Q: I am designing a control system for a wastewater treatment plant. I plan to use two technologies to measure level in the digester. The digester is about 90 feet high. The first instrument is a 6-GHz radar, and the other is a differential pressure transmitter (DPT). The issue with the DPT is a very long, low-pressure impulse line that will need to be filled with a fluid, preferably some kind of very low-freezing-point oil or glycol.
Instead of having this 90-ft long impulse line, my plan is to not connect the low side of the DPT to the digester, but leave it open to atmosphere, and then add a pressure transmitter on top of the digester. The DCS system will then subtract the pressure measured by the DPT and the pressure transmitter to obtain the differential pressure to calculate level in the digester. Do you see any issue in this?
A: The error contributions of each of your transmitters will be at least 0.1% FS and can often be additive. When the maximum level is 90 ft (39 psi) and assuming a vapor pressure of 100 psig, your system will result in an error of 0.1 + 0.04 = 0.24 psi = 6.65 ins. Add to that the errors if the vapor pressure is over 100 psi, or if the vendors overstated their accuracy, or the barometric pressure varies, or the sludge temperature, density or anaerobic activity changes, and foaming or zero shift occurs, etc. The 0.1% FS error will increase with time unless you have means for accurate and frequent recalibration. So, you can easily have an error of a foot. If you can live with that, your scheme is OK.
The other option (Figure 1) is to use one DPT with either a chemical seal (filled with non-freeze fluid), or if you have compressed air available, use a pressure repeater on the low-pressure side. With this system, you can cut the error in half.
Whichever you pick, both need extended diaphragms that fill the nozzle (flush to the ID of the digester) on both the high- and the low-pressure nozzles. The pressure repeater requires more maintenance, and in some locations, the availability of compressed air or nitrogen is also a limitation. Therefore, I tend to use chemically sealed DP cells, such as the Rosemount 1199.
A: In one of my earlier projects, we had a similar problem. And, being a corrosive service, we were using a diaphragm seal type DPT. As suggested by vendors, diaphragm seal lengths have a maximum height of 30 meters, above which the response time is very high. Hence, we went ahead with the same approach, using two pressure transmitters, and doing the difference calculation in the DCS.
A: Two parameters will help guide your choices:
First, what is the required accuracy of the level measurement? Is it ±1 ins. (25 mm) or ±3 in. (75 mm)?
Second, what is the static pressure in the vessel? Let's say it's 100 psig (about 7 barg). Then both the upper and lower transmitters will need to measure 100 psig± (half of your required level accuracy). Is measuring 100 ± 0.02 psi (for the 1-in. case) plausible? It might be with the right accuracy transmitter and long-term stability specification (you probably don't want to be compelled to check calibration once a week, for example).
If the static pressure is 2 psig, then it becomes very plausible with garden-variety pressure instruments, I think, with 1% combined uncertainty from drift, temperature effects, calibration accuracy, etc.