Head versus pressure

There s a language barrier between the pump manufacturers and the pump users. They use different terminology. Pump users, the operators and mechanics, use pressure gauges that read in psi, pounds per square 

To understand pumps and analyze their problems, its neee,ssary to dominate the formula that changes feet of head (H) into psi. This is explained in Chapter 2, but here is a brief review  

If the liquid is water, the specific gravity is 1.00. VVe see that two factors separate psi from head in feet . First is the 2.31 conversion factor, and second, the specific gravity. 

The pump companies develop their curves using head in feet (H), because when they make a new pump, they don t know the ultimate service of the pump (they don t know the liquid that the pump will be pumping), but they do know how many feet of elevation the pump can raise that liquid. This is why it s necessary to specify pumps in feet of head and not in psi. Let s begin by exploring the H-Q curve of the pump, using feet of head. 

On the graph, if point A repre.sents 10ft of head at 0-gpm, and if point F repre.sents 10 gpm at 0 ft of head, then point C on the curve represents 8 ft of head at 6-gpm. Fdere we. see that the pump is always on its curve. The pump can operate at any point on this curve from point A to point F . At any. specific head, this pump will pump a. specific How, or gpm corresponding to the head. 

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Measurement versus Equipment Performance Pumps that are in reasonable condition typically operate within 5 percent of their pump curve. Consequently, pressures and flows that are inconsistent with the pump curve imply that the indicated flow and/or pressure are incorrecl . Figure 30-16 shows a single impeller curve plotted as head versus flow. The point shown is inconsistent with the pump operation. Therefore, that pair of flow and pressure measurements is not validated and should not be used in the subsequent steps. 

Figure 12-461, Part 3. Stage performance of a compressor is usually represented in a pressure coefficient, n, or M, and efficiency, t), versus Q/N (capacity vs. speed). A given impeller stage design will have a different characteristic depending on the relationship of its operating speed to the inlet sonic velocity of the gas. For higher ratios of speed to sonic velocity, N/A , the head or pressure coefficient curve will be steeper at flows higher than the design. (Used by permission Bui. 423, 1992. Dresser-Rand Company.)...

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. 

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. 

The design type refers to variations in equipment configuration (e.g., fixed head versus floating head in a heat exchanger). The adjustment for material of construction is used principally to account for the use of alloy steel instead of carbon steel. The pressure rating factor allows adjusting costs for pressures other than the refer-... 

Fig. 11. Pressure/flow-rate characteristics of extruder (capacity versus pressure drop across the head) in processing of polypropylene filled by 10 % (by mass) of chalk (a) and 20 % (by mass) of asbestos (b) at a temperature in the head equal to 210 °C and amplitude of reciprocatingrotary vibration of the core, degr.— 0 2 — 4 3 — 11.5 — 22.3...

Characteristic Curves. The pressure head versus flow relationship for a centrifugal pump, called the characteristic curve, and illustrated in Figure 13, depends very much upon the design of the impeller, its vanes and the volute casing. 

Shape of the operating characteristic of the machine e.g. pressure (head) versus liquid flow rate in a pump. 

Where the driven machine is a centrifugal type i.e. pump or compressor, the shaft power may be taken as that which occurs at the end of curve operating point. This rule-of-thumb point is defined as being 125% of the power required at the maximum operating efficiency point on the designed curve of pressure (head) versus fluid flow rate, at the rated shaft speed. 

It is extremely important that the system resistence or characteristics be fully known before selecting a control system. Figure 19-9 shows a plot of pressure versus capacity for a constant speed Compressor. Shown on this plot also are three different types of system characteristics. A compressor operating against a fixed head or pressure could have a system characteristic defined by... 

Centrifugal-pump performance follows its hydraulic curve (i.e., head versus flow rate). Therefore, any variation in the total backpressure of the system causes a change in the pump s flow or output. Because pumps are designed to operate at their Best Efficiency Point (BEP), they become more and more unstable as they are forced to operate at any other point because of changes in total system pressure, or head (TSH). This instability has a direct impact on centrifugal-pump performance, reliability, operating costs, and required maintenance. 

The purpose of the pump is to produce a certain flow against a certain pressure. This is done at a certain efficiency. The optimum point at which the efficiency is at a maximum is called the best efficiency point. For every size or design of pump, there is a best efficiency point at a given speed. The performance of the pump is plotted on a curve of head versus flow (Figures 8-3 and 8-4) By combining different sizes of pumps on a single chart, a pump tomb chart is produced (Figure 8-5). 

It is necessary to have pump and compressor curves in order to do the required calculations. In this example, we will use equations for the pump curves. These equations can be obtained by fitting a polynomial to the curves provided by pump manufacturers. As discussed in Chapter 18. pump curves are usually expressed as pressure head versus volumetric flowrate. This is so that they can be used for a liquid of any density. In this section, pressure head and volumetric flowrate have been converted to absolute pressure and mass flowrate using the density of the fluids involved. Pump P-201 operates at only one speed, and an equation for the pump curve is... 

In the regenerated catalyst standpipe, a 40 Ib/ft (640 kg/m ) catalyst density versus a 25.4 Ib/ft (407 kg/m ) density produces 3 psi (20,7 Kj,) more pressure head, again allowing an increase in circulation or a reduction in the regenerator pressure (gaining more combustion airi... 

Traditional amphiphiles contain a hydrophilic head group and the hydrophobic hydrocarbon chain(s). The molecules are spread at molecular areas greater (-2-10 times) than that to which they will be compressed. The record of surface pressure (II) versus molecular area (A) at constant temperature as the barrier is moved forward to compress the monolayer is known as an isotherm, which is analogous to P-V isotherms for bulk substances. H-A isotherm data provide information on the molecular packing, the monolayer stability as de-... 

Fig. 14.19 shows a typical set of pressure versus time curves obtained from tests on a rocket motor. When the I/D ratio defined in Fig. 14.19 is increased, the head-end chamber pressure is increased drastically immediately after the ignition stage. These grains are seven-pointed-star-shaped neutral-burning grains (diameter D = 114 mm), and are made of an AP-Al-CMDB propellant with the composition nc(0-25), ng(0-31), Ita(0-08), ap(0-27), and ai(0 09). The ratio of the initial burning surface area (Ayg) to the nozzle throat area (Aj), = AygjAp and the ratio of the... 

Detonation in Slurry Explosives. Cook, in his book, pp 316-21, described under the heading "Water-Compatible Explosives properties of slurry explosives developed by M.A. Cook.St H.E. Farnam. These expls were intended for use in large diameter underwater blasting at Iron Ore Company of Canada s Knob Lake operation. The success of these expls brought out the importance of pressure and density on the products of detonation. Table 12.21 of Cook s book gave computed properties of three dry versus water soaked slurry mixtures at AN/TNT ratios of zero, 1.0 8c 3.25. It was of interest to note that the computed (dry basis) available energy A of the TNT in slurry with 27% water was 17% greater... 

Figure A3.3 is a plot of average void fraction, a, versus the dimensionless superficial velocity, for the different flow regimes and values of C0. The correlations presented here may overestimate level swell for pure vapour pressure systems if there is a non-boiling region (in which static head suppresses boiling) at the bottom of the reactor. This is conservative for relief system sizing and is discussed further by DIERS151.

The analogy between three- and two-dimensional phase diagrams can be carried much further. Monomolecular amphiphilic films show ordered phases similar to three-dimensional systems [579], The phases of an amphiphilic monolayer can be detected most conveniently in pressure-area (7r-versus-OA) isotherms. These may look different for different substances. The behavior of simple amphiphilic molecules, like long-chain alcohols, amines, or acids, was extensively investigated (reviews Refs. [580,581]). In monolayers so-called mesophases can occur. In a mesophase the tail groups are ordered over relatively large areas, while the order in the hydrophilic head groups is only over a much smaller distances. 

Example 13.4. The result of a typical X-ray measurement is shown in Fig. 13.10 for a galactocerebroside [605], The plot on the left side shows the normalized reflected X-ray beam intensity versus the incident angle a for two different film pressures. The pressure-area isotherm is shown in the inset, together with the points of measurement a and b. On the right side are the extracted electron density profiles normal to the film surface taken at the same film pressures. At 0 A we find the monolayer surface (top of the alkyl chains), a depth of -40 A corresponds to pure water. In between is the film. The measurement is so sensitive that we even find two different electron densities within the monolayer. This is illustrated by the dashed boxes denoted by film 1 and film 2 (shown for curve b only) which represents the simplified electron density distribution in the so-called two-box model. A box is defined as a part in the film of a certain thickness where the electron density is constant. In the two-box model the film is divided into two layers. Film 1 represents the hydrocarbon tails, film 2 corresponds to the mean electron density of the head groups. 

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