Constant Pressure VFD on well tankfill application

[kwired]

Well, we've been supplying VSD pump systems into the water industry for quite a few years. Most of these have been for water companies/authorities supplying potable water. Pumps in pumping stations pumping treated mains water - lots of it and quite often over long distances. The London Water Ring Main is 50 miles for example. One of the more important parameters they have to control is pressure - you don't want too many burst water mains under the streets of London......

Pressure is measured remotely from the drive. In the early days we had external PID controllers. Nowadays, a PID loop is often part of the drive internal package. The PID loop can be used to any measurable parameter including, but not exclusively, pressure.

Perhaps that goes some way to explaining why I think calling it a Constant Pressure VFD is a bit.........odd.

You need to get out of industrial application mode - these "constant pressure controllers" are for small domestic wells typically only up to 5 horsepower. The controls used before they started making these drives was just a direct acting across the line control method, with user seeing pressure vary from say 30 PSI at "cut in" up to 60 PSI at cut out. Now replace simple pressure switch and/or associated motor contactor with this constant pressure control scheme and you can set output for 45 PSI and unless you are calling for more demand then the pump can deliver - you pretty much get a fairly constant 45 PSI in the same system - plus you get soft start/stop and motor only runs at needed speed when demand is low.

Can you use some other drive, a pressure transducer and set it up to work here - yes, these drives just happen to be packaged with only selected options/settings readily available to the user as they were only intended to be used for a fairly specific application instead of being considered a general purpose drive. Plus the well installers that typically sell them don't need to know much about drives in general, just follow set up instructions, most parameters you find on a general purpose drive they don't understand are already programmed in and are not something they have easy access to - just the typical parameters that may need changed for their more specific application.

 

[ptonsparky]

The village my shop is in recently put in a VFD on one of their pumps to maintain the level in the water tower. It has a pressure transducer but all it really does is attempt to maintain a certain level of water in the tank. X feet of water = Y PSI = Z ma. The tank itself is not pressurized with a captured air space.

Advantage 1 is no float switches to maintain or freeze up/down and pressure in the town is maintained at a fairly constant level. High water usage turns additional pumps on in a lead lag pattern.

 

[kwired]

The village my shop is in recently put in a VFD on one of their pumps to maintain the level in the water tower. It has a pressure transducer but all it really does is attempt to maintain a certain level of water in the tank. X feet of water = Y PSI = Z ma. The tank itself is not pressurized with a captured air space.

Advantage 1 is no float switches to maintain or freeze up/down and pressure in the town is maintained at a fairly constant level. High water usage turns additional pumps on in a lead lag pattern.

A point I tried to make earlier. I have worked in municipal well houses that were not using VFD control on the pump yet still use pressure switch(es) to control water level in an elevated tank. No wiring to 100' tall tank, no float switches to have to climb tower to maintain, it is all in close proximity to the pump, controller, and all inside the heated well house, and they have been doing it that way successfully for years. I don't see why a pressure transducer still couldn't be used to control tank level was my main point, though other questions about the need for a VFD on the pump may still be worth asking though.

 

[GoldDigger]

Whether or not a pressure switch can be used effectively to control water level really depends on the tank. For a domestic vented storage tank with a difference of only three feet from empty to full you are not likely to find a pressure switch with that small a hysteresis.
For a tank with a twenty foot rise from empty to full it is an easier task.
With an analog transducer instead if a switch it is easier still.

 

[ActionDave]

A point I tried to make earlier. I have worked in municipal well houses that were not using VFD control on the pump yet still use pressure switch(es) to control water level in an elevated tank. No wiring to 100' tall tank, no float switches to have to climb tower to maintain, it is all in close proximity to the pump, controller, and all inside the heated well house, and they have been doing it that way successfully for years. I don't see why a pressure transducer still couldn't be used to control tank level was my main point, though other questions about the need for a VFD on the pump may still be worth asking though.

How do they ensure the tank turns over i.e. use enough existing water in the tank before bringing fresh water in?

 

[ptonsparky]

Whether or not a pressure switch can be used effectively to control water level really depends on the tank. For a domestic vented storage tank with a difference of only three feet from empty to full you are not likely to find a pressure switch with that small a hysteresis.
For a tank with a twenty foot rise from empty to full it is an easier task.
With an analog transducer instead if a switch it is easier still.

[h=1]PTD25-20-0100WCH

PRESS XMTR 0 TO 100inWC 4-20mA 1/4in MNPT M12 PLUG[/h]

I think I could even do it.

 

[Besoeker]

You need to get out of industrial application mode - these "constant pressure controllers" are for small domestic wells typically only up to 5 horsepower. The controls used before they started making these drives was just a direct acting across the line control method, with user seeing pressure vary from say 30 PSI at "cut in" up to 60 PSI at cut out. Now replace simple pressure switch and/or associated motor contactor with this constant pressure control scheme and you can set output for 45 PSI and unless you are calling for more demand then the pump can deliver - you pretty much get a fairly constant 45 PSI in the same system - plus you get soft start/stop and motor only runs at needed speed when demand is low.

Can you use some other drive, a pressure transducer and set it up to work here - yes, these drives just happen to be packaged with only selected options/settings readily available to the user as they were only intended to be used for a fairly specific application instead of being considered a general purpose drive. Plus the well installers that typically sell them don't need to know much about drives in general, just follow set up instructions, most parameters you find on a general purpose drive they don't understand are already programmed in and are not something they have easy access to - just the typical parameters that may need changed for their more specific application.

Yes. Constant pressure controllers. Constant pressure control scheme. But Constant pressure VFD???

It's a system. The VFD is part of that system.

You pose the question:
"Can you use some other drive, a pressure transducer and set it up to work here"

In response I ask:
Could you control the pressure without a pressure transducer?
Where would you fit that pressure transducer?
Does it matter whether it is domestic or industrial?

 

[GoldDigger]

Could you control the pressure without a pressure transducer?
Where would you fit that pressure transducer?
Does it matter whether it is domestic or industrial?

Constant pressure main or booster pumps for residential scale use use a purely mechanical pressure reducing valve mounted on the outlet of the pump. The valve has a maximum pressure differential and a minimum flow rate for regulation, so it needs to be sized correctly to the pump and to the required flow rates.
The pump is a centrifugal type and so is not harmed by the flow restriction during continuous operation.
I have not seen this sort of pressure regulation offered for use with a submersible deep well pump though.


When the well has a low recharge rate and low depth of water above the pump (in-well storage), it is not uncommon to throttle the output of the pump with a simple valve or flow limiter to allow the pump to run without any risk of sucking air.

 

[Besoeker]

Constant pressure main or booster pumps for residential scale use use a purely mechanical pressure reducing valve mounted on the outlet of the pump.

Yes, that was also used in pumping stations. Rotork Valves.
It's an inefficient means of control with the pressure losses across the valve.
That was one of the reasons to introduce VFDs - much improved efficiency.

 

[kwired]

Yes. Constant pressure controllers. Constant pressure control scheme. But Constant pressure VFD???

It's a system. The VFD is part of that system.

You pose the question:
"Can you use some other drive, a pressure transducer and set it up to work here"

In response I ask:
Could you control the pressure without a pressure transducer?
Where would you fit that pressure transducer?
Does it matter whether it is domestic or industrial?

I am willing to bet that "constant pressure VFD" is more so slang then a name that anyone is marketing a product under. Constant pressure controller or something very similar is the name I recall seeing in product information.

 

[kwired]

How do they ensure the tank turns over i.e. use enough existing water in the tank before bringing fresh water in?

Don't know.

May not even be that much of a problem chlorination seems like it should solve some of that issue. Also if you somehow design inlet/outlet to cause some turbulence when there is flow it may help take care of some of that issue - just a thought but really have no clue if that is true or not.

 

[Besoeker]

I am willing to bet that "constant pressure VFD" is more so slang then a name that anyone is marketing a product under. Constant pressure controller or something very similar is the name I recall seeing in product information.

Or a promotional sales line.

 

[ActionDave]

Don't know.

May not even be that much of a problem chlorination seems like it should solve some of that issue. Also if you somehow design inlet/outlet to cause some turbulence when there is flow it may help take care of some of that issue - just a thought but really have no clue if that is true or not.

I was curious because there is a tank in my fair city that has some issues with controls. Right now it stays topped off instead of cycling at least half way. Water in it gets stale even though it is chlorinated.

 

[kwired]

I was curious because there is a tank in my fair city that has some issues with controls. Right now it stays topped off instead of cycling at least half way. Water in it gets stale even though it is chlorinated.

I'm not certain how much tank level differential there is between cut in and cut out on the ones I have been around, but they typically have separate pressure switches for cut in and for cut out.