EXECUTIVES & NON-EXECUTIVES CENTRIFUGAL PUMPS FOR EXECUTIVES & NON-EXECUTIVES SSNMOHAPATRA

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A. Stationary Components A centrifugal pump has two main components:             Rotating component comprising of an impeller and a shaft          Stationary component comprising of a casing, casing cover and bearings. A. Stationary Components  o       Casing §         Casings are generally of two types: volute and circular. The impellers are fitted inside the casings. §         Volute casings build a higher head; circular casings are used for low head and high capacity. A volute is a curved funnel increasing in area to the discharge port. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid. SSNMOHAPATRA

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         Circular casing have stationary diffusion vanes surrounding the impeller periphery that convert velocity energy to pressure energy. Conventionally the diffusers are applied to multi-stage pumps.        Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. Split casing implies two or more parts are fastened together. When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. §         Casing Wear rings: The wear ring acts as the seal between the casing and the impeller. SSNMOHAPATRA

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o       Suction & Discharge Nozzle: The suction & discharge nozzles are part of the casings itself. They commonly have following configurations. §        End suction/Top discharge - The suction nozzle is located at the end of, and concentric to, the shaft while the discharge nozzle is located at the top of the case perpendicular to the shaft. Pump is always of an overhung type and typically has lower NPSHr because the liquid feeds directly into the impeller eye. §    Top suction Top discharge nozzle -The suction and discharge nozzles are located at the top of the case perpendicular to the shaft. Pump can either be overhung type or between-bearing type but is always a radially split case pump. §     Side suction / Side discharge nozzles - The suction and discharge nozzles are located at the sides of the case perpendicular to the shaft. Pump can either be an axially or radially split case type. SSNMOHAPATRA

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WORKING PRINCIPLE The key idea is that the energy created by the centrifugal force is kinetic energy. The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid. This kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle the liquid further decelerates and its velocity is converted to pressure according to Bernoulli’s principle. SSNMOHAPATRA

One fact that must always be remembered: A pump does not create pressure, it only provides flow. Pressure is a just an indication of the amount of resistance to flow. SSNMOHAPATRA

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MINIMUM CONTINUOUS FLOW Minimum continuous flow is the lowest flow rate at which a centrifugal pump may be operated on a continuous basis without exceeding the design constraints of the manufacturer or the equipment owner. Thermal stability, mechanical integrity and process control are the three primary factors that contribute to the determination of allowable continuous minimum flow. A minimum continuous flow should be independently determined for each factor. The highest minimum flow rate resulting from this analysis dictates the minimum flow rate for a pump. SSNMOHAPATRA

o Seal chamber and Stuffing box           Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. §         When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. SSNMOHAPATRA

Throat Bushing: The bottom or inside end of the chamber is provided with a stationary device called throat bushing that forms a restrictive close clearance around the sleeve (or shaft) between the seal and the impeller. § Throttle bushing refers to a device that forms a restrictive close clearance around the sleeve (or shaft) at the outboard end of a mechanical seal gland. Bearing housing encloses the bearings mounted on the shaft. The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. § The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water. SSNMOHAPATRA

B. Rotating Components o Impeller is the main rotating part that provides the centrifugal acceleration to the fluid. They are often classified in many ways. ·  Based on major direction of flow in reference to the axis of rotation o       Radial flow o       Axial flow o       Mixed flow ·  Based on suction type o       Single-suction: Liquid inlet on one side. Double-suction: Inlet to the impeller symmetrically from both sides. ·  Based on mechanical construction o       Closed: Shrouds or sidewall enclosing the vanes. o       Open: No shrouds or wall to enclose the vanes. SSNMOHAPATRA

Axial  Flow  Pumps The impeller pushes the liquid in a direction parallel to the pump shaft.   Axial  flow  pumps  are  sometimes  called  propeller  pumps  because  they  operate essentially the same as the propeller of a boat.  The impeller of a typical axial flow pump shown in Figure. An axial flow pump can be designed as a suction pump that draws water in through one end and discharges it out the top of the pump. However, axial flow pumps are not typically used for suction lift applications. Axial flow pumps are typically used in high flow rate, low lift applications. A mixed flow pump similar to a turbine pump may be used as a well pump provided the well is not too deep. SSNMOHAPATRA

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Impeller Wear rings: Wear ring provides an easily and economically renewable leakage joint between the impeller and the casing. If clearance becomes too large, the pump efficiency will be lowered causing heat and vibration problems. Shaft The basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts. It must do this job with a deflection less than the minimum clearance between the rotating and stationary parts. § Shaft Sleeve Pump shafts are usually protected from erosion, corrosion and wear at seal chambers, leakage joints, internal bearings and in the waterways by renewable sleeves. Unless otherwise specified, a shaft sleeve of wear, corrosion, and erosion-resistant material shall be provided to protect the shaft. SSNMOHAPATRA

· Net Positive Suction Head Required, NPSHr When discussing centrifugal pumps, the two most important head terms are NPSHr and NPSHa. ·  Net Positive Suction Head Required, NPSHr NPSH is one of the most widely used and least understood terms associated with pumps. Understanding the significance of NPSH is very much essential during installation as well as operation of the pumps. Pumps can pump only liquids, not vapors Rise in temperature and fall in pressure induces vaporization NPSH as measure to prevent liquid vaporization NPSHr is a function of pump design NPSHr increases as capacity increases NPSHa is a function of system design SSNMOHAPATRA

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The fifth requirement is attention towards the Mechanical seal. Five Basic Requirements for Trouble-Free Operation of Centrifugal Pumps The first requirement is that no cavitation of the pump occurs throughout the broad operating range. Discharge pressure Vibration Motor Current The second requirement is that a certain minimum continuous flow is always maintained during operation. The third requirement is that bearings should be adequately lubricated. The fourth requirement is the pump should be properly aligned with the driver. The fifth requirement is attention towards the Mechanical seal. SSNMOHAPATRA

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UNDERSTANDING MECHANICAL SEAL API PLANS SSNMOHAPATRA

Plan #32.Flushing to remove unwanted fluid in the stuffing box. A clean liquid from an outside source is brought into the stuffing box through a regulating valve at one atmosphere (15 psi. or 1 bar) higher than stuffing box pressure. The liquid should be brought in at the bottom of the stuffing box to insure thorough cleaning. All of this flushing liquid will eventually go into, and dilute your product. SSNMOHAPATRA

Plan #53/52 Barrier/ buffer fluid using an internal pumping ring If the pressure of the fluid between the seals is higher than stuffing box pressure we call it barrier fluid. If the pressure is lower than stuffing box pressure we call it buffer fluid The buffer/barrier liquid can be circulated either by forced circulation, a pumping ring or convection (each a different API plan number). The method that you will use will be dictated by the heat being generated by the pressure in the stuffing box, the shaft speed and shaft size. Since heat generation is also a function of seal design, all seal manufacturers provide charts or graphs to give you the correct guidelines that will insure the proper amount of circulation between the seals. If you elect to use a forced circulation system be sure to introduce the fluid into the bottom of the stuffing box or gland connection and out the top. This arrangement will insure that the space between the seals is vented allowing proper cooling to take place. SSNMOHAPATRA

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Plan 01 Discharge recirculation to pressurize the pump stuffing box The only legitimate use of this discharge recirculation line is to pressurize the stuffing box to prevent a liquid from vaporizing, and that is where it is recommended. (Only for very clean fluid) Plan 02. The stuffing box is dead-ended. Heating or cooling fluid is circulated through the stuffing box jacket. This is the best method of controlling the stuffing box temperature when the pump is not running. Plan 62. External fluid source providing a quench (steam is the most common quench fluid). Typically used with a close fitting bushing in the rear of the gland to prevent the steam from entering into the bearing cavity. A good environmental control. We discussed it earlier SSNMOHAPATRA

Plan 11. A line is connected from the discharge side of the pump and recirculated through an orifice into the gland flush connection. Orifices are hard to size and since many pumping fluids contain solids orifices are easy to clog. Plan 12. A line is connected from the discharge side of the pump and recirculated through a strainer and control orifice to the gland flush connection. If solids are present in the fluid the strainer will frequently clog along with the orifice Plan 13. A line is connected from the bottom of the stuffing box, through a flow control orifice, to the suction piping. Controlling the size of the orifice to get exactly the right flow is difficult. In most cases you will not need the orifice SSNMOHAPATRA

Plan 21. Discharge recirculation through a flow control orifice and cooler into the seal chamber. You are cooling with high-pressure, pump discharge fluid. Not too good an idea! Hot, high-pressure fluids can be dangerous if the cooler or its inlet and outlet-line fittings leak. There are better cooling methods that utilize lower fluid pressure. Jacketing, barrier fluid and quenching come to mind. Also note that with this arrangement you only get the cooling effect when the pump is running. This could cause a premature seal failure when the pump stops. Plan 23. A pumping ring is installed in either the stuffing box or within the mechanical seal components that will pump the stuffing box fluid through an external cooler and then back to the stuffing box. This is a good arrangement because it uses lower pressure cooling fluid. You find this arrangement on the stuffing box of many boiler feed pump sealing applications. Plan 31. Discharge recirculation through a cyclone separator to the stuffing box, Cyclone separators are not very effective in removing the solids that fail mechanical seals. I would not waste my time with this one. SSNMOHAPATRA

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