising. The suction flow was 265 Tons per day and when the RGC discharge pressure dropped, the suction flow also dropped to 255 Tons per day.
 
Answers
02/07/2013 A: Narendra Naidu, NWR, nbpnnaidu@gmail.com
Where does the MGC discharge join on the RGC suction, i.e. d/s of the suction flow meter or upstream?
29/05/2011 A: jagjit singh, indian oil, singhj5@iocl.co.in
while you are taking other compressor in line and mug discharge flow increases from 25 to 35 knm3/hr due to rise in mug discharge pressure restricting the rgc suction flow. take mug spill back in manual mode and adjust flow accordingly to avoid such problem
20/03/2010 A: jagadeesh rajasekharan, bharat petroleum corporation Ltd, jag_nair@hotmail.com
I presume there is a pressure control on the RGC Suction Drum which opens the RGC spillback control valve in a low RGC suction drum pressure to maintain the pressure and opens the Relief or RGC suction drum top pressure control valve to flare in case of high pressure in RGC suction drum.
When you loaded the MGC2 keeping MGC1 online the suction flow and suction pressure in the RGC Suction drum must have gone above the set value of the Relief/RGC suction drum top pressure Control valve. This may cause sudden drop in RGC suction drum and the RGC spillback in order to maintain the RGC suction drum pressure opens in ramp mode causing less flow towards the Reactor side activating low hydrogen flow trip.
I suggest to keep a constant watch on the RGC suction drum while changeover of MGCs are made. Also loading has to be synchronised so as to reduce the loading of MGC1 in steps of 25 % parallel with increasing of MGC2 loading.
19/03/2010 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Assuming that your compressor suction pressure did not change, it sounds like something happened that dropped the mol wt of the make up hydrogen (ie. purity increased). As the mol wt of gas to a centrifugal compressor decreases the delta P it develops goes down as does the power demand. Its analogous to a centrifugal pump. If the density of the fluid being pumped decreases the head stays constant but the amount of pressure developed by that head decreases along with the pump horsepower. What purity is the purity of your normal make up hydrogen?
19/03/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Well, the volume amount of hydrogen (very low mol wt.) increased a lot in the recycle gas as you loaded up the 2nd MGC. That dropped the spgr. significantly, reducing the discharge head (pressure) commensurately. Simultaneously, the power draw of the RGC dropped because the lighter weight gas did not take as much energy to compress. The lower discharge pressure may not have been enough to maintain forward flow in the recycle loop (there is significant time delay in the process loop--several seconds before a change in RGC discharge pressure is reflected in the suction drum of the RCG. During this time delay, it is quite possible the real mass flow rate through the RCG dropped to close to or below the 'surge line' (a condition where the rotor blades are aerodynamically stalled, and there is no forward flow of the gas). The 'low flow' shut-down is probably adjusted for a 'normal' RCG sp.gr. and did not consider the reduced sp.gr. in calculating the mass flow rate.

In essence, a good calculation (mass flow rate) used incorrect data and shut the unit down thinking the recycle gas rate (#-moles/hr) had dropped below safe level, or the RCG reached a 'surge' condition and tripped.
19/03/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
This is interesting. I wish I could see the instrumentation on a P&ID. Was the suction pressure (Separator pressure) steady during this period of changes to the recycle gas discharge pressure and flow rate? Is the recycle gas flow rate on flow control?