Suspect Xmitter Problems?

I currently offshore on a platform doing a pre-start check of some equipment and I have a pressure xmitter that's causing the Underrange bit to cycle repeatedly. I put a trend on the bit and was watching it go crazy.

The xmitter is reading zero pressure... can't see it cycle much on the raw values....

Put a 789 on the loop from the xmitter to the Analog Input Card and it stayed steady and the Underrange Bit stayed low.

Has anyone else ran it to a xmitter act like this?

Moved the input to a spare Analog Input and the issue followed it.. switched out the card with a spare... same thing. Checked the wiring okay.

I'm beginning to think it's a bad xmitter although I'm reading correct values from it when we pressure it up. BTW, this only happens when we're at 0 psi.

Any words of wisdom?

Thanks
Bobby

You did not mentioned what kind of PLC and xmitter you are talking about. My experience lay with A-B PLC and mostly Rosemount and E+H transmitters. In analog module configuration you may disable underrange and/or overrange alarms. You getting alarm mostly due to unstability of you analog signal. It may flactuate in a range too small to be displayed on your transmitter. Just take it as given - it goes below 4 mA. So good luck in your analog module configuration.

Hey! Go outside and wave!

Didn't know you guys were in the gulf. I don't use any under range or over range bits here. If your analog input stops at 4 ma, that's all you need. Sometimes these transmitters will float just a tad.

I've found it better (in the past) if somebody needs underrange, use a set value below 4 ma, like 3.75 ma or something. The bit that cycles just below 4 ma can be useless at times.

Other option is to set the underrange on a timer, give it, say 5-10 seconds. You know for sure you have an underrange unless the float is that long.

ALmost all 'smart' transmitters have a failsafe setting, where the transmitter will output either a low value, like 3.7mA, or a high value, like 21.8 mA, if the internal diagnostics determine that there's a critical failure resulting in the inability to provide an actual PV.

Whether the failsafe setting is high or low depends a DIP switch, fuse link setting, and maybe HART can set it, but I'm not sure about HART.

Some, but not all PLCs have AI cards that can be ranged to account for the failsafe condition.

This is not an answer to the OP's question regarding the underrange bit, although I suspect that the under or over range bit could be used to detect a failsafe condition.

Dan

The problem with using AB's low and high range bits is that they are precisely outside the 4-20 ma limit. I don't use them. Transmitters aren't as precise as AB's cards!

sounds like you need to do a calibration on the transmitter. if it still does it after that then just set the zero to where it shows 1 psi. nothing in the gulf is critical to the point where 1 psi makes a difference.

Hey bud... misery loves company?

I'm out in deepwater on Chevron Blind Faith, South of New Orleans.

This is some AB Flex I/O Analog. I'm supporting someone else's program. This is picking off Compressor Discharge Pressure and they're test HH, H, L, LL and Underange for Xmitter Failure. If it flags for Xmitter Failure it opens the other Alarm/Shutdowns and prevents them from flagging so you don't get a incorrect shutdown flag on the HMI. Out of umpteen hundred Rosemount Xmitters this is the only one flaking out on me like this.

I&E Tech hooked up the HART and recalibrated the xmitter and verified scaling.

I'm thinking I'm going to de-bounce it like you reccomended and go on. The cycle on the underrange bit going high and low is mere msecs... nothing long.

Pretty weather the last few days... working on my farmers tan on the galley deck.

Thanks for everyone's two cents.... Once again you guys rock!

Bobby

tomalbright said:

Hey! Go outside and wave!

Didn't know you guys were in the gulf. I don't use any under range or over range bits here. If your analog input stops at 4 ma, that's all you need. Sometimes these transmitters will float just a tad.

I've found it better (in the past) if somebody needs underrange, use a set value below 4 ma, like 3.75 ma or something. The bit that cycles just below 4 ma can be useless at times.

Other option is to set the underrange on a timer, give it, say 5-10 seconds. You know for sure you have an underrange unless the float is that long.

Click to expand...

Nice! I'm a little closer to shore.

I don't think we could get by with that scheme; sounds like an interesting play with the specs. So far, we can't disable anything unless it's maintenance, let alone an automatic OOS on active alarms. If the transmitter was out for any length of time, that would be a problem. Just my opinion...

I've worked with I & E Tech up here setting up a whacked out platform after a storm, they have some pretty capable guys.

>>Put a 789 on the loop from the xmitter to the Analog Input Card and it stayed steady and the Underrange Bit stayed low.<<

You said it stayed steady.... I'm assuming that means 4mA or above... did you try to put your meter on to record the minimum reading...

Also, when you say that the underrange bit stayed low with the meter in line, do you mean that there wasn't problem. Is there a minimum resistance requirement to your xmtr? Do you have a small (<100 ohm) resister, that you can put in series?

-MUR

Xmitter Failure is a SD. Since the signal is driven low "opening up" the other flags prevents a false positive on the other AL & SD associated with the Xmitter. But the end effect is the same... a SD.

Operators have the option of putting the system in Bypass for X number of minutes, but after the Bypass Timer is DN the system is "armed" again.

The platform is having all types of fun bringing the Turbine Generators online. I'm glad I'm not there to support that equipment. Just a lowly VRU "cookie cutter" type o' thang I'm here for.

This platform hasn't seen "first oil" yet, but it's around the corner......... maybe <smile>.


Thanks again for your input!
Bobby

tomalbright said:

Nice! I'm a little closer to shore.

I don't think we could get by with that scheme; sounds like an interesting play with the specs. So far, we can't disable anything unless it's maintenance, let alone an automatic OOS on active alarms. If the transmitter was out for any length of time, that would be a problem. Just my opinion...

I've worked with I & E Tech up here setting up a whacked out platform after a storm, they have some pretty capable guys.

Click to expand...