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Our Deep Catalytic Cracking unit Wet Gas Compressor (WGC) seems to be an over-designed one as its operating at the minimum governing speed(MGS) with the anti-surge valves around 35% open even at 90% plant capacity. Due to this the speed/performance controller is operated in manual mode making it difficult to control the reactor pressure.
Is it possible to change/lower the MGS of turbine so as to bring the speed control in the unit's operating range and thus keeping the reactor pressure control in AUTO. If not, is it advisable to tune the ASVs so that they close at a lower load?
 
Answers
22/02/2016 A: Eric Hennings, Technip Stone & Webster, EHennings@technip.com
Kindly consult with vendor in regard to lowering of minimum governor.
Modifications to WGC are often expensive. It may be more economic to live with the existing machine.
Consider the following: (a) lower suction pressure to increase volumetric flow and improve LPG yield in FCC (b) increase Gas Plant pressure for increased LPG recovery from off gas. (Alternately reduce naphtha recycle at same LPG recovery to save utilities.)
08/02/2016 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
Determine the reason for the minimum governor speed. It may be because of critical speed or more likely lubrication requirements. I am assuming it has an internal driven lube oil pump. As long as your oil pressure is sufficient and you are no where near the critical speed I see no reason not to lower the minimum speed.... I wouldn't touch the ASV settings they are set for a reason.
04/02/2016 A: Vijay Beleri, Rateau energy systems, v_beleri@hotmail.com
Some points to check before you can run at lower speeds:
What are the critical speeds of the compressor train, and individual units?
Operating speeds should be nowhere close to the critical speed.
Guess this data must be available from the torsional analysis of the train.
What make of governor are you using?.
04/02/2016 A: NS Murthy, Suez, murthy.ns.ext@suez.com
It is a typical process control need. Check with your instrument team on the possibility of operating WGC on fixed rpm (like motor operated) and place the reactor pr control on anti surge valve after ensuring that surge limit is 'far away'. Also, optimize WGC discharge pr (I.e. GASCON operation) so that there is some more room for the machine away from surge limits.
04/02/2016 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
You will use more energy but allow automatic control if you replace the control valve (or trim) with a smaller one and increase the RPM, running at 50% open on the control valve.