SVX9000 common alarm issue

[Johnny mechanic]

We are getting an alarm on our SCADA system triggered from RO1 (relay output 1) of our SVX9000, which is a VFD common alarm (description in our SCADA system). This alarm seems to come in when the pump is ramping down, then it will clear it self and run when it gets a call again. We may have 10-20 alarms over a 6 hour period. How can i see what this RO1 is tied to?

 

[don_resqcapt19]

You will have to get the VFD manual and look at what that relay output it programmed to tell you.

 

[Jraef]

You will have to get the VFD manual and look at what that relay output it programmed to tell you.

Correct. But as a gross generalization, the "Common Alarm" usually just means it is programmed to change state when any one of the many drive protection functions are activated. So where you would need to look is in the "Fault History" of the drive to see what the fault was. In the SVX9000 that is in the "M4 Fault History Menu', it stores the last 30 faults, starting with the latest.

Past experience would lead me to believe you are getting a Regen situation when you are decelerating and the drive is tripping on DC Bus Over Voltage, then resetting itself. Not uncommon, but it means something is either not working right in your pump system, like a check valve not closing to prevent back-flow, or the drive's Decel programming is incorrect for the application. For example if you have a Pump Control Valve that closes slowly to prevent water hammer (common), then the Decel time of the VFD must ensure that the drive makes the pump still put out enough static head to keep the fluid from flowing backward before the valve closes. If the decel is too fast or gets to a speed at which the pump provides insufficient static head, the flow goes backward, spins the pump backward and because the motor is still energized via the VFD, the motor becomes a generator and charges up the DC bus of the drive.

 

[tom baker]

If you have a pump control valve, or cla-Val, I have them to be troublesome as they are mechanical opening - closing. The open or close needle valves get out of adjustment. We removed the pump control valves when VFDs were installed, used check valves and adjusted ramp times in vfd.

 

[Jraef]

If you have a pump control valve, or cla-Val, I have them to be troublesome as they are mechanical opening - closing. The open or close needle valves get out of adjustment. We removed the pump control valves when VFDs were installed, used check valves and adjusted ramp times in vfd.

I’ve done that too. The potential problem though is that if power fails, you have lost control of the pump so the valve slams, causing water hammer.

 

[GoldDigger]

I’ve done that too. The potential problem though is that if power fails, you have lost control of the pump so the valve slams, causing water hammer.

I may be missing something, but if there are no energized valves, only check valves, I do not see how losing control of the pump ramp time leads to a valve slamming.
I can see the pump itself becoming a high flow resitance element, causing a pressure surge if not a water hammer, but not the valve doing anything.

 

[gadfly56]

I may be missing something, but if there are no energized valves, only check valves, I do not see how losing control of the pump ramp time leads to a valve slamming.
I can see the pump itself becoming a high flow resitance element, causing a pressure surge if not a water hammer, but not the valve doing anything.

Check valves can slam too, especially if they are spring loaded, which some are.

 

[GoldDigger]

Check valves can slam too, especially if they are spring loaded, which some are.

But if they slam when the flow rate drops to zero, there will be no water hammer, yes?
If the check valve closes while there is still flow in the normal direction, wouldn't the water hammer pressure increase simply force the valve back open?

 

[Jraef]

I may be missing something, but if there are no energized valves, only check valves, I do not see how losing control of the pump ramp time leads to a valve slamming.
I can see the pump itself becoming a high flow resitance element, causing a pressure surge if not a water hammer, but not the valve doing anything.

Losing control in that the pump just shuts off immediately instead of decelerates slowly, so the flow instantly reverses, the valve slams closed and traps the kinetic energy in the water column, which is what causes water hammer.

 

[GoldDigger]

Losing control in that the pump just shuts off immediately instead of decelerates slowly, so the flow instantly reverses, the valve slams closed and traps the kinetic energy in the water column, which is what causes water hammer.

I just do not see how any significant flow velocity in the reverse direction could build up before the check valve closes. The valve should close very close to the time the forward velocity drops to zero.
If for some reason there is a long delay of free reverse flow before the valve closes, your point would apply.

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[Jraef]

The amount of flow reversal is not "significant", it's only the amount of the travel of the check valve. But that's enough, because all of the mass of the water in that pipe suddenly stops and since liquids can't compress, that kinetic energy in the formerly moving liquid becomes a shock wave.