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.
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 composition and temperature measurements the transportation delay can be huge because the pertinent velocity is the fluid velocity (e.g. 0.5 to 5 fps).
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 down stream 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.
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 this week. I will hold off on my personal favorites of composition and temperature till next week.
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.
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.