The vessel level trend in Figure 4 exhibits a cycle with a period of approximately 200 min. The total duration of the trend is only 12 hr., though, which isn't long enough to be certain a cycle definitely exists. However, let's assume it does. A TI of 24 min is far shorter than half the period of the cycle. Reducing Kc to 2 %/% from 46 %/% mandates a significant increase in TI; so it seems logical that decreasing Kc to 0.4 %/% from 2 %/% also should require an increase in TI.
Figure 5 presents the performance for a Kc of 0.4 %/% and a TI of 120 min, which is twice the residence time of the vessel. A cycle still might exist but it's not as evident or as regular as the one in Figure 4. Even so, its period is roughly 3 hr, so a TI of 2 hr exceeds half the period of the cycle.
The performance in Figure 5 generally is preferable to that in Figure 4 (with a TI of 24 min). The variations both in vessel level and discharge valve opening (and consequently discharge flow) clearly are less than in Figure 4. This underscores the need to use long values of TI when Kc is very low.
LONG RESET TIMES
Have you ever seen a controller tuned with a TI of 120 min? If not, there's a very good reason. With conventional controllers, the maximum possible TI usually was around 30 min. With early distributed control systems, the maximum possible TI was comparable (maybe up to 45 min). Even today, some digital control products impose the format of XX.XX on TI, thus limiting the maximum to 99.99 min.
If the required TI exceeds the maximum imposed by the control system, you can either set TI to the maximum allowed by the control system or remove the reset action entirely, which results in proportional-only control. The latter is preferable.
Although advocated by some for level control applications, proportional-only control isn't popular. Most controller implementations permit the reset mode to be "turned off" by entering an appropriate value for the reset tuning coefficient. A value for MR is computed when the loop is switched from manual to automatic. Thereafter, you must make any changes in the value of MR manually. However, manually adjusting MR raises two issues:
Entering a value of MR. Some controller implementations neither display the value of MR nor permit entering a value for MR. Given the rare use of proportional-only control, this isn't surprising. Fortunately, workarounds usually are possible.
Manual reset. In years past, a practice known as "manual reset" basically involved adjusting MR. However, as most loops are PI or proportional-integral-derivative, very few operators now are trained to make adjustments in MR.
While proportional-only control is perfectly acceptable for most level applications, these two obstacles are difficult to overcome.
CECIL L. SMITH is president of Cecil L. Smith, Inc., Baton Rouge, La. E-mail him at email@example.com. This article is based on concepts from his book "Practical Process Control," published by John Wiley & Sons.