2000 Hook-up Book

Testing Steam Traps

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

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

SYSTEM DESIGN

Pressure Reducing Valve

or malfunction until the leakage area exceeds that needed by the condensate load. The cost then begins and reaches the maxi mum 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. 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 conden sate 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.

D

Strainer

Steam Supply

Pressure Gauge

Spira-tec Loss Detector

Test Trap

A

C

B

Drain

Drain

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 thermo dynamic 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

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