Archive for category Pressure
A process plant’s technician was mystified about how to get a typical gauge pressure transmitter to read in the vacuum range. “All our gauges are 0 to 30 inches mercury, and that’s what we need to transmitter output to be. But the transmitter you sent us just stays around 4mA when we pull a vacuum.”
We walked out to the reactor vessel to look at the installation. The transmitter’s Low side port was open, its high side port was plumbed into a tee along with a conventional bourdon tube pressure gauge reading gauge pressure vacuum.
I could see why he was confused. The mechanical gauge goes from 0 to 30. I asked what range he used to configure the pressure transmitter. His answer, “0 to 30 inches mercury, same as the mechanical gauge.”
So, what was happening?
Lots of people like the pushbuttons on industrial pressure transmitters because the basic settings that every transmitter needs can be set up without a HART communicator. This includes things like the tag name, engineering units, LRV (Lower Range Value, the zero, or what 4.0mA represents), URV (Upper Range Value, the span, or what 20.0mA represents) and damping (an average or filter factor that dampens noise).
On the new Honeywell ST700/ST800 series smart transmitters, the tag name and engineering units are easy to configure and self explanatory, but I seem to stumble when setting up the LRV and URV because I’m faced with a non-descript choice. There’s two sets of options (under Transmitter Setup, not Calibration):
OK, either configures an LRV or a URV value, but which is which? What’s the difference?
Earlier this week, I took a call from a customer having problems with his pressure transmitter. He’d figured out that his last transmitter, installed in an area with nearly continuous wash-downs, had failed because of water intrusion. The bigger problem was that it wasn’t coming from a loose cover, a conduit pipe, or a leaky conduit connection. In his case, the water came into the transmitter through a vent hole and messed up the measurement cell electronics.
So he asked me to recommend a 3000 PSI transmitter that could survive the washdown conditions.
A customer needed to re-range a Siemens SITRANS P DS III transmitter without applying pressure. He’d read my earlier blog post about using the pushbuttons to program the DS III (Pressure Transmitter Pushbuttons to the Rescue!), and where to find the Z-fold instruction sheet.
All was going to plan, until he started looking for the settings for lower range value (LRV) and upper range value (URV). And then, in an all-too-familiar scenario, he hit a roadblock. The instructions don’t actually mention URV and LRV, the terms he’s familiar with, by name or acronym. So what’s a guy to do?
People have asked me about setting fault alarms in level transmitter analog signals at 2mA or 3mA levels. What they typically don’t understand is that a two-wire transmitter uses the electrical current below 3.6 mA for its own power and operation. So, a 2.0 mA or 3.0 mA fault indication just isn’t possible. At these low currents, there wouldn’t be enough power to generate the fault indication signal and to keep the transmitter functioning properly. Read the rest of this entry »
Posted by danstips in Air Bubbler, Communications, Configuration, HART, Honeywell, Installation, Level, Level Technology, Paperless Recorders, Pressure, Pressure Switches, pressure switches, Pressure Transmitters, Siemens, Software, Switches, Transmitters, Trendview X-Series Paperless Recorders, Troubleshooting, United Electric on February 2, 2012
Yesterday and today, people all over Chicago and the Midwest were looking at pictures from last year. We had 22″ of snowfall in one day. The roads were closed. The airports were closed. Even the Lesman offices were closed. And today’s weather forecast? 45°… in Chicago… in February.
This morning in Pennsylvania, a groundhog named Phil came out, looked back, and saw… his shadow.
All this looking back made me a little reflective myself. I’ve been writing this blog for about 6 months now. So I thought I’d take a minute and recap the articles people keep coming back to read:
Pressure instruments in contact with the process can take a real beating. Process fluids can corrode the wetted parts and destroy the sensing element. Media the solidifies can clog the pressure-sensing port. Or, an installed instrument can affect the process by providing a spot for media remnants to remain after cleaning and purging. Specialized chemical seals deal with these pressure sensing issues.
Here’s a list of questions to ask about your application that will determine if you need a chemical seal with your gauge, switch, or transmitter.