solenoid valve fault tracing by norgren
DESCRIPTION
How to make a trouble shooting in solenoid valve, make it simple and fast if everybody knows the basic philosophy of pneumatic valvesTRANSCRIPT
Fault Tracing
For pneumatic and electro-pneumatic systems
Contents
Safety Fault effect Common faults Component fault tracing Solenoids Switches
Monitored conditions Fault tracing procedure Simulated system
pneumatic
Simulated faults
Click the section to advance directly to it
Introduction
Valves
Simulated system PLC
System fault tracing
Introduction
Modern component design and manufacture coupled with planned preventive maintenance provide a high level of performance and reliability
In the event of a machine or system break down, the heavy cost of lost production time makes it essential to restore normal running in the minimum time
If a machine or system shows a fall off in performance or stops working, there is a race to trace and correct the fault as fast as possible
A methodical approach and experience of likely problems is the pre-requisite of the fault tracing engineer
Safety in fault tracing
Safety in fault finding
In any fault tracing exercise personal safety and the safety of others is paramount
Work should be carried out using approved practices and observing relevant legislation
Ideally all electrical and pneumatic power will be isolated, pressure exhausted and moving parts mechanically locked
Safety in fault finding
In practice it may be necessary to have the machine or device partly or fully powered up to locate a fault
To gain access to test a suspect device, it may also be necessary to remove and override the guards.
This clearly presents many dangers and great caution and awareness is required by the fault finding team
Fault finders must keep well clear of the path of all actuators, mechanisms and other hazardous moving parts
Electrical equipment should be checked using only the proper test instruments
Safety in fault finding
DANGER! Jammed actuator
The actuator will be pre-exhausted
Clearing the jam will cause the actuator to impact to the end of its stroke
This will be faster than you can react to remove your hand
Ensure the actuator is de-pressurised and use a tool to clear the jam
Safety in fault finding
DANGER! Live trip valves and limit switches
Accidentally touching or leaning on a limit valve or switch while testing a machine can generate a signal
This can cause an actuator or sequence of actuators to operate unexpectedly
Safety in fault finding
DANGER! An exhausted system can leave some components with trapped pressure
An actuator controlled by a 3 position valve may be pressurised regardless of whether the main pressure has been exhausted or not
Removing a connection to one end or overriding the valve will cause sudden actuator movement
Click the illustration to start and stop animation
Safety in fault finding
Some of the dangers to consider include:
Sudden exhausts to the face:
noise can injure the ears ejected particles can
injure the eyes Electric shock Moving mechanical parts
Fault effect
Symptoms of a fault
In a typical pneumatic or electro-pneumatic machine a fault will first be apparent due to one or more of the following:
Poor performance, slow Faulty product, inaccurate Machine stops
A fault tracing and correction procedure must be
put into action
Common faults and solutions
Common faults and solutions
SymptomThe machine is working but is falling off in performance due to slower operation
Possible Causes Upstream flow
restriction or air starvation
Downstream flow restriction
Lack of lubrication
Possible Solutions If weak supply pressure
Fit larger pipe Install local receiver Install larger
compressor If strong supply pressure
Check for kinked tube downstream and renew
Lubricate mechanisms Fit air line lubricator
Common faults and solutions
SymptomOne actuator is falling off in performance due to slower operation
Possible Causes Flow regulators set too
low Tube kinked Lack of lubrication Out of alignment Objects in actuator path Piston rod bent Barrel dented
Possible Solutions Readjust flow regulators Renew tube Fit air line lubricator Clean, realign and
lubricate mechanisms Replace or repair
actuator
Common faults and solutions
SymptomFaulty product due to machine adjustments
Possible Causes Stops or mechanisms
out of adjustment Limit valve or sensor
moved out of position causing a short stroke
Failed ON sensor causing a skipped step in the sequence
Insufficient power to a stamping or pressing actuator
Possible Solutions Readjust mechanisms,
stops and limit sensors Check suppressor and
circuit then replace sensor
Modify the logic control to stop the machine in the event of any sensor failing ON
Increase pressure to the actuator or replace the actuator with a larger one
Common faults and solutions
SymptomMachine stops
Possible Causes Failed pneumatic or
electrical power supply Limit valve or sensor
moved out of position Mechanical jam
preventing an actuator from operating a limit switch
Failed to OFF sensor breaking the sequence
Possible Solutions Re-establish power
supplies, reset machine and restart
Adjust and tighten sensor
Ensure pressure is released from both sides of the actuator before clearing a jam
Check suppressor and electrical circuit then replace sensor
Component fault tracing
Solenoids
Solenoid pilot valves
Fault: the pneumatic output is continuously ON when the valve coil is de-energised
Check the manual override (if fitted) it may have been left in the ON (1) position
0 1
Solenoid pilot valves
Fault: The pneumatic output is OFF when the coil should be ON
Check the electrical supply at the plug
If this is OFF trace the supply back to the source
If ON the fault may be: low voltage mechanical fault
preventing the armature pulling in
burnt out coil
DC
OFF
200mV
2V
20V
1000V
200V
750V
200V
20V
2V
200mV
200uA2mA
200mA
20mA20A
200mA20mA20A
2mA 200uA
DC
AC
20M
2M
200K
20K
2K
200
AC
20A mA uA COM V
FUSED
UNFUSED
!!!
1000Vdc 750Vac MAX
250mA MAX10A cont20A 30sec MAX
AUTO POWER OFF
-TECHIDM91ISO
Solenoid pilot valves
A suppressor fitted within the plug may fail
If so it is likely to fail open circuit
This will not stop the coil from working but will leave it unsuppressed
This could damage the circuit that is switching the coil
Short circuit failure will blow a fuse or damage the switching circuit
+24 V 0 V
Solenoid pilot valves
A weak or broken spring will prevent the armature from sealing the supply, causing air to flow to the outlet and exhaust
Alternatively the armature may be held in the up position by the air pressure after the coil is de-energised
2
3
1
Solenoid pilot valves
Cut or deeply embedded seats, can cause leakage but needs to be extensive before the valve is unusable
12
3
Solenoid pilot valves
Coils must be firmly fixed to the solenoid stem
For a.c. solenoids the reaction of the alternating magnetic field causes axial reactive forces on the coil
If the coil is loose it will vibrate
There will be heat build up, less efficient holding of the armature and eventual failure
Click the illustration to start and stop animation
Solenoid pilot valves
Solenoids valves have matched coils and stems according to coil power
A low power coil will not pull in the armature against the stronger spring of a high power stem assembly
A high power coil causes slamming of the armature on a low power stem resulting in premature seal failure
1.6
0 1
1.6
2W = 1.0mm orifice diameter 6W = 1.6mm orifice diameter8VA = 1.6mm orifice diameter
24V =
6W
Solenoid pilot valves
Solenoid coils are continuously rated which means they can be left energised indefinitely
When energised for a long time it is normal for a coil to become too hot to hold comfortably
Overheating can result if coils are continuously energised in a confined space with no means of ventilation
100%ED
Switches
Reed switches
Fault: Switch contacts permanently made
1. Used directly with an unsuppressed solenoid coil where the arcing has fused the contacts together
2. Too high a current has passed through the contacts
a surge current caused by very long leads (capacitive coupling)
too high a load
L + V
0 VLoad
Extended leadsStandard leads
470µH
+24 V 0 V
Fit a suppressor
Fit an inductor
Reed switches
Indicating light (l.e.d.) permanently ON although the switch functions normally
1. On three wire types this is most likely due to the brown and black wire being reversed
2. If this is not the problem it is likely the contacts have failed closed
BlueCoil
Black0 V
Brown + V
0 V
Protection diode
BlueCoil
Black0 V
Brown + V
0 V
Protection diode
Reed switches
Fault: Switch not turning ON when actuators piston is at the end of stroke
The switch is positioned too far to one end on the actuator
It is beyond the range of the magnet
Click the illustration to start and stop animation
Reed switches
Fault: System not responding to a switch at the actuator’s mid-stroke position
It is likely that the piston speed is too fast
The bandwidth of the magnetic field is approximately 6mm.
If this distance is covered in less time than the response time of the equipment being signalled then it will not be successful
Click the illustration to start and stop animation
Limit switches
Fault: The switch mechanism appears to be moving but the switch is not changing over
1.This may be due to a cam operating the roller in the pre travel band only
re-adjust to include the operating travel
2. Broken contact due to vibration and mechanical fatigue
3. Burnt contacts due to unsuppressed loads
Operatingtravel
Pre-travel
Valves
Power Valves
Fault: Valve spool not changing position
Low signal pressure The minimum operating
pressure can be from 1bar to 3 bar depending on the valve type
Power Valves
Fault: Valve spool not changing position
Opposed signals on bi-stable types (fault with other part of the control)
The signal on the left has not been removed so the new signal on the right has no effect
Check the valve giving the left signal and it’s control
Power Valves
Fault: Valve spool not changing position
Sticking spool due to incorrect lubrication (swelling of seals)
Lubricating a valve with a non-compatible oil can cause the seals to swell and grip the spool
Some non-compatible oils when dried out leave a residue that can glue the spool in position
Power Valves
Fault: Valve spool not changing position
Jammed spool due to the use of a long threaded fitting which has damaged the valve bore
Use only fittings designed for connection to pneumatic valve ports
Power valve
Fault: Slow changeover of the spool
A blocked vent hole The space behind the
pistons needs to breath If the valve is installed
firmly against a flat surface the breathing hole can be blocked
The restriction can cause slow or incomplete operation of the spool
Power valve
Fault: Valve spool not resetting
For mono stable valves this may be due to a broken spring
Power valve
Fault: Air escaping continually from exhaust ports
Leaking due to damaged or worn seals
For 5/2 valves this may alternatively be caused by leaking actuator seals
Simulated Faults
Simulated circuit
This simple circuit simulates an application
Click on the circuit to see it in operation
The lever valve is operated and the cylinder operates automatically
The lever valve is reset to stop the cylinder in the fully instoked position
The next series of slides show the effect on this circuit of various faults
Click the illustration to start and stop animation
Fault simulation 1
Click the circuit for fault 1 The cylinder stops in the
outstroked position where it should be operating the trip valve
The trip valve (a1) has not operated and cannot signal the 5/2 valve
The fault is likely to be loose mountings of valve (a1) so that it is not being tripped properly
Click the illustration to start and stop animation
Fault simulation 2
Click the circuit for fault 2 The cylinder stops in the
outstroked position operating the trip valve
The trip valve (a1) has operated but is not signalling the 5/2 valve
The fault is likely to be a kinked or trapped tube
Click the illustration to start and stop animation
Fault simulation 3
Click the circuit for fault 3 The cylinder completes
one cycle but cannot start the next cycle
The trip valve (a1) has operated but not reset
The 5/2 valve has opposed signals so remains in the reset state
The fault is likely to be jamming of the valve operating mechanism against an obstruction
Click the illustration to start and stop animation
Fault simulation 4
Click the circuit for fault 4 The piston rod stops in a
part stroked position The piston rod has
jammed against an obstruction
The cylinder is pre-exhausted and will impact to the end of stroke when the obstruction is cleared
Freeing the jam while there is pressure applied is dangerous
Click the illustration to start and stop animation
Systems Fault Tracing
System fault tracing
With a failed system it is usually difficult to know where to start the search
The fault tracing process is one of back tracking from the event that should have happened but has not
It may be local such as a jammed actuator or the back tracking process may take you through several stages of logic before the fault is revealed
Resources
The speed with which faults can be located within a system depend on the following facts:
How well the engineer knows and understands the machine
The number of monitored conditions
The quality of the documentation and if it is up to date
Sunday, April 9, 2023
Monitored conditions
Pressure gauges show the presence and level of pressure
Pressure indicators show the presence of pressure
Lamps show the presence of electrical power
LEDs show that a solenoid is receiving power or that a reed switch or sensor is turned ON
2
4 6
8
10
4080
120
lbf/in2
bar
Monitored conditions
In the absence of monitored conditions the fault tracing procedure will take a little longer:
Pneumatic connections have to be tapped to check for the presence of pressure
Electrical connections have to be accessed to check for voltage
OFF
200mV
2V
20V
1000V
200V
750V
200V
20V
2V
200mV
200
uA
2mA
200mA
20mA
20A
200mA
20mA
20A2mA 200
uA
DC
AC
20M
2M
200K
20K
2K
200
AC
20A mA uA COM V
FUSED
UNFUSED
!!!
1000Vdc
750Vac
MAX
250mA MAX10A cont20A 30sec MAX
AUTO POWER OFF
-TECH IDM91ISO
Fault tracing procedure
1. Identify the actuator that has failed to complete or make it’s stroke
2. Establish the direction of movement at this step, outstroke + or instroke –
3. Inspect the state of the inputs and outputs of the valve controlling the actuator
4. Are these correct for the intended stroke ?
If YES: the fault is with the actuator or between valve and actuator
jammed actuator flow regulator sealed tube squashed
If NO: are the inputs correct ?
If YES the fault is with the valve
If NO follow the incorrect signal path to find the fault
Inputs and Outputs
It will help if you become familiar with the pattern of inputs and outputs at a power valve
When a system stops the pattern will be a guide to the fault
Few systems will have such convenient monitoring indicators as shown on the next slides but the pattern can be established
Where push in fitting are used, pushing the tube towards the valve but not releasing the collet will give a springy or slack reaction indicating whether pressure is present or not
Building blocks
All circuits are made from basic building blocks
The circuits illustrated are single actuator building blocks pneumatic and electo-pneumatic inputs
Click each circuit to animate and observe the changing pattern of monitored conditions, inputs (signals) and outputs (power)
Click again to stop or start at different steps
Click the illustration to start and stop animation
Inputs / outputs for 5/2 Valve
These are the four patterns that exist normally during the operation of an actuator
If an actuator fails to move, patterns 1 or 2 suggest a jammed actuator or low pressure, patterns 3 or 4 suggest a failed signal
If an actuator stops at part stroke, any of these patterns suggest a jammed actuator
+ –
10
+ –
10
+ –
10
+ –
10
1 2
3 4
All combinations are shown on the next slide with suggested reasons why the actuator has not operated
5/2 Valve i/o patterns
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
+ –
10
Jammed or lost signal 0 ?
Jammed or lost signal 1 ?
Jammed at part - stroke ?
Jammed at part + stroke ?
Signal 1 not released ?
Signal 0 not released ?
Valve supply lost ?
Valve supply lost ?
Valve supply lost ?
Supply lost plus false signal ?
Valve not changing position ?
Valve not changing position ?
Split actuator or valve seal ?
Split actuator or valve seal ?
Split actuator or valve seal ?
Split seals plus false signal ?
Back tracking to the fault
The back tracking procedure is identified with the next two example simulations
1. System with pneumatic logic 2. System with PLC logic
Fault simulation (pneumatic logic)
Click the circuit to animate the sequence A+B+B-C+C-A- It stops near the end of the third cycle Follow the fault tracing procedure
1. Actuator C should have moved minus
2&3. The inputs are not correct follow either of these
4. The cascade valve has not changed. The lower input is not correct
5. Follow the path of this signal. This comes from valve c1. It can be seen the roller is missing and the valve has not operated
Click the illustration to start and stop animation
Fault simulation (PLC Logic)
Click to animate A+A-B+B-C+C-A+B+C+ABC-ABC+A-B-C- It stops at the end of the last A- Follow the fault tracing procedure
1. Actuator B should have moved minus
2. The input a0 from the previous movement A minus is not ON
3. As a result the output to send B minus has not been given
4. Following the fault path to the a0 sensor reveals that it has slipped out of range of the piston in actuator A
Click the illustration to start and stop animation
End