Refresher course on what makes a valve package reliable
When considering control issues in a modern process plant, it is easy to get confused by the complexity of it all. The control system is usually the part plant owners concentrate on most. However, it is sometimes easy to forget that the last control element – the control valve – sets the baseline for control. If the baseline is good, then it forms a good foundation for smooth plant operation. So what are the things both an experienced engineer and a fresh graduate should keep in mind about control valves?
Control valve package
The control valve package and the way it is built are the first crucial factors to consider. The control valve package consists of a valve, an actuator and a positioner. Ideally, all these components should come from the same manufacturer to guarantee that the package is designed and tested to work well. If a multivendor package is selected, the one who takes the lead responsibility for the overall performance should be carefully considered – which is not a minor matter for the final control element.
Valve material selection
When it comes to the centerpiece of the control valve package – the valve – its material should be selected based on the specific application needs. The trim type matters most for efficient control. In a modern control valve, there are typically tens of different types of trims from which to choose. They vary from normal trims to multi-hole Tendril-type trims (below the illustration on the left) – and all the way to multi-channel Omega-type trims (below the illustration on the right) that are used in the most demanding applications.
Variations in flow characteristics and capacities then need to be considered for the basic trim. It is important to make this selection carefully to achieve the best possible control performance. Even though some modifications to the control can be made with modern digital positioners, they cannot do more than fine-tune the behavior if the mistakes have already been made in the trim selection.
The muscle moving the valve – an actuator
An actuator is the muscle that moves the valve. In many cases, a pneumatic diaphragm actuator is used because air is safe to use in almost any process and is easily available from any plant. The diaphragm actuator offers very low friction and smooth operation. It is critical to make sure that the equipment is well tested and has a long operational lifetime. A well-designed actuator can provide hundreds of thousands of cycles of care-free operation.
Positioners for accurate control
Positioners have been developing rapidly over the past few years. Nowadays, the digital positioner is the standard for most control valves. Digital positioners, like Neles NDX®, offer high air capacity for a fast response and reduce the need for additional instrumentation.
In addition, digital positioners are filled with features that enable end users to achieve accurate control. Autocalibration and different performance settings are included to help out with this. Furthermore, intelligent diagnostics can check the valve condition and predict the right timing for service.
Valve sizing software
Control valve sizing is another area where modern software provides a big benefit compared with the old-school way. In the past decades, the rule of thumb was that the control valve should be one or two sizes smaller than the pipeline – that’s it. Today’s selection and sizing software, like Nelprof™, take into account the piping, the properties of the flow medium and the valve’s installed behavior to select the correct valve according to the application. It’s crucial, for instance, to analyze the installed behavior and not just the inherent Cv curve.
Basics remain unchanged
In the future, many innovations in electronics and software will help support the control valve. But the basics will remain unchanged – a valve’s mechanical features, especially the trim, as well as the valve selection and the overall control valve package quality will always have a key role.
Therefore, it is essential to entrust your control valve selection to a reliable and knowledgeable supplier. At Metso, we continuously prove that by delivering decades-long, field-proven solutions and high professionalism, we are able to make a true difference for our customers.
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