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Api vs ansi pump
 

Api vs ansi pump

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    Api vs ansi pump Api vs ansi pump Document Transcript

    • Which processANSI pump is right for your application? vsI API? By Ross C. Mackay, Contributing Editor n the world of pumps, there are two types of horizontal end suction centrifugal pumps that are more commonly used than all the others put In view of these figures, it is apparent that the API pumps should be considered for higher pres- sure and temperature services than the lighter dutytogether. They are the ANSI pump that is designed ANSI pump.and built to the standards of the American NationalStandards Institute, and the API pump that meets The Liquidsthe requirements of the American Petroleum In considering the various types of liquids han-Institute Standard 610 for General Refinery Service. dled by these pumps, the fluids in the petrochemi- Over the years, ANSI designs have become the cal industry are frequently classified aspreferred style of end suction pumps, not only for Hydrocarbons, Aggressive Chemicals and Mildchemical process applications, but also for water Chemicals.and other less aggressive services. The ANSI Hydrocarbons are petroleum-based productsStandard provides for dimensional interchangeabil- that are frequently classified as light, intermediateity of pumps from one manufacturer to another. or heavy. At atmospheric pressure and temperature, The API pump is almost the exclusive choice light hydrocarbons tend to vaporize, intermediatefor applications in the oil refin- hydrocarbons are liquid, andery industry, where it handles heavy hydrocarbons are highlyhigher temperatures and pressure Taking these fac- viscous or even solid.applications of a more aggressive Aggressive Chemicals includenature. While this specification tors into account, strong acids, alkalines or oxidiz-also deals with some vertical ing agents that are destructive toshaft models, this article will you can finish up both equipment and the environ-focus on the more widely used ment. They are also dangerous tohorizontal designs. These single with a profitable plant personnel if allowed to leak.stage pumps are designed with a Mild Chemicals are generallyradially split casing to accommo- and reliable easy to handle, and are not detri- mental to either equipment ordate a back pullout arrangementfor ease of maintenance. The operating system. the environment.major difference between the Many of these liquids cantwo styles is reflected in the casing pressure design produce toxic fluid exposure and vapors if they areratings, which are as follows: allowed to leak out of a pump. Vapor release is a common danger with hydrocarbons that vaporize at ANSI Pump Rating = 300 PSIG at 300° F atmospheric conditions or other chemicals that may API Pump Rating = 750 PSIG at 500° F be exposed to very high operating temperatures.36 MARCH 2004 www.pump-zone.com PUMPS & SYSTEMS
    • If a vapor release is exposed to a spark, the vapor discussed on this problem in recent years. This loadcloud may even explode or catch fires. is at its maximum when the pump is run at the Consequently, in handling these liquids, we shutoff condition, and gradually decreases as themust be extremely aware of much more than envi- flow rate approaches the Best Efficiency Pointronmental damage and pumping efficiency. We (BEP). If the pump operates beyond the BEP, themust also be very conscious about personal safety. load increases again, but in the opposite directionTherefore, the choice between the ANSI pump and on the same plane. Examination of the resultantthe API pump must take into account the specific shaft deflection problems has indicated that thefluid properties, as well as the operating conditions. radial plane on which the out-of-balance load acts The main difference between these choices is is approximately 60° counterclockwise from thepredominantly a result of the differences in casing cut-water of the volute.design.Volute Cases Both pump styles have a radial split casing, andmost ANSI pumps and some API pumps employ asingle volute design of the interior passages. This isparticularly evident in the smaller sizes that involvelow-flow rates and lower specific speeds of theimpeller. As shown in Figure 1, the area of the voluteincreases at a rate that is proportional to the rate ofdischarge from the impeller, thus producing a con-stant velocity at the periphery of the impeller. Thisvelocity energy is then changed into a pressure ener-gy by the time the fluid enters the discharge nozzle. Figure 2. Double volute case Most of the larger API pumps are produced with a double volute design to reduce these loads on high-flow and high-head units. (See Figure 2.) This is accomplished by balancing the opposing out-of- balance loads from each volute. While the cost of this is a slight reduction in efficiency, it is consid- ered a small price to pay for the increased reliabili- ty that ensues. Another casing feature found in many API pumps is the top suction/top discharge arrange- ment, which has also been used in a slightly differ-Figure 1. Single volute case ent configuration in a vertical inline pump design. In this arrangement with a horizontal pump, the The peculiar shape of the volute also produces suction nozzle is located at the top of the casingan uneven pressure distribution around the adjacent to the discharge nozzle, rather than on theimpeller, which in turn results in an imbalance of end. On the vertical inline design, the suction noz-the thrust loads around the impeller and at right zle is once again on the side, but now it is oppositeangles to the shaft. This load must be accommodat- to the discharge nozzle, thus creating the “inline”ed by the shaft and bearings, and much has been appearance.PUMPS & SYSTEMS www.pump-zone.com MARCH 2004 37
    • path from the suction flange to the eye of the impeller. Back Cover Arrangements One of the major differences between the ANSI and API pump cas- ings is in the manner in which the back cover is secured to the casing. In the ANSI design shown in Figure 3, the back cover and gasket are held against the pump casing by the bearing frame adaptor, which is most frequently supplied in cast iron. This usually results in a gap between the mating faces of the frame adaptor and the pump casing that has the Figure 3. Typical ANSI pump potential to permit uneven torquing of the bolts. In the event of a higher- The drawback of this design is, for most of than-normal pressurization of the casing by the these pumps, that the NPSH required is often con- process system, this may cause a fracture of the siderably greater than it would be in the end suction adaptor arrangement. More NPSH is needed in order to The API design in Figure 4 bolts the back cover accommodate the friction losses in the tortuous directly to the casing and uses a confined controlledCircle 392 on Reader Service Card 38 MARCH 2004 www.pump-zone.com PUMPS & SYSTEMS
    • March04PUMPS&SYSp26-41 2/17/04 10:45 AM Page 39 compression gasket with metal to metal fits. The adaptor is bolted indepen- dently to the back cover and does not play a part in the pressure boundary of the pump casing. Mounting Feet Another difference be- tween the two pump styles is the configuration of the mounting feet. All ANSI pump casings are mounted on feet project- Figure 4. Typical API pump ing from the underside of the casing and bolted to the baseplate. If these pumps are used on high-temperature applications, the casing On the other hand, API pumps are mounted at will expand upwards from the mounting feet and the horizontal centerline of the casing on feet pro- cause severe thermal stresses in the casing that will jecting from each side of the casing and bolted to detrimentally affect the reliability of the pump. pedestals that form part of the baseplate. This Operation at lower temperatures will not be affect- arrangement provides the API pump with the ed by this feature. advantage of being able to operate with pumpage at Circle 519 on Reader Service Card PUMPS & SYSTEMS www.pump-zone.com MARCH 2004 39
    • elevated temperatures. As the pump comes up to vices is also evident in the bearing housings of the temperature in such cases, any expansion of the API pumps, which tend to be much more robust in metal will be above and below the casing centerline, design and also accommodate cooling jackets with and will exert minimal amounts of stress to the cas- a greater capacity of cooling water. ing, thus contributing to optimum reliability of the pump. Materials of Construction The ability to handle higher temperature ser- Pump manufacturers can provide ANSI and API pumps in a wide assortment of mate- rials, the selection of which depends on the operating stress and effects, as well as the type of wear from the product being pumped. The most In order to common materials used in these cen- low NPSHR achieve suction de - values, an end trifugal pumps are: sign enclosed im incorporating an pe eye and in ller with a large jection bo standard res is • Cast iron • Ductile iron • Bronze • Carbon and low alloy steels such as 4140 • Chrome steels such as 11%, 12% or 13% • Martenistic stainless steels in the 400 series • Precipitation hardening stain- less steels like 17-4 PH • Austenitic stainless steels like the 300 series or alloy 20 • Duplex stainless steels such as CD4MCu • Other more exotic alloys such as Hastelloy, Titanium, etc. Repair Considerations It is important to remember, before any repair procedures are performed on any pump compo- nent, that the material of construc- tion must be accurately identified by means of the appropriate tests. Prior to any repairs being conduct- ed on a pump casing, it is alsoCircle 210 on Reader Service Card advisable to consider the economic advantage of the repair under con- sideration. Smaller and medium-sized ANSI pumps are designed with a high degree of interchangeability and produced in volume. Conse- quently, it can frequently be more cost effective to replace the entire 40 MARCH 2004 www.pump-zone.com PUMPS & SYSTEMS
    • pump rather than a combination of the impeller, Contributing Editor Ross Mackay has over 40casing and back cover. In addition, both the indi- years in the pump industry—many of them workingvidual parts and complete pumps are available fairly with both ANSI and API pump applications. Today,quickly. This can make it more cost effective to he specializes in helping companies increase theirreplace rather than repair the parts, unless the wet pump reliability and reduce operating and mainte-ends are made of the more exotic alloys. It is clear, nance costs through consulting and education. He canin the case of non-metallic pumps (which may also be reached at 1-800-465-6260 or through his web siteconform to ANSI standards), that the components at www.rossmackay.commust be replaced, as they general-ly cannot be repaired. API pumps, however, are gen-erally more economical to repairthan to replace. These units areusually installed in more ruggedduties and hazardous applicationsin refineries or other petrochemi-cal industries, and are conse-quently more durable and moreexpensive. Delivery periods arealso frequently longer, and theparts more costly than their ANSIequivalents—particularly the casesand impellers. This makes it very temptingto source these parts from anafter-market supplier rather thanthe Original Equipment Manu-facturer (OEM). It should benoted, though, that the majorparts of a centrifugal pump (i.e.the casing, the impeller and theback cover) are all cast from pat-terns involving intricate hydraulicdesigns, which are of a proprietarynature. These parts are also theones that provide the hydraulicperformance of the pump. Whilethe parts might be available fromafter-market suppliers at slightlylower prices than they are fromthe OEM, that cost saving willfade into insignificance if thepump does not meet its hydraulicperformance. Your OEM can Circle 537 on Reader Service Cardaccept the responsibility for thesubsequent hydraulic perfor-mance of these replacement parts. By taking these factors intoaccount, you can finish up withreliable pumps that have beenproperly selected to provide a prof-itable and reliable operating system. P&SPUMPS & SYSTEMS www.pump-zone.com MARCH 2004 41