COMMUNICATIONS maketh the intelligent valve

Smart instrumentation and digital fieldbuses are changing the face (and functionality) of automatic valves BY wally stommes

`Smart systems may be configured for each valve, and locally perform diagnostics via a few inputs – the benefits are many’

Valve monitors fulfil a vital rOle as part of automated valve systems – they verify valve position. Without them, problems may occur – with the computer merely assuming proper valve function. A batch of ingredients may be ruined simply because a valve that should have been open was actually stuck closed. Equally, lives may be put at risk.

So valve monitors are being used ever more extensively. In fact, in recent years, valves have had to become more controllable and reliable, and offer reduced maintenance requirements to fit in with shut-down periods. Safety and the environment have also become real concerns – and these too have dramatically changed valve monitors.

Naturally, valve systems have to communicate a number of parameters, and alert operators to a range of problems. So they are fast turning into intelligence centres for valves.

Today, systems are being used to better evaluate a variety of valve performance characteristics. On modulating valves, the control signal is compared to the valve position to determine valve stiction. If excessive stiction is apparent, costs may be increasing and quality compromised. During start-up, a performance review also allows engineers to quickly determine if the valve system is functioning as it should.

Tracking flow vs valve travel may also provide clues to valve wear and process performance. Curves compared over time will indicate changes in, for example, flow at the same valve position. With a reference valve travel vs flow baseline, significant deviations can be alerted to operators as a potential problem. Also, as performance history is accumulated, a greater understanding may be gained of optimal valve repair and/or replacement timing.

Predictive diagnostics

Then again, by measuring actuator travel speed and pneumatic air pressure, valve torque requirements may also be determined on on/off valves. Significant changes then provide warning of excessive valve wear and the potential for catastrophic failure. Combining this data with life cycle specifications, temperature levels and other variables, an algorithm may be developed to determine a maintenance and/or replacement regime.

Another predictor of valve performance is packing gland pressure level. Increasing pressure between the primary and secondary packings is indicative of primary seal failure – increasing the potential for stem leakage, which results in fugitive emissions.

A particularly low cost method to predict problems at the actuator is checking pneumatic supply pressure. If it’s below a threshold, the actuator simply won’t be able to power the valve open. Alerting personnel to this fact allows maintenance to remedy the problem before it translates into downtime.

Contemporary monitoring systems are now incorporating smart technology to analyse valve performance in the field. Smart systems may be configured for each valve and locally perform diagnostics via a few inputs. The benefits are many, including the ability to keep algorithms and processing power at the valve system without burdening the PLC or DCS. Should an alert condition or deviation occur, they can transmit findings to the control system on a need-to-know basis.

Care must be taken to select the appropriate communications. On modulating applications, HART is commonly used over the 4-20 mA wires for data interchange. However, because HART is not multi-drop, it may be cost prohibitive. Alternatively, there are proprietary bus protocols. But although you get the benefits of a fully digital multi-drop system, you are also locked in to the manufacturer concerned.

Standardised digital communications are desperately needed. And, the good news is that there have been enough developments recently to lead the author to conclude that they are now approaching fast.

Fieldbus Foundation has completed testing on a final standard, and suppliers like National Instruments, Softing and Ship Star are now offering chips and support systems. Others, such as LonWorks and Profibus, are also available. Device bus systems, like DeviceNet and SDS, are also fully tested, with manufacturers racing to bring field instruments to market.

Installing smart systems

At the simplest level AS-Interface and Seriplex have already been field proven, and a variety of manufacturers are offering appropriate products. Although varying in sophistication, all offer digital multi-drop comms – and thus significant installation savings.

Beyond the usual benefits which accrue from digital bus networks, there are several critical points for valve automation systems. Typically, harnessing smart systems you can integrate control input data, position status and auxiliary sensor inputs, such as air supply pressure and packing gland pressure. With the on-board intelligence and the digital comms, field-centred predictive diagnostics becomes a reality.

Further, with digital bus systems it becomes viable to perform control at the valve. This may be switching solenoids for on/off control with an auxiliary power bus, or operating an I/P on a positioner. And thus, a number of intermediate control possibilities emerge.

Possibilities include:

* Fill control (dribble control) – the capability to top-off tanks or hoppers by holding the valve in a predetermined third position.

* Flow damping – the ability to rapidly close large butterfly valves without water hammer. This can be done by quickly closing during part of the travel and then slowing.

* Reducing thermal shock – opening steam valves slightly to a third position until the piping has heated sufficiently; then fully opening the valve to allow maximum flow.

* Emergency shut-down checking – valves may be closed slightly (to 85%) and reopened to check for emergency shut down functionality without shutting down the process.

In all of these, more control is being exercised with minimal additional cost – provided the field device is already intelligent and controllable. Clearly, standard fieldbuses are the key, and they’re coming. So what can we do to prepare for their arrival?

* Prepare a training program to get all of your key instrument engineers up to speed on digital systems. A number of fieldbus user groups are providing seminars, and much of the information is generic. Industry Shows also provide useful forums from which to explore how they differ.

* Consider intrinsically safe (IS) systems where possible for hazardous areas. The familiarity gained with IS electrical practices and their low energy philosophy lend themselves to digital comms.

* Use a pay-back period of no more than three years when considering proprietary, or HART-based comms. Standard fieldbus promises to offer so much more that it may be worth designing systems over the next two years with bus upgrades in mind.

* Voice your ideas and concerns for instruments running on fieldbus to valve and instrument suppliers. They are now developing network-based products – make sure they are designing with you in mind.

Valve comms and control will undergo dramatic changes over the next five years. Those able to harness the changes will reap significant benefits.

The time to begin making investments which should translate into returns in the late `90s is now.

* The Author is with StoneL Corp.