PROPULSION CHEMICAL
How to Measure Pipe Size for Unthreaded Pipe and Pipe Fittings
Feed system for amateur rocket engine testing consists of
- Tank to store liquid fuel.
- Regulated supply of high pressure nitrogen gas to force the fuel from the tank into the engine.
- Regulated supply of high pressure gaseous oxygen.
- Control device for regulating propellant flow rates.
What is a fuel tank?
- Closed vessel
- Contains the liquid fuel at moderate pressure (300-500psi)
Considerations when choosing a tank
Important
- Nearly all tanks are thin wall pressure vessels made for aircraft service
- Additional outlets or welding to the tank wall could seriously weaken the tank
- In ALL cases, tank must be hydrostatically tested to be at least 1.5 times the desired operating pressure before use in the rocket engine feed system
The tank should have enough ports, or the tank plumbing should be so arranged so that the following are available:
- Safety Relief Valve (Spring loaded or burst disc)
- Gas Inlet Port
- Load and Vent Port
- Fuel outlet and drain
- P is the pressure in the tank (1.5 times the desired operating pressure)
- D is the outside diameter of the tank
- T_w is the wall thickeness
- S is the allowable stress
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By the size of the rocket engine -
By the desired operating time
What is the purpose of propellant control valves
- Allow the operator to start and then manually remote-control the flow of each propellant into the engine What are the requirements for propellant control valves
- Stainless steel needle valves with Teflon packing or seals
- Should be mounted near the tanks and engine of the test stand and operated remotely by means of valve stem extensions (Test Stand notes coming soon)
- Valves should be sized to afford a range of flow control with minimum pressure drop across the valve
This includes
- Fuel tank vent and fill valve
- Drain valve
- Nitrogen Purge Valve
- Inexpensive, high quality ball valves are highly recommended for these functions as they offer positive shut-off , easy operation with handle indication of on or off, and full lin operating
- Brass or stainless steel valve bodies with Teflon seals are acceptable, and the valves may be line or panel mounted
What is the purpose of check valves?
- Permit fluid flow in one direction
Important
What is recommended for check valves
- ¼-inches line size is recommended for all functions shown below (with exception of gaseous oxygen line check valve which should feature metal-to-metal seats and be at least 3/8 inch line size
- Check valves
MUSTbe thoroughly cleaned prior to use and tested to ensure that the valve is working properly
What is the purpose of the relief valves?
- Prevents tank failure in the event of over-pressurization.
- This may occur if the gaseous nitrogen regulator failed to function or shut-off properly
What are the recommendation for a relief valves?
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Adjustable spring-loaded relief valve is recommended because:
- (+) It may be set to different pressures as feed system uses change.
- (+) It does not have to be replaced.
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Alternative device: Burst Disc
- (+) Ruptures at a preset pressure and relieves the overpressure in the tank.
- (-) Requires replacement after actuation and is not pressure adjustable.
- (-) Different disc must be used for each pressure range desired.
What is the purpose of a fuel filter?
- Filter out particles down to ten microns in size
- Fuel injection holes on small liquid-fuel rocket engines are easily plugged with contaminants from the fuel tank and control system
What is the recommendations for pressure gauges
- Bronze Bourdon-tube pressure gauges
- Offer accuracy, ruggedness, low cost
- When cleaned, they are fully compatible with gaseous oxygen or hydrocarbon fuel (like ethanol)
Important
- Small high pressure gauges (2.5 or 3 inch diameter) should be used by the amateur builder for measuring pressure in the high pressure gas cylinders or manifolds
Gauges for Fuel, Oxygen, Water and Combustion Chamber Pressure
- Should be AT LEAST 3.5 inch diameter for easy reading from a distance
- 3.5 Acaloy gauges of Helicoid are recommended because of their reliability and low cost
- These gauges are easily panel mounted and make a neat test stand installation
What does it mean by plumbing?
- Refers to the flow tubes and fittings used to collect the component discussed previously.
What are some plumbing recommendations?
Note
- ¼ inch diameter stainless steel tubing for the Fuel and Nitrogen Systems
- 3/8 inches diameter stainless tubing for oxygen line
- Flare fitting with metal-to-metal seats are recommended for joining tubing to other components
- ¼ and 3/8 inch diameter copper tubing cab be used for fuel, oxygen and nitrogen supply system, but it is not as desirable as stainless steel is more easily flared
The amateur builder should use only good flaring tools and should form or bend tubing only with a tube bender
- Use Teflon tape
- Use Teflon tap where fittings screw into fuel tank, valve, or other components having pipe threads!
- No other pipe thread compound should be used, especially on gaseous oxygen components
🛠️ Material option for Injector
🛠️ High Temperature Characteristics of Stainless Steel
🛠️Design Factor versus Safety Factor
Note
Uni-Bell recommends that engineers maintain the practice of using 2.0 as the minimum safety factor for design
🛠️Swagelok Gaugeable Tube Fittings and Adapter Fittings
🛠️How to Identify and Measure Pipe Fittings
Note
See How to Identify and Measure Pipe Fittings for more
🛠️How to Measure Pipe Size for Unthreaded Pipe and Pipe Fittings
Note
Download and Print this Sheet
🛠️Selection Criteria for Pipe Fittings
Pipe fittings are to be chosen considering certain factors. They are as follows:
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Connection types: When purchasing pipe fittings, you should be aware of the fact that a fitting can have two different connector types. One end of the fitting might be female threaded while the other female threaded. One end might be male slip while the other end is threaded, in the case of plastic fittings. They can also have matching ends which can accommodate any requirement.
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Materials of construction: As a rule, the pipe fitting should be of the same material as the material used in the making of the pipe in which it is to be fitted. However, in some cases, materials conforming to certain codes or standards can also be used in pipes of another material.
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Check for flow: To keep the flow consistent, the ends of pipe fittings should be slightly larger than the rest of the pipe so that they can accommodate connections without narrowing the inner diameter (ID) of the pipe.
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Type of fitting: Besides pipe materials, pipe fittings are identified by the type of fitting — threaded or slip, male or female.
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Size: When measuring the size of pipe fittings, it is to be noted that the male threaded fittings are measured to the outside edge or OD, while female fittings are measured to the inside edge of the inlet or ID.
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Thickness: Just as pipes are available in a number of different thicknesses or "schedules", so also the pipe fittings.
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Design: Each pipe or tube is designed to carry certain specific types of fluids, liquids, gases, chemicals under varying conditions. Accordingly, the pipe fittings are also available in variety of designs.
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Standards and codes: There are certain standards and codes set by various organizations by which the different pipe fittings are graded. For example, ASTM, ASME, BSP etc. are certain standards assigned to pipe fittings and those standards dictate their use.
Tapered Thread
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Tapered threads have a constant thread pitch, but the thread diameter varies along the axis of the thread body.
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However, due to the helical nature of the threads, a clearance remains between the thread mating points where fluid can escape.
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A thread sealant is often included to minimize the leak, often either a Teflon Tape or a thread-locking additive.
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NPT and BSP are the two most common tapered threads, each of which uses a different thread pitch.
Straight Thread: Gasket
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Straight threads, or parallel threads, have no taper in their threads.
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Straight threads typically follow either the SAE or BSPP thread conventions, but there are also metric and ANSI/ASME options that frequently arise.
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The first sealing mechanism with straight threads is via gaskets. In this method, an o-ring or a gasket insert is included to create the liquid seal.
Straight Thread: Flared
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Another mechanism of sealing with straight threads is a flared fitting body.
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The flared sections typically have a good surface finish to allow for intimate mating and therefore minimize leak probability.
Straight Thread: Compression
- Compression fittings are similar in concept to the flared fitting, except instead of the flared section being attached to the fitting body, it manifests itself using a set of ferrules and a compression nut which mate with the system tubing.
