Full Steam Ahead: Comparison of Steam Conditioning Valves

 Successful steam conditioning and the performance of its related products improves plant performance, which is why selecting a steam conditioning valve is important. A pressure reducing valve (PRV) with a desuperheater may seem smarter and seems to provide the lowest possible cost. However, it’s best to think in terms of a valve’s total cost which includes hardware, installation, and the operation involved. This is why installing a steam conditioning valve can help to lower costs overall, providing better system performance.

Function Comparison

To desuperheat steam efficiently, water must be injected uniformly as possible in the smallest size of droplets. This can be achieved with a well-designed desuperheater, whether it’s integrated with a PRV or installed separately.

The problem is achieving an efficient mixing between steam and water and at the same time minimizing the risk of spraying on the pipe wall. This is best done if the desuperheater is an integrated part of the PRV, and utilizes some of the available pressure drop to create a flow pattern suitable for mixing with a pressure-reducing pipe in the outlet the shape of a basket. Around the outlet of the pressure-reducing pipes that are only drilled in the cylindrical part, the most turbulent conditions exist and since turbulence works like a mixer, this is the best possible position to install the nozzles.

There is also the fact that velocity between the pressure-reducing pipe and the wall is higher than the velocity downstream, resulting in smaller drops since it creates a resulting drag force which disturbs and breaks up the water into fine droplets. The higher the velocity in the mixing point between water and steam the smaller the droplets will be. Since the evaporation time of droplets is proportional to a droplet diameter to the power of 2, it is very important to integrate a desuperheater and PRV if a short distance to bends and temperature sensor is required. The turndown of a steam conditioning valve is at least twice (and often four times) as good as the separate type.

The result is that desuperheating becomes more efficient if it is integrated as a steam conditioning valve. Not only is there a higher turn down, but also less straight pipe downstream, and a liner is not required since the flow pattern works as a fluid mechanical liner.

The temperature sensor can be located closer to the desuperheater position, which is important (especially in part load) to get a reasonable response time for the controller.

Installation Comparison

Installation is easier and quicker as shown in the following example, which is based on: P 1 = 20 bar (a), T 1= 300 C P 2 = 5 bar (a), T 2 = saturation 0 inlet 200, 0 outlet 400.

If the pressure class is increased, the on-site cost also increases. Also, it’s important to remember that water injection in the outlet of a valve will reduce noise emitting from valve outlet.

For PRV + desuperheater

A representative cost on-site including tools is USD 40/Hr which results in 39 x 40 = USD $1,560.

If a cone is required after the PRV (which is common) an additional price for the cone must also be added. Additionally, if the steam conditioning valve is used in a bend, this cost must also be considered in a comparison of installation; a typical price for the bend is USD $650 and for the cone USD $800.

In many cases, non-destructive testing (or X-ray at approximately USD $250) for each weld may also be added.

A representative cost on-site including tools is USD 40/Hr which results in 78 x $40 = USD $3,120.

For PRV + Cone + Desuperheater+ Bend

A representative cost on-site including tools is USD 40/Hr which results in 117 x $40 = USD $4,680.

For PRV + Cone + Desuperheater

Conclusion

The assumption that a lower price tag equals savings doesn’t always hold true. When selecting the correct valve for a steam conditioning application, a PRV and desuperheater may appear the more cost-effective choice, but the extras as well as the additional time for site work always need to be considered. Installation, hardware and operation should always be included when making a final decision that could make or break your power plant’s efficiency.

Published in SOLUTIONS Spring 2003