1、_ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising there
2、from, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions. Copyright 2008 SAE International All rights reserved. No part of this publication m
3、ay be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA)
4、 Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org AIR5829 AEROSPACE INFORMATION REPORT Issued 2008-02 Air in Aircraft Hydraulic Systems RATIONALE This Aerospace Information Report is prepared to aid engineers and technicians in the understanding of the forms and sou
5、rces of air in hydraulic systems and discusses design and maintenance alternatives for reduction or elimination of the effects of air on hydraulic system performance. TABLE OF CONTENTS 1. SCOPE 3 2. REFERENCES 3 2.1 Applicable Documents 3 2.1.1 SAE Publications. 3 2.1.2 U.S. Government Publications
6、3 2.2 Definitions . 4 3. TECHNICAL DISCUSSION 4 3.1 Forms of Air in Closed Hydraulic Systems . 4 3.1.1 Dissolved Air . 4 3.1.2 Entrained Air . 4 3.1.3 Free Air . 4 3.2 Sources of Air in Hydraulic Systems. 5 3.2.1 Maintenance 5 3.2.2 Operation 5 3.2.3 Non-operation . 6 3.2.4 Leakage 6 4. EFFECTS OF A
7、IR IN HYDRAULIC SYSTEMS 6 4.1 Reduced stiffness of actuators 6 4.2 Reduced Pump Efficiency. 8 4.3 Heat Generation at Pumps . 8 4.4 Failure of Pumps to Produce Pressure. 8 4.5 Excessive or Erratic Volume Change in Reservoirs . 8 4.6 Cavitation 8 5. DETERMINATION OF AIR CONTENT IN HYDRAULIC SYSTEMS 9
8、5.1 Reservoir Volume Change 9 5.2 Commercial Air Measuring Devices 9 6. DESIGN FOR AIR IN HYDRAULIC SYSTEMS 9 6.1 Reservoir Pressurization. 9 6.2 Orientation of Fittings and Hose in Inlet Lines 9 6.3 Elevation of Pumps Relative to Reservoirs or Inlet Fluid Columns 12 6.4 Restrictor Design 12 6.5 Man
9、ual or Automatic Bleed Valves. 12 6.6 Self-sealing Couplings 13 6.7 Installation of Accumulators 13 6.8 Installation of Reservoirs. 13 SAE AIR5829 - 2 - 6.9 Reservoir Design 13 6.9.1 Separated Type. 13 6.9.2 Non-separated 13 6.10 Filter Bowl Removal Shut-off Valves. 13 6.11 Pump Inlet Pressure . 14
10、6.12 Maintenance Practice Related to Air 14 6.12.1 Prevent Loss of Fluid 14 6.12.2 Filling After Component or Tube Replacement. 14 6.12.3 Operation of Ground Cart in Open Loop Mode. 14 6.12.4 Use Hydraulic Fluid Purifiers. 14 7. NOTES 14 FIGURE 1 AIR SOLUBILITY OF HYDRAULIC FLUIDS 5 FIGURE 2A ADIABA
11、TIC SECANT BULK MODULUS FOR MIL-PRF-83282 WITH FREE AIR 7 FIGURE 2B ADIABATIC SECANT BULK MODULUS FOR MIL-PRF-83282 WITH FREE AIR (ISO). 8 FIGURE 3 PRESSURE MAINTAINING BOOTSTRAP PRESSURE CIRCUITS . 10 FIGURE 4 PUMP INLET INSTALLATIONS . 11 SAE AIR5829 - 3 - 1. SCOPE This SAE Aerospace Information R
12、eport (AIR) discusses the forms that air may take in aircraft hydraulic systems. Further, the effects of the various air forms on system operation are addressed. Recommended system design to prevent air effects and maintenance procedures to prevent and remove air are provided. Nitrogen leakage from
13、accumulators is also a source of gas in hydraulic systems and may compose a portion of the “air” in the hydraulic system. The term “air” in this report does not differentiate between a gas composed strictly of normal atmospheric air or one that includes a mixture of additional nitrogen as well. The
14、discussions of the report apply equally with any proportions of atmospheric air and nitrogen in the system. 2. REFERENCES 2.1 Applicable Documents The following publications form a part of this document to the extent specified herein. The latest issue of SAE publications shall apply. The applicable
15、issue of other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regulation
16、s unless a specific exemption has been obtained. 2.1.1 SAE Publications Available from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel: 877-606-7323 (inside USA and Canada) or 724-776-4970 (outside USA), www.sae.org. AS1241 Fire Resistant Phosphate Ester Hydraulic Fluid for
17、 Aircraft AIR1362 Aerospace Hydraulic Fluids Physical Properties AIR1922 System Integration Factors That Affect Hydraulic Pump Life ARP4752 Aerospace - Design and Installation of Commercial Transport Aircraft Hydraulic Systems AS5440 Hydraulic Systems, Aircraft, Design and Installation Requirements
18、for AIR5451 A Guide to Landing Gear System Integration 2.1.2 U.S. Government Publications Available from the Standardization Document Order Desk, 700 Robbins Avenue, Building 4D, Philadelphia, PA 19111-5094. Tel: 215-697-2179. http:/assist.daps.dla.mil/quicksearch/. MIL-PRF-5606 Military Specificati
19、on Hydraulic Fluid, Petroleum Base; Aircraft, Missile and Ordnance, NATO Code Number H-515 (Inactive for New Design) MIL-PRF-83282 Performance Specification Hydraulic Fluid, Fire Resistant Synthetic Hydrocarbon, Metric, NATO Code Number H-537 MIL-PRF-87257 Military Specification Hydraulic Fluid, Fir
20、e Resistant; Low Temperature, Synthetic Hydrocarbon Base, Aircraft and Missile, NATO Code Number H-538 SAE AIR5829 - 4 - 2.2 Definitions AIR: Normal earth atmosphere composed of 79% nitrogen and 21% oxygen with other gasses in minor (1%) amounts. HENRYS LAW: The weight of gas absorbed is directly pr
21、oportional at equilibrium to the gas pressure above the fluid. OPEN-LOOP: This term refers to the use of a hydraulic ground cart attached to the aircraft hydraulic system with the control valve on the cart set to operate the aircraft system using the ground cart reservoir as a fluid source. CLOSED-L
22、OOP: This term refers to the use of a hydraulic ground cart attached to the aircraft hydraulic system with ground cart valves set to use the aircraft reservoir as a fluid source. 3. TECHNICAL DISCUSSION 3.1 Forms of Air in Closed Hydraulic Systems 3.1.1 Dissolved Air Air is dissolved in hydraulic fl
23、uids proportional to the pressure. Henrys Law applies to the solution of air in hydraulic fluids in relation to pressure. Dissolved gas content as a function of temperature is less predictable, increasing at lower temperatures and decreasing at higher temperatures. At one atmosphere and 68 F (20 ),
24、a phosphate ester hydraulic fluid, AS1241, can have 9.0 percent of air dissolved by volume at 100 percent saturation. Hydrocarbon hydraulic fluids, MIL-PRF-5606, MIL-PRF-83282 and MIL-PRF-87257, can have 11.8 percent of air under the same conditions. As an example, a regional commercial jet hydrauli
25、c system may contain 24 gallons of phosphate ester fluid. If this fluid were saturated at one atmosphere, there would be approximately 2.2 gallons by volume of air dissolved in the fluid. At one atmosphere and above, the air in the fluid remains dissolved and does not affect system operation. If the
26、 local pressure at any point drops below one atmosphere, the air within that local area will proportionately come out of solution. Figure 1 shows the air solubility of several hydraulic fluids at low pressure. Air in solution is not considered to affect performance of hydraulic components. 3.1.2 Ent
27、rained Air Entrained air appears in fluid as froth, foam or bubbles. This is the first state of air when it comes out of solution. For example, in a sealed container containing hydraulic fluid at one atmosphere, reducing the pressure will cause air to come out of solution. This air will appear as mi
28、croscopic bubbles. If the container is maintained at the lower pressure condition for a sufficient time, the bubbles will join into larger bubbles and eventually rise to the top of the container. If the pressure is increased sufficiently, the air will begin to dissolve back into the fluid at any air
29、 to fluid interface. Entrained air in sufficient quantity will affect hydraulic system performance. 3.1.3 Free Air Free air is the accumulation of air bubbles at the local highest point in a component or tube. Free air will only be present if the container is left undisturbed, without fluid motion,
30、for a sufficient time to allow entrained air to coalesce into a visible volume of air. Free air in sufficient quantity will affect hydraulic system performance. Free air can exist in the form of foam, consisting of thousands of minute bubbles adhering together. SAE AIR5829 - 5 - Air Solubility of Hy
31、draulic Fluids 70 F (21 C)0.01.02.03.04.05.06.07.08.09.010.011.012.013.014.015.00 0.2 0.4 0.6 0.8 1 1.2Pressure atm (MPa = atm *0.101325) Vair_percentMIL-PRF-5606, MIL-PRF-83232, MIL-PRF-87257, AS1241FIGURE 1 - AIR SOLUBILITY OF HYDRAULIC FLUIDS 3.2 Sources of Air in Hydraulic Systems 3.2.1 Maintena
32、nce When the hydraulic system is disassembled at any point, the fluid in the component or tube drains by gravity unless means are taken to prevent fluid loss. The volume created by the loss of fluid is replaced by air. When the system is reassembled the air will remain in the closed system unless st
33、eps are taken to displace the air with hydraulic fluid, e.g. bleed the system. 3.2.2 Operation Throughout the hydraulic system, air is constantly going into solution and coming out of solution as the local pressure increases and decreases and the local temperature decreases and increases. In portion
34、s of the system at maximum system operating pressure, all the air may be dissolved in the fluid. As pressure decreases when flowing through a component, the dissolved air will come out of solution and occur as entrained air until the local pressure increases again. When the aircraft is shut down and
35、 the hydraulic system is depressurized, all of the air in the system comes out of solution until the local pressure stabilizes. As a rule air will come out of solution much more quickly than it will return into solution. In the presence of a vacuum, release of dissolved air is rapid. In a controlled
36、 test, about 75 percent of dissolved air was released within two minutes; the remaining 25 percent required one-half up to one hour. A significant source of entrained air in a system may be traced to restrictors that reduce the fluid pressure below the critical point. Results have been reported of t
37、ests showing release of dissolved air with fluid flow through improperly designed restrictors. SAE AIR5829 - 6 - 3.2.3 Non-operation When a hydraulic system is shut-down and non-operational, system pressure reduces. As fluid cools and contracts, a vacuum may develop at various locations within the s
38、ystem causing air to come out of solution. The local pressure may be greater than, equal to or less than atmospheric pressure when the system is non-operational. The pressure will change with temperature rise or fall or be subject to variation with buffeting of control surfaces by wind or inertial l
39、oads if the aircraft is being towed. At each local pressure state, the air which comes out of solution will collect at the local high point(s) within each component and tube. Since the amount of air is proportional to the fluid volume, the greatest single volume of free air within the non-operationa
40、l system will be in the system reservoir. Depending upon the attitude, length, and diameter of tubes, significant amounts of free air may collect at the high points in the hydraulic system. 3.2.4 Leakage Leakage of nitrogen across piston seals in accumulators can be a source of gas into the hydrauli
41、c system. Example: Given a 50 cu in (819 cc) accumulator at 70 F (21 C), precharged to 1500 psig (10.34 MPa), and subject to leakage into the hydraulic system, resulted in 40 psig (0.28 MPa) pressure loss. The amount of nitrogen leaked into the hydraulic system would be about 65 cu in (1065 cc). On
42、a servicing gage with 200 psi (1.38 MPa) increments, a loss of 40 psig (0.28 MPa) would not be easily observed. Another source of nitrogen or air leakage into hydraulic systems is across the piston separator in low pressure accumulators, for example, a nitrogen pressurized separator type hydraulic r
43、eservoir. Valves that vent to atmosphere, such as the reservoir overboard relief valve may leak air across the valve with a vacuum in the hydraulic system side. Other sources that have been hypothesized are reverse leakage across pump and motor shaft seals and flight control actuator rod seals when
44、the surface is buffeted and deflected by wind loads. There is not general agreement on the mechanics and probability of the latter three sources of air. 4. EFFECTS OF AIR IN HYDRAULIC SYSTEMS The following discussion relates only to free or entrained air within the hydraulic system. 4.1 Reduced stif
45、fness of actuators The stiffness of a cylinder with a free or entrained air/oil mixture is degraded compared to a homogenous column of fluid. In this case, the stiffness of the cylinder is determined by the lower bulk modulus of an air-oil mixture, rather than the relatively high bulk modulus of oil
46、 with no free or entrained air. Reduced stiffness affects the dynamic performance of the actuator and the flight control surfaces it may be powering. Also, the energy going into the fluid re-dissolves the air, so less of the energy is moving the fluid. Figures 2A and 2B show the adiabatic secant bul
47、k modulus for MIL-PRF-83282 fluid at 3000 psi (20.7 MPa) with various percentages of free air. The bulk modulus for no free air is taken from AIR1362. The graphs highlight the 100 000 psi (689.5 MPa) level which is often used as a minimum allowable bulk modulus for stiffness design. The bulk modulus
48、 is reduced to 100 000 psi (689.5 MPa) at only 0.5 percent free air at 276 F (135.56 C). The graphs assume saturated fluid with adiabatic compression of the free air. The equation used to compute the bulk modulus with free air is: SAE AIR5829 - 7 - ()()()o12k12112e1PPPP1PP+=where: e= bulk modulus of
49、 mixture, lb/in2(MPa) o= bulk modulus of oil at P2with no air, lb/in2(MPa) P2= compression pressure, lb/in2(MPa) P1= reference pressure for o, lb/in2(MPa) = decimal percent free air k = ratio of the specific heat at constant pressure to the specific heat at constant volume for air All pressures are absolute Bulk modulus of saturated oil is not dependent on the amount of dissolved air it contains Adiabatic Sec
