27.11.2Exclusive valve sequencing

Other applications for split-ranged control valves call for a form of valve sequencing where both valves are fully closed at a 50% controller output signal, with one valve opening fully as the controller output drives toward 100% and the other valve opening fully as the controller output goes to 0%. The nature of this valve sequencing is to have an “either-or” throttled path for process fluid. That is, either process fluid flows through one valve or through the other, but never through both at the same time.

A practical example of this form of split-ranging is reagent feed for a pH neutralization process, where the pH value of process liquid is brought closer to neutral by the addition of either acid or caustic reagent liquids:

ATO

Acid

pH controller (indicating)

AIC AT

Mixer

pH analyzer

Here, a pH analyzer monitors the pH value of the liquid solution and a single pH controller commands two reagent valves to open when needed. If the process pH begins to increase, the controller output signal increases as well (direct action) to open up the acid valve. The addition of acid to the mixture will have the e ect of lowering the mixture’s pH value. Conversely, if the process pH begins to decrease, the controller output signal will decrease as well, closing the acid valve and then opening the caustic valve. The addition of caustic to the mixture will have the e ect of raising the mixture’s pH value.

27.11.3Progressive valve sequencing

A third form of control valve sequencing is used to expand the operating range of flow control for some fluid beyond that which a single control valve could muster. Once again pH control provides a suitable example to illustrate an application of this form of sequencing.

pH is an especially challenging application of process control because the dynamic range of the process is enormous. Each unit of pH value change represents a ten-fold change in hydrogen ion concentration within the process liquid. This means the di erence in ion concentration between a process liquid having a value of 10 pH and a process liquid having a value of 7 pH (a pH di erence of 3) is a factor of one thousand (103)! Consequently, the flow rate of reagent necessary to neutralize a process liquid stream may vary widely. It is quite possible that a control valve sized to throttle minimum flow will simply be too small to meet the demands of high flow when needed. Yet, a control valve sized large enough to meet the maximum flow rate may be too large to precisely “turn down” when just a trickle of reagent is needed.

This same general control problem was encountered by automotive engineers in the days when carburetors were used to mix gasoline with air prior to combustion in an engine. A carburetor is a mechanical air flow control device using a “butterfly” valve element to throttle air flow into the engine, and a venturi element producing vacuum to aspirate fuel droplets into the air stream to create an air-fuel mixture. A carburetor with a butterfly valve and flow tube sized to idle well and respond to the needs of in-town driving would not flow enough air to provide good high-speed performance. Conversely, a large carburetor suitable for driving at racing speeds would o er poor control at lowspeed and idling operation. Their solution to this problem was the progressive carburetor, having two butterfly valves to throttle the flow of air into the engine. One butterfly valve passed low amounts of air flow only, while a larger butterfly valve opened up only when the accelerator pedal was nearly at its maximum position. The combination of two di erently-sized butterfly valves – progressively opened – gave drivers the best of both worlds. Now, an automobile engine could perform well both at low power levels and at high power levels.

On a fundamental level, the problem faced in pH control as well as by early automotive engineers is the same thing: insu cient rangeability. Some processes demand a greater range of control than any single valve can deliver, and it is within these processes that a pair of progressively-sequenced control valves is a valid solution.

Applying this solution to a pH control process where the incoming liquid always has a high pH value, and must be neutralized with acid:

pH controller (indicating)

AIC AT

Mixer

pH analyzer