A while ago, I got a call from a customer who was having trouble with a differential pressure transmitter. He was using a draft range DP transmitter to measure the pressure in a combustion chamber, so it could be controlled with a damper. He had one port connected to the combustion chamber with impulse tubing, and the other (low side) was left open to the atmosphere.
He’d noticed that when a fork truck or other vehicle sped past the furnace – the transmitter was mounted next to a traffic lane — it cause the furnace pressure to momentarily dip downward, and cause the damper to oscillate.
He figured out that the air movement provided by the passing vehicle was creating a momentary pressure pulse on the low side port. These air movements were creating difficulty in maintaining furnace pressure.
So, he asked me, “How can we dampen the effect of the momentary pressure pulse?”
Recently, a customer and I were talking about his Honeywell X-series paperless recorder, and how the information was appearing in TrendManager PC software.
The graphs were showing data with the original pen scales, 0° to 500° F. But the recorder rarely, if ever, read below 100° or above 400°, so there was a lot of “white space” on the trend chart.
Could we do anything about it?
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?
Unexpected moisture in your process lines can cause costly problems.
From corrosion, rust, and oxidation to frozen control lines, failed pneumatic systems, and increased wear on moving parts, the presence of unwanted water in your process lines and instrumentation can be incredibly damaging.
My associate over at RAECO spells it all out for you. Check it out!
Originally posted on Tom's Tips:
When someone mentions issues with moisture, the first thing that comes to my mind is mold. If your basement has ever flooded, you can relate. This article is not about that.
In the manufacturing and process world, moisture can cause significant problems, even in small concentrations. The phrase “water and oil don’t mix” came to being for a reason (well, technically it can under the right pressure, but you get the idea).
Water doesn’t always play well with others. It can cause corrosion in valves, pipes and motors. It will react with a number of chemicals like lithium, sodium, silver, ammonia, chlorine, hydrogen sulfide and sulfur dioxide. It can destroy catalysts in hydrocarbon and petrochemical processing. And of course, it will freeze.
So what does that mean to your processes?
- If you have pneumatic controls and tools, it can cause corrosion and can also plug pneumatic orifices, valves and actuators
- If you…
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More and more people are using Modbus to get data from their instruments and controllers back into their control systems for reporting, alarming and troubleshooting.
And while I can’t be there to help you set up your Modbus master, I can give you 13 rules and some general practice advice for communicating to any Modbus RTU device.
Last month, I presented a webinar for our customers. At the end of Control 101, when we opened the session up for questions, a customer asked about making the choice between 4-20 mA and digital signal outputs. It got me thinking about this list I put together a while back — one that I pull out whenever customers ask me “Is 4-20 mA still valid?”
The answer: Yes, it is. And here are the 18 reasons I came up with… so far. Read the rest of this entry »