Dynamic head loss

Pressure allowance across a control valve for good operability 20 to 50% of the dynamic head loss or 70 to 140 kPa... 

The selection of the pump cannot be separated from the design of the complete piping system. The total head required will be the sum of the dynamic head due to friction losses in the piping, fittings, valves and process equipment, and any static head due to differences in elevation. 

The literature has used two names for this subject total dynamic head or total developed head (H or TDH). Let us derive TDH first by considering the system connected in parallel between points 1 and 2. Since the connection is parallel, the head losses across each of the pumps are equal and the head given to the fluid in each of the pumps are also equal. Thus, for our analysis, let us choose any pump such as the one with inlet g. From fluid mechanics, the energy equation between the points is... 

Example 4.1 It is desired to pump a wastewater to an elevation of 30 m above a sump. Friction losses and velocity at the discharge side of the pump system are estimated to be 20 m and 1.30 m/s, respectively. The operating drive is to be 1200 rpm. Suction friction loss is 1.03 m the diameter of the suction and discharge lines are 250 and 225 mm, respectively. The vertical distance from the sump pool level to the pump centerline is 2 m. (a) If the temperature is 20°C, has cavitation occurred (b) What are the inlet and outlet manometric heads (c) What are the inlet and outlet total dynamic heads From the values of the idh and odh, calculate TDH. 

Estimating the size of the smallest length scale is relatively simple. One could use computational fluid dynamic modeling techniques or estimate them based on the power input to the system (head loss) and the mass of the fluid being powered. For example, in pipe flow, the energy dissipation rate is a function of the total head loss in the flow h, the volumetric flow rate Q, the density of the solution p, and the mass of the solution m, which in this case is the mass of fluid contained within the pipe [Equation (4.1-3)]... 

The dynamic (or frictional) head loss h may be calculated from... 

Method for calculation of major losses of liquids. First determine fluid properties such as the density, and dynamic viscosity at the operating temperature. Determine the inner diameter of the pipe, and evaluate its absolute roughness based on Table 20.3. Then calculate the Reynolds number for average velocity of the liquid. Afterwards, either use the Moody chart to evaluate the Fanning friction factor based on the Reynolds number and relative roughness, or compute the Colebrook equation by successive iterations. Finally, use the Darcy-Weisbach equation to determine the friction head loss. 

The tailings from a small mine are pumped at a weight concentration of 40%. They consist of crushed rock at a specific gravity of 3.2. The c/gs of the particles is 1mm. For a flow rate of 280 m /hr, a smooth high-density polyethylene pipe with an internal diameter of 138 mm is selected. Using Newitt s method as expressed By equations 4.27 and 4.29, determine the head loss due to the presence of solids, assuming a dynamic viscosity of 1.8 cP. 

Viscous and inertial frictional pressure losses and dynamic head pressure losses arise due to propellant flowing down the channel to the exit as indicated in the red arrows in Figure 3.13. Consider the vertically oriented LAD channel as shown in Figures 3.1 and 3.13, with the origin attached to the bottom center of the channel. The list of assumptions and corresponding implications used to solve for the viscous pressure drop inside the channel are as follows ... 

At high flow velocities, the inertial resistance factor, 2, can be viewed as a loss per unit length along the flow direction, thereby allowing the pressure drop to be specified as a function of dynamic head. 

The static pressure difference will be independent of the fluid flow-rate. The dynamic loss will increase as the flow-rate is increased. It will be roughly proportional to the flow-rate squared, see equation 5.3. The system curve, or operating line, is a plot of the total pressure head versus the liquid flow-rate. The operating point of a centrifugal pump can be found by plotting the system curve on the pump s characteristic curve, see Example 5.3. 

Net positive suction head (kPa) 850.0 848.0 Control valve (% dynamic loss) 68. 0... 

The differential head of the circulation water pump is relatively small, since dynamic losses are modest (short vertical pipe and a low AP spray nozzle) and the hydrauhc head is small, only about 6 m (20 ft) from the basin to the elevation of the spray header. Combined, the pumping energy demand is about 35 percent that for an equivalent CT application. The capital cost for this complete water system is also relatively small. The pumps and motors are smaller, the piping has a smaller diameter and is much shorter, and the required piping structural support is almost negligible, compared to an equivalent CT application. WSAC fan horsepower is typically about 25 percent less than that for an equivalent CT. 

The segments of the buildup are (a) the equivalent clear liquid bead on the tray h Ll (b) any hydraulic gradient A caused by resistance to liquid flow across the tray, which usually is not significant for sieve trays, (c) liquid head equivalent to pressure loss due to flow under the downcomer apron. A. and (d) total pressure loss across the tray above, necessarily included to maintain the dynamic pressure balance between point A (just above the floor of tray 3) and point B in the vapor space above tray 2. 

---