A valve positioner positions the valve in response to input signals from the process control system (or programmable logic controller). Most systems currently use either analog 4-20 mA or digital bus-based communications to digital valve controllers, which are highly advanced digital positioners with significant capabilities beyond just positioning the valve.
Whether the signal is analog or digital, the controller converts that input to a pneumatic signal (I/P). This provides the force (air pressure) to move the valve to a new position and keep it there. If the air supply coming into the valve controller is dirty, the performance of the controller can be affected, resulting in poor valve operation.
Vibration is another major cause of valve and positioner related issues. High vibration applications can cause damage to the valve controller feedback linkages, resulting in process instability.
Overcoming Dirty Air
It might seem that micron-sized particles in plant air supplies would not affect valve operation, but they do. Dirty air is probably the number one contributor to control valve loop tripping and unplanned shutdowns. The incoming air must pass through small orifices in the I/P, a part of the unit that converts the input signal to the pressure needed to move a valve. Tiny airborne particles can combine with moisture or oils droplets and build up inside an orifice, causing variations in the pneumatic signal that can result in unreliable valve operation.
Since the quality of plant air varies significantly, it is necessary to design the instrumentation to minimize the effects of dirty air at the control valve assembly. An additional tool is the diagnostics in HART® and fieldbus communicating digital valve controllers to obtain advance information about the condition of valve (see below).
One design to mitigate effects of dirty air uses orifices made of sapphire, which is actually grown around a pin to the required size, instead of machined steel orifices (Figure 1). This material is perfectly smooth and essentially frictionless, so no machining is necessary. Standard steel machined orifices have small tooling grooving present to which the particulate can adhere. The walls at the air entry point offer another potential site for dirt buildup. That area was designed to create a much larger space for incoming air, significantly reducing the possibility of moisture-laden dirt collecting on the walls and altering the air flow.
More than a quarter million instruments are in use today with sapphire orifices with few reported failures worldwide due to dirt buildup in the instrument since 2007.
Better air filtration is another solution that should be implemented where dirty plant air is a known problem. The ISA Standard 7.0.01 defines specifically how air should be filtered and dried. Unfortunately, most plants cannot meet these specifications consistently. Because digital valve controllers provide much tighter control than standard positioners, and reduced bleed is a major trend in the industry, orifices in the instruments in general have decreased in size. As a result, many DVC manufacturers are suggesting the use of filters with a greater filtration capability. It is common to see digital valve controllers installed with filters in the 2 to 5 micron range that also have the coalescing ability of removing moisture.
High vibration caused by equipment operating near the control valve assembly can also be a problem, causing wear and damage to the linkage that provides feedback on the valve position. The control instrument needs to correctly sense the actual valve position to control it accurately based on input signals received from the process control system.
One solution making these linkages less susceptible to vibration is a tough coating applied to the linkage. When cured, the surface can be hard and friction-free. Vibration will then minimally affect the linkage, reducing the risk of a process disturbance.
An even better solution is to remove that linkage altogether, replacing the mechanical connection with a linkage-less non-contact magnetic type of feedback element. Wear is eliminated while valve position accuracy is retained. A substantial number of DVCs (Figure 2) are currently operating in the field using this technology.
Predicting Valve Issues
In addition to their primary positioning function, digital valve controllers monitor and store a great deal of information about the control valve assemblies on which they are mounted. This diagnostic information can be accessed in various ways, providing predictive intelligence that helps plant personnel determine what service may be needed and when it should be done to prevent unexpected malfunctions and possible unplanned downtime.
Diagnostic information can be retrieved from valve controllers in the field by connecting to an instrument (point-to-point) and extracting the available data, depending on the type of controller. HART and fieldbus instruments yield a large amount of useful information, such as supply pressure diagnostics, which can indicate a problem with plant air.