Top Twelve Mistakes in pH System Design

The extreme capability of the pH measurement and the broad spectrum of applications and process conditions create exceptional requirements for electrode design, mixing uniformity, control valve precision, deadtime minimization, nonlinearity compensation, and control strategies. pH loops are often the most challenging loops in the plant. Slight deficiencies in the mechanical, piping, process, measurement, and control system design and installation can cause the pH loop to oscillate and fail to meet objectives.

In order for the control system to work, special attention must be paid to the design and installation of electrodes, transmitter, controller, control valve(s), piping, and mixing equipment. The process, mechanical, and instrument engineers all must be alert to special system requirements from the inception of the project. The top twelve mistakes are summarized below. Ten of these are illustrated in the Figure, which shows the use of a horizontal tank, gravity flow reagent, and a ball valve without a positioner.

Top Twelve Mistakes Made Every Day in pH System Design

1. Incorrect or missing titration curve
2. Absence of a plan to handle failures, startup, and shutdown
3. Insufficient number of stages of neutralization (inadequate rangeability and sensitivity)
4. Improper vessel geometry and agitation patterns (excessive equipment dead time)
5. Backfilled reagent dip tube (excessive reagent delivery delay)
6. Incorrect location of reagent injection point (short circuiting)
7. Gravity flow reagent (excessive reagent delivery delay)
8. Incorrect location of reagent control valve (excessive reagent delivery delay)
9. Control valve with excessive stick-slip (poor sensitivity and large limit cycle)
10. Electrodes submersed in vessel (coating and maintainability problems)
11. Electrodes located in pump suction (bubbles, clumps, and wrenches)
12. Electrodes located too far downstream (excessive measurement delay)

For my tutorial “pH Measurement and Control Opportunities” attend ISA Automation Week