Do you have a PFD or P&ID that you could share? That would be great.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

Here is a crude sketch of the different possibilities:

Put a tank on a high roof, or on a tower.
That will provide a nice constant pressure supply for the users.
Then all you have to worry about is refilling the tank when the level falls a bit.

Mike Halloran
Pembroke Pines, FL, USA

A tower probably costs a heck of a lot more than a VFD these days.

Keith Cress
kcress - http://www.flaminsystems.com

Yeah.. the tank is already in place, moving it is not an option

Hi Azza,
Without a check valve on the pump discharge, there is no way to hold pressure on the line, there therefore your comment " the line does not stay pressurized, but rather falls to low values (~0.5 barg) fairly quickly once the pump is turned off. " Even with a check valve, a small leak will quickly bleed the pressure off. I suppose that you could install a bladder tank. along with a check-valve, to keep the pressure up, and then start-stop the pump on pressure limits.
BTW, how large is this pump (in terms of motor NP hp)? Obviously if the pump is too large then the bladder tank cost could become excessive.
BTW, does the tank have a level transmitter which could turn the pump off on lo-lo level? This might save the pump if the tank becomes empty. I guess there is not flow-transmitter on the pump discharge?
There is no downside to frequent starting / stopping the motor as it is VFD controlled.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

GroovyGuy, hey thanks for looking over my drawing :)

There actually is a check valve (and isolation valve), but I neglected to draw it in a rush. I will post a picture of the setup later. There is also a level sensor on the tank w/ H/L/LL setpoints. As you noted, the LL cuts off the pump. Motor size is 7.5 HP (5.5 kW). There is no flow sensor currently, but that is something that could easily be added as I have reserved space for it.

Ah, so you think using acceleration function of the VFD will allow frequent start/stops? That was one of my concerns. The other is picking the low pressure ON-point...

Howdy Azza,
Is the motor inverter-duty rated, most motors theses days are rated as such. If not, you should consider installing a dV/dt filter on the drive output.
I see no reason why you could not start-stop the pump as often as required to suit demand.
I don't see the ON-pressure setting being an issue, but rather I see the OFF-pressure setting being more of an issue. I would suggest that for starters, that ON-pressure limit < 75% of nominal would be OK. The real trick will be to select an OFF-pressure limit. ie Don't forget that the VFD will try to keep the pressure close to the normal setpoint, and the only way to ensure that the system can hit the OFF-pressure limit is to ensure that the VFD minimum speed is high enough to ensure a high pressure can be obtained during low-flow or no-flow conditions. This can be a bit tricky to set up. Having a Flow-transmitter helps a lot ( ie you can simply stop the pump on no-flow ).

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

Often a small bladder tank is included on the outlet side of the pump in these cases. It allows the water to start flowing the instant it's called for giving the pump some finite time to spool-up. Systems control to pressure looked at next to the little bladder tank which also provides some pressure signal filtering. I've seen the commercial packages turn down the VFD briefly (almost unnoticeably) every 60 seconds or so to see if the pressure drops meaning a demand is really occurring. Since the VFD is PID'ing to the pressure it can not actually tell when there is no longer flow. That's how it's done avoiding flow sensors.

Keith Cress
kcress - http://www.flaminsystems.com

GG, itsmoked, thank you for your valuable advice! Here is a picture of the pumps, as promised. (Should have used a tank outlet isolation valve, but alas. Will add next time I take the piping apart)



The bit about stopping the pump ... I must admit I did not think of that. Somehow I assumed that even with PID control there would be some degree of a spike when all outlets are closed, but before the controller corrects. With this new info in mind, I am quite open to the flow sensor. Now, all I gotta do is source them...hahaha.

I looked into the dV/dt filters. They are quite beefy, aren't they? The ones from the maker of my drive (Toshiba) are about the same size as the inverter itself and weigh... get ready... 30 kg (65 lbs)! Does this sound about right? Where would you mount this thing? Not in the cabinet, right? Also, my cables are < 50m (150 ft), is it still justified?

Not sure if my motors are inverter rated, probably not, unless it comes standard somehow. The fan certainly runs slower if the frequency is down. I've attached the nameplate, if there's an easy way to tell.



About the bladder tank, how big would it have to be for a 15 m3/h (65 gpm) flow?

Hi Azza;
See the following link for Gould's bladder tanks;
http://goulds.com/tanks/
WRT size I would spec their largest standard size of 115usg.

The dV/dT filter that I had in mind can be found at;
http://www.transcoil.com/Products/V1k-dv-dt-Motor-...
These filters are small (ie 8lbs for an 11A filter in an open type configuration).
I would contact your pump/motor OEM and ask if the motors are VFD rated. I'd be surprised if it wasn't so rated. If the motors are in fact VFD rated, that together with your short motor feeder length (of 50m), I would recommend that a filter is not required. However, if the motors are not rated for VFD duty, I would recommend that the filters be installed.

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

Hi Azza; Groovy's got some nice links there and I pretty much agree with his filter suggestions.

I don't know if those motors are VFD rated. One thing that can confuse you is if you ask a maker, "Is this VFD rated?" they will almost always tell you NO you need to pay 2X the normal price for that. Their fall-back argument is always that if there isn't an auxiliary motor running the fan then how could it be VFD rated? In pumps you never need that aux fan since the motor is going to have to spin at least 50% of the rated speed to make a centrifugal pump do anything and when it is running at 50% the load is more than covered by the existing shaft mounted fan.

As for the tank as a comparison a residential little pressure tank is usually on the order of a gallon and it has to contend with faucets banged on and off. If your plant can have a pressure dip without a problem you could go pretty small. If the run is long and big then you'd want a bigger tank that can help buffer that big inertial bang if the bottler bangs off not having a soft valve. In that case I'd probably go bigger too though you might not need 150 gallons.

A problem with a flow sensor is that if it has to see a large flow it may be lousy at seeing a small flow like a hand washing dribble, whereas that would cause a significant pressure change.

Keith Cress
kcress - http://www.flaminsystems.com

GG, itsmoked, did some pump control trials today. The results are exactly as you described. Quite easy to set the low ON pressure. But it is the high STOP pressure that is more of a problem. Under PID control, the frequency just kept getting turned down after all outlets were closed. Eventually it got down to 0.8 Hz and stayed there steady. Tried not using PID at all, and just blasting the motor 100% with only the low/hi alarms being active. This sort-of worked. For any one valve I could pick good start/stop points where the alarms could reliably start/stop pumps as valve was opened and closed. However this approach did not scale to even two valves. Or opening 1/2 or 1/4 of the way. What would have worked for one, did not work for another. With partial opening pressure spikes were a nuisance and led to cycling.

Looks like I need a flow sensor. Any preference between "paddle" and "thermal flow" types?

I will try to find out about the VFD rating of the motors, except these vendors are not very knowledgeable (sigh). The bladder tank sounds like a solid idea. Wish I had thought of it earlier. I guess it will be the next option to explore after trying with the flow sensor.

Thanks for the feedback. Nice looking system BTW. Clean piping.

I believe your VFD should have a do-not-run-below-this-speed parameter setting where you can have it quit at ~10 or 20Hz.

I also believe what you need is not a flow sensor but a controller that does... essentially what I described above looking at just the pressure. A very small PLC or something similar. Even an Arduino would do it.

I suspect the thermal type might be more applicable to your (future failed) attempt at trying to control this with flow.

BTW Using a flow sensor, how exactly do you get any measured flow to actually start the pumps in the first place?

Keith Cress
kcress - http://www.flaminsystems.com

itsmoked, yes, forgot to mention in the update:

Tried setting the minimum frequency. The "steady state" under the testing conditions was 47.5 Hz. Tried setting the low frequency limit on the inverter up to 40 Hz without being able to cause pressure to rise when all valves were closed. Maybe this effect could be achieved by going higher, but I don't see it as a viable solution. The actuation range would be severely limited, and you might as well just run the pump at full power (which did not work out). Oh, and control is much less stable with this limit in place.

Here is my plan with the flow sensor. The low pressure would be sensed by the pressure sensor with some hysteresis. This low pressure alarm will act as a start button being held. While the rising pressure clears the hysteresis, the flow is expected to pick up enough to trigger the flow sensor, which will act as a stop button being held unpressed by the flow. Thus, pressure to start, flow to stop. Does this make any sense?

Howdy Azza,
The only issue that I see with using a low-flow condition to stop the pump is finding a flow-transmitter that is accurate at low-flow conditions to be sufficiently reliable. First, you must decide at what low-flow condition you will stop the pump (ie 1.0 usg / minute)
Options include, in no particular order;
- vortex type
- paddle-wheel type
- ultrasonic type
- magnetic (mag-flow) type
- thermal type
- other??
Perhaps someone with more experience with FTs could offer a solution.
You should not discount itsmoked's suggestion, ie;
I've seen the commercial packages turn down the VFD briefly (almost unnoticeably) every 60 seconds or so to see if the pressure drops meaning a demand is really occurring. Since the VFD is PID'ing to the pressure it can not actually tell when there is no longer flow. That's how it's done avoiding flow sensors.
The above suggestion is actually pretty slick, and you don't need any additional hardware. Of course you will need a PLC to implement the required logic, but I assume that you have one of these about somewhere, correct?
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

"Of course you will need a PLC to implement the required logic"

Ah, yes, the one thing I do not have. I made this panel in an old-fashioned style.. ie, all relays. I guess I was trying to keep it simple, haha. (If I did it again, I would certainly use a PLC).

The flow sensor I am currently looking at is a paddle type. Seems it can read fairly low depending on the model. SMC IF3. I believe this to be the easiest option for me right now. It will fit into the schematic with minimal mods.

Looks like I will be adding a "smart relay" to the project. With this addition, itsmoked's trick about slowing down the VFD comes to mind. May I ask some additional questions about it?

Quote (itsmoked)

I've seen the commercial packages turn down the VFD briefly (almost unnoticeably) every 60 seconds or so to see if the pressure drops meaning a demand is really occurring.

Is the implementation part of the PLC code, ie, turn off pid, slow down the frequency, if pressure < threshold then ... else ..., or is it more of a physical perturbation to jolt the pid controller out of a local minimum? An example of the latter would be - vfd is driving from external (pid control) signal and we go and override that input and set the frequency to 1 Hz for a fraction of a second, and then return back to external frequency input (and see what the pid controller does in response)?

It's a PLC implementation like so:

1) Has a steady state pressure been reached(This could be at zero flow)? No - goto 1; Yes - goto 2
2) Yes - start a 60 second timer. goto 3
3) Time-out? No - goto 4; Yes - goto 5
4) Still steady-state? Yes - goto 3; No - clear timer and goto 1
5) Reduce the speed 30%. goto 6
6) With tank bigness/flowsmallest consideration, is the pressure dropping? Yes - return 30% goto 1; No - goto 7
7) Spool pump down to stopped.
8) Pressure below X? Yes - goto 1; No - goto 8

This scheme requires a check valve or the pressure will never stabilize because as soon as you reduce pump speed the pressure will sag to whatever the tank head is at any particular moment - looking like 'water consumption'. Of course you need a check-valve anyway so you never get process water back-feeding to the source or tank.

Keith Cress
kcress - http://www.flaminsystems.com

itsmoked, thank you for a detailed explanation. A couple more questions:

1. The PID algorithm is being performed by the VFD itself, not the above program, correct?

2. Reducing the speed involves disabling/overriding PID, correct?

3. And lastly:

Quote (itsmoked)

With tank bigness/flowsmallest consideration, is the pressure dropping? Yes - return 30% goto 1; No - goto 7

The pressure drop check, does it have to be fancy? Like a running average of the first derivative? Or can it be simple - just drop speed for 1/2 sec, then check that the pressure value also dropped from the steady state by more than X% ?

Quote:

1. The PID algorithm is being performed by the VFD itself, not the above program, correct?

Yes. Though it's slow enough you could do it in most PLCs if you'd rather.

Quote:

2. Reducing the speed involves disabling/overriding PID, correct?

It can probably be done several ways successfully but typically you'd just change the setpoint the PID is controlling too and whatever PID would just control to the lower setpoint. If the check valve is there then the tank pressure will start dropping down to whatever that lower setpoint is. However, if the tank pressure actually does drop ANY then there is still demand. i.e. it doesn't matter how you actually change the pump output it's only important that you actually reduce the output pressure at the pump's output so you can discover a reducing pressure at the isolated (by the check valve) pressure tank.

Quote:

The pressure drop check, does it have to be fancy? Like a running average of the first derivative? Or can it be simple - just drop speed for 1/2 sec, then check that the pressure value also dropped from the steady state by more than X% ?

Just the simple percentage after a given delay. That's why I mentioned the tank/flow aspect. If you have a monster pressure tank but only a water saving drinking fountain running you may not detect it in less than 30 seconds. You want a tank picked to limit a big drop during the test but to also pickup a small user. I'd guess you want something in the 1/2 to 2 gallon range. (I'm not intimate with your various draws.)

Keith Cress
kcress - http://www.flaminsystems.com

itsmoked, I just completed the implementation of your suggestion. And I must say.... It works fantastic! I did not expect it work as well as it did. I owe you a beer, no make that a keg!

Thank you!

As an example:
Working pressure = 20 - 30 PSIG
Maximum pressure when pump is running at minimum speed = 45 PSIG
Set a pressure switch to start the pump when the pressure drops to 20 PSIG
Set a pressure switch to stop the pump when the pressure rises to 40 PSIG
Use a pressure transmitter to control the pump at 30 PSIG when it is running.
When the flow stops the pump will continue running until the pressure reaches 40 PSIG and the pump stops.
When flow starts or the pressure leaks down to 20 PSIG the pump will start and be controlled at 30 PSIG.
Avoiding the flow control saves money and avoids an additional failure mode.
You don't want to control the flow when the demand is variable.
Rather than a flow transmitter a simple flow switch could be used.
Also, a flow switch or transmitter adds cost and an additional failure mode.
BUT
Consider that when there is no water use for an extended period of time and the pressure slowly bleeds down to zero.
Then when a valve is opened there is no pressure and no flow. No signal to start the pump.
Also,
OR
Use a soft start, a bladder tank and pressure switches as has been done successfully for many decades before VFDs were developed.
Actually, with a large enough bladder tank you will not even need a soft start.
There are probably millions of systems in use using only a DOL pump, a pressure switch and a bladder tank.
I draw water from a well and that is the system I have. I doubt that I could tell any difference in performance if I went to a high tech VFD system.
There are VFD well pumps available for specific issues.
1> A water use license often limits the maximum pumping rate, and the VFD is used to set the maximum pumping rate at that location.
(Example: The pump, as installed delivers 15 GPM. The license allows only 10 GPM. The pump installer adjusts the VFD frequency to deliver a maximum of 10 GPM, and walks away. The VFD always runs at that frequency.)
2> The VFD allows a three phase motor to be used with a single phase supply.
The VFD frequency is set and forget, not varied.
Engineers love to gild the lily but:
The tried and true KISS solution is to use a bladder tank, a DOL pump and a pressure switch.
Bill
--------------------
"Why not the best?"
Jimmy Carter

amazing azza; !!!! I somehow missed your 11 DEC post and just saw it now.
I'm very pleased it worked well for you. Hope your system is getting hands off for you finally.


Yo Bill! I think I see an issue with your scheme:

Quote:

Set a pressure switch to stop the pump when the pressure rises to 40 PSIG
Use a pressure transmitter to control the pump at 30 PSIG when it is running.

With fed-back control to 30 PSIG why would the system EVER achieve 40 PSIG to shut-off?

Keith Cress
kcress - http://www.flaminsystems.com

You would limit the lower frequency of the VFD.

Quote (Bill)

Maximum pressure when pump is running at minimum speed = 45 PSIG

I see the VFD solution as a good way to supply a needed pressure and/or volume boost to an under performing system.
I would use the VFD system as a supplementary system, never as the primary system.
(Large municipal systems may be excepted.)
That said, congratulations on a successful solution. lps

Bill
--------------------
"Why not the best?"
Jimmy Carter

Ah I get that now Bill. I like it.

I believe the VFD based system azza has put together is superior to a simple pressure tank system for his commercial production system. I hate water pressure tanks! They crap out constantly and drive me bonkers. I despise the pressure cycling they cause too. I'd spend 10X the money to avoid the waterlogged pains-in-the-butt. That said, they certainly are simple and straight forward (until they water log..)

Keith Cress
kcress - http://www.flaminsystems.com

Hi Keith. While I don't always agree with you I always respect you, your knowledge and your experience.
Not to argue, but to share experience:
Yes, I have seen waterlogged systems. Generally, but not always, cheap, poor quality equipment.
However there are millions of pressure tank systems in daily use. There are both bladder systems and air cushion systems that use a snifter valve to maintain an air cushion in the top portion of the pressure tank.
Water pressure tanks are ubiquitous in rural areas where well water is used.
Most work well for many years.
I did notice more frequent failures with the bladder tanks available in Central America.
The professional well drillers and system suppliers in rural Canada and I hope in the rural USA use much better quality bladder tanks.
Good quality pressure tank systems last for decades.
We just get called to the failures and most of those are cheap, poor quality equipment.
I have been in my present place for under 10 years but the pressure tank system is over 20 years old. I am expecting a tank failre in the next 5 or 10 years.
My pressure cycles between 30 PSIG and 40 PSIG. We don't notice the pressure variation. I have seen systens with an undersized pump where the pressure drops well below the cut in pressure with heavy use, but that is not fault of the pressure tank, it is a case of the pump is too small to hold the pressure up no matter what control is used.
Most of the systems that I have seen that could benefit from a VFD booster are mains systems with chronic low pressure, but ample volume.
As I said, not to argue or contradict but to share the reasons for my point of view.
Respectfully yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

Bill, thx for the LPS.

Working in filter systems around here where we're extracting either manganese, arsenic, iron, or cleaning up hideous surface water for domestic consumption I've seen lots of bladder tank based systems. Some with as many as 8 pressure tanks tied together. Around here they sure aren't cheap! I'd expect an equivalent VFD based system to cost considerably less in those big multi-tank jobs but I'm not sure as that usually isn't my problem in the installations.

In my personal experience I'm a bit of a water nut and have a large RO system under my kitchen sink. We have two flows from it. One is polished RO and the other is re-mineralized RO. Both have to pile up in 3 gallon bladder tanks under the sink to be available on-demand for cleaning, cooking, or drinking. The RO strips rubber smell from its tank and so needs a follow-on carbon polishing filter. The Re-min side doesn't pull anything from its tank and so is served up straight from the tank. Part of my hatred of bladder tanks stems from these under-sink tanks. It seems I have to do battle with them about every two to four years. It's also annoying because there's always this period (I think I'm entering just this week) where something seems fishy about the standby volume. It then takes about a month to come to the conclusion that one has gone logged. The whole thing is annoying - but really hard to rid of the tanks.

Keith Cress
kcress - http://www.flaminsystems.com

I see where you're coming from Keith.
A bladderless tank won't work for you either. The snifter valve needs the negative pressure from the suction side of a pump to charge with air on each pumping cycle.
I should have mentioned that down south I saw several el-cheapo bladder tanks fail in less than a year. The short cycling would burn out the pump motor. The local expert would replace the pump. The new pump would burn out in a few days. Then they would call me.
I am interested in a new place with a whole house RO system. (a big house and lots of out buildings and 160 acres for Will's horses.)
If I can swing a deal this will be my first experience with RO.
Did you have a horse at one time?

Bill
--------------------
"Why not the best?"
Jimmy Carter

Bill; Thanks for explaining the bladderless tank/snifter scheme. That's a new one to me.

WOW, ouch with that short cycling stuff. Now that you remind me, my old boss' house did that the minute he went on vacation. :)

That new place sounds like da'bomb. I hope you can pull it off!
Whole house RO! That will be something. Must be some lame ground water there. If you land that place make sure you don't just drink RO water as it can imbalance your electrolyte balance. We figured that out after a while. You'll want a re-min cartridge for your kitchen drinking tap. Talk to me if you land there.

Keith Cress
kcress - http://www.flaminsystems.com

We drink bottled water.
When I moved into our present place about 9 years ago, I asked the neighbour across the street how the well water was locally.
"Fine. No problems. I've been drinking it all my life and look at me."
I looked at him and immediately started buying bottled water.
In 9 years I don't think that I have ingested as much as a litre of well water.
On the subject of the snifter valve.
I must correct myself. What is commonly called a snifter valve in some areas is actually an AVC (Air Volume Control) valve.
The snifter valve injects a small volume of air at each cycle. With a snifter valve it is common to get air discharge with the water out of the facets.
The AVC valve has restrictive porting so that when the water level in the tank is above the level of the valve, the valve works normally.
As the volume of air in the tank increase so that the level is below the level of the valve, the action of the valve is much reduced.
Both the snifter and the AVC do the same job but the AVC does it much better with almost no air sputtering with the water.

An Air Volume Control Valve.

Bill
--------------------
"Why not the best?"
Jimmy Carter

I had no idea either of those valves existed. I really like the idea that a tank could work without the stupid bladder. The thing that creeps me out the most about bladder tanks is that once the bladder is leaking just what putricity are you ingesting that lived on the 'other side' of the bladder? With a straight air-in-the-tank there is no normally hidden side. Though, thinking about it, it might make sense to use a HEPA filter to prevent cooties from being invited directly into the tank water. I'm guessing you need a compressed air source for either of those valves?

Keith Cress
kcress - http://www.flaminsystems.com

I'll throw my 2 cents in for the general durability of bladder tanks and the alternative high-maintenance, high risk bladderless tanks.

I have 44 years of service from the 30 gallon bladder tank in my house. The neighbor's replaced his bladderless tank 23 years ago and is still using the same replacement bladder tank. Neither he nor I have ever had to 'add air'.

A couple months ago I stopped by a friends house and I could hear the pump cycle each time a faucet was tuned on. We went down to the basement and sure enough, it was bladderless tank with no air cushion. The relay didn't burn out quick enough, he didn't believe me when I told him his pump is jeopardy, and a month later he had to replace his submersible pump.

I have come to the conclusion that a bladderless tank is the 'contractor' tank, cheaper than a bladder tank and the contractor doesn't live in the house, the contractor just low bids.

I'm guessing that the quality for those 3 gallon under-the-sink bladder tanks is a different quality class than the rural home domestic water supply 30, 40, 50 gallon bladder tanks if you get annual or bi-annual failures.

You're probably right Dan. Little tanks = cheap. Even though they aren't.

Keith Cress
kcress - http://www.flaminsystems.com

Another thing Keith. There are two types of ACV valves.
One type adds air only when the water level rises above the level of the valve. It may be identified by the diaphragm shape.
The other type of ACV has a works in conjunction with a snifter valve and has a float inside the tank. It works by bleeding of some of the excess air supplied by the snifter valve when the water level drops below the level of the valve.
Neither type of valve needs a a compressed air supply. The snifter valve is ahead of the check valve and allows some air to enter the pump housing when the pump shuts down and drains back and the pressure in the housing goes negative.
The air injecting type of ACV has a pilot line to the pump.
When the pump stops, the negative pressure pulls the diaphragm back and charges the valve with air through the small check valve.
There is a restriction so that very little water is drawn back into the valve.
When the water level is below the level of the valve, there will be some compressed air in the chamber. when the pump stops and the diaphragm is drawn back, this air expands and little or no air is drawn into the chamber.
I imagine that those bladders are factory filled with nitrogen or with clean air.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Pump always on and sized peak flow and pressure required to satisfy your highest pressure drop

Your piping loop should be a full sized loop and lead all the way back to the original tank

At the end of your pipe run, after it ties into your last water connection, you put a back pressure regulating valve, set to maintain pressure in the loop behind it to whatever your highest pressure drop consumer is. Back-pressure regulating valve will be closed when everything is open, but as consumers close off it will see pressure behind it build up and then will allow flow through until pressure in the loop behind drops below whatever pressure you set the spring to.

If everything was off, all your flow would just continue circulating, and that pressure would be available in your line anytime anything opens.

GT-EGR, thank you very much for sharing!

May I ask a follow up question? Why not do it the other way around: put the regulating valve after the pump but before the first consumer and "short-circuit" back to tank if the line is sufficiently pressurized (ie, no consumer is open). I can think of stagnation being a problem in outer corners of the distribution system. Are there other concerns?

Keeping the loop circulating as you mentioned would be the main reason - there are plenty of technical concerns about avoiding stagnated water in piping depending upon how long of a time period you may go

Otherwise it sounds like you are describing the same scenario, Im just used to backpressure regulating valves being at the end of the loop.

I would recommend you do both with the one right by the pump discharge being the system pressure relief valve instead.

Thank you for the insight, GT-EGR

Amazing azza I am attempting to this same project and have read the whole thread. I have some confusion on what sensors/drive/PLC you have determined works best. I first tried the whole floweter setup but found the meter to not be responsive enough and sometimes just doesn't work. My setup now is that the the pumps run 100% of the time with a recirculation loop. I am wanting to only run the pump when needed by machine or operator.

jwill;

You need a PLC. Do you have any experience with PLCs? If so what make?

It will need to support an analog input.

It will either need to have an analog output to pass the needed speed to the VFD or it could use a protocol like MODBUS to command the VFD's speed.

You will need a VFD capable of running the pump motor. (The pump motor must be threee phase.)

You will need a pressure transducer that spans from 0.0 to the pressure you want to operate at plus a 25 or 50% so any accidental water hammer doesn't wreck the pressure transducer. The transducer analog out needs to match the PLC's analog in offering. Preferably both should be 4-20mA as that's typically easier to run as a sheilded twisted pair.

You need a check valve sized for the piping.

You need a small pressure tank.
~1/2 gallon for 3/4" water line.
~1 gallon for a 1" water line.
~2 gallon for a 1-1/2" water line.
~5 gallon for a 2" water line.

You'll also want things like RUN/STOP switches to disable it all.**

If I was going to do this I'd use inexpensive but quality Automation Direct stuff and I could get everything there except the pressure tank. I'd choose the CLICK! series PLC and get the CPU version that does both the Analog in and the Analog out. I'd use the newer Ethernet CPUs as they're really sweet and allow modifying the logic without even stopping the ladder execution.

I would also use the 4" color touch screen HMI (enormous bang-for-the-buck) so I didn't need switches** and so I could display lots of things like the pressure and the pressure setting and a page of good stuff like the PID settings while developing the PLC code of the application.

You may also want to use the standard flow meter with a pulse-out to show the actual flow occurring on the HMI since the above system does not know or actually care about flow. The pulse-out flow signal will be fine for showing water consumption even though it's completely useless for controlling the pump. You could totalize the flow too.

Operational note: I've recently learned not to run a VFD on a standard motor at less than 1/4 of full speed, or better, 1/3 of full speed as it can overheat the motor while pumping absolutely nothing anyway. So, keep that in mind in whatever algorithm you come up with.

Keith Cress
kcress - http://www.flaminsystems.com

Hi Keith. Is there any reason why you can't use the P function of a PID controller directly to the PLC? Set the PLC to either run at a minimum of 35% or stop the motor when the input falls below about 35%.

Bill
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"Why not the best?"
Jimmy Carter

Hey Bill.

Possibly, certainly worth a try, however a lot of very popular lower-end drives don't have PID control. Often, too, they have to be in vector mode before they allow the PID to run. Conversely there are a couple of drives out there that have PLCs built into them that would probably handle this job. Because of the fairly non-linear aspect of centrifugal pumping I'm not clear on how a typical drive's P would manage. I'd like to try it sometime and almost did last month. But, after a week of turmoil on a very low dollar job I just went with a Grundfos SCALA2. I was seriously impressed with everything about it. Worked like a charm. ~$600+ bucks though, but that was less than me gathering everything, putting it together and writing the PLC stuff. The SCALA2 has a tank but it's so small and optimized that you can't find it in its stylized package. You can tell it's doing exactly what I described above. It's so quiet you can't hear it standing in the redwoods about twenty feet away.

PIDs in PLCs turn out to be pretty simple too. You can grass-roots one in about 5 ladder rungs. I've done many that way as the PLCs I use usually don't offer native PID functionality and upgrading to one that costs 50% more just so they do kinda galls me. :)

Keith Cress
kcress - http://www.flaminsystems.com

The unit I suggested will output a signal to control a VFD.
We used Ramp and Soak controllers to dry softwood lumber years ago. CRT (Constant Rising Temperature) drying schedules were popular. Before that we used large case pneumatic controllers with a cam turning with the circular recording chart to drive the set point up scale. Changing a schedule for a different species meant removing and replacing the set-point cam.
Part of our design and build task was detailing and having a cam about 10" in diameter cut from a thin aluminum sheet.

Bill
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"Why not the best?"
Jimmy Carter

Keith I’m not too keen on PLC and have never programmed one myself. I was thinking that all this could have been done using pressure switch/transducer with p&id loop on a drive. I definitely didn’t think I would have to purchase and additional tank. I’m just not sure this process will work for mine. I may need to drop back and punt so to speak.

Hi jwill; I'd be interested to know if that worked. I think boundary conditions will cause a problem with your scheme because it can leave the pump idling with little flow actually occurring.

You could probably do that with a large pressure tank in the, I don't know, maybe 50 gallon rsnge and then use the VFD'd pump exactly like a normal ON/OFF pressure switch operated water system but instead using a spooling UP/DOWN pump instead of a bang ON/OFF. It would take some tinkering.

Keith Cress
kcress - http://www.flaminsystems.com

Keith I was thinking that as the pressure drops too low.(determined by the flow of the pump) Than the drive could kick in the motor and run until an “over-pressurized” state and then shutoff. My thinking is if the machines or operators are using it that the pump would stay running. My problem I think I would have is pressure drop through the check valve too quickly than the pump would run just as much and I would get no benefit from it.

After some further investigating online I did find a flow switch with a controller. Do you think this could be viable option either with or without the pressure switch? Or would I experience some other irregular running issues. I am worried that maybe the flow wouldn't be enough to engage the flow switch. Which is why I was leaning to the pressure switch.

Could you tell us some more about your plant?

Flow?

Lowest service flow?

Highest expected flow?

Does the flow stop entirely?

Fed from a well, or a tank, or municipal water supply?

Is this a booster system or does it provide most or all the pressure?

What kind of facility is it, an apartment complex, a single dwelling, a cannery?




Keith Cress
kcress - http://www.flaminsystems.com

Hi guys, I suppose a progress update is overdue, as I have gone through a few iterations on the design of this thing.

The first way I had it running was with an external temp/process controller acting as a PID controller. It has a nice display for SP/PV and an easy to use interface. The start signal came from a low alarm and the stop signal was done via the "sleep mode" of the VFD. (Sleep mode is when the drive turns off if the command frequncy falls below a certain threshold. Very similar to what waross was mentioning). This worked, but the temp controller was not a very sensitive controller, and if you closed off the consumer slowly, the PID output did not fall as expected of it (this is undesired behaviour). Perhaps if I used the VFD's internal PID option it would have worked better, but I did not investigate it further.

The second way was with PLC assistance. I added a PLC, obviously :) If the pump was running, the program would periodically force-drop the drive frequency. The start signal came from a low alarm as before, but now the stop signal was driven by another low alarm that resulted (or not) from the momentary frequency drop. This worked surprisingly well (thanks itsmoked!), and I ran it this way for close to a year.

But I just can't stop messing with it. Recently, I was changing tank level sensing from using internal relays on a flowline ultrasonic level sensor (those relays are crap btw, don't use them. The sensor itself is ok, just don't use the relays) to a plain old analog pressure reading, seeing how I had a PLC now. I decided to also try using the built-in VFD PID functions instead of relying on an external controller.

Thus, the third way fed the process value signal directly to the VFD, which did its own PID. The VFD also forwarded this process value to the temperature controller, which was now only functioning as a display and providing alarms. The VFDs PID capabilities turned out to be *far* better than the temperature controller's, albeit a tad more difficult to puzzle through. This is how I am running it now.

Great feedback azza!

My thought was that a temp controller PID is not going to understand the cubic X³ response of the system (centrifugal pump) it was controlling very well.

Do you have a pressure tank involved?

What period do you use to slow the pump for checking for continued flow?

Did your VFD PID have a "Pump Mode" you're using?

In regards to the VFD's PID are you running P.I.D. or just P.I. or just P? Did you figure out how to tune it?

Keith Cress
kcress - http://www.flaminsystems.com

Did you set up the proportional band on the original PID or use the default setting?
The proportional band setting has a great influence on the response characteristics.

Bill
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"Why not the best?"
Jimmy Carter

The main problem with the temp controller turned out to be its integration (and differentiation) time slice. On paper it claims 150 ms update cycle, but after using it I think that applies only to the proportional term. Integration appears to be done over 1 second slices. This leads to overcompensation. The only thing you can really do in this situation is turn down Ki. But even so, it was a totally usable system if you didn't mind an occasional oscillation around the setpoint.

I do not have a pressure tank. The flow checks are done once every minute. The VFD PID itself does not have a "pump" setting. It only concerns itself with things immediately related to PID, such as: PV<->frequency mapping, Kp/Ki/Kd settings, and such. There is one weird setting - response time. One can adjust the response time to be slower or faster (with K's being fixed). But this is claimed to be used for things like conveying, whereas for pumps, the manual specifically instructs to use a slower response. Being myself, of course I tried the turbo mode :) It tripped the drive as it tried to spin up too fast, heh. Didn't sound very pleasant either. The "pump" mode is present as a macro and affects the V/F curve (linear vs quadratic vs vector), overload threshold and minimum frequency limit. I did not investigate how, if any, this changes the PID response. But, yes, I do have it set to "pump".

With regard to PID settings, I played with 3 combinations P, PI, PID and found that the D term was almost never useful in this application. In the end I settled on PI. My tuning method is Ziegler–Nichols-ish. First, the controller is configured for P-only and the Kp (gain) is increased until oscillations appear. From there I turn it back down to where it is stable, but still has a large offset. Then I start adding the I term. Again, keep adding until it shows signs of instability, then go back to an intermediate setting.

Thanks for those fascinating details!

What VFD did you use?

Keith Cress
kcress - http://www.flaminsystems.com

Glad to be able to share it with you all :)

For the water treatment plant I use Toshiba VF-S15's. Very happy with them. The waste water plant will use Siemens's, it will be an interesting head-to-head.

I'm a believer in KISS, Keep It SSimple.
I start with P only. It may not be perfect, and may show deviations on the recording chart but often it produces satisfactory results.
eg; 10% proportional band with 5% offset will always be within 5% under normal operation.
Don't use Integral unless you must.
Derivative is mostly for very slow acting processes. eg; Temperature control with large thermal masses.
By contrast there are probably millions of water supply systems running on on-off control with deadbands or differentials of 30% to 50% of full scale.
How much effect will a 5% deviation have on wash-down hose performance for example?
Bill
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"Why not the best?"
Jimmy Carter

More excellent points Bill.

Thanks Azza. I've also found Mitsubishi drives to be nice too.

Keith Cress
kcress - http://www.flaminsystems.com