Make The Right Opening Move

Readers suggest how to relate valve position to flow.

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THIS MONTH'S PUZZLER
We want to increase the recycle flow from our sulfuric acid alkylation wash tank. The mixture going through the recycle is mostly alkylate (butanes, iso-butanes and propanes) along with 98% sulfuric acid. We're having trouble linking the percent open of the control valve with the actual flow measured across an orifice plate (Figure 1). Why are we having problems? Do we need a larger valve?
 
FIRST CHECK THE DATA SHEETS
As your flow changes, the upstream and downstream pressure available for the valve to operate will change as well. This will impact the flow calculated by using valve position. If you are using constant upstream and assumed downstream pressures to get flow from the valve position, it may not be very accurate. Besides pressure, phase is also important.
 
Make sure that your fluid doesn't change phase upstream of the control valve or the flow meter. This will make measurements inaccurate. Make sure that both the flow meter and valve are sized for the same type of fluid, generally 100% liquid or 100% vapor.

Make sure all the stream properties for specifying the orifice plate and valve are consistent. Check the flow transmitter calibration as well. Temperatures, pressures, density differences, etc. will impact flow comparisons between the flow meter and the valve position calculation. And, finally, verify valve position on your distributed control system with its position in the field to be sure that the positioner does not need recalibration or replacement.

Once you have validated and, if needed, corrected any of these items, go back to see if the orifice measurement and valve position calculation matches more closely than before. If not, you may need to determine if your flow regime supports using an orifice plate for flow measurement. Verify if you are in the laminar or turbulent flow regime. You may need to select a different flow measuring device depending whether you are in the laminar or turbulent flow regime.
Eric M. Roy, principal engineer
Westlake Chemical, Sulfur, La.

GIRD FOR A TOUGH TASK
Based on the curve, the valve position is fairly high if the flow is increased more than about 25%. Because the valve flow coefficient, Cv, goes up dramatically as a centrifugal pump requires more head to overcome the pipe flow resistance, the percentage open also increases in a similar dramatic fashion (Cv = Q×(SG/DP)0.5) [Q being flow rate; SG specific gravity; and P pressure]. Most control valves are equal percentage and therefore shouldn't operate in their normal range above 70%. You could experiment with a linear valve or one with a characteristic curve that's in between. Determining the current valve performance is difficult. There's no way to match the actual flow to the valve position on a timeline. They're separated in space and valves are far too complicated.

Tuning, though often a problem, and an easy solution, probably isn't the problem here. The control valve is a complex machine consisting of more than a plug and a port; there's the actuator with its spring, solenoids, seals, etc., and the linkage in between the valve stem and the actuator.

You might try averaging the flow meter output and the control valve position to steady the trends. While this may help, there is a problem: the valve and flow meter will have built-in control filters that will never cooperate effectively.

Given the service, a sticky residue could build up on the plug, causing stiction between the plug and seat. A review of the maintenance files can verify this. There also could be problems with the valve stem seals. The spring could be corroded by exposure to acid. All of these resistances could cause the valve to move in a stepwise fashion rather than a smooth glide. The resistance, working together with the control signal, can conspire to create hysteresis, an oscillation that results in added response deadtime. If averaging the percent open, choose a midpoint on a hypotenuse of the backside of the step.

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