ELECTROSTATIC CHARGE MEASUREMENT

MESURE DE CHARGE ELECTROSTATIQUE

English Abstract

ABSTRACT
In apparatus for continuously monitoring the electrostatic
charge transport rate or the charge density in a stream of
insulating material flowing through a pipeline, a charge
collection chamber and a volumetric flowmeter are connected
into the pipeline. The chamber is electrically isolated from
the pipeline and connected to earth through an electrometer.
The chamber is constructed to ensure turbulent flow within
the chamber, the cross-sectional area being enlarged in relation
to the pipeline to produce a substantial reduction in linear
flow rate. When calibrated for a particular material, such as
a hydrocarbon fuel, a value for charge density is derived from
measurements of the flow rate and of the current measured between
chamber and earth.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for continuously monitoring the electrostatic charge
transport rate in a stream of insulating material flowing through a pipeline,
comprising: a charge collection chamber having an electrically conductive
wall; inlet and outlet means for connecting said chamber into a pipeline to
enable continuous flow of said material therethrough, including means to
maintain said chamber in electrical isolation from said pipeline, at least
part of the length of said chamber being of enlarged cross-sectional area
relative to the pipeline for which said inlet means is designed and said
chamber having a transition portion between said input means and said enlarged
cross-sectional area which is effective to permit turbulence in the ensuing
flow and the length of the chamber being predetermined in relation to the
characteristics of the stream to be monitored such that in operation the
residence time of an element of the stream within the chamber is appreciably
less than the relaxation time constant of the material; and means for connect-
ing said conductive wall to earth potential; and means for measuring the flow
of current in the earth connecting means.
2. Apparatus according to claim 1 in which the chamber is substantial-
ly enclosed by an electrostatic shield having earth connection means.
3. Apparatus according to claim 1 or claim 2 in which the chamber is
of generally cylindrical form having an internal diameter in the range of
ratios from <IMG> to <IMG> with respect to the internal diameter of the inlet
means.
4. Apparatus according to claim 1 or 2 including volumetric flow
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metering means for deriving a value of flow rate in the pipeline to enable
a value for charge density in the stream to be derived from the charge
transport rate.
5. Apparatus according to claim 1 or 2 having means responsive to the
value of charge transport rate or of charge density to control the operation
of charge neutralisation means for the treatment of the material in the pipe-
line.
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Description

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This invention relates to the measurement of electrostatic
charge and is of particular application to the monitoring of
charge density in a stream of insulating material flowing
through a pipeline.
The generation and transport of electrostatic charge by
insulating material flowing through a conveyor system is a
general hazard but one of particular concern in the handling
of volatile hydrocarbon fuels. It is important to be able
to monitor charge density so that known methods of neutralising
the charge may be put into effect. In considering the possible
means of measurement, however, the imprecision of the term
'insulating' when used in this context must be borne in mind.
Thus, a form of monitoring apparatus may be envisaged which is
suitable for materials from which only a negligible quantity of
charge is released to the wall of a test chamber during the
period of measurement. Such materials have a long relaxation
time. Other materials such as some kinds of liquid fuel, for
which on the basis of conventional conductivity mea#urements
a long relaxation time would also be predicted, in fact
demonstrate comparatively rapid relaxation. It is an object
of the invention to provide apparatus suitable for measurement
on materials of the latter kind.
According to the invention an apparatus for continuously
monitoring the electrostatic charge transport rate in a stream of
insulating material flowing through a pipeline comprises a chamber
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having an electrically isolnted condllctive wall, the chamber
having inlet and outlet means for connecting the chamber into
the pipeline, means for connecting the conductive wall to
earth potential, and means for measuring the flow of current
in the earth connecting means, the chamber being of enlarged
cross-sectional area relative to the pipeline for which the
inlet means is designed such that the linear flow rate of the
material is caused to be substantially reduced when the stream
enters the chamber and providing a form of transition between
the inlet and chamber cross-sections such that the ensuing
pe ~m i ~tc61~
flow is ~ed to be turbulent, the arrangement being such that
the value of the current is a measure of the charge transport
rate.
The relationship between the measured value of current
and the charge density can be derived for each material from a
knowledge of the volumetric flow rate.
Preferably the chamber is substantially enclosed by an
electrostatic shield'having provision for connection to earth.
The chamber may be generally cylindrical in form having a
diameter in the range of ratios from ~ to ~ i with respect to
the diameter of the inlet means. The square root ~otation is used
to emphasise the cross-sectional area relationship.
Preferably the length of the chamber is so selected in relation
to the characteristics of the stream to be monitored that in
operation the residence time of an element of the stream within
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the chamber is appreciably less than the relaxation time
constant of the material.
It is apparent from the li-terature of research into
the electrical properties of for example liquid fuels
flowing in pipes thatcomplexand incompletely understood
relationships govern the exchange of charge between an
insulating fluid and the containing wall. The present
invention is based on the recognition that a test chamber
may be specified such that the value of charge density in
the insulating material may be reliably derived from a
predetermined relationship between the value of charge
density and the observed rate of flow of charge to the
wall.
It is not necessary to consider in detail the various
mechanisms which are believed to be involved in the gain
and loss of charge by a moving fluid in contact with
the conducting but electrically isolated wall of a pipe.
Such mechanisms result generally in a net gain of charge
by the fluid, the rate of gain being termed the ~streaming
current'. Since the effect of surface contact is important
if not predominant in determining the charge mechanisms the
fluid bulk can only be considered electrically homogeneous
if the flow is turbulent but this condition can reasonably be
assumed for high delivery rate pipelines. If the streaming
current in a very long pipe is denoted by i1' the linear
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velocity of the stream by u and the diameter of the pipe by
d the generally agreed relationship is expressed by
i1 = k.u d
The exponents a and b are each greater than unity and may
themselves depend on the values of u and d, but precise
values are irrelevant to the present discussion and we may
assume a _ b = 2, 90 that
i1 = k.u d ......................... (1)
' The flowing fluid therefore continues to accumulate charge
in each region where streaming current enters the flow and
at any point along the length of the pipe the cumulative
charge den~ity may be denoted by dq. The rate of accumulation
may be non-uniform since obsta:cles such as filters in the
pipe are prolific ~ources of charge. (The term 'streaming
current' is sometimes used to refer to the product of dqx
volumetric flow rate but in this specification the product is
designated 'charge transport rate'.)
The progress of discharge is also complex when a volume
of stationary fluid~ having a charge density dq , is exposed
to a conductive wall which is maintained at earth potential,
Con~idered as a dielectric the fluid would be expected to show
an exponential decay of charge density dependent on the ratio
Or the conductivity and relati-e per~ittivity. ~or high-purity
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fuels this ratio would yield a very long time sonstant. In
practice processes such as ion recombination may operate to
reduce the time constant to values in the order of one second.
The rate of decay may no longer be strictly exponential but
it is convenient to allot a time constant (or relaxation time)
T in the conventional sense. Thus after the elapse of a time t,
the charge density d~t is given by the expression:
dqt = dqO.e i ........................ (2)
Relaxation must similarly be taken into account when the
generation of streaming current 1 in a ~hort length of pipe at
earth potential is considered. Equation (1) is then modified
to give:
i = k.u2.d2.(1 _ e -t/T) ( )
where t is now the time of flow through the pipe.
The application of equations (2) and (3) to the determination
of charge density in a test chamber will be demonstrated but an
embodiment of the invention will first be described with reference
to the accompanying drawings in whi~ch:
Figure 1 represents a fuel delivery pipeline in which is
installed a monitoring apparatus in accordance with the invention, and
Figure 2 represents schematically the apparatus of Figure 1
with additionally means for neutralising charge in the pipeline.
Referring to Figure 1 a generally cylindrical metallic test
chamber 10 is provided at one end with an inlet coupling 12 for a
pipeline 14 and at the other end with a similar outlet coupling 16.
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Each coupling 12, 16 i9 mounted in an insulating bush 18 80 that
the chamber 10 remains electrically isolated when it is connected
into the pipeline 14. An electrostatic screen 20 i~ also mounted
from the bushes 18 coaxially with and almost completely enclosing
the chamber 10 but electrically isolated from the chamber 10and
from the pipeline 14. The screen Z0 is ~rovided with an earthing
connector 22 and the chamber 10 carries a terminal 24 which is
accessible for electrical connection through a hole in the wall
of the screen 20. The chamber 10 is designed for use with a
plpeline 14 of specified diameter, the couplings 12, 16 being
of similar diameter to the pipeline 14 and the chamber 10 having
a diameter over the greater part of its length which is three
times the diameter of the pipeline 14. The tran~ition between
the diameter of the coupling 12 and the maximum diameter of the
chamber 10 is made abruptly as indicated by the form of shaulder 25
for which the preferred internal angle exceeds 60 and any
stre4mlining of the profile should be avoided. There will be
implicit in the specification of the test chamber 10 for use with
a particular diameter of pipeline~ an expectation of a specific
range of flow rates. From this information the length of the
chamber 10 is preferably calculated so that the residence time
for an element of fluid entering the chamber 10 at the mean flow
rate is appreciably less than the relaxation time for the fluid.
A flowmeter 26 of any conventional form is installed in the pipeline
~; 25 so that the volumetric flow rate can be determined at the time of
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1087Z47
each charge measurement. In Figure 1 flowmeter 26 is shown
downstream of chamber 10 but an upstream position is equally
suitable.
In carrying out the measurement an alectrometer 28 is
connected between the terminal 24 and earth and fluid i8 allowed
to flow through the chamber 10 for a period sufficiently long
to enable the charge distribution to stabilise. The value of
current indicated by the electrometer 28 and the rate of flow
are then recorded. For purposes of standardisation it is necessary
to have obtained at least one direct measurement of charge density
by conventional means for the same fluid together with the
corresponding value of current indicated by the electrometer 28.
Measurements made on different occasions will not be validly
comparable if it is possible for air to be trapped in chamber 10.
In order to exclude this possibility it is desirable therefore
that chamber 10 should be mounted vertically with the inlet at
the lower end.
With reference to Figure 2 one example of a control system
is shown schematically in which a pipeline installation 14 includes,
in the manner of Figure 1, a test chamber 10 with electrostatic
screen 20, electrometer 28 and volumetric flow meter 26. Pipeline 14
also passes through a charge neutraliser 30 located upstream of the
chamber 10. Neutraliser 30 operates in a known way in respon~e to
the ou1:put from a controller 32 to neutralise the charge content of
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10~7247
the stream as measured at each instant. Controller 32 receives, by means
of any necessary transducing or ampli~ying elements, signals which represent
the value of current flowing through electrometer 28 and the rate of flow
recorded by meter 26. Controller 32 includes means for deriving from the
two input signals an ~utput signal to determine the degree of neutralisation
required to compensate for the total charge content or to reduce it to a
safe level.
Experimental observation has provided evidence of the relationship
between the value of charge density in the pipeline and the value of current
indicated by electrometer 28 for a medium-conductivity fuel which is in
accord with the following analysis:
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It will be apparent from the earlier discus~ion that a relaxation
current i collected by the chamber 10 i9 drawn from the charge
accumulated by the fluid before entry to the chamber 10 but i8
diminished by any new component of streaming currentl which may
be generated in the passage of the fluid through the chamber 10.
If the volume flow rate is denoted by v' and the charge density
of fluid entering the chamber 10 by dq , then i = (v'.dq - i ).
By substitution from equations (2) and (3) and by introducing
the respective values u1 and d1 for the linear flow rate in the
chamber 10 and the diameter of the chamber 10:
i = (vl.dq - k.u1 .d1 ) (1 - e / ).
It will be seen that, subject to the exponential decrement, since
the value of u1 is dependent on 1/d12, the value of i can be made
to correspond almost directly to charge density by making the
diameter d1 large. It isthought that the measurement i9 best
made by selecting the length of the chamber 10 so that the residence
time of an element of fluid within the chamber is appreciably less
than the relaxation time constant of the fluid. The effect of each
element of fluid is then sensed only during the initial stage of the
relaxation curve when the rate of discharge is highest.
Consideration of the factors which determine the value l has
shown theoretically that the measurement proposed represents the
cumulative charge density by a sample which will generally be small
and will depend on the ratio t/T, and that the flow continues with
the initial charge diminished by that amount.
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The test procedure may be summarised as follows:
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(1) For calibration purposes establish the relationship
between main stream charge density and collector current
for the fluid concerned.
(2) Confirm that the inflow and outflow charge densities are
not significantly different. The t/T relationship must
then be satisfactory whether the effective value of T
is known or not
(3) Measure the volumetric rate of flow
(4) Measure collector current. Derive charge density or
charge transport rate from (1) and (3).
It will be appreciated that in the test structure described,
the diameter d1 of the test chamber i9 envisaged as being
significantly enlarged in relation to the diameter d of the pipelineS
the consequent reduction in the generation of streaming current to
a fraction d /d1 of that in the pipe combined with the turbulent
flow conditions enables a valid measurement of charge density to be
made. It is thought that while some degree of advantage will be
obtained for any diameter ratio d1/d which is greater than unity,
this will be small for ratios appreciably belo ~ and will not be
usefully enhanced for ratios~significantly greater than ~
Similarly the length of the test chamber may advantageously be such
that the value of t ]ies below 0.5T although the standardisation
procedure will of course accommodate other values.
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10137247
Economy in the installation required for refuelling demands
the use of the smallest pipeline at the highe~t possible flow rate
which is con~idered electrically safe. There is nonetheless an
inherent risk and the use of a test meter of the kind and in the
manner described enables the fuel charge density or the charge
transport rate to be monitored continuously under the control of
an unskilled observer to provide warning of danger. The value of
the monitored parameter may of course provide an input to an
automatic warning system or the automatic operation of a charge
neutralisation process or other appropriate proced~re may '
be arranged. The use of a cl~osed,loop i9 indicated in
Figure 2; alternatively a neutraliser located downstream of the
test position may be u~ed as an open loop control.
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