Quantcast Meter Types

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(b) Temperature compensation is recommended if there is any chance of
metering superheated steam.  S u p e r h e a t i n g can occur in a variety of
s i t u a t i o n s :  downstream of malfunctioning desuperheaters, downstream of
p r e s s u r e reducing stations. a n d downstream of certain piping configurations.
( c ) Installation must be to ASME standards and/or manufacturer's
s p e c i f i c a t i o n .  One of the most common metering errors is that the meter is
placed where it is convenient, not where it should be. IF THERE IS NO
( d ) I n s p e c t t h e m e t e r s i t e .  D o not rely on drawings to establish line
size and operating pressure.  C o m m o n l y , m e t e r s a r e o r d e r e d f o r a p i p e s i z e
l i s t e d in mechanical drawings.  T h e n , when the true size of the line is
d i s c o v e r e d , the meters are made-to-fit.  S i m i l a r l y , b o i l e r p l a n t s o f t e n
operate at a pressure lower than designed.
( e ) Maintenance. A steam meter, l i k e any other piece of equipment, must
b e maintained. Bearings wear, o r i f i c e s erode, and data recording equipment
m u s t be periodically resealed.  A steam meter left to operate without regular
m a i n t e n a n c e cannot be trusted to give reliable results.
3 . 2 Meter Types.  Steam meters are categorized into three main types:
d i f f e r e n t i a l p r e s s u r e , t u r b i n e (figures 3-11 and 3-12), and vortex shedding
( f i g u r e 3-13).  Since each type has advantages and disadvantages, the
comparisons shown in table 33 can be considered before selection.
3 . 2 . 1 O r i f i c e M e t e r s .  O r i f i c e plate meters operate on a differential
p r e s s u r e principle.  A differential pressure (DP), proportional to the steam
f l o w , is measured across a knife-edged orifice plate (figure 3-14). DP varies
a s a function of the square of the flow.  The following items are
characteristics of orifice meters.
( a ) Accuracy.  A c c u r a c y within the specified flow range is %1 percent
f u l l scale, but turndown ratio is limited, typically 3:1.
( b ) S i z i n g .  S i z i n g i s c r i t i c a l b e c a u s e t h e d i f f e re n t i a l p r e s s u r e i s
s m a l l and the limited turndown ratio requires an accurate estimate of flow
( c ) C a l i b r a t i o n .  Calibration is relatively simple and accurate. It can
b e performed with a manometer and utilizes standard procedures familiar to
m o s t instrument technicians.
( d ) Maintenance.  E r o s i o n of the knife edge is inevitable and
r e p l a c e m e n t of the orifice is periodically required. Seasonal changes in flow
m a y require change of the orifice to maintain accuracy within the flow range.
( e ) O u t p u t .  O r i f i c e type meters typically output a pneumatic signal to
a mechanical ring balance.  T h e mechanical system is relatively crude and
d i f f i c u l t t o c a l i b r a t e .  P r e s s u r e compensation is also difficult. Another
t y p e DP transmitter converts the pneumatic signal to an electric analog signal
that will drive strip chart recorder or data logger. This type transmitter is
easily pressure compensated.


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