Deptt. of Irrigation and Drainage Engineering, Dr. PDKV, Akola Power Requirement M. U. Kale Assistant Professor Deptt. of Irrigation and Drainage Engineering, Dr. PDKV, Akola

Friction head in pipe system A friction exist on both the suction and discharge sides of a pump friction head varies with Rate of flow of water Pipe size Condition of interior of pipe and Material of which the pipe is made

loss of head due to friction is given by

Friction head in pipe system Friction coefficient ‘f’ depends upon the smoothness or roughness of the pipe surface Value of ‘f’ is less if the pipe is new and smooth

Friction loss in pipe fittings and pump accessories

Total pumping head Also called as total head is the energy imparted to the water by the pump often expressed in the unit of length Two situations in pumping Free surface of the source of water supply is below the centre line of the pump Free surface of the source of water supply is above the centre line of the pump

Various terms used in designating pressure heads in pumping sets Free surface of the source of water supply is below the centre line of the pump Stastic suction lift (hss) – It is the vertical distance from the free suction water level (pumping water level) to the centre line of the pump Total suction lift (Hs) – It is the sum of static suction lift and the losses due to friction in the suction pipe and fittings, including the entrance losses at the inlet to the suction pipe (hfs) Hs= hss + hfs

Static delivery head (hd) – Also called as static delivery lift It is vertical distance between the centre line of the pump and the point of free discharge or the level of the free surface of the discharge liquid Delivery head (Hd) – It is sum of static delivery head and friction losses in the delivery pipe (hfd) Hd= hd + hfd Velocity head (Hv) – It is the pressure required to create the velocity of flow in the pipe v2 Hv = ------- 2g Total head (Hv) – It is the sum of total suction head, delivery head and velocity head H = Hs + Hd + (v2d/2g) + (v2s/2g)

Various terms used in designating pressure heads in pumping sets b) Free surface of the source of water supply is above the centre line of the pump Stastic suction head (hs) – It is the difference in elevation between the centre line of the pump and the level of water at the source of pumping. Total suction lift (Hts) – It is given by static suction head minus all the friction losses in the pipe and fittings and entrance losses in the suction pipe. Hts= hs + hfs

H = Hd + v2d/2g - Hts - (v2s/2g) Static delivery head (hd) – Also called as static delivery lift It is vertical distance between the centre line of the pump and the point of free discharge or the level of the free surface of the discharge liquid Delivery head (Hd) – It is sum of static delivery head and friction losses in the delivery pipe (hfd) Hd= hd + hfd Velocity head (Hv) – It is the pressure required to create the velocity of flow in the pipe v2 Hv = ------- 2g Total head (Hv) – It is the sum of delivery head and velocity head minus the total suction head. H = Hd + v2d/2g - Hts - (v2s/2g)

This phenomenon is called as cavitation. If delivery and suction pipe are of same diameter, total head Cavitation – While pumping water, if the pressure at any point inside a pump drops below the vapour pressure, corresponding to the temperature of the liquid, the liquid will vaporise and form the cavities of vapour. These bubbles of the vapour are carried along with the stream until a region of higher pressure is reached, when they collapse or explode with tremendous shock on adjacent walls. This phenomenon is called as cavitation. H = Hd - Hts

Sudden in-rush of liquid into the cavity created by the collapsed vapour bubbles causes mechanical destruction or erosion Apart from this, corrosion also occurs due to chemical reaction between the gases and metal and additional destruction of the metal takes place. There is an accompanying noise, varying from a low rumbling to loud knocks and resultant heavy vibration of the pumping unit. The energy required to accelerate the flow of water to fill the hollow spaces results in loss of power. Thus, cavitation is accompanied by a reduction in efficiency of the pump.

Net Positive Suction Head (NPSH) NPSHR is the amount of energy required to move the water into the eye of the impeller Cavitation will occur when the hydraulic head at the pump inlet is too low for its operation The pressure head needed at the pump inlet may be termed the net positive suction head The total net positive suction head = net positive suction head + entrance and other friction losses in the suction piping

When the pumping water level is below the pump inlet, the total net positive suction head is equal to the atmospheric pressure minus the vertical distance from the pumping water level to the pump inlet. When the pumping water level is above the pump inlet, the total net positive suction head is equal to the atmospheric pressure plus the vertical distance from the pumping water level to the pump inlet. NPSH is a characteristic of the pump. The total available NPSH can be increased by placing the pump inlet at a lower level in the well or operating the pump with a higher pumping level in the well.

Negative Pressure The negative pressure head, also called as suction Suction is the head below the atmospheric pressure head. This draws the fluid into a pipe or pump chamber by producing a partial vacuum. The water in the well is forced up and into the pump by the atmospheric pressure acting on the free surface of water in the well

Maximum Suction Lift When water is moving into a pump, maximum suction lift is limited by four factors i.e. atmospheric pressure, vapour pressure, head loss due to friction and NPSH of the pump itself. Hs = Ha – Hf – es – NPSH - Fs Where, Hs = maximum practical suction lift, m Ha = atmospheric pressure at the water surface, m es = saturated vapour pressure of water, m Fs = factor of safety, 0.6 m An approximate correction of for altitude is a reduction of 0.36 m for each 300 m in altitude

Various terms concerning to power requirement in pumping Water Horse Power (WHP) It is theoretical power required for pumping. It is expressed as Discharge, l/s X Total head, m WHP = -------------------------------------------------- 76 Discharge, m3/s X Total head, m 0.076

Shaft Horse Power (SHP) It is power required at the pump shaft. It is expressed as Water Horse Power SHP = --------------------------------------- Pump Efficiency Pump efficiency = -------------------------------------- Shaft Horse Power

Various terms concerning to power requirement in pumping c) Brake Horse Power (BHP) It is actual horsepower to be supplied by the engine or electric motor for driving a pump. For monoblock and other direct driven pumps, BHP = SHP For other indirect drives, Water horse power BHP = -------------------------------------------------- Pump efficiency X Drive efficiency

d) Input Horse Power (BHP) It is expressed as Water Horse Power IHP = --------------------------------------------------------- Pump efficiency X Drive efficiency X Motor / engine efficiency BHP X 0.746 Kilowatt input to electric motor = -------------------------------------- Motor Efficiency

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