Available gage pressure at pump inlet with flow

Absolute available dynamic and static head at pump inlet with flow

Net positive suction head (NPSH) or maximum head available for surpressing cavitation in the pump (Hs)a flange

Pressure reference line


FIGURE 10-7. Definition of pump suction head.

selecting centrifugal pumps for rocket application has been to select the highest shaft speed that gives a pump with a low value of (Hs)r, does not require excessive tank pressurization or other design complications, and thereby permits relatively lightweight pump design. This places a premium on pumps with good suction characteristics.

There have been some low-thrust, low-flow, experimental engines that have used positive displacement pumps, such as diaphragm pumps, piston pumps, or rotary displacement pumps (gear and vane pumps). For low values of Ns these pumps have much better efficiencies, but their discharge pressures fluctuate with each stroke and they are noisy.

One method to provide a lightweight turbopump with minimal tank pressure is to use an inducer, which is a special pump impeller usually on the same shaft and rotating at the same speed as the main impeller. It has a low head rise and therefore a relatively high specific speed. Inducer impellers are immediately upstream of the main impeller. They are basically axial flow pumps with a spiral impeller, and many will operate under slightly cavitating conditions. The inducer stage's head rise (typically, 2 to 10% of the total pump head) has to be just large enough to suppress cavitation in the main pump impeller; this allows a smaller, lighter, higher-speed main pump. Figures 10-3 and 10-8 show an inducer and Ref. 10-8 describes the testing of one of them.

FIGURE 10-8. Fuel pump inducer impeller of the Space Shuttle main engine low-pressure fuel turbopump. It has a diameter about 10 in., a nominal hydrogen flow of 148.6 lbm/sec, a suction pressure of 30 psi, a discharge pressure of 280 psi at 15,765 rpm, an efficiency of 77%, and a suction specific speed of 39,000 when tested with water. (Courtesy of The Boeing Company, Rocketdyne Propulsion and Power.)

FIGURE 10-8. Fuel pump inducer impeller of the Space Shuttle main engine low-pressure fuel turbopump. It has a diameter about 10 in., a nominal hydrogen flow of 148.6 lbm/sec, a suction pressure of 30 psi, a discharge pressure of 280 psi at 15,765 rpm, an efficiency of 77%, and a suction specific speed of 39,000 when tested with water. (Courtesy of The Boeing Company, Rocketdyne Propulsion and Power.)

In some rockets the inert mass of the turbopump and tank system can be further reduced by putting the inducer impeller into a separate low-power, low-speed booster turbopump, driven by its own separate turbine. In the Space Shuttle main engine there are two such low-pressure-rise turbopumps, as shown in the flow diagram of Fig. 6-4 and the engine view of Fig. 6-1. This allows the inducer impeller to be operated at an optimum (lower) shaft speed.

Influence of Propellants. For the same power and mass flow, the pump head is inversely proportional to the propellant density. Since pumps are basically constant-volume flow machines, the propellant with the highest density requires less head, less power and thus allows a smaller pump assembly.

Because many of the propellants are dangerous to handle, special provision has to be made to prevent any leakage through the shaft seals. With spontaneously ignitable propellants the leakages can lead to fires in the pump compartment and may cause explosions. Multiple seals are often used with a drainage provision that safely removes or disposes of any propellants that flow past the first seal. Inert-gas purges of seals have also been used to remove hazardous propellant vapors. The sealing of corrosive propellants puts very severe requirements on the sealing materials and design. With cryogenic propellants the pump bearings are usually lubricated by the propellant, since lubricating oil would freeze at the low pump hardware temperature.

Centrifugal pumps should operate at the highest possible pump efficiency. This efficiency increases with the volume flow rate and reaches a maximum value of about 90% for very large flows (above 0.05 m3/sec) and specific speeds above about 2500 (see Refs. 6-1 and 10-9). Most propellant pump efficiencies are between 30 and 70%. The pump efficiency is reduced by surface roughness of casing and impellers, the power consumed by seals, bearings, and stuffing boxes, and by excessive wear ring leakage and poor hydraulic design. The pump efficiency rjP is defined as the fluid power divided by the pump shaft power PP\

A correction factor of 550 ft-lbf/hp has to be added if Pp is given in horsepower, H in feet, and Q in ft3/sec. When using propellants, the pump power has to be multiplied by the density ratio if the required power for water tests is to be determined.

Example 10-1. Determine the shaft speed and the overall impeller dimensions for a liquid oxygen pump which delivers 500 lb/sec of propellant at a discharge pressure of 1000 psia and a suction pressure of 14.7 psia. The oxygen tank is pressurized to 35 psia. Neglect the friction in the suction pipe and the suction head changes due to acceleration and propellant consumption. The initial tank level is 15 ft above the pump suction inlet.

SOLUTION. The density of liquid oxygen is 71.2 lbm/ft3 at its boiling point. The volume flow will be 500/71.2 = 7.022 ft3/sec. The vapor pressure of the oxygen is 1 atm = 14.7 psi= 29.8 ft. The suction head is 35 x 144/71.2 = 70.8 ft. From Eq. 10-8 the available suction head is 70.8 + 14.7 = 85.5 ft. The available suction head above vapor pressure is (HS)A = 70.8 + 14.7 - 0 - 29.8 = 55.7 ft. The discharge head is 1000 x 144/71.2 = 2022 ft. The head delivered by the pump is then 2022 - 85.5 = 1937 ft.

The required suction head will be taken as 80% of the available suction head in order to provide a margin of safety for cavitation (HS)R = 0.80 x 85.5 = 68.4 ft. Assume a suction specific speed of 15,000, a reasonable value if no test data are available. From Eq. 10-7 solve for the shaft speed N:

S = 2\.2N/QI{Hsf^ = 21.27V v/7.022/68.4°'75 = 15,000 Solve for N = 6350 rpm or 664.7 rad/sec.

The specific speed, from Eq. 10-3, is

Ns = 21.2AV/2/tf3/4 = 21.2 x 6350V7.022/19370 75 = 1222

According to Table 10-2, the impeller shape for this value of Ns will be a Francis type. The impeller discharge diameter D2 can be evaluated from the tip speed by Eq. 10-4:

u = fy/2g0AH = 1.0v2 x 32.2 x 1937 = 353 ft/ sec D2 = 353 x 2/664.7 = 1.062 ft = 12.75 in.

The impeller inlet diameter Dt can be found from Eq. 10-5 by assuming a typical inlet velocity of 15 ft/sec and a shaft cross section 5.10 in.2 (2.548 in. diameter).

A = \nD] + 5.10 = 67.41 + 5.10 = 72.51 in.2 D\ = 9.61 in. (internal flow passage diameter)

This is enough data to draw a preliminary sketch of the impeller. Turbines

The turbine must provide adequate shaft power for driving the propellant pumps (and sometimes also auxiliaries) at the desired speed and torque. The turbine derives its energy from the expansion of a gaseous working fluid through fixed nozzles and rotating blades. The blades are mounted on disks to the shaft. The gas is expanded to a high, nearly tangential, velocity and through inclined nozzles and then flows through specially shaped blades, where the gas energy is converted into tangential forces on each blade. These forces cause the turbine wheel to rotate (see Refs.10-1 and 10-11).

Classification and Description. The majority of turbines have blades at the periphery of a turbine disk and the gas flow is axial, similarly in concept to the axial flow pattern shown for pumps in Table 10-2 and the single-stage turbine of Fig. 10-1. However, there are a few turbines with radial flow (particularly at high shaft speeds), such as the one shown in Fig. 10-2. Ideally there are two types of axial flow turbines of interest to rocket pump drives: impulse turbines and reaction turbines, as sketched in Fig. 10-9. In an impulse turbine the enthalpy of the working fluid is converted into kinetic energy within the first set of stationary turbine nozzles and not in the rotating blade elements. Highvelocity gases are delivered (in essentially a tangential direction) to the rotating blades, and blade rotation takes place as a result of the impulse imparted by the momentum of the fluid stream of high kinetic energy to the rotating blades which are mounted on the turbine disk. The velocity-staged impulse turbine has a stationary set of blades which changes the flow direction after the gas leaves the first set of rototating blades and directs the gas to enter a second set of rotating blades in which the working fluid gives up further energy to the turbine wheel. In a pressure-staged impulse turbine, the expansion of the gas takes place in all the stationary rows of blades. In a reaction turbine the expansion of the gas is roughly evenly split between the rotating and stationary blade elements. The high pressure drop available for the expansion of the turbine working fluid in a gas generator cycles favors simple, lightweight one- or two-stage impulse turbines for high thrust engines. Many rocket turbines are neither pure impulse nor reaction turbines, but often are fairly close to an impulse turbine with a small reaction in the rotating vanes.






Single-stage, single-row impulse turbine

Single-stage, single-row impulse turbine

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