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Condensate Level Switches For Steam Service

Situation: A steam condensate level switch operates only occasionally; usually when a control system component (valve, D/P cell, computer, etc.) has failed. It is the last line of defense against a catastrophic failure such as a steam explosion, burnout, turbine damage, etc. It must always work reliably.

Solution: A device of the simplest design, which has the longest history of no-failure reliability, will be the best choice. Delta Controls’ Series 700 mechanical float and displacer actuated level switches fit the description and are the best choice for these applications. They have been in worldwide steam condensate service for over 25 years without a failure.

You must select the proper sensing element and body to insure that the inherent reliability is achieved. The basis and rules for hardware selection follow.

Saturated condition is defined as when the steam liquid and vapor phases are in equilibrium and neither condensation nor evaporation is occurring. There is a corresponding temperature at which saturation conditions occur for every specific pressure reading.

The density of the steam vapor varies with the pressure and becomes a significant factor at 650 PSIG and higher. The weight of the vapor pushes down while the weight of the condensate pushes up on the sensing element. The actual net SPG lifting the element is the SPG of the condensate minus the SPG of the vapor. This value must be equal to or higher than the “MIN SPG” spec for the sensing element selected.

It is possible to “superheat” the vapor to a higher than saturation temperature at the system pressure. In this case, the density of the vapor is lower than it is at saturation conditions. The minimum SPG difference will always occur at saturated conditions; therefore the sensing device will see a higher net SPG under superheated conditions and will continue to work reliably at higher temperatures.

The rule is: Size the sensing element (float or Displacer) for saturated conditions at the highest working pressure; Be sure that it will also physically contain the highest temperature and maximum pressure to be encountered in the applications. Contact Shreveport Engineering for any Application Assistance that you may need, or with any questions that you may have.

The table below shows the density in terms of SPG, which is based on the density of water at 77° F (25° C) and 1 atmosphere (standard temperature and pressure reference conditions) Linear interpolation for pressures other than the steam / condensate saturation conditions shown will be adequate in most cases.

Saturated Steam Conditions SPG @ Saturation Saturated SPG @ Saturation
PSIG °F Bar °C Liq Vap Net PSIG ° F Bar °C Liq Vap Net
15 250 1.0 122 0.944 0.001 0.941 750 512 51.7 267 0.773 0.027 0.746
50 298 3.4 149 0.918 0.002 0.916 850 527 58.6 275 0.758 0.030 0.728
100 358 6.9 171 0.896 0.004 0.892 900 534 62.1 279 0.751 0.033 0.718
125 353 8.6 179 0.888 0.005 0.883 1000 545 69.0 285 0.741 0.036 0.705
150 366 10.3 187 0.880 0.005 0.874 1250 572 86.2 300 0.711 0.046 0.665
250 406 17.2 209 0.854 0.009 0.845 1500 596 103.4 313 0.681 0.058 0.623
400 447 27.6 232 0.825 0.014 0.811 1750 617 120.7 381 0.656 0.072 0.583
450 459 37.0 238 0.816 0.016 0.800 1800 621 124.1 327 0.647 0.073 0.573
500 471 34.5 244 0.808 0.018 0.790 2000 636 137.9 336 0.622 0.085 0.537
550 479 40.0 248 0.800 0.019 0.781 2250 652 155.2 344 0.597 0.098 0.499
600 490 41.4 254 0.792 0.022 0.771 2500 668 172.4 353 0.557 0.122 0.435
650 497 44.8 258 0.784 0.023 0.761 2750 682 190.0 361 0.516 0.147 0.369
700 506 48.3 263 0.777 0.025 0.746 3000 695 207.0 368 0.462 0.186 0.276
              3135 703 216.0 373 0.393 0.212 0.181

@ 3191.5 PSIG (220 Bar) and 705.4° F (374.1° C); both liquid and vapor = 0.3181 SPG density units and have the same density, which is 19.84 lbs/ft3 (318.1Kg/M3)

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