THIS MONTH’S PUZZLER
Our unit superintendent is concerned about a multistage centrifugal gas compressor. It’s fed by a separator and discharges to an absorber; there’s a pressure control valve at the compressor suction and a flow control valve in a recycle line from the compressor discharge. Ever since the first startup after the last turnaround, the motor current unexpectedly surges during startups and other upset conditions before settling into generally steady service. The packing in the absorber was replaced during that turnaround. What could be causing the problem? Should the superintendent be concerned? Can we do anything so we can safely limp through until next year’s turnaround?
CHANGE THE START-UP PROCEDURE
The following steps may be taken to avoid unwanted surging: 1) before starting the compressor close the suction and discharge valve and open the bypass/recycle valve so that there will be the same load; 2) purge the system before starting the compressor, unloading it to minimize motor current; 3) start the motor ? it will draw maximum current at the time of starting [compared to regular operation], which is a normal phenomenon; 4) after proper lubrication, the gas compressor should be loaded slowly in stages, i.e., 25%, 50% and 100%, allowing some stabilization time. This procedure will take care of unwanted surging and frequent tripping of motor. The motor may be diagnosed for desired current output and leakages, etc.
C. M. Pakhale, superintendent
Oil India Ltd., Duliajan, India
CHECK THE CURVE
I suspect the compressor is running off its curve and surging. The new packing in the absorber has less pressure drop than the old packing. I would consider adding pressure drop between the compressor and absorber via a manual valve partially closed or installing a flow orifice. There are no clues in the problem to suggest a dynamics/tuning problem with the control valve on the suction.
Mike Gentilcore, principal research engineer
Covidien, Hazelwood, Mo.
LOOK AT PRESSURE DROPS
This surge may be because of high resistance on the discharge side. While starting the compressor keep flow valve on discharge line throttled full open — more than what you normally would. As far as column packing is concerned, I suggest checking for the column pressure profile before and after the packing is changed. If the pressure drop is more than it should be, record it for future changes. But as I understand the problem was there even before the change of packing, so this particular the problem may be the opening of the valve. Maybe PID setting change can be accommodated. It may happen the pressure control valve is undersized.
Surender Khtri, process engineer
L&T Engineering - E&C Division, Faridabad, India
CONSIDER TWO POSSIBLE CAUSES
There are two solution based on following two conditions:
Condition 1: The packing replaced is not permeable [e.g., too small, wrong type], creating a vacuum on the suction side of the compressor. In turn, this increases the motor current draw. Eventually, the packing will be destroyed, thus allowing normal air flow, bringing motor current within limits after some time.
Solution 1: Check the differential pressure between absorber chamber and the discharge line connecting to compressor. Pressure inside the chamber should be high when compressor is “off” and should reduce little when the compressor is “on.” Open the absorber to check the packing.
Condition 2: The initial starting torque current of motor is much higher than running-condition current.
Solution 2: Consider using a variable frequency drive; look at the settings available in drives at low frequency and low voltage. The torque can be increased and it will conserve energy. Also, the smooth operation of drive will absorb the sudden surge current. Do one check, though. Verify that the outlet valve of discharge is open 100% and inlet valve [the pressure regulator] opens gradually.
Abhay Singh Gour, process control engineer
N.I.T., Trichy, India
LOOK AT CAUSE OF POWER DEMAND
A motor power surge is the result of power demand from the compressor. This
could be due to:
The packing could be “floating,” or expanding, because of gas upward flow. Packing demands a minimum upward flow of gas [i.e., saltation velocity], below this minimum gas flow it tends to “settle” and add resistance or pressure drop to the upward gas flow. This resistance could increase the discharge pressure and consequently delay (say, by 1 second) opening of the recycle valve. If real time trend analysis is available in the distributed control system, check the trends for current, absorber differential temperature and flow across the compressor. You will also want to identify which parameter moved first and how they are related to each other.
Another possible cause could be the unusual pressure drops in the system. Inspect the check valve at the compressor discharge or at the inlet of the absorber and verify the movement of its spindle. The movement may be restricted, sometimes by residual materials left in the piping during replacement. Test or measure the pressure drop across the absorber at 100% load before and after the packing change-over.
Investigating these two causes can serve as a starting point.
Anirudh Rajendra Kumar, chief process engineer
Viva Methanol LFTZ Enterprise, Eurochem Corp., Lagos, Nigeria
IMPLEMENT CASCADE CONTROL
Are the pressure and flow control tracking one another? You might try cascading the pressure and flow with pressure being the primary. The flow should false load during startup and upset condition to avoid flow changes and compressor surging.
Porter Yarbrough, engineer
Bayer MaterialScience, LLC, Pittsburgh
We’ve just discovered what appears to be a design flaw during the first hazard and operability study involving solvent extraction evaporators. The evaporation system is a three-effect backward-feed design using long-tube vertical evaporators. The viscosities should be moderate, i.e., below 100 cP; there’s only a slight possibility of solid build-up unless the bulk temperature falls below 100°F. With backward feed, 150-psig steam enters the first unit as the feed while a dilute concentration of ferric chloride and hydrochloric acid enters the third unit. The product, a concentrated aqueous solution of ferric chloride, exits the first effect. It’s passed with the evaporated steam from the first unit to the second and then to the third, where it’s condensed and decanted after the final effect. The problem is the level control and pump cavitation. Operating at the design capacity of 50 gpm, the pump can empty the reservoir, i.e., the receiver, below the pump in less than a minute. This problem is most severe during startup. At this stage of the design what can we do to make this process operate more smoothly? Unfortunately, the evaporators, which require expensive materials, already have been ordered.
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