The leak started as a weep at the toe of a weld on a ten inch white liquor header, was logged as housekeeping, and eleven days later opened into a spray that put two people in hospital with caustic burns. The pipe was carbon steel, which is the correct material for white liquor, and it had been welded to a procedure that the mill had used for twenty years. What it had not been given was a post-weld heat treatment, because the spool had been fabricated during a shutdown and the oven was booked. The cracking that followed was caustic stress corrosion, and it began in the heat affected zone exactly where the residual stress was highest.
A kraft pulp mill is one of the few places where a ball valve meets four different chemical environments within a hundred metres of pipe, and where the correct material for one of them is the wrong material for the next. White liquor at pH 14 cracks carbon steel welds unless they are stress relieved. Black liquor at 75 percent solids is not a liquid below 100 degrees C and will seize a valve that cools down. Green liquor carries dregs that grind a polymer seat away in a year. And the bleach plant, forty metres away, runs at pH 2 with chlorine dioxide, where the stainless steels used everywhere else in the mill pit within months.
1. Three liquors, and only one of them is a corrosion problem
The white liquor that comes out of the causticisers is the mill's cooking chemical: roughly 90 to 110 grams per litre of sodium hydroxide as effective alkali, 25 to 40 grams per litre of sodium sulphide, and 20 to 30 grams per litre of sodium carbonate, at 85 to 95 degrees C. It is a hot, strongly alkaline, sulphide-bearing stream. Its problem is not wastage but cracking, and the cracking is stress corrosion, which means it gives no warning and no wall loss before it fails.
Green liquor is the same chemistry one step earlier, dissolved from the recovery boiler smelt: 100 to 130 grams per litre sodium carbonate, 30 to 45 grams per litre sodium sulphide, a little hydroxide, at a similar temperature, and carrying 20 to 300 milligrams per litre of suspended dregs. Its problem is abrasion at the seat and fouling in the cavity, because those dregs are calcium carbonate, carbon and sulphide particles that are small enough to travel and hard enough to wear.
Black liquor is the mill's fuel and its most difficult fluid. Weak black liquor from the washers is 12 to 20 percent solids at 80 to 95 degrees C and behaves like dirty water. After evaporation it is 65 to 80 percent solids at 110 to 135 degrees C and behaves like a hot paste whose viscosity changes by an order of magnitude over a 15 kelvin window. Its problem is rheology, and a valve that is metallurgically perfect will still fail if it lets the liquor cool.
| Stream | What is in it | Temperature | pH | Dominant failure | Usual body |
|---|---|---|---|---|---|
| White liquor | NaOH 90 to 110 g/L, Na2S 25 to 40 g/L | 85 to 95 C | 13.5 to 14 | Caustic stress corrosion at welds | Carbon steel, stress relieved |
| Green liquor | Na2CO3 100 to 130 g/L, Na2S, dregs 20 to 300 mg/L | 85 to 95 C | 12 to 13 | Seat abrasion, cavity fouling | Carbon steel or 316L |
| Weak black liquor | 12 to 20 percent solids, sulphate, organics | 80 to 95 C | 11 to 12 | Fouling, mild corrosion | Carbon steel |
| Heavy black liquor | 65 to 80 percent solids, viscosity 200 to 5,000 cP | 110 to 135 C | 11 to 12 | Solidification, seizure, fire | Carbon steel, steam jacketed |
| Lime mud slurry | CaCO3 30 to 50 percent solids | 80 to 90 C | 12 to 13 | Abrasion of seat and ball | Cast iron or carbon steel |
| Chlorine dioxide stage | ClO2 5 to 15 g/L, chloride 2 to 8 g/L | 45 to 60 C | 2 to 4 | Pitting and crevice attack | Titanium grade 2 |
2. Caustic stress corrosion: the failure that starts at a weld
Three conditions have to coexist for caustic cracking: a susceptible alloy under tensile stress, a caustic concentration above a few percent, and a temperature above a threshold that falls as the concentration rises. In white liquor all three are present by design, and the only variable the designer controls is stress. That is why carbon steel is the correct material and also why the post-weld heat treatment is not optional.
The threshold is commonly quoted in the region of 50 to 60 degrees C for stressed carbon steel in 5 to 15 percent caustic, which puts white liquor at 90 degrees C well above it. Stress relief at 600 to 650 degrees C, with the whole circumference of the weld and a band either side, is the standard mitigation and it is written into every mill's welding procedure. It is also the step that gets skipped when a spool is fabricated in a hurry, and the failure appears twelve to thirty months later, which is long enough for the paperwork to have been filed and forgotten.
The cast valve body is a special case, because a casting cannot be stress relieved in the same way as a pipe and often carries weld repairs that reintroduce residual stress. A valve with a weld-repaired body in white liquor is a candidate for cracking at the repair, and the specification should require the repair map, the heat treatment record and a hardness check on the repaired area, with a limit around 200 HB. Austenitic stainless is not a way out: 316L cracks in caustic above roughly 60 degrees C, sometimes faster than carbon steel because its thermal expansion puts more stress on the weld. Nickel 200 is immune and is used for concentrated caustic above 120 degrees C, at a price that confines it to small valves on sampling and dosing lines. Duplex 2205 has better caustic resistance than 316L and is a reasonable upgrade for a valve body that cannot be stress relieved, but it is not immune and it does not remove the need for a controlled welding procedure.
3. Black liquor is a solid with a temperature window
At 65 percent solids and 110 degrees C, heavy black liquor has a viscosity in the range of 200 to 1,000 centipoise. At 70 percent solids the number can reach several thousand, and below about 95 degrees C the material stops behaving as a fluid and starts behaving as a paste that will not drain. Below 80 degrees C it can set hard enough to require a hammer. This is not a marginal effect at the design point; it is the defining property of the stream, and it sets a hard rule for every valve in the line: the body must stay above about 105 degrees C at all times, including during a shutdown.
The standard answer is a steam jacket on the body, fed at 3 to 6 bar, which gives a metal temperature of 140 to 160 degrees C. Electric tracing is used where steam is not available at the point, and it is less forgiving because a failed circuit is invisible until the valve seizes. Insulation covers the body and stops at the bonnet, so that the packing does not cook, which is the same rule as any high temperature valve and is routinely ignored on black liquor because the whole line is insulated for heat conservation and the bonnet gets wrapped with it.
The consequence of a cold valve is not a slow stroke. The liquor in the cavity sets, the ball is surrounded by a solid plug, and the next stroke either shears the stem key, twists a square drive, or tears the seat out of its retainer. A mill that has had one of these events adds a rule to its shutdown procedure: black liquor valves are cycled once before the line cools, and the jacket steam stays on through the outage. That rule costs almost nothing and prevents the most expensive failure mode in the liquor area.
4. A set-and-reheat cycle costs more torque than ten years of service
The damage from a single cooldown is measurable. An eight inch Class 150 valve in 72 percent solids black liquor at 120 degrees C turns at about 1,200 newton metres when the line is hot and clean. After one event in which the liquor set in the cavity and was then reheated, the breakaway torque on the same valve is typically 3,000 to 4,000 newton metres, because the re-solidified liquor has filled the seat clearance and the cavity, and the first stroke has to shear it rather than wipe it. That is a factor of three from one event, and it is not recoverable by turning the valve a few times; the seat has been deformed and the cavity is now packed with carbonised material.
This is the number that should drive actuator sizing in the liquor area, not the clean torque from the vendor's catalogue. A spring return actuator selected at 1.5 times the clean breakaway will stall on the second stroke after a cooldown. Sizing at 2.5 times, and stating the torque basis in the datasheet, is the only way to keep the valve operable through a realistic operating history.
5. Green liquor dregs: abrasion on a schedule
Dregs are the insoluble fraction of the smelt: calcium carbonate, unburned carbon, metal sulphides and refractory fragments, at 20 to 300 milligrams per litre and mostly under 100 micrometres. In a pipe at two metres per second they cause negligible wear. In a valve seat they do not, because the flow accelerates through the seat gap and because the particle is trapped between two hard surfaces that are closing on it.
A polymer seat in green liquor rounds at the leading edge within six to twelve months, and once the edge is rounded the valve no longer holds a tight shutoff, which in a liquor circuit means continuous dilution and a causticising balance that drifts. A metal seat faced with a cobalt based alloy in the 400 to 450 HV range, or with tungsten carbide applied as a coating and reaching 1,000 to 1,200 HV, lasts several times longer and degrades gracefully rather than suddenly. The ball should be hard faced to match, because an untreated stainless ball wears in a band that then leaks regardless of the seat condition.
The same reasoning applies to the lime mud slurry from the causticisers, at 30 to 50 percent solids, where the abrasion is an order of magnitude higher and where a ball valve is chosen for its straight-through bore rather than for its sealing. In that duty a metal seated valve with a generous clearance, or a pinch or knife gate valve, is often the better answer, and a ball valve is specified because the piping standard says ball valve.
6. The bleach plant is a different plant
Forty metres from the white liquor header, the bleach plant runs a chlorine dioxide stage at pH 2 to 4, 45 to 60 degrees C, with 5 to 15 grams per litre of ClO2 and 2 to 8 grams per litre of chloride. This is an oxidising, low pH, high chloride environment, and it defeats 316L by pitting and crevice attack in a matter of months, especially under a gasket or in a thread. Grade 2 titanium is the material the stage is normally built in, and it is one of the few places in a mill where titanium is genuinely the right answer rather than an expensive reflex.
Two cautions apply. Titanium is not suitable for dry chlorine gas, where it can ignite, so the material choice has to be tied to the phase and the water content of the stream rather than to the word chlorine. And duplex stainless, which is an excellent choice elsewhere in the mill, should not be substituted grade for grade in a D stage without a corrosion test in the actual liquor, because the oxidising chloride chemistry of that stage is not the environment the grade was developed for. The safe rule is that the bleach plant is specified from its own material list, drawn by someone who has looked at the chloride and the oxidant together, and not from the mill's general piping specification.
Elastomers follow the same logic. Chlorine dioxide attacks most hydrocarbon rubbers, so seats are PTFE and seals are perfluoroelastomer or encapsulated, and the valve is specified with a fugitive emission class because a bleach plant leak is a chlorine exposure as well as a production loss.
7. Trapped liquor, steam tracing and a cavity that becomes a pressure vessel
A ball valve in the closed position isolates a cavity full of liquid. Heat that cavity and the liquid expands against a rigid body. At 120 degrees C the expansion coefficient of water multiplied into its bulk modulus comes to roughly 1.5 bar per kelvin, about three times the room temperature value, so a 30 kelvin rise from steam tracing puts 45 bar into a cavity that is rated for 19. That is a Class 150 valve being asked to hold Class 300 pressure from the inside, with no relief path and no instrument to show it.
The mitigation is either a cavity relief hole on the upstream side of the seat, which lets the excess escape into the line, or a self-relieving seat design that does the same thing through the seat ring, or an external relief connection on a double block and bleed arrangement. What must not happen is a cavity relief that quietly becomes a cross-contamination path. In a mill, the classic case is a valve between white liquor and water where a relieved cavity lets caustic into a cooling water line, and the symptom is a rising pH in a circuit nobody is monitoring.
Steam tracing makes this worse in a way that is easy to overlook, because the jacket is designed to keep the body hot and is usually left on permanently. A valve that is closed for a week with the jacket at 150 degrees C is a small autoclave. The design review question is not whether the cavity relieves, but where it relieves to and what is on the other side.
8. Galling, lubricity and stainless on stainless
White liquor has almost no lubricity. It is a hot caustic solution with dissolved sulphide, and it will wash any grease off a stem in a few strokes. A stainless stem in a stainless gland in that service galls, and the failure is progressive: the first sign is a rising actuator air consumption, then a slow stroke, then a stem that will not move at all and a valve that has to be cut out. The fix is a material pair rather than a lubricant: a cobalt-based hard facing on the stem, a Nitronic 60 gland, or a hard chrome plating, and a stem surface finish fine enough that the oxide does not key into it.
The same applies to the ball and seat in a metal seated valve. Two identical stainless surfaces in sliding contact with a caustic film between them will pick up and tear. Differential hardness, or a dissimilar pair, is what makes a metal seated valve survive a hundred thousand cycles in liquor service.
9. Actuator sizing in a mill that runs at minus 30
Pulp mills are frequently in cold places, and the liquor lines are frequently outdoors between buildings. An actuator sized at the compressor's rated pressure and at 20 degrees C will under-deliver in February, because the air supply pressure drops along a long header, because the seals stiffen, and because the spring force changes with temperature. The sizing calculation should use the pressure at the actuator inlet on the worst day, which is often 4.5 bar rather than 6, and the lowest ambient the site has recorded, which in northern mills is minus 30 or lower.
The same calculation should use a torque basis of 2.0 to 2.5 times the clean breakaway for black liquor and 1.5 times for white and green liquor, and it should state the basis in the datasheet so that the next person who reviews the valve knows what was assumed. A valve that is correct but under-actuated is indistinguishable from a valve that is wrong, and it fails in the same place: mid-stroke, with the line partly open.
10. Clauses for the data sheet
The material matrix below is the short version of this article and is worth attaching to a purchase requisition, because it prevents the most common error in mill valve procurement, which is applying one material rule to a whole area.
| Service | Body | Ball and stem | Seat | Clause that matters |
|---|---|---|---|---|
| White liquor, above 80 C | Carbon steel, stress relieved | 316L with hard facing | PTFE or PEEK | Heat treatment record, hardness under 200 HB |
| Green liquor with dregs | Carbon steel or 316L | Stellite 6 overlay | Metal, tungsten carbide | Leakage class B, hard facing thickness |
| Heavy black liquor | Carbon steel, steam jacketed | 316L, hard chrome stem | Metal or PEEK | Jacket pressure, traceability, fire-safe |
| Lime mud slurry | Carbon steel, wear resistant | Hard faced | Metal, generous clearance | Solids tolerance, torque at 2.5 times |
| Chlorine dioxide stage | Titanium grade 2 | Titanium | PTFE | No copper or iron in the wetted path |
| Digester blow line | Carbon steel, hard faced | Tungsten carbide | Metal | Erosion allowance, cycle count |
Beyond the matrix, four documents should be required. A material certificate to EN 10204 3.1 for every wetted part, with positive material identification on the finished valve rather than on the mill's own test bar. A post-weld heat treatment chart for any carbon steel body that has been repaired. A jacketing drawing that shows the steam connections, the drain and the insulation break at the bonnet. And an actuator sizing sheet that states the air pressure, the ambient temperature and the torque multiple used.
The valve works that supplies these orders cuts from 15 millimetres to 1,400 millimetres of bore, which is the half inch to 56 inch range, in pressure classes that run from ASME 150 through 2500, with a temperature window from minus 196 to 650 degrees C. Shell testing runs at 1.5 times the cold working pressure and seat testing at 1.1 times, every unit, and a quotation with a drawing follows a complete datasheet inside three working hours.
11. What the mill learns the hard way
Three lessons recur in every mill that has operated a liquor area for twenty years. The first is that the material rule for white liquor is about stress, not about corrosion, and that a stress relief record is worth more than a higher alloy. The second is that black liquor failures are thermal, and that keeping a valve hot through a shutdown costs less than any repair. The third is that the bleach plant is a separate specification, and that a mill which buys valves for the whole site from one material list will have a corrosion problem exactly where the list was least applicable.
None of these is difficult. All of them depend on the datasheet carrying the stream chemistry, the temperature history and the cycle count, rather than the size and the pressure class alone.
12. Frequently Asked Questions
Why does carbon steel crack in white liquor when it is the recommended material?
Because the recommendation assumes the steel is stress relieved after welding. Caustic stress corrosion needs three things at once: a caustic concentration, a temperature above roughly 50 to 60 degrees C, and tensile stress. White liquor at 90 degrees C supplies the first two, so the third has to be removed by heat treatment at 600 to 650 degrees C across the weld and a band either side. A spool or a cast body with a weld repair that missed the furnace will crack in the heat affected zone twelve to thirty months later, with no wall loss to warn anyone.
What happens if heavy black liquor cools inside a valve?
It sets. Above 65 percent solids the material has no flow below about 95 degrees C, and below 80 degrees C it can be hard enough to need a hammer. The cavity fills, the seat clearance fills, and the next stroke has to shear a solid plug, which typically triples the breakaway torque and can twist a stem or tear a seat retainer. The countermeasures are a steam jacket held on through shutdowns, insulation that stops at the bonnet, and a procedure that cycles the valve once before the line cools.
Can duplex stainless replace 316L in the bleach plant?
Not by substitution without a corrosion test. The chlorine dioxide stage is an oxidising, low pH, high chloride environment, and it is titanium territory. Duplex has better chloride resistance than 316L and is used in some mills in the first bleaching stage, but its performance depends on the specific chloride level, temperature and oxidant concentration, and a grade for grade swap made on the basis of a higher pitting resistance number has failed in practice. Specify the bleach plant from its own material list.
Why does a ball valve cavity overpressure in a steam traced line?
Because the closed cavity is liquid full and rigid. At 120 degrees C each kelvin of heating adds about 1.5 bar inside it, since water expands by roughly 7.5 x 10^-4 per kelvin against a modulus near 2 GPa, so 30 kelvin of tracing heat adds roughly 45 bar inside a body rated for 19. The relief has to be designed: an upstream cavity relief hole, a self-relieving seat, or an external connection. The second question is where the relief discharges, because a relieved cavity between caustic and water is a cross-contamination path as well as a pressure path.
How should the actuator be sized for liquor service?
At 2.0 to 2.5 times the clean breakaway torque for black liquor, and 1.5 times for white and green liquor, using the air pressure available at the actuator on the worst day rather than the compressor rating, and the lowest recorded ambient temperature. A spring return actuator is weakest at the end of the return stroke, so the spring is selected against the seated breakout torque, not the running torque. The basis should be written into the datasheet so that it can be checked later.
