Valves And Pipe Lines

Valves control the flows of liquids and gases and pipes conduct these fluids to the intended components. There are no rocket engines without them. There are many different types of valves. All have to be reliable, lightweight, leakproof, and must withstand intensive vibrations and very loud noises. Table 6-6 gives several key classification categories for rocket engine valves. Any one engine will use only some of the valves listed here.

The art of designing and making valves is based, to a large extent, on experience. A single chapter cannot do justice to it by describing valve design and operation. References 6-1 and 6-2 decribe the design of specific valves, lines, and joints. Often the design details, such as clearance, seat materials, or opening time delay present development difficulties. With many of these valves, any leakage or valve failure can cause a failure of the rocket unit itself. All valves are tested for two qualities prior to installation; they are tested for leaks—through the seat and also through the glands—and for functional soundness or performance.

The propellant valves in high thrust units handle relatively large flows at high service pressures. Therefore, the forces necessary to actuate the valves are large. Hydraulic or pneumatic pressure, controlled by pilot valves, operates the larger valves; these pilot valves are in turn actuated by a solenoid or a mechanical linkage. Essentially this is a means of power boost.

6.9. VALVES AND PIPE LINES 233 TABLE 6-6. Classification of Valves Used in Liquid Propellant Rocket Engines

1. Fluid, fuel; oxidizer; cold pressurized gas; hot turbine gas.

2. Application or Use: main propellant control; thrust chamber valve (dual or single); bleed; drain; fill; by-pass; preliminary stage flow; pilot valve; safety valve; overboard dump; regulator; gas generator control; sequence control; isolation of propellant or high-pressure gas prior to start.

3. Mode of Actuation: automatically operated (by solenoid, pilot valve, trip mechanism, pyrotechnic, etc.); manually operated; pressure-operated by air, gas, propellant, or hydraulic fluid (e.g., check valve, tank vent valve, pressure regulator, relief valve), with or without position feedback, rotary or linear actuator.

4. The flow magnitude determines the size of the valve.

5. Duty cycle: single or multiple pulse operation; reusable for other flights; long or short life.

6. Valve Type: normally open; normally closed; normally partly open; two-way; three-way, with/without valve position feedback; ball valve, gate valve, butterfly type, spring loaded.

7. Temperature and pressure allow classification by high, low, or cryogenic temperature fluids, or high or low pressure or vacuum capability.

8. Accessible or not accessible to inspection, servicing, or replacement of valve or its seal.

Two valves commonly used in pressurized feed systems are isolation valves (when shut, they isolate or shut off a portion of the propulsion system) and latch valves', they require power for brief periods during movements, such as to open or shut, but need no power when latched or fastened into position.

A very simple and very light valve is a burst diaphragm. It is essentially a circular disk of material which blocks a pipeline and is designed so that it will fail and burst at a predetermined pressure differential. Burst diaphragms are positive seals and prevent leakage, but they can be used only once. The German Wasserfall antiaircraft missile used four burst disks; two were in high pressure air lines and two were in the propellant lines.

Figure 6-14 shows a main liquid oxygen valve. It is normally closed, rotary actuated, cryogenic, high pressure, high flow, reusable ball valve, allowing continuous throtting, a controlled rate of opening through a crank and hydraulic piston (not shown), with a position feedback and anti-icing controls.

Pressure regulators are special valves which are used frequently to regulate gas pressures. Usually the discharge pressure is regulated to a predetermined standard pressure value by continuously throttling the flow, using a piston, flexible diaphragm, or electromagnet as the actuating mechanism. Regulators can be seen in Figs. 1-3 and 6-13.

The various fluids in a rocket engine are conveyed by pipes or lines, usually made of metal and joined by fittings or welds. Their design must provide for

Closed

Seal liftoff

Hydraulic actuator housing

Thermal insulator

Shaft seal assembly bearing

Intergrai ball/shaft/cams

Bearing

Cam follower lnlej pin and bearing

FIGURE 6-14. The SSME main oxidizer valve is a low-pressure drop ball valve representative of high-pessure large valves used in rocket engines. The ball and its integral shaft rotate in two bearings. The seal is a machined plastic ring spring-loaded by a bellows against the inlet side of the ball. Two cams on the shaft lift the seal a short distance off the ball within the first few degrees of ball rotation. The ball is rotated by a precision hydraulic actuator (not shown) through an insulating coupling. (Courtesy of The Boeing Company, Rocketdyne Propulsion and Power.)

Section A-A

Hydraulic actuator housing

Thermal insulator

Bearing

Cam follower lnlej pin and bearing

Shaft seal assembly bearing

Intergrai ball/shaft/cams

Closed

Seal liftoff

Open

FIGURE 6-14. The SSME main oxidizer valve is a low-pressure drop ball valve representative of high-pessure large valves used in rocket engines. The ball and its integral shaft rotate in two bearings. The seal is a machined plastic ring spring-loaded by a bellows against the inlet side of the ball. Two cams on the shaft lift the seal a short distance off the ball within the first few degrees of ball rotation. The ball is rotated by a precision hydraulic actuator (not shown) through an insulating coupling. (Courtesy of The Boeing Company, Rocketdyne Propulsion and Power.)

thermal expansion and provide support to minimize vibration effects. For gimballed thrust chambers it is necessary to provide flexibility in the piping to allow the thrust axis to be rotated through a small angle, typically ±3 to 10°. This flexibility is provided by flexible pipe joints and/or by allowing pipes to deflect when using two or more right-angle turns in the lines. The high-pressure propellant feed lines of the SSME have both flexible joints and right-angle bends, as shown in Figs 6-1 and 6-15. This joint has flexible bellows as a seal and a universal joint-type mechanical linkage with two sets of bearings for carrying the separating loads imposed by the high pressure.

Sudden closing of valves can cause water hammer in the pipelines, leading to unexpected pressure rises which can be destructive to propellant system components. An analysis of this water hammer phenomenon will allow determination of the approximate maximum pressure (Refs. 6-15 and 6-16). The friction of the pipe and the branching of pipelines reduce this maximum pressure.

Flexible Joints For Chamber
FIGURE 6-15. Flexible high-pressure joint with external gimbal rings for a high-pressure hot turbine exhaust gas. (Courtesy of The Boeing Company, Rocketdyne Propulsion and Power.)

Water hammer can also occur when admitting the initial flow of high-pressure propellant into evacuated pipes. The pipes are under vacuum to remove air and prevent the forming of gas bubbles in the propellant flow, which can cause combustion problems.

Many liquid rocket engines have filters in their lines. This is necessary to prevent dirt, particles, or debris, such as small pieces from burst diaphragms, from entering precision valves or regulators (where debris can cause a malfunction) or from plugging small injection holes, which could cause hot streaks in the combustion gases, in turn causing a thrust chamber failure.

Occasionally a convergent-divergent venturi section, with a sonic velocity at its throat, is placed into one or both of the liquid propellant lines. The merits are that it maintains constant flow and prevents pressure disturbances from traveling upstream. This can include the propagating of chamber pressure oscillations or coupling with thrust chamber combustion instabilities. The venturi section can also help in minimizing some water hammer effects in a system with multiple banks of thrust chambers.

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