Mechanical Engineer''''s Reference Book 2011 Part 14 doc

Seals and sealing 15/67 0 Check all metal components for physical damage. Check seal faces for scratches, nicks or visible imperfec- tions. 0 Check secondary seals for cuts, nicks. tears, and chemical attack. Some elastomers are attacked by common fluids such as ozone, water and mineral oils. Storage Mechanical seals should be stored in the protective packaging supplied by the manufacturer. The packaged seal should be kept in an area free from dirt. excessive moisture, high humidity and extreme cold. Good ventilation is also recommended. starting a pump against a closed valve, can be avoided or macle less damaging - again the seal manufacturer may be able to help. 15.2.4.4 Seal selection Selecting the most appropriate sealing system for a specified duty can be a difficult exercise which is best left in the hands of a reputable seal manufacturer unless particular company expertise and experience is available. In addition to the primary seal, secondary containment and ancillary equipment may be required and the manufacturer may also be able to make other suggestions for improving the reliability of the final design. The basic steps involved in seal selection are worth knowing and will aid the liaison betwen manufacturer and user. The manufacturer will require the following data to make a primary seal selection: 0 Precise and complete seal housing dimensions 0 Running pressure including the seal chamber pressure, if o Running temperature 0 Physical and chemical properties of the sealed fluid o Expected life 0 Required leakage Specification of any secondary containment and ancillary systems will require further information regarding the sealed fluid (e.g. auto-ignition point, toxicity, flammability, tendency to decompose, tendency to crystallize, percentage of solids). Company or other regulations regarding permitted leakage levels should also be consulted. For a more extensive guide to seal selection the reader is referred to reference 24. known 15.2.4 $ Seal installation Training The fitting of mechanical seals is a skilled job and should be carried out by trained personnel. Site surveys indicate that between 25% and 40% of all seal failures may be attributable to incorrect fitting. Cartridge seals reduce the risk of fitting errors significantly and their use is to be encouraged. All reputable mechanical seal manufacturers offer training courses on seal installation and the investment of time and money for making use of these services will inevitably return dividen,ds in reduced premature seal failures in the field. Handliizg The rules for handling mechanical seals are: Q Obey any specific instructions in the literature enclosed @ Avoid mechanical damage or shock as many seal compo- @ Do not place the sealing faces down on dirty, unyielding 0 Unpack the seal carefully; shrunk-wrap packaging should e Check that the seal supplied matches the seal specified for 0 File all relevant technical inforimation supplied by the seal @ Transfer fitting recommendations into company mainte- with the seal. nents are brittle or fragile. surfaces. be cut off carefully. the duty. manufacturer. nance procedures. Inspeclion While detailed checks cannot usually be made on-site. the seal should be inspected prior to installation for any sulperficial damage: e Check materials specifications against duty (manufacturers provide references of material codes). Fitting Seal fitting is best carried out in a clean environment and, if possible, the pump should be removed in entirety to a workshop for stripping down and rebuilding. This practice has a number of attractions: 0 The pump components, especially seals and bearings, are not exposed to the elements. 0 The existing seal may be removed more carefully and inspected for damage. The relatively benign and clean conditions of a workshop are more conducive to good fitting practice. Prior to fitting the seal the pump should be checked for misalignments which may be harmful to the seal. Checks should include: shaft balance, shaft run-out, seal chamber squareness, and concentricity of the shaft to the seal chamber. The seal manufacturer will be able to give a comprehensive list of recommended checks; alternatively, they are catalogued in Section 9.2 of reference 24. Most mechanical seals are supplied with detailed fitting instructions and these should be carefully followed. There are some fitting requirements common to all seals: Check seal envelope dimensions carefully, particularly the components which dictate the compression length of the loading member. Over-compression of a seal will probably lead to premature failure. 0 Avoid twists and kinks in any O-rings. PTFE O-rings should be softened in boiling water immediately prior to fitting. 0 Seal faces must be kept clean. Any grease or foreign matter on the faces should be wiped off using lint-free tissue soaked in a suitable solvent such as propanol. 0 Check that any ancillary equipment is cleaned and properly commissioned. Piping connections should be inspected for conformance to the seal manufacturer's drawing. 0 Ensure adequate coupling alignment - this is very impor- tant for long seal life. The seal manufacturer will recom- mend appropriate tolerances. 0 Avoid excessive pipe strains arising from misalignments between the pump flanges and pipework. Coupling align- ment should be rechecked after connecting the pipework. 0 Check that seal flushing systems are operating correctly and that valves are open. If possible, vent the seal chamber at start-up. 15.2.4.6 Seal failures Defining seal failure is difficult and depends to large extent on the nature of the sealed fluid and the practice of the seal operator. Most seals are removed because of excessive leak- age although sometimes it is necessary to inspect if the seal is running hot or squealing. A pump outage caused by a failing seal is obviously irritating to users but they should ensure that vital evidence, which may reveal the reasons for failure, is not lost when the failed seal is removed. Careful records of seal failures are valuable aids to effective troubleshooting and 15/68 Plant engineering should be made systematically by the trained fitter as the seal is removed. Process conditions prevailing at the time of failure should be logged - many seal failures can be linked to changes causing the seal to experience off-design conditions; for example, low flow rates through the pump may cause cavita- tion and excessive vibration at the seal, Leakage may be due to the failure of any of the seals including the secondary seals - these should be carefully in- spected as they are removed. Do not handle the rubbing faces before visual inspection. Check the faces for obvious damage, including chemical attack. In addition, note any of the follow- ing if evident: Thermal distress including surface cracking and discolora- 0 Solids build-up both in the sealing interface and on the sides Surface pitting and erosion - a magnifying glass can be a A summary of common failure modes and corrective action is given in Table 15.17 and further information can be found in references 25 and 26. tion. of the faces. useful aid. Acknowledgements The author gratefully acknowledges the British Hydromecha- nics Research Group Limited for permission to publish the foregoing text. 15.2.5 Clearance seals 15.2.5.1 Introduction Clearance seals tend to be purpose-designed for use in particu- lar rotary applications where it is not possible to use lip or mechanical seals. Their principal virtues are high reliability and long life compared to other rotary seal types. The price for these advantages is relatively high leakage for pressurized duties even when tight radial and axial tolerances are achieved. The principal areas of use are: steam and gas leakage control, especially for turbines and compressors, large water turbines, grease seals for bearings, high-pressure and/or high-speed reciprocating applications (e.g. diesel fuel injector pumps), and some high-pressure water pumps. Most high-duty clearance seals are built into the piece of machinery by the manufacturer; consequently there are few commercially avail- able units. In terms of geometry the seals range from simple fixed and floating bushings to complex labyrinth and visco- seals. Clearance seals are used frequently as a secondary or back-up device (e.g. throttle bush) to limit leakage in the event of primary seal failure. In these cases the primary seal would often be a mechanical seal. 15.2.5.2 Characteristics The high reliability and long life of clearance seals are important features. Given a correct initial set-up the life of the seal is usually only limited by wear caused by abrasives in the pumped fluid or contact between the rotating and stationary components due to shaft run-out. In most cases this process is very gradual. Leakage from clearance seals is usually high compared with contact seals - very tight axial and radial tolerances are re- quired to approach contact seal performance in this regard. However, tight-toleranced seals are more sensitive devices and may be adversely affected by deflections induced hydrau- lically and thermally in both seal and machine, and also by machine misalignment and external vibration. Tribologically compatible materials are required since contact is likely at some time. The unit cost is generally higher than more standardized seal types, an exception being grease retainers which are very low unit-cost items. In general, the choice between individual types of seal will be a balance between cost, life and leakage. Table 15.18 outlines the four main types of clearance seal and their relative merits. 15.2.5.3 Seal types Fixed bushing The fixed bushing, shown diagrammatically in Figure 15.114, is the simplest design of clearance seal. Its main virtue lies in its low cost. One-piece fixed bushings are mainly used as pump wear rings, as balance drums on multi-stage pumps and throttle bushes as a secondary back-up to other rotary seals. Tolerance requirements can be very tight, raising the true cost of an apparently low-cost seal. This problem is usually overcome by adopting multi-segment designs and/or a floating bush (see below). The correct choice of materials is very important. Typically, for water duties bushes are segmented and may be manufac- tured from carbon-graphite and run on a bronze or carbon- steel shaft sleeve. On a clean process liquid the life of the bush may be between 5 and 10 years. If abrasives are present then a flame-hardened or nitrided stainless steel sleeve is recom- mended and possibly a compatible babbitt bush-lining. The fixed bushing is characterized by high leakage, which is highly dependent on the radial clearance and relative eccen- tricity. The amount of leakage can be predicted for both laminar and turbulent flow conditions with compressible and incompressible fluids: Laminar flow: Theoretical calculation as shown in Table 15.19 Turbulentflow: Empirical data as given in Figures 15.115 and 15.116 Fixed bushings are commonly used as an auxiliary seal on centrifugal pumps to minimize leakage in the event of primary seal failure. The diametral clearance to BS 6836 should be no greater than: Shaft diameter 100 i 0.2 Bearing material Leakage dependson ccmpatible with (clearance) and (eccentricity) I Atmosphere Id 1-24 <o.i% xa Y Figure 15.114 Typical fixed bushing seal Seals and sealing 15/69 Table 15.17 Mechanical seal failure modes (copyright BHR Group Ltd) Symptoms High Over- High Special conditions leak heat wear Seize Failure mechanism Action Viscosity of fluid high high low Low-lubricity fluid Speed of sliding high high high low Pressure differential low high high Fluid temperature high high high Abrasives in fluid Crystallizable fluid Polymerizable fluid Salt solutions Ionic fluid Nun-Newtonian fluids, suspensiosns. colloids, etc Sterilization cycle Cleaning cycle Ozone, radiation exposure to sunlight Flushing etc. Auxiliary cooling Double smeals Seal faces rough Bellows type seal Single-spring seal drive Vibration present Housing flexes due to pressure or temperature changes Seal flatness poor X X X Excessive frictional heating, x x x x Excessive frictional heating, seal x x x Poor hydrodynamic lubrication, film vaporizes distorts solid contact X X X X Surface seize or 'pick-up' X X X Excessive frictional heating, film vaporizes Thermal stress cracking of the face X xxxx Thermal distortion of seal x x x Poor hydrodynamic lubrication, solid contact Fluid pumped by seal against pressure X X X Hydrodynamic film overloaded (XI x x X X Seal or housing distorting X X X Elastomer secondary seal overheated Distortion of seal faces Thermal stress cracking of the faces X X Solids in interface film X X Crystals form at seal face. secondary seal jams X X Solids form at seal face, secondary seal jams x x x x Corrosion damages seal faces Fluid behaves unpredictably, leakage may be reversed (XI X X High temp. or solvents incompatible with seal materials, especially elastome1 Seal materials (rubber) fail x x x x Stoppage in auxiliary circuit x x x x Pressure build-up between seals if there is no provision for pressure control x x x x Asperities make solid contact X Floating seal member vibrates X X x x Face separation unstable x x x x Seal faces out of alignment, Spring ineffective due to wrong shaft rotation direction non-uniform wear X Excessive seal gap Provide cooling Provide cooling Use faces with good boundary lubrication capacity Use faces with good boundary lubrication capacity Reduce face loading Provide cooling Use material with higher conductivity or higher tensile strength Provide cooling Use face with good boundary lubrication capacity Try reversing seal to redirect flow Modify area ratio of seal to reduce load Stiffen seal and/or housing Use a high-temperature rubber Reduce thermal stresses, e.g. by cooliag the seal Avoid rapid temperature changes or large temperature gradients Circulate clean fluid round seal Raise temperatnre of flush fluid outside seal Raise temperature of flush fluid outside seal Select resistant materials Try reversing seal to redirect flow in acceptable direction Use compatible materials Protect seal from exposure, consider other materials Overhaul auxiliaries Provide pressure control Lap or grind faces Fit damping device to bellows Reverse motor, or change spring Try to reduce vibration, avoid bellows seals, fit damper Stiffen housing and/or mount seal flexibly Lap faces flatter 15/70 Plant engineering Table 15.18 Types of clearance seal (copyright BHR Group Limited) Seal type Ad vantages Disadvantages Application areas Labyrinth High speed, high temperature. ‘Zero’ wear if shaft located correctly. Visco-seal Fixed bush High speed, high temperature. ‘Zero‘ wear. Zero leakage at design speed. Gas or liquid seal. Relatively low cost. Simple design. Floating bushing ‘Self-adjusting’ clearance seal. (most commonly segmented) precision required. Will wear in to shaft. so less Relatively low cost. Large diameters can be accommodated relatively economically. High cost. High precision, axially and radially. Static and dynamic leakage. Usually gas seal only. High cost, high precision. radially. Uni-directional leaks when stopped. Gas ingestion can reduce effectiveness. High leakage. Pressure/speed limited. Have ‘finite’ life compared with true clearance seals. Material choice limited to good bearing combinations: seal material generally, carbon, bronzes or babbitt. Gas turbines Gas compressors Steam turbines Specialized pumps and compressors. e.g., sodium pumps. Pump wear rings. Very low pressure water seals Low-duty gas turbines and compressors. Water pumps and turbines (particularly large diameters) Table 15.19 Leakage from a bushing seal -laminar flow q = volumetric flow ratelunit pressure 77 = absolute viscosity gradientlunit periphery Fluid incompressible Fluid conipressiblea . ,3 s-1 27ra(Ps - Pa) Q= c3 c3 (ps + Pa) q = - . (1 + 1.5~~) I 0 c q=-’- 7; Axial bush 12s 249 Pa Radial bush . ., c3 12s 4= 27ra(Ps - Pa) Q= q m3 s-’ (0 - b) (a - b) c3 a log, - 247i a 4= b (a - b) . a For Mach number < 1.0. i.e. fluid velocity = local velocity of sound. V If shaft rotates. onset of Taylor vortices limits validity of formula to L< 41.3 (where V = surface speed and v = kinematic viscosity). A variation on the fixed clearance seal is the centrifugal liquid barrier seal in which the centrifugal action of the rotating component is sufficient to create a pressure differen- tial to oppose the leakage of sealed fluid. An example of a seal of this type is the hydrodynamic disk seal shown in Figure 15.117 which may be a single or multi-stage device. The performance of this seal type is reported in Merry and Thew.27 Floating bushing The floating bushing, shown in Figure 15.118, is a nominally self-aligning version of the fixed bush. It therefore requires less accurate precision in its installation. It is relatively low cost and, in segmented form, is particularly suited to large diameter seals. Its main areas of application are for low-duty gas turbines and compressors, low-pressure water pumps and water turbines. Leakage is relatively high com- pared with other seals unless very tight tolerances can be held. Estimates of leakage may be made using the figures given for fixed bushings. The floating bush is somewhat more limited in its pressure and speed capabilities and, as for fixed bushings, the choice of materials is vitally important. Figure 1!5.115 ILeakage flow in a fixed bushing seal 0.01 I 0-4 10-~ 10-2 lo-' 1 Clearance (in.) Figure 15.116 Effect of viscosity and clearance on leakage from a bushing seal Seals and sealing 15/71 15/72 Plant engineering Two-stage hds One-stage H DS I / , , Fluid outlet _ Figure 15.117 Hydrodynamic disk seal. (Copyright BHR Group Ltd) P (a) Ld Figure 15.118 Typical floating bushing seals Floating bushing seals in the non-contacting gland-ring arrangement are commonly used on steam and gas applica- tions. Typically the gland rings are manufactured from carbon- graphite which are often segmented. An example of a carbon gland is shown in Figure 15.119; in this case the rings are of bevel section and spring loaded to enable compensation for ring wear. The bore of the rings is designed to match the shaft diameter at the operating temperature in order to keep leakage to a minimum. Carbon gland rings can also be used in other arrangements for water turbines; typically the rings may be used in a tenon-jointed form, as opposed wedge rings or as hydrostatic radial face seals. A special type of floating bush seal is the controlled clearance rotary seal shown in Figure 15.120. The seal is designed to be self-energizing and self-compensating. Hy- drostatic and hydrodynamic forces resulting from the pressure Segmental carbor, rings Leak off Garter spring 1 Pres Figure 15.119 Bevel-section carbon gland. (Source: Morganite Special Carbons Ltd) of the sealed liquid and shaft rotation, respectively, are used to create a pressure wedge in the film between the flexible inner ring and the shaft. The result is a tapered film with a very small exit clearance, thus effecting a seal. Labyrinth seal The labyrinth seal is the most widely used clearance seal type. In its basic forms it resembles the examples shown in Figure 15.121 but is also to be found in a plethora of different designs, some of which may be purchased 'off the shelf'. The leakage from labyrinth seals is typically about half that of bushing seals due to the increased flow resistance caused by the eddies which are set up in the grooves between the vanes. The vane axial spacing is commonly about twenty times the vane lip clearance - the compromise here is between achieving sufficiently large grooves to promote strong eddies and limiting the axial length of the seal to manageable Seals and sealing 15/73 BLADE SPACING, in Figure 15.120 Controlled-clearance rotary seal. (Source: James Walker and Co. Ltd) 0 1.0 2.0 3.0 4.0 5.0 6.0 BLADE SPACING. mrn Ambient pressure = latmos.; blade thickness = 0.0055in: radial clearance = 0.005in; pressure ratio = 0.551; temperature 310K , Figure 15.122 Performance of a ty?ical labyrinth seal Figure 15.121 Typical labyrinth seals dimensions. Typical leakage performance is shown in Figure 15.122. which indicates the influence of blade spacing and the number 'of blades. For design purposes the user is referred to Figure 1.5.123. Labyrinth seals are capable of very high speeds and high temperatures with nominally unlimited life. Consequently, they are used on gas sealing duties, turbines and compressors, and for steam turbines. They are also available as bearing grease seals as an alternative to lip seals. Their main draw- backs are, in general, relatively high cost and the high axial and radial precision required in their installation and opera- tion. As for other clearance seals, the materials of construction should be tribologically compatible as a precaution against rubbing contact between the vanes and the rotor. An addi- tional factor may also be creep, especially in high-speed turbines where stresses are large. A convenient material arrangement is to use metal fins and a carbon bush as shown in Figure 15.124. Alternatively, a metal foil honeycomb may be used made, for example, from stainless or Nimonic steets. Visco-seal The visco-seal or wind-back seal is basically a fixed bush with a helical groove cut either into the shaft or the bore of the bush as shown in Figure 15.125. The effect of the helix is to pump the sealed fluid back into the sealed system as the shaft is rotated. The seal therefore works best for viscous fluids or for high rotational speeds. The seal is essentially a single speed, uni-directional, device. It is designed to give inward pumping action perfectly match- ing the leakage flow with the net result of zero leakage at what is known as the sealing pressure. Excess pressure will result in leakage while at low pressures the seal runs partially dry and air may be pumped inwards. Typical performance and design criteria are summarized in Figure 15.126. There is no sealing action when the shaft is stationary and it may be necessary to fit an auxiliary static seal. This may lift off automatically when the shaft rotates. A development of the visco-seal is the barrier visco-seal in which a pair of seals are installed back-to-back as shown in Figure 15.127. The resulting pressure barrier which builds up between the seals may be used to buffer the sealed fluid from atmosphere, Compatibility between sealed fluid (usually a gas) and buffer fluid (usually a liquid) must be established. In tests using grease as the barrier fluid a pressure of 10 bar was sealed by a 13 mm diameter seal running at 1000 rpm. 15/74 Plant engineering I 1 N E E - VI Cn =?. E 0 0 Ilc = 0.375 n = number of blades A 3 4 0 2.0 1.8 1.6 1.4 1.2 1.0 9 < 0.8 0.6 0.4 0.2 n c m P max. - P atmos. TEMPERATURE 17.3 For other temperatures multiply Cm/A by = t( GEOMETRY For other values of s/c or l/c multiply Qm/A by 9: STEPPED LABYRINTH For a stepped labyrinth use an 'effective' n value, kn: O1.5 2 2 Q LL > W E 1.0 2 Y 05 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 10.0 1 IC Figure 15.123 Calculation of leakage for a labyrinth seal 9 The visco-seal is suitable for high-temperature applications where the pressure is low to moderate. However, it requires a high degree of radial and axial precision and its initial cost is relatively high. Its main areas of application are for special- duty very long-life pumps or high-speed rotary compressors. Further information can be found in references 28 and 29. Acknowledgements The author gratefully acknowledges the British Hydromecha- nits Research Group Limited for permission to publish the foregoing text, Boilers and waste-heat recovery 15/75 Segmental carbon rings Leak off - Pressur Figure 15.127 Barrier visco-seal Figure 15.124 Carbon labyrinth gland. (Source: Morganite Special Carbons Ltd) be considered. There are eight categories of boiler avaiiable. In order of rated output these are: Cast iron sectional boilers Steel boilers Electrode boilers Steam generators Vertical shell boilers Horizontal shell boilers Water tube boilers Fluid bed boilers Density OF fluid = p Viscosity of fluid = Q Figure 15.125 Visco-seal 1 .o 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 L: L I /,,,I IIU 1 02 103 104 (Re) Figure 15.126 Typical visco-seal performance 15.3 Boilers and waste-heat recovery 15.3.1 'Types of boilers This section covers industrial boilers, therefore only units of 500 kg h-' of steam, or equivalent hot water, and above will 15.3.1 .I Cast iron sectional boilers These are used for hot water services with a maximum operating pressure of 5 bar and a maximum output in the order of 1500 kW. Site assembly of the unit is necessary which will consist of a bank of cast iron sections. Each section has internal waterways. The sections are assembled with screwed or taper nipples at top and bottom for water circulation and sealing between the sections to contain the products of combustion. Tie rods compress the sections together. A standard section may be used to give a range of outputs dependent upon the number of sections used. After assembly of the sections the mountings, insulation and combustion appliance are fitted. This system makes them suitable for locations where it is impractical to deliver a package unit. Models are available for use with liquid, gaseous and solid fuels. 15.3.1.2 Steel boilers These are similar in rated outputs to the cast iron sectional boiler. Construction is of rolled steel annular drums for the pressure vessel. They may be in either a vertical or a hori- zontal configuration depending upon the manufacturer. 15.3.1.3 Electrode boilers These are available for steam raising up to 3600 kg h-l. Normal working pressure would be 10 bar but higher pressures are available. Construction is a vertical pattern pressure shell containing the electrodes. The length of the electrodes controls the maximum and minimum water level. The electrical resistance of the water allows a current to flow through the water which in turn boils and releases steam. Since water has to be present within the electrode system, lack of water cannot burn out the boiler. The main advantage with these units is that they may be [...]... 15.3.3.3 Convection Convective heat transmission occurs within a fluid and between a fluid and a surface, by virtue of relative movement of the fluid particles, that is, by mass transfer Heat exchange between fluid particles in mixing and between fluid particles and a surface is by conduction The overall rate of heat transfer in convection is, however, also dependent on the capacity of the fluid for... conservative calculation 1098 1133 1166 1198 1227 1255 1281 1305 1328 1348 1367 1384 140 0 141 3 Table 15.21 Transport properties: gas oil products of combustion Temp Spec heat ("C) (J kg-' K-') Viscosit (kg m-'s-' x 106) Conductivity S p vol (W m-' K-' (m3 kg-') x 103) 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 140 0 20.37 24.34 27.93 31.21 34.25 37.09 39.44 42.3 44.71 47.01 49.22 51.35 53.40... Steel Tubes Circular Flanges for Pipe, Valves and Fittings Bolting for Flanges and Pressure Containing Purposes Heating ventilation and air conditioning 15/91 ASME 1989 Part 1 Power Boilers ASME 1989 Part 2 Material Specification ASME 19139 Part 8 Pressure Vessel Division 1 Design Code 15.4 Heating, ventilation and air conditioning 15.4.1 Heating 15.4.1.1 Statutory heating regulations Except for some defined... water- and fireside surfaces of the boiler is important All boilers will have an inspection opening or manway on the top of the shell with inspection openings in the lower part Some larger boilers will have a manway in the lower part of the shell or end plate With a three-pass wet-back boiler all tube cleaning and maintenance is carried 15/78 Plant engineering out from the front The front smokebox... 1181 1207 1232 1255 1276 1294 1311 1326 1339 1351 1.05 1.332 1.613 1.895 2.176 2.458 2.87 3.02 3.302 3.583 3.865 4 .146 4.428 4.709 Table 15.23 Transport properties: bit coal products of combustion Temp Spec heat (J kg-' K-') ("C) 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 140 0 1031 1065 1096 1125 1152 1177 1201 1222 1242 1259 1275 1289 1301 1311 Viscosit (kg m-'s-' x 106) Conductivity... and, in shell boilers, on the reversal chamber tubeplate Waterside heat transfer on the major part of the convective heating surface in these units is by convection without boiling 15.3.3.7 Further reading A good introduction to the vast literature on the science and technology of heat transfer, with 87 further references, is given in Rose, J W and Cooper J R , Technical Data on Fuel 7th edn British... rate by the following formula: F Blowdown rate = -x 100% B-F where F = the total dissolved solids content of the feed in parts per million allowing for the mixture of make-up and condensate plus any chemical treatment and B = the maxi- mum recommended solids content for boiler water in parts per million While vitally necessary, blowdown can be expensive in terms of lost heat Therefore a point will be reached... operational load 15.3.15 Design Standards for pressure vessels, pipes and flanges BS 779 BS 855 15.3 .14 Energy conservation Energy conservation in the boiler house can be considered in two areas One is the selection and installation of suitable equipment and the second is good operation and management 15.3 .14. 1 Plant installation The boiler, flues and chimney, pipework and hotwell where installed should... Furnace heat transfer Heat transfer in the furnace is mainly by radiation from the incandescent particles in the flame, and from hot radiating gases such as carbon dioxide and water vapour The detailed theoretical prediction of overall radiation exchange is complicated by a number of factors such as carbon particle and dust distributions, and temperature variations in three-dimensional mixing This is... expansion tank and causing boiling On no account should an open vent be provided for this type of system 3 Gas pressurization with spill tank This form consists of a pressure cylinder maintained partly filled with water and partly with gas (usually nitrogen) which is topped up from pressure bottles Water expansion is usually arranged to discharge from the system through a pressure-control valve into a spill . virtue of relative movement of the fluid particles, that is, by mass transfer. Heat exchange between fluid particles in mixing and between fluid particles and a surface is by conduction 44.43 76.86 1000 1348 46.75 81.86 1100 1367 48.98 86.6 1200 1384 51.13 91.08 1300 140 0 53.2 95.31 140 0 141 3 55.2 99.25 sp. vol. (m3 kg-l) Temp. Spec. heat Viscosit Conductivity Sp list of recommended checks; alternatively, they are catalogued in Section 9.2 of reference 24. Most mechanical seals are supplied with detailed fitting instructions and these should be