How to Specify Lined Valves for FGD and Limestone Slurry Service?

Quick Answer:

Match the valve type and liner construction to the slurry chemistry, solids, temperature, pressure, and operating duty.

Butterfly valves are commonly used for many FGD duties, but abrasive service and flushing requirements also need consideration.

YOUFUMI offers PTFE/PFA/FEP-lined butterfly valves, including a lug design listed for slurry applications.

Why Is FGD Such a Severe Test for a Valve Lining?

Wet limestone flue gas desulfurization (FGD) exposes a valve to three challenges at the same time: chloride-bearing liquid, suspended limestone and gypsum solids, and changing temperatures. Due to this combination, choosing a lining becomes more complicated than just checking the chemical compatibility.

As water circulates through the FGD process, chlorides can build up. A barrier is then created between the process fluids and the metal beneath by Fluoropolymers like PTFE, PFA, and FEP. This protection, however, depends on the lining remaining intact. If mechanical or abrasive damage exposes the substrate, the process fluid can reach the metal.

A different type of wear is created by gypsum and limestone particles. These travel through the valve with the slurry and consistently make contact with wetted surfaces. The severity of the wear can increase further where flow velocity increases or particles strike the same area repeatedly.

You also need to account for temperature changes, as fluoropolymers contract and expand differently from the surrounding metal body. The lining and its retention can suffer from additional stress if temperature changes occur repeatedly. For a flue gas desulfurization valve, these effects need to be considered together. A corrosion resistant valve still needs a design that protects the liner from excessive slurry wear or mechanical stress.

Why Are Butterfly Valves Used in FGD Slurry Service?

In FGD piping, butterfly valves provide several practical benefits. For one, they are compact and function well in large pipe sizes. Additionally, they have relatively simple geometry compared to other valve types. Certain designs, like a concentric butterfly valve, also avoid the deep body cavities found in some other valve types, leaving fewer places for slurry to collect.

Due to these characteristics, lined butterfly valves are useful for many isolation duties. As the FGD piping gets larger, the compact face-to-face dimensions of these valves can be particularly useful. The downside is that the butterfly disc remains in the flow even when the valve is completely open.

In dense or highly abrasive slurry, the disc and sealing area can suffer from increased wear because of continuous particle contact. That’s why a knife gate or slurry valve may be more appropriate for some heavy-solids isolation duties, as the gate retracts from the flow path when open.

Duty matters as well when selecting a limestone slurry valve. A butterfly valve used mainly for isolation sees different conditions from one used to regulate flow. Partially closing the disc changes the flow around it and can increase localized velocity and wear.

Valve size also affects the choice. FGD systems can use large slurry lines, where the compact construction of a butterfly valve becomes practical compared with bulkier valve designs. However, increasing diameter also increases the area exposed to the slurry. The disc, seat, and lining still need to be selected for the expected solids and flow conditions.

YOUFUMI’s lug lined butterfly valve lists slurries among its applications. Our wafer, lug, and flanged designs are available with PTFE, PFA, or FEP lining. The butterfly valve catalog also shows a lined disc, fluoropolymer seat, and elastomer backing behind the seat.

How Much Lining Thickness Does an FGD Valve Need?

No universal lining thickness guarantees a prolonged life in FGD service. A thicker lining can provide additional material before wear reaches the substrate. It can also increase the distance that process chemicals have to travel through the barrier.

All of that, however, doesn’t change the fact that thickness is only one part of the design. As the temperature changes, metal valve bodies and fluoropolymers expand at varying rates. Sustained mechanical load can also cause the lining to experience creep. That’s why it’s important to ask how the manufacturer forms, supports, and retains the lining along with its nominal thickness.

The location of the lining matters as much as the nominal thickness. Slurry does not wear every wetted surface evenly. Areas with accelerated flow or those exposed to direct particle impact can experience greater wear than less exposed surfaces. A thickness value on its own therefore tells you little about how the lining will perform unless you also know how the valve geometry exposes it to the slurry.

This is why simply specifying the thickest available liner is not a reliable approach. The lining needs to work with the valve’s geometry and expected operating conditions.

Liner thickness also cannot compensate for operating outside the valve’s pressure-temperature limits. YOUFUMI’s butterfly valve catalog gives temperature ranges for PTFE, PFA, and FEP alongside a pressure-temperature curve showing how allowable pressure changes as temperature rises.

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How Should FGD Slurry Valves Be Flushed and Maintained?

Flushing becomes important when slurry stops moving through the line. Gypsum and limestone solids can settle during stagnant periods, allowing deposits to form around internal valve surfaces.

A scrubber valve can become harder to cycle once these deposits form. Material around the closure or seat area can interfere with shutoff and place additional mechanical load on the sealing surfaces.

Clear-water flushing before a standby period or planned shutdown can remove residual slurry before it settles. Make sure the flushing frequency follows the process rather than going with a fixed calendar interval.

The general claim that a specific flushing schedule will double valve life is not supported by available evidence. Two questions are more useful when setting the maintenance schedule: how long does the line remain stagnant, and how quickly do the solids settle?

Flush-water conditions matter as well. The water must be compatible with the process and valve materials, and large temperature differences should be considered before introducing it into hot equipment. The goal is to clear residual slurry before it has time to settle and harden around the valve.