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Article # 0001

Considerations When Installing a Steam Conditioning Valve

 By Richard L. Jones, PE

A steam conditioning valve controls both steam pressure reduction and steam temperature reduction in a single body.  Applications include steam letdown stations, steam turbine bypass, and steam dump to condenser.  These applications are normally considered to be some of the most severe services of any valves in a modern steam plant. 

 

Process Conditions

When sizing steam conditioning valves, it is imperative that all process conditions be organized into the worst-case combination.  These conditions can then be used to determine the maximum and minimum steam velocities and steam Cv and the maximum and minimum spraywater flow rates and water Cv.   Often overlooked are start-up conditions where the combination of inlet pressure and flow rate can often be the determining factor in the steam conditioning valve’s required steam Cv. 

The available spray-water must be approximately 80 psi to 150 psi higher than the steam pressure.  The pressure differential is necessary to allow the steam conditioning valve’s spray nozzle to create a fine mist of water droplets with high surface area to water volume ratio. This ensures the fastest evaporation of the water and the least chance of water fallout.

Boiler Feedwater and Condensate are frequently used as the desuperheating water source for the steam conditioning valve.  Both sources are normally at temperatures well above ambient which is an advantage.  Hotter spray droplets will evaporate more quickly and thus have less chance of falling out of suspension.  

The steam outlet temperature should normally be a specified at no less than 7-10 degrees above the steam’s saturation temperature.  Applications with the set-point near saturation require greater evaluation to ensure proper operation and no water fall-out.

 

Valve Features

When evaluating the various steam conditioning valve designs to determine the best one for an application, there are a number of key considerations to consider.  The valve’s steam and spraywater turn-down ratios are very important.  The first ratio refers to the valve’s maximum Cv divided by its minimum controllable Cv. The second ratio refers to the maximum and minimum spray-water flow rates with which the nozzle can produce consistently fine droplets.  There is a third and most important ratio, the system turn-down.  This ratio is determined by the maximum and minimum steam flow rates with the applicable process conditions in which the steam pressure and temperature can be properly controlled without spray water droplet fallout.  

Some smaller and low pressure steam conditioning valves have cast steel bodies with integral flanges.  Most steam conditioning valves in larger sizes and above ANSI 600# Class are manufactured from forgings.  This allows the manufacturer to custom build the valve with any combination of inlet and outlet size, material, and ANSI Pressure Class.  This not only simplifies the system design and installation, but is also an essential factor in minimizing the system noise.  Many of the applications for steam conditioning valves include high pressure drops.  In these cases, the determining sizing factor of conventional cast body globe style control valves often becomes the commonly accepted body velocity limit of 0.33 to 0.5 Mach.  This can result in the requirement of a larger valve body than that would otherwise be required based on only on the required Cv. 

The forged-body steam conditioning valves are butt-weld ends as standard.  They do not have to be removed from the piping system as all service can be performed through the valve bonnet.  Additionally, the forged-body construction is designed to last for the life of the plant.

Steam conditioning valves can be either globe or angle design.  However, in most severe service applications the angle design is recommended.  The angle-body valve provides several distinct advantages, primarily in simplicity and thus cost and in the ability to deal with the generated noise.  

The steam conditioning valves can usually be installed in almost any orientation except with the actuator pointing downward below horizontal.  The water connection can be located in any position 360 degrees around the centerline.  And, in cases where the steam conditioning valve is installed in the low point of a line, a drain connection is added to the low point on the high pressure part of the valve. 

Always necessary for use with a steam conditioning valve is a spraywater control valve.  This valve controls the flow of the spraywater for desuperheating and should be located as close to the steam conditioning valve as possible. 

 

System Design     

It is necessary that the steam velocity at the outlet of the valve stay above the minimum recommended value provided by the manufacturer.  This minimum velocity is necessary to ensure that there is adequate turbulence and mixing for complete evaporation of the spray water droplets and no water fallout.  The normally provided recommended minimum flow rate is based on outlet steam velocity and pipe size, and is meant to be a conservative value.  In some cases, the recommended minimum flow rate can be reduced depending on other factors such as the percentage of water being added, the difference in enthalpy between the water and the steam, the difference between the steam saturation temperature, the orientation, the difference between the temperature set-point and saturation, and the spray-water temperature. In cases where the steam flow and velocity fall below the recommended values, there is a possibility that the spray-water droplets will not completely evaporate but will fall out of suspension and recombine at the bottom of the pipe.  Then, as the temperature set point is not achieved, the controller will ultimately open the spray water control valve fully.  The result is often a condensate load which exceeds the capacity of the available condensate removal system/steam traps.  The excess water may pool significantly and cause water hammer, thermal stress cracking, and erosion of the piping system, especially at elbows, and any restrictions in the steam flow. 

Special care must be also be exercised when the outlet steam pressure is very low.  At low pressures, the density of the steam is very low and it is thus more difficult to maintain the spray-water droplets in suspension for the time necessary for them to evaporate.

Steam Conditioning Valves should be installed with straight runs at both the inlet and the outlet.  Upstream, the length should be approximately 5-7 pipe diameters.  Downstream, the required minimum recommended length is a function of the outlet steam velocity.  In some cases, the downstream straight run can be reduced somewhat after review of the water temperature, the difference between the steam saturation temperature and the set-point, the percentage of water being added, the orientation of the installation, and the type of application.

The failure to eliminate water from the steam line is the greatest potential risk to steam conditioning installations.  The water may be condensate that collects upstream or downstream of a closed steam conditioning valve or spray-water that leaks into the outlet side of the valve.  Both possibilities must be considered when designing the condensate removal system as failure to remove the water can lead to severe water hammer and even catastrophic damage to the steam conditioning valve and the downstream piping system.                

 The standard control for steam conditioning valve is feed-back control.  This type of control is characterized by downstream pressure and temperature sensors.

In some steam conditioning applications, there may not be adequate piping length available in which to place a temperature sensor for feed-back temperature control.  For example, in a dump to condenser application, the steam conditioning valve may be located very close to the condenser, too close for the use of feed-back temperature control.  Pressure is controlled from a downstream pressure sensor in the normal manner.  Temperature control is accomplished using inputs from the inlet water pressure and temperature, the inlet steam pressure and temperature, and the steam pressure between the valve and sparger inlet.  The flowrate can be calculated from the pressure between the valve and sparger as they are proportional.  These values can then be used in the process algorithm programmed into the Controller to result in the proper position of the spraywater valve to achieve the desired outlet steam temperature. 

In steam conditioning applications, the temperature reduction does not occur immediately at the point of water injection.  For this reason, the piping material immediately downstream of the valve and for a minimum distance equal to the recommended straight run, should be specified based on the steam temperature assuming no desuperheating water is added.  There should also be a fail-safe shutdown of the steam conditioning valve in the case of loss of cooling water or over-temperature of the outlet steam line. 

There are many factors to consider when designing a steam conditioning system.  However, with the proper advanced planning a successful installation is readily attainable. 

About the Author

Richard Jones, PE is the President of Richard. L Jones, Inc. and a 1975 Nuclear Engineering graduate of Texas A&M University.

Article # 0001         TEST QUESTIONS:

1.   A steam conditioning valve may be ... 

  1. of globe or angle design.

  2. installed in the feedwater piping.

  3. installed with the actuator below horizontal.

  4. All of the above.

2.   A steam conditioning valve controls the ...

  1. steam temperature reduction.

  2. steam pressure reduction.

  3. neither a or b.

  4. both a and b.

3.   Which of the following process conditions should be considered when sizing a steam conditioning valve?

  1. The available spray water pressure.

  2. The steam outlet temperature.

  3. The maximum and minimum steam velocities.

  4. All of the above.

4.   What characterizes the feed-back control for the steam conditioning valve?

  1. a control box near the steam conditioning valve

  2. upstream turbulence sensor

  3. downstream pressure and temperature sensors

  4. All of the above.

5.   Where should the spraywater control valve be located?

  1. as far away from the steam conditioning valve as possible

  2. at the center of the steam conditioning pipe run

  3. as close to the steam conditioning valve as possible

  4. All of the above.

6.   What factors can reduce the recommended minimum flow rate?

  1. percentage of water being added

  2. difference in enthalpy between the water and the steam

  3. the difference between the set-point temperature and steam saturation temperature

  4. All of the above.

7.   When outlet steam pressure is very low what happens? 

  1. the density of the steam becomes too high

  2. the temperature rises

  3. spray-water droplets do not stay in suspension

  4. All of the above.

8.   What standard end do forged-body steam conditioning valves have?

  1. butt-weld

  2. threaded

  3. flanged

  4. All of the above.

9.   The failure to eliminate water from the steam line is the greatest potential risk to what?

  1. steam conditioning installations

  2. rusting through the pipe

  3. explosion

  4. All of the above.

10.   Why are most larger steam conditioning valves manufactured from forgings?

  1. simplifies the system design and installation

  2. essential factor in minimizing the system noise.

  3. allows the manufacturer to custom build the valve

  4. All of the above.

 

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