Patent Number: 046769463
Section: summary

FIELD OF THE INVENTION This invention relates to thermal insulating blankets and in particular to insulating blankets useful in containment buildings of nuclear power plants. BACKGROUND OF THE INVENTION First-generation nuclear power plants utilized metallic, reflective-type insulation for pipes and equipment located inside the containment buildings. It was found that this reflective insulation did not produce the promised reduction in heat loss and resulted in reduced efficiency and increased operating costs. Further, during maintenance shutdowns this fragile metal insulation was damaged by foot traffic and/or handling making it difficult or impossible to reinstall. Since each piece was custom-made, replacements could not be readily obtained during the scheduled shutdown period. In addition, this insulation did not lend itself to easy inspection of pipe welds, which must be performed at regular intervals. As a result of these problems, nuclear power plants began to replace this metal insulation with removable blankets, particularly in areas where inspection of pipe welds was required. A potential problem arises, however, with use of blanket insulation during a loss of coolant accident (LOCA). During a LOCA, the blankets may be subjected to high energy liquid jets (i.e., subcooled liquid or steamwater mixtures at high pressure) which enter the containment area. If the jet stream impinges on the blankets, the fibrous blanket material may be torn lose generating debris which can clog the protective screen of the emergency core cooling system (ECCS) sump and thereby impair recirculation of water from the sump. The first blanket system accepted by the United States Nuclear Regulatory Commission consists of a two-inch thick blanket of very light density glass fiber fillers totally encapsulated in a glass fiber cloth envelope. The blankets are used in either single or double layer construction depending upon the pipe temperature. The cloth envelopes are formed with sewn seams and, in the case of double layer construction, the seams in the two layers are staggered with respect to one another. Velcro fastenings are used to attach the blankets to the pipes and/or adjacent blankets. An outer jacket of 26-gauge (0.01875 inch thick) stainless steel fastened by suitcase latches is sometimes used to protect the blankets. These fragile blankets are intended to be completely destroyed in the event of a LOCA with the hope that the residue from the blankets will be small enough so as not to block the sump screen and, if necessary, to pass through the recirculating pump. Another known insulating blanket used in nuclear containment buildings consists of a glass fiber filler layer having a waterproof sheet of ERCO-SIL 36S (sold by Eastern Refractories Company, Inc., Belmont, Mass.) on its outer surface (i.e., disposed away from the pipe). The remaining sides of the filler layer are covered by glass fiber cloth. Metal clips consisting of stainless steel hog rings connect the cloth to the waterproof sheet. This design is much stronger than the first design and is intended to resist damage during a LOCA. Other known insulating blankets used in nuclear containment buildings are described in Durgin, W. W., and Noreika, J. F., "The Susceptibility Of Fibrous Insulation Pillows To Debris Formation Under Exposure To Energetic Jet Flows," NUREG/CR-3710 (SAND 83-7008), Alden Research Laboratory and Sandia National Laboratory, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555. A first blanket described therein consists of 16 lb/ft.sup.3 mineral wool with a cover of asbestos cloth coated with one-half mil of Mylar (registered trademark of E. I. DuPont de Nemours & Co., Wilmington, Del.). A second consists of 11 lb/ft.sup.3 needle packed fiberglass layers covered with stainless knitted mesh on one side (close to the pipe) and silicone glass cloth on the other (outer) side. A third consists of 11 lb/ft.sup.3 needle packed fiberglass layers covered with 18 ounce fiberglass cloth. These three blankets were tested for damage and failure (loss of blanket material) under jet streams of up to 65 psi applied at an angle of 45.degree. and 90.degree. . The third blanket exhibited the greatest resistance to damage and failure. In the field of thermal and acoustic insulation blankets for exhaust systems in gas transfer plants, oil rigs, refineries and the like, it has been suggested to provide an insulation blanket comprising a ceramic fiber core approximately one and one-half inches in thickness, a thin lead sheet adjacent the core for noise insulation, a thin stainless steel foil/fiberglass cloth laminate between the core and insulated object for excluding moisture, a silicone rubber coated fiberglass cloth completely surrounding the core and other layers, and a flexible stainless steel mesh stocking enveloping the entire assembly to provide additional protection (U.S. Pat. No. 4,442,585). These blankets are wrapped tightly around pipes by means of lacing anchors and second layers are provided around the joints to render the edges significantly impermeable to thermal and acoustic radiation. SUMMARY OF THE INVENTION It is an object of this invention to provide a thermal insulating blanket useful for insulating pipes and equipment inside the containment building of a nuclear power plant and which will resist damage during a loss of coolant accident (LOCA). Another object is to provide an insulating blanket having an outer casing that will dissipate the force of a high force liquid stream striking the blanket so as to prevent the blanket from tearing. A further object is to provide an insulating blanket which if torn during a LOCA will not clog the protective screen of the emergency coolant recirculating sump. Another object is to provide an insulating blanket that will be cut into small pieces if torn by a high force liquid stream. Another object is to provide removable and reusable, flexible blanket insulation for pipes and equipment. According to this invention an insulating blanket is provided having an inner surface positionable adjacent the object to be insulated and an opposing outer surface. The blanket consists of a filler layer of thermal insulating fibers, a waterproof sheet covering one surface of the filler layer, and a wire mesh casing surrounding the filler layer and sheet to form the blanket. The waterproof sheet is positioned between the outer surface of the blanket and the filler layer. The wire mesh casing serves to dissipate high force liquid jet streams which may strike the outer surface of the blanket during a LOCA. Further, if the jet stream tears the blanket, the wire mesh serves to cut up the filler layer as it passes through the mesh so that the torn blanket pieces will not clog the protective screen of the emergency coolant recirculating sump. In a preferred embodiment, a flexible wire mesh septum lies in substantially parallel relationship between two layers of glass fiber wool or ceramic fiber wool. The mesh septum increases the strength of the blanket and aids in cutting up the filler layer as it passes through the septum if torn by a high force liquid stream. A flexible waterproof sheet, consisting of a fine wire mesh screen embedded in silicone rubber, covers the outer surface of the filler layer. A flexible wire mesh casing surrounding the filler layer and waterproof sheet has of from about 40 to about 100 regularly spaced apertures per square inch and is made from stainless steel or Inconel wire having a diameter of from about 0.005 to about 0.015 inches. More preferably, the casing consists of knitted Inconel mesh having about 60 apertures per square inch and made from about 0.011 inch diameter wire. According to the process of the invention, a thermal insulating blanket is applied to pipes and equipment in the containment building of a nuclear power station. The insulation resists damage if struck by a high force liquid stream during a LOCA and is cut into small pieces which will not clog the protective screens of the emergency coolant sump if torn during a LOCA. The insulating blanket has an inner surface positioned adjacent the pipes and equipment and an opposing outer surface, and consists of a layer of thermal insulating fibers and a wire mesh casing covering at least one surface of the filler layer to form the outer surface of the blanket. The casing dissipates high force liquid streams which strike the outer surface of the blanket and cuts up the filler layer as it passes through the casing if torn by a jet stream.