Measurement Location Tips

The question for the day is where to locate measurements. My first choice would be a Caribbean island but if the plant is not there, the sensing or sample lines and the associated transportation delays would be quite long. The additional loop dead time would cause all sorts of performance problems that might take years to troubleshoot. I might even have to move to the island. Hmm, sounds good for my golden years.

I have seen sensor locations chosen based purely on cost or ease of accessibility rather than performance. Technicians need to be able to safely remove and replace sensors but many of the accessibility issues can be reduced by better performance and by smart instruments, maintenance software systems, and ultimately wireless communication.

When considering the location in terms of loop performance the objectives are to:

1. Insure a representative measurement point

2. Decrease measurement noise

3. Increase sensor reliability

4. Reduce sensor fouling

5. Improve sensor response time

6. Decrease transportation delay

Maintenance and operations are often aware of items 2 through 4. Item 1 requires some process understanding, item 5 requires some testing or info from the vendor, and item 6 can be simply estimated as the length of the path to the sensor divided by the velocity.

For liquid pressure and flow measurements, the delay is negligible except for very long distribution lines because the pertinent velocity is a pressure wave traveling at the speed of sound in the fluid. For liquid composition and temperature measurements the transportation delay can be large because the pertinent velocity is 0.5 to 5 fps. For gas measurements the delay is a lot less since the velocities are 10 to 50 times larger but may still be a concern because the process dynamics are quite fast for throttling of feeds for gas unit operations and fuels for furnaces.

For nearly all liquid process measurements, a partially full pipe line is bad news. Air pockets in the meter, sensing lines, and at the sensor can cause failure to meet all 6 of the performance objectives noted. Trapped bubbles can become pockets. Unless there are some extenuating circumstances, sensors should be located upstream of control valves but downstream of pumps to help insure the line is full and to minimize bubbles from flashing vapors. I prefer pump strainers rather than sensors to catch solids and wrenches.

For desuperheaters, the measurement location must be far enough downstream to ensure there are no water droplets. It takes about 0.1 seconds for water to vaporize and another 0.1 seconds for the stream to be mixed. This does not sound like much of a requirement for distance from the outlet of the desuperheater to the sensor until you realize the pipeline velocity might be 250 feet per second.

To get the point, increase the signal to noise ratio and reliability, not get all fouled up, and improve the speed for making decisions on how to improve location, let’s consider briefly the measurement of flow and pressure.

For flow measurement, the next most important consideration, particularly for vortex and differential head meters, is to make sure the velocity profile is uniform. Erratic and changing flow profiles cause poor measurement repeatability and noise. Flow meters must be located upstream of control valves with sufficiently long straight run upstream and downstream. The number of pipe diameters of straight run needed depends upon the meter and the piping details such as elbows, gate valves, and fittings. The number increases for flows at the low end of the meter capacity. A good but old resource for the straight run and instrumentation installation requirements in general is the Manual on Installation of Refinery Instruments and Control Systems – Part I (API RP 550).

Magmeters have a very minimal and Coriolis meters have essentially no straight run requirements although I wouldn’t recommend flanging any meter directly to a valve. A properly selected and installed Coriolis meter provides the most representative, accurate, and noise free flow measurement. It offers true mass flow independent of composition, something you cannot achieve even with even the fanciest pressure and temperature compensation.

The rate of coating build up tends to decreases as the fluid velocity increases. Since many meters have a rangeability based on minimum velocity, sizes that are too large will tend be get more fouled besides erratic at low flows. On the other hand there may be maximum velocity and meter sizes that are too small may have excessive pressure drop or cause excessive erosion when solids are present.

For pressure, the direct close coupled mounting of the sensor to the process eliminates the sensing lines and associated concerns about accumulation of solids and vapor pockets, freezing from inadequate winterization, and vaporization and cooking from excessive heat tracing.

For temperature and composition measurement transportation and mixing delays can be significant.. For conveyors and paper machines, the delay is the equipment length divided by its speed. For sample and process piping, the delay is the total length of piping including fittings divided by the fluid velocity. For extruders, heat exchangers, and static mixers, the delay is the equipment volume divided by the total volumetric flow. For agitated vessels, the mixing delay is the turnover time (liquid volume divided by the summation of the volumetric feed, recirculation flow, and agitator pumping rate). For dip tubes, the transportation delay is the dip tube volume divided by the volumetric dip tube drain flow. It gets worse. When the control valve closes to the dip tube, the stuff inside the dip tube slowly migrates into the vessel. For sensitive systems such as pH, there can be a noticeable drift for several hours.

Is this the whole story for process dead time?

Based on transportation delay and conventional wisdom, it would be best to locate an electrode or thermowell in the vessel rather than a recirculation line. Is this always the case? The electrode and thermowell time constant can be larger by 20 or more seconds in a vessel because the local fluid velocity is typically less than 0.5 fps compared to the 5 fps in a recirculation line. If the measurement location in the recirculation line is within 20 feet of the vessel, you are only talking about 4 seconds of delay. If you consider the electrode or thermowell tip may be too close to the wall and is much more likely to foul and develop a coating at low velocities, you have the scenario for sensor time constants greater than 60 seconds besides the need for more frequent cleaning. If there are not any safety issues, I prefer the faster and cleaner sensor you get in a recirculation line (e.g. elbow about 20 pipe diameters downstream of pump with sensor tip near the center of the line). For exothermic reactors, temperature sensors inside the reactor may be required to eliminate any possibility of pump or recirculation valve failure preventing a sensor from seeing the process temperature.

It is important to remember the loop dead time is not just process transportation and mixing delays. You need to add sensor time constants, final element dynamics (delays, lags, deadband, and resolution), signal filter time constants, and digital scan and execution times.

The final point here is that a measurement location chosen purely based on accessibility is a self-fulfilling prophecy in that maintenance will need more accessibility because of more performance problems. The more frequent service, removal, and handling of sensors have important safety implications.

See the January 2008 Control Talk Column “Straight Talk” and the November/December 2010 InTech article “Temperature measurement, control key to plant performance” for more details on this and other considerations for sensor location.