Water distribution controls for a small plant 6

[amazing azza]

Hello friends, I am planning a water distribution system for a small plant. There are a number of machines that use water (ie. continuously if machine is on), and there are a number of sinks and hose stations which may be opened at any moment for an arbitrary length of time.

What are some of the typical ways to control supplying water to this mix of users? Here is what I can think of:

1. Have the supply pump be on 100% of the time and install an overpressure valve at the pump outlet that sends pump output back into the water tank. This way the pressure in the supply line is always equal to (or less than) the rating of the overpressure valve.

2. Have the supply pump be on 100% of the time and pump water in a loop around the plant that leads back into the water tank. This way the supply line is always pressurized at the pressure that the pump can deliver. (Naturally, this value would be chosen wisely).

3. Have the supply pump be on 100% of the time and have a control valve at pump outlet that sends the output of the pump back into the water tank depending on the pressure in the line. Using PID control, the pressure in the supply line would be kept constant.

4. Install a pressure switch on the pump outlet such that if someone opens a valve (or machine turns on), the pressure in the line will fall and the pump will turn on (maybe with inverter control). Pump would turn off when the pressure in the line spikes as the valve is eventually closed.

I experimented with #4 a bit, but found that in real life the line does not stay pressurized, but rather falls to low values (~0.5 barg) fairly quickly once the pump is turned off.

 

[amazing azza]

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:

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% ?

 

[itsmoked]

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.

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.

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

 

[amazing azza]

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!

 

[waross]

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

 

[itsmoked]

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:

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

 

[waross]

You would limit the lower frequency of the VFD.

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

 

[itsmoked]

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

 

[waross]

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

 

[itsmoked]

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

 

[waross]

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

 

[itsmoked]

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

 

[waross]

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

 

[itsmoked]

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

 

[danw2]

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.

 

[itsmoked]

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

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

 

[GT-EGR]

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.

 

[amazing azza]

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?

 

[GT-EGR]

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.

 

[amazing azza]

Thank you for the insight, GT-EGR