Use of Ceramic Control Valves in Evaporated Ammonia Lime Milk Systems

Abstract

The lime milk delivery pipeline and throttling control valves are prone to significant erosion. This study analyzes and compares the designs of the imported ceramic eccentric rotary control valve, as well as the domestic double-seal ceramic O-shaped and standard double-seal ceramic control valves. Based on field applications, the erosion resistance and regulatory stability of different valve designs under identical operating conditions are tested and evaluated. A selection guide for lime milk control valves is provided to address internal leakage within the valve body and extend its service life.

In the annual production of 1.5 million tons of soda ash at China National Salt Qinghai Kunlun Alkali Industry Co., Ltd., there are seven sieve plate ammonia distillation towers, each with a distillation capacity of 1020-1070 m³/h and a lime milk flow rate of 420 m³/h, with a single tower lime milk flow of approximately 60 m³/h. Lime milk is pumped from the lime workshop to the pre-ash bucket for the decomposition of ammonium salts in the distillation liquor. During actual production, factors such as high limestone impurities or inadequate calcination result in the presence of significant solid particulates in the lime milk, such as sand. Under operating pressure, these solid particles cause substantial erosion of the delivery pipeline and throttling control valves. This paper compares the scouring resistance and performance of lime milk control valves with varying designs and materials.

1. Use of Imported Ceramic Eccentric Rotary control valves

In August 2011, the 1-million-ton soda ash project at Kunlun Alkali Industry Co., Ltd. was completed and commissioned. The seven FV32la-g lime milk flow control valves used in the project's initial phase were imported ceramic eccentric rotary valves manufactured by Fisher. After four years of operation, the valve core and sealing surfaces had suffered severe erosion, resulting in significant internal leakage. As a result, it became impossible to adjust and control the valve at small openings, which significantly impacted the regulation of lime milk flow. Figure 1 illustrates the wear on the valve body.

Figure 1a illustrates that when the medium passes through the eccentric rotary valve, pressure is concentrated on the side of the valve core facing the flow direction. Figure 1b depicts the medium flowing through the gap between the valve core and the valve seat, primarily on the side where the core faces the medium inflow. As the medium exits, the small gap induces a throttling effect, resulting in a high flow rate. In conclusion, the side of the valve core facing the medium inflow is prone to damage and erosion of both the valve body and sealing surfaces. Erosion primarily occurs on the side of the valve body where the medium flows through, specifically where the valve core closes.

Figure 1 Wear gap of valve core and sealing surface

This control valve differs from a standard valve in the following ways:

• The sealing surface of the eccentric rotary valve is oriented opposite the flow direction to prevent erosion of the valve body after throttling the lime milk flow. The valve cavity is relatively large, which reduces erosion as the medium enters the valve body.
• The valve core and the medium outlet flange have a compact design, and adjusting the medium flow direction does not cause wear on the valve body and seals. However, erosion in the outlet pipe is more pronounced.

Additionally, the inlet and outlet connections of the first valve were incorrectly connected during installation due to an installation error, resulting in the erosion and damage to the valve body within four months of operation. Despite multiple repair attempts, the issue remained unresolved, and the valve has been decommissioned. The short section of the outlet pipeline, composed of cast iron, is still severely eroded and is expected to require replacement after approximately 200 days of continuous operation.

2. Application of Double-Sealed Ceramic O-Shaped control valve from a Chinese Manufacturer

On July 20, 2017, a gray milk valve was replaced with a double-sealed ceramic O-shaped control valve from a Chinese manufacturer. The valve has been operating continuously for 14 months. On September 19, 2018, the valve body experienced leakage. The valve was promptly replaced. Upon disassembly and inspection, the ball core, valve seat, valve stem, and valve body showed signs of erosion, as shown in Figure 2.

Figure 2 Scouring of O-shaped valve core and valve cavity

After thoroughly inspecting the valve body, flushing components, and flow patterns, the flow properties of the medium were analyzed. Under normal operating conditions, the valve opening is typically maintained between 20% and 30% to meet process control requirements. As the fluid passes through the valve cavity, the small valve opening results in high flow rates at the inlet and outlet, causing turbulence and the formation of vortices that impact the valve body cavity, valve seat, and valve stem. The ceramic components are subjected to prolonged, repeated erosion from the vortices. The wear rate increases significantly, and erosion becomes more severe as the valve opening decreases.

Figure 3 Schematic diagram of vortex

When the ceramic ball valve is used to regulate gray milk flow, the valve opening typically ranges from 20% to 35%, with the valve core angle usually between 22° and 26°. This results in a crescent-shaped gap forming on the throttling surface between the valve core and valve seat. As gray milk, containing solid particles smaller than 5 microns, passes through the throttling surface, the flow rate increases significantly, leading to substantial erosion and wear of the valve core, valve seat, and sealing surface. As the valve core and valve seat wear, the flow throttling area increases. To maintain the desired flow rate, the control valve must reduce its opening, further accelerating the wear on the valve core and valve seat. Even when the hard alloy is repaired or the sealing surface is replaced, the service life remains extremely short.

3. Use of Double-Sealed Ceramic control valves

After consulting with the manufacturer regarding the issues outlined above, as well as feedback on the specific valve core scouring medium and the factors influencing the valve core, the valve structure was optimized, and a ceramic control valve was installed. The design of the solid valve core is shown in Figure 4.

• The original valve core’s circular flow surface was modified to a V-shape, with the triangular surface aligned along the centerline of the medium's outflow. This modification results in an approximately equal percentage flow characteristic, helping to minimize the vortex formed when the medium passes through the valve cavity and impacts the internal surface of the valve body.
• The inner diameter of the valve cavity was reduced from DN150 to DN125. This ensures that the valve core opening remains between 40% and 65% during normal operation, helping to reduce the scouring force caused by the accelerated flow rate when the valve is partially opened.

The ceramic control valve was commissioned in September 2018, and the valve position has remained stable at approximately 51%, with the valve core angle typically operating in a half-open position. At this point, the throttling surface between the valve core and valve seat forms a triangular shape, significantly reducing the flow rate and flow deviation. The system has remained stable in operation.

Figure 4 V-shaped valve core of ceramic control valves 

4. Conclusion

Based on the analysis of the performance of the two distinct valve core structures, the following considerations should guide the selection of a gray milk control valve:

• It is essential to fully understand the process parameters. Additionally, the KV value of the control valve should be accurately calculated based on these parameters to avoid compromising performance and minimizing wear on the seal and valve core caused by a small valve opening.
• Throttling of gray milk through the control valve inevitably causes scouring. A well-designed valve core structure, such as a V-shaped double-seal ceramic control valve, can minimize scouring of the valve body.
• The sealing surface of the ceramic eccentric rotary valve faces opposite the flow direction of the medium, and the valve body is smaller. While scouring of the valve body is minimal, pipeline erosion can be significant. It is advisable to use ceramic, wear-resistant pipes downstream of the valve.
• Consider factors such as market price, cost-effectiveness, and the availability of spare parts.