Ask Greg McMillan

Greg's Response:

If the reagent pipeline is partially filled or empty or a dip tube is backfilled with process fluid, the regent delivery delay becomes the biggest source of dead time in a pH loop for the small reagent flows commonly required for neutralization. Whenever a reagent control valve closes or a metering pump stops, the reagent continues to drain into the process, and process fluid can be forced back up into the reagent injection or dip tube. Even if hydraulics do not promote much drainage or backfilling of the dip tubes, ion migration from high to low concentrations will proceed until an equilibrium is reached between the concentrations in the reagent tube and the process volume. As a result, the pH will continue to be driven by the drainage and migration of reagent after the control valve closes or the metering pump stops. If the valve is closed or the pump is stopped for a long time, when the valve opens or the pump starts, it must flush out process components in the tubes before the reagent gets into the volume. The worst-case delivery time delay is the volume of the backfilled tube divided by the reagent flow. Because dip tubes are designed to be large enough to withstand agitation and the design standard for normal flows is to take the dip tube down toward the impeller, the reagent delivery delay can be several orders of magnitude larger than the turnover time.

If reagent piping is totally filled with a constant concentration of noncompressible liquid reagent, a change in valve position initiates a change in reagent flow within a second or two. An automated on-off isolation valve close-coupled to the process connection that closes when the control valve throttle position is below a reasonable minimum helps keep reagent lines pressurized and full of reagent. Injection of reagent into recirculation lines with a high flow rate just before entry into the vessel instead of via dip tubes offers a tremendous decrease in reagent delivery delay.

Dynamic simulations with piping and dip tube dynamics are needed to determine the best liquid reagent delivery design.

For much more knowledge, see the ISA book Advanced pH Measurement and Control Fourth Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).