The roll of paper coming off a paper machine is like a historical document. Whatever was happening in the process during production is captured in the sheet at each moment. If stock flows to the headbox were cycling, then there will be corresponding cycles in the finished product basis weight throughout the roll. If chemical additive flow ratios were not maintained, then sheet properties will vary throughout the roll.
Flow variability impacts the plant in other areas as well. Pulp mills, utilities areas, and stock prep systems will all suffer from efficiency losses when flow control is not properly maintained. The effects can be far-reaching. For example, imagine the energy wasted if boiler feedwater flow oscillates. Rather than producing a steady supply of constant-pressure steam, the boiler will also cycle, causing steam header pressures to cycle, causing every steam consumer in the plant to cycle. The loss in plant efficiency can be as much as 1% of annual energy consumption – that’s a lot of wasted energy and quite a large sum of money from a single poorly-performing flow.
Several sources of flow control issues
Good flow control depends on good measurement, good control, and good valve performance. If any one of these components suffers, the flow will not be well-managed, and this will directly impact the process.
The flow measurement itself can be the source of efficiency losses. For example, a noisy flow measurement means that the controller will move a lot, forcing the valve to move a lot. This valve movement in response to noise adds no value. In fact, it consumes a lot of expensive compressed air, and amplifies the instrument noise, turning it into a real process upset. Real-world experience shows that this sort of noise-induced movement can also reduce valve life by more than half.
Controller tuning and coordination between flow loops is also important to peak efficiency. As a general rule, every oscillating controller is causing a loss of efficiency. When a controller swings +/- 5% around the setpoint, there is a 1.7% loss of process efficiency. It should therefore be a big priority to stabilize controls and capture the efficiency benefits.
Valve performance can also directly impact efficiency. Valve sizing directly affects pumping energy. Mechanical valve issues, like stiction, can induce process cycles and reduce efficiency in the same way as controller tuning issues. Whenever you address any of the before mentioned issues, you are also positively affecting the environmental performance of your process.
Finding the root cause of flow control problems
With hundreds of process flows to manage, it can be difficult to find and prioritize all the flow control issues. Fortunately, automated tools like Metso’s Expertune PlantTriage can both find and prioritize the issues quickly. At the same time, these digital tools are helping us improve the sustainability of operations by optimizing energy and raw material consumption.
It is also critically important to be able to directly evaluate the true cause of the issue. The figure below shows the live valve signature for two different cycling controllers. The one on the left comes from a flow controller that oscillates due to a tuning problem. The one on the right shows the controller oscillating due to valve stiction. By knowing which is which, you can dispatch the right person to fix the right problem. There is no point in tuning a loop when it is the valve’s air supply that needs attention!