I was surprised by a pump indicating shutdown/interlocked (using DC blocks) when the setpoint and output were already set to the passive states. This did not make sense to me as the interlock can't accomplish anything more than is already being accomplished except preventing the output from being set to an active state if the setpoint changed.
The specific scenario is needing to block flow before stopping the pump to prevent backflow, but interlocking the pump off if the valve used to block flow is closed and the pump is running deadheaded for too long.
I realize the DC block has likely always worked like this, but I can't seem to understand why it should work that way. I'm just curious if there is some rationale or use case that explains why it works this way. I find knowing the history of things makes me more effective at applying.
In reply to HumairaF:
In reply to Brian Hrankowsky:
Don't we all. And just wait until you hear their stories in person!
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Rachelle McWright: Business Development Manager, Dynamic Simulation: U.S. Gulf Coast
In reply to Rachelle McWright:
I'm the tall one...
Will reply when back on a computer and not on a phone
In reply to Matt Stoner:
Andre Dicaire
In reply to Andre Dicaire:
Hi Andre, The way our module is designed, the DC block is always in cascade and we use the cascade input (wired from a calc block). The interlock condition has a few seconds delay to allow us to command the motor to stop from a phase before the interlock trips. The motor has a VFD and the VFD stop was configured for a ramped stop. the deceleration rate selected was slow (on the order of 7 seconds to go from the normal speed down to 0). What I swear I saw was the DC block get to stopping, a few seconds later the interlock timed out, the DC block then indicated shutdown/interlocked, then the VFD finally turned off the run status feedback - but by then it was too late and we have the interlock alarm. I am very confident the DC block was executing the passive command (output was stop) before the interlock condition timed out and tripped.
I am also on 14.LTS
Trying to keep my answers shorter, so I split them in two posts. Clever no?
Actually, there's no history to add here as it looks like the DC works as it always did. There are a lot of options for the DC block on how it can be configured to drive its outputs but there must be an explanation for your observation.
Here's a before and after shot of DC block while in passive state and Interlock goes to 0:
The FAIL parameter is shown as an output and remains set to CLEAR.
Your scenario states that you interlock the PUMP to the state of an OUTFLOW Block Valve, so that the PUMP stops if the valve is closed, to prevent deadheading the PUMP, but the PUMP still deadheads for too long, i.e. it takes too long for the pump to stop. Is that still an issue?
This sounds like a process design issue. Is the block valve there to serve the PUMP and prevent back flow or is it used to block flow to downstream process? If it is for the later, shouldn't you normally stop the pump first and interlock the block valve to close at the appropriate time so that back flow is prevented without deadheading the Pump? Interlocking Pump to the block valve is still required incase it is closed inappropriately, but normal operation should sequence the two.