For example, if the flow through a pipe can be from 0 – 10,000 gpm, and you are adjusting the speed of a VFD from 0-100%, the starting gain and reset would need to be 0.004 and 0.02 instead of 0.4 and 2.0. If the spans are different, corrections would have to be made to the parameters themselves. Some controllers handle tuning parameters based on percent of span, while others do not make this correction. These recommended starting parameters are based on the input and output ranges being the same. Loops where the PV changes slowly, or changes its direction of movement due to change in OP (temperature and level in vessels with slow turnover) typically need high gain (3 – 100) and low reset (0.05 – 0.3). Loops where the PV changes quickly due to a change in OP (flow, or pressure or level in vessels with fast turnover) should have low P-gain (perhaps 0.2) and higher reset (1.5 – 10 rpm).
These sometimes need moderate-to-high gain and less integral. For the latter, a characterization is more subtle – you want to characterize the slope of the PV for various OPs instead of its value. For others (such as level) the direction (slope rather than value) of the PV is relative to the OP. These should be characterized as shown above – they typically need more integral and minimal P-gain. Generally these PVs begin to show the result of an OP change immediately (even if the time constant to complete the change is long) and do not need any derivative.Īnother categorization of PVs: some (such as flow) increase when the OP increases and decrease when the OP decreases. These tend to have a significant delay between a change in OP and the beginning of the change in PV (#2 above), and therefore might benefit from derivative action.īulk properties describe the state of the fluid as a whole so that it all changes everywhere in a pipe or vessel (for practical purposes) simultaneously. Particle properties include temperature, pH, conductivity, etc. Particle properties are those where a fluid in a pipe may have different properties in different areas, so that the fluid must be mixed or moved to change the property at the PV measurement point.
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