Technical Note: KTN-104

 

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Why pneumatic actuators occasionally operate jerky, bumpy, or even just move to one end?

A general overview


CAUTION
Technical Notes presented here are for general reference only. Information contained in these notes may not be applicable to your specific situations or these notes are unlikely to present all the relevant issues or constraints associated with your specific situations. Hence caution should be exercised before using any information contained in these notes.

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Table of Contents

  1. Background
  2. Typical Jerky Behavior
  3. Effects of Jerky Motion
  4. Identifying the Source
  5. Alternate Solutions
  6. Conclusion

Background

Air pressure is widely used in motion control applications primarily because of favorable economics and its ease of generation and control. In valve actuators, air pressure is used in cylinders and air motors. Most pneumatic actuators use some form of piston cylinders or diaphragms with great success.  In rare cases, a pneumatic actuator is seen to provide jerky output motion.  This note is an A general short overview of this particular problem.

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Typical Jerky Behavior

Some commonly observed actuator (mis)behavior patterns are shown below:

  • When air pressure is applied to open the actuator, a "unusual" delay is observed to build up air pressure. As the pressure builds up, the actuator opens with a "thud" and accelerates towards open position.  Air pressure drops considerably until stabilized, based on flow control valve setting.
    This behavior may be attributed to stuck or wedged gate or plug. Once the gate is released from the seat, load and hence the pressure required is very low. Air expands resulting in an accelerated motion in opening direction. In rare cases, an internally rusted cylinder (from water in air supply) may also create similar effect. If exhausting air on closing side is metered with a speed control valve ( called meter-out control), the amount of acceleration can be limited.
     
  • When the ball or disk or gate of the valve is of heavy weight (having large moment of inertia), the opening motion may be superimposed by oscillatory motion of the actuator. ( The actuator seems to open at different speeds- faster and slower in repeated manner). The oscillation frequency may be slower when cylinder piston is at around the middle.
    This behavior is because of the "spring-mass" effect created by the springiness of air and the inertia of the gate along with poor damping in the system.
     
  • The actuator moves fast, and then slow in repeatedly but random fashion, without any identifiable pattern - simply stated actuator moves in jerky motion.
    This behavior may be attributed to stick-slip motion with predominantly coulomb friction.  The origin for the friction could be from the valve or cylinder because of misalignment, poor surface finish, overlay, rubber coat, galling, loss of friction-reducing coating, boundary layer effects, contaminants and slurries, gouging, heat related expansion, pressure locks, and other specific factors.
     
  • The actuator moves in a single frequency or in beats with apparent constant amplitude. Normal hunting or limit cycle oscillations come into this category. This behavior can be seen more in actuators with pneumatic positioners or "bang-bang" servos.
    This behavior can be attributed to what is known as "non-linearities" in the actuator system. These non-lineraities can include deadband, loose fittings such as in keyways and mounting bolts. This behavior can be seen in electrical and hydraulic actuators as well with its own unique characteristics.
     
  • Another type of unwanted actuator motion could be from hydrodynamic or Bernoulli forces created in the valve.
    Essentially in these circumstances, the fluid medium generates a (negative or over running) torque in the same direction as the actuator. In these cases, the actuator, as it moves, will suddenly "zip" to an end. Even in modulating application, the actuator may simply move to one end, when it is supposed to be at some intermediate position based on the input command signal.
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Effects of Jerky Motion

Depending on the severity of these unwanted actuator motion as well as on a host of other factors such as how often the actuator is operated,  the jerky motion may have the following deleterious effects:

  • Safety of personnel.
  • Increased wear and tear in plug and actuator internals.
  • Potential fatigue life issues for valve, actuator and associated appurtenance.
  • Transfer of vibration to pipeline and other location.
  • Noise related issues.
  • Performance degradation.
  • Heat generation.
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Identifying the Source

Once the actuator is in place, if the unit is accessible, then a 'non-intrusive' and logical approach may be used to find a solution. Non-intrusive here implies that the valve is not removed from the line or the actuator itself is not disassembled.

Measurements and observations of actuator parameters and behavior may give key information to the nature of the actuator problem.  such information may include:

  • Pressure readings in static and dynamic conditions.
    Since pressure readings provide the load behavior, this information can be a key indicator of the problem.
     
  • Effect of different settings of flow or speed control valves.
    Tightening the flow increases damping which results in lower amplitude oscillations when problem is associated with high inertia of the plug.
     
  • Frequency of oscillation
    Fixed amplitude oscillations can generally attributed to loose mechanical connections or deadband.
     
  • Type of noise, vibration, or jerky motion in conjunction with actuator position.
    If the actuator "jumps" at frequent intervals it could be due to stick-slip or dry friction.
  • Effect of inactivity of actuator for long time on jerky motion.
    When a plug or a gate stays in contact with another surface for long time with very fine clearance, as in the case of a closed valve, depending on fluid medium and impurities, the boundary layer in these clearances may slowly grow and cause an apparent  solidification of the clearances. In other words, if we assume a constant pressure between two ends of a capillary tube, given enough time, the flow through the capillary will decrease or stop as if the capillary is plugged.  Observation of this phenomena, sometimes called 'obliteration', was reported to have been made initially by a team of MIT scientists in 1922. The boundary layer solidification can be broken by disturbing the surface, but will start all over again once left undisturbed. One of the techniques used in hydraulics industry to alleviate this problem is use of a dither signal, a high frequency, low amplitude oscillation in the area affected by this phenomena.
     
  • Unique motion such as Stick-slip (saw tooth motion) indicating heavy coulomb friction.
    Stick-slip is the classical coulomb friction where it takes more force to move a stationary part as opposed to a reduced force for a moving unit.  In other words, static friction is larger than dynamic friction.  In an air actuator, once the valve moves, friction is reduced which results in an expansion of air and higher actuator speed. When higher load is agin encountered by the actuator, it slows down or temporarily stops, again repeating the "saw-tooth" motion.  The motion is somewhat analogous to the "squealing" of chalk on a black board.
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Alternate Solutions

If intrusive or non-intrusive evaluations cannot solve the specific problem, alternate means can be sought. Any alternate solution, again, depends on the particular behavior of the valve.
For example, basic problems such as misalignment, lack of required lubricants, loose mechanical connections or worn out internals have to be fixed when they are the source for the problem.

Certain problems can be fixed by alternate techniques. For example, Kanair has supplied modulating hydraulic actuators to replace pneumatic actuators used in a power plant damper application.  In this case, under certain load, a pneumatic positioning damper/louver (actuator) will simply move to one end without faithfully following the input position command. It was not possible to predict when this happened because of the multitude of flow factors involved. The load changes along with the compressibility of the air simply overran the actuator. 
Kanair hydraulic actuator was designed to account for any negative aerodynamic forces and keep the damper position as commanded from the control room.

In another case of a pneumatic actuator on a butterfly valve, a cost effective method was to use an air over oil system. In an air over oil system, air pressure is applied on top of the oil in tanks and the actuator cylinder is operated with hydraulic oil.  The relative stiffness provided by the oil eliminates the problems associated with the compressibility of air.

When properly designed, hydraulic actuator by virtue of high oil stiffness (represented by bulk modulus) can handle heavy inertial load without undesirable oscillations or jerky motion.  It is also easier to dampen a misbehaving hydraulic actuator using techniques such as orifice controls

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Conclusion

This note provided an overview on the jerky motions sometimes observed in pneumatic actuators. These notes are unlikely to present all the relevant issues or constraints associated with your specific situation. Hence caution should be exercised before using any information contained in these notes.

 

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A kanair technical note : KTN-104.
Copyright 2003 Kanair, Inc. All rights reserved.
Revision 0: 11/20/03.