PUMPS Pumps are the ‘Hearts’ of any process industry – Hydro Power Station being one of them. Performance of pumps is directly linked with the efficiency, reliability and availability of a Hydro Power Station. One of the ways to ensure good performance of pumps is to maintain them in a planned manner which, in turn, generates a need for trained people. Definition of Pumps: Pump is the machine that lifts liquids, moves them from place to place, and pressurizes them from a number of useful tasks. Pumps are mechanical devices used to motivate liquids and gases, for carrying and conveying through pipes from point ‘A’ to any other given point. Classification of Pumps: Basically pumps are of three types: • Reciprocating. • Rotary, and • Centrifugal. Reciprocating Pump: In this type energy is added to the liquid by the to and fro movement of piston, plunger, diaphragm etc. They can be subdivided into following varieties: • Piston Pumps • Plunger Pumps • Ram Pumps • Diaphragm Pumps Rotary Pumps: Here the pumping action is caused by relative movement of roting and stationary elements of the pump. The can be subdivided into following categories: • Gear Pump • Screw Pump • Vane Pump • Lobe Pump Most of the pumps installed the Power Stations are of centrifugal type. Centrifugal Pump: This type of pump have an impeller housed in a suitable shaped casing so that when the impeller rotates momentum is applied to liquid in the pump casing transporting it from the inlet to the outlet side by changing velocity into pressure energy.
Centrifugal pumps can be put in different categories: a) b) c) d) e)
Horizontal. Vertical. Multistage. End suction. Split casing.
Definition of centrifugal pump: A machining which receives water at some central point and flings it there from with the ultimate result, that it is raised from one point to another point. Head-wise categorization is as under: i) Head: 0’-20’ ii) Head: 0’-80’ iii) Head: 0’-350’ Iv) Head: 0’-4000’ Specifications:
Very Low Lift Pumps. Low Lift Pumps. Medium Lift Pumps. High Lift Pump.
Pumps are commonly rated by horsepower, flow rate, outlet pressure in feet (or meters) of head, inlet suction in suction feet (or meters) of head. The head can be simplified as the number of feet or meters the pump can raise or lower a column of water at atmospheric pressure. From an initial design point of view, engineers often use a quantity termed the specific speed to identify the most suitable pump type for a particular combination of flow rate and head. Construction of centrifugal pumps These consist of mainly three parts: • • •
Casing. Impeller - Single eye or Double eye. Shaft or spindle.
Stationary parts consist of casing, bearing and stuffing box and rotating parts consist of the impeller and the shaft. a) Casing: Casing or Housing is the main casting which is firstly used to restrain the water into an approximately circular or spiral path and secondly to collect the water, as it is delivered from the periphery of the impeller. It is provided with the inlet ‘Suction’, the outlet ‘Discharge’ and with a stuffing box to permit the projection of the spindle or a shaft.
b) Impeller: It can be described as a wheel having equally spaced blades or vanes, arranged around the shaft. At one side is the inlet or eye. From the eye, the blades run in a curved path to the outer edge of the wheel. Classification: Impellers can be classified in various types as detailed below: a) Type of Suction: i) Single Suction. ii) Double Suction. b) Form of Vanes: i) Radial Flow. ii) Mixed Flow. iii) Axial Flow. c) Mechanical Design i) Over Hung. ii) Open. iii) Semi Open. iv) Closed. c) Shaft: The basic function of the shaft is to transmit the torque and supporting the impeller and other rotating parts. The impeller is keyed to the shaft which is supported on either ends by bearings. d) Shaft Sleeves: Pump shafts are usually protected from erosion, corrosion and wear at stuffing box, leakage joints, and internal bearings and in the water ways by renewable sleeves. The most common function of the shaft sleeves is that of protecting the shaft from wear at stuffing box. e) Bearings: Bearings are used to reduce frictional force and to keep the shaft in correct alignment with the stationary parts under the action of radial and transverse loads. f) Pump Sealing: Pump seals are provided to prevent any leakage at the point where the pump shaft passes out through the casing. The pump seals also prevent air leakage into the pump, if the pump pressure is less than atmospheric and if the pressure is above atmosphere the function is to prevent the liquid leaking out of the pump. Basically two type of sealing are used: i) ii)
Stuffing box. Mechanical seal.
g) Stuffing Box: stuffing box consists of a number of rings of packing around the pump shaft housed inside a cylindrical cage between the pump casing and the pump shaft. Pump Maintenance: The usual intervals for preventive maintenance are monthly, quarterly, half yearly and annually. i)
Daily observation of pump Operation: The operator on duty should made daily inspection and any irregularities in operation of a pump should be repaired accordingly. This applies particularly to
changes in the sound of running pump, abrupt changes in bearing temperatures and stuffing box leakage. A check of pressure gauges and of the flow meter should be checked hourly. If reading instruments are provided, a daily check should be made to determine whether the capacity, pressure or power consumption indicate that further inspection is required. Stages of Major Pump check-up: i) ii) iii) iv)
Stripping the pump identification and marking of parts. General inspection of parts removed. Specific fault detection. Rectification of fault : i) ii) iii) iv) v) vi) vii) viii) ix)
Bearing housing. Bearing replacement. Neck ring clearance. Impeller condition. Changing gland packing. Check shaft for straightness. Keyways & keep. Shaft sleeves tolerance. Condition of nuts and bolts.
i) Condition of gland and gland housing. ii) Assembly of pumps. iii) Pump alignment. Faults and trouble shooting: Some faults that can be diagnosed by observations are the below listed: i) Pump does not deliver liquid. ii) Pump does not deliver rated quantity. iii) Pump does not generate rated delivery pressure. iv) Pump loses liquid. v) Pump over loads driving unit. vi) Excessive vibration. vii) Bearing over heating. viii) Bearing wear rapidly. ix) Stuffing box leakage excessive. x) Packing with short life. Check list for commissioning of pump after major overhauling Ensure the following: Trial run of motor is completed. i) Fabrication of suction and discharge pipe is completed. ii) All temporary supports are removed.
iii) iv) v) vi) vii) viii) ix) x) xi) xii)
Alignment is checked. Bearings are cleaned and greased. Pump and motor are coupled. Coupling bolts are tightened properly. Rotor is free. Pressure gauges and thermometers are mounted. Coupling guard is fixed and is not touching coupling. Suitable glands are assembled. Over tightening of glands should be avoided. Cooling water circuit is thorough and no leakage in the system. Both the lines i.e. suction side & delivery side are properly flushed. Men and materials are cleared/removed from the site of work.
Ensure Control Room Engineer is informed about readiness of pump for trial run. Ensure that enough water (or any fluid which the pump will handle) is available. Some basics points to be checked before starting of pumps in day to day use / application:i) Ensure power supply on control panel. Circuit healthy indication should be available on control panel. ii) Ensure water at suction or in the sump tank. iii) Check the coupling. iv) Check the discharge valve. v) Check leakage from stuffing box. vi) Switch off the motor heater or heating lamps.