Digital valve controllers help drive performance optimisation programmes

Norman Render, Fisher Instrument Business Unit Manager, Emerson Process Management, explains how digital valve controllers fitted to critical process valves can help increase plant availability and efficiency and reduce maintenance costs across the valves’ complete lifecycle.

Energy costs account for between 40-60% of total production costs – depending on the industry. Due to increasing energy demands, the UK’s Chemical Industry Association is predicting that electricity is expected to make up 70% of costs by 2020, up from its current 60%*.  This means there is a greater need than ever to drive optimisation initiatives to reduce operating costs and remain competitive.

Optimisation programmes tend to focus on high end initiatives such as advanced process control.  Whilst the status and performance of control valves might not be at the top of the target list for engineers trying to reduce operating costs, valves are crucial to good final control and therefore provide one of the biggest opportunities to help drive performance optimisation programmes.

Studies have shown that approximately two-thirds of the control loops in manufacturing plants are poorly tuned, malfunctioning, or operating in manual.  Poorly performing control valves can be a prime reason for loops running in manual, resulting in less than optimal set-points.  This can lead to out-of-specification product, increasing waste or rework and cause plants to run below capacity due to the increased safety margin needed.  In addition, valves that are not performing correctly can cause process upsets that lead to unplanned shutdowns, resulting in poor plant availability. Poorly performing control valves also make it difficult to take advantage of advanced process control strategies.

Digital valve controllers

Digital valve controllers have been around since the mid 1990’s. Based on microprocessor technology, these instruments enable users to monitor valve performance on-line, to collect and trend performance data over time, and to better predict or plan valve maintenance. The information they provide can help improve process safety, minimise variability, plan smarter maintenance, and reduce process downtime – all aspects of an optimisation strategy.  A single digital valve controller can replace three traditional instruments, namely the position controller, position transmitter, and open/close switches.

Emerson’s Fisher® FIELDVUE DVC6200 Series Digital Valve Controller, for example, mounts to any make of pneumatic actuator and provides the valves actual position and operating characteristics. Performance diagnostics run continuously, analysing valve and actuator data while the process remains on-line. If a problem is detected, information is automatically directed to the appropriate personnel, allowing more informed decisions to be taken.  That includes whether the process can continue operating or whether the problem is serious enough to warrant taking the process offline.

Emerson’s software is used to present information in a consistent, easy‐to‐interpret interface that provides the capability to configure, calibrate, and diagnose the instruments from a centralised location. Emerson’s software for example, is available in a variety of configurations and can be integrated into the maintenance systems of most DCS manufacturers.  It is this flexibility, including the ability to be fitted to any control valve and be integrated into most DCS systems that brings value to end users in terms of standardisation and reduced inventories.

Figure 2: Emerson’s software presents performance information in a consistent, easy-to-interpret interface wherever it’s needed

For applications in hard to reach areas of the plant, wireless adapters can be easily connected to provide access to the otherwise ‘stranded’ valve diagnostics available from the digital valve controller.  WirelessHART technology also negates the need for upgrading non-smart DCS I/O, putting valve diagnostics at the fingertips of anyone wishing to use them.

Whole lifecycle protection

Digital valve controllers provide the opportunity to obtain an array of valuable diagnostic information that can be used to improve plant performance and maximise availability throughout the lifecycle of the control valve. The valve lifecycle itself can be divided up as follows:

1. Start-up & Commissioning
2. Normal Operation (Predictive Maintenance)
3. Abnormal Situation (Trouble shooting)
4. Turnarounds (Corrective Maintenance)

Within each part of the lifecycle the digital valve controller supports plant personnel in different ways.

Figure 3: Typical Control Valve Lifecycle

Start-up & Commissioning

When installing a new valve, the first step is to scan and record its baseline signatures, enabling the ‘as-shipped’ performance to be compared with the valve”s current operating condition.

All device related maintenance − including repairs and configuration, is automatically recorded in an instrumentation database, providing an audit trail or history of events that can be tracked and viewed by tag number. Some manufacturers have secure access to a personal account that provides a database of valve information. These web-based tools can help with preparations for upcoming shutdowns, help follow up on the service history of a valve and assign documents and valve data to the secure, personalised database.

When the valve has been installed, the commissioning engineer can validate that the valve is operating as intended with good dynamic response – for example, that it will fully stroke and can quickly and accurately respond to small input signal changes.  Objectives are:

• Ensure that the valve assembly is ready for start-up
• Find and fix operational and performance issues prior to start-up
• Optimise performance and set up meaningful alerts
• Establish a baseline reference for future troubleshooting

Fisher valve with digital valve controller and WirelessHART adapter enables quick, cost effective access to diagnostics

Application example – Start-up of polysilicon plant

Digital valve controllers and diagnostic software helped with early identification of a valve problem at a new polysilicon plant. This related to resilient-seated butterfly valves and related accessories that included EPDM-coated seats and disks, FIELDVUE instruments, and actuators that exhibited incorrect torque output.

The valves were installed in lined piping that resulted in an uneven load on the flanged area of the resilient seat. This created pinch points, where the disk would catch and stick in the EPDM seat. When plant personnel bumped the air pressure to move the valves, the result was torn EPDM seats. The problem had not been identified until diagnostics indicated increased friction during the valve stroke. Identifying the problem early enabled the valves to be repaired, verifying their performance and avoiding later problems when the plant was on-line.

Normal Operation

During normal operation, control valves fitted with a digital valve controller can be continually monitored from a central control room. Performance diagnostics are used to monitor Key Performance Indicators (KPI’s) such as friction, deadband or mass air flow whilst the process is still on-line.  They give a non-intrusive indication for valve, actuator and instrument health with trending capabilities to show how that health is changing over time.  As control valve performance is key to the performance of the loop and therefore the whole process, knowing when performance worsens is key to maintaining operational efficiency.

Application example – Control of oxygen to reactor at chemical plant

As part of analysing the performance of a critical valve supplying oxygen into a reactor, its travel deviations were recorded to determine the cause of process variations. The performance diagnostics showed that vibration was the most likely cause of the process variations.

Until these readings were taken and analysed, the plant piping group were unaware they had a vibration problem. By changing the mounting and strengthening the supports and expansion joints the problem was resolved.

Abnormal Situations

Should a problem be detected, the diagnostic capabilities can be used to locate the source of the problem before it is necessary to shut the process down.  Performing these tasks remotely avoids sending personnel into hazardous locations unless absolutely necessary.  This drives improved teamwork between Process, Operations, I&E Engineering and Maintenance with the objective of increasing process availability & reliability through:

• Problem detection
• Severity
• Identification
• Cause
• Corrective action

Application example – Cooling water valves at polyethylene production plant

Emerson’s digital valve controllers monitor valve performance and deliver detailed information about the health of cooling water control valves. The software is used to quiz the devices for additional information or to run advanced diagnostic tests and these include Travel Deviation alerts which are a primary indicator of control valve problems.

A Travel Deviation alert is triggered when the control valve has not moved to its required position within a certain period of time. This alert can indicate many potential faults, such as an increase in friction, air leaks, mechanical faults or a physical blockage within the control valve.  In this particular example the control valves were required to move to within 5% of the desired position within five seconds. If this does not occur, a travel deviation alert is sent.

In one incident, a travel deviation alert occurred on an 8-inch butterfly valve used to control the flow of cooling water to a reactor. The valve should have closed, but it remained open and unmoving. The maintenance team disassembled it and found a jagged, 8-inch-long metal fragment stuck inside that was preventing its closure.
Without the diagnostics’ alert, the valve would have gone unchecked until the reactor was restarted, which would have resulted in a potentially long and costly delay to production. Worse, the fragment might have travelled downstream where it could have caused extensive mechanical damage to more expensive assets.

Turnarounds

When a planned shutdown or turnaround occurs, the diagnostic capabilities can be utilised to:

• Improve turnaround effectiveness & efficiency (can reduce number of valves pulled, and shutdown pre-work can pinpoint issues before plant comes down)
• Ensure quality of repair
• Re-establish performance benchmarks and validate that valve is fit for service (avoid false start-ups)

Following a turnaround the lifecycle restarts as we are now back in normal operating mode.

Application example – Fluidised Cat Cracker Unit (FCCU) at refinery

Maintenance personnel at a refinery found that many of the control valves that had been identified as ‘poorly performing’ and added to a priority fix list, were actually in good shape when they were pulled from the line and inspected.

Adding digital valve controllers to the control valves on the FCCU enables plant personnel to remotely communicate directly with the smart positioners. Performance diagnostics provide on-line, real-time data about the valve’s status and operating condition. Using these on-line diagnostic tools, plant maintenance teams are able to accurately determine which of the valves on the priority fix list actually need to be pulled and repaired before the process came down.

This meant that over the next three scheduled outages, only 28 of the 253 valves on the priority fix list were pulled for repairs. This saved the refinery more than 900 hours of labour (4 hours per valve x 225 valves) and an estimated £100,000 in labour costs alone.

In addition, refinery team members saved about £1800 per valve in parts and labour for every valve not pulled. That represents an additional saving or avoided cost of £405,000 (225 valves x £1800) over the three outages.

Summary

For most companies, process control valves should be categorised as critical assets and users should look to protect their investment across the whole lifecycle.

Replacing traditional analogue valve controllers with digital valve controllers enables critical valves to be continuously monitored. When combined with performance monitoring and diagnostic software, start-up and commissioning are simplified, safety is enhanced and process variability minimised.

In addition, on-line performance monitoring and diagnostics enable problems to be detected early – enabling more efficient maintenance procedures to be established – reducing unscheduled downtime and providing maintenance savings.

Most importantly, the additional functions provided by digital valve controllers can be used to support performance optimisation programmes – helping process companies meet their business objectives by improving plant availability, increasing process efficiency and reducing operating costs – all of which means being more competitive in the global market.

Emerson Process Management

Can be contacted via E-Mail: InfoCentral@EmersonProcess.com
or visit www.Fieldvue.com