Re: [foxboro] Compensated Level Indication
- From: Terry Doucet <doucet427@xxxxxxxxxxx>
- To: <foxboro@xxxxxxxxxxxxx>
- Date: Wed, 1 Sep 2010 11:50:38 -0400
The major d/p transmitter manufacturers will offer a matched set of diaphrams
(equal volume on both the Hi and Lo side of the d/p transmitter). This negates
any worry about wet legs vaporizing liquid and since the volumes are matched,
ambient temperature changes have less impact on dp error. The cost is a little
higher but well worth it to keep a constant pressure on the higer elevation
level tap.
Density of liquid propane is going to change with temperature much more than
with pressure as long as your pressure can maintain the liquid and not allow
flashing to vapor. If you can assure liquid is always present, then the higher
elevation pressure tap should correct for changes in pressure in your surge
tank.
You need an RTD in the tank to measur liquid temperature. For the ranges of
temperature that you expect to see inside the tank, check the propane tables to
see how linear is the response of density to temperature. If it is reasonably
linear then you can use a calc block to correct the "indicated level" to true
level. If the response is dramatically non-linear then you may have to use a
characterizer block to get a value of density for a specific temperature and
send that to the CALC block to correct the indicated level.
You also need to have a feel for the economic costs of the bad indicated level.
Afterall the definition of a surge tank is to allow things to smooth out over
time.
If there is any chance that control of the level will be required at a later
date, then you can look at in-flow and out-flow to work towards a 3-element
controller.
Terry
> Subject: Re: [foxboro] Compensated Level Indication
> Date: Wed, 1 Sep 2010 10:02:30 -0400
> From: E.Steinbrecher@xxxxxxxxxxxx
> To: foxboro@xxxxxxxxxxxxx
>
> I have to agree with Mr. Riccardi. Unless you have bubbles to deal with
> in a propane tank like you do in a steam drum, the two legs of a dP
> instrument ( one in the top and one in the bottom) should work, both for
> level and mass since you have a density data as related to temperature.
>
>
>
> >Nick Steinbrecher
> >
>
> -----Original Message-----
> From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx]
> On Behalf Of Putman, Robert
> Sent: Wednesday, September 01, 2010 7:59 AM
> To: foxboro@xxxxxxxxxxxxx
> Subject: Re: [foxboro] Compensated Level Indication
>
> I agree with Corey's comments that this issue should be very similar to
> power boiler drum level control. As I recall, boiler drums typically
> have a choice between single element and three element control. There
> should be significant information on the web on how this is done.
> Foxboro also should be a resource. At one time they had an excellent
> group in Foxboro, MA that worked on boiler controls.
>
> Taking an educated guess - As temperature goes down density increases
> which should drive level down. However, at the same time the vessel
> should contract which should make the level increase. Not sure what the
> net result of this is but it could be tested.
>
> Also, remember that levels are integrating processes with a different
> set of tuning rules than flows, temperatures, or pressures.
>
> Best of luck,
> Bob
>
> -----Original Message-----
> From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx]
> On Behalf Of Corey R Clingo
> Sent: Tuesday, August 31, 2010 11:28 PM
> To: foxboro@xxxxxxxxxxxxx
> Subject: Re: [foxboro] Compensated Level Indication
>
> Got too occupied to respond today, but I did this once upon a time on a
> steam drum on a fired boiler. If I recall correctly, we used steam
> tables
> to determine the vapor and liquid densities based on temperature and
> pressure and used those with the dP leg liquid heights to calculate
> actual
> level based on P, T, and dP (not even sure we used T, as it had less
> effect for the conditions we saw than the pressure). It probably had
> the
> most effect under normal operating conditions when boiler demand
> changed,
> and the level fluctuated due to "shrink and swell" effects. If you need
> details I can dust the Brand H cobwebs off of my brain and get the
> particulars.
>
> For propane, I wouldn't think you would need to go to such great
> lengths,
> unless the density changes are great over your operating range (I don't
> have the physical properties of propane handy, though). If not, just do
> the density calculations ahead of time for "typical" operating
> conditions
> and calibrate the transmitter accordingly.
>
>
> Corey Clingo
> BASF
>
>
> P.S I'm an electrical guy by training, so take my chemistry with a mole
> or
> two of NaCl.
>
>
>
>
>
> "Joseph M. Riccardi" <Joe@xxxxxxxxxxxxx>
> Sent by: foxboro-bounce@xxxxxxxxxxxxx
> 08/31/2010 07:32 PM
> Please respond to
> foxboro@xxxxxxxxxxxxx
>
>
> To
> <foxboro@xxxxxxxxxxxxx>
> cc
>
> Subject
> Re: [foxboro] Compensated Level Indication
>
>
>
>
>
>
> Troy,
>
> I have been waiting for all of the measurement experts to chime in
> before
> possibly embarrassing myself but it seems they are few and far between.
> Another dying breed? I am certainly no expert but I'll throw my 2 cents
> on
> the table to start the bidding process.
>
> In a closed tank pressurized system, the only pressure compensation
> required
> on the level measurement is from the pressure in the vapor space above
> the
> liquid; no? Isn't the low pressure side of the dP level transmitter
> connected to the top of the tank; this is called the reference leg?
> This
> reference leg can be dry or wet depending on the application and/or
> installation. Changes to the vapor pressure are automatically
> compensated
> for in the same way as the effect of barometric pressure changes are
> canceled out in open tanks (low pressure side of the dP level
> transmitter
> is
> open to atmosphere).
>
> Temperature changes affect the Specific Gravity but I think this is a
> straight forward calculation that must be included.
>
> Any help so far or should I fold them...
>
>
> Joseph M. Riccardi
> DCS Services - Industrial Process Control
>
> North-Central Office (OH, PA, MI, IN, WV Area)
> South-East Office (FL, GA, AL, SC, NC Area)
>
> "To give real service you must add something that cannot be bought or
> measured with money; and that is sincerity and integrity." - Donald A.
> Adams
>
>
> -----Original Message-----
> From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx]
> On
> Behalf Of Brazell, Troy L
> Sent: Tuesday, August 31, 2010 12:12 PM
> To: foxboro@xxxxxxxxxxxxx
> Subject: [foxboro] Compensated Level Indication
>
> All knowing list,
>
> I have had a request thrown at me that I have never had before.
>
> This plant is asking for pressure and temperature compensated level
> indication on a propane refrigerant surge tank.
>
> I have the dP level, head pressure and vapor temperature. Has anyone
> done
> this?
>
> Thanks
>
> Troy
>
> **************************
>
> Troy Brazell
>
> DCP Midstream
>
> ISA CCST
>
> Sr. Process Control Analyst
>
> Office 405-605-3877
>
> Cell 405-301-2994
>
> tlbrazell@xxxxxxxxxxxxxxxx
>
> **************************
>
>
>
>
>
>
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> Systems (formerly The Foxboro Company). Use the info you obtain here at
> your own risks. Read http://www.thecassandraproject.org/disclaimer.html
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> _______________________________________________________________________
> This mailing list is neither sponsored nor endorsed by Invensys Process
> Systems (formerly The Foxboro Company). Use the info you obtain here at
> your own risks. Read http://www.thecassandraproject.org/disclaimer.html
>
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_______________________________________________________________________
This mailing list is neither sponsored nor endorsed by Invensys Process
Systems (formerly The Foxboro Company). Use the info you obtain here at
your own risks. Read http://www.thecassandraproject.org/disclaimer.html
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