2000 Hook-up Book - Spirax Sarco

say seven (7) years. This means
that after the first seven years of
the life of the plant, in any year an
average of almost 15% of the
traps will fail. With an annual
maintenance campaign, some of
the traps will fail just after being
checked and some just before the
next check. On average, the 15%
can be said to have failed for half
the year, or 7-1/2% of traps failed
for the whole year.
Now, most of the traps in any
installation, on the mains drip and
tracer installations are probably
1/2" or 3/4" size and most of them
are oversized, perhaps by a factor
of up to 10 or more. Let us assume
that the condenste load is as high
as 25% of the capacity of the trap.
If the trap were to fail wide open,
then some 75% of the valve orifice
would be available for steam flow.
The steam loss then averages
75% of 7-1/2% of the steam flow
capacity of the whole trap population,
or about 5.62%.
The steam flow through a
wide open seat clearly depends
on both pressure differentials and
orifice sizes, and orifice sizes in a
given size of trap such as 1/2"
usually are reduced as the
designed working pressure
increases.
Estimating Trap Steam Loss
Steam loss through a failed open
trap blowing to atmosphere can
be determined from a variant of
the Napier formula as follows:
Steam Flow in lbs/hr =
24.24 X Pa X D 2
Where:
Pa = Pressure in psi absolute
D = Diameter of trap orifice
in inches
By multiplying the steam loss
by hours of operation, steam cost
(typically $6.00 per 1,000
pounds), and by the number of
failed traps, total cost of steam
system loss may be estimated.
The formula above should
not be used to directly compare
potential steam loss of one type
Testing Steam Traps
Table 17: Steam Flow through Orifices Discharging to Atmosphere
Steam flow, lb/h, when steam gauge pressure is
Diameter 2 5 10 15 25 50 75 100 125 150 200 250 300
(inches) psi psi psi psi psi psi psi psi psi psi psi psi psi
1/32 .31 .47 .58 .70 .94 1.53 2.12 2.7 3.3 3.9 5.1 6.3 7.4
1/16 1.25 1.86 2.3 2.8 3.8 6.1 8.5 10.8 13.2 15.6 20.3 25.1 29.8
3/32 2.81 4.20 5.3 6.3 8.45 13.8 19.1 24.4 29.7 35.1 45.7 56.4 67.0
1/8 4.5 7.5 9.4 11.2 15.0 24.5 34.0 43.4 52.9 62.4 81.3 100 119
5/32 7.8 11.7 14.6 17.6 23.5 38.3 53.1 67.9 82.7 97.4 127 156 186
3/16 11.2 16.7 21.0 25.3 33.8 55.1 76.4 97.7 119 140 183 226 268
7/32 15.3 22.9 28.7 34.4 46.0 75.0 104 133 162 191 249 307 365
1/4 20.0 29.8 37.4 45.0 60.1 98.0 136 173 212 250 325 401 477
9/32 25.2 37.8 47.4 56.9 76.1 124 172 220 268 316 412 507 603
5/16 31.2 46.6 58.5 70.3 94.0 153 212 272 331 390 508 627 745
11/32 37.7 56.4 70.7 85.1 114 185 257 329 400 472 615 758 901
3/8 44.9 67.1 84.2 101 135 221 306 391 476 561 732 902 1073
13/32 52.7 78.8 98.8 119 159 259 359 459 559 659 859 1059 1259
7/16 61.1 91.4 115 138 184 300 416 532 648 764 996 1228 1460
15/32 70.2 105 131 158 211 344 478 611 744 877 1144 1410 1676
1/2 79.8 119 150 180 241 392 544 695 847 998 1301 1604 1907
Figure 60
Steam Trap Test Rig
Pressure
Reducing Valve
Drain
Pressure
Gauge
A
Strainer
C B
D
Spira-tec
Loss
Detector
Drain
Steam Supply
Test
Trap
To
Atmosphere
of trap against another because
of differences in failure modes. In
those that fail open only the
inverted bucket trap orifice blows
full open. Thermostatic types usually
fail with their orifice at least
partially obstructed by the valve,
and flow through thermodynamic
types is a function of many passageways
and must be related to
an equivalent pass area. In every
case, no trap begins losing steam
through wear or malfunction until
Inexpensive test stand may be
used to test steam trap operation.
Valves A, B, C, and D are closed and
the trap is attached. Valve C is
cracked and valve D is slowly opened.
The pressure-reducing valve is
adjusted to the rated pressure of the
trap being tested, valve C is closed,
and valve A is opened slowly, allowing
condensate flow to the trap until it is
discharged. Valve B is then partially
opened to allow the condensate to
drain out, unloading the trap. Under
this final condition, the trap must
close with a tight shutoff. With some
trap configurations, a small amount of
condensate may remain downstream
of the trap orifice. Slow evaporation of
this condensate will cause small
amounts of flash steam to flow from
the discharge of the trap even though
shutoff is absolute.
the leakage area exceeds that
needed by the condensate load.
The cost then begins and reaches
the maximum calculated only
when the trap fails completely.
The object is, of course to prevent
it from reaching that stage. The
steam system always functions
best when traps are selected that
are best for the application and
checked on a regular basis to
control losses.
57
SYSTEM DESIGN