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Article # 0016
Design Guidelines for a Successful Steam Desuperheater Installation
By Richard L. Jones, PE
Desuperheaters are generally utilized to reduce the temperature of superheated steam to a desired setpoint for the protection of downstream piping and equipment or for the supply of saturated steam for heat transfer purposes. This steam temperature reduction is occasionally accomplished by using a heat-exchanger with no contact between the fluids but is most frequently accomplished by the addition of water to the steam. The success of a desuperheater installation is dependant on selecting the best available equipment and the incorporation of proven desuperheater system design guidelines.
Water injection desuperheaters generally fall into one of the following three categories:
a) surface absorption b) mechanical atomization including venture and c) steam atomization. An absorption desuperheater consists of a vessel in which the wetted internal surfaces are in contact with the steam flow. The superheated steam absorbs the water thereby lowering the temperature of the steam. The absorption type is generally characterized by high initial cost, large pressure drop, ability to reduce steam temperature to saturation, small space requirements, and very high system turndown rangeability.
The mechanically atomized desuperheater is characterized by cooling water being injected directly into the steam flow. In some cases, the desuperheater internals will include a venture section designed to create turbulence and better mixing of the water at the point of injection. Venturi type desuperheaters are frequently able to operate with water pressure equal to or only slightly higher than the steam pressure. These units are usually characterized by low system turndown rangeability, high water fall-out rate, and the need for the set point to be a minimum of 15-20 degrees F above saturation.
Other mechanical designs are in essence a spray nozzle inserted into the steam pipe with a focus on generating the best possible spray pattern and the smallest possible water droplets. These units are characterized by relatively low initial cost, moderate system turndown rangeability, the need for relatively high steam velocities, set points limited to approximately 15-20 degrees F above saturation, and large space requirements.
The steam atomized desuperheater utilizes a higher pressure “atomizing” steam in conjunction with the cooling water. The atomizing steam adds heat to the cooling water thereby speeding its change in state to a gas as well as atomizing it mechanically into a finer sized mist of droplets than is possible by only mechanical injection through a nozzle. The finer mist results in faster absorption, reduced space requirements, less dependence on steam pipeline velocities, and higher system turndown.
Selecting the best desuperheater for the application requires a thorough understanding of the application and the features, weaknesses and limitations of the different type desuperheaters. However, once the best desuperheater has been selected, the system must be designed according to the manufacturer’s recommendations and proven guidelines to ensure proper system performance.
Proper desuperheater system design requires a complete knowledge of all anticipated operating conditions and an understanding of the main parameters affecting the system’s performance. Included in these parameters are: a) turndown, b) steam and water pressure drops, c) straight runs, d) distance to the temperature sensor, and e) suitable drain.
There are several “turndown” ratios involved in desuperheating, First, the manufacturer generally provides a turndown for the desuperheater. This turndown is the ratio of the maximum water flow to minimum water flow for which the desuperheater is able to generate consistently suitable and small droplets. Another turndown is that of the water flow control valve. It may be a standard control valve or may be an integral part of the desuperheater. In either case, the valve has a turndown ratio of maximum to minimum controllable water flow. The most important turndown associated with a desuperheater is the system turndown. This is the ratio of maximum steam flow to minimum steam flow for the installed desuperheater for which controllable desuperheating can be accomplished. Typically, system turndowns are limited to approximately 15:1 for mechanical desuperheaters and approximately 30:1 for steam atomized desuperheaters.
Many other factors affect the available system turndown. Included are line size and its interrelated steam velocity, spray water temperature, and the presence of a liner. Steam in a smaller pipe has greater turbulence at a given velocity than in a larger pipe. Therefore, the minimum velocity necessary to prevent water fallout is lower in the smaller pipe than in a larger pipe. Thus, the maximum possible system turndown in smaller pipe is often greater than that possible in larger pipe. Also, if the cooling water is hotter and close to the saturation temperature of the steam, it will more readily flash and evaporate thereby speeding the steam cooling and reducing the chance of water droplet fallout at a given steam velocity. Liners can further improve the systems turndown. They are typically installed to protect the base pipe from contact by the relatively cool water droplets with resulting thermal stress and/or to create a short section of reduced inside diameter piping through which the steam is accelerated with increasing turbulence and better mixing and thus faster and better droplet evaporation.
Different style desuperheaters have varying steam pressure drops. Venturi type result in relatively hig steam pressure drops. Mechanical injection through nozzles can result in little or non-existent pressure drops. Additionally, the spray water typically needs to take a 75 psi minimum pressure drop through the spray water control valve and spray nozzle in order to have decent control and create adequately small droplets.
Straight runs upstream and downstream of a desuperheater are commonly required. The upstream straight run is usually recommended as a function of the pipe diameter, typically 3-5 pipe diameters. This is to ensure that the steam is in a somewhat consistent and homogeneous state and not swirling or cork-screwing when the spray water is injected. The downstream straight run is best determined by computing the required evaporation time of the droplets or using a commonly accepted time factor and multiplying it times the maximum steam velocity. The result is the minimum recommended straight run necessary to ensure that the water droplets have evaporated before coming into contact with an elbow and becoming recombined into a pool of water.
Similar to the recommended minimum downstream straight run, the recommended distance to the temperature sensor requires that “all” droplets have been evaporated. This distance is therefore always past the minimum recommended straight run. Additionally, it is acceptable and even advantageous to have an elbow between it and the desuperheater. The temperature measurements must be made on dry steam as any droplets could result in a false reading and poor temperature control.
A suitable drain downstream of a desuperheater is an essential safety precaution. Changing process conditions, failed equipment, and failed temperature control system can lead to catastrophic loss of control and excessive water spray and buildup at the bottom of the steam pipe. It is imperative that such buildup be avoided and a properly designed drip leg with a steam trap and emergency manual atmospheric drain valve are always recommended. The atmospheric drain is also a means for understanding the state of operation of the desuperheater when troubleshooting the system for problems.
Knowledge of the available desuperheater equipment and its correct operation plus incorporation of proper system design parameters is essential to ensure a safe and successful desuperheater installation.
Richard L. Jones, PE is the President of Richard L. Jones, Inc. and a 1975 Nuclear Engineering graduate of Texas A&M University.
Article # 0016 TEST QUESTIONS:
1. What are desuperheaters generally used for?
To reduce the temperature of superheated steam.
To remove a superheater coil.
To improve the steam quality.
All of the above
2. Which of the following are categories of water injection desuperheaters?
saturated output
condensing steam
steam atomization
All of the above
3. An absorption desuperheater consists of a vessel with...?
a large surface area for heat transfer.
only one steam inlet and one steam outlet.
wetted internal surfaces in contact with the steam flow.
None of the above
4. The absorption type desuperheater is generally characterized by ...?
small space requirements
very high system turndown rangeability
large pressure drop
All of the above
5. The mechanically atomized desuperheater is characterized by ...?
cooling water being injected directly into the steam flow
very high system turndown rangeability
a large surface area for heat transfer.
All of the above
6. Which of the following factors affect the available system turndown?
spray water temperature
line size / steam velocity
the presence of a liner.
All of the above
7. Why are straight runs required upstream and downstream of a desuperheater.
to ensure that the steam is in a somewhat consistent and homogeneous state when it reaches the desuperheater.
to ensure that the water droplets have evaporated before coming into contact with an elbow.
Both a. and b.
None of the above
8. Typically, system turndowns are limited to approximately ___ for mechanical desuperheaters
5:1
15:1
30:1
All of the above
9. Typically, system turndowns are limited to approximately ___ for steam atomized desuperheaters.
5:1
15:1
30:1
All of the above
10. What parameter(s) must be known for proper desuperheater system design?
Boiler size in lb/hr.
Boiler fuel.
turndown
All of the above
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