by Tom Whitaker, Industrial Insulation Group

and Gordon H. Hart, P.E., Artek Engineering, LLC

Calcium silicate is used to insulate high temperature pipes and equipment and for fire endurance applications.  It is manufactured and sold in three different forms: preformed block, preformed pipe, and board.  Today’s calcium silicate manufactured in North America is noted for its high compressive strength, corrosion inhibiting properties, high temperature structural integrity, and a formulation free of unsafe materials.  It can withstand continuous temperatures up to either 1200°F (Type I, for pipe and block) or 1700°F (Type II, fire endurance boards).  Structural calcium silicate is available for applications requiring higher temperature resistance and greater strength but is not covered in this publication.

Calcium silicate first evolved about 1950 from two earlier types of high temperature thermal insulation: 85% magnesium carbonate and pure asbestos insulation. At one time there were as many as eight manufacturing plants in North America making use of a number of different manufacturing processes.  When first developed, calcium silicate insulation was typically reinforced with asbestos fibers.  By the end of 1972, the majority of North American manufacturers had switched the reinforcing fibers to glass fiber, plant fibers, cotton linters, or rayon.  Today, North American manufactured calcium silicate contains no asbestos. 

 

When industrial facilities started asbestos insulation abatement programs in the 1970s, asbestos-free calcium silicate was widely used as the replacement material.  It came to be used on piping and equipment at oil refineries, petrochemical plants, power plants, steam distribution lines, and other high temperature applications requiring a high strength insulation material.  Today, there are only two North American manufacturing plants producing calcium silicate insulation although there are other plants throughout the world.

Calcium silicate is made from amorphous silica, lime, reinforcing fibers and other additives that are mixed with water in a batch mixing tank to form a slurry.  This slurry is pumped to the pre-heater where it is heated to boiling and quickly poured into molds.  After a few minutes of setting in the molds, the material is removed as a solid which is wet and fragile.  These formed but uncured pieces are placed into an indurator (i.e., a sort of steam pressure cooker), for several hours, where the chemical reaction takes place to form calcium silicate.  The pieces are then removed from the indurator and placed into a drying oven.  After drying, the pieces are removed from the drying oven and trimmed, slit into two or more pieces, and packaged in cartons.  The entire process is a relatively low energy process as the highest temperature reached is only about 380° F, that being in the indurator.  

The molded, cured insulation material is essentially a crystalline formation with more air space than solid space (> 90% air).  Millions of tiny air spaces separated from one another by low thermal conductivity crystalline walls give calcium silicate its insulating characteristics.  It is from these small spaces that calcium silicate obtains its insulating properties. In particular, very little infrared (IR) radiation is able to pass through the material, making it a highly effective high temperature insulation material.

There is an ASTM International material specification, ASTM C533, “Standard Specification for Calcium Silicate Block and Pipe Thermal Insulation”, that establishes minimum acceptable standards for both Types I and II.  Type I is rated to a maximum use temperature of 1200° F and has a maximum density of 15 lbs/ft³ whereas Type II is rated to 1700° F and has a maximum density of 22 lbs/ft³.  The as-manufactured compressive strength for both types is greater than 100 psi, at a 5% deformation, the highest of any non-structural high temperature insulation material included in the ASTM materials specifications.  The maximum linear shrinkage, after exposure to the maximum use temperature, is only 2% for both types.  The flexural strength is greater than 50 psi for both types of materials.  Both the flame spread and smoke developed index are 0, per ASTM E84, since the material does not contribute to combustion in any manner.  Maximum allowable mass loss values in the ASTM specification are 20% and 40% after tumbling for 10 minutes and 20 minutes, respectively, demonstrating its resistance to breakage.  Maximum allowable thermal conductivity values at various corresponding mean temperature values, given in Table 1, are plotted below to demonstrate an important characteristic, namely a relatively flat thermal curve that does not increase greatly with increasing mean temperature (i.e., the thermal conductivity does not even double with an increase in mean temperature from 100° F to 700° F).

 

Not all product characteristics are fully defined in ASTM C533 yet there are some that are nevertheless very important.  For example, thermal conductivity and compressive strength are not adversely affected after testing for maximum use temperature in accordance with ASTM C411.  North American calcium silicate is formulated and manufactured to inhibit corrosion under insulation (CUI) on both stainless and carbon steel.  This material is also classified as non-combustible per ASTM E136 and when used as part of a system with jacketing and sufficient thickness, it can effectively provide fire endurance performance such as protecting piping and tanks from a 2000°F hydrocarbon fire.

Calcium silicate insulation is typically covered with a protective jacketing.  This may be conventional aluminum sheet, stainless steel sheet, poly vinyl chloride (PVC) sheet, glass cloth with weather barrier mastic, or a multi-ply laminate.  To prevent intrusion of water, a bead of sealant should be used on sheet metal jacketing overlaps.  

Table 1: Physical Properties of calcium silicate pipe and block 

from ASTM C533

Physical Properties (ASTM standard test)	Type I (pipe, block and board)	Type II (board only)
Maximum Use Temperature, Deg. F	1200	1700
Density, maximum, pcf (C167)	15	22
Flexural strength, minimum, psi (pipe: C203, block: C446)	50	50
Compressive strength, minimum, psi (C165)	100	100
Mass loss by tumbling, maximum, % (C421)
After first 10 minutes	20	20
After second 10 minutes	40	40
Soaking heat linear shrinkage, max., % (C356)	2	2
Hot surface performance at 1200 degrees F (C411, C447)
warpage, maximum, in.
cracking	No thru cracks	No thru cracks
Apparent thermal conductivity, maximum (Btu-in/h-ft²-degreesF) at Mean Tempterature of (deg. F) (pipe: C335, block: C177, both: C1045)
100	0.41	0.50
200	0.45	0.54
300	0.50	0.58
400	0.55	0.61
500	0.60	0.64
600	0.66	0.67
700	0.71	0.70
800		0.73
900		0.72
1000		0.77
Surface Burning Characteristics (E84)
Flame spread index, max	0	0
Smoke density, max	0	0
Non-Combustability (E136)	Passes	Passes
Non-Corrosive to Austenitic Stainless Steel (C795)	Passes	Passes

Calcium silicate is typically applied on high temperature (> 250°F) piping and equipment in industrial facilities such as chemical plants, refineries, steam transfer lines, and steam electric power plants.  Since it is a rigid material that has a relatively flat thermal conductivity curve, extremely high compressive strength, high flexural strength, a Class A rating for Flame Spread/Smoke Developed, and is non-combustible (ASTM E136), it is widely used in high temperature, industrial applications subject to physical abuse.  In addition, the North American products are manufactured with a process and formulation to inhibit corrosion on the steel pipe and equipment surfaces.

Due to its high compressive strength (> 100 psi), high flexural strength (> 50 psi), and resistance to damage from tumbling, plus its ability to maintain those properties over time up to its rated 1200ºF, calcium silicate can withstand considerable physical abuse without loss of insulating efficiency.  Horizontal pipes and equipment are particularly vulnerable where workers may step on and lean ladders against the insulated pipes. In addition, the structural integrity of calcium silicate is often a design consideration where pipe and equipment are expected to vibrate on a continuing basis.  Calcium silicate can withstand vibration induced by high-temperature steam flow around internal pipe obstructions such as valve internals, measuring devices, and flow restriction orifices. .

As with other types of thermal insulation, calcium silicate is a very sustainable material. This is based on input energy used during its manufacture compared to the energy saved over its life.

Example based on 3E Plus®:

• 8” NPS Pipe operating at 800°F for 95% of the year in a 60°F environment with 5 mph wind

• 3” calcium silicate insulation covered with aluminum jacket

Compared to uninsulated pipe, the calcium silicate provides a heat loss reduction of 97.44% resulting in predicted annual energy savings of  88.5 million Btus per lineal foot of pipe.

The energy required to manufacture a lineal foot of calcium silicate is so relatively small (about 154,000 Btu / LF) that the ratio of energy used to energy saved is 575:1 for one year and 11,500:1 for 20 years.

As the world struggles with the supply of energy and its related cost, conservation is an important element.  All insulation integrated into a properly designed system, to include calcium silicate, can be and is a major contributor to conservation.