The Idiots' Guide to
Highways Maintenance
Copyright © 2000/08, C.J.Summers
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SURFACE
DRESSING SPRAYER, DEPOT SPRAY-BAR TEST
CONTENTS
INTRODUCTION
THE TESTING TRAY
THE TEST
DIPPING THE TEST TRAY
CALCULATING
THE SPEED OF THE SPRAYER TO CONTROL RATE OF SPREAD OF BINDER
INTRODUCTION
I have kindly been supplied with up to date
photographs of the Depot Tray Test, by the company Recomac.
This images show the procedure for
testing and calibration of a spray bar to ensure even spraying distribution
across its spraying width and to determine the rate at which the binder is
discharged.
The test has
been around for many years, thirty that I know of, but many of us still regard
the depot tray test as an essential operation.
I have always regarded this test
as fairly fundamental in checking that the sprayer gives even distribution of binder across its spraying width, and to
determine the rate at which it sprays binder on to the road, at a
standard temperature of binder and pressure within the tanker.
However it is possible in the near future this basic yet excellent
test will cease to be a requirement in calibrating a surface
dressing sprayer, but some will retain it anyway as they regard it as an
important part of the whole surface dressing process.
It is possible the depot tray test will be replaced by the on site "carpet tile" test
which I believe is a far more fiddly and less accurate way of accurately calibrating a spray-bar.
It will be interesting to see what documents are published, and
by whom, and what they contain before final comment can be made.
But, as I understand it, the "depot spray-bar test" is still
a current requirement of most surface dressing contracts.
However, just because a test is no longer contained in a current British
Standard, or a requirement of a Sector Scheme document, it does
not prevent you from including a "similar" test in your
contract document if you feel the test is an important factor in
ensuring the accuracy of the equipment employed in surface
dressing.
Full details of the depot spray-bar test, and how it is to be
performed and recorded are to be found in :-
B.S.
1707 : Hot binder distributors for road surface dressing
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THE TESTING TRAY
This shows the the testing tray, in position, clean and ready for the
test, with its many individual sections. |
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THE TEST
The sprayer is
reversed into position with the spray-bar exactly over the testing
tray.
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The sprayer is then operated for exactly one minute, at
normal operating temperature and pressure.
The pressure gauges above the spray bar being monitored during the test,
and the temperature of the binder is recorded. |
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DIPPING THE TEST TRAY
After running the spray-bar for one minute the sprayer is
moved forward.
The level of binder is measured ( "dipped" ) in
each separate compartment, and the depth recorded.
From these dip measurements, and in obvious cases by eye,
the distribution pattern of the spray-bar can be
determined.
If it does not meet the requirements of the specification
the sprayer will not be issued with a certificate.
Also the amount of binder sprayed in one minute can be
determined, and this allows you to calculate to output of
the spray-bar.
Once you know the output of the spray bar it is possible
to compile the "speed chart" for the particular
spray bar to determine the forward speed the sprayer
needs to travel to spray a particular amount of binder
per square metre.
N.B. the spray rate of the spray-bar will alter according
to the viscosity of particular binder you are spraying,
so more than one spray-bar test may be necessary if you
intend using several binders for differing site/traffic
conditions.
And of course viscosity of any particular binder will change if the
temperature at which it is sprayed changes. |
CALCULATING
THE SPEED OF THE SPRAYER TO CONTROL RATE OF SPREAD OF BINDER
It is necessary to know the rate of discharge of the spray bar for
each grade of binder it is proposed to use so that you can establish a
"speed chart" for the sprayer to achieve particular amounts
of binder sprayed on each square metre.
Speed (metres per min.) = rate of binder delivery in litres per min. /
[spray width (metres) x binder depth (mms.)]
Example
:-
Speed (metres per min.) = 215
litres per minute
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[2.134 metres x 1.2mms.]
= 84 metres per minute
1 litre per sq. metre = 1.0mm.
depth of binder *** 1.5 litres per sq. metre = 1.5mm. depth of binder
*** ETC.
Note :- With emulsions this will be the depth of emulsion, but the emulsion
is only 67% bitumen, so 1.5 litres of emulsion will give a 1.5mm.
layer of emulsion, but only a 1mm. layer of bitumen after
"breaking".
But do not start getting your calculators out as the presence of the
water in the emulsion is already allowed for in the rates of spread of
binder quoted in Road Note 39.
All you now need to ensure accurate rates of spread of
binder is an accurate speedometer (5th. wheel) on the sprayer, for the
pressure in the tank to remain stable at the pressure at the time of
the spray bar test, and of course the viscosity of the binder to
remain the constant.
Viscosity of the binder will depend upon consistent product from the
supplier and the temperature at which it is sprayed. |
For further
information on spray-bar testing, speed chart calculation,
and surface dressing in general press ----> HERE
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