Head-flow curve

Certain pump designs use an internal bypass orifice port to alter head-flow curve. High liquid velocities often erode the orifice, causing the pump to go farther out on the pump curve. The system head curve increase corrects the flow back up the curve. 

If a rising head flow curve is specified (see 5.1.13), the negative tolerance specified here shall be allowed only if the ... 

FIGURE 31 Typical head flow curve for mixing impeller and draft tube with corresponding system curves. 

When considering centrifugal machines it is important to base the motor rating on the end of curve condition of the driven machine, because in practice the machine may need to run at this extreme condition for a reasonably long period of time. This condition is generally defined as 125% of the capacity of the machine at the maximum working efficiency point on the head-flow curve for the designed shaft speed. 

The COAST computer program is used to calculate the reactor coolant flow coast down transient for any combination of active and inactive pumps and forward or reverse flow in the hot or cold legs. The equations of conservation of momentum are written for each of the flow paths of the COAST model assuming unsteady one-dimensional flow of an incompressible fluid. The equation of conservation of mass is written for the appropriate nodal points. Pressure losses due to friction, and geometric losses are assumed proportional to the flow velocity squared. Pump dynamics are modelled using a head-flow curve for a pump at fiill speed and using four-quadrant curves, which are parametric diagrams of pump head and torque on coordinates of speed versus flow, for a pump at other than full speed. 

The sump pumps must have periodic tests which adequately indicate their actual discharge capacity in an emergency situation. Pump head/flow curves may be used in conjunction with reduced flow testing if full flow testing is not possible. The curves to be used must have been previously verified by testing at the full flow level or have been provided by the vendor for the specific model/type installed. Pump test procedures must identify test acceptance criteria (i.e. acceptable head/flow curves and conditions). 

Caution should be taken to prevent excessive acceleration. One solution is to provide for opening of the compressor discharge vent valve to increase the compressor flow and increase blower horsepower. This is effective with a centrifugal compressor, however, not with an axial compressor unless the compressor is provided with adjustable stator vanes that are reliable. This can be seen from the performance curve. The head versus flow curve for a given vane setting is extremely steep and opening the vent valve is ineffective. However, if the vanes operate fully open and the vent valve opens, the combined effect is satisfactory. The vent valve must not be oversized. 

The performance curve can also be shifted to match the process requirements by variable inlet guide vanes. Located at the compressor inlet, these vanes change the direction of the velocity entering the first-stage impeller. By changing the angle at which these vanes direct the flow at the impeller, the shape of the head capacity curve can be changed. 

Under cavitating conditions a pump will perform below its head-performance curve at any particular flow rate. Although the pump may operate under cavitation conditions, it will often be noisy because of collapsing vapor bubbles and severe pitting, and erosion of the impeller often results. This damage can become so severe as to completely destroy the impeller and create excessive clearances in the casing. To avoid these problems, the fol-iotving are a few situations to watch ... 

Check the system (or loop) instability by using the Ledinegg criterion with an average lumped channel pressure drop. If it does not satisfy the Ledinegg stability criterion, one or more of the three remedies can be taken orifice the inlet, increase the steepness of the pump head-versus-flow curve or increase the resistance of the downcomer of a natural-circulation loop. 

Such performance curves are normally determined by the manufacturer from operating data using water at 60°F. Note from Eq. (8-6) that the head is independent of fluid properties, although from Eq. (8-4) the power is proportional to the fluid density (as is the developed pressure). The horsepower curves in Fig. 8-2 indicate the motor horsepower required to pump water at 60° F and must be corrected for density when operating with other fluids and/or at other temperatures. Actually, it is better to use Eq. (8-4) to calculate the required motor horsepower from the values of the head, flow rate, and efficiency at the operating point. The curves on Fig. 8-2 labeled minimum NPSH refer to the cavitation characteristics of the pump, which will be discussed later. 

The head-flow rate curve of a centrifugal compressor often has a maximum as shown on Figure 3.21, similar to the pump curve of... 

Fig. 10. Pressure/flow-rate characteristics ( tentative flow curves ) of angular circular extrusion head (the design is given in Fig. 8) in molding of polypropylene with MFI = 0.5 g/10 min under conditions of core rotation at a speed of, min-1 1 — 0 2 — 10 3 — 20 4 - 30 5 — 40 6 — 50 7 - 60 8 - 70...

The centrifugal compressor is a machine that converts the momentum of gas into a pressure head. The compressor pressure ratio (PD/P,) varies inversely with mass flow (W). For a compressor running at constant speed (co), constant inlet temperature (Tj), and constant molecular weight, the discharge pressure may be plotted against mass flow (Curve I in... 

When two or more pumps operate in parallel, the combined head-capacity curve is obtained by adding up their individual capacities at each discharge head, as illustrated in Figure 2.122. The total capacity of the pump station is found at the intersection of the combined head-capacity curve with the system head curve. When constant-speed pumps are used in parallel, the added increments of pumping can be started and stopped automatically on the basis of flow. 

Worn pump head-capacity curve i- Capacity loss (steep system curvd) —Capacity toss (fiat system curve) Flow rote... 

For each t5rpe of flow considered, a head loss curve was plotted, representing the variations in total head loss as a function of mean flow rate [7, 8, 32]. These curves are a valuable tool in analyzing and classifying the phenomena likely to occur in this type of flow. 

The characteristic curve of extrudate flow including adherence to the walls, and hence representative of shghtly to moderately entangled polymer flow in sudden two-dimensional or axisymmetrical contractions [7, 32], is represented in Fig 2. It shows a slope discontinuity above a certain pressm-e level, which depends on the pol3uner-die pair considered. With low flow rates, the flow is stable. Indeed, for these regimes, allowing for entrance effects, the flow curve is in fact representative of the shear rheometry of the polymer imder consideration, at low shear rates [34]. The slope discontinuity of the head loss curve indicates a modification in the structure of flow. It will be seen that this corresponds to the triggering of a hydrodynamic instability upstream of the contraction. 

CHARACTERISTIC CURVES HEAD-CAPACITY RELATION. The plots of actual head, total power consumption, and efficiency vs. volumetric flow rate are called the characteristic curves of a pump. Such curves are illustrated schematically in Fig. 8.12. In Fig. 8,12a, the theoretical head-flow rate (often called head-capacity) relation is a straight line, in accordance with Eq. f8.21) the actual developed head is considerably less and drops precipitously to zero as the rate increases to a certain value in any given pump. This is known as the zero-head flow rate it is the maximum flow the pump can deliver under any conditions. The rated or optimum operating flow rate is, of course, less than this. 

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