Building a maintenance management program for valves

Diagnostics from smart valve actuators using HART communication can save maintenance costs and improve plant reliability when used in a comprehensive maintenance management program. This is often the first big victory of a program.

04/17/2014


Valves that do not have direct HART I/O can send their diagnostic information via a wireless add-on device. Courtesy: Monsanto Muscatine, the 2012 HART Plant of the Year recipient.While most smart devices are field instruments, smart valve actuators have the same capabilities, and often users launch a maintenance program for valves ahead of instruments. HART Plant of the Year recipients bear this out: Dow, Monsanto, and MOL all used HART diagnostics to improve plant reliability and save money on valve maintenance. Valves are far more maintenance intensive than instrumentation and have more wear-prone moving parts than a typical flowmeter or pressure sensor.

The ability of a given valve to function can have a huge impact on a process unit's operation, depending on where it's located. Its ability to move as expected and control as needed makes all the difference, so diagnostic information is a huge advantage for reliability. HART Communication can also supply real-time valve position feedback to operators, confirming that a valve is in the position that the control system says it is.

When a valve maintenance program in a large facility is thought out well, implemented with care, and with the right individuals at the helm, a full-time valve engineer can easily be worth $1 million or more when the costs of repair and replacement are combined with improved operations and plant availability.

Establishing criticality

Unless your plant has an unlimited budget, you will likely find yourself constrained as to how many valves can have smart actuators or positioners installed. So which valves are the most important? Criticality has two main elements: the importance that a given valve has in a unit's operation, and the likelihood that that it will malfunction.

If you look at the P&ID for a process unit, you can identify certain valves that have to operate 24/7 for the unit to produce, and it may be a large number. If one of those valves fails, the unit goes down, or if it doesn't perform correctly, product goes out of spec. Plant designers often install backup units in parallel on equipment like pumps so either can do the job, but this is not a common practice with valves. Those critical valves working without backup deserve special attention because you can't reroute the flow and you can't do without them.

The same thought applies when you have to perform valve maintenance. Can that valve be repaired while the unit is still running. If everything has to be shut down to take it out or fix it in place, that valve is also critical.

Probability of failure

Operators can perform step tests on valves to ensure that they are controlling correctly, but that is only part of the picture. Such tests may not show that a friction problem is developing, and every time it moves, it requires more force. Sooner or later the friction will overcome the force and the valve will malfunction.There are valves that can be installed in a process unit and operate for years with no trouble. Where pressure drop isn't excessive and the product is benign, there is little wear and tear in daily operation. But this isn't always the case. Products can be corrosive, erosive, or otherwise aggressive, and there is nothing you can do beyond finding the best possible valve design and most durable materials.

Difficult service valves are also prime candidates to be retrofitted to become smart because they are subject to the kind of abuse that shortens lives. A worn stem or one where sticky product has accumulated can be difficult to move, and this is a simple thing for the smart actuator to measure and monitor over time. The system can warn operators of changes while there is still time to do something about it.

Planning turnarounds

Looking at the HART diagnostics for that valve gives a much different picture. The yellow line represents the amount of force that the valve should need to move based on historical performance. The red and blue lines show what is actually happening now. That valve needs to be serviced soon before it causes an unplanned outage.If your plant is one that runs continuously and can do so for years at a time, sooner or later it will have to be shut down for some period of time to fix accumulating problems. Generally this turnaround period has to be as short as possible, so there is much planning in the months and even years leading up to it. Everything has to be staged and ready to follow the repair and refurbishment schedule. That list will include valves since many will have to be repaired or replaced during the outage.

Turnaround planners must know exactly which valves need attention, and what kind of attention so appropriate units and parts can be ordered in advance. Valve diagnostics can help identify those valves and determine what needs to be done. When the turnaround clock is ticking, that is not the time to be discovering valves that need more attention than realized, or some that are scheduled for replacement are actually performing just fine.

In the repair shop

Valves that have been repaired, either in-place or in the valve shop, should be tested before returning to service. A smart actuator / positioner can measure and record the valve movement and make sure it is operating within acceptable parameters. This requires an appropriate testing procedure for each type of valve with a database where historical information can be stored.

That historical information is what helps identify developing problems. When the amount of torque required to turn a stem increases slowly over time or takes a sudden jump, trouble is not far away. Keeping that information where it can be accessed easily is a critical element of a larger maintenance program.

Establishing a program

After some time in the shop, that valve looks like it is operating as it should. The opening and closing pressures are in a tight group along with the ideal standard performance indicator. HART can be used on the bench to verify correct performance before reinstallation.Automated diagnostic capabilities for valves and other field devices available through HART communication can provide huge benefits at many levels. Effective diagnostic tools can make these activities quick and easy to perform from a central control facility or even a remote location. The concept of taking the data to an expert instead of taking the expert to the problem is a powerful and underutilized concept, but depends on proper implementation of a good set of diagnostic tools and effective work processes.

Overview of barriers

Given the large incentives and low capital cost for asset management, it would seem that this technology would be widely used. The reality is far different. Let's take a look at those barriers and suggest some possible resolutions.

The first impediment has been the technology barrier. There is no question that device diagnostic technologies like HART work, but the tools available to put that information to work for asset management have traditionally been incomplete, hard to use, and poorly integrated. The good news is that the tools are improving and many users have demonstrated effective ways to use them. The bad news is that effective use of the tools and the engineering skills necessary for effective deployment are rare.

The second problem for asset management is management. Technical problems have solutions given management support, but managers have to be rewarded for doing so. That kind of program requires good metrics, good reporting, and effective audits to produce a scorecard. In the absence of a good management system, poor performance is often treated as a technology problem, or just the normal state of things.

Who's supposed to be acting on the diagnostic information? Maintenance? Can you tell a maintenance guy to look at an incomprehensible mass of unprioritized diagnostic data and decide what he should do based on that? Properly implemented systems can provide order and priority to make the data useful to engineering and maintenance organizations.


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