Choosing Control Valves for the Pulp and Paper Industry

In the pulp and paper industry, key production processes involve handling a range of media, including pulp slurry, steam, water, wastewater, and black liquor. Each of these media has distinct properties, necessitating careful selection and configuration of control valves for different stages of production. This article explores the unique requirements for control valves in the paper industry, covering aspects such as media characteristics, valve selection criteria, pulp slurry analysis, and practical applications.

Media Characteristics Analysis

During paper production, most media are non-corrosive, except for the pulp in the bleaching stage, which contains small amounts of chlorine ions, and black liquor, which is alkaline in nature. Pipeline and container pressures typically do not exceed 1.6 MPa, with steam having the highest temperature among the media. For steam at 1.6 MPa, the saturation temperature can reach 203°C. These conditions indicate that high-temperature, high-pressure, corrosion resistance, and explosion-proof features are generally unnecessary for control valves in this industry.
The stability of media properties allows for simplified selection calculations, omitting complex factors such as flashing, cavitation, and high differential pressure noise that are often essential in other industries. This simplifies valve selection and reduces performance demands, optimizing both maintenance and operational costs.

Control Valve Selection Requirements

For water, steam, and gas control in the paper industry, single-seat, double-seat, and sleeve control valves are commonly used, as they are in many other industries. However, handling pulp slurry and black liquor requires specially designed rotary V-notch ball valves, O-shaped control ball valves, and butterfly valves. This selection accommodates the specific needs of paper production.

1. Single-Seat, Double-Seat, and Sleeve Control Valves

Single-seat, double-seat, and sleeve valves are suitable for controlling the flow of pure liquids and gases. These valves are straightforward in design and offer high precision, ideal for regulating single-phase fluids like water and steam. Single-seat valves provide excellent sealing, though they require high flow capacity, whereas double-seat valves balance force and are suitable for high-flow applications. Sleeve valves combine the benefits of single-seat and double-seat designs, offering balanced flow capacity and control precision.

2. V-Notch Ball Valves and O-Shaped Control Ball Valves

For handling pulp slurry and black liquor, single-seat and double-seat valves are unsuitable due to their tortuous flow paths, which can lead to clogging from the fibers and particles in the media. In contrast, V-notch and O-shaped ball valves have clear flow paths that prevent pulp buildup. These valves regulate flow by rotating the ball, providing high flow capacity, excellent sealing, and a shearing function that can cut through impurities. V-notch ball valves offer precise control of pulp flow, while O-shaped ball valves are suitable for on-off control of slurry flow. These valves are widely used in the paper industry, especially for slurry and black liquor containing fibers and solid particles.

Pulp Slurry Characteristics and Valve Control Requirements

Pulp slurry, a mixture of water and fibers, represents a two-phase flow unique to the paper industry. It differs significantly from pure liquids and gases, particularly in pressure loss behavior. At low flow rates, slurry pressure loss remains stable, but as flow rate increases, the pressure loss fluctuates unpredictably. Typically, slurry pressure loss is higher than that of water, but at high flow rates, it can be lower than water's pressure loss. This indicates that standard liquid formulas for determining valve flow coefficients may not accurately reflect the true characteristics of slurry flow.
Research shows that slurry requires a larger flow coefficient than water. Past methods for calculating slurry flow coefficients often used rough approximations, which may not accurately capture slurry's complex flow dynamics. Therefore, a margin in the flow coefficient is necessary when selecting valves for slurry applications.

Control Valve Design and Selection in Practice

In practical engineering design, pulp concentration, valve pressure differential, and other parameters frequently fluctuate. Therefore, control valves must have sufficient flow capacity to accommodate potential variations in flow. Typically, V-notch ball valves are chosen and sized based on normal process data, allowing the valve to operate within a target opening range (such as 60%) to meet dynamic process demands.
V-notch ball valves offer a high rangeability of up to 1:300 and an equal percentage flow characteristic, which allows them to handle a wide range of flow variations, ensuring stable flow control. In practice, V-notch ball valves have proven effective in adapting to large fluctuations in slurry flow, meeting the needs of paper production.

Conclusion

In the paper industry, the unique properties of media require specific considerations for control valve selection and use. Thoughtful valve selection not only optimizes control performance but also helps reduce production costs. Single-seat, double-seat, and sleeve valves are ideal for water, steam, and gas control, while V-notch and O-shaped ball valves, with their superior shearing and flow capabilities, are ideal for pulp slurry and black liquor. Further research and practical applications show that using V-notch ball valves effectively handles variations in slurry flow, ensuring stable operation in the paper production process.