a. A proof pressure
test is normally applied to hose for a
specified period of time. On new hose, the
proof pressure is usually 50% of the minimum
specified burst except for woven jacket fire
hose where the proof pressure is twice the
service test pressure marked on the hose (67%
of specified minimum burst). Hydrostatic tests
performed on fire hose in service should be
no higher than the service test pressure referred
to above. The regulation of these pressures
is extremely important so that no deteriorating
stresses will be applied, thus weakening a
normal hose.
b. A hold test,
when required, is a means of determining whether
weakness will develop under a given pressure
for a specified period of time.Percent length
change elongation or contraction) is the difference
between the length at 10 psi {0.069 MPa)(except
wire braided or wire spiraled) and that at
the proof pressure times 100 divided by the
length at 10 psi (0.069 MPa). Elongation occurs
if the length of the hose under the proof
pressure is greater than at a pressure of
10 psi (0.069 MPa). Contraction occurs if
the length at the proof pressure is less than
at 10 psi (0.069 MPa).
In testing wire braided or spiraled hose,
the proof pressure is applied and the length
recorded. The pressure is then released and,
at the end of 30 seconds, the length is measured;
the measure- ment obtained is termed the "original
length."
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c. Percent change in outside
diameter or circum ference is the difference
between the outside diameter or circumference
at 10 psi (0.069 MPa) and that obtained under
the proof pressure times 100 divided by the
outside diameter or circum ference at 10 psi
(0.069 MPa). Expansion occurs if the measurement
at the proof pressure is greater than at 10
psi (0.069 MPa). Contraction occurs if the
measurement at the proof pressure is less
than at 10 psi (0.069 MPa).
d. Warp is the deviation
from the straight line drawn from fitting
to fitting; the maximum deviation from this
line is warp. First, a measurement is taken
at 10 psi (0.069 MPa) and then again at the
proof pressure. The difference between the
two, in inches, is the warp. Normally this
is a feature measured on woven jacket fire
hose only.
e. Rise is a measure
of the height a hose rises from the surface
of the test table while under pressure. The
difference between the rise at 10 psi (0.069
MPa) and at the proof pressure is reported
to the nearest 0.25 inch (6.4 mm). Normally,
this is a feature measured on woven jacket
fire hose only.
f. Twist is a rotation
of the free end of the hose while under pressure.
A first reading is taken at 10 psi (0.069
MPa) and a second reading at proof pressure.
The difference, in degrees, between the 10
psi (0.069 MPa) base and that at the proof
pressure is the twist. Twist is reported as
right twist (to tighten couplings) or left
twist. Standing at the pressure inlet and
looking toward the free end of a hose, a clockwise
turning is right twist and counterclockwise
is left twist.
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g. Kink test is
a measure of the ability of woven jacket hose
to withstand a momentary pressure while the
hose is bent back sharply on itself at a point
approximately 18 inches (457mm) from one end.
Test is made at pressures ranging from 62%
of the proof pressure on sizes 3 inches (76
mm) and 3.5 inches (89 mm) to 87% on sizes
under 3 inches (76 mm). This is a test applied
to woven jacket fire hose only.
h. Volumetric
expansion test is applicable only to specific
types of hose, such as hydraulic or power
steering hose, and is a measure of its volumetric
expansion under ranges of internal pressure.
Design Considerations
In designing hose, it is customary to develop
a design ratio, which is a ratio between the
minimum burst and the maximum working pressure.
Burst test data is compiled and the minimum
value is established by accepted statistical
techniques. This is done as a check on theoretical
calculations, based on the strength of reinforcing
materials and on the characteristics of the
method of fabrication.
Minimum burst values are used as one factor
in the establishment of a reasonable and safe
maximum work pressure.
MAXIMUM WORKING PRESSURE IS ONE OF THE ESSENTIAL
OPERATING CHARACTERISTICS THAT A HOSE USER
MUST KNOW AND RESPECT TO ASSURE SATISFACTORY
SERVICE AND OPTIMUM LIFE.
It should be noted that design ratios are
dependent on more than the minimum burst.
The hose technologist must anticipate natural
decay in strength of reinforcing materials,
and the accelerated decay induced by the anticipated
environments in which the hose will be used
and the dynamic situations that a hose might
likely encounter in service.
Including all considerations, the following
recommended design ratios are given for newly
manufactured hose:
1. Water Hose up to 150 psi WP: 3:1
2. Hose for all other liquids, solid materials
suspended in liquids or air, and water hose
over 150 psi WP: 4:1
3. Hose for compressed air and other gases:
4:1
4. Hose for liquid media that immediately
changes into gas under standard atmospheric
conditions: 5:1
5. Steam Hose: 10:1
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METHODS FOR MEASURING
ELECTRICAL RESISTANCE OF HOSE
Method 1-Plug Method
Sample Preparation
1. Cut a 24 inch (610mm) sample from the length
of hose to be tested unless the size and length
of the hose is such that it is necessary to
test the entire length.
2. Buff both the tube and cover for 1.5 inch
(38 mm) on each end with 120 grit emery paper.
3. Insert either polished steel or copper
plugs having diameters equal to the I.D. of
the hose into each end for a distance of one
inch (25mm).
4. Clamp the plugs in the hose with a polished
0.500 inch (12.7mm) reusable band clamp. Place
the bands 0.250 inch (6.4mm) from the end
of the hose and tighten firmly.
5. After the clamps are applied, allow the
hose to rest for at least ten minutes before
the test is made.
Test Procedure
To measure the resistivity of the hose, lay
the entire assembly out straight on a nonconducting
surface. Place the ohmmeter electrodes on
the plugs in the hose to record the tube resistivity.
If the cover resistivity is desired, place
the electrodes on the clamps. This method
tends to eliminate the greatest number of
variables, yet it is simple to set up and
make the test.
The resistance between the plugs or clamps
shall be measured with a megohm meter (having
a range of 0.08 to 10.6 megohms, utilizing
a measuring voltage of 500 volts across the
unknown resistance) which has previously been
standardized against a known resistance.
The resistance of any hose shall be reported
as ohms per foot of hose, as determined by
dividing the total resistance by number of
feet of hose between clamps.
Editors Note:
The above method is an example of a test method
not currently and specifically covered by
an ASTM test method. It is, however, under
development by ISO TC45, Subcommittee 1, Working
Group 4, under the title, "Rubber and
Plastic Hoses -Determination of Electrical
Properties."
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Method 2- Nail or "Pot
Room" Method
Conduct test on non-conducting surfaces, and
at normal room temperature and humidity. Assure
that test setup is as shown below.
Test setup for determining electrical resistance.
Test Procedure
Conduct test as follows:
1. Cut sample hose, 24 inches long
2. Assure that both inside & outside of
hose are free of oil, dirt, etc.
3. Pierce sample ends with clean nails, as
shown.
4. Connect nails to 1000 volt DC power source
& megohm meter or 1000 volt "megger,"
as shown.
5. Record total resistance, in megohms.
6. Measure "test length" as shown.
7. Divide total resistance by test length
to get megohms per inch.
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