Re: [foxboro] General Instrument Question
- From: Corey R Clingo <corey.clingo@xxxxxxxx>
- To: foxboro@xxxxxxxxxxxxx
- Date: Wed, 8 Aug 2007 13:41:32 -0500
The meter in question was measuring 30 °C nitric oxide. We also have
several in steam service (60# and 235#). We have lost a few sensors, but
the primary failure item on the steam ones is the U-shaped gasket around
the sensor. It leaks and steam can then travel up to the meter
electronics, where it condenses and causes meter failure.
The NO application was somewhat difficult, needing an oversize meter to
avoid pressure drop. Consequently, we needed a relatively high turndown.
And the noise, small as it was, was actually there; it was gas pressure
fluctuations from a compressor bank, but those were separated from the
meter by a packed tower and 200' of pipe (we found this out by closing a
block valve upstream of the meter). The pipe itself did not vibrate.
We originally bought the Foxboro because of its claimed turndown ratio in
the PSS, ~40:1. That turned out to essentially be a lie (or marketing,
but I guess there is little difference between those), because the manual
states that the low flow cut-in should be raised several "steps" above
that 40:1 minimum for good performance. We kept having to raise it and
raise it beyond that to get rid of the noise, to a point that we were
barely able to start up. We finally had to stop raising it and use the
aforementioned block valve as part of the startup procedure; manually
close block valve, reset the NO system shutdown logic, have operator at
console open control valve some, have outside operator open block valve
quickly to get established flow before noise spiked the reading and
tripped the plant again.
And Foxboro is the only vortex vendor I've seen that has a process-exposed
sensor. This has caused us problems on the NO meter because we have to
clear the line (which requires shutting down the entire plant) to replace
the sensor. We initially accepted the exposed sensor because of the
turndown we thought we would be able to achieve.
We got several different "solutions" from different TAC people, some of
which contradicted each other. We replaced every part in it except the
meter body. We had a field service guy come down and wire up a brand new
one for us after they claimed our wiring practices were faulty (that
installation worked less than a year). We relocated the transmitter head
to the meter body to eliminate the remote preamp and eliminate the
low-power signal runs. Nothing permanently fixed it.
After several years of fighting it, TAC making things up as they went
along, and the inherent disadvantages of the exposed sensor, we gave up
and bought the Rosemount. Its sensor is separated from the process by a
welded chamber, so there are no leaking gaskets to worry about. It has
more configuration knobs to tweak if you need them (e.g., low-pass filter
frequency and amplitude detection threshold are independently tunable),
but we let the meter figure everything out based on gas density and it has
worked great, with a low flow cut-in about half where we left off with the
Foxboro. Block valve is no longer required for startup.
As for the Micromotion, I don't know if the gas/liquid split is consistent
(I suspect not) ,but the coil drive is consistently at or above 100%, so
it is apparently not always able to pick up a signal. It certainly isn't
Micromotion's fault. I'm sort of waiting for operations to get tired of
it, get tired of my "I told you so's", and fix the piping :)
Corey Clingo
BASF Corporation
<tom.vandewater@xxxxxxxxxxxxxx>
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08/08/2007 11:57 AM
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Re: [foxboro] General Instrument Question
Corey,
We also had intermittent problems with Foxboro's Vortex meters.
We liked using them in our 150# steam applications instead of DP
cell/orifice plates but we experienced unpredictable signal noise on
multiple occasions and had to replace the sensors too often in order to
solve the problems. I've never really trusted them since then but they
may have solved that issue.
As to the statement about two phase flow measurement with
Coriolis meters, I found this statement on the Emerson site:
"The effect that mixtures of liquid and gas phase will have on meter
accuracy will depend on the design of the flowmeter and the precise
process conditions. Micro Motion offers Coriolis flowmeters that perform
extremely well under many of the conditions of gas entrained in liquid.
Even more important, Micro Motion offers customers a tremendous amount
of application experience to assist in helping customers install
Coriolis flowmeters for the best possible performance in these types of
applications. Although large quantities of gas in a liquid flow might
influence the measurement, it is important to know that dual phase
fluids will not permanently damage the meter."
In reading this you can assume that you probably can't expect to
buy one off the shelf and have it perform with accuracy in a two-phase
application. You probably need application support from the vendor
also. But if your two phase split is consistent it should be possible
to give a good representation of mass flow.
I remember at the advent of Micro-Motion meters that the
sensing/transmitter unit had to be solidly mounted on top of about 3 yds
of concrete to eliminate line vibration. We calibrated them by pumping
water through them into 55 gallon drums and then hauled them to a weigh
station. They were indeed very accurate. They have progressed a long way
since then and can be installed in about any location/orientation and
they are "dead nuts" on. They are really quite impressive.
Tom VandeWater
Control Systems Developer/Analyst
Dow Corning Corporation
Carrollton, KY USA
-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx]
On Behalf Of Corey R Clingo
Sent: Wednesday, August 08, 2007 11:37 AM
To: foxboro@xxxxxxxxxxxxx
Subject: Re: [foxboro] General Instrument Question
We too have numerous Micromotion meters, and some of them we bought=20
specifically to get a density reading along with the flow. They work=20
well. You can get them with a variety of outputs; we typically use two=20
4-20mA, but they can be had with HART, FF, RS-485 Modbus, etc.
I think some other plants out here have some E+H ones measuring density=20
also.
As for Foxboro's claim to be able to measure 2-phase flow, that would be
useful in a few applications I know of, but I would have to have one in
my=20
process for awhile to believe that. I'd be leery of doing the test,=20
however, as the last time I had a problem with a Foxboro meter, TAC was=20
not very helpful and the ultimate solution was to remove it and buy a=20
Rosemount. That meter was a vortex, and coriolis meters are even more=20
complex.
Corey Clingo
BASF Corporation
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