What would happen if I asked various people in our industry, “Which control valve in your plant will you have to repair in the next turnaround?”
You’d hear “I don’t know” a lot.
Usually in turnarounds, operators pull control valve without knowing if they need corrective maintenance. Once they verify the control valve conditions, they’ll replace items or whole valves, even those in good shape, spending money on unnecessary fixes.
Most valve positioners today provide diagnostics to tell maintenance crews when to take preventive or predictive actions. Sadly, many companies ignore this information and waste money in their turnarounds.
Reducing control valve maintenance costs
So how can you know which valve to repair in the next turnaround? By controlling the life cycle of your valves and analyzing the diagnostics from your valve positioner. In conventional plants with 4–20 mA communication, how can you use 100 percent of the potential from your SMART field devices?
Smart (i.e, intelligent, digital) valve positioners perform the same basic functions as a traditional valve positioner, but they have expanded functionalities. Like any “smart” device, a smart positioner includes a small computer that enables additional capabilities.The capabilities beyond positioning are what make smart positioners unique and valuable, but also what can make them intimidating. Smart positioners make the basic positioning functionality across your plant more accurate and reliable. Every positioner can be calibrated exactly the same and that calibration can be maintained, which provides more accurate control to setpoint and thus optimum process control.Smart positioners enable accurate calibration. Users often specify an input signal with a larger range than necessary to compensate for inaccurate positioner calibration. In the case of analog 4–20-mA inputs, users will drop the input to well below 4 mA and then adjust it to exceed 20 mA to ensure the valve shuts off and travels from 0% to 100%.
An example of how to monitor controlled equipment is the intelligent electro-pneumatic valve positioner. Control valves vary in dimensions, from pipe sizes a fraction of an inch to a few feet in diameter. Valves control the flow rate of various types of abrasive, corrosive or viscous substances and are therefore subjected to wear and tear. State-of-the-art electro-pneumatic positioners not only perform extensive self diagnostics but also determine the condition of the physical valves or flaps which they control. At first the positioner “learns” the correct behavior of a “good” valve. Then during normal plant operation it detects any deviation from this “normal” behavior. By this principle one can detect valve stickiness, pneumatic leaks (e.g. torn membrane), breakage of the valve cone, deposit build-ups on the valve cone or seat, wear and tear of the valve cone or seat and various other mechanical deterioration. To the operator the valve may still appear to be functioning correctly at this stage, but on the maintenance station the valve is indicated as having a problem and signals the degree of seriousness of damage. Maintenance can immediately be planned and the necessary repair actions executed before serious damage is caused.
By using wireless to send all HART data to the plant asset management (PAM) system. Yes, in this scenario, we want to use wireless where we already have cable, cutting the cost of the PAM system.
You can also try a multiplexer . Multiplexer is a device that has multiple inputs and a single line output. The select lines determine which input is connected to the output, and also to increase the amount of data that can be sent over a network within certain time. It is also called a data selector.You can also consider changing your programmable logic controller (PLC) cards, for example from analog to HART. Sometimes these options can trim costs or make your PAM system run faster. You can install the wireless solution in parallel with 4–20 mA communication. It won’t interfere with or influence the signal, either, which makes installing it easier and decreases costs.
Of course, you’ll need the right software to access all data from your devices. Maybe you didn’t know, but most brands use proprietary software to grant access to the valve positioner. However, you might have a way around that. Getting diagnostics directly from field devices without proprietary software requires a brand that can send all functions and diagnostics directly to the device type manager (DTM) or device description (DD) files.
Even when a company has a digital network like PROFIBUS DP/PA or FOUNDATION Fieldbus, it may still fall short of reaching its potential with the network. Indeed, often plants use their digital field networks as simple analog networks for remote access, losing their diagnostics.
In these cases, the companies must review their maintenance systems. A PAM system provides access to all diagnostics from each field device. And predictive maintenance gives you the key to getting the most value from a PAM system. If you can analyze your diagnostics before your device develops a problem, then you can act to prevent that problem.
Maybe you want to hire an external service to verify your valves with a universal tool throughout your plant. You can do that, but a PAM system will probably serve you better. With it, you’ll receive data from your devices all the time, thereby opening the door to predictive maintenance.
Original publication at Visaya Solutions on medium.com