Condensate Level Switches For Steam Service

Situation: A steam condensate level switch only operates 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 application(s). Contact Delta Controls 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.

@ 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).