Re: [foxboro] General Instrument Question
- From: "Ken Heywood" <kheywood@xxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Wed, 8 Aug 2007 17:03:53 -0400
Here are some thoughts from Harold Skinner, an old steam and boiler guy
...
"The standard Foxboro Vortex sensor is a piezio crystal (like in a
telephone mouthpiece). The sensor is rated for 400 deg. F (233 psig
saturated steam). It has two problems. It rolls-over and dies at 410
deg., and deteriorates quickly above 400. I also suspect, after having
replaced several, that never operated over 400 deg., that the sensor
deteriorates at high (allowed) temperatures, and looses performance over
time. This may be caused by deterioration of the fill-fluid (silicone)
which causes out gassing and bubbles to form within the capsule (a
problem capillary seal d/p level transmitters have.) The fill fluid
passes the pressure pulses from the shed vortices to the piezio crystal
to produce an electrical signal.
The solution, was to develop the un-filled capsule (high-temperature,
700 deg. F). Any transmitter can be retrofitted, but they require a
pre-amplifier and can not measure quite as low a flow rate as the
standard sensor. More theory in Foxboro TI 27-65b. (They also
re-designed the standard capsule, going to grounding one side of the
crystal, to minimize signal noise; and they switched manufacturers.)
Also, the old sensor had a problem with high velocity flows (as steam
can be) in that it's ability to measure above a certain frequency was
the limiting factor in sizing full-scale flows, not the vortex principle
itself. Don't know if this has ever been fixed (the new model 84 was
supposed to address this with a new sensor technology) but it is not yet
out. Foxboro eventually modified their computer sizing programs in
recognition of this limitation. (I think it was at about 200 Hz. I
have seen it happen even on air flow. The signal just drops-out, or can
be very erratic - yet returns to good readings as the flow decreases.)
Another problem I am tracking with another brand of vortex meters, on
natural gas flow at a power-generating plant, is the effect of changing
turbulence on the accuracy, as the flow changes. Could this be a
problem on steam?
Coriolis mass flow. A vibrating (or oscillating) tube measures density
from the frequency of oscillation. From this additional signal they can
also calculate volume flow. It is not a new technique, as density
meters were on the market using this approach, even before the mass
meters. Rosemount had a good video illustration on their web site
The problem with two-phase flows is: how do you set up a flow lab to
calibrate the meter (no manufacturer has a good answerer to this one)
and how does the meter tell the difference between a change in flow rate
vs. a change in the ratio?
Foxboro touted their original design as being more tolerant of entrained
air (to a higher percentage that the competition) then came out with the
CFT50 which was supposed to be even better. E+H, whose meter doesn't
seem to do well with entrained air and inhomogeneous mixtures, had
derogatory comments about Foxboro's limitations. The perfect technology
may not exist."
Best Regards,
Ken Heywood
Process Control Services, Inc.
-----Original Message-----
From: Balmer, Robert D. [mailto:robert_balmer@xxxxxxx]
Sent: Wednesday, August 08, 2007 9:08 AM
To: Foxboro DCS Mail List
Subject: [foxboro] General Instrument Question
Content-Type: text/plain;
charset=us-ascii
Content-Transfer-Encoding: quoted-printable I am involved on a project
in China and there is a control engineer over there saying they can use
a mass flow meter to measure density. I have never heard of such a
thing, has anyone out there in control world seen such an application.
If so could you point me to some literature on how it is accomplished?
Regards,
=20
Robert Balmer
=20
Senior Applications Analyst/Programmer
ISA CCST
319 463 2206
Robert_Balmer@xxxxxxx
=20
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