Digital valve positioners can improve valve operation while lowering operating and maintenance costs in a process plant. This article discusses the process control loop with a specific focus on the control valve and its role in these systems.
The article will then look at valve types very briefly before focusing on digital positioners. It will examine the expectations that the user should have with regards to digital positioners, for example, whether they are an individual in the engineering team that will have to spec out positioners or an end-user.
A number of considerations for selecting a good digital positioner will also be explored, looking at what potential buyers should be looking for, features that would be beneficial in the longer term and the importance of making purchases with a view to long-term plans and use of digital positioners.
Repeatable and sustainable performance are key factors when selecting a digital positioner. This is also very important when considering control valves because the positioner is the driving force behind the entire assembly that ensures the valve performs in line with expectations.
This article will also look at valve automation and how to best put together the positioner and control valves before finally looking at options around the predictive and preventative diagnostics that digital positioners provide.
Ongoing digitization means that these diagnostics are being embraced by industry more and more and they are becoming a very important feature of digital positioners and other instrumentation.
Fundamentals of the Process Control Loop
The central point of the process control loop is the control valve. The positioner is an essential part of the assembly and the process control loop consists of a sensor and a transmitter that is reporting information about the process; for example, what is in the process pipe, or measurement of flow, pressure or temperature.
That signal is fed back to the controller (for example, a control system such as DSC or PLC) and then from the controller, the output signal is relayed as a set point signal at 4-20 mA.
Before continuing, it is worth outlining the importance of the 4-20 mA and HART options. The 4-20 mA signal is an analog signal and this is the typical input signal into digital valve positioners as well as a basic electro-pneumatic valve positioner that does not include a microprocessor.
This serves as the control signal, which is the position demand signal, as well as powering the positioner.
HART is a digital communication method that is superimposed onto these 4-20 mA wires. Most customers are using the 4-20 mA signal and their positioner has HART capabilities, but they are not making use of these.
The advantages of using HART are linked to the overall benefits of using a digital valve positioner. Using HART communication results in improved accuracy, better stability of the control valve, better process availability and cost and energy savings when compared to an older style analog-type positioner.
The control valve positioner requires an air supply and an energy source. The purpose of the valve positioner is to pressurize the actuator in such a way that the valve movement corresponds to the setpoint from the controller.
When this is done correctly, the valve is able to regulate the process medium and position the valve according to the set point. The setpoint is determined by the control loop, set via the controller.
The valve assembly consists of the valve that is controlling the process variable, the actuator and the positioner. Several actuator types are available on the market. One of the most prominent types is the diaphragm type actuator.
These are typically single-acting, using a spring function to achieve a single-acting failsafe. In the event of any problems such as loss of air or loss of signal, this type of valve is designed to be able to safely adjust the process to a safe condition.
Another common type of actuator is the piston-type actuator, which can be single-acting or double-acting. Single-acting piston-type actuators use a spring function to allow the piston to retract or extend depending on which direction is required.
A double-acting actuator is generally double-acting without a spring function and this could require a specific type of positioner.
Positioners found on the market are designed to be able to work with a single-acting actuator such as a linear rising STEM type of actuator or a piston-type actuator with a spring function for rotary use that can be switched between linear or rotary modes.
The user must decide whether the application is ‘failsafe’ or ‘fail in place,’ and this is done through the design of the positioner. Users must also decide between double-acting types of actuators or piston types and whether a linear or rotary valve type is the best option for their application.
Loss of control signals is a serious issue that should be considered and because piston-type actuators are pneumatic, consideration should be given to what happens if the system loses its compressed air or whether there should be safety functions built into the valve assembly to move the valve to a more safe position.
Digital Positioners: Key Considerations
Microprocessor-based digital positioners often make use of a 4-20 mA input signal with HART communication. Other communication capabilities are available such as the Foundation Fieldbus or Profibus PA, but HART communication with 4-20 mA as the setpoint signal is one of the most common.
Digital positioners should at least have a front-facing display with push buttons, allowing the user to perform the setup of the device. Manufacturers do not always include a display, but this does add a lot of value for the end-user because they can control the positioner without the need to use separate software or programming tools.
Users can also access the device directly for fault-finding and error diagnosis purposes.
As well as easy input options, every smart positioner or digital positioner should have a self-calibrating mode, sometimes referred to as an auto-tune or an auto-adjust function.
The design of the positioner could incorporate very basic or even advanced auto-tune or self-calibrating functionality, but this is an important factor in the positioner being able to properly adapt to the valve assembly.
This self-calibrating functionality also enables easy, fast and repeatable commissioning of the positioner to the actuator assembly, meaning that this process does not require any human intervention. This automation also helps minimize inaccuracies due to different individuals calibrating instruments in different ways.
It is important to consider is the fact that positioners do not always have tuning parameters and that the device may or may not have parameters settable for both the up direction versus the down direction or clockwise versus counter-clockwise.
This is notable because the valve behaviors and dynamics taking place in the control valve are often asymmetrical, largely due to the difference in dynamics moving up or down versus clockwise and counter-clockwise. A good positioner should offer independent parameter functions for open and closed directions.
Performance considerations are also central to selecting the right digital positioner. There are several design types available in the marketplace, but one of the most common types uses the pneumatic relay principle of control.
Here, the mechanism inside the positioner converts the electrical signal to an analog signal (pneumatic pressure), meaning that the electrical input signal is passed via a microprocessor to the pneumatic relay, keeping the movement of the air out to the valve.
The pneumatic relay type is the preferred relay type because it offers excellent dynamic performance and low air consumption – two highly beneficial features when selecting an appropriate digital positioner.
When considering air capability and air handling, it is important to think about how quickly the valve assembly responds to the setpoint signal. This is very much related to the air capacity capability, with higher capacity ensuring very fast movements, even for small setpoint changes.
Fast response times ensure that the valve can keep up with the demand of the process control loop, avoiding potential wastage of materials in instances where the valve struggles to reach the set point.
Air supply pressure is as important as air capacity because it is the air supply pressure that allows the valve assembly to achieve its desired force and torque. This is also very important for the seat load.
An ideal device will allow a wide range of supply pressure to the instrument. To provide an example of the typical capability of performance, a design can be up to 50 kilograms an hour, which equates to about 23 scfm of air capacity.
Air supply pressure, in this case, would be 145 psi. This example would ensure that a wide range of supply pressure is available from the compressors. There is no limitation here, but this must be regulated down to the appropriate level of the valve assembly.
A good digital positioner should offer low air consumption, with contemporary positioners offering air consumption as low as 0.015 scfm due to the relay mechanism design. Low air consumption is an important factor for ensuring energy savings.
The device’s inputs and output should also be considered. Typical digital positioners will receive a 4-20 mA input. This could be HART 5 or HART 7, which is the latest communication level of HART. The device may require an emergency shutdown capability or even partial stroke functionality.
Its output should include the ability to provide a feedback signal of some kind and this could be HART digital or an analog signal providing the setpoint feedback to the control room for verification of the valve position.
Other functions are available such as alarm diagnostics, travel alarms for open and closed directions and these may be useful when working with discreet signals coming from the control valve that verify that the valve has reached its desired point, is open or closed.
Different versions of ABB’s digital positioners are available for use in hazardous areas and these are able to meet the requirements of any hazardous area agency regulations. These instruments offer intrinsically safe design, small compact housing, lightweight, cost-effectiveness and the option of heavier duty, flameproof or explosion-proof housing where this is required.
A key function of any digital positioner is its ability to adapt to the control valve. This process requires the use of a function referred to as self-calibration, auto-tune or self-tune.
This is a function that allows the digital positioner to be adapted to the valve assembly and its ability to achieve this successfully is dependent on a number of criteria.
The ultimate goal of calibration is to make it easy for the user to apply the position to the valve, regardless of whether the valve is linear or rotary or whether the valve has to be clockwise, counter-clockwise, direct or reverse acting.
A digital positioner’s calibration function takes the user through a pre-defined menu structure, prompting them to select options via a series of yes or no questions.
At the end of the selection criteria, the user simply presses the button on the device and it will run through an entire self-calibration procedure, meaning the positioner will then be able to adapt to the valve based upon the criteria that the user has programmed via the setup menu.
Calibrating the Digital Positioner
The way the positioner adapts to the value is important because the positioner is required to find what is referred to as the ‘control parameters.’ Control parameters are the internal functions of the positioner’s microprocessor and these functions are integral to the device’s ability to adapt to the control valve.
The size of the valve, how much air capacity is required, or the pressure that is coming into the device are all automatically adjusted during the unit’s self-calibration process. These calibration settings are saved into the device, ensuring the self-calibration process is repeatable and accurate.
A range of control parameters is common in any digital positioner, but these parameters sometimes use different names. Examples include deadband, zones, KP or gain value, derivative time, TV, or dead time.
When calibrating a digital positioner, the setpoint is the main target to be achieved. The area around the setpoint is known as a deadband and this term refers to how accurate the positioner has to be to find its setpoint or the positioning point within the deadband. The tighter the deadband, the more precise the positioner is.
The zone parameter offers the positioner the ability to move towards the deadband and ultimately to the set point. These are values that the positioner automatically finds – users do not have to set them up beforehand.
The positioner’s microprocessor technology will calculate and determine these parameters at the touch of a button, making setup much easier for the user.
The ultimate final objective of the calibration process is enabling the positioner to track a changing set point very accurately, rapidly and without overshooting the setpoint. This step is perhaps the most important because users need to ensure that the valve reaches the setpoint quickly.
This links back to the importance of air capacity because a valve assembly that does offer enough air capacity based upon the size of the actuator may not deliver a fast enough response time in terms of reaching the set point.
When selecting the functionalities of the hardware of the instrument, there are other factors to consider, particularly around what the device should do in the event of a set point signal that fails and whether or not it should move to a safe position under these circumstances.
This step is straightforward when using a single-acting spring diaphragm-type valve, but double-acting valves or valves with no spring assist require the positioner to also take action.
This may take the form of an emergency shutdown or an ESD function that allows the control room to exercise an emergency shutdown of a signal to the device; for example, shutting the valve, closing or opening a door, depending on the process requirements.
It is important to consider how robust the design of the device is based upon its vibration immunity for process applications and resistance to ambient conditions. Different positioners offer different capabilities in these respects, with one common example being the adaptive tuner.
An adaptive tuner allows the digital positioner to exercise its digital functionality to optimize itself. No human intervention is required in this process and the function of the adaptive tuner is to automatically optimize the parameters of the device so that the funnel process runs smoothly.
For example, if there is erratic behavior in the assembly, switching on the adaptive function will allow the control valve to find the ideal control parameters for that process, eventually reaching an exactly controlled steady behavior.
It is also important to consider the ease of calibrating large numbers of digital positioners, where customers have complex or sophisticated network requirements.
In this instance, the laptop or tablet that contains the software tool should be able to connect via point-to-point communication. This means users simply need to connect a HART modem onto the analog 4-20 mA signals to allow them to communicate point to point, one instrument at a time.
Working with a bus system where there are multiple positioners requires a different approach and users should connect to a point in the network where their communication device can scan all the devices that is on that loop.
Remote IO capability is useful here, where all the devices are connected to a centralized IO unit, allowing users to scan the entire network at once. The user can then communicate with each instrument from a central point as you wish. This is also possible with a multiplexer type of setup.
Appropriate Positioner Mounting
A mounting kit is used to attach the positioner to the actuator and attaching the positioner to an actuator to a control valve is often a highly specialized process. This process is known as ‘valve automation’ because it involves automating the valve by attaching and adding a positioner.
Depending upon the actuator’s construction and type, the mounting kit will consist of a bracket to secure the positioner to the actuator and a coupling or small lever arm and associated linkage to connect the motion of the actuator to the positioner.
The choice of the mounting kit depends upon the make, model and physical construction of the particular actuator that the positioner is being attached to.
The mounting kit serves two functions: it physically secures the positioner to the actuator via a bracket while ensuring that the positioner is able to detect the motion of the actuator and know the position of the actuator and valve.
It does this via a feedback shaft on the positioner, connected to the motion of the actuator via a coupling or linkage.
The choice of mounting kit will vary depending on the type of actuator. With linear-style actuators, the mounting kit consists of a bracket, a small lever arm and some small linkage pieces to translate the valve motion to the positioner.
A rotary-style actuator is a simpler setup in terms of mounting the positioner, simply requiring a bracket and a coupling. The mounting kit cost is often overlooked when potential users are looking at a control valve package, but a mounting kit typically costs between $200 and $300.
While the type of mounting kit required depends on the particular make, model and design of the actuator, there are some industry standards involved in valve automation. These standards are not currently commonly used, but their use is increasing.
Examples of these standards include Namur or DIN IEC, whereby the construction of the actuator is defined in terms of how accessories like the positioner can be attached. Automating types of valves that meet these standards and actuators is, of course, much easier than working with actuators that do not conform to the industry standard.
Control valve packages are provided to customers in a number of ways, the most common of which being a complete package provided by the control valve manufacturer or a subcontracted organization.
That control valve manufacturer produces the valve and actuator and while the manufacturer may or may not produce their own positioner, they provide the whole control valve package to the customer. This package includes the valve, actuator and positioner, with these components already mounted and tested.
This approach is highly convenient for a customer who does not need to be concerned about the mounting kit because the work has already been done by others.
Even when purchasing a pre-packaged solution, it is important to be aware that it is possible to specify the type of positioner used and that it may be prudent to select a higher-end or high-performance positioner, or depending on the application, a less expensive, lower-end type of positioner.
Not specifying the control package may lead to disappointment if the positioner does not have the required features. Another approach to selecting control valve packages with positioners is to contact a local valve shop. These outlets specialize in providing control valve packages to customers.
Alternatively, customers may be replacing an existing positioner on an actuator with a different type of positioner and this might require a new and different mounting kit. All of these scenarios require the customer to carefully weigh up their options in terms of positioners and mounting kits.
Getting the best performance from state-of-the-art digital positioners and ensuring that the digital positioner can do its job requires a precise transfer of the valve motion and movement to the positioner so that the positioner knows where the valve is. This is done via the mounting kit.
It is very important that the mounting kit is designed well and fits properly because any deadband in the mounting pieces, linkage or coupling will adversely affect the positioner’s performance.
Asset Monitoring and Optimization of the Digital Positioner
There is increasing demand from customers end-users to utilize smart technology and to be better able to provide information about what is happening in the field. A typical example would include a tablet or a computer connected with a modem across the 4-20 mA signal lines that is communicating with an instrument in the field.
This could also be embedded in the DCS system, which means the software would be running as part of a DCS that can handle HART IO-type capabilities.
There are some new industry guidelines being set up by FieldComm Group – a group defining communication protocols and how instrumentation should be able to communicate with other instruments, cloud-based systems and controllers.
One technology that is evolving rapidly is Field Device Integration (FDI), which is a standardized technology developed by the FieldComm Group. These tools will allow easy configuration, commissioning, diagnostics and maintenance of HART instruments and other Fieldbus or Profibus type devices.
This functional tool facilitates quick scanning of the field instrumentation, allowing users to transfer data onto the tablet or the computer with ease as well as offline and online setting up of the device. The same tool could also be utilized for device health reporting, making it an ideal asset for proactive and predictive maintenance functions.
ABB offers the Field Information Manager (FIM) tool, which is a single device, tool and software driver designed for setting up devices as well as diagnostics.
A comprehensive range of diagnostic functions is available, but one of the most important diagnostic considerations for control valve type applications is the predictive maintenance of positioner’s travel counters over time.
For example, monitoring the stroke counter, past issues with regards to timeout, problems with the setpoint and how many times this has happened. It is also important to consider the use of a controlled deviation to evaluate potential problems within the assembly, such as friction and striction.
Software tools allow the user to look at all of these predictive maintenance options, examine current operating conditions and make decisions based upon that information.
Proactive maintenance type categories are often more comprehensive and these tools could include histograms inside the positioner’s memory to provide information valve leakages.
If the diaphragm or piston leaks, proactive maintenance tools could provide information on the plug wear or seed wear of the valve, the response time of the assembly, speed over position, or valve signatures.
Monitoring all of these factors and potential issues allows users to make decisions and to be able to fix things before they break, causing the valve to be taken out of line.
This allows maintenance to take place in a more manageable way, during a regular maintenance interval, for example, rather than an emergency shutdown scenario that could stop the whole process. Overall, these features allow users to extend the availability of the control valve, keeping this running as long as possible without issue.
Summary and Conclusions
Digital value positions have the potential to greatly improve valve performance and operation while lowering operating costs.
There are a number of considerations and objectives that most organizations want to achieve for their control loop processes. Improving control valve accuracy is essential because this will reduce the process variability.
This, in turn, leads to savings on raw materials, increasing the profitability for the project or the final product yield. The use of digital positioners can help ensure this accuracy.
The stability of the control valve assembly is important because it reduces the wear on the valve. The more stable the valve is over time, the less wear occurs on the packing, diaphragm, O-rings and other components.
Stable, accurate control of the valve facilitates increased availability and extended lifetime of the control valve assembly.
The longer the valve is kept running at its optimum performance, the longer the period between maintenance intervals of the control valve, resulting in reduced cost implications when the valve does have to be taken out of operation.
It is also possible to achieve significant savings by employing technology that has very low air consumption. Compressed air is an expensive commodity and by reducing the losses from air-consuming devices, positioners can lower that cost.
All of the factors outlined here are important when selecting a digital valve positioner and it is essential that potential users evaluate these factors against their objectives, making sure that the selection fits the valve and the process control.