Emerson Exchange 365

In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Mohd Zhafran A. Hamid.

Mohd Zhafran’s First Question

If you have selected a control valve whose installed flow characteristics significantly deviates from linear (either by mistake or forced to select due to certain circumstances), what is a practical way in the field after installation to linearize the installed flow characteristic?

Greg McMillan’s Answer

You need a sensitive flow measurement to identify the installed flow characteristic online. If you have a flow measurement and make changes in the manual controller output 5 times larger than dead band or resolution limit spaced out by a time interval greater than the response time, the slope of the installed flow characteristic is the change in per cent flow divided by the change in per cent signal. You need at least 20 points identified on the installed flow characteristic.

A signal characterizer is then inserted on the controller output to convert the flow in percent of scale to percent signal to get a piecewise linear fit that would linearize the characteristic so far as the controller is concerned. The controller output and linearized signal to the valve should be displayed. This linearization can be done in a positioner, but I prefer it being done in the DCS or PLC for better visibility and maintainability. For much more on signal characterizers see my Control Talk blog Unexpected benefits of signal characterizers.

Mohd Zhafran’s Second Question

I recently read the addendum “Valve Response Truth or Consequences” in Greg’s article How to specify valves and positioners that do not compromise control. I am curious for fast loop whereby the control valve is used with volume booster but without positioner, how come you can move the stem/shaft by hand only even though the valve size is big. Would you mind sharing the overall schematic? Also, would you also share the schematic of using positioner with booster and booster bypass?

Greg McMillan’s Answer

Positive feedback from a very sensitive booster outlet port is greatly assisting attempts to move the shaft either manually or due to fluid forces on a butterfly disk as described in item 5 of my Control Talk blog Missed opportunities in process control – Part 6. There is a schematic of the proper installation in slide 18 of the ISA Mentor Program webinar How to Get the Most out of Control Valves. I don’t have a schematic of the wrong thing to do where the volume booster input is connected to current to pneumatic transducer (I/P) output.

For new high pressure diaphragm actuators or boosters with lower outlet port sensitivity, this may not happen since diaphragm flexure and consequential change in pressure from change in actuator volume may be less than booster outlet port sensitivity but it is not worth the risk in my book. The rule positioners should not be used on fast loops is mostly bogus as explained in my point 4 in the same Control Talk blog.  If you need a response time faster than 0.5 seconds, you should use a variable frequency drive with a pulse width modulated inverter.

Mohd Zhafran’s Third Question

Greg highlighted the importance to specify valve gain requirement. Is there any publicly available modeling software that we design engineer can utilize to perform valve gain analysis? So far, I have encountered only one valve manufacturer that provides control valve sizing software (publicly available) with feature of valve gain graph. This manufacturer calculates process model based on the principle that the pressure losses in a piping system are approximately equal to flow squared.

Greg McMillan’s Answer

The Control Talk column Why and how to establish installed flow characteristic describes how one practitioner uses Excel to compute the installed flow characteristic. The analysis of all the friction losses in a piping system can be quite complicated because of the effect of process fluid properties and fouling determined by process conditions and operating history and the piping system including fittings, elbows, inline equipment (e.g., heat exchangers and filters), and valves.

A dynamic model in a Digital Twin that includes system pressure drops and the effect of fouling and the ability to enter the inherent flow characteristic perhaps by a piecewise linear fit can show how the valve gain changes for more complex and realistic scenarios. Ideally, there would be flow and pressure measurements to show key pressure drops particularly where fouling is a concern so that resistance coefficients can be back calculated. 

The fouling of heat transfer surfaces can be detected by an increase in the difference needed between the process and utility temperature to compensate for the decrease in heat transfer coefficient. A slow ramp of the valve signal followed by a slow ramp in a flow measurement could reveal the installed flow characteristic by a plot of flow ramp versus the signal ramp assuming there are no pressure disturbances and flow measurement has sufficient signal to noise ratio and rangeability.