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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
SUITABLE LOCATION FOR THE METER, THE PIPING SHOULD BE RECONFIGURED.
( 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
range.
( 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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