Steam Conditioning Valves and Desuperheaters

What Are Desuperheating and Steam Conditioning?

Superheated steam is steam heated above its saturation temperature. In heat transfer applications, at turbine outlets or in process steam distribution, the degree of superheat often has to be reduced, because steam close to saturation transfers heat more efficiently in exchangers and protects equipment from excessive temperature. A desuperheater sprays water into the steam as fine droplets; as the water evaporates it absorbs heat from the steam and the steam temperature falls.

A steam conditioning valve reduces both steam pressure and temperature in one device. Pressure control is a closed feedback loop, while temperature control can be feedback or feedforward depending on the application. Turbine bypass, process steam and steam pressure reducing stations are typical uses of these valves.

In both cases success depends not only on the equipment but on the whole system: pipe lengths, sensor locations, spray water pressure and temperature, drains, insulation and preheating must be designed together. Unevaporated droplets can separate at pipe bends, degrade control quality and cause thermal shock in piping elements; correct layout at design stage is therefore as important as equipment selection.

How Does a Desuperheater Work?

A desuperheater uses the heat of vaporization of the injected water to lower steam temperature. Fast and complete evaporation requires correct atomization and mixing.

1.      Temperature measurement: A temperature sensor placed far enough downstream measures the temperature of the mixed steam.

2.      Spray water control: The controller sets the opening of the spray water control valve according to the setpoint; water is fed to the desuperheater at a pressure above steam line pressure.

3.      Atomization: Water is introduced as fine droplets by mechanical nozzles, variable geometry nozzles, high-pressure atomizing steam or a venturi section.

4.      Evaporation and mixing: Droplets evaporate in the steam flow and the temperature becomes uniform. Around 80% of the injected water is expected to evaporate within the initial straight pipe run.

Feedback temperature control

Closed-loop feedback control is used when temperature can be measured accurately and consistently. Bringing steam temperature very close to saturation can leave water droplets in the steam because of the uneven temperature profile across the flow section, which disturbs measurement. It is therefore recommended to keep the setpoint at least 6 °C above saturation temperature.

Feedforward temperature control

Feedforward control is used when temperature cannot be measured accurately, a faster response is needed or variables change disproportionately. An algorithm running in a PLC or DCS determines the required water quantity from an enthalpy balance calculated with inlet and outlet steam pressure and temperature and spray water temperature; the measured spray water flow serves as feedback.

Desuperheater Types

Desuperheaters come in different types according to how the water is atomized and how the unit is installed in the line.

·         Fixed geometry mechanical atomization: A simple design with one or more fixed nozzles for nearly constant loads. Installed through a side flange on lines of 6 in and larger; maximum unit Cv 3.8.

·         Variable geometry mechanical atomization: A back-pressure-activated design with one, two or three nozzles for moderate load changes. Lines of 8 in and larger; maximum unit Cv 15.0.

·         Heavy-duty variable geometry type: For applications such as boiler interstage attemperation with high thermal cycling, high steam velocity and flow-induced vibration; integral thermal liner and a nozzle mount optimized against vibration. Maximum unit Cv 15.0.

·         Steam-assisted atomization: Uses high-pressure steam for rapid, complete atomization; for low-velocity steam lines and applications needing high rangeability. Maximum unit Cv 9.97.

·         Venturi ring type: Injects water at the outlet of a venturi section; mounted between two flanges with no moving parts. Suited to low-velocity steam with moderate load variation; maximum unit Cv 9.48.

·         Steam conditioning valve: A control valve that combines pressure and temperature reduction in one body; for turbine bypass and process steam applications.

Technical Specifications

Key technical data of the desuperheater family:

Pressure and temperature limits and standard limits must not be exceeded; the type is selected by load change, steam velocity and water conditions.

Key Advantages

·         Accurate temperature control: Steam temperature can be brought to within 6 °C of saturation.

·         Wide rangeability: Up to 50:1 inherent rangeability to follow changing loads.

·         Type selection per application: Different atomization types from constant load to heavy duty with high thermal cycling.

·         Equipment protection: Prevents excessively superheated steam from damaging heat exchangers and process equipment.

·         Wide size range: Solutions for steam lines from 1 in to 60 in.

Applications

Desuperheaters and steam conditioning valves are used in systems where steam temperature or pressure must be reduced in a controlled way.

Power plants

Boiler interstage attemperation, turbine bypass systems and steam conditioning during startup.

Process steam

Controlling steam temperature close to saturation for heat exchangers and process equipment.

Steam pressure reducing stations

Temperature control downstream of a pressure reducing valve; water injection close enough that turbulence speeds up mixing.

Design and Installation Criteria

The performance of steam conditioning equipment should be treated as a system, together with its physical environment and service conditions.

Straight pipe and sensor distances

·         Approximate first straight pipe length after the desuperheater: 0.1 times the maximum steam outlet velocity

·         Temperature sensor distance: 0.2 times velocity below 15% spray water, 0.3 times above

·         Residual superheat above 11 °C shortens the required length

·         Injecting water within three pipe diameters of the pressure reducing valve lets turbulence speed up mixing

Spray water

·         Water pressure should be a minimum of 150 psid higher than the outlet steam pressure; a 500–1000 psid differential improves performance

·         Water above 93 °C speeds up evaporation; water flow rises by 1–1.5% for each 28 °C increase

·         Water flow above 15% of steam flow slows evaporation

·         A strainer in the 40–100 mesh range is recommended ahead of the nozzles

Drains, insulation and preheating

·         Inlet and outlet drains where condensation or incomplete evaporation is possible; drip leg at the lowest point downstream

·         Full insulation of the valve and spray water valve, including the bonnet surface

·         Preheating when the difference between live steam and valve body exceeds 100 °C

Noise and vibration

·         Avoid multiple elbows directly upstream and downstream of the valve

·         Noise above 110 dBA may cause structural fatigue

·         Use a long-radius elbow instead of a tee for the first change of direction

·         Do not use the valve body or actuator as a fixed anchor point; orient the actuator vertically

Standards

Desuperheater connections and pressure classes are defined by the following standards:

Overpressure protection with a relief valve must be provided according to applicable regulations and good engineering practice.

Steam Systems with TLY Enerji

Most problems in desuperheater projects come not from the equipment itself but from system design: insufficient straight pipe, a sensor measuring before evaporation is complete, low water pressure or missing drains. TLY Enerji therefore approaches desuperheater selection together with pipe layout and control strategy.

Desuperheater type and nozzle arrangement are defined according to steam and water conditions; straight pipe and sensor distances are calculated, the spray water control valve is sized and a feedback or feedforward control approach is proposed. Insulation, drain and preheating requirements are written into the project documentation, and commissioning and site support are planned according to project scope. Where existing systems show temperature oscillation or pipe damage, layout and control strategy are re-evaluated.

·         Steam system and control strategy analysis

·         Desuperheater type and nozzle selection

·         Straight pipe and sensor distance calculation

·         Spray water control valve sizing

·         Commissioning coordination

Frequently Asked Questions

What is the difference between a desuperheater and a steam conditioning valve?

A desuperheater only sprays water into the flow to reduce steam temperature; pressure reduction is done by a separate valve. A steam conditioning valve combines pressure and temperature reduction in one control element. Steam conditioning valves are preferred where both pressure and temperature control are needed together, such as turbine bypass.

How close to saturation can steam temperature be brought?

Desuperheaters can control steam temperature to within 6 °C of saturation temperature. In feedback control, however, it is recommended to keep the setpoint at least 6 °C above saturation; otherwise temperature differences across the flow section may leave water droplets in the steam and make temperature measurement misleading.

Which desuperheater type should I choose?

The choice depends on load changes, steam velocity and water conditions. Fixed nozzle types suit nearly constant loads and variable geometry types moderate load changes. Steam-assisted atomization is considered for low-velocity steam and high rangeability, and the venturi ring type for low-velocity lines where no moving parts are wanted. The heavy-duty type is preferred for services with high thermal cycling.

What should the spray water pressure be?

Water entering the spray water control valve should be a minimum of 150 psid higher than the outlet steam pressure. Smaller differentials can reduce rangeability and cause incomplete atomization of the droplets. A 500–1000 psid differential improves performance, but the risk of cavitation at high differentials must be taken into account.

How far downstream should the temperature sensor be placed?

The sensor should be where the water has fully evaporated and is evenly distributed across the pipe. For an estimate, use 0.2 times the maximum outlet velocity when spray water is below 15% of steam flow and 0.3 times when above. The exact distance depends on water temperature, residual superheat, water quantity and pipe geometry.

Why are drains necessary in desuperheater systems?

Condensation while the system is idle or incomplete evaporation leads to free water collecting in the line. Water upstream can damage valve internals, while unevaporated water downstream can damage piping and instruments and upset temperature measurement. Drains upstream and downstream, with a drip leg at the lowest downstream point, are therefore recommended.