Reliable control valves – foundation for process performance and uptime
Director, Control Valves Product Center
When considering control performance 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, one should not forget that the last control element – the control valve – sets the baseline for accurate control. Only if the baseline is good, it forms a good foundation for smooth plant operation and performance. So, what are the things to keep in mind about control valves?
The control valve and the way it is built are the first crucial factors to consider. The control valve unit consists of a valve, an actuator and a positioner. Ideally, all these components should come from the same manufacturer to guarantee that the whole unit 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.
Valve material selection
When it comes to the centerpiece of the control valve unit – the valve – its materials should be selected based on the specific application needs and conditions. Selecting the correct control valve trim type is important for and accurate control performance and operation. Typically, there are 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 a 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, 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 the 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 quality will always have a key role.
Therefore, it is essential to entrust your control valve selection to a reliable and knowledgeable supplier. At Neles, 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.
This blog post has been up-dated in July 2020, due to company name change to Neles.
What do you look for in a control valve supplier? When it comes to control valve requirements no two are ever the same. It is not just the pressure and temperature of the process or the required controllability, but also issues such as the need to prevent cavitation, to control noise, to keep within emission limits, etc. This means selecting a control valve always requires due care and attention.
Control valve requirements often differ depending on the process area, application and end user specifications. Process conditions and the range of parameters also vary widely, including pressure, temperature, controllability, cavitation risk, environmental impact, noise and more. Plus, priorities may differ. An end user may want to know more about a valve’s performance, controllability and total life-cycle cost, whereas an EPC contractor may be more concerned about the CAPEX-related costs and delivery times. How do you know where to start – and where to turn for help with quality information you can rely on?
Successful control valve sizing and selection depends on knowing the actual process conditions in which the valve is to be installed. This information is rarely available firsthand. The more assumptions one has to make on flow conditions, the less accurate the control valve sizing is going to be.