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Old December 12th, 2017, 12:05 AM   #1
Bullzi
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Pump Control Ideas

Hi Everyone,
Looking for ideas on a new project. Here are the details:
PLC - CompactLogix L33
VFD - PowerFlex 753
Ethernet/IP Control

We are unloading rail cars of Gasoline and Diesel and putting it into storage tanks. I have Pressure Transmitters on the pipe on the inlet to the pump and on the outlet from the pump. Those run into the PLC Analog Card.

The pump requires a minimum NPSHr of 6 ft at lower flows up to 8.5 ft at full flow. I have converted that to psi to give me a range of 1.9 to 2.95 psi.

I have done projects with this VFD but it was more simple control, based on the # of pump request. Never done one where I was looking at a dynamic input like pressure.

What I want to do is to use the PT's, PLC and VFD's to control the pump speed so I can pump as fast as I can without cavitating the pump.

As the rail car pumps down the NPSHa will drop so obviously I will need to slow down the pump. I was just looking for some ideas. I could just do some kind of range thing like:
Inlet Press >3psi run full speed
Inlet Press between 2 & 3 run and 3/4 speed
and so on
But I as thinking that there has to be a better more fluid solution for doing this. Maybe PID?

So if you have any experience in doing something like this I would love to hear how you handled it.
Thanks for the help!!
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Old December 12th, 2017, 01:38 AM   #2
OkiePC
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PID would probably work well here using the inlet pressure as the PV and with a setpoint set with a small safety factor above the minimum allowed for smooth pump operation. Once it is well tuned, you could probably trim the setpoint down even closer to that minimum value and get the fastest performance from the system.

You will probably need to filter the analog input from the PT if it is in the moving stream of fluid. It is likely to bounce due to vibrations in that stream. I have only done water and fluids with SG near or above 1.0 so not sure how the fluid dynamics of fuel might pose unique challenges I am not aware of.

I would definitely have shutdown fault logic based on the inlet pressure being below a setting somewhere below the minimum PID SP and above the mechanical safe limit, but tied to a debounce timer to avoid nuisance trips.
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Old December 12th, 2017, 10:29 AM   #3
Bullzi
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Thanks Oki,
I have all my analog scaling in a timed task that runs every second. Do you think that is long enough of a delay?

I have never used PID in a PLC so this will be a first for me. What would you recomend for starting values of P,I & D?
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Old December 12th, 2017, 11:53 AM   #4
James Mcquade
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Bullzi,

I don't know your degree or background so I am going to give my opinion.

Since you mentioned Railcar tankers with gasoline and diesel, you must adhere to class 1, div 1 rules in NFPA70. you must also look and NFPA70e - are flash, and nfpa 496 - purging and pressurization of enclosures, nfpa 79
- electrical standard for industrial machinery (may not apply here), and I think there is a requirement for unloading pads (staging area).

I also recommend that a PE (if you are not one or have on in the staff) to review your design and stamp it to avoid any future issues if an incident occurs.

regards,
james
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Old December 12th, 2017, 01:48 PM   #5
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I don't know that I would even use a PLC, there are very smart VFD's that have PID's built into their software.

We use VFD's with PID's setup all over the oil industry and they work great.

A standalone VFD just might be cheaper also??
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Old December 12th, 2017, 03:45 PM   #6
Bullzi
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James - I am not a PE but we have one on the project. I have been working with Petroleum products for almost 30 years so I fully understand your concerns. The VFD and Control panel are outside of the Classified Area.

damica1 - Yea we looked at the ABB drives with the "Pump Smart" Technology and they were over $2,000 more expensive each than the AB PowerFlex. That is saying something when you are more money that AB!!

I like functionality they had but for $8,000 (4 x $2,000) I can write a lot of logic. So I guess I am trying to emulate some of that functionality in my PLC code.

Thanks for the input!!
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Old December 12th, 2017, 04:01 PM   #7
VAN
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Quote:
Originally Posted by Bullzi View Post
So I guess I am trying to emulate some of that functionality in my PLC code.
Does this project have an HMI?
Can you build a quick HMI just as a proof of concept?
Can you inhibit the IO cards and build a simulation routine (this maps IO so that when an input comes on the code turns an output on and you can simulate the system running)?

I always think it's easiest to just simulate a system and figure out what to anticipate.
So, pump turns on the PSI should rise this can be a simple add based off a timer that the pump is on and the output of the pump (or CV) would make the added value rise/fall. This would let you get some base PID parameters in and start a "tuned" PID. Or even prove the system works as you think it should.

I'm a big fan of simulation code.
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Old December 12th, 2017, 04:20 PM   #8
Tom Jenkins
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A couple of thoughts.

NPSH is typically absolute pressure (psia) not gauge pressure. Make sure you include approximate barometric pressure into your considerations.

NPSH is based on preventing cavitation, which is the formation of vapor bubbles at the impeller. It will depend on temperature, so make sure you have adequate safety margin for reasonable temperature variations.

Most pump curves show NPSHR for water. Gasoline and Diesel have different vapor pressures, and both will differ from water. Make sure your NPSHR is appropriate for the worst case fluid or somehow differentiate between them.

I wouldn't control the pump with a PID based on the suction side, particularly if this is a flooded suction pump. I would run based on another constraint, such as discharge pressure or flow rate if that matters, or else run flat out until I approach the NPSH limit. Then I would simply increment the speed down until NPSH is OK. I would use a simple deadband control like a home thermostat.
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Old December 12th, 2017, 05:01 PM   #9
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I agree with Tom above. PID on inlet pressure is probably not the best approach, particularly if the liquid level in the rail car has a greater effect on the pressure than the pump speed.
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Old December 12th, 2017, 05:45 PM   #10
James Mcquade
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so more thoughts to consider

use line and load reactors for 480 volts if the motor length id over 50 ft. I would use them for less than 50 ft.
use shielded vfd cable for the motor leads to reduce noise in the cabinet.

regards,
james
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