Tribology Handbook 2 2010 Part 8 pot

625 Packed glands The main applications of packed glands are for sealing the stems of valves, the shafts of rotary pumps and the plungers of reciprocating pumps. With a correct choice ofgland design and packing material they can operate for extended periods with the minimum need for adjustment. VALVE STEMS Valve stem packings use up to 5 rings of packing material as in Figure 25.1. For high temperature/high pressure steam, moulded rings of expanded graphite foil material are commonly used. This gives low valve stem friction. To reduce the risk of extrusion of the lamellar graphite during frequent valve operation, the end rings of the packing can be made from graphite/yarn filament. Materials of this type only compress in service by a small amount and can provide a virtually maintenance free valve packing if used with live loading as shown in Figure 25.2. ROTARY PUMPS Rotary pump glands commonly use up to 5 rings of packing material. For most applications up to a PV of 150 bar m/sec (sealed pressure X shaft surface speed) a simple design as in Figure 25.3 is adequate. In most pumps the pressure at the gland will be 5 bar or less and those with pressures over 10 bar will be exceptional. At PV values over 150 bar m/sec direct water cooling or jacket cooling are usually necessary and typical arrange- ments are shown in Figure 25.4 and 25.5. When pumping abrasive or toxic fluids there may be a need to provide a flushing fluid entry at the fluid end of the glands, as in Figure 25.6, or a high pressure barrier fluid which is usually injected near the centre of the gland as in Figure 25.7. Figure 25.1 A typical valve stem packing DISC SPRING STACK STUD SPACER SLEEVE BRAIDED GRAPHITE EXPANDED GRAPHITE BRAIDED GRAPH IT€ Figure 25.2 A valve stem packing using spring loading to maintain compression of the valve packing and avoid leakage LIQUID GLAND FOLLOWER Figure 25.3 A general duty rotary pump gland 625.1 Packed glands B25 LANTERN RING COOLING WATER IN LET Figure 25.4 ,A gland packing with direct cooling via a lantern ring FLUSHING FLUID Figure 25.6 A packing gland with a flushing fluid system RECIPROCATING PUMPS Reciprocating pumps also use typically 5 packing rings. However due to the increased risk of extrusion of the packing due ito the combination of high pressure and reciprocating movement, an1 i extrusion elements are usually incorporated in the gland. Self adjusting glands can be used on reciprocating pumps but the spring loading for compression take up must act in the same direction as the fluid pressure loading, as shown in Figure 25.10. JACKET COOLING JACKET COOLING PTF E ANTI-EXTRUSION RING uI f u Figure 25.5 A gland packing with a cooling jacket for high temperature applications BARRIER FLUID I Figure 25.7 A packed gland with a barrier fluid system Figure 25.8 A reciprocating pump gland with PTFE anti-extrusion washers between the packing rings Figure 25.10 A reciprocating pump gland with internal spring loading to maintain compression of the packing Figure 25.9 A reciprocating pump gland with an anti-extrusion moulded hard fabric lip seal B25.2 B25 Packed glands PACKING MATERIALS Table 25.1 Materials for use in packed glands Material Maximum operating temperature “C Special properties Typical application5 Expanded graphite foil 550°C Low friction, self lubrication, Valve stems 2500°C in non-oxidising low compression set and constituents. Available as rings environments contains no volatile Graphite/yarn filament 550°C Available as cross plaited square section lengths. Resistant to extrusion Valve stems Aramid (Kevlar) fibre 250°C Tough and abrasion resistant Valve stems and pumps PTFE filament 250°C Low friction and good chemical resistance Valve stems Pumps at surface speeds below 10 m/s Hybrid graphite/PTFE yarn 250°C Particularly suitable for high speed rotary shafts. Close bush clearances needed to reduce risk of extrusion. Good resistance to abrasives. Pump shafts for speeds of the order of 25 m/s Ramie 120°C Good water resistance ~~~ ~ __ Rotary and reciprocating water pumps Note: Many of these packings can be provided with a central rubber core which can increase their elasticity and thus assist in maintaining the gland compression. Their application depends on the temperature and chemical resistance of the type of rubber used. PACKING DIMENSIONS AND FITTING Typical gland dimensions are shown in Figure 25.1 1 and packing sizes in Table 25.2. Table 25.2 Typical radial housing widths in relation to shaft diameters. All dimensions in mm All packings except expanded graphite Exponded graphite Shajl diameter Housing radial width Shaft diameter Housing radial width up to 12 above 12 to 18 18 to 25 25 to 50 50 to 90 90 to 150 150 3 5 6.5 8 10 12.5 15 up to 18 3 above 18 to 75 5 150 and above 10 75 to 150 7.5 825.3 Packed glands B25 A=7W r FIRST OBSTRUCTION / P- ROTATING SHAFT (DEPTH 7W APPLIES WHEN LANTERN GLAND IS USED) RECIPROCATING SHAFT Figure 25.11 Typical gland dimensions for rotating and reciprocating shafts Pump shafts, valve stems and reciprocating rams should have a surface finish of better than 0.4 pm Ra. Their hardness should not be less than 250 Brinell. Rings should be cut with square butt joins and each fitted individually with joins staggered at a minimum of 90". After applying a small degree of compression to the complete set, gland nuts must be slackened off to finger tight prior to start up. Once running, any excessive leakage can then be gradually reduced by repeated small degrees of adjustment. The major cause of packing failure is excessive compression, particularly at the initial fitting stage. Further advice may be obtained from packing manufacturers. B25.4 _______~ B26 Mechanical piston rod packinqs The figure shows a typical general arrangement of a FLANGE STUD mechanical rod packing assembly. The packing (sealing) rings are free to move radially in the cups and are given an axial clearance appropriate to the materials used (see Table 26.2). The back clearance is in the range of 1 to 5 mm. (& to in). The diametral clearance of the cups is chosen to prevent contact with the rod; it lies typically in the range 1 to 5 mm (A to 6 in). The sealing faces on the rings and cups are accurately ground or lapped. The case material can be cast iron, carbon steel, stain- less steel or bronze to suit the chemical conditions. It may be drilled to provide lubricant feed to the packing, to vent leakage gas or to provide water cooling. The rings are held in contact with the rod by spring pressure; sealing action however, depends on gas forces which hold the rings radially in contact with the rod and axially against the next cup. PRESS END CONNECTION GASKE Figure 26.1 General arrangement of a typical mechanical piston rod packing assembly SELECTION OF TYPE OF PACKING 1 Pressure breaker Description Three-piece ring with bore matching rod. Total circumfe- rential clearance 0.25 mm. Garter spring to ensure contact with rod. Applications Used in first one or two compartments next to high pressure, when sealing pressure above 35 bar (500 psi) to reduce pressure and pressure fluctuations on sealing rings. 2 Radial cut/Tangential cut pair Description The radial cut ring is mounted on the high pressure side. (Two tangential cut rings can be used when there is a reversing pressure drop.) The rings are pegged to prevent the radial slots from lining up. Garter springs are fitted to ensure rod contact. Ring bores match the rod. Applications The standard design of segmental packing. Used for both metallic and filled PTFE packings. _~._______. . TANGENT 'IAL OR RADIAL CUT RING B26.1 Mechanical piston rod packings B26 3 Unequal segment ring Description The rings are pegged to prevent the gaps lining up. Garter springs are fitted to ensure rod contact. The bore of the larger segment matches the rod. Applications Rather more robust than tangentially cut rings (2) and hence more suitable for carbon-graphite packings. 4 Contracting rod packing Description Cast iron L-ring with bronze or white metal inner ring or three piece packing with filled PTFE and metallic back-up ring. Contact with rod maintained by ring tension. Rings pegged to prevent the gaps from lining up. Note: this style of packing has to be assembled over the end of the rod. Applications Used for both metallic and filled PTFE packings. 5 Cone ring Description Three ring seal-each ring in three segments with bore, matching rod. Cone angle ranging from 75" at pressure end to 45" at atmosphere end. Applications Used for both metallic and filled PTFE packings. B26.2 B26 Mechanical piston rod packings DESIGN OF PACKING ARRANGEMENT Number of sealing rings There is no theoretical basis for determining the number of sealing rings. Table 26.1 gives values that are typical of good practice: Table 26.1 The number of sealing rings for various pressures Pressure No. of sets of sealing rings up to 10 bar (150 p.s.i.) 3 10-20 bar (150-250 p.s.i.) 4 20-35 bar (250-500 psi.) 5 35-70 bar (500-1000 p.s.i.) 6 70-1 50 bar (1OOC-2000 p.s.i.) 8 above 150 bar (2000 p.s.i.) 9-12 Piston rods Rod material is chosen for strength or chemical resistance. Carbon, low alloy and high chromium steels are suitable. For the harder packings (lead bronze and cast iron) hardened rods should be used; treatment can be flame or induction hardening, or nitriding. Chrome plating or high chromium steel is used for chemical resistance. Surface finish Metal and filled PTFE packing 0.2-0.4 pm R, (8-16 pin cla) . Carbodgraphite and metal/graphite sinters 0.1-0.2 pm R, (4-8 pin cla) Dimensional tolerances Diameter Taper over stroked length * 0.01 mm (0.0005 in)’ Out-of-roundness 0.025 mm (0.001 in) f0.05 mm (+0.002 in) -0.05 mm (-0.002 in) Notes: 1 With Type 4 packings increase number of sealing rings by 50-100%. be adequate. breakers (Type 1) in addition, on the pressure side. ‘2 With Type 5 packings four sets of sealing rings should 3 Above 35 bar (500 psi) use one or two pressure Packing materials Table 26.2 The types of packing material and their applications Material Rod hardness Axial clearance Applications (1) Lead-bronze 250 BHN min 0.08-0.12 mm Optimum material with high thermal conductivity and good (0.003-0.005 in) lubricated bearing properties. Used where chemical conditions allow. Suitable for pressures up to 3000 bar (50 000 psi.) (2) Flake graphite 400 BHN min 0.08-0.12 mm (0.003-0.005 in) Cheaper alternative to (1); bore may be tin coated to assist grey cast iron running in. Suitable up to 70 bar (1000 p.s.i.) for lubricated operation (3) White metal not critical 0.08-0.12 mm (0.003-0.005 in) Used where (1) and (2) not suitable because of chemical (Babbitt) conditions. Preferred material for high chromium steel and chrome-plated rods. Max. pressure 350 bar (5000 psi). Max. temperature 120°C (4) Filled PTFE 400 BHN min 0.4-0.5 mm Suitable for unlubricated and marginally lubricated operation (0.015-0.020 in) as well as fully lubricated. Very good chemical resistance. Above 25 bar (400 psi.) a lead bronze backing ring (0.1/0.2 mm) clear of rod should be used to give support and improved heat removal (5) Reinforced p.f. not critical 0.25-0.4 mm Used with sour hydrocarbon gases and where lubricant may resin (0.010-0.015 in) be thinned by solvents in gas stream (6) Carbon-graphite 400 BHN min 0.03-0.06 mm (0.0014.002 in) Used with carbon-graphite piston rings. Must be kept oil free. Suitable up to 350°C (7) Graphite/metal 250 BHN min 0.08-0. 12 mm Alternative to (4) and (6) sinter (0.003-0.005 in) B26.3 Mechanical piston rod packings B26 FllTilNG AND RUNNING IN 1. Cleanliness is essential so that cups bear squarely together and to prevent scuffing or damage at start up. 2. Handle segments carefully to avoid damage during assembly. 3. Check packings float freely in cups. 4. With lubricated packings, check that plenty of oil is present before starting to run-in. Oil line must have a check valve between the lubricator and the packing. Manually fill the oil lines before starting. Use maxi- mum lubrication feed rate during run-in. 5. If the temperature of the rod rises excessively (say above 100°C) during run-in, stop and allow to cool and then re-start run-in. 6. Run in with short no-load period. B26.4 B27 Soft piston seals SELECTION AND DESIGN Table 27.1 Guidance on the selection of basic types l&e name Distributor ‘u’ CUP ‘0’ rinz External-fitted to piston, sealing in bore COMPRESSION Internal-fitted in housing, sealing on piston or rod Simple housing design Good Good Poor Very good Low wear rate Very good Good Good Poor High stability (resistance to roll) Good Fair Very good Po01 Low friction Fair Fair Fair Good Resistance to extrusion Good Good Good Fair Availability in small sizes Fair Good Poor Very good Availability in large sizes Good Fair Good Good Bidirectional sealing Single-acting only. Use in pairs back-to-back for double-acting. ‘Non-return’ valve action can be useful pairs Effective but usually used in Remarks Do not allow heel to touch mating surface Use correct fits and guided piston, etc. Avoid parting line flash on the sealing except under high pressure. If seal too soft for pressure, lip may curl away Unsuitable for from surface rotational movement Application notes acetal resin, nylon, PTFE, glass fibre/PTFE or metal bearings. To prevent mixing of unlike fluids, e.g. aeration of oil, use two seals and vent the space in between to atmosphere. Long lips take up wear better and improve stability but increase friction. Use plastic back-up rings to reduce extrusion at high pressures. The use of a thin oil will reduce wear but may increase friction. For pneumatic assemblies use light grease which may contain colloidal graphite or MoS2. Choose light hydraulic oil for mist lubrication. Avoid metal-to-metal contact due to side loading or piston weight. If seals will not maintain concentricity use B27.1 VENT TO ATMOSPHERE Soft piston seals B27 Table 27.2 Seals derived from basic types Table 27.3 Special seals ~~ Double-acting, one-piece, narrow width, but preswre can be trapped between lips and seal may jam. Needs composire piston Similar, but no pressure trap and can be fitted to one-piece piston Derived from ‘0’ ring. Less tendency to roll. Improved and multiple sealing surfaces. Sealing forces reduced and parting line flash removed from working surface Multiple sealing lips to obviate leakage due to curl _. . ~ e~ ‘W’ section. Good for hydraulic applications and high pressures. Can be used internally or externally Material Rubber Dynamic seal on piston ‘\“‘ Register between body Polyethylene sections Static seal in body sections Dynamic seal on piston Piston head seal Polyethylene SPRING Fits ‘0’ ring groove. Usually Use internally or F’TFE externally. Suitable for rotational movement. Table 27.4 Mating surface materials Materials ryPe Finish Remarks 0.6 pm max. 0.2 to 0.4 pm (8 to 16 pin) preferred 0.2 pm rnax. 0.05 to 0.1 prn (2 to 4 ,pin) preferred Best untreated materials. Improve High cost. J with use. Brass As drawn Copper As drawn J Liable to SCUE and corrode. J Low cost Aluminium As drawn alloy Polished J Anodised / / J Low cost. Short life Abrasive, therefore polish Hard anodised Corrodes. Mild steel As drawn Corrodes readily Very abrasive, polish before and Honed Hard chrome J after plating. High cost. plated Used mostly for piston rods Ground d J ~~ Stainless steel Ground Notes: Anodising and plating can be porous to air causing apparent seal leakage. The finish on the seal housing can be 0.8 pm. Use rust prevention treatment for mild steel in storage. B27.2 [...]... 141 2. 4 2. 1 1.6 20 6 177 149 2. 6 2. 0 1.6 2. 8 2. 3 - - - - - -9 2. 2 I 9 1.4 1 98 1 72 2.1 1.6 1.3 1.a 3.4 2. 8 2. 2 - - Thermal capacity (J/kg "C) at 30°C 60°C 100°C 188 0 1990 21 20 186 0 1960 21 00 185 0 1910 20 80 I960 20 20 21 70 1910 20 10 21 50 188 0 1990 21 20 Thermal conductivity (Wm/m2 "C) at 30°C 60°C 100°C 0.1 32 0.131 0. 127 0.130 0. 1 28 0. 125 0. 1 28 0. 126 0. 123 0.133 0.131 0. 127 0.131 0. 129 0. 126 0. 1 28 0. 126 ... 0.0074 370 23 26 51 1.1 1.6 2. 7 MVI light machine oil 0 .8 82 0.0039 385 4 37 59 0 .2 2.1 2. 3 MVI heavy machine oil 0.910 0.0075 440 8 37 54 0.4 2. 7 3.1 HVI light machine oil 0 .87 1 0.0043 405 6 26 68 0.3 1.4 1.7 HVI heavy machine oil 0 .88 3 0.0091 520 7 23 70 0.4 1 .8 2. 2 0 42 58 0 2. 8 2. 8 ~~ HVI cylinder oil 0 .89 9 0. 026 8 685 Medicinal white oil 0 .89 0 0.0065 445 R EFlNlNG Distillation Refining processes Lubricants... ("C) 25 0 300 I IO 22 0 320 305 26 0 370 Maximum temperature in presence ofoxygen ("C) 21 0 24 0 I10 180 25 0 23 0 20 0 310 Maximum temperature due to decrease in viscosity ("C) I50 180 100 20 0 25 0 28 0 Minimum temperature due to increase in viscosity ("C) -35 -65 - 55 - 50 -30 -65 -20 -60 Density (g/ml) 0.91 1.01 1. 12 0.97 1.06 I 04 1. 02 1 .88 Viscosity index 145 140 0 20 0 175 195 160 100-300 Flash point ("C) 23 0... machine Light machine Heauy machine cy1inde' 8 62 88 0 89 7 8 62 87 5 89 1 81 0 Density (kg/m3)at 25 °C Viscositv (rnNs/mZ) at 30°C 18. 6 45.0 171 42. 0 153 60°C 6.3 12. 0 31 13.5 34 135 100°C 2. 4 3.9 7.5 4.3 9.1 27 Dynamic viscosity index 92 68 38 109 96 96 Kinematic viscositv index 45 45 43 98 95 95 -40 -29 Pour point, "G -43 Pressure-viscosity coefficient (,mZ/N lo8) at x 30°C 60°C 100°C Isentropic secant bulk... Table 2. 3 presents examples on a number of typical oils Table 2. 3 Typical structural group analyses (courtesy: Institution of Mechanical Engineers) Spec@ grauip at 15~6°C Oil pppc LVI spindle oil Viscosip Ns/m2 at 100°C 0.0 027 0. 926 % 28 0 % % cA Mean molecular weighf cN cP RA RN RT 46 0 .8 1.4 2. 2 32 22 ~~~~ ~ ~~ LVI heavy machine oil 0.943 0.0074 370 23 26 51 1.1 1.6 2. 7 MVI light machine oil 0 .8 82 0.0039... 566 IP 1 32 Min usable temp Available with extreme Use protection pressure additive Cost Ojjicial specijcation "C ( O F ) Max usable temp "C (OF) Lithium 175 (350) -75 (-100) 120 (25 0) Yes Yes Rolling bearings High DTD 55 98 (XG 28 7) MIL 6 -23 82 7 A Clay None 55 (-65) Not Yes applicable+ Yes Rolling bearings High DTD 55 98 (XG 28 7) 175 (350)f No Rolling bearings Very high DTD 5579 (XG 29 2) MIL-G -25 760A Cost... (XG 27 9) MIL G- 10 924 B Sodium 20 5 (400) (conventional) 0 ( 32) 150-175 Yes (30c~350) No Glands, seals Low low-medium speed rolling bearings - Sodium and calcium (mixed) 150 (300) -40 (-40) 120 -150 (25 0-300) Yes No High-speed rolling bearings Medium DEF 22 61A (XG 27 1) MIL-L-7711A Lithium 175 (350) -40 (-40) 150 (300) Yes Yes All rolling bearings Medium DEF STAN 91- 12 (XG 27 1) DEF STAN 91 - 28 (XG 27 4)... 1 32 Min usable temp "C("F) Max usable temp "C("F) Available with Rust extreme protection pressure additiue Lime (calcium) 90 (190) -20 (0) 60 (140) - Lime (calcium) heat stable 99.5 (21 0) -20 (0) or -55 (-65) 140 ( 28 0) * 80 (175) * Use Cost Oj'icial speciJication Yes General purpose Low BS 322 3 (Some- Yes times) General purpose and rolling bearings Low BS 322 3 DEF STAN 91-17 (LG 28 0) DEF STAN 91 -27 ... Fast (20 00 r.p.m.) A specially selected ultra-clean grease required Veryf a s t (over 20 00 7.p.m.) ~~ - Miniature (under 10 mm) Small (20 -40 mim) Medium (65 mm) Calcium (LG 28 0 type) Calcium or lithium (XG 27 1 type) Lithium (XG 27 1 type) (Lithium (XC 27 1 type)) (Lithium (XG 27 4 type)) Sodaxalcium oil type Large (100 mm) Lithium/oil (XG 27 4 or XG 27 1 type) Lithium (XG 27 4 type) Lithium (XG 27 4... 34 48 50 000 20 000 10000 20 00 1000 500 10.0 PHYSICAL PROPERTIES 8. 0' Viscosity-.Temperature 6.0 5.0 4.0 Figure 2. 4 illustrates the variation of viscosity with temperature for a series of oils with kinematic viscosity index of 95 (dynamic viscosity index 93) Figure 2. 2 shows the difference between 150 Grade I S 0 34 48 oils with KVIs of 0 and 95 Vi scosity Pressure c2 3.0 -20 Temperature, "C Figure 2 2 . HVI heavy machine oil 0 .88 3 0.0091 520 7 23 70 0.4 1 .8 2. 2 ~~ HVI cylinder oil 0 .89 9 0. 026 8 685 Medicinal white oil 0 .89 0 0.0065 445 0 42 58 0 2. 8 2. 8 R EFl Nl NG Distillation. Mean Ns/m2 molecular at 100°C weighf Oil pppc LVI spindle oil 0. 926 0.0 027 28 0 22 32 46 0 .8 1.4 2. 2 ~~~~ ~ ~~ LVI heavy machine oil 0.943 0.0074 370 23 26 51 1.1 1.6 2. 7 MVI light. light machine oil 0 .8 82 0.0039 385 4 37 59 0 .2 2. 1 2. 3 MVI heavy machine oil 0.910 0.0075 440 8 37 54 0.4 2. 7 3.1 HVI light machine oil 0 .87 1 0.0043 405 6 26 68 0.3 1.4 1.7 HVI