Spiraflo Steam Flowmeters - Spirax Sarco

2
The Spiraflo steam meter with density compensation
is designed to give very accurate results,
regardless of variations in steam pressure
How it works
The Spiraflo steam meter operates on the wellestablished
spring loaded variable area principle.
A circular plate positioned in the flow is free to move
against the action of a heavy duty precision made
torsion spring. To determine the plate angle, a rotary
transformer of special design is incorporated into the
unit. This converts the plate angle into an electrical
output which is proportional to the flow. The device
operates within the pressurised body of the transducer,
thus eliminating the need for any rotary seals. The
result is a robust design, well suited to the rigours
of steam service. It offers good turndown, high
accuracy and repeatability.
An in-built temperature probe provides information
which enables the flow computer to compensate for
the pressure variations which are commonplace in
steam systems. This density compensation is
essential in achieving accurate measurement
of steam flow.
The addition of a pressure transmitter enables the
Spiraflo to be used for superheated steam as well as
saturated steam systems.
Density compensation
It is rare for the pressure in a steam system to
remain absolutely constant. Unless this variation is
taken into account, flow measurement errors will
occur. The automatic
density compensation
offered by the Spiraflo
system completely
eliminates these errors
1 000
900
800
Flowrate
and allows accurate
700
metering whatever the
600
steam pressure. This is 500
shown in the diagram.
400
300
200
Flowrate, kg / h
PTX
RVDT - Rotary variable differential
transformer
RTD - Resistance temperature
detector
PTX - Pressure transmitter
Signal conditioning unit
(the signal conditioning unit converts
the signal into a form suitable for
use in the flow computer)
Flow computer
Spring loaded plate
RVDT
The Spiraflo system components
100
0
0 1 2 3
Cumulative error
4
Hours elapsed
5 6 7
0
8
In this example a simple non-compensated meter is set for 6 bar g. The actual pressure in the system varies
through the day and unless this is allowed for, by the end of the day, very significant errors can arise. This is
typical of many steam systems.
RTD
Actual
system
pressure
10
8
Pressure bar g
6
4
2
2 500
2 000
1 500
1 000
500
0
Cumulative error, kg