How do you control pump speed from output pressure

For starters, check out that (a) the motor will sustain VVVF operation and
(b) that the flow can be controlled stably by reducing the motor speed. High
static head applications can be very hard to stabilise with variable speed.
If you can advise the static head between the pump and the discharge it
should be possible to give an indication on this, but there's no substitute
for trying it. The pump manufacturer may be able to assist, but don't count
on it. Assuming that both these aspects are resolvable, then pressure
control should be pretty straightforward using available devices. The
simplest way to handle the turnoff would be to do it when the 3 tanks are
all full, presumably the pressure solution is nice because you don't need
signals from the tanks back to the pump. With pressure control, what  would
happen would be that the pressure would rise somewhat when the last flow was
cut off, and the VSD would then back the pump speed off further. This might
be a usable approach, but that sort of thing can be a bit chancey. It would
need to be trialled. Another possibility would be a flow switch in the
common line, you wouldn't need a continuous measurement which would keep
costs down.

One other variable that may help or may hinder is entrapped air in the
lines, if the system geometry allows that to occur. That can influence your
dynamics significantly, making for variable loop characteristics. Been
caught by it more than once.

Controlling the pump motor frequency sounds like a complex and expensive
solution.
If you were to introduce a suitably chosen fluid resistance element at a far
end location, the flow rate would decrease here. The negative factor to this
suggestion is that a higher pressure would be reflected to the pump and
could cause it to turn off prematurely. However, with a well chosen value of
resistance, you might achieve the required result. ... The emphasis here is
on 'well chosen'. You might have to experiment until you achieve an
acceptable resistance value. The fluid resistance component might be
implemented in the form of a manually adjustable valve, or even a few meters
of a reduced diameter pipe, placed at a strategically located point. A
partially closed gate valve might work. ... The member storage tanks would
require a longer period of time to fill, and the pump would  have to run for
an extended period. However, you would not have the excessive flow rates to
contend with.

neilr wrote:

Controlling the motor speed might be hard. Depending on the pump type,
flow vs. rpm might be very non-linear, so control might be unstable.
There are three last tanks, and the controller must be stable for each.

How important is efficiency? A bypass valve that shunts excess flow back
to the well when two tanks are full is an inefficient solution that is
simple to install and inexpensive. The cost would include either sensors
in the tanks to determine when they have shut and two-out-of-three
logic, or velocity sensors at the meters to open the bypass when any one
goes out of range.

Jerry
--
Engineering is the art of making what you want from things you can get.
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Jerry,After further consideration of your approach, and that of John
Popelish I could try this method without to much effort. We already
have a pressure relief valve [set at approx 850kpa] at the pump.This
was installed to protect the pipeline[class 9 pipe= 900kpa]  from
excess pressure and possibility of bursting.If I was to set this relief
valve at say 400kpa and by-pass some of the excess flow back into the
river while the last one or two tanks are filling Switching off the
pump would be by sensing the increased pressure or flow rate back into
the river once the last tank has filled.

neilr wrote:

It sounds like a plan! You might consider two relief valves, one of
which opens when only one tank is filling, the other, when all tanks are
full. It's time to shut the pump when the second valve opens. A pressure
switch might replace the second relief valve.

You might need to throttle the outlet(s) of the relief valve(s) to
ensure stable operation. In that case, an unthrottled valve to protect
the pipe might be prudent.

Good luck.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

One can use the following approach to estimate the reduction in pump
pressure needed  to keep your water meters from exceeding their maximum
rated flow rate.  Using the familiar (Q, flow rate) = K x (square
root(P1 - P2))equation one can estimate K for your main supply line.  K
is a proportionality constant for your main supply line, P1 is your
pump's output pressure and P2 will be assumed to be zero for this
estimate.  This is a reasonable assumption as when your pump is first
turned on and all of your member tanks are filling simultaneously the
equivalent flow resistance of multiple branch lines is very low. You
didn't specify what the flow rating of your pump was so for this
estimate I will use a 25 gpm, 94.6 liters per minute, rating.  For P1 I
will use 400 kpa, 58 psi, as it is the only number given in your
original post.  Then:
K = 94.6 / (sgrt(400)) = 4.73   Then say the maximum rated flow rate of
you water meters is 5 gpm,18.93 liters per minute. For a worse case
analysis assume P2 is zero.  Then if you wanted to limit the flow rate
in your main supply line to 18.93 liters per minute when the last tank
is filling your pump's output pressure must be less than ((18.93 /4.73)
x (18.93 / 4.73)) = 16 kpa, 2.32 psi.  This might appear to be very low
but don't forget the pressure drop is proportional to the square of the
flow rate.  Flow resistance in your branch lines will help.  However,
you said your water meters were seeing flow rates far in excess of
their rating so there can't be very much flow resistance in the branch
lines in your present system.  My suggestion is the best way to keep
your water meters from exceeding their maximum rated flow rate is to
add a flow restrictor in every branch line.  The restrictor should be
sized so that a water meter's maximum rated flow rate is not exceeded
when full system pressure is applied to a branch line.
Howard

neilr wrote:

Have you considered a bypass control valve at the pump that
recirculates the water back to the storage at the pump inlet, to
control pressure?  You could detect the last tank full condition by
watching the valve position.  When it has been open enough to be
carrying the full flow for some period of time, turn the pump off.
The pressure setpoint could be high enough that, when all 3 tanks are
filling, the control valve would be fully closed, so it would not much
affect the total fill time.

My own thoughts exactly!

Kelvin B. Hales
Kelvin Hales Associates Limited
Consulting Process Control Engineers
Web: www.khace.com

Kelvin Hales wrote:


Even though it may not be as elegant as a variable speed drive, it
would probably be easier to maintain without a maintenance engineer.

John Popelish wrote:

Isn't that basically what I wrote way back? How could I have put it more
clearly?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:

Not much.  You got there first, but I didn't see your reply before
posting mine.

Because controlling the speed of a three phase AC motor is difficult
and expensive alternate schemes like installing an accumulator at the
pump should be investigated.  You could then use one set of pressure
switches and on off control to keep the pressure in the accumulator
within a suitable band.  Then once all member storage tanks are full
and the pump stops running for say 30 minutes or more use a timer to
lock out your pump until it is reset the following night.

I don't know what you consider expensive, but you can purchase a 3
phase AC drive (inverter) for a 4kW motor for around $500. I'm sure
there is currently a starter and circuit protection (fuses, overload)
which could be easily replaced physically by a Variable Frequency Drive
with proper fusing. Use an analog signal from a P/I transducer to
control the speed of the motor. All of these other bypasses and whatnot
seem extremely labor intensive. Using an inverter would be a simple
"out of the box" electrical installation.

gnoge wrote:

Do you have any idea how motor speed translates to flow/head at a
particular head/flow? Does anybody? How will severe nonlinearities here
affect loop stability?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

gnoge wrote:

This is more in line with my approach.Grundfos have an off the shelf
solution but involves completely replacing the existing control panel
containing relays,timers,c/ct breakers etc with a new control panel
containing 3 phase AC drive[inverter] as well as relays timers,c/cct
breakers,[ certainly more than approx.$2000.]My preferred choice would
be to retrofit existing control panel with the necessary sensors
/controllers.

Hello All.
what i would like to say with confidence that it is definitely not
expensive to control the speed  ( via frequency control ) of motor by
using a drive.
The product in which i am expert can do this very well. It needs only
PID function which will control the speed as per the set point and
error value. Also the auto off function is there available. For more
information please feel free to e mail anytime at
vbaundhekar@ra.rockwell.com
Where ever your application is in the world we are ready to help you !
Thanks.
Vijay

vijay.aundhekar@gmail.com wrote:

That would be a loop within a very non-linear loop. The final controlled
variable is not motor speed, but hydraulic pressure. There is not only
the pump nonlinearity to consider, but the dynamics of the piping system.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Try study this system
http://www.lowara.com/product.php/4724

Thank you. Any controllers for submersible pumps.