Introduction to Compression Packing

Transcription

1 Exercise E250-S03-EXR-DFT.wpd Introduction to Compression Packing Objectives Given DAC #250, several types of compression packings, and the packing vendor specification information, identify the environmental conditions in which the packing can be exposed. Performance Standard Identify all the italicized terms with 100% accuracy. Match with 100% accuracy the correct compression packing with the process application in which it will be used. Foundation Competencies None Required Background Reading None Tools Required Pencil Exercise Data Sheet Components Required DAC #250 Various compression packings Design Assistance Corporation 03-1

2 Introductory Discussion The purpose of the packing in a valve is to prevent the leakage of the process material to the exterior atmosphere past the stem of the valve. The valve packing is contained in the valve bonnet. The earliest packing consisted of cotton, oakum or other fibrous waste that was literally stuffed in the enclosure, hence the term stuffing box, and then lubricated. A stuffing box is used to control the leakage at the point where the valve stem penetrates the valve body. A plain stuffing box is made up of two primary components, packing and a gland follower. Figure 1 Valve Components The majority of packing is available in two general forms: compression and die cut. In this exercise you will look at compression packing, also referred to as braided packing. Compression packings, so called because of the manner in which it performs the sealing function, are made from relatively soft, pliable materials. They consist of a number of rings that are inserted into the annular space between the stem and bonnet (the stuffing box). By tightening a follower against the top, pressure is transmitted to the packing set, expanding the rings radially against the side of the stuffing box and the valve stem, effecting a seal. The compression packings are formed by either twisting or braiding individual strands of yarns together to obtain either a round or square finished product. Each type of construction has a particular set of advantages and some disadvantages when compared to another construction. The braided packings are sold as either 2007 Design Assistance Corporation 03-2

3 pre-cut rings or in continuous ropes that are rolled on a spool and must be cut and bent around a rod/shaft and compressed into a suitable shape. Leakage through the packing material is prevented by the lubricant contained in the packing. As the packing is compressed, it must have the ability to deform in order to seal. It must also have a certain ruggedness of construction so that it may be readily cut into rings and assembled into the stuffing box without serious breakage or deformation. All compression packings are impregnated with a variety of lubricants. The lubricants provide a resiliency that allows the packing to deform and recover under slight mechanical deficiencies such as shaft deflection. Approximately 30% of the total volume of compression packing is lubricant. The four principal types of compression packing construction are: Twisted Square-braided Braid-over-braid Interlocked-braided Twisted compression packing, shown in Figure 2, is made from multiple yarns, rovings, or metallic ribbons by turning the individual strands around each other at a constant number of turns per foot (or per meter). The materials are twisted together or around a core to obtain the desired size. The twisted strands are lubricated with mineral oil and graphite. Figure 2 Twisted Compression Packing Twisted packing is not as strong as the braided type, but it is quite easy to install. Usually it is round in cross-section but in the case of the metallic ribbons, it is normally formed into a square section during the twisting operation. One size of packing can be used for several stuffing box sizes, because of its twisted construction. Strands from a larger size can be untwisted and removed so that the remaining packing will fit a smaller size stuffing box. When metallic materials are used in the packing, they can be made to resist 2007 Design Assistance Corporation 03-3

4 high temperatures and pressures, to resist the penetration of fluids, and to conform to the irregularities of worn equipment. The twisted type packing is primarily for smaller valves. Square-braided packing, also known as plait braid packing, is made of asbestos, cotton, plastic, or leather either alone or in combination and sometimes include metal wires such as lead or copper. This type of packing, shown in Figure 3, is processed on equipment where strands (normally 8 or 16) pass over and under strands running in the same direction. Figure 3 Square-Braided Packing This form allows the wearing away of individual fibers and also retains pockets of lubricant between each plait. The loose structure makes the packing quite soft. The packing's softness permits flexibility and easy adjustment and makes it usable on old or worn equipment. Resulting packings are usually supplied in square cross section, as shown in Figure 4, but rectangular sizes can also be braided by this method. It usually contains a high percentage of lubricant (grease- or oil-impregnated). This form of packing is normally suited to centrifugal pump applications rather than valve service. Figure 4 Square- Braided Packing 2007 Design Assistance Corporation 03-4

5 Braid-over-braid packing, also known as round braid packing, is shown in Figure 5. It is manufactured by round braiding machines that braid tubular jackets using yarns rovings, ribbons and other forms of various materials, either alone or in combination. Size is obtained by braiding jackets one over the other (braid-over-braid). The braided jackets are impregnated with lubricants. Figure 5 Braid-Over-Braid Packing This type of packing is sometimes braided over a lead core, as shown in Figure 6, to give better shape-holding characteristics. The finished packings are usually calendered into a square or rectangular cross section. Figure 6 Braid-Over-Core Packing Braid-over-braid and braid-over-core packings are quite dense and particularly suitable for valve stem sealing. When wire reinforced, as shown in Figure 7, they are normally used in high pressure applications. Braid-over-braid, when not reinforced with wire, is quite conformable and thus is useful in low pressure situations. Figure 7 Wire-Reinforced Braid-Over-Braid Packing 2007 Design Assistance Corporation 03-5

6 Interlock braided packing, shown in Figure 8, is usually more dense and impermeable than square braided packing. It is often used for the manufacture of valve seals. Yarns, rovings (untwisted bundles of fibers), ribbons and other forms of various materials, either alone or in combination, are processed on equipment where the strands criss-cross from the surface diagonally through the body of the packing. Each strand is strongly locked by other strands to form a solid integral structure that cannot easily ravel or come apart in service. There are no jackets to wear through, and no plaits to come loose. Figure 8 Interlock Braided Packing In normally constructed interlocking braid, there is a relatively even density throughout the braid. It is relatively dense, yet it is flexible and conformable. These characteristics make it the most universal braid for use in valves at the highest pressures. Interlock braided packing may be formed as square braids, braid-over-braid, and twist or laminated, wrapped and/or crimped. It is typically utilized in devices with rotating, reciprocating or oscillating motion. However, it also can be used as a static seal. Square interlock braided packing is used on pumps and valves in lowpressure operations, while braid-over-braid constructions are generally used for high-pressure applications, especially in valve stems and expansion joints. Braidover-core constructions, however, are used for applications where greater resilience is required, such as for sealing shafts on pumps or valve stems on valves in nuclear power plants. Figure 9 Interlock Braided Packing 2007 Design Assistance Corporation 03-6

7 Most compression packing can function without the injection of additional lubricant and/or cooling fluid, and most of the lubricant required for satisfactory performance is inherent in the raw material and the braid. Packing is used in a wide variety of process applications in industry. The process applications vary in temperature, pressure, and ph. The packing material must be such that it can handle the environment in which it is exposed. Many manmade fibers are available today, which allow for very economic and effective sealing solutions. Below is an overview of the most common packing materials. These materials can be used singly or braided in combination with two different styles to achieve a particular performance level to satisfy a unique application. Glass Glass fibers have superior thermal properties, dimensional stability, and tensile strength. They resist most chemicals and can be formulated to resist strong acids. Glass fibers are available in filament yarns suitable for braiding and are treated with chemicals to protect the fibers. Treatments have been developed such as the use of PTFE (polytetrafluoroethylene, or fluorocarbon, commonly known as Teflon ) resin impregnation to enhance the service applications of these glass fibers. Glass will not burn. The high thermal conductivity of glass fiber dissipates heat more rapidly than organic fibers. The most common glass fiber is called E-glass. E-glass retains 75% of its tensile strength at 650F. Common solvents, oils, petroleum distillates, bleaches and most organic chemicals do not affect glass fibers. Strong acids however will corrode all of the commercially available glass fibers Design Assistance Corporation 03-7

8 Carbon Graphite These fibers are commonly used in braided packings. The difference between these two materials is the crystalline structure. Graphite operates at a higher temperature because of its higher thermal conductivity. The high stability of these fibers normally dictates their use in severe sealing applications. These packings are generally the most expensive of the packing family, but in most applications the less frequent replacement actually makes them more cost effective to use. Aramid Fibers These are synthetic fibers called aromatic polyamids that were given the generic name aramid. One such popular fiber is Dupont Kevlar. These fibers have excellent resistance to high temperatures accompanied by high tensile strength, which make them perfect for rugged services. In general, aramid fibers have good chemical resistance, but are not as effective as the fluorocarbon fibers (PTFE for example). Acids, such as hydrochloric and nitric acids, will significantly reduce their tensile strength. Strong alkalis, like sodium hydroxide, will also degrade aramid fibers. In applications such as braided packings, where the aramid fibers are tightly braided and have been impregnated with PTFE, the chemical resistance can be greatly improved Design Assistance Corporation 03-8

9 PTFE (fluorocarbon) Fibers These fibers have an especially high resistance to chemicals and heat, as well as exceptional low levels of friction and adhesion. Packing braided from PTFE fibers offers outstanding performance in highly corrosive chemical environments, as well as under less severe conditions. It is virtually indestructible, has a low coefficient of friction, high compression strength, and good dimensional stability, is self-lubricating, and gives continuous service to 550F. PTFE packings are more expensive as compared to synthetic fibers such as aramid or acrylics, but in many applications, their value offsets the initial installation costs. Metallic Packing materials such as lead, copper, and aluminum are readily moldable and heat conductive. Lubricated lead rings are often used by themselves or combined with other materials to equalize load deformation on installed packing, or used as antiextrusion rings in high pressure pump applications. Copper is used primarily as end rings on slow reciprocating, worn and non-conductive shafts. Successful sealing with compression packing is a function of two important and related factors: The service parameters need to be properly matched to the ability of the packing chosen. Refer to the packing vendor data sheets and selection guides to match the packing styles to temperatures, pressures, ph ranges and the media to be sealed. The correct size packing must be used to ensure the proper seal Design Assistance Corporation 03-9

10 The proper selection of packing materials is dependent on the operating conditions of the equipment. Six parameters of the equipment must be known before a proper packing recommendation can be made. The acronym "STAMPS" is used to gather this information: S = Size - cross section T = Temperature of the media being sealed A = Application - type of equipment (i.e., pumps, valves, mixers, etc.) M = Media - process material being sealed P = Pressure of media being sealed S = Speed - shaft speed in FPM (pumps only) Packing performance can be affected by many factors, including: Frequency of stem movement System pressure and temperature Fluid medium Stem and stuffing box surface finish Stuffing box depth and diameter Accuracy of stem guidance Vibration Quaility of installation Packing composition Gland pressure In this exercise you will identify different types of packing and determine which type is best suited for particular applications Design Assistance Corporation 03-10

11 Performance Steps Step 1. 3 Identify the different / 8" bulk braided packing types that are provided with the DAC : Identify the Palemetto No. 1347AF, PTFE Impregnated Syn-Tex Packing. Identify the Palemetto No. 1359, Aramid Corner eptfe/graphite Packing. Step 2. Step 3. Identify the Palemetto No. 1588, Pulpak Papermill Packing. Identify the Palemetto No. 4062AF, Wire Inserted Steam Valve Packing. Identify the Palemetto No. 5000W, Braided Flexible Graphite Steam Valve Service. Using the attached Palmetto Packing specification sheets, identify which of the packings identified in Step 1 is best suited for the applications listed below. Identify the packing that is best suited for use with caustic white liquor (pulp and paper industry). Identify the packing that is best suited for use with steam at a pressure of 650 psi. Identify the packing that is best suited for use with extreme steam pressure and temperature. Identify the packing that is best suited for use with abrasive slurries. Identify the packing that is best suited for use with non-staining substances and allows for valve stem misalignment. Return the to its initial starting condition. Return all packing to its normal storage location Design Assistance Corporation 03-11

12 Return all books and drawings to their normal storage location. Ensure that your work area is neat and clean. Note: Ask your instructor to inspect your training station. The instructor will verify the work area is clean and clear of clutter before accepting your station and initialing the exercise data sheet. Have the instructor initial your Exercise Data Sheet Design Assistance Corporation 03-12

13 Summary Congratulations! You have demonstrated your ability to identify various types of braided valve packing and their applications. Your knowledge of the braided packings will be needed as you continue with the exercises that follow. Optional Tasks Identify other types of braided valve packings and their applications Design Assistance Corporation 03-13

14 Resources Electrical Power Research Institute, Valve Stem Packing Improvements. Waltham, MA: Foster-Miller, Inc. European Sealing Association, Sealing Technology - BAT guidance notes. ESA Publication No. 014/05 th Thomas Bieber and Carl A. Nelson, Audel Millwrights & Mechanics Guide, 5 Ed. Hoboken, NJ: Wiley Publishing, Inc Design Assistance Corporation 03-14

15 Review Questions Name: Date: Answer the following review questions. Circle the correct response(s) or write your answers in the space provided below. 1. Which of the following best describes twisted compression packing? (a) Each strand is strongly locked by other strands to form a solid integral structure that cannot easily ravel or come apart in service. There are no jackets to wear through, and no plaits to come loose. (b) Strands pass over and under strands running in the same direction. Loose structure makes the packing soft. (c) Manufactured by round braiding machines that braid tubular jackets of various materials, either alone or in combination. Packings are quite dense and particularly suitable for valve stem sealing. (d) Once size fits all - strands from a larger size packing can be removed so that the remaining packing will fit a smaller stuffing box. 2. Which of the following best describes interlocked compression packing? (a) Each strand is strongly locked by other strands to form a solid integral structure that cannot easily ravel or come apart in service. There are no jackets to wear through, and no plaits to come loose. (b) Strands pass over and under strands running in the same direction. Loose structure makes the packing soft. (c) Manufactured by round braiding machines that braid tubular jackets of various materials, either alone or in combination. Packings are quite dense and particularly suitable for valve stem sealing. (d) Once size fits all - strands from a larger size packing can be removed so that the remaining packing will fit a smaller stuffing box Design Assistance Corporation 03-15

16 3. What type of service parameters should be considered when choosing the type of compression packing? 4. What factors can affect the performance of packing? 2007 Design Assistance Corporation 03-16

17 Exercise E250-S03-EXR-DFT.wpd Introduction to Compression Packing Exercise Data Sheet Name (s): Date Completed: Initials: 3 Step 1. Identify the different / 8" bulk braided packing types that are provided with the DAC. Palemetto No. 1347AF, PTFE Impregnated Syn-Tex packing identified. Palemetto No. 1359, Aramid Corner eptfe/graphite packing identified. Palemetto No. 1588, Pulpak Papermill packing identified. Palemetto No. 4062AF, Wire Inserted Steam Valve packing identified. Palemetto No. 5000W, Braided Flexible Graphite Steam Valve Service packing identified Design Assistance Corporation 03-17

18 Step 2. Using the attached Palmetto Packing specification sheets, identify which of the above packings is best suited for the applications listed below. The packings best suited for use with caustic white liquor (pulp and paper industry) are: The packings best suited for use with steam at a pressure of 650 psi are: The packings best suited for use with extreme steam pressure and temperature are: 2007 Design Assistance Corporation 03-18

19 The packings best suited for use with abrasive slurries are: The packings best suited for use with non-staining materials and allows for valve stem misalignment are: Step 3. Return the to its initial starting condition. Returned all packing to its normal storage location. Returned all books and drawings to their normal storage location. Work area is neat and clean. Instructor s initials: Design Assistance Corporation 03-19