Statistical analysis may help. The idea would be to group together the valve response points with corresponding flow rates. Then, with statistically relevant sets, say, about 20 points minimum, it might be possible to estimate the percent open matching the flow. An analysis of this sort is useless if the valve and the flow meter are significantly far apart geographically. With that said, this will allow you to generate a flow versus Cv curve for the valve.
The next step will be to use this information in a hydraulic simulation to estimate the valve position with higher flow. The solution probably involves an iteration to adjust for changes in physical properties through the valve but the answer should tell you if you're below a reasonable position for the valve, say, 80% for a typical globe valve. Of course, this approach assumes the pumps providing pressure to the tank are sufficient — that's a different question.
Dirk Willard, lead process engineer
Fluor Global Services, Inver Grove Heights, Minn.
CONSIDER A SOFT VALVE POSITION CONTROLLER
To illustrate how to use the valve position to control the flow process value, we will use the following nomenclature: FC.PV for flow process value (PV); FC.OP for flow control output (%); FC.VP for flow control valve position (%); and FC.SP for flow process value set point.
The valve output flattening with time suggests that the FC.PV is overshooting the missing FC.SP. With the valve position behaving the way it is, the FC.PV will be difficult to control its set point with FC.OP as the manipulated variable.
The underlying issue here is control of the flow process value FC.PV with the valve output FC.OP given the variability in the valve position FC.VP for changing value of the valve output FC.OP. This can arise depending on the internal valve mechanism of actuation. Most certainly, this flow control valve doesn't have a valve positioner but needs one. A soft valve position controller solution is outlined without the expense of a hardware-based valve position controller.
It is a simple model-based control solution involving the causal relationship chain: FC.OP => FC.VP => FC.PV. It's clear from the causal chain that to control FC.PV it is pivotal to control FC.VP. Only in a well-behaving FC.VP, would controlling FC.PV with FC.OP work reliably.
A soft valve position controller requires several models. These can be identified in real time with the controller in closed loop or in manual. The soft controller can compensate for valve stiction as well.
Both FC.VP and FC.PV regulation can operate simultaneously albeit with different cycle times if necessary. (The valve position control loop is faster than the flow process value control loop. A fuller description of the details relating to the basis of the soft valve position controller can be found at www.unified-control.com/VirtualSoftValvePositioner.html).
The soft valve position controller provides an alternative to a hardware-based solution and one that would work with any valve in a number of ways with little expense and maintenance.
Din Attarwala, president
Unified Control Technology Corp., Princeton, N.J.
An explosion killed a welder who was using a torch in an attempt to cut loose an anchor bolt on top of a polymer atmospheric overflow tank, in preparation for removing the tank's agitator. The original work permit covered removal of the anchor bolts by pneumatic tool but the bolts were rusted to the nuts and wouldn't budge. Because the window for removing the agitator was closing, maintenance higher-ups okayed using the torch. The welder thought he could get at the bolts from the outside, away from the agitator shaft, which didn't include a seal. The clearance space between the tank and agitator shaft was open; calculations showed the monomer didn't produce sufficient vapor pressure to form a flammable mixture at normal temperatures — so, the vessel was classified as an uncovered process. The overflow tank was connected by vent to two others. Lockout procedures were deemed safe: all the intake lines were isolated except for the common vent. The plant continued to operate, filling the adjacent overflow tank while the welder worked. Hydrocarbon sniffing took place before the welder began but not during the welding. Safety decided the mishap was the welder's fault. What do you think?
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