Ratio Control

Ratio Control Principles

Overview

This module focuses on ratio control systems, their operation, troubleshooting, configuration, and tuning. Ratio control maintains a controlled variable at a fixed ratio to another variable, optimizing production and minimizing operator intervention and off-specification product.

Objectives:

1. Describe the advantages and applications of ratio control.

2. Draw a block diagram of a ratio control system.

3. Describe methods for tuning ratio control systems.

Ratio Control Principle

Ratio control maintains a controlled variable at a fixed ratio to another variable. The goal is to maintain a third variable at a desired value. For example, diluting heavy oil with condensate to achieve the correct viscosity for pumping.

In a scenario where heavy oil is diluted with condensate, the heavy oil flow is controlled by a level controller (LC-200) in a surge vessel, while the condensate flow is managed by a flow controller (FC-100). The desired ratio of condensate to heavy oil is, for instance, 0.3 to achieve the correct viscosity.

Example:

• Heavy Oil Flow Rate: Measured by FT-200

• Condensate Flow Rate: Controlled by FC-100

• Desired Ratio: 0.3 (Condensate/Heavy Oil)

Manual Adjustments

Without ratio control, operators must constantly monitor and adjust the condensate flow rate (FC-100) based on changes in heavy oil flow, pipeline demand, or viscosity variations, leading to inefficiencies and off-specification product.

Advantages of Ratio Control

• Minimizes operator intervention.

• Reduces off-specification product.

• Enhances efficiency and cost-effectiveness.

Ratio Control Schemes

1. Direct Ratio Control

The direct ratio control scheme directly controls the ratio between two flow streams. The controlled process variable is the ratio itself. It involves a flow relay to calculate the ratio by dividing one stream by the other.

• Wild Flow ($q_w$): Uncontrolled flow (e.g., heavy oil) measured by FT-200.

• Controlled Flow ($q_c$): Manipulated flow (e.g., condensate) measured by FT-100.

• Ratio (R): Calculated by relay FY-100 as $R = \frac{q<em>c}{q</em>w}$. The ratio becomes the process variable (PV) for the ratio controller.

• The flow ratio controller (FFC-100) adjusts the condensate flow to maintain the PV at the required setpoint (SP), ensuring the desired viscosity.

Disadvantage:

• Built-in nonlinearity due to the dividing relay, making it less common.

2. Indirect Ratio Control

The indirect ratio control scheme calculates the required flow rate of the manipulated stream by multiplying by the ratio. The calculated value becomes the setpoint for a flow controller. Here, the controlled process variable is NOT the ratio.

• The wild flow ($q_w$) of heavy oil is measured by FT-200.

• The flow of condensate ($q_c$) is measured by FT-100 and controlled by FC-100.

• Relay FY-200 multiplies the wild flow by the desired ratio to obtain the required controlled flow rate: $q<em>c = R \times q</em>w$. The output serves as the remote setpoint (RSP) for FC-100.

Advantage:

• No built-in nonlinearities, making it a preferred scheme.

Indirect Ratio Control with Feedback Trim

This scheme integrates a viscosity controller (AC-100) to adjust the ratio automatically. The output of AC-100 is the required ratio to maintain the viscosity at the desired setpoint. The relay FY-200 multiplies the wild flow by this ratio to determine the controlled flow rate ($q<em>c = R \times q</em>w$). This setup allows the system to automatically adjust for viscosity changes.

Applications of Ratio Control

Ratio control is widely used for blending, mixing, and feed controls in chemical reactions. Specific industrial applications include:

• Mixing heavy oil and condensate to achieve a required viscosity.

• Maintaining fuel-to-air ratios for combustion control.

• Blending gasoline to meet product specifications.

Mixing

In a mixing scheme, such as diluting heavy oil with condensate, the flow of heavy oil is the wild flow dictated by the pipeline, while the condensate flow is the controlled flow. A density controller (DC-100) outputs the ratio because of the correlation between density and viscosity.

Example Calculation:

• Required Ratio: 0.3

• Wild Flow Rate: 1500 kg/hr

• Output of Ratio Station (FY-100): $q<em>c = R \times q</em>w = 0.3 \times 1500 \text{ kg/hr} = 450 \text{ kg/hr}$

• The remote setpoint (RSP) for the condensate flow controller (FC-100) is 450 kg/hr.

Ratio Calculations

When engineering units are used, the required ratio is the ratio of controlled flow to wild flow: $R = \frac{\text{Controlled flow}}{\text{Wild flow}} = \frac{q<em>c}{q</em>w}$

For percentage signals, the fractional ratio ($R_f$) is calculated from the fractional flow:

$R<em>f = \frac{\text{Controlled flow / Span}}{\text{Wild flow / Span}} = R \times \frac{\text{Span}</em>c}{\text{Span}_w}$

Fuel to Air Ratio

Ratio control is crucial in combustion processes to maintain appropriate fuel-to-air mixtures, ensuring efficient and safe operation. Four primary metering control schemes are used:

1. Series

2. Parallel

3. Parallel Cross-Limited

4. Characterizing the Air Flow Signal

Series Metering Control

In series metering control, the firing demand signal goes only to the air controller. The fuel flow is the wild flow, and its setpoint is the firing demand signal from a temperature controller (TC-100). The airflow is the controlled variable, with its setpoint derived from the fuel flow transmitter (FT-100) multiplied by the air/fuel ratio via relay FY-200A, trimmed by an oxygen controller (AC-100) through relay FY-200B.

Series metering control ensures excess air at steady-state conditions but may fail during disturbances, potentially leading to excess fuel. It is less commonly used due to complexity in ensuring excess air under all conditions.

Parallel Metering Control

Parallel metering control sends the firing demand signal to both the fuel and air controllers, either using a ratio in engineering units or a fractional ratio.

Engineering Ratio

In parallel metering control using an engineering ratio, the temperature controller (TC-100) outputs the firing rate demand, which serves as the setpoint for the fuel flow. The airflow setpoint is derived from TC-100's output multiplied by the air/fuel ratio in engineering units, provided by relay FY-200A.

Example:

• Required Air/Fuel Ratio: 10.6

• Fuel Flow Demand: 84 sm³/hr (from TC-100)

• Airflow Setpoint: $10.6 \times 84 \text{ sm}^3/\text{hr} = 890.4 \text{ sm}^3/\text{hr}$

Fractional Ratio

Using a fractional ratio, the firing rate demand signal is a percentage. The airflow setpoint is the firing demand multiplied by the fractional air/fuel ratio ($R_f$).

• $R<em>f = R \times \frac{\text{Span}</em>c}{\text{Span}_w}$

Example:

• Required Fractional Ratio: 0.848

• Firing Demand: 70%

• Airflow Setpoint: $0.848 \times 70\% = 59.36\%$, corresponding to 890.4 sm³/hr.

Parallel Metering Control with Oxygen Trim

This setup incorporates an oxygen controller (AC-100) to correct the calculated ratio for optimal furnace operation. The oxygen trim provides adjustment to the airflow controller's setpoint. The relays are used to characterize the signal for each stream to optimize the control. A high/low limit for the ratio should be considered to protect from over or under firing depending on the constraints of the system and process goals.

Parallel Metered Cross-Limited Control

Cross-limited control enhances safety and efficiency by preventing fuel flow increases before airflow increases and vice versa. High and low selectors ensure that the air and fuel flows respond appropriately to changes in demand. The cross-limited controls are designed with specific constraints and process safety considerations.

The addition of a bias ensures that the signals are selected appropriately, adding fixed signal as a minimal value based on the lowest safe value in the system.

Characterizing the Airflow Signal

Characterizing the airflow signal involves calibrating a multiplier relay to achieve a specific output percentage for a given input percentage. This is typically applied to parallel cross-limited control schemes to fine-tune the air/fuel ratio.

Blending

Batch vs. In-Line Blending

• Batch or Sequential Blending: Measures one component at a time without using ratios.

• In-Line or Continuous Blending: Simultaneously blends all components using ratios.

Analog In-Line Blending

Analog in-line blending blends products in proportion to the flow rates of each stream. Flow rate controllers regulate each stream based on analog signals.

The stream flow rates are controlled to give the correct ratio of components in the product.

Digital Blending

Digital blending uses quantity controllers to blend products based on the amount of each component. Pulse flowmeters measures each pulse (count) to indicate a precise amount. The digital blending involves streams blended at set ratios.

Side-Stream Blending

Side-stream blending mixes a stream with another before the mixture is measured, often used for blending ethanol with gasoline. Because the traditional blending measures each component separately does not work with this type of blend, side-stream are optimized for these types of considerations.

Stream 3 is the side-blended stream, which connects into the product flow stream upstream of the flowmeter of Stream 1.

Block Diagrams

A block diagram models each element of the control loop using transfer functions, the block diagram gives insight on how a control loop operates. Static gains can model each element and provide information on the control loop's steady state response only.

Direct Ratio Control Block Diagram

Direct Ratio Control Scheme has nonlinearity caused by the dividing relay (FY-100), which calculates the ratio between the two streams.

Nonlinearity Analysis

A control loop static gain changes if the loop is nonlinear. If the flow rates are variable and not consistent, the valve opening can partially negate the effects from changes to the wild flow to keep the loop static gain similar.

Indirect Ratio Control Block Diagram

The PV for the flow controller (FC-100) is the condensate flow (controlled flow). The RSP for the flow controller comes from the output of the flow relay (FY-200), which multiplies the heavy oil flow (wild flow) by the ratio to calculate the required condensate flow rate. This control scheme does not introduce any nonlinearity into the condensate control loop.

Nonlinearity is not a concern as it does not cause instability.

Cascade Control

Cascade control scheme is a control scheme where the output of the primary controller is used as the setpoint of the secondary controller. Primary controller is the viscosity controller (AC-100), and its output is used to calculate the setpoint for the secondary controller (FC-100).

Tuning Ratio Control Systems

Tuning Considerations

Ratio control systems range from simple to complex. Additional components and controls require the process to be tuned based on a cascade control strategy.

Indirect Ratio Control Tuning Steps:

1. Condensate flow controller (FC-100) is tuned first, which is the secondary controller of the cascade system.

2. Viscosity controller (AC-100) is tuned last with the secondary controller in automatic.

Controller must constantly respond to changes in its setpoint, needs to react fast until it is unstable. In the viscosity (AC-100), it must be tuned conservatively and slower.

Considerations:

• Integral action is required because offset is not acceptable.

• Derivative function is not recommended.

Parallel Metered Fuel Air Ratio Control Tuning Steps:

1. Air and fuel flow controller (FC-100 and FC-200) are tuned first as they are the secondary controllers of the cascade system.

2. Firing demand controller (TC-100) is tuned second, with the air and fuel controllers in automatic.

3. Feedback trim controller (AC-100) is tuned last.

Considerations:

• Both controllers must be tuned to have the same speed of response.

• Tuning the fuel flow control (FC-100), the airflow controller (FC-200) needs to be in cascade mode with an RSP so that it ensures excess air at all times.

• Needs to be at least five times slower than the secondary loops to prevent loop interaction.

System Configuration

Ratio station consists of function blocks with a specialized PID-RATIO controller that combines the capabilities of the ratio station with a PID controller. The function blocks can trim the ratio and allows you to change/enter the ratio.

The RATIO function block can operate in manual, automatic, or cascade mode, displaying the current ratio. When programming a ratio control system, the considerations are mainly on manual operation, bumpless transfer, and reset windup.

Manual Operation

The operator is able to take direct control over the process by placing the secondary controller in manual. By taking this, it gives the operator direct control over the FCE and adjusting secondary controller output.

Bumpless Transfer

There is a need for bumpless transfer when switching controllers or RATIO between modes. Sudden step change in the signal by switching modes has to be avoided.

Reset Windup

The design must prevent reset windup. PID external feedback and anti-reset windup options provides by the PID manufacturer provides a means for its functionality.