Chapter 1

Introduction

A pump is a device used for the purpose of transferring fluids and/or gases, from one place to another, by mechanical action, either rotary or reciprocating.Pump types generally fall into two main categories – Rotodynamic and Positive DisplacementThe Rotodynamic pump transfers rotating mechanical energy into kinetic energy in the form of fluid velocity and pressure. The Centrifugal pump is a type of rotodynamic pump which utilises centrifugal force to transfer the fluid being pumped. This project focuses on Centrigual Pumps used for liquids such as acids, solvents, abrasive solvents, saline water, caustic, coal ash slurry, etc..

Fig.. 1PrincipleLiquid enters the pump at the centre or eye of the impeller (Fig..1). Usually the impeller rotates at a speed of 1200-3600 rpm. The speed of the impeller creates a centrifugal force that throws the liquid to the outer edge at a high velocity. It leaves the impeller at high velocity and enters the volute, which is an enlarged chamber where the velocity is quickly reduced. This velocity reduction results in pressure increase. The amount of pressure an impeller develops depends upon its diameter and speed at which it rotates. The large diameter impeller operating at a higher speed will develop a highest pressure. The pressure developed by the impeller is limited by the materials of which the impeller is made. If a single impeller will not develop the pressure required, two or more impellers can be installed in series to increase the pressure rise across the pump. A pump with three impellers can be compared with three pumps which operate in series. There is no theoretical limit to the number of impellers which can be installed in a pump. However, horizontal pumps seldom have more than eight impellers in one casing. If this is not enough to produce a desired pressure, a second pump will be used.

1

Chapter 2 Materials, Construction and Function of important parts

Materials:

Since these pumps are used in a corrosive environment, different materials are used in their construction, for parts such as impellers, casing and stuffing box including back plate (pressure plate) and expeller. Following materials are used. (for composition see sheet 1 attached)

Code Material of Cast UsesCF8M SS 316 SolventsCF8 SS 304 SolventsWCB Cast Steel Water

CN7M Alloy -20H2SO4; used for leeching processes

CF3M SS 316L SolventsCD4Mcu CD4MCu Slightly abrasive solvents

CK-20 Carpenter 20H2SO4; used for leeching processes

Grade 4A,5A,6A Duplex SS Saline water

N12MV Hastelloy BHCl & mixtures containing high % of HCl

CW12MW Hastelloy CHCl & mixtures containing high % of HCl

CZ-100 Nickel (100%)Caustic at elevated temperatures

M-35 MonelCoal ash Slurry (highly erosive materials)

Ni-Hard (Ni- Resistance) Ni-Hard Hydrofluoric acid

Construction and Function of parts:2

Fig.. 2

The main parts of a centrifugal pump are the Bearing Housing, Bearings, Casing, Stuffing Box including Pressure Plate and Expeller, Impeller, Shaft and Seals, Fig.. 2.

Bearing Housing:

This holds the 2 bearings and oil to provide lubrication for the bearings and for cooling. More the amount of oil, more cooling is provided. But if the entire block is filled with oil then the bearings get over heated and may be ruined. The extra volume that is provided at the bottom of the bearing housing gives place to the extra oil. The bearing itself is only 1/4th immersed in the oil.Constant oiler maintains the oil level.

Bearings:The bearings help:

To reduce friction when the impeller is rotating and to take the radial loads. To support the shaft so that it does not wobble inside the pump casing. To prevent lateral movement of the shaft so that the rotating parts do not touch the pump

casing.

3

Casing:A clearance is provided between the casing and housing. This clearance helps in the flow. The smaller the clearance, the more easily the pump can deliver the liquid and thus improve the efficiency of the pump. If there is no clearance then there will not be any place for the impeller to rotate. Plastic pumps have higher clearance to avoid fusion at high temperatures.

Stuffing Box and Expeller:The 2 side holes are provided for cooling.Types:

Conventional Large Bore Seal Chamber: Large bore provides increased life of seals through improved lubrication and cooling of faces. Available with internal bypass to provide circulation of liquids using external flush. Ideal for conventional or cartridge single mechanical seals in conjunction with a flush and throat bushing at the bottom of the chamber.

Large Bore Seal Chambers: Enlarged bore seal chambers with increased radial clearance between the mechanical seal and seal chamber wall provide better circulation of liquid to and from the seal faces. Improved lubrication and heat removal of seal faces extend seal life and lower maintenance costs.

Stuffing box expeller pressure plate

Expeller is found between the stuffing box and the pressure plate. It is only used for liquids which have slurry. It has vanes which protect the shaft from the slurry.

As the expeller rotates, the vanes impart forces to the mixture of

the fluid and solid within the cavity causing it to rotate about the shaft. Centrifugal action associated with this rotation flings the mixture outward but the expeller housing boundaries confine the outward motion. The net result of this is the creation of a

low pressure area about the shaft. Air fills the low pressure area about the shaft and a concentric mixture gas area is formed at some diameter within the expeller vanes. This interface is essential as it forms a distinct barrier between the mixture and the shaft. It also protects the seal from exposure to abrasives during pump operation. It comes with an attached sleeve. For pumps which have expellers, there is no need for a separate sleeve. The mechanical seal cover is attached on the sleeve of the expeller. Stationary part of seal is placed on the seal cover then inserted on the expeller sleeve on top of the stuffing box. It is tightly locked on. Rotary part is inserted on the expeller sleeve.

4

O ring is inserted on the shaft and then the entire stuffing box arrangement is bolted and fixed with a key. Then impeller (+ O ring) and casing (+ gasket) is bolted and set with the 6 screws on the back plate. Then the rotary part of the seal is set in place and screwed tight (after compressing it and checking that the U pack or V pack do not prevent the springs from returning to their original position).Notes:This setting of the rotary seal is only done in pumps which have expellers. Normally, the rotary part of the seal is set on the sleeve according to the calculations made.

Impellers:Semi-Open and Closed ImpellersClosed impellers provide more head and are more efficient. They give the liquid a channel to flow in. The liquid flows into the centre and is immediately discharged upwards. Used for clear liquids like solvents. They develop more pressure but have a lower capacity.

Fig. 4.

Semi-open impellers are used for thicker liquids and liquids with solid particles. They take 20-30% slurry. These liquids get stuck in the closed impellers. Semi-open impellers are better than open impellers in terms of efficiency and head. They also have a greater suction power. They do not develop as much pressure as closed impellers but have a higher capacity than them.

Fig. 5.

5

Seals (Gland + Mechanical):Gland Packing:

Should not be over tightened as then all leakage will be prevented. A small amount of liquid lubricates the packing and thus reduces friction and wear. It maintains the desired compression. A sleeve is sometimes fitted to the shaft in stuffing box region. This type of packing should not be used for suction lift conditions as air may be drawn into the pump through the stuffing box thus causing the pump to lose suction.

Pumps designed to operate under

suction lift conditions use sealing or injection type of Fig. 6. packing arrangement as it uses the liquid to

help seal the packing gland and prevent air being drawn into the pump. This sealing liquid either comes from the discharge side of the pump (internally sealed) or from an external source (externally sealed).Lantern ring is usually used in distributing the sealing liquid within the stuffing box. Made of CTF (carbon filled Teflon), GFT (glass filled Teflon), brass and bronze and is normally positioned mid-way in the stuffing box. They are present around the sleeve and prevent scoring on the sleeve. Used for the injection of liquid and flushing of liquid. If pump handles slurry containing solid particles then the sealing system should be of internal type with both hard faces like SiC-SiC or TC-TC with Plan 02-62. For clear liquids (acids), sealing system should be of external type. By a quench type gland, water, oil or other fluids can be injected into the gland to remove heat from the shaft thus limiting heat transfer to the bearing frame. This permits the operating temperature of the pump to stay higher than the limits of the bearing and lubricant design. This same gland can be used to prevent the escape of a toxic or volatile liquid into the air and the pump.Material of gland: Cotton, asbestos, flax, etc. woven or braided to form a continuous length. It is reinforced with wire strands which strengthens the material and helps it retain its shape. Made in the form of a ‘V’ section and installed with the open part of the ‘V’ facing the liquid being pumped. In this position, the pressure of the liquid in the pump tends to expand the packing and helps it to fit on the shaft.

6

Mechanical Seals:

When leakage of the pumped liquid is not acceptable, mechanical seals replace the gland packing. But emissions are not 100% prevented.Rotary part of the mechanical seal is pressed against the stationary seal ring by the combined action of the spring and the pressure of the liquid. Spring present in the rotary part of the seal helps in maintaining contact pressure between the faces of the rotating and stationary parts of the seal. Before the seal is assembled on the shaft, the stationary part of the seal + seal cover arrangement is inserted on the rotary part + sleeve and compressed a few times to make sure that the U Packs (or V Packs) present are not preventing the springs from returning to their original position. 1 mm clearance is provided between the seal and the shaft on either side of its diameter to prevent jamming and also the liquid is properly circulated.

Fig. 7. U packs and V packs are made out of elastomers for the purpose of sealing. If liquid is a vegetable oil, then ARO VV (Vitton) or ARO TT (Teflon) is used for the U pack or V pack. ARO indicates single spring, RO indicates multi spring. Single spring seals have U packs and multi spring seals have V packs. U pack has a U shaped cross section whereas a V pack has a V shaped cross section. V packs are thinner than U packs. They resemble O rings.It is used to protect the shaft and bearing from the liquid. Ceramic seals and SiC (alpha sintered i.e. it does not contain silica) used for chemicals as they do not get heated easily whereas a metallic seal would require constant cooling. The rings that are provided on either side of the ceramic seal are there to make it compressible as the seal is dense and not malleable.The rotary part of the mechanical seal is generally made out of carbon.If the liquid is an acid, then Tungsten Carbide (TC) is used.

Advantages of Mechanical Seals: Limited leakage of product and hence a reduction in emissions providing a safe

environment. Reduced friction and power loss Elimination of shaft and sleeve wear Reduced maintenance costs Ability to seal higher pressures in more corrosive environments

Assembly: Stationary part of seal fits into the seal cover. The seal cover seal can either be an O ring or a

gasket. This prevents leakage from the joint between the stuffing box and seal plate.

7

O ring is inserted. It prevents leakage between the stationary seal ring and seal plate Rotary part of the seal is secured to the sleeve according to calculations. The thickness of the

seal cover is calculated and that much place is left on the sleeve when the rotary part of the mechanical seal is inserted on the sleeve. The rotary part is tightened onto the sleeve after it is placed in its correct position.

O ring is inserted. It prevents leakage between the rotary part of the seal and shaft. Thrust collar is secured by shaft and takes the reaction thrust of the spring. Entire arrangement is bolted onto the stuffing box and then onto the shaft.

Types of Mechanical Seals:

Fig. 8 Pusher: Incorporates secondary seals that move axially along a shaft or sleeve to maintain

contact at the seal faces. This feature compensates for seal face wear and wobbles due to misalignment.

o Advantages: Cheap Commercially available in a wide range of sizes and conFig.urations.

o Disadvantage: Prone to secondary seal hang-up and fretting of the shaft of the sleeve.

Fig. 9 Non-pusher: This seal does not have to move along the shaft or sleeve to maintain sleeve

contact. Also known as bellows.o Advantages:

Ability to handle high and low temperature Does not require a secondary seal

o Disadvantage: Bellows have a thin cross section and thus must be upgraded for use in

corrosive environments.

Fig. 10

8

Balanced: These seals have higher pressure limits, lower seal face loading and generate less heat. This makes them well suited to handle liquids with poor lubrication and high vapour pressures such as light hydrocarbons. Balancing a mechanical seal involves a simple design change which reduces the hydraulic forces acting too close to the seal faces.

Fig. 11 Unbalanced:

o Advantages: Cheap Leaks less More stable when subjected to vibrations, misalignment and cavitations.

o Disadvantage: Low pressure limit

Fig. 12 Cartridge: The mechanical seal pre-mounted on a sleeve including the gland and fits directly

over the special model shaft or shaft sleeve. (single, double, tandem). Seal faces are made from SiC. These seals are used for hard liquids or those liquids which contain slurry.

o Advantage: No requirement for usual seal setting measurement for installation. Lowers maintenance costs Reduces seal setting errors

9

Sealing Arrangements:

Fig. 13 Single inside: Balanced to withstand high seal environment pressures. For clear non-

corrosive and corrosive liquids with satisfactory lubricating properties where cost of operation does not exceed that of a double seal.

Single outside: For extremely corrosive liquids with good lubricating properties, an outside seal offers an economical alternative to the expensive metal required for an inside seal to resist corrosion.

o Disadvantage: It is exposed outside the pump making it vulnerable to damage from impact

and hydraulic pressure works to open seal faces so they have low pressure limits (balanced or unbalanced)

10

Fig. 14

Fig. 15

11

Fig. 16

Fig. 17 Double (dual pressurized): These are for liquids that are not compatible with a single seal

(toxic, hazardous, have suspended abrasives or corrosives which require costly materials)o Advantages:

It has 5 times the life of a single seal. The metal inner seal parts are never exposed to the liquid being pumped

thus viscous abrasive or thermosetting liquids are easily sealed without a need for expensive metallurgy.

Its life is virtually unaffected by process upset conditions during pump operations.

12

Double Gas Barrier (Pressurized Dual Gas): Very similar to cartridge double seals. Involves an inert gas like nitrogen to act as a surface lubricant and coolant. The gas barrier seal uses nitrogen or air as a harmless and inexpensive barrier fluid that helps prevent product emissions to the atmosphere and fully complies with emission regulations. The double gas barrier seal should be considered for use on toxic or hazardous liquids that are regulated or in situations where increase reliability is required on an application.

Tandem (Dual Unpressurized): Used for volatile, toxic, carcinogenic, hazardous liquids. It eliminates icing and freezing of light hydrocarbons and other liquids which could fall below the atmospheric freezing point of water in air (32°F i.e. 0°c). Buffer liquids are ethylene glycol, methanol, propanol. If primary seal fails then the outboard seal can take over and function until maintenance of the equipment can be scheduled.

If there is a leakage in the mechanical seal, the pump is disassembled and the casing and seal cover is removed. Reasons for leakage:

O ring is broken. If there is a gap between the rotary and stationary part of the seal. This happens if the U

Pack (or V Pack) prevents the spring (in the rotary part of the seal) from returning to its original position thus causing a gap. This mostly happens in single spring seals.

If the pipe has impurities. If the seal face is broken (seal face: black rings on the seal).

Cooling Line (Flush):

Fig. 18A cooling line is provided (Plan 11-62) for the mechanical seal. It is made out of CF8M. The liquid chosen by the customer (that is given direction by the pump) flows through the line. The liquid flows under the stationary part of the mechanical seal onto the sleeve till the seal face of the rotary part. It expands the bellows present in the rotary part. The spring present in the rotary part force the liquid

13

back thus forming a film between the rotary and stationary part of the seal. This film needs to be present at all times for the proper working of the seal. The motion of the impeller provides the pressure with which the liquid present flows to the seal.If the pump is used for long periods of time and then suddenly drained out, the seal becomes dry. This in addition to the heat generated due to the friction (between the seal faces) can cause major damage. The cooling line provides the liquid thus preventing the dryness.

Fig. 19A cooling line is not needed (Plan 02-62) for mechanical seals made out of SiC (Silicon Carbide). When the mechanical seal is made out of SiC, there is a slight gap between the stationary part of the seal and the seal cover. This gap provides the space for the liquid to cool the seal and thus cooling line (aka flush) is not required.

Coupling:A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not allow disconnection of shafts during operation. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. By careful selection, installation and maintenance of couplings, substantial savings can be made in reduced maintenance costs and downtime.Couplings are made out of RSS (a type of rubber). A tire type coupling is more secure as it has less chances of breaking due to pressure.A cover is provided for the spider and distance piece for the protection of the user. (Refer to Fig. 21)

14

Chapter 3

Assembly of Pumps and Use of Jigs

Assembly:

Fig. 20Bearing + Shaft:Bearings are heated on a bearing heater for approximately 3 minutes allowing them to expand (in microns). They are then fitted on the shaft. When they cool, they contract and fit onto the shaft. This process is called Press Fitting. A circlip locks the outside bearing along with a star washer (CF8) and locknut. The bearing is then put in the bearing cover. An O ring is inserted to lock the bearing cover. The arrangement is then bolted inside the bearing housing. Telescopic cover is the back plate (provides movement). It is fitted over the in-house bearing.Notes:The O ring is made out of Vitton, EPDM (ethylene propylene diene monomer (M-class) rubber), PTFE (elastomer). (Refer to “Seals”)

Oil Seal:The oil seal is lubricated and inserted on the other side of the bearing cover. A deflector is inserted on top of the oil seal to protect it from the liquid (if the oil seal is made out of rubber).Notes:It is made of brass for international delivery and out of rubber for national delivery. Rubber oil seals are not accepted abroad.

15

For SPP, oil seal is made from Teflon (Polytetrafluoroethylene).The deflector is made out of stainless steel.

Sleeve + Stuffing Box + Mechanical Seal + Impeller + Casing:Rotary part of the seal is put on the sleeve and bolted into the stuffing box. Stationary part of the mechanical seal is put in the sleeve cover. This arrangement is them bolted onto the stuffing box. Gasket is used to lock it in place and then the entire arrangement is inserted on the shaft. Key locks it. Impeller is fitted in place and is locked onto the shaft by a locknut. Another gasket is used to lock the stuffing box to the casing. The entire arrangement is set in place by the 6 screws present on the back plate. Notes:Seal cover/gland plate:It is made out of a material compatible to that of the liquid being pumped.There are many different shapes depending upon the shape of the stuffing box.

MotorThe shaft (at the bearing housing end of the pump) is connected to a coupling. The flowchart below describes the connection to the motor.Shaft Coupling Spider Distance Piece Spider Coupling Shaft (Motor)

Fig. 21

FrameThe pump and motor are bolted onto a frame. The centre of the motor and pump is calculated for proper alignment Notes:The frame is made out of MS7 (Mild Steel).

16

Jig:

During drilling of an object, a jig is used. It can hold many stuffing boxes together. A cover is inserted on top and bolted. The holes provided on the jig fixture are used as a guide for drilling holes. It is made out of mild steel.Note:Eye bolt used to pick up jigs and other such heavy parts.

17

Chapter 4

Testing and Balancing of Pumps

Testing Done with raw water. The required head and efficiency for the liquid that will be used is

calculated and accordingly the results are made. (pH, corrosion, does it vapourise, viscosity, sp. Gravity)

Voltage and current fixed according to ASME standards. Head losses due to bends in pipe layouts in the industry are more thus head should be

slightly higher than that specified by the customer. Flow is controlled by the valve Lower pipe – suction; upper pipe – delivery Power of motor measured in HP and kW Priming done before tested. In priming, the entire pipe line is filled with water. The pump

and motor are connected. Priming is done to evacuate any air present in the pipes and the pump (Refer to “Priming and Pumping of STP”)

After testing, the pump is left on for 15-20 minutes for a light fluid and for 30-45 minutes for a heavy fluid. This is done to check if there are any leakages in the seal and whether the pump gets heated, done manually, no instrument is used.

All the values are displayed on the computer screen. Installed software helps in determining the readings.

7 readings are taken so that the performance curve is smooth. More the readings, smoother the curve.

The customer specifies the amount of head with respect to the flow. (with respect to water, not the liquid being pumped) If the head is more than that required, the impeller diameter is trimmed. If the flow is less than that required, the impeller is replaced by one of a larger diameter.

Notes:The larger the impeller diameter, more the flow and therefore the head will increase.Trimming of the impeller diameter is done by grinding.According to pump affinity laws, the diameter of the impeller can be calculated based on flow and head.Refer to testing sheets (calculation + performance) for all calculations and graphs.

Hydro Testing Casing and back plate (stuffing box) is tested here. They are assembled together and bolted Suction port is blocked The entire arrangement is immersed in water Air at a certain pressure is allowed to enter through the delivery port If bubbles are seen then there is a leakage and the part is sent for welding and machining

again.

Soap Test:18

In some places, a soap test is carried out. The assembly is done and sprayed with soap. Then air is allowed to enter at a certain pressure. If there is a leakage, then soap bubbles will form.Notes:Sometimes the arrangements are tested with water. Instead of air, water is allowed to flow through the delivery port. The arrangement is not immersed in water, it is left outside. It there is a leakage, the water can be seen coming out through the defect in the part. This testing is done for pumps that use thick and dense liquids (STP). Air, being light, will pass through any gaps present and even if there is no defect, bubbles will be seen. As such, water is used instead.

150 psi – Normal air pressure360 psi – Used for certain pumps (USA)

NPSH: Net Positive Suction Head: It is the minimum requirement for the pump to perform its duties. It happens at the suction side of the pump, including what goes on in the eye of the impeller. (Unit: m) It depends upon:

Suction piping and connections. Elevation of liquid in the suction side Absolute pressure of the fluid in the suction piping Velocity of the fluid Temperature of the fluid

NPSHR: Net Positive Suction Head Required: It is the energy in the liquid required to overcome the friction losses from the suction nozzle to the eye of the impeller without causing vaporization. This means that it is the minimum suction pressure necessary to keep the pumped fluid in a liquid state and for safe, reliable operation. According to the standards of the Hydraulic Institute, a suction lift test is performed on the pump and the pressure in the suction vessel is lowered to the point where the pump suffers a 3% loss in total head. This point is called the NPSHR of the pump.NPSHA: Net Positive Suction Head Available: It is the energy in the fluid at the suction connection of the pump over and above the liquid’s vapour system. It only deals with the suction side of the pump. NPSHA is present in the system and can be calculated.Pressure can be converted to head using the following equation:

H= 10.2 x PressureSpecificGravity

…………………………………………………………………………………………………………… (i)

Where H is the head in meters and pressure is in bar (1 bar = 14.5 PSI)

NPSHA = Ha + Hs – Hvp – Hf ……….………………………………………………………………………………… (ii)

Where:

Ha is the atmospheric head at sea level. If tank is pressurized then it is calculated according to equation (i).

Hs is the static head of the fluid level in the suction vessel to the pump centerline. It can be positive or negative.

Hvp is the liquid vapor pressure head. It depends upon the temperature of the liquid. The vapor pressure is found from thermodynamic tables and is substituted in equation (i) to find the liquid vapour pressure head.

19

Hf is the friction losses in the suction piping and connections. It can be found from the friction tables for pipes and fittings. For submerged and deepwell pumps, Hf = 0.

BalancingBalancing of the impeller is done with respect to its weight.

Acceptableunbalance= ImpellerWeight xuImpeller Radius

u = 20 for 2900 rpm = 40 for 1440 rpm

Balancing is done with a dial indicator. (At 2 different angles (positions), the weight is measured. If it is not the same everywhere, then the impeller is unbalanced. All the readings are shown on the computer screen. Unbalance is corrected by trimming the impeller diameter.

20

Chapter 5

Types of Pumps

Thermic Pump

It has a cooling plate and no sleeve. Heat barrier plate for better heat dissipation. Used for liquids at high temperatures (max 350° C) and vegetable oils. They are mainly used

in boilers for transfer process. Closed impellers for higher efficiency. High head No cooling line. Cooling fins are provided to eliminate use of external cooling system. Bearing housing and stuffing box have a different design from those used normally. Oil lubricated bearing housing only with ball bearings provided on the product side and the

drive side to ensure ease of maintenance and better service life. Use SiC-Carbon or Lecrolloy-Carbon seals. (Spring: Wave type; Bellows: rubber) Holes are provided on either side of the adaptor for a thermosyphon pot. For high temperature liquids and liquids posing environmental and health hazards in the

chemical and pharmaceutical industry, single mechanical seals are not recommended. For such applications a double mechanical seal with a pressurized thermosyphon pot circulating the barrier fluid are used. The barrier fluid recirculates through the seal by heat convection

Thermosyphon pot is used for pressurizing double and tandem mechanical seals. It provides a barrier fluid for the necessary lubrication and cooling to seal faces to increase seal life. The pot is equipped with a cooling coil inside the shell to bring down the temperature of barrier fluid coming from the seal to the pot.

Solvent Transfer Pumps (STP)

21

Impeller cover and cage provide the stronger pull and give stronger head (high head, low capacity)

Vertical Excellent for handling solvents from barrels and from tank farms to the charging vessels. Suction and discharge ports on either side Available in SS 316 and cast iron with all wetted parts made from SS 316 Impeller is also called floating impeller Can easily be transferred from place to place An open impeller has thin vanes. If any solid impurity is present in the liquid (due to

defects in the pipe) then the vanes bend and the entire part has to be replaced. (Refer to “Flushing the Pipes”)

Available only with internally mounted mechanical seals with the rotary faces made from carbon and high aluminum ceramic (Al2O3).

Advantages:o Low NPSH required (Refer “NPSH”).o Pump can take negative lift from 3 to 5 meterso Life lubricating bearings, covered from both the ends so minor leakage from pump side

does not affect the bearings.o The pump runs at a low speed thus low maintenance required.o Motor required is vertical flange type and is directly coupled to the pump thus coupling

alignment is easier making the installation easiero Low overhang shaft arrangement gives minimum deflection at the seal faces.o Floating type impeller reduces noise and vibration.o Seal cooled and flushed by fluid itselfo It can be fitted as a valve thus does not require any suction or delivery pumping efforts

(Refer to “Priming and Pumping of STP”). Self priming

Assembly of Solvent Transfer Pump:

22

Grease bearings are attached on the shaft and the shaft is inserted into the motor stool. A cup is placed on top and the stationary part of the seal is fitted there. The rotary part is inserted on top. No sleeve is present. Then chamber (with suction and delivery ports) is put on top. Clearance is given. Then impeller cage is set and locked on top. Open impeller is placed. Chamber is bolted according to the setting of the impeller (setting of impeller is done on 4 bolts). Teflon rope is used as sealing over the impeller cage. Impeller cover is fitted and bolted. A valve is attached to the cover for draining.Notes:Teflon rope is used instead of a gasket as sealing because the rope can be compressed so it fits tightly over the cage. It fits like a gasket but seals better. Very soft.No oil is present as grease bearings are used.A tiny hole is provided in the chamber to check mechanical seal leakage.Valve is present as this is a self priming pump and the extra water during priming can be drained out.

Priming and Pumping of STP:The pump is partially filled with the liquid. The rotation of the impeller throws the liquid out between the blades towards the periphery by centrifugal force and at the same time imparts velocity to the liquid in the volute passage. The air being lighter remains in the center of the cage. The volute has its maximum cross-sectional area between points at the bottom section of the impeller cage and decrease in area in either direction from this point. As a pair of blades approach the discharge, the liquid is forced (by centripetal action) between them, down towards the center due to the decrease cross-sectional area of the volute. This provides a positive liquid piston action, pushing the air at the centre out through the discharge port and into the discharge pipe. The space between the blades as they pass the extreme end of the discharge port is now completely filled with liquid which is retained therein until the bridge or sealing surface between the discharge and suction ports has been crossed. Once, past the bridge, the liquid between the impeller blades is thrown out into the volute passage (which has an increase in cross-sectional area). This provides the positive liquid piston action away from the centre of the pump, reducing the pressure over the suction port and air in the suction pipe is forced up into the pump as a result. This action continues until all the air has been evacuated from the suction line. This eccentric liquid ring effect amounts to definite suction and discharge strokes of liquid piston within the pump and is the reason for its positive timing action. After all the air has been evacuated from the suction line, the pump commences and will continue to pump the liquid on exactly the same basis. If the pump breaks suction, it will pump air until suction line is again submerged and then pick up the liquid again. No foot or check valves are needed. It has great air handling capacity. Minor leaks in the suction line, loop in the suction line or bubbles present in the liquid flowing through the pipe do not effect operation. When the pump is stopped, the liquid is retained in the pump casing and the pump is ready without any further priming to start pumping.

Self Priming Pumps (SPP) Has an open impeller Can be single state & double state (double state: 2 impellers; used to increase head) Horizontal Called “Self Priming” because open impeller is always immersed in the liquid Only needs to be primed once Can easily be transferred from place to place.

Chapter 623

Preventive measures and Troubleshooting

Flushing the PipesIn companies where new pipes are installed, a process called flushing needs to be carried out. These new pipes have been welded together and thus there may be many defects (such as burrs, holes, etc.). These defects need to be removed. The pipes are connected to the pump and the liquid is allowed to flow thus removing the defects. This process is called flushing.But the mechanical seal used in the pump is very delicate. If there is any solid particle in the liquid, the seal will crack. As such, the companies install glands (+ gland pushers) during the flushing process. Once the defects have been removed, the mechanical seals are installed.

Problems:1. Cavitation and vapor lock occur when gas is present in the pump. A few gas bubbles will

cause cavitation. More will cause vapor lock. Cavitation: Cavitation occurs when the liquid entering a pump contains a few bubbles of gas. When the gas flows through the impeller with the liquid and its pressure is increased in the pump, some or all of the gas liquefies (the vapor bubbles collapse). A high centripetal force results from this collapse and may cause severe vibration and possible pump damage. The pump will continue to pump liquid, but it will be noisy and may vibrate. This may also occur because of inadequate NPSH.Vapour Lock: Vapour lock occurs when gas enters the pump. The pump will compress the gas a slight amount, but not nearly enough for the gas to flow out the discharge line. The trapped gas prevents liquid from entering the pump. The effect is that no liquid flows through the pump.Solution

Procedure to start the pump after cavitation or vapor lock occurs:o Close a valve in the discharge line.o Open valve in suction line.o Open casing vent valve until a steady stream of liquid comes out.o Start the pump and observe the discharge pressure. It should rapidly increase and

then level off.o Slowly open the valve in the discharge line.o Close the valve in the vent line.

2. Vibrations: Although a certain amount of noise is to be expected from centrifugal pumps and their drivers, unusually high noise levels (in excess of 100 dB) or particularly high frequencies (whine or squeal) can be an early indicator of potential mechanical failures or vibration problems in centrifugal pumps. Causes of vibrations are of major concern because of the damage to the pump and piping that generally results from it. Vibrations in pumps may be a result of improper installation or maintenance, incorrect application, hydraulic interaction with the piping system, or design and manufacturing flaws. SolutionAs there can be many causes of vibrations, the solution depends on the cause of vibration.The common types of vibration tests fall into 2 categories:

o Natural excitation signature analysis tests: Running of the pump at a steady operating condition and collecting data from pairs of transducers at important locations to determine vibration amplitude VS. frequency plot spectrum.

24

o Shutdown and startup transients using peak average: In the tests the pump runs up and down in speed slowly, while documenting the frequency spectrum signature changes due to forced response and instabilities occurring in the pumping system.

o In addition to these tests, experimental modal analysis (EMA) has been found to provide information that helps in determining and eliminating vibration problems, particularly if these problems are a result of resonance.

Chapter 7

Casting

25

Investa uses sand and investment casting methods. Investment casting is better than sand casting as it gives a better finishing but it requires a lot of time. Investment casting:

Wax poured into the die and allowed to solidify. Gates, runners, risers made out of wax are attached to the part by heating the wax and allowing it to cool.

It is coated with slurry forming the pre-coat. It is then covered with Zircon sand which makes it harder and provides smoothness. 16/30 Grade sand forms the secondary coat (4 layers) and 30/80 Grade sand provides strength. The reason behind this is to prevent the investment from breaking.

Dewaxing is done at 200°C in hot wax leaving a hollow shell called the Investment. The hollow shell is baked at 1000°C and in this hot condition, the metal is poured. If the

investment is not heated, then the temperature difference causes damage to the shell. The investment is removed by a vibrating machine. The part is then cut by arc welding and a lathe machine is used to remove the gates,

runners, etc. Shot blasting is carried out to fill the holes or gaps that might be present in the cast. The

blast powder is of a tiny size (like sand particles) and it is applied with high pressure. Belt grinders and pneumatic tools are used for finishing. Sand blasting, glass blasting and vibro-finishing (using stones) provide a smooth surface

finish. Finished product

Notes:Parts weighing more than 50-60 kilograms cannot be made using investment casting as it is too heavy for a person to carry. These parts are made using sand casting.

Bibliography

1. http://www.roymech.co.uk2. Centrifugal Pumps Design and Application - Val S. Lobanoff & Rober R. Ross

26

3. Rolon Seals Catalogue4. Investa Pumps Catalogue5. Investa Maintenance Manual6. Alfa Laval Pump Handbook7. Methods of Investigation and Solution of Stress, Vibration and Noise Problems in

Pumps – William D. Marscher

27