1.7.2 Main characteristics of pump work
Work of each pump in the installation composition (dr.1.22) characterizes following parameters: giving Q, pressure H, power N, coefficient of useful action η and vacuumetric height of absorption Hvac.
Air water lifts (erlifts) for reception and giving of water use pressed air and thus demands on building of compressor stations. Erlifts are simple, reliable on construction and in exploitation, but coefficient of useful action is quite low 0,25-0,35.
Stream pumps are wide used for pumping out of the sediment from water receiving and water deposit constructions, drawing off air from drawing tubes during starting of big centrifugal pumps, in warmth supply and for mixing and circulation of water.
Drawing 1.22. Scheme of centrifugal pump installation: 1 – pressure plumbing; 2 – oven door; 3 – inverse valve; 4 – manometer; 5 – pump; 6 – vacuum meter; 7 – absorbing plumbing; 8 – receiving valve; Hgr –is geometric height of absorption; Hg – geometric height of water lifting; H – full pump pressure; hrs – pressure losses in absorbing plumbing; h gn losses of pressure in the pressure plumbing.
Giving of pump Q (l/sec, m3/hour) – is fluid volume, which is given by pump per time unit.
Pump stress H (m) is water stream energy increasing, which flows through the pump. There are differs manmetric and needful stresses. Monomeric stress is determined by readings of manometer and vacuum meter.
where HM and HB are readings of manometer and vacuum meter, leaded to the axis of pump, Pa; Vk and Vn – velocities of fluid moving in the stress and absorbing branch pipes, m/sec; g – acceleration of free falling, m/sec2; - density of the fluid, kg/m3.
If the pump works with backwater, then the stress:
Where H I are readings of manometer on absorbing line, m.
Necessary stress of the pump consists on geometric height of water lifting and losses of stress from the side of absorption and forcing. For installation, which is represented on the drawing 1.22 is necessary stress:
Power of pump N (Vt), that are consumed for certain Q and H:
where η is coefficient of useful pump action , which is taken by data of production plant and takes into consideration all losses of energy in the pump (hydraulic, volumetric, mechanical), connected with water giving.
Power of the drive NDV (Vt) is set bigger than power of the pump in the case of overloading from unaccounted situations:
Where k = 1,1=1,25 – coefficient of power reserve, which is taken dependently on engine’s power.
In case of connection of the pump with engine through middle power of drive:
Where hпер – CUA of transmission.
There are differed vacuumetric and geometric heights of absorption.
Vacuumetric height of absorption is difference of atmospheric and vacuumetric pressures, expressed in meters of fluid pillar:
Patm – atmospheric pressure , Pa; P1 – absolute pressure in absorption pump cavity, Pa.
Geometric height of absorption Hg is vertical distance from free water level in the source to horizontal axis, which comes through the point of absorption of cavity of pump with minimal pressure.
Allowable geometrical height of absorption:
Where Pn is pressure of saturated vapor on temperature of fluid which is pumping, Pa; h is allowable cavitation reserve (exceeding of full stress in absorption pipe of pump under the stress of saturated vapor on the certain temperature), m; hg are losses of stress in absorption plumbing.
Pump characteristics is dependence of stress, power, CUA and allowable height of absorption during pump giving for certain frequency of rotating n and diameter D of the working circle. There are analytical and graphical characteristics of pump. On working graphical characteristics of pumps on the curve Q-H there are shower by wave kines recommended area of pump usage, which correspond to the most stable and economical work regime, and also maximal meaning of CUA.
Graphical characteristics of pump Д 1600-90 with rotation frequency of working circle n=1400 min-1 is expressed on the drawing 1.23. Characteristics of the pump are lead to the constant maximal rotation frequency of the working circle n.
Drawing 1.23. Characteristics of centrifugal pump of the mark Д 1600-90 n= 1450 min-1.
During pump working with other rotation frequency n, stress, giving, power and allowable vacuumetric height of absorption for the pump are calculated by formulas:
Widening of branch of pump application is also possible to realize by cost of decreasing of working circle (10-20 % from its maximal diameter). Coefficient of useful action η during rotating frequency change in wide borders (up to 50%) and during turning of working circle up to 20% changes too little, and these changes in practical calculations may be not taken into consideration. For previous choose of pumps it is comfortably to use summary pump characteristics (dr.1.24).
Drawing 1.24. Summary characteristics of pumps of K and KM types.
According to characteristics pump is selected for giving of fluid by given expenses and necessary stress. It is necessary to know plumbing’ characteristics (or plumbing system), which is determined as sum of geometric height of water lifting and stress losses:
Graphically characteristics of the plumbing are represented as parabola with top on the ordinate axis, located on the distance Hg from y-axis. For determination of optimal pump work regime with data plumbing will be compatible characteristics of pump and plumbing.
On the picture 1.25 represented characteristics of the pump Q-H. Checking parallel to the axis Q straight line CD on the distance Hg from it and adding to Hg value Σhnor, which corresponds to one or another values or one or another expenses Q, will get characteristics of the plumbing CE. Point 1 of intersection of pump and plumbing characteristics is called “working point”. It characterizes giving Q1, CUA, η, pump power N1 during working on the given plumbing. Pump should be chosen in such a way, that working point located in the borders of highest CUA.
On the drawing 1.25 it is shown work of one pump on one plumbing. In project solutions may be different cases: work of two or more pumps on one or more drainages. On drawing 1.26 it is shown parallel work of two identical pumps on one drainage. In this case characteristics of one pump are doubled on the same stresses, i.e. Q=Q1+Q2 H=H1=H2. CUA of two identical parallel working pumps is equal to CUA of one pump and corresponds to point 3. Power of each of working pumps is marked by point 6.
Drawing 1.25. Common works of pump and plumbing
Drawing 1.26. Characteristics of parallel working of two identical pumps.
On absorbing line of each centrifugal pump, usually, installed bolt and vacuum meter, and on stress – inverse valve for protection on the pump from hydraulic strike and prevention of inverse movement of water; bolt for expenses regulation and pump starting up; manometer for determination of stress determination.