A new vision for maintaining pneumatic valves

01 February 2007

In the few seconds it takes to open or close a pneumatic valve, a rich set of operational data are available to those prepared to observe them. This new valve sensor package is designed to capture and interpret these data, and warn plant operators when something is going wrong.

Traditionally, solenoid activated pneumatic valves are considered simple devices. They are either in one of two states, open or closed, and should not be at any stage in between. As a consequence of their simplicity, there is a scarcity of operational measurements that maintenance engineers can make on them. They can, of course, verify with electromechanical switches if the valve is open or closed, but this is more for
operational purposes than diagnostic measurement. It provides some measure of comfort to control room operators, who understandably want to know if something is flowing through a pipe or not.

What else could a maintenance engineer do? He could wire a counter to keep track of the number of opening and closing cycles, this would give some measure of valve usage. Or, if he has the right timing equipment, can measure the interval it takes for the valve to open or close. Valve travel time is usually short;
for small valves it might be a few hundred milliseconds and for large valves it may be on the order of one or
two seconds or minutes. But since it’s virtually the only diagnostic measurement that can be made, it might
be worthwhile measuring it accurately.

Why is that? Presumably, if over a period of time (several months or a year or longer) one compares valve travel time measurements, and the amount of time it takes the valve to open trends to longer and longer values, something is going wrong. It may be sticking, for example. Some deposits might be building up on the mechanical parts.

Alas, valve travel time is not always an accurate measurement of a valve’s condition, says Adrie Robbeson, who is Product Manager EMEA in Control and Network Solutions for Tyco Valves & Controls. He points out that the travel time depends on the compressed air supply, which can vary a few percentage points during the day. Some travel time data may be useful for diagnostic purposes but over the long run it is not a clear and reliable method.

Today’s diagnostics
According to Adrie Robbeson, we have inherited a world of process automation that is vastly different from the one we lived in 20 years ago. The main difference is that the proliferation of intelligent devices coupled with high speed communication buses have given engineers and managers access to a wealth of information about their plant resources that they didn’t have before. ‘From the common mobile phone to
sophisticated enterprise resource planning (ERP) systems, the success of all of these systems depends on
communication and intelligence—at speeds which were not even contemplated twenty years ago,’ he says.

Unfortunately, the basic automated pneumatic valve package, as it is currently installed, has not kept up the
pace of this rapid intelligence plus high speed communications evolution. Most valves utilise discrete monitoring and control with little or no intelligence. What is there to be intelligent about, in an on/off valve?

Some progress has been made. Proximity switches have replaced older style electromechanical switches; these provide more reliable and longer-lasting verification of the open and closed positions. And high-speed data buses such as AS-Interface are being deployed more and more, even though these industrial fieldbuses are used primarily for operational purposes, that is, to provide the on or off signal and verify the valve’s position. But at the end of the day, maintenance is carried out pretty much in the way it has always been done: according to a planned schedule, which may or may not reflect the individual units’ real requirements. This can be a costly exercise, as Mr. Robbeson points out, when a valve package is removed from service, disassembled, and inspected—only to find out that no maintenance is required.

While this state of affairs may have been acceptable 20 years ago, times have changed, he admits, and the process manager now requires more detailed information in order to make the best maintenance decision, one that is based on facts and not hunches.

‘Process managers require increased efficiency through diagnostic information, predictive maintenance and reduced downtime, and most importantly they need advance warning of any deterioration of elements within the system,’ he says. ‘In addition, the demands on process operators have ensured that new types of installations are required, including Emergency Shut Down (ESD) with Partial Stoke Testing (PST).’

The way forward
Several years ago Mr. Robbeson and his group at Tyco began taking a look at ways to improve maintenance on pneumatic valves. Where could they get the information they needed to tell what was going on in a valve? Basically, it’s a sensor problem. Where can you put the right sensors, so at the right time they can acquire the right data that will be useful for maintenance analysis?

Since the only action a valve ever takes is when it opens and closes, this must be the ‘right time’ to acquire data. What would be the ‘right’ data? One could capture the exact position of the valve, as a function of time, and also the compressed air pressure, also as a function of time, and monitor these as variables that might change over the lifetime of the valve.

The Tyco research group set up equipment to monitor these variables and to no one’s surprise, yes they did
change over a period of time and yes, anomalies in valve operation would show up as visible changes in the
position and pressure curves. Of course, it is one thing to monitor the intricacies of the operation of an instrument in the laboratory, and a quite different situation to monitor the same instrument on the plant floor under industrially hardened conditions. So Tyco’s product development people had to get involved, helping to build a sensor package that would be small enough to easily mount on the top of valve positioners and yet withstand harsh factory environments.

The XamineR
The results of Tyco’s product development is a package called XamineR. It is slightly larger than a video tape or paperback book. In the centre part is the Hall effect sensor which measures the exact valve position and gives precise position feedback (not just the end points—the ‘open’ and ‘closed’ positions—as was previously done with valve positioners). On one side are the pressure sensors, that measure the input and exhaust pressures, as well as the differential pressure between the A and B ports. (The A port is the air inlet to open the valve; the B port to close it. Spring return valves use only one port, in which case the differential pressure measurement is the same as the air supply measurement.) On the other side of XamineR is the fully encapsulated electronics package that contains the transmitter to transmit the data, either by a serial line, a high speed fieldbus, or a wireless technology like Bluetooth.

XtractoR software
Along with the hardware, Tyco developed a software package called XtractoR to calibrate, commission,
interrogate, and maintain the XamineR. Using a high speed communications mechanism like Bluetooth, XtractoR collects raw sensor data from XamineR and displays them as in figures 1 and 2, which are time plots of the valve’s opening and closing. The red line is the position of the valve; the dark green is the air supply; the blue is the differential air pressure and the yellow is the exhaust. These lines will shape differently for different valves and over time, will change as a given valve is used more and is more exposed to the environment.

Beta testing has revealed a number of different characteristics to look for in the curve shapes than indicate potential problems. As of this writing, Tyco engineers have logged about 40 specific ‘errors’ in signatures that can be used to alert operators. Process operators, of course, do not want to spend their time looking at valve signatures and characteristic curves to detect early deterioration of their elements within the system, so Tyco is developing higher level software that uploads data from XtractoR and formats them into easy-toscan alert screens. The valve population management tool will help both Tyco and plant engineers to quickly ascertain developing problems.

Since many pneumatic valves are installed in emergency shutdown systems, and thus are rarely used, Tyco has developed Partial Stroke Testing (PST) procedures for XamineR that work with the valve moving only 20% to 30% into the closed direction. The PST is a completely automated procedure that takes only a few seconds.

XamineR can also provide Electronic Software Definition (ESD), fully time and date stamped and validated, in order to provide authenticated evidence of actions for any health and safety body.

Diagnostics in graphical format
Figure 1 is an example of a graphic representation of valve diagnostics which the XamineR and XtractoR software combination can supply. The data from the sensor package are
transmitted directly to a PC via BlueTooth or a serial line and displayed in graphical format. This particular torque graph shows readings from a valve in good operating condition; perhaps it is a brand new valve in which case this would be the ‘signature’ of the valve, used for comparison purposes. The graph shows that after a very short hesitation while it breaks away from its seat, the valve position (red) moves smoothly to its fully open position in less than two seconds.

If a valve has a problem, or is developing problems, this graph would clearly indicate it. For example, a valve
with a typical opening problem would break away from its seat and then, after slight initial movement, would stall, with nothing happening. The red line would stay at the bottom of the graph instead of rising. Then suddenly a few milliseconds later it would start to open and go to the full opening position. The moment that the actuator air pressure exhibits a peak may also be an indication of the valve opening problem.

Tyco engineers have been studying the diagrams from many different valves with a variety of defects. In general, they have discovered three areas of interest in the diagrams, all of which can be interpreted and acted upon for the benefit of future system integrity:

a) Prior to breakaway;
b) During the valve movement; and
c) After reaching the end position.

The development of the XamineR field device and XtractoR software in combination will enable Tyco Flow
Control service engineers to download data from the device and upload these data to a Valve Population Management tool. They will be able to do this in real time.

Tyco says its goal is to present the complete picture of the installed valve population, together with a
performance and service history of all automated valves installed in the organisation’s manufacturing plants. If they have accumulated the data in a proper database, they will also have a performance and service history of all automated valves installed in their organisation’s plants, no matter where they are located around the globe.This will allow the plant’s maintenance management to assess the performance
of various automated systems under different process conditions and in various locations, all from a single point.

Tyco Flow Control says it has been developing interfaces, both for pneumatic and electric, for all the most
common network technologies, including HART, AS-Interface, DeviceNet, Profibus and Foundation Fieldbus. The company is a member of all of these organisations and is involved in the Field Device Type (FDT) / Device Type Manager (DTM) initiative. A DTM is under development for XamineR based on Profibus/Foundation Fieldbus.

Tyco Flow Control
www.tycoflowcontrol.com


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