Head loss

Friction head loss Friction match Friction materials... 

In order to select the pipe size, the pressure loss is calculated and velocity limitations are estabHshed. The most important equations for calculation of pressure drop for single-phase (Hquid or vapor) Newtonian fluids (viscosity independent of the rate of shear) are those for the deterrnination of the Reynolds number, and the head loss, (16—18). 

The flow resistance of pipe fittings (elbows, tees, etc) and valves is expressed in terms of either an equivalent length of straight pipe or velocity head loss (head loss = Kv /2g ). Most handbooks and manufacturers pubHcations dealing with fluid flow incorporate either tables of equivalent lengths for fittings and valves or K values for velocity head loss. Inasmuch as the velocity in the equipment is generally much lower than in the pipe, a pressure loss equal to at least one velocity head occurs when the fluid is accelerated to the pipe velocity. 

There are also many empirical formulas used for calculatiag the friction head loss in piping systems. These must be used carefuUy because many are based on the properties of specific fluids and are not appHcable over a broad range of fluids, temperatures, and pressures. For example, the Ha2en and Wdhams formula widely used for water flow ... 

Although it has been common practice to specify the pressure loss in ordinary valves in terms of either equivalent length of straight pipe of the same size or velocity head loss, it is becoming more common to specify flow rate and pressure drop characteristics in the same terms as has been the practice for valves designed specifically for control service, namely, in terms of the valve coefficient, C. The flow coefficient of a valve is defined as the volume of Hquid at a specified density that flows through the fully opened valve with a unit pressure drop, eg, = 1 when 3.79 L/min (1 gal /min) pass through the valve... 

Mechanically cleaned racks allow smaller clear openings because head loss does not become so high. Mechanical cleaning can be intermittent or continuous. Intermittent cleaning is cycled by float-operated switches controlled by a float in the influent channel. 

V = superficial velocity based upon the gross area of the screen K = velocity head loss... 

FIG. 8-82 Installed flow characteristic as a function of percent of total system head allocated to the control valve (assuming constant head pump, no elevation head loss, and an R equal 30 equal-percentage inherent characteristic). 

Poor circulation due to higher than expected head losses... 

The heights of head losses in Eq. (14-104) should be in consistent units, e.g., milhmeters or inches of liquid under operating conditions on the plate. 

For valve plates, values of K and Ko depend on whether the valves are fully open. They also depend on the shape and weight of the valves. Vendors of valve plates make K and Ko data (or their equivalent) readily available. An analysis of valve plate pressure drop has been reported by Bolles [Chem. Eng. Pmgr. 72(9), 43 (1976)], and typical dry head loss data, shown in Fig. 14-31, are taken from that worlc. 

Loss under Downcomer The head loss under the downcomer apron, as millimeters of hquid, may be estimated from... 

The rated discharge is at a static head of /7j and a motor h.p. P. In the process of controlling the discharge from Q to and Qy, the valve is throttled, which increases the head loss of the system (or system resistance) from A/ji to and H,yy respectively. The operating point on the curve now shifts from point A[ to /It... 

The definition of NPSHr may change in the future. A pump is in a definite state of cavitation with the 3% total head loss definition. Many pump users want a more explicit definition of NPSHr, and higher NPSHa safety margins to avoid inadequate NPSHa and cavitation altogether. 

The calculation of the overall stage efficiency must also include losses encountered in the diffuser. Thus, the overall actual adiabatic head attained will be the actual adiabatic head of the impeller minus the head losses encountered in the diffuser from wake caused by the impeller blade the loss of part of the kinetic head at the exit of the diffuser (A(/ed), and the loss of head from frictional forces (A(/osf) encountered in the vaned or vaneless diffuser space... 

The adiabatic head that is actually available at the rotor discharge is equal to the theoretical head minus the head losses from the shock in the rotor, the incidence loss, the blade loadings and profile losses, the clearance between the rotor and the shroud, and the secondary losses encountered in the flow passage... 

E = Head loss due to friction in feet of flowing fluid... 

These contribute to the flowing head loss in a pipe. However, there are many situations where by chance, or on purpose, u /2g head is converted to PV or vice versa. 

For good control where possible, make the control valve take 50%-60% of the system flowing head loss. 

NPSH is the pressure available at the pump suction nozzle after vapor pressure is subtracted. It is expressed in terms of liquid head. It thus reflects the amount of head loss that the pump can sustain internally before the vapor pressure is reached. The manufacturer will specify the NPSH that his pump requires for the operating range of flows when handling water. This same NPSH is normally used for other liquids. 

GPM = Column liquid loading, gal/min Hj,. = Downcomer backup, inches of liquid hi = Condensing side film coefficient, Btu/hrft °F H,(j = Head loss under downcomer, inches of liquid H v = Weir height, ins. 

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