Cooling steam to reduce superheated steam temperature
Steam Desuperheaters & Desuperheating
A Steam Desuperheater is designed to reduce the temperature of super heated steam to produce lower operating temperatures. The majority of installations fall into these
- Power plant requirements for desuperheated steam supplying units having limited operating temperatures such as auxiliaries, heating systems, heat exchangers and more recently, dump stations.
- To improve heat transfer of surface-type exchangers.
- Reduction and control of super heated steam, where excess temperatures will harm the product.
- Use on boilers, either between super heater stages or at boiler exit, to control super heat temperatures at partial loads.
- Use as bypass of bleeder or back pressure turbines to maintain balance between process steam and power requirements.
- Miscellaneous applications where balancing or make-up steam is required in reduced pressure systems in refineries and process plants.
In operation the majority of our Desuperheaters require the water pressure to be equal to the main line steam pressure with the exception of the Type 6910 and Type 6905 where the water pressure must be at a greater pressure to the main line steam. Water is syphoned in to the combining tube assembly, where it is heated and then mixed with a small proportion of steam passing through the throat region. This is then introduced in to the steam line as a finely atomised mist which alleviates the requirements of thermal liners and impingement shields.
Desuperheater selection is usually defined by the operating characteristics and required duty. Our different types of desuperheater can be seen below:
Type 6940 & 6950 Venturi Desuperheater
These desuperheaters are recommended for use under a wide range of conditions, including steady and variable flows. They can be installed horizontally or vertically upwards. When installed facing vertically upwards with steam flow ascending, turn down ratios can be increased substantially.
Venturi desuperheaters are normally used in areas where atomizing steam is not available. Turn down ratio is dependent upon a wide variety of factors, such as installation (horizontal or vertical), amount of residual super heat, and piping. Depending on exact flow conditions, units are capable of 50% to 5% flow variation with typical pressure drop across the unit varying between 0.13 Bar and 0.7 Bar.
Type 6952 & 6953 Attemperator Desuperheater
Attemperator Desuperheaters are normally used for relatively steady load conditions where pressure losses must be minimised, again they can be mounted either horizontally or vertically upwards, and when installed facing vertically upwards with steam flow ascending turn down ratios can be increased substantially.
These desuperheaters are a modification of the venturi-type unit, although without the venturi tail. While it is less costly and has negligible pressure losses, it normally does not have the rangeability of the venturi-type unit. Actual turn down ratio is dependent upon a wide variety of factors, such as installation, amount of residual and initial super heat, piping, etc. Normal flow variation is 75% to 15% of flow.
Type 6970 Atomising Desuperheater
Type 6970 Desuperheaters serve a wide range of applications. In a combined pressure reducing desuperheating station where flow rates vary widely, this unit (when equipped with adequate controls) provides dependable operation with turn-down ratios in excess of 50:1 depending on exact operating conditions. These desuperheaters are recommended for use where sufficient high pressure steam is available to provide the atomising steam supply.
In the Type 6970, an ejector action is used to entrain excess condensate deposited in to the pipeline, this is an important innovation and a feature of this type of unit. Very few problems are encountered while operating desuperheaters at normal pipeline velocities, however research has proved conclusively that at low pipeline velocities (encountered at 1/50 up to a quarter of normal flow) un-vaporised liquid can settle out of a horizontal stream. In addition to this when the duty is required to approach saturation temperature, it becomes impossible to completely vaporise the cooling liquid. Therefore while super heated steam is still flowing through the pipeline, water accumulates in the bottom of the line. Since this keeps the required temperature from being reached, a control valve will continue to supply excess water into the pipe line while attempting to maintain the required temperature.
Type 6970 overcomes these complications by recycling excess water back into the atomising device. The water that is added through the control valve is therefore limited to the amount required for desuperheating. High pressure steam enters through the ejector steam nozzle which is precisely designed for each application. This high pressure steam entrains the mixture of fresh and excess cooling water through the water inlet line and atomises the water, which is then discharged into the super heated steam line at saturation temperature. The preheating reduces the time required to evaporate the liquid, and the consequent small particle size and turbulent stream improves heat transfer. At low flows the return line entrains excess water and at high flows where the desuperheater is operating at maximum duty, the return line entrains small amounts of steam which helps to pre-heater to the cooling water.
Type 6910 Surface Absorption Desuperheater
Type 6910 Desuperheaters reduce steam temperature by forcing it in to contact with a large number of wetted metal reaction rings. This type of unit is generally used where space limitations and requirements of minimum water carryover in the down stream pipe work are stipulated. Normally used in the marine, food processing and drying industries, we have seen units that have been in operation for over 20 years with minimum service required.
These desuperheaters can be used where very wide turn down ratios are required, in fact the turn down for this type of desuperheater is usually limited by the range of the control valves supplying the unit. Saturation temperature or a percentage of wet steam at the outlet is possible with the Type 6910, and in general the temperature control bulb can be placed at 1.5 meters or less from discharge of the desuperheater, this is not possible with other types of Desuperheaters.
Type 6905 Mechanical Dump Desuperheater
Type 6905 units use two or more special spray nozzles, or several rings of spray nozzles to atomize water and produce a large heat transfer contact area necessary for rapidly cooling the steam. As the droplets are relatively coarse and the steam temperature entering the desuperheater is normally above 400 Degrees Celsius, we usually recommend and supply a thermal sleeve welded on to the unit which extends approximately 0.3 meters into the downstream piping. Water pressure to this type of unit differs from all other units in that it is required at a minimum of 1.7 Bar above main steam line pressure.
Desuperheater Control System (Typical Layout for Type 6970)
Click to enlarge (opens new window)
A desuperheater is not a single piece of equipment that succeeds or fails on its own, but is only one of several distinct system components. For a successful application, system engineering is a must. Neglect of any one component may result in system failure, no matter how excellent the design and engineering application of the other components. Therefore, all of the control components must be carefully selected for the specific application to be handled.
System Valves and Controls
- Steam Pressure Reducing Valve - This valve must have a turndown somewhat greater than that of the system; it must respond to plus and minus control signals even at maximum and minimum flow rates. These valves are selected for a useful control range of 20% to 80% of maximum flow. They are normally an equal- percentage type or have equal-percentage characteristics. This type of valve has the best inherent flow characteristic and range needed for proportional control.
- Water Control Valve - This valve must have sufficient rangeability to meet the application. Two valves in parallel may be needed to get this range - one large and one small. Where a large water pressure differential is encountered, be careful of possible cavitation. Consider using a pressure control valve upstream of the main flow valve.
- Atomising Steam Shutoff Valve - This valve is used only with Type 6970 desuperheaters in the on- off, not throttling mode, as in above valves. The valve is sized on a constant flow, dependent on unit size and a nominal pressure drop.
- Temperature Controller - Must have an adjustable proportional control band wide enough to match response characteristics of the entire desuperheater system. Automatic reset prevents drift in control point. Rate action is seldom needed, but if it is provided, complete cut off should be possible.
- Pressure Controller - Must prevent large pressure variations which might interfere with temperature control, therefore it needs an adjustable throttling range and automatic reset.
- Control Valve Actuators - Pneumatically operated control valve actuators are the most popular types in use, but electric, hydraulic and manual actuators are also used. The spring and diaphragm pneumatic actuator is most commonly specified due to its dependability and simplicity of design. Pneumatically operated piston actuators provide an integral positioner capability and high stem force output for demanding service conditions.
- Temperature Switches - They are used in connection with alarm systems for high and low temperatures. The temperature sensor uses the expansion principle in which the fluid or element in the sensing bulb reacts to the line temperature.
- Pressure Switches - These switches, are generally used in connection with alarm systems for high and low pressures. The alarm can be either audio or visual. Pressure applied to the sensor actuates a mechanism and its movement is then used to control the operation of an electrical snap acting switch, or other actuating medium.
Important Note: A control system cannot successfully hold a temperature that is no higher above saturation than the controller's degree of sensitivity and dead band.