Calcium Silicate Insulation Pipe Cover: How Pipe Sections Are Made

A calcium silicate insulation pipe cover is made by forming a lime, silica and fiber slurry in split cylindrical molds, curing it in an autoclave so tobermorite or xonotlite crystals form, drying it, then trimming it to standard pipe sizes. It is supplied as half sections or curved segments sized to ASTM C585; calcium silicate is typically 1 to 3 in thick per layer.

This article is about production: how pipe sections and segments are formed and finished, how the size and thickness series affects the mold list, and what to check on every batch. Product performance, such as thermal conductivity and service temperature, is covered in calcium silicate insulation properties, and the comparison with other materials in calcium silicate vs perlite pipe insulation.

Pipe Sections, Segments and Block: The Product Forms

Calcium silicate pipe and block insulation, often called calsil in the trade, comes in three basic forms: pipe sections, curved segments and flat block or board. The National Insulation Association (NIA, 2013) describes calcium silicate pipe insulation as “hollow cylinder shapes, split in half lengthwise, or as curved segments,” typically 36 in long, with single-layer thicknesses from 1 to 3 in and thicker insulation “supplied as nested sections.” Block is supplied 36 in long, in widths of 6, 12 and 18 in and thicknesses of 1 to 4 in, with grooved block for large curved surfaces.

The product standards name the same forms:

China’s national standard for calcium silicate thermal insulation is GB/T 10699; if your buyers specify it, check the current edition for its product forms, density classes and tolerances.

For a producer, the forms decide the molds. Half sections cover small and medium pipe; curved segments usually cover large pipe and vessels, where a single half shell becomes awkward to form, autoclave and ship without breakage; block covers flat surfaces and can be machined into special shapes. Which temperature grade and density each form needs is a product question, covered in our guide to calcium silicate insulation properties and thermal conductivity.

How a Calcium Silicate Insulation Pipe Cover Is Made

The production sequence for a calcium silicate insulation pipe cover is the same as for insulation board up to forming: boards and pipe sections are made from the same slurry and formulation, and only the presses and molds differ. The main steps are:

1. Raw material preparation. Lime is slaked, quartz sand or another silica source is wet-ground, and glass fiber or pulp is dispersed. All the products are asbestos-free.
2. Batching and slurry. Lime milk, silica slurry, fiber and water are weighed and mixed in a high-speed mixer, then held in stirred tanks.
3. Hydrothermal synthesis (high-temperature grades only). For the 1,000 °C and 1,050 °C grades, a stirred pressure reactor first synthesizes xonotlite slurry. The 650 °C tobermorite grade skips this step (the static process).
4. Forming. Slurry is filled into split steel molds around a core and dewatered under pressure. Large sizes are formed as segments.
5. Demolding and green handling. The green shells are released and placed on cars. They are weak at this stage, so handling causes much of the early breakage.
6. Autoclave curing. Steam curing completes the crystal formation and gives the section its strength.
7. Drying. Hot air removes the water left in the pores.
8. Machining and finishing. Ends are trimmed, bores are turned on large sections, and shapes for fittings are cut.
9. Inspection and packing. Dimensions, density and strength are checked, and the sections are packed in matched pairs.

The full equipment list, section by section, is in our calcium silicate plant setup guide.

Forming Pipe Sections and Curved Segments

Forming decides the dimensions, the density and much of the scrap rate of a pipe cover. There are two established approaches, and they follow the process route.

Casting in arcuate molds (static process)

In the older static route, the lime–silica slurry gels and hardens in the mold. A 1966 US patent describes it plainly: blocks are formed in deeper containers, while for pipe sections “generally shallow arcuate mold halves are used in conjunction with cylindrical cores” (US 3,231,657). The same patent notes that density “depends essentially upon the amount of water” and reports a piece of about 12 lb/ft³ (about 190 kg/m³, our conversion).

Filter-press molding (dynamic process)

In the dynamic route, xonotlite slurry made in a stirred reactor is shaped by filter-press molding. A 1981 US patent gives a usual press-filter molding pressure range of 1 to 200 kg/cm² G, followed by steam curing at 5 to 50 kg/cm² G and drying (150 °C for 8 hours in its examples), with very light products around 0.10 g/cm³ (US 4,298,561). The wide range shows that pressure is set per product: together with slurry solids, it decides how much water is squeezed out and so how dense the section is.

Pipe-section forming on our reference designs

On our reference designs, pipe sections are formed in split steel molds made to your drawings, with slurry injection, an optional extruder for semi-dry forming, a lathe for the bore of large-diameter sections and a green-body cutter. In our Middle East project, the second line added a 200 t pipe-section press with 16 molds made to the client’s own drawings.

Mold fit is the detail that causes trouble. On the same project, slurry leaked during pressing because of mold fit, and pressing time was extended from 1 to 5 minutes at a lower pressing speed. Press and molds should be designed and checked together; for pipe sections, the molds are made to your drawings.

Autoclaving and Drying Pipe Sections

Autoclaving forms the crystals that set the service temperature; drying removes the water that would otherwise raise conductivity and weight. Both have to be gentle enough not to crack curved, thin-walled shapes.

• Autoclave. On our reference designs, autoclaves of 2 m diameter and 21.5 m length operate at about 1.2 MPa and 190 °C. In our Middle East line, boards are autoclaved at 190–200 °C for 6–20 hours. Pipe-section schedules are set for each section size during commissioning. The 1966 patent above cites cycles from one hour up to 15 hours at pressures from 75 psig upward, and warns that superheat above 600 °F could leave products with “stress and/or strain defects including obvious cracking” (US 3,231,657).
• Drying. In the same Middle East line, boards are dried with hot air at 100–180 °C to below 5% moisture. The drying section on our reference designs is a tunnel dryer with hot-air furnace, fans and dryer cars.

Pipe sections dry less evenly than flat board: the inner and outer surfaces are exposed differently, and a curved shell can distort if one side dries faster. Car loading and airflow should be designed for the section sizes you make, not copied from a board dryer.

ASTM C585 Sizes: What They Mean for Your Mold List

ASTM C585 is the dimensional standard behind most pipe insulation size tables. Its scope covers preformed insulation “supplied in cylindrical sections” and lists recommended single-layer inner and outer diameters for nominal wall thicknesses from ½ to 5 in (13–127 mm) for pipe insulation in general, to fit over standard sizes of pipe and tubing (ASTM C585-22). Inch-pound values are the standard; SI values are for information only.

Three points from the standard matter to a producer:

1. Its purpose is to limit the size range. It aims “to minimize the number of insulation sizes and thicknesses to be manufactured and stocked” (ASTM C585-22).
2. Nominal is not actual. The inner and outer diameters are fixed so sections fit over pipe and tube. The actual wall thickness of a given nominal size can differ from the nominal figure, so take dimensions from the standard’s tables, not from simple arithmetic. Check the current edition for the full table.
3. Nesting is not guaranteed. The standard warns that its dimensions “do not necessarily permit application of one thickness of pipe insulation over another” and that wide outer diameter tolerances “will prevent many pipe and tube insulations from nesting” (ASTM C585-22).

Estimating the mold program

The size series multiplies quickly. As an illustrative estimate, a producer offering 20 pipe sizes in 4 wall thicknesses needs 80 size combinations, each with its own mold set, and more sets for the fast-moving sizes. That count does not yet include the molds for curved segments, fittings and other special shapes. That is why many plants start with the sizes their local refineries, power plants and process industries actually order, then add molds as demand is proven. Ask your customers for their pipe schedules before the mold list is fixed.

Machining, Fittings and Finishing

Finishing turns a cured shell into a section that fits. The usual operations are end trimming to length, bore turning for large-diameter sections, cutting of segments, and machining of shapes for valves, elbows and tees. The NIA (2013) notes that valve and fitting insulation can be fabricated from standard sections, and that calcium silicate is “normally finished with a metal or fabric jacket for appearance and weather protection.”

Two production points follow:

• Keep halves together. Pairs from the same mold and pressing fit best, so mark and pack them as pairs.
• Plan for trim and broken green ware. Machining and early breakage produce scrap. Plan dust collection at the saws and lathes, and water recycling for slurry and press water.

Quality Checks for Calcium Silicate Pipe Sections

Industrial buyers ask for density, thermal conductivity, compressive and flexural strength and linear shrinkage, tested batch by batch, so the laboratory should be part of the line from day one. For pipe sections, dimensional checks against the size table your customers use matter as much as material tests.

• Material tests. ASTM C533 lists apparent thermal conductivity, linear shrinkage after heat soaking, flexural and compressive strength, mass loss by tumbling, hot-surface performance, surface burning characteristics, stress corrosion performance and moisture content among its test methods (ASTM C533-17(2023)). Its three types are classified mainly by maximum-use temperature and density (NIA, 2013); check the current edition for the limits.
• Stainless steel service. Industrial buyers often also ask for ASTM C795, the specification for non-metallic thermal insulation for use in contact with austenitic stainless steel. It requires the insulation to pass the preproduction stress corrosion test of ASTM C692 and the chemical requirements of ASTM C871 (ASTM C795-08(2023)), so raw materials and process water have to be controlled with it in mind.
• Dimensions. Inner diameter, wall thickness, length and the fit of each pair should be checked against ASTM C585 or the table named in the order.

A complete line includes a compression and flexural tester, a thermal conductivity tester, a drying oven and balance for density and moisture, and a high-temperature furnace for linear shrinkage. For how the results compare across grades, see calcium silicate insulation properties.

Planning Pipe Section Capacity

Pipe section output is set by molds, cars and autoclave capacity, not by the press rating alone. A pipe section usually takes more autoclave and dryer space per cubic meter of product than flat board, because of its hollow shape.

The single lines we coordinate typically range from 10,000 to 40,000 m³ a year. If high-temperature pipe sections are in your plan, include the reactor section from the start, because a static-process line cannot make the 1,000 °C and 1,050 °C grades. Steam also needs planning: size the boiler for every unit that draws steam at the same time.

Raw materials matter as much as the molds: the purity, fineness and reactivity of your local silica and lime decide how the crystals develop, and with them density, thermal conductivity and linear shrinkage. The general logic of lime–silica chemistry is explained in calcium silicate board raw materials, although insulation formulations differ from building board formulations.

FAQ

What is a calcium silicate insulation pipe cover?

It is preformed calcium silicate insulation shaped to fit around a pipe, usually as two half sections split lengthwise, or as curved segments for large pipe. It is made from lime, silica and reinforcing fiber, cured in an autoclave and dried. ASTM C533 covers it in the United States and EN 14306 in Europe. Grades and thermal performance are explained in calcium silicate insulation properties.

Is there a calcium silicate pipe insulation thickness chart?

The size tables come from ASTM C585, which lists single-layer inner and outer diameters for nominal wall thicknesses from ½ to 5 in for pipe insulation in general. For calcium silicate, the National Insulation Association describes single-layer thicknesses of 1 to 3 in, with thicker insulation supplied as nested sections. Use the current ASTM C585 tables for actual diameters.

How are calcium silicate pipe sections formed?

Slurry is filled into split steel molds around a core and dewatered under pressure, or, in the older static route, cast in arcuate mold halves where it hardens before autoclaving. High-temperature grades use xonotlite slurry from a stirred reactor, shaped by filter-press molding. Large pipe sizes are formed as curved segments.

Can one line make both pipe sections and board?

Yes. Both are made from the same slurry and formulation; you add the presses and molds for each product. Pipe-section molds are made to your drawings, so define the pipe sizes and wall thicknesses you will sell before the molds are ordered.

Why do calcium silicate pipe sections crack in production?

Common causes are rough handling of weak green shells, poor mold fit, too fast heating in the autoclave and uneven drying of curved shapes. A historical patent links excessive superheat in curing to cracking. Cracking problems are usually traced by reviewing the slurry formula, the autoclave schedule and the drying airflow together.

Planning a Calcium Silicate Pipe Insulation Plant?

The equipment for raw materials, forming, autoclaving, drying and finishing, for board and pipe sections in 650 °C, 1,000 °C and 1,050 °C grades, is set out on our calcium silicate insulation production line page. The calcium silicate plant setup guide covers the planning sequence, from raw material tests to commissioning.

Sources

• ASTM C585-22 Standard Practice for Inner and Outer Diameters of Thermal Insulation for Nominal Sizes of Pipe and Tubing – ASTM International, 2022: https://store.astm.org/c0585-22.html
• ASTM C533-17(2023) Standard Specification for Calcium Silicate Block and Pipe Thermal Insulation – ASTM International, 2023: https://store.astm.org/c0533-17r23.html
• ASTM C795-08(2023) Standard Specification for Thermal Insulation for Use in Contact with Austenitic Stainless Steel – ASTM International, 2023: https://store.astm.org/c0795-08r23.html
• EVS-EN 14306:2015 Thermal insulation products for building equipment and industrial installations – Factory made calcium silicate (CS) products – Specification – CEN; Estonian Centre for Standardisation and Accreditation, 2015: https://www.evs.ee/en/evs-en-14306-2015
• ISO 8143:2010 Thermal insulation products for building equipment and industrial installations – Calcium silicate products – ISO, 2010: https://www.iso.org/standard/44974.html
• Exploring Insulation Materials: Granular Insulations, Insulation Outlook – National Insulation Association, 2013: https://insulation.org/io/articles/exploring-insulation-materials-granular-insulations/
• US 3,231,657 A, Method of curing calcium silicate insulating materials – US patent, 1966: https://patents.google.com/patent/US3231657A/en
• US 4,298,561 A, Process for preparing calcium silicate shaped product – US patent, 1981: https://patents.google.com/patent/US4298561