Process Engineering Equipment Handbook 2009 Part 15 doc

Pumps P-233 open to the atmosphere or the absolute pressure in the closed tank or condenser from which the pump takes liquid. In a deaerating heater, it is normally the saturated pressure at the existing water temperature. H z is the height in feet of the fluid surface above or below the pump centerline. If above, it is considered to be plus since the suction head is then increased; if below, it is minus. H vp is the head corresponding to the vapor pressure at the existing temperature of the liquid. H f is the head lost because of friction and turbulence between the surface of the liquid and the pump suction flange. In designing a pump installation and purchasing a pump, there are two types of NPSH to be considered. One is the available NPSH of the system, and the other is the required NPSH of the pump to be placed in the system. The former is determined by the plant designer and is based upon the pump location, fluid temperature, etc., while the latter is based upon suppression pump tests of the manufacturer. To secure satisfactory operating conditions, the available NPSH must be greater than the required NPSH. In higher-energy pumps such as boiler- feed pumps, values of NPSHA should be 1.5 to 2.0 times larger than NPSHR (as normally measured with a 3 percent drop in head). The calculation of available NPSH will be illustrated by two examples. The head corresponding to a given pressure is given by the equation H p = 2.31 p/sg, where p is the pressure in pounds per square inch and sg the specific gravity of the liquid. Assume that water at 80°F is to be pumped from a sump. The unit is located at an altitude of 800 ft above sea level, and the suction lift (from water surface to pump centerline) is 7 ft. The pipe losses amount to 1 ft head. What is the available NPSH? The atmospheric pressure at an altitude of 800 ft is 14.27 psia. The specific gravity of the water at 80°F is 0.9984, and the vapor pressure is 0.5069 psia. Determine the available NPSH of a condensate pump drawing water from a condenser in which a 28-in vacuum, referred to a 30-in barometer, is maintained. The friction and turbulence head loss in the piping is estimated to be 2 ft. The minimum height of water in the condenser above the pump centerline is 5 ft. The absolute pressure in the condenser is 30 - 28 = 2 inHg, or 0.982 lb/in 2 . The corresponding specific gravity is 0.9945. A third example is that of a deaerating heater having a water level 180 ft above the pump centerline. The water temperature is 350°F. The pipe friction loss is HHH H pzvp f =+- -= +- -=228 5 228 2 3 NPSH ft HH f z ==15ft ft HH p pvp == = ¥ = 2 31 2 31 0 982 0 9945 228 . . sg ft HHH H pzvp f =+- -= -=32 97 7 1 17 1 23 80 .NPSH ft H f = 1ft H p vp == ¥ = 2 31 2 31 0 5069 0 9984 117 . . sg ft H z =- () 7 ft negative since it is a lift H p p == ¥ = 2 31 2 31 14 27 0 9984 32 97 . . sg ft 12 ft. Since the water in the deareator is in a saturated condition, this absolute pressure on the surface liquid equals the vapor pressure at 350°F. Then The required NPSH must be determined in most cases by means of suppression tests. The Hydraulic Institute has prepared a series of diagrams to estimate this required head (see Figs. P-243 to P-246 inclusive). These diagrams are not to be considered as the highest values that can be obtained by careful design, but they may be used for estimating as they represent average results of good present-day practice. The use of the diagrams is simple and may be illustrated by an example. A double- suction pump operating at 3600 rpm delivers 1000 gal/min against a total head of 200 ft. What should the minimum NPSH be for satisfactory operation? The specific speed as found from Fig. P-234 is 2200. By referring to Fig. P-243, the point corresponding to this specific speed for double-suction pumps and a total dynamic head of 200 ft gives a 12-ft suction lift as the safe maximum. If the same conditions NPSH ft =+- - = ¥ +- ¥ -= HHH H pzvp f 2 31 134 6 0 892 180 2 31 134 6 0 892 12 168 . . P-234 Pumps FIG. P-243 Upper limits of specific speeds: double-suction pumps handling clear water at 85°F at sea level. (Source: Hydraulic Institute.) Pumps P-235 were applied to a single-suction pump with the shaft through the eye of the impeller, the safe minimum suction condition would require at least a 1 ft positive head (i.e., the suction head would have to be at least +1 ft rather than -12 ft; hence, the required NPSH would be 35 ft instead of 22 ft). These curves are based upon handling clear water at 85°F and sea-level barometric pressure. If the water temperature is higher, the difference in head corresponding to the difference in vapor pressures between 85°F and the temperature of the water pumped should be subtracted from the suction lift or added to the suction head. Also, if the unit is to be located above sea level, the difference in head corresponding to the difference in atmospheric pressures should be subtracted from the suction lift or added to the suction head. Thus, in the above example, if the water temperature is 140°F and the plant is located at an altitude of 2000 ft, the correction for vapor pressure will be 2.889 - 0.596 = 2.293 lb/in 2 , and the correction for altitude will be 14.69 - 13.66 = 1.03 lb/in 2 . The corresponding head change will be 2.31 (2.293 + 1.03)/0.9850 = 7.8 ft. For the double-suction pump the maximum suction lift would be 12.0 - 7.8 = 4.2 ft, and for the single-suction pump the positive suction head would have to be 1.0 + 7.8 = 8.8 ft. FIG. P-244 Upper limits of specific speeds: single-suction shaft through eye pumps handling clear water at 85°F at sea level. (Source: Hydraulic Institute.) A series of diagrams (Figs. P-247 through P-249) have been prepared by the Hydraulic Institute to determine NPSH on the basis of the flow, operating speed, and discharge pressure for hot-water and condensate pumps. They may also be used to find the maximum permissible flow for a given available NPSH. Hot water. Two curves, Figs. P-247 and P-248, have been prepared for pumps handling hot water at temperatures of 212°F and above. These curves show the recommended minimum NPSH in feet for different design capacities and speeds. Figure P-247 applies to single-suction pumps and Fig. P-248 to double-suction pumps. These curves serve as guides in determining the NPSH for hot-water pumps and do not necessarily represent absolute minimum values. Net positive suction head for condensate pumps. Figure P-249 indicates NPSH for condensate pumps with the shaft passing through the eye of the impeller. It applies to pumps having a maximum of three stages, the lower scale representing single- suction pumps and the upper scale double-suction pumps or pumps with a double- suction first-stage impeller. For single-suction overhung impellers the curve may be used by dividing the specified capacity, if 400 gal/min or less, by 1.2, and if greater than 400 gal/min, by 1.15. P-236 Pumps FIG. P-245 Upper limits of specific speeds: single-suction overhung impeller pumps handling clear water at 85°F at sea level. (Source: Hydraulic Institute.) Pumps P-237 The curve may be used for capacities and speeds other than shown by the relation that, for a definite NPSH, the product of rpm ¥÷gal/ —— — min remains constant. Minimum flow-through pump. The difference between the power put into a centrifugal pump and the useful power performed by the pump, or the difference between the brake and water horsepowers, is converted into heat, most of which appears as increased temperature of the water. The bhp curve of a pump rated at 500 gal/min against a 2600-ft head and the corresponding water-horsepower curve are shown in Fig. P-250. If we neglect bearing losses, which are minor, the difference between these curves at any capacity represents the horsepower absorbed by the water in the form of heat. Multiplying these differences by 42.4 gives the Btu generated in the pump per minute. Dividing these values by the flow in pounds per minute gives the temperature rise at each capacity. This curve is plotted in Fig. P-250. This temperature rise is generally not important in single-stage pumps, particularly if they are handling cold water, but for pumps handling hot liquids, such as boiler-feed pumps, it may become a serious matter. Then the resulting rapid temperature rise may cause the internal rotating parts to expand more rapidly than the heavier encircling parts so that severe rubbing may occur, or the impeller may even FIG. P-246 Upper limits of specific speeds: single-suction mixed- and axial-flow pumps handling clear water at 85°F at sea level. (Source: Hydraulic Institute.) become loose on the shaft. Also the temperature of the water may rise to a point at which the water flashes into steam, causing the pump to become vapor bound. The temperature rise in a pump may be calculated from the formula where Dt = temperature rise, °F h= overall pump efficiency, expressed as a decimal c = specific heat of fluid being pumped (equals 1.0 for water) H = total head of pump, ft Dt H c = - () 1 778 h h P-238 Pumps FIG. P-247 Net positive suction head for centrifugal hot-water pumps, single suction. (Source: Hydraulic Institute.) FIG. P-248 Net positive suction head for centrifugal hot-water pumps, double suction, first stage. (Source: Hydraulic Institute.) Pumps P-239 FIG. P-249 Capacity and speed limitations for condensate pumps with the shaft through the eye of the impeller. (Source: Hydraulic Institute.) FIG. P-250 Pump characteristics. (Source: Demag Delaval.) The allowable temperature rise of the water before it flashes into steam depends upon the suction conditions, or NPSH, of the pump. NPSH is the net head above the vapor pressure corresponding to the temperature of the liquid handled. As outlined in the subsection “Net Positive Suction Head,” every installation has two types of NPSH: that available in the installation and that required by the pump. The maximum allowable vapor pressure at the pump inlet is found by converting the available NPSH into pounds per square inch and adding this to the vapor pressure corresponding to the temperature of the liquid being handled. This pressure is the vapor pressure corresponding to the temperature to which the liquid may be raised before it will flash into vapor. After the allowable temperature rise has been established, an approximate minimum safe continuous-flow efficiency can be obtained by rewriting the previous equation in the form where H so = heat at no flow, or shutoff head c = specific heat of liquid The flow corresponding to this efficiency is found on the pump-performance curves. In boiler-feed pumps having single-suction impellers, all facing in the same direction, a leak-off balancing arrangement is used to compensate hydraulic thrust. If the balancing leak-off flow is returned to the suction of the pump, flashing can occur at extremely low rates of delivery. Therefore, the balancing flow is frequently piped to an open heater in the feed system of the pump, where, by flashing, the temperature of the water will be reduced to that corresponding to the pressure and there should be no valve of any kind between the junction of the balancing connection and the heater. Many installations do pipe the balance flow to the pump suction line. With the advent of the larger, higher-energy boiler-feed pumps of the 1960s and 1970s, it became apparent that the higher vibration and pressure pulsations occurring at partial capacities would affect the minimum-flow setting. Consequently, minimum-flow values of 25 percent of the flow at best efficiency became commonplace, with higher levels in special cases. Recirculation connection. The minimum flow through a pump is directed through a recirculation line from a discharge-line connection to the suction source. This line has a recirculation orifice designed to pass the required minimum flow or has a modulating-pressure breakdown valve. The recirculation connection is in the pump discharge line between the discharge nozzle and the check valve. All valves in the recirculation line must be open whenever the pump is operating under any of the following conditions: 1. Low flows 2. Starting pump 3. Stopping pump The valves should be opened or closed either manually or by automatic controls. When automatic controls are used, they should be checked at initial starting and occasionally thereafter during starting procedures. Figure P-251 shows a diagram of the piping arrangement. h= () + H tc H so so 778 D P-240 Pumps Pumps P-241 Materials for pumping various liquids. The materials used for pumps must be suitable for the liquids handled to prevent excessive corrosion. The Hydraulic Institute Standards give the materials to be used for the more common liquids and should be consulted for selecting the applicable material combination. From the standpoint of materials, pumps may be divided into three basic types: standard fitted (combination of iron and bronze), all iron, and all bronze. Other materials, including corrosion-resisting steels, are listed in the subject standards. Table P-28 gives a summary of the various materials used for centrifugal pumps. If the liquid to be handled is an electrolyte, the use of dissimilar metals in close combination, especially those that are widely separated in the galvanic series, should be avoided insofar as possible. The use of bronze and iron in the same pump handling seawater will greatly accelerate the corrosion of the cast iron parts. A table of the galvanic series is given in Table P-29. Pump application General. As outlined in the subsection “Classification,” there are on the market a multitude of centrifugal-pump types. Some of the basic types will be mentioned here. Single-stage, double-suction pump. See Fig. P-252. This type of centrifugal pump is the most common one and is used for general service in industrial and municipal plants. In the larger sizes, the use of these pumps is almost universal for municipal-water distribution, and pumps of this type are in service in practically every major city of the United States. For heads up to 300 ft or higher, single-stage pumps are used, while for higher heads two or more units are arranged in series. Figure P-253 shows an installation of two motor-driven units arranged in series. FIG. P-251 Recirculating connections for boiler-feed service. (Source: Demag Delaval.) [...]... Max Max Min Max Min Max Min Max Min Gravity, API Min 0 .15 420 625 2.2 1.4 35 0.10 0.35 e 675 40 (4.3) 26 20 0.50 0.10 125 45 (26.4) (5.8) 130 or legal 1.00 0.10 150 40 150 or legal 2.00f 300 45 (32.1) (81) (638) (92) Reprinted by permission from Commercial Standard CS 12–48 on Fuel Oils of U.S Department of Commerce a Recognizing the necessity for... number of factors besides physical unbalance of the rotating parts may cause vibration Among these are: 1 Resonance between the unit and its foundation or piping or resonance within the unit due to natural frequency of the pump, the motor frame, the motorsupporting pedestal, or the foundation Resonant vibrations may also be caused by other equipment in operation in the area 2 Operation at or near a... for most of the equipment involved in the aerospace age It is the workhorse of the rapidly expanding fluid-power industry, which is providing much of the energy-transfer systems for today’s highly sophisticated machines and tools It is finding ever-widening use in diversified fields of application such as Navy and marine fuel-oil service, marine cargo, oil burners, crude oil, chemical processing, and lubricating... for most of the equipment involved in the aerospace age It is the workhorse of the rapidly expanding fluid-power industry, which is providing much of the energy-transfer systems for today’s highly sophisticated machines and tools It is finding ever-widening use in diversified fields of application such as Navy and marine fuel-oil service, marine cargo, oil burners, crude oil, chemical processing, and lubricating... Society Standards Designations* ASTM† ACI AISI A48, Classes 20, 25, 30, 35, 40, and 50 A339, A395, and A396 B143, 1B and 2A; B144, 3A; B145, 4A A216—WCB A217—C5 A296—CA15 A296—CB30 A296—CC50 A296—CF-8 A296—CF-8M A296—60T CA15 CB30 CC50 CF-8 CF-8M CN-7M 1,030 501 410 446 304 316 A439 Remarks Gray iron—six grades Nodular cast iron—five grades Tin bronze—six grades (includes two... greater For such applications, double-volute pumps are used (Fig P-261) In the double-volute casing, the water leaving the impeller is collected in two similar volutes, the tongues of which are set 180° apart The two volutes merge into a common outlet to form the discharge of the pump Hydraulic forces (indicated by Double-volute pumps P-248 Pumps FIG P-261 Double-volute pump (Source: Demag Delaval.) FIG... delivered a Air leaks in suction or stuffing boxes b Speed too low* c Discharge head higher than anticipated d Suction life too high (Check with gauges Check for clogged suction line or screen.) e Impeller partially plugged up f Not enough suction head for hot water g Mechanical defects: (1) Wearing rings worn (2) Impeller damaged (3) Casing packing defective h Foot valve too small i Foot valve or suction... either an outside source or a high-pressure portion of the pump Packing is held in the stuffing box by packing glands These are usually split, making it possible to remove them without taking the pump apart Quench glands are used when the liquid being pumped exceeds a safe margin on its vapor pressure Glands are pulled into place by gland bolts Frequently these are swing bolts, making disassembly easier... pressures ranging up through 10,000 lb/in2 and handling viscosities from less than 1 cSt to more than 1,000,000 SSU The rotary pump is quite often defined as a positive-displacement type by most authoritative engineering references because of the general employment of characteristic close-running clearances that substantially limit internal leakage It might be more logical and technically correct to drop the... past, the rubbing velocity of the contact surfaces has been a limitation, these velocities increased in the 1970s to 230 ft/s or more, and mechanical-seal applications have increased in boiler-feed pumps, particularly in Europe In boiler-feed-pump applications that exceed the limitations of packing or mechanical seals, serrated bushing seals and multifloating ring seals are generally used Examples of the . the resulting rapid temperature rise may cause the internal rotating parts to expand more rapidly than the heavier encircling parts so that severe rubbing may occur, or the impeller may even FIG by ASTM Specifications) A216—WCB . . . 1,030 Carbon steel A217—C5 . . . 501 5% chromium steel A296—CA15 CA15 410 13% chromium steel A296—CB30 CB30 . . . 20% chromium steel A296—CC50 CC50 446 28% chromium. plotted in Fig. P-250. This temperature rise is generally not important in single-stage pumps, particularly if they are handling cold water, but for pumps handling hot liquids, such as boiler-feed