We are looking to add a remote turbine speed indicator to our turbine for use during a water wash.
For ease of installation, we are attempting to duplicate a signal coming from the MK-V Comm Link to use as a drive signal for the RPM meter. Can anyone point me in the right direction on how to duplicate this signal?
Our first thought was to use the TNH_RPM signal directly from the MK-V, however it is difficult to reach the cable tray that would be needed to run those lines. Much easier would be to run a wire from the Control Room (DCS) to the water wash cab to provide this signal. DCS point name is T1TNHRPM.UNIT1 (Emerson Ovation). Thanks in advance.
If the signal is present in the DCS (Emerson Ovation), AND you have a spare 4-20 mA output signal channel and can configure the channel to use the signal to drive the output, that would seem to be the best solution since you can't easily get to the cable tray for the Mark V signals.
You can run a twisted, shielded pair from either the Mark V OR the Emerson Ovation to the Water Wash Skid in any cable tray--as long as it's a low-level wiring cable tray. The Mark V could easily be configured to drive an output of <C> processor with the signal, BUT since cranking occurs at low speed the output signal will be a small analog value. And, the same would be true of the Emerson Ovation output if it is driven by TNH_RPM.
There are ways around this, or, a simple switch could be installed at the meter in the Water Wash skid to open the circuit when the unit is NOT being washed. (A better solution would be to use a 100- or 250-ohm resistor in place of the meter when the unit wasn't being washed--so you would require a double-pole, double-throw switch and a resistor to accomplish that. (The reason for the resistor is to have a load on the output channel of either the Mark V or the Emerson Ovation so neither complains about an open circuit with the unit is not being washed.) And, again, since cranking occurs at about 20% speed (or so), the output of the 4-20 mA channel will be only about 7.2 mA. Probably good enough for a digital meter, but an analog needle won't move very much (20% of scale).
Again, you can run the twisted, shielded pair cable in any cable tray, as long as it's a low-level signal cable tray. Doesn't have to be a GT low level signal cable tray. If there's room in a more accessible cable tray that's even for 120 VAC signals and you can keep the cable away from the other cables by an inch or more, you MIGHT be able to get away with that. It all depends on how much current is running in the cables, and how often it is being switched.
Hope this helps!