Mark V Scan rates


Thread Starter

William Monson

I am running some 7FA turbines at a power plant that use the Ametek PG 3000 series pressure transmitters. These are great if you have the time to monitor and calibrate them frequently, as they drift a bit but last forever.

I am considering replacing these with another suitable SMART device but have had some questions about the potential change:

1 Is a SMART transmitter response time fast enough?
2 How can I determine the scan rate of Mark V?
3 What SMART device will take the heat and vibration?

I see rousemount has some good details about response time in their manuals (and on I have been buying a lot of Autrol because of the low cost but not as good of response time literature.

It is a tough decision to replace a known good transmitter with the unknown, but we are a slim-operations facility. 2 Technicians, 1 mechanic, 1 electrician (30 total employees) at a 6 unit 1200MW facility. Drift is tough to keep up with when looking at positioners, pressures, temps, etc.

Any one have some suggestions or experience with such changeouts?
William Monson,

SMART transmitters are NOT recommended for:

1) P2 pressure sensing
2) CPD pressure sensing

They just aren't fast enough--especially for CPD monitoring for F-class GE-design heavy duty gas turbines.

The Ametek units have been known to be plagued by drift problems, but have you tried contacting the manufacturer to see if they have a newer, better model which doesn't suffer from the same issue?

I will say many sites have replaced their transmitters with SMART transmitters with no ill effects noted--to date. Not a particularly good idea on F-class units, again, but it has been done. If the units are not under any kind of long-term service agreement with GE or some other service company you are certainly free to do this--as long as you understand and are willing to accept the risks. And they are higher with F-class machines--because they just run at the friggin' limit of the materials. And the monitor CPD to detect compressor issues--and they have been known to have a couple of compressor issues....

The Mark V still does the P2 pressure control in firmware and so requires a fast sensor. Mark VIs and Mark VIes usually do the P2 pressure control in the application code (not at the firmware level)--HOWEVER, application code usually runs at a 40 ms scan rate, so SMART transmitters for Mark VIs and Mark VIes for P2 pressure sensing are probably not the worst idea.

The Mark V scan rate is specified in a file that's used by the Control Sequence Compiler. For GE-design heavy duty gas turbines, the fastest scan rate is 8 Hz, if I recall correctly, and most heavy duty gas turbines run at this rate. I think the file name is MSTR_SEQ.CFG; it's another one of the many ASCII text files.

I believe GE bought Druck and is now supplying Druck pressure transmitters with new units and many retrofits. They are good devices--but their problem is that they require a good deal of tubing re-work and conduit/wire re-work to install on an older turbine (they're not the same size and don't have the same electrical connections). Of course, we all know how fond GE is of their equipment and parts, and how much money they require to separate them from their spare parts. So, often that's not an option.

If your site is under a long-term service agreement with some vendor, you might ask them for recommendations. But, in general--unless the SMART transmitter response times have improved greatly, they're not a good choice for this application.

If you're willing to use a Rosemount or other manufacturer's device that is not SMART, they generally have a higher response time that is acceptable.

Write back to let us know how you fare!