88WC Fault Display on HMI

Hello Professionals,

When the turbine (GE frame 9E, 115MW) is running and online, and we have 2 units of 88WC (cooling water motor 1&2) where one of them is running. For instance 88WC-01 is running and set to lead. It is observed that 88WC-01 would trip (display fault) after a short while and then switch running status to 88WC-02 while its still leading. Same happens when 88WC-02 is set to lead. It trips after a while and switches the running status to 88WC-01 to run while 88WC-02 still leads and displays fault on the HMI. We have checked the contactors for arcs or partial contacts and even changed out contactor kits in both motor withdrawal buckets, but the problem still persists.

Please your advice would be appreciated.
Hello Sir

I would like to help with some advises for this case. You did not tell by which reason 88WC 1 or 2 tripped?

I don't have the p&ids for the Water cooling systems on the frame 9E, but philosophy is the same for this system. Try to have look on process diagrams and see with local indication what alarms or trip can be occurred.

Then You should check the logic regarding the pumps change over logic.

I did it on Mark6e in my last project. We have been faced for some override between master or slave from local panel which inducted false selection and operation from DCS view (local/remote selection master/slave lead/lag. Problem was solved by troubleshooting the signals exchanged between motor controls centers and DCS AND for sure modifying and adapting process diagram and DCS logics .

As soon as you can tell us alarms or trip occurred by getting some trend views can be very useful and key to solve your problem.

Hope this post can help you,

What OTHER Process Alarms are annunciated when the Cooling Water pump changes over? And, what is the timing of the alarms (before or after the lead pump trips)?

Specifically, is there a Cooling Water Pump Motor Overload alarm, or an Auxiliary Motor Overload alarm?

When did this problem start--after a maintenance outage, or a unit trip from load, or?

Were either the cooling water pump motors or the cooling water pumps recently replaced or refurbished?

Is the unit experiencing higher-than-normal, or lower-than-normal, L.O. temperatures?

Does the unit have a generator that uses water to cool the generator (either by cooling air or hydrogen)? If so, does the generator seem to be experiencing higher-than-normal, or lower-than-normal, temperatures?

Typically, GE and its packagers do not trip motors on electrical overloads (their reasoning is that it's better to burn up the motor than to lose the unit--the operator should be capable of making a decision if an overload alarm is annunciated and take appropriate action based on site practices and/or procedures). Some turbines had all or most of the auxiliary motor overload devices connected together in a single input to the Mark* which resulted in a single Process Alarm, Auxiliary Motor Overload. It was the operator's responsibility to go out to the location where the motor starters were located and determine which overload had actuated, and then take appropriate action.

Many sites with other, older GE-design heavy duty gas turbines vehemently objected to this practice (of "ganging" all the auxiliary motor overloads into a single input and having to send someone to determine which overload had actuated) and so many turbines were provided with individual auxiliary motor overload alarms to make it easier for the operator to quickly know which motor(s) had experienced an overload.

Cooling water pump motors are not <i>usually</i> subject to overload, because if flow is restricted either at the suction or the discharge the amperage (current) draw is usually reduced--not increased. And, if the cooling water discharge pressure (from either pump) is low (per a pressure switch) then usually there is only a Process Alarm to that effect and it is the operator's responsibility to investigate and resolve the low cooling water discharge pump pressure.

Since we can't examine the actual logic or application code running in the Mark* turbine control system at your site we can only make general statements (as above).

It seems odd that both cooling water pump motors are experiencing similar problems, which means that it's most likely that something is amiss with the cooling water system. There is usually at least one orifice in a GE-design cooling water system to control the flow-rate, and it is conceivable that if the orifice were removed (or not re-installed after a maintenance outage) that the flow-rate of each pump could increase to the level that caused an overload--but, AGAIN, GE and its packagers don't usually trip motors on overloads (for the reason cited above). However, some sites have INSISTED that the auxiliary motors be tripped on overload, or have modified the motor starter control circuit to do that. We can't know what has or has not been done at your site, so you're going to have to investigate and let us know what you find.

It's also interesting--and odd--that no other auxiliary motors are experiencing a similar problem, which, again, points to an issue with the cooling water system. Or the logic/application code may have been modified in an unusual way. Or, some combination of the two.

When a problem like this happens it's best to try to think back to when the problem started, and if it started after a maintenance outage, then try to think what was done to the cooling water system during the maintenance outage. Also, it's best to examine all of the Process Alarms which are present on the Mark* to try to find out if there is something else going on which could be related.

Again, it's been my experience that the cooling water pump motors are usually larger than required and rarely, if ever, experience overloads (which would be detected in the starter). And if the suction of the pump or the discharge of the pump (both of which usually have some kind of strainer in them) are choked (plugged; dirty) then the flow-rate of cooling water will decrease which will cause the current drawn by the pump motors to decrease (that's usually how most centrifugal pumps work--the higher the flow-rate through the pump the more current drawn by the pump motor; the lower the flow-rate through the pump the lower the current drawn by the motor). So, choked (plugged) strainers will usually result in lower-than-normal motor currents, not higher.

Hope this helps! Please write back to let us know what you find or with more details.