FGD Density Meter Selection Guide: Complete Point-by-Point

Steps

1. Point 1 — Limestone slurry preparation tank / discharge

   Typical range 10–30 % solids, coarse and highly abrasive, agitator-driven air entrainment. Density here sets the reagent feed to the absorber, so an error propagates into desulfurisation efficiency and reagent cost at the same time. Recommended: PS7000 — the sensor face sits flush with the bore, so nothing protrudes into an abrasive stream, and the gas index output lets an operator distinguish a genuine concentration rise from agitator-entrained air. Install on the discharge line rather than in the tank, and prefer a vertical rising section. If a coarse-ground limestone circuit varies its grind, note that attenuation-based methods are more sensitive to that than impedance-based ones.

2. Point 2 — Absorber gypsum slurry recirculation main

   The largest lines in the system, commonly DN400–DN800, running 15–25 % gypsum solids with substantial entrained oxidation air. Bore size alone eliminates several options: transmission-type instruments are bounded around DN500. Recommended: PS7000, which covers DN50–DN1000. This is the point where the pump-running interlock matters most, because recirculation pumps cycle and the stagnant-line artefact is easy to mistake for a process excursion. Where the line cannot be tapped at all and diameter permits, a clamp-on PS7010 is the alternative — but confirm the air content first, because oxidation air is exactly what a transmission path tolerates least.

3. Point 3 — Gypsum discharge / dewatering feed

   Higher solids, typically 20–50 %, feeding hydrocyclones and vacuum belt filters. Density controls the dewatering system and therefore gypsum moisture and saleability. Abrasion is at its worst here. Recommended: PS7000 on the discharge pump outlet, but not immediately at the pump flange — allow straight run so that turbulence and pump-entrained air have settled. If the measurement is used to control filter feed rate, short-term repeatability matters more than absolute accuracy, which is worth stating in the specification because it can widen the feasible set.

4. Point 4 — FGD wastewater discharge

   Low solids, chemically aggressive, often high chloride, and increasingly subject to discharge limits that make the measurement a compliance record rather than a control input. Materials selection dominates here: confirm wetted parts against the actual water analysis, not against a generic FGD specification, because chloride content varies widely between plants and coal sources. For low-solids service a PS7400 tuning fork with an appropriate wetted material is often a better fit than a slurry instrument, since the solids loading no longer justifies one.

5. Point 5 — Ammonia water / urea solution for SCR

   This is a different measurement problem from everything above: a clean binary solution, no solids, and concentration rather than slurry density is what matters. Typical duties are 20 % ammonia water or 40 % urea, where concentration drives NOx removal and reagent cost. Recommended: PS7020, using sound speed, which resolves clean binary solutions far more finely than any slurry method — three accuracy classes are offered, corresponding to ±0.0005 / ±0.001 / ±0.005 g/cm³. Two prerequisites must be confirmed at enquiry: the solution must be bubble-free, and the sound-speed-to-concentration curve must be monotonic across your working range. Ammonia is also a safety consideration, so confirm the area classification and which certification scheme applies.

6. Point 6 — AFT tower / high-concentration recirculation slurry

   Advanced flue-gas treatment towers run higher concentrations than a conventional absorber and often combine functions in one vessel. Two field lessons apply. First, mounting on the tower wall rather than in a pipe puts the sensor where local conditions may not represent the bulk — if there is a recirculation line, measure there instead. Second, where aeration or dual pumps operate near the sensor, bubbles raise the reading and add noise; with an energy-based diagnostic, high gas content and high solids can look similar, so the gas index output is not a luxury here but the thing that lets you tell them apart. Recommended: PS7000 on a recirculation line, with the pump-running interlock configured.