Temperature Control on Sales Oil Coolers (Over-cool/Under-cool)

Heres the situation..... 

I have a control loop (TIC-3148) that consists of one Glycol cooled exchanger, that cools the oil from a Vertical Stabilizer to the tank farm. We currently control this loop by PID control with the Oil Flow through the cooler Fed Forward, with a filter block, Lead lag and dead time block. I have tried manually tuning and,Insight, and cant seem to get this to stop over cooling and undercooling (as you can see in the attached trend) Is there some kind of FRSPID_OPT or STRUCTURE that i am missing? Below are some of the Process Disturbances i have noticed 

1. Flow through the cooler: I have tuned the Flow to be fairly constant through the oil trains but the composition is still changing (Slugs of Oil) and the odd time the stabilizer gets a huge push of oil

2. Cold Glycol Temperature: when we have hot weather it will affect the glycol cooling capacity so the Glycol flow demand will go up in order to cool the same capacity of Oil 

I also find that there is loop interaction between the Min Flow controller and the Shipping controller when production is low, and the system starts "batching". The stabilizer closes off shipping, the re-circulation valve opens until it builds level, then starts opening the shipping valve and closing the re-circulation and the Oil cooler will see a big rush of flow and almost high temps and shuts down. This is only a problem when production is low and i believe we can fix by split ranging between the two valves to minimize loop interaction

Any thoughts?Metal

Attached below is a Graphic, Control studio and a trend for clarification 

 

12 Replies

  • If Hot Glycol is reused after cooling. Isn't it just sufficient to slow TIC-3148 down?
    Also do you need to maintain continuous shipping flow?
  • In reply to Junggyu Yu:

    Yes i want TIC -3148 to be slow and steady but due to the nonlinear response and disturbances, I find the temperature will go above 65 degrees and shut in our shipping when the Controller is too slow. My hopes are for small Fluctuations in Controller output, a steady PV, and have the controller Ramp when it needs to(High Flow rates). The error is okay if it swings about Plus/minus 5 deg C. Maybe I am seeing this wrong.... for now it is tuned to keep the temp from hitting 65 Deg ( +22DegC from SP, sometimes the PV makes it all the way to 62DegC).

    My Current Tuning Values are
    Gain:1.5
    Reset:50
    Rate:10
    FF Gain:0.15

    Are you saying i should increase the Reset??
    I do not need to maintain continuous shipping flow, we just find it is easier on the Oil Coolers to have a stead differential pressure, instead of pushing high amounts of oil, to no oil.
  • Shamusk,
    First we must make sure the temperature valve is performing well. Does it have a positioner? The valve performance can also be verified with the glycol flow, FI-3151. Could you send a history plot that includes the TC PV, SP and OUT; FI-3151 and FI-1605. The TC-3148 can be in Auto for this plot but zoom in for 1-2 cycles so we can tell what is going on. If possible, perform a series of 1% steps to check the valve, wait about 60 seconds on each step, 2 steps up, 4 steps down, 2 steps up. Send a plot of this with scaling of the variables that we can see movement.
    Also, with a constant oil flow, you should be able to put TC-3148 in Manual and the temp should line out. Then, perform two manual output steps (2-5% if possible), a step either up or down, wait until the temperature lines out, them back to the original value and let temperature line out. Then send the recording of the above variables.

    If the Temperature Transmitter is very far downstream of the cooler, the dead time might be excessive which would impact the tuning. Once I see the trends, I can make a better judgement on the tuning.
    James
  • In reply to James Beall:

    Shamusk,
    I just noticed that when you reduced the FF_GAIN the swing of the PID/OUT reduced greatly. In addition to the above questions, what is the value of the FF_SCALE and the range of FI-1305 (oil through the cooler). If possible, I recommend that you turn-off the FFWD and get the tuning resolved (I agree with the comment that the Integral may be too fast (Reset too small).
    James
  • In reply to James Beall:

    James Beall,

    Thanks so much for the reply. I am assuming that the positioner is an old one since it is part of the old plant, but I will look into this when I make a field trip. I did the testing you recommended and i got some trends that i will attach below. I also took a disturbance trend for FF calculations, But FF is disabled for now 

    The value of the FF scale on TIC-3148 is 0-150m3/Hr

    FI-1305 Scale= 0-150m3/Hr

    Example1: This Trend shows how the system is currently tuned/controlling...

    • FI-3151=Glycol Flow
    • FI-1605=Oil Flow 
    • TIC-3148= Temp Controller

    Example 2: The Requested valve performance indicator 

    • I manipulated the Temp Controller valve output as requested 
    • The Glycol Flow shows some issues with ether the valve or flow meter??

    Example 3: No disturbances (Oil Flow Steady) and manipulate the Temperature controller output (3%) step change

    • Noticed that the Flow becomes unsteady and noisy. Not really sure why... i still need to figure this out. Oil Viscosity due to high temperature causes the Flow meter to read incorrectly??
    • Temperature Steady state when Output is maniputlated: 54.4Dec C Took about 11min 
    • Temperature steady state when put back to normal: 44.8Deg C took 20.5 min 

    Example 4: (extra Info) How the Controllers reacted from being put into auto with current tuning values 

    • Done With testing and switched back to Auto 

    Example 5: Flow Disturbance 

    • I actually Tripped the Pumps on this one because it was so close to steady state (6m3/Hr set-point change with temp controller in manual)

    Let me know if you need more information! 

  • In reply to Shamusk:

    I forgot to ask! What is the PV_SCALE of TIC-3148?
  • In reply to Shamusk:

    Shamusk,

    I think the glycol valve has problems.  It looks like it has overshoot and an excessive delay upon a reversal in direction.  However, it does appear to responding in some manner to 1% changes.  You should have it checked but we can proceed!

    Ok, let calculate the loop dynamics.

    Up Step:  Process (loop) Gain = %PV Change/%Output Change = ~(54.4 - 42.5 Deg C)*(100%/150 Deg C) / 3% out = 2.6 %PV/%OUT

                    Dead Time ~30 seconds

                    Time Constant = Tss/4 = 12 min/4 = 3 min = 180 seconds   (my estimate)

    Down Step:  Process (loop) Gain = %PV Change/%Output Change = ~(48.8 - 54.4 Deg C)*(100%/150 Deg C) / 3% out = 1.24 %PV/%OUT

                    Dead Time ~60 seconds

                    Time Constant = Tss/4 = 16 min/4 = 4 min = 240 seconds   (my estimate)

    To be on the robust side of tuning, let's use the Highest Process Gain (2.6) and the slowest dynamics (Dead Time=60 seconds, TC=240 Seconds).

    I'll use Lambda Tuning for First Order Self Regulating Process, Lambda = 3* Dead Time = 180 Seconds.

         Reset= Time Constant = 240 seconds

        Gain = Reset / [(Process Gain)(Lambda + Dead Time) = 240/[(2.60)(240+60)] = 0.31

        Rate = 0 seconds

    I'd be concerned about using FFWD until you get the production oil FT responding better.  Also, it looks like in the test where you increase the Production Oil flow from 

    When you do, it looks like you should have the following FF_GAIN, based on a FF_SCALE of 0-150 M3/hr.  Looks like a ~4 Deg C increase in Temperature when the Production Oil was increased 6 m3/hr.

         FF_GAIN max = -(gain vs ffwd variable) /(process loop gain) = -[(4 deg C*100%/150 Deg C)/ (6 m3/hr *100%/150 m3/hr)] /(-2.6) = 0.25

         I generally start with about 1/2 of max FF_GAIN and increase based on performance.  So, start with ~0.15 and increase slowly as dictated by performance!

    Don't forget to have the glycol valve checked!  Let me know how this works out.  

    Note the temperature response had the typical 2nd order response so some derivative (20-50 seconds) would be helpful but let's see how this works first!

    James 

  • In reply to James Beall:

    James Beall

    WOW! Thank you very much for the calculations, this helps myself and the plant alot. I applied the PID values recommended and i will leave the FFWD off for now. Will let you know how it controls in the mean time

    I have noticed that the Min Flow and the Oil Flow out have Loop interaction and those spikes seen in the oil flow to sales is caused by the min flow controller making adjustments. However this is only an issue when production through the unit is low and the min flow is actually needed. I will have to bring this up with our engineering group and get some kind of split range control strategy implemented to cancel the loop interaction. I was thinking complementary split range control should work?

    We have scheduled Instrumentation to take a look at all the Loops involved (Including the Glycol Valve) and check for instrument error (In Two Weeks) so stay tuned for how that goes.

    Question: Why no Derivative action?

    Cheers,
    Shamus Kelly
  • In reply to Shamusk:

    Shamus,

    I did not calculate a derivative because I could not see the Temperature Trend on the step test trends well enough to estimate the second order time constant, which would dictate the Derivative (Rate) setting.  Since I could see the second order, I mentioned that you might try adding a Rate setting of ~20-50 seconds (start at 20, move to 50 if beneficial).  With some run time to confirm stability, you might also try increasing the controller gain from 0.31 towards 0.61 if it shows benefit.  My initial tuning was pretty mild.  The FFWD will be very beneficial if the production oil flow meter can be fixed!

    Here are links to Lambda Tuning articles that I wrote in the ISA Intech magazine.

    I'm not sure how the minimum flow scheme is configured currently.  Perhaps the erratic flow reading is causing some problems with the current scheme.

    Good luck!  Let me know how it goes!

    James

  • In reply to James Beall:

    Here's an update regarding this tuning issue:
    I stepped away from this problem until the instrumentation could be checked. So as of Jan 20th, 2021 the WO to investigate all the Instruments associated with this system is complete. A couple interesting factors were found in the report below;


    Sales Cooler Loop (115E): Good
    Notes:
    1. There is a Thermowell located on the inlet to the cooler that currently has a local Temperature gauge.
    2. Temperature (read off the local gauge) was 92.5. Temperature on the vessel TI-3105 read 103.5 Deg C
    3. Valve Trim on LY-2109 was changed in Aug-Sept by TECH from quick opening to equal Percentage.

    TIT-3148
    Make: RTD
    Installation: Thermowell
    DCS Range: 0-150 Deg C

    Calibration:
    0% = 5.65 Deg C
    50% =80.6 Deg C
    100% = 155 Deg C
    75% = 118.32 Deg C
    25% =43.19 Deg C

    Report:
    1. Instrument Tech Attempted tightening the Field JB termination; no change in RDT calibration.
    2. Instrument Tech attempted checking and tightening the Marshalling connection (charm); no change
    3. Added a Compensation in the Delta V module to subtract 5.5 Deg C from the AI Block

    TY-3148 Positioner
    Make: CVS Smart Positioner (Feedback available)
    *Note* valve was rebuilt and replaced by Techs a couple months ago valve looked responsive but did not test for stiction

    FIT-3151
    Make: Rosemount DP
    Installation: ½ inch tubing from Orifice plate
    DCS Range: 0-75 m3/Hr

    Calibration:
    0% = -0.027%
    50% = 50.03%
    100% = 100.072%
    75% = 75.05%
    25% = 24.959%

    Report:
    1. Blew down lines before calibration; lines looked clean and flowed freely.



    Sales Flow: Good
    FIT-1605
    Make: Krone Ultasonic Flow meter Installed inline.
    Installation: Flow meter is strapped on fairly far away downstream of the valve (should have a Dead Time)
    DCS Range: 0-150m3/Hr
    *NOTE* Tech verified the Meter was installed properly. Cannot do much more.

    LY-2109
    *This valve was serviced and the trim was replaced from quick opening to equal percent by TECH. This valve is deemed working

    Gas Boot Pressure Loop: Not Acceptable

    PIT-2110
    Make:Old Rosemount “dumb” transmitter
    Installation: Glycol Filled to prevent oil from plugging up the lines
    Calibration:
    *Did not get to record values because this was done online and we did not have time to verify back to DCS. Instrument required calibration because it was out of calibration.
    *MOC will be Created to add two redundant smart transmitters

    PY-2110:
    Make: Old style “dumb” I/P
    Installation: An adjustable regulator exist on the inlet and the pressure control loop exist on the outlet and only controls the outlet pressure. Consists of an old Fisher Pneumatic positioner

    Calibration:
    25% = 10%
    75% = 45%
    100% = 75%
    50% = 43%
    0% = 0%

    *TECH is making a follow up to replace the positioner with a CVS Smart positioner. Will update Engineering with the new data sheet
  • In reply to James Beall:

    James,

    Hope all is well,

    Thank you for  the ISA links. Here is how the loop looks with just throwing some Tuning values in. As you can see it is handling the Flow (upset) fairly well. I found it went to +/- 4Deg C (Previously +/- 15Deg c and would almost trip the unit)

    The next problem to solve here is the level swings in the stabilizer, which will take some time to figure out and find a feasible solution. I've definitely learned that you have to get all your loops checked for errors before trying to tune....

    My current Tuning settings that I would like to trim up and actually come up with a calculated Tune instead of numbers thrown together:

    Gain: 0.31

    Reset: 160 

    Rate: 5

    PV Filter TC: 6

    FF Gain: 0.4 

    Filtering on the FF signal: 20 lead time and 10 time constant filtering 

    Thanks so much, 

    ShamusK