Control-Tips from Systems Control Inc and Yehia El-Ibiary

CONTROL-TIPS by Yehia El-Ibiary

These Tips are published periodically to help the engineering community

ALL RIGHTS
RESERVED

Control Tips by Dr. Yehia El-Ibiary

1. Increasing Damping in a Hydraulic Control System.
2. Stabilizing a Hydraulic Position Control System.
3. Shaping the Response of a Hydraulic Control System
using Pole Placement technique.
4. Sizing up Hydraulic and Electrical Drives
5. Vibration Monitoring
6. Hot Fan Maintenance Tip
7. Sizing up Heaters of Hydraulic Systems

4. Sizing up Hydraulic and Electrical Drives

Sizing up hydraulic or electrical motors and drives
is key to well performing control systems.
One common error in sizing these drives is basing
the size on the steady state load requirement and
overlooking the entire duty cycle of the load.
The transient behavior of the load is key to proper
sizing of the drives.
For example accelerating an inertial load will
impose significant torque requirement that need to
be analysed.
If there is a speed reduction, reflected inertia needs
to be calculated.
If the system operates outside, torque calculation
needs to reflect the effect of low temperature on
starting the load.
Altitude of the system should also be taken into
account in sizing up the drives.
Load Locus plot can be used very effectively in
mapping the operational requirement of the load.
Once the plot is created the drive size is selected
such that it meets the maximum needed power.
This will ensure that the system will start and run
properly under all operational and environmental
conditions.

5. Vibration Monitoring

Monitoring vibration on critical equipment like
fans, motors, bearings, gear reducers, pumps is a
must.
One way of finding how critical is the component
is by studying the cost of down time on the
operation of the plant.
The cost of down time provides the designer key

information about how much to spend on
protecting this component from failing.

There are two ways of monitoring vibration. One

way is to have periodic monitoring on a regular
basis where a vibration specialist comes and
check the vibration level and provides a report on
the condition of the equipment.

The disadvantage of this method is there is no

guarantee that the equipment will not fail during
the interval when vibration is not monitored.

The second method is to monitor the vibration

level on- line continuously. Alarms and shutdown
levels can be set using the ISO standards.

The advantage of on-line monitoring is to provide

immediate protection in case something
unforeseen happens. On fans for example a build
up of deposits can flake off causing severe
vibration that can ruin the fan and can shut down
the plant.

Another advantage of on-line monitoring is the

monitoring of the trend in vibration level. If the
trend is up then a vibration expert can be hired to
conduct a comprehensive analysis on the
machine to determine the cause of the increased
vibration level before it is too late.

So the conclusion is: To ensure that no costly

down time does not occur, a combination of on-
line monitoring of vibration plus a selective
comprehensive vibration analysis will be needed.
This will be the most economical way of providing
an unplanned down time.

Systems Control Inc can help you select the right

monitoring system for your plant.

6. Hot Fans Maintenance Tip

Hot fans are used to force hot gases and are
critical to the operation of plants. If they stop the
plants shut down. A shot down can cost the
plants a lot of money in lost revenue.
During operation the fan shaft temperature goes
up because of the hot gas.
As long as the fan is rotating the hot shaft does
not cause any problem. If the fan however is shut
down a problem can arise that can affect the
operation of the fan very drastically.
Because of the weight of the fan and the high
temperature of the shaft, the weight of fan can
cause the shaft to deflect and cause it to have a
permanent set.
The magnitude of the deflection will depend on
the size of the shaft, the span, the temperature of
the shaft and the weight of the fan.
When the fan is started again the deflection
causes the fan to vibrate due to the unbalance
created by the now eccentric shaft.
Depending on the severity of the deflection the
fan bearings can be damaged over a period of
time. The more severe is the unbalance the faster
the damage will occur.
In order to avoid this situation it is recommended
to add a small turning gear made of a suitably
sized motor, a reducer, and an over running
clutch to one end of the shaft.
The turning gear can then be used to run the fan
during the cool down period after the main motor
is shut down. This would allow even cooling of
the shaft thus preventing a permanent deflection
of the shaft.
If you need help sizing a turning gear Systems
Control Inc can help.

7. Sizing Up Heaters of Hydraulic Systems

If a hydraulic systems is located outside an oil heater
is usually required in cold climates to maintain the oil
at suitable temperature and for proper start up.
One of the common mistakes is to size the heater
based on steady state analysis.
The problem with basing the heater size using only
steady state analysis is the time it takes to heat up
the oil to operating temperature. Imagine waiting 12
hours for the heater to heat up the tank before you
can start the system.
Over sizing the heater is not a solution since there is
a limit on the heat density "watts/square inch" so that
the oil does not catch fire. Adding more heaters can
cause the cost of the system to go up needlessly.
To properly size the heater for quick start up the time
constant of the heater and the tank must be
calculated. This would involve finding out the mass
of the oil, the surface area of the tank, the specific
heat of the oil, the time required to start up the
system, and using transient analysis to find the right
heater size.
If you need help with start up applications, Systems
Control Inc can help