BS 7294:1990 download.Method for DEtermination of flow-rate characteristics of pneumatic fluid power components.
1 Scope
BS 7294 specifies a method for testing pneumatic fluid power components which use compressible fluids, i.e. gases, to enable their flow-rate characteristics under steady-state conditions to be compared.
It specifies requirements for the test installation, the test procedure and the presentation of results.
Accuracy of measurement is divided into two classes (A and B) which are explained in Annex A.
General background information is given in Annex B and the basic theoretical equations are given in Annex C. Guidance as to the use of practical units for the presentation of results is given in Annex D.
BS 7294 generally applies to those fluid power components up to and including 20 mm nominal bore used with compressible fluids (gases), the internal flow passages of which remain constant during testing. Examples of such components are
a) directional control valves, flow control valves, quick exhaust valves, etc.;
b) moving part logic devices.
It may also apply to components larger than 20 mm nominal bore but this may require the provision of exceptionally large flow generating equipment.
Two test methods are described according to the type of component. There are also two sets of characteristic constants: C and b; and A and s, respectively (as defined in 3.2 to 3.5). These may be calculated from the results.
The first set of characteristics (C and b) applies to cases where comparison of similar components is required, or when calculations of pressure and flow involve a single component only.
The second set of characteristics (A and s) is necessary when the flow behaviour of several components which are connected in series is to be estimated. This set may also be used as an optional alternative to the first set for simple flow calculations and for comparison of components.
BS 7294 does not, apply to components which exchange energy with the fluid (gas). for example cylinders, accumulators. etc.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of BS 7294. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on BS 7294 are encouraged to investigate the possibility of applying the most recent editions of the standards listed below. Members of JEC and ISO maintain registers of currently valid International Standards. Iso 228-1:1982, Pipe threads where pressure-tight joints are not made on the threads — Part 1: Designation, dimensions and tolerances. Iso 261:1973, Iso general purpose metric screw threads — General plan.
Iso 1179:1981, Pipe connections, threaded to ISO 228-1, for plain end steel and other metal tubes in industrial applications.
ISO 5598:1985, Fluid power systems and components — Vocabulary.
3 Definitions
For the purposes of BS 7294, the definitions given in ISO 5598 and the following definitions apply. It should be borne in mind, however, that the following definitions may differ from those given in other specific International Standards.
choked flow
occurrence when upstream pressure, Pi’ is high in relation to the downstream pressure, P2’ such that the velocity in some part of the component becomes sonic. The mass flow of the gas is proportional to the upstream pressure, Pit and independent of the downstream pressure, p9
3.2
critical pressure ratio, b
pressure ratio (p21p1) below which flow becomes choked
3.3
sonic conductance, C
mass flow rate through the component. q11, divided by the product of the upstream pressure,p1, and the mass density at standard conditions , (see Table 2) when the flow is choked, i.e.
4.2 The numerals used as subscripts and the
asterisk (*) used as a superscript to the symbols
listed in Table 1 are as specified in Table 2.
4.3 The graphical symbols used in Figure 1 and
Figure 2 are in accordance with ISO 1219.
5 Test installation
5.1 Test circuit for components with inlet and outlet ports
A suitable test circuit as shown in Figure 1 shall be used.
5.2 Test circuit for components which exhaust directly to atmosphere
A suitable test circuit as shown in Figure 2 shall be used.
NOTE Figure 1 and Figure 2 illustrate basic circuits which do not incorporate all the safety devices necessary to protect against damage in the event of component failure. It is important that those responsible for carrying out the test give due consideration to safeguarding both personnel and equipment.
5.6 Special requirements
5.6.1 When the component under test has ports which are not threaded and other means of connecting to pipes or hoses are used, measuring tubes having internal diameters which correspond to the appropriate pipe or hose internal diameters shall be used.
5.6.2 If these diameters do not correspond, measuring tubes of the next largest internal diameter in the range shall be used.
5.6.3 When the component under test has ports which differ in size, measuring tubes which are suited to the relevant port shall be used.
6 Test procedures
6.1 Test conditions
6.1.1 Gas supply
6.1.1.1 The gas used shall be stated in the test report.
6.1.1.2 The gas shall be filtered and conditioned to comply with the recommendations of the test component manufacturer.
6.1.2 Checks
Periodically check that the pressure-tapping holes are not blocked by liquids or solid particles.
6.1.3 Test measurements
6.1.3.1 Each set of test readings shall be recorded after steady-state conditions have been reached.
6.1.3.2 The variations in upstream parameters shall he within the tolerances stated in Table 5.
7 Presentation of test results
7.1 All measurements and the results of calculations shall he tabulated by the testing agency and, where specified or when appropriate, shall also be presented graphically as described in 6.2.2.4.
7.2 The following performance characteristics related to flow-rate capacity and flow, which are calculated in accordance with 6.3, shall be stated; from these characteristics the performance of the component can be predicted and compared, either in the form a) and b), or the form a) or b):
a) sonic conductance, C, and critical pressure ratio, b;
NOTE Parameters Cand b will be valid only for the gas used in the test.
b) effective area, A. and coefficient of compressibility effect, s.
NOTE Parameters will also be valid only for the gas used in the test.
7.3 The class of measurement accuracy, i.e. A or B from Annex A, shall be stated and the calibration record shall be available.
8 Identification statement (Reference to BS 7294)
Use the following statement in test reports, catalogues and sales literature when electing to comply with BS 7294:
‘Test for the determination of flow-rate characteristics conforms to ISO 6358, Pneumatic fluid power — components using compressible fluids — Determ mat ion of flow-rate characteristics.”