Automatic restart for centre pivot irrigation systems

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					   Automatic restart for centre pivot irrigation systems
                                 By Hal Werner

Automatic restart is the process of returning the centre pivot system back to a
safe running condition without being present or providing manual intervention.

This includes restarting the pump, pressurising the pipeline and system, and
returning the safety controls and centre pivot to the “run” operation. Where used,
centrifugal pumps will need controls to ensure that the pump is primed before the
system is restarted. Water hammer, a rapid change in pressure caused by sudden
changes of water flow in pipeline, such as starting and stopping pumps, is a
consideration when restarting irrigation systems. This is especially true for units
that have high pumping lifts and/or long pipelines. Water hammer control can be
designed for almost any system. Restart systems can vary from simple,
economical packages to complex, expensive control systems. Where the well and
pump are near the centre pivot, restart packages generally are easy and
economical to retrofit. A common package involves installing two timers – first
to delay the restart cycle so that the pump does not start immediately when power
is restored and the second to delay the safety shut-down sequence until water and
pressure are restored. Several factors contribute to the need for additional
equipment and controls when restarting centre pivot systems. These include: the
type of pump, length of pipeline, elevation difference from the pump to the pivot,
pipe pressure rating and pivot operation pressure.

Advantages

A major advantage of automatic restart is that it maximises pumping time.
Operators may not be able to restart the irrigation system immediately after
power is restored, thereby decreasing available pumping time. During periods of
high crop water needs, which often occur at the same time as electric peak load
management, loss of pumping time could be costly if the crop runs short on
water. Another advantage is the convenience of automatic restart after loss of
power. Irrigation systems are often several kilometers away from home
operations, and frequent trips to check on status and then manually restart the
system are time-consuming and costly. Automatic restart may even add
sufficient convenience for systems close to home base, especially where the
investment to add the controls is minimal. Finally, properly installed automatic
restart systems can assure the operator that the equipment is protected from the
hazards of either attended or unattended starts. Electrical control and valves,
where needed, can prevent problems with water hammer. Equipment for
automatic priming of centrifugal pumps can even simplify manual starting of
those units.
Disadvantages

A disadvantage of automatic restart is that some irrigation systems can be costly
to convert when pumps need priming and water hammer is probable. Equipment
to restart complex irrigation systems could cost several thousand rands. Another
disadvantage is that complex irrigation systems require specific design solutions.
Improper selection and installation of a control package may not prevent water
hammer and could increase the potential for damage to the system.

Water hammer

Water hammer in irrigation pipelines is similar to banging of household pipes
except that the result of water hammer is much larger in irrigation lines and can
be very damaging. Uncontrolled water hammer can explode pipeline, bust pumps
and blow pivots apart. Horror stories abound about water hammer problems that
were caused by poor design or inadvertent operation. Water hammer can exceed
400 percent of the normal operating pressure of the system and can easily exceed
the pressure rating of the pipe. Negative water hammer pressure is also possible
and can collapse pipelines. Short pipelines have less water hammer potential
than longer pipelines. Factors that affect water hammer are pipeline length, pipe
size and material, flow-rate, (m3/h), how fast the flow-rate (velocity) changes,
and whether there is trapped air in the pipeline. Pipeline water velocity is directly
related to flow-rate and, thus, to water hammer. It is recommended that water
velocity be less than 0,3m/s when filling pipelines. Maximum velocity once the
line is full should be less than 1,5 m/s. Air pockets in a pipeline can cause water
hammer when the air compresses moves along the line. Water hammer control
can be designed for almost any system. Two general solutions are available for
controlling water hammer – various automatic valves or motor speed controllers.

Th type of equipment to use is based on each specific application and should be
designed and installed by a qualified dealer. Costs can range from a couple of
thousand rands for a single pivot application to several thousand rands for more
complex systems.

Two-stage valves: The two-stage valve is an automatic valve installed in the
pipeline that is set to open in two steps. The first step restricts flow during filling
or refilling. The second stage is normally full open for unrestricted flow.
Switching between stages should be regulated and can be accomplished using a
timer or differential pressure sensor. The two-stage valve is often the simplest
and cheapest of the automatic valves, but offers the least flexible control.

Line fill valves: The line fill valve is an automatic valve that controls the actual
filling rate of the pipeline. Flow is restricted when filling or refilling and then
gradually is opened until the system is filled and pressurised.

Surge anticipator valves : The surge anticipator valve incorporates a sensing
device that detects the onset of surges or water hammer. It then opens quickly to
discharge water and lessen the effect of the pressure surge. While other options
for controlling water hammer reduce flow as the pipeline is filling, only the surge
anticipator valve is designed to detect and discharge surges.
Motor speed controllers : Frequency-regulated motor speed controllers allow
selection and automation of a wider range of motor speeds, so that the output of
the pump can be adjusted to regulate filling and refilling pipelines. Motor/pump
speed can be reduced during initial filling, then slowly speeded up to full
operating speed. Changes in speed can be accomplished using timers or feedback
from pressure/flow sensors.

MP priming

It is important that all air is evacuated from the suction line pump before it is
started. An air-tight check is needed in the pipeline just downstream from the
pump. All fittings and seals must be air-tight. Otherwise, priming will be
difficult, if not impossible. An automatic priming system is required for
automatic restart of centrifugal pumps. It is also helpful for initial priming of
pumps and for repriming when needed.

Several automatic priming solutions are available, including commercial
packages or systems combining various off-the-shelf components that can often
be installed more economically. Two basic types of systems are common, the
vacuum priming method and the pump fill method. The pump fill method can
only be used on suction lines with foot valves while the vacuum method can be
used with or without foot valves. The pump fill method uses a small submersible
or sump pump to fill the suction line and pump. An air release valve lets air
escape and a pressure or water sensor switches power from the sump pump to the
irrigation pump. The check valve on the pump discharge must have sufficient
back pressure to activate the sensor. The vacuum method requires an air-tight
pump system from the water level to the discharge check valve. The vacuum
evacuates the air from the suction line and pump and fills them with water. A
water sensor on the top of the irrigation pump switches power from the vacuum
pump to the irrigation pump.

Equipment solution examples

Example#1: A well and turbine pump delivers 80 m3/h to a centre pivot. The
centre pivot has been converted to lower pressure (270 kPa) and is located 10 m
from the well. A restart system would be an automatic restart package from a
pivot supplier and would include safety timers and controls. No water hammer
control is needed because of the proximity of the pump to the pivot and the lower
operating pressure. Total cost for the package is about R2 500 to R4 000.

Example #2: A centrifugal pump delivers 400 m3/h from the river to two centre
pivots that are 1 200 m away and 30 m higher than the river. Pump pressure is
900 kPa to get a pivot operation pressure of 400 kPa. The pipe is ten-inch Class
160 PVC.

Automatic priming would be needed. Cost of controls, vacuum pump and
installation is about R4 800 to R8 000. Water hammer control is necessary for
automatic restart. The cost of the system would be about R24 000 to R32 000,
which includes an automatic valve, safety timers and controls.
Consideration and precautions

Some older centre pivot systems may not be suitable for retrofitting for automatic
restart. If the centre pivot is plagued with problems and shut-downs, automatic
restart may only aggravate the situation. A good time to add automatic restart is
when components of the system are repaired or replaced. Always make sure that
the safety shut-down controls can override the restart controls and that the total
control system operates as designed. It is important to have controls that will try
to restart the system no more than two or three times when other safety faults
exist. Many centre pivot systems that have been converted from high to low
pressure may have an adequate safety factor for restart without fear of damage
from water hammer, especially those with short pipelines. Any system that is
presently started without manual control of valving is an excellent candidate for
automatic restart, and automatic valves may not be needed. The elevation
difference between the pump and centre pivot will govern the type of water
hammer control needed. Pipelines that slope uphill should remain filled during
shut-downs unless there are leaks. A single automatic valve will often suffice.
However, systems that have downhill sloping pipelines may empty during a shut-
down and may need more than one automatic valve to adequately protect against
water hammer problems. It may not be desirable to use automatic restart with
some high-pressure systems or with very high lifts. Irrigation systems with
complex combinations of pumps, piping and/or centre pivots pose unique
problems for automatic restart and should be designed by a qualified dealer.

Reprinted courtesy of Irrigation Journal

				
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posted:1/26/2011
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