Steam
Flow Meters
Ensuring energy efficiency through
effective monitoring of steam.
The steam boiler house is an area where significant
financial savings can be made through energy
efficiency. Fiscal monitoring of energy used
is vital for optimization of processes and planning
investment in machinery. Despite this the boiler
house rarely attracts the interest from management,
and therefore the investment, that it deserves.
The majority of boiler houses do monitor the
flow of steam from the boiler house to process,
but often the accuracy of the measurement system
is questionable. In such a harsh environment,
most traditional flow measurement instrumentation
suffers from wear, corrosion and fatigue. If
it is has been in place for a long time without
proper maintenance and/ or has not been calibrated
for a long time, then measurement errors are
significant. For example, without maintenance,
a traditional orifice plate is likely to be
eroded by steam flow to a point the reading
has no relevant meaning within a few years,
or even months.
Measuring the mass flow of saturated steam,
which at the measuring point is invariably wet,
is not easy. The selection of the measurement
element is critical to ensure long-term accuracy.
This article provides a quick reference guide
to the wide range of technologies available
and is designed to help those making the selection
find the right solution for their budget and
application.
Steam measurement technology
Steam flow is a harsh environment and the presence
of water droplets and other impurities flowing
at high velocity precludes the use of some types
of flow meters. Flow measurement technologies
that are useable in the steam environment are:
Orifice plate, averaging pitot, shedding vortex,
variable area, venturi, nozzle and (to a lesser
extent) corriolis.
Orifice plate, variable area, venturi and nozzles
type meters fall into the same category and
all rely on the pressure drop caused by the
increase in velocity as the fluid passes through
a restriction. The differential between upstream
and down stream pressures is proportional to
the flow velocity.
The averaging pitot tube is positioned across
the pipe diameter to measure the impact and
suction pressure of the fluid. This also induces
a differential pressure, which is proportional
to the flow velocity, but it does so without
affecting the velocity of the flow.
The shedding vortex meter has a similar profile
to the averaging pitot tube but it relies on
the oscillating vortex eddies shed by a non-streamlined
body positioned in the flow. The number of eddies
is proportional to the flow velocity.
The Corriolis meter uses a straight or bent
tube, which is forced to vibrate at its natural
frequency. As the fluid to be measured flows
through the tube it accelerates causing the
tube to twist. This twisting motion is proportional
to the flow velocity.
Selecting a steam meter
The purpose of a steam flow system is to move
energy to where it is to be used. It follows,
therefore, that the introduction of a restriction
into the pipe as part of the metering equipment
reduces the efficiency of the system.
This basic principle brings into question the
use of the traditional orifice plate device,
and particularly variable area meters, which
introduce an unrecoverable pressure drop into
the system. The consequent energy cost can be
substantial; this type of device is energy inefficient
by today's standards.
In contrast, shedding vortex meters and averaging
pitot tubes create negligible line loss - less
than 70mbar (one inch of water) on an equivalent
10 bar system (i.e. is less than 0.7%).
Fixed and variable orifice plates
Orifice plates are prone to wear. In the harsh,
wet steam environment, the sharp-edge of the
hole in the plate is soon eroded. Without frequent
servicing and regular replacement, the orifice
plate quickly looses its accuracy.
Variable devices also suffer mechanical wear,
and some components may be affected by temperature
change. Although compensation for these components
can be built-in, long term accuracy is compromised.
Do not be misled by the figures that manufacturers
of variable orifice plates quote for 'turndown'.
(e.g. 100:1) The limitations of the steam minimum
and maximum flow, and the effect of actual site
conditions rarely allow this figure to be achieved.
Calculate the maximum expected flow and the
minimum achievable flow for the meter; this
will give the actual available turndown for
the application which has nothing to do with
the figure quoted for the meter.
Shedding vortex
Vortex shedding meters present a small profile
to the flow and therefore the pressure drop
is negligible making them energy efficient.
These devices give a practical turndown of 30:1
although they do not tolerate low flow rates
so this turndown may not be realised on all
applications. Vortex meters are susceptible
to error from the effect of mechanical noise.
The units exhibit long-term accuracy although
wear can take place making the vortex less defined,
but this is not a significant problem.
Averaging pitot tube
Unlike other flowmeters, installation of an
averaging pitot tube is straightforward, requiring
minimum downtime. The only incursion into the
pipe is a one-inch weldolet into which the pitot
is screwed. This and the fact that it introduces
a negligible pressure drop into the system make
it highly energy efficient and a popular choice
for a many common steam measurement applications.
When selecting a pitot tube, remember that
shape is important; only those with a square
cross-section retain accuracy over the full
measurement range. The accuracy of those with
round bars is affected by changes in flow.
The averaging pitot tube has no minimum flow
velocity; it measures even very low flows accurately,
and has a turndown of up to 100:1. The critical
factor in a low velocity steam flow measurement
is the capability of associated pressure transmitter.
The 'turndown' of the combined system is usually
quoted at about l0:1.but this is easily expandable
to 100:1 by using two differential pressure
cells.
Corriolis
Corriolis is the most accurate of all the measurement
technologies under discussion. It has a very
wide measurement range, but it is expensive.
Where very high accuracy is paramount, regardless
of cost, this is the best choice.
A steam flow meter is a significant investment,
which can provide essential long-term plant
and process information. Choosing a flowmeter
is a minefield for the uninitiated and it is
quite easy to be misguided by the wealth of
data put out by manufacturers.
Turndown ratio is often seen as the major selection
criteria, however the turndown specified by
the manufacturer is for maximum and minimum
flow conditions for the instrument not the application.
In practical terms advertised turndown is seldom
realised because the maximum flow at site is
to low. Furthermore flow meters can be used
on liquids and gas and the advertised criteria
covers all applications which and may not be
applicable to steam use.
Long-term accuracy can only be guaranteed by
a measurement device that is not prone to wear.
Orifice plates and variable area devices loose
their accuracy over time, particularly in a
dirty, harsh environment like steam.
Averaging pitot tubes and shedding vortex devices
have fixed characteristics, ensuring accurate
measurement over a long period of time.
For more information, please contact:
Solartron Mobrey Ltd.
Tel: +44(0) 1753 756600
Fax: +44(0) 1753 823589
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