How Desuperheater Nozzle Testing Prevents or Predicts Failures

Summary

Desuperheaters are a mission-critical component in the steam process loops found in many process plants. On the surface, the task is simple: spray water into the steam to control the temperature and protect downstream equipment. Unfortunately, that function is much more difficult than it might appear due to the constantly changing dynamics of steam load variations and temperature swings. Failure to spray enough water can lead to significant damage due to overheating of downstream equipment, while water overspray or leakage leads to cracked pipes and quench damage. Both these conditions can lead to unplanned outages and expensive repairs.

The heart of a desuperheater is its spray nozzle as it must handle a wide variation of inflow rate, temperature and pressure —yet consistently deliver a highly atomized water spray that is easily evaporated. Unfortunately, these nozzles are often installed and then largely forgotten until they fail, resulting in expensive repairs and downtime. This article describes how to create a cost-effective nozzle testing program that can pay for itself many times over.
 

The desuperheat process

A typical combined cycle power plant utilizes the hot exhaust gas from the combustion turbine to generate steam, which drives multi-stage turbines (Figure 1). The temperature of the steam can vary significantly as the plant load changes, particularly during low load conditions, so steam desuperheaters are installed in the feed lines to some steam headers between superheaters and reheaters to add water and reduce temperature as required. This is just one example of the many types of steam process loops that use desuperheaters.

Desuperheater nozzles

There are many different designs for desuperheaters, but all depend on carefully designed and positioned nozzles which spray water into the process steam. Ideally, this spray is atomized as finely as possible so the water can quickly evaporate before it reaches pipe walls or downstream equipment. Like the desuperheater itself, there are host of different nozzle designs, but each strives to emit a complete hollow cone of tiny water droplets (Figure 4 center). Unfortunately, the nozzles can wear or plug over time, creating spray pattern gaps and solid water stream patterns that impinge on piping walls and crack piping (Figure 4 left). Clogged nozzles can also severely restrict flow, limiting the desuperheater’s ability to control temperature.

An inexpensive and easy solution

One way to avoid the problems described above is to simply check the nozzles on a routine basis. Emerson suggests nozzles should be inspected and tested every 18 to 24 months, and they should usually be replaced every 24 to 36 months, or when they exhibit a change in performance.

It is not hard to remove and test nozzles, and many process plants simply contract the work to accredited service providers who can either flow test nozzles in their local facilities, or bring test stands into the plant to perform onsite testing and repairs (Figure 5). Setting up a nozzle test and maintenance program is not difficult, but many take a reactive approach to their desuperheater maintenance, leading to more costly downtime. Accredited service providers can address these and other issues by delivering the test equipment and expertise to mitigate desuperheater performance issues before they become major problems.

Nozzle testing case study

One 860 megawatt combined cycled plant incorporated a desuperheater operating at 2,130 pounds per square inch gauge and 250 degrees Fahrenheit. The plant noticed the valve responsible for controlling the spraywater to the desuperheater was running nearly fully closed, yet the temperature downstream of the desuperheater was being measured well below specification. An Emerson Accredited Service Provider suggested the unit be taken out of service and immediately flow tested, but the plant continued to run. Three weeks later, a weld joint cracked on the downstream piping and the unit had to take an unscheduled 10-day outage to repair the damage. The cost to pull and flow test the nozzles was less than $13,000. The cost to take down the plant and repair the piping damage was $250,000.

In the wake of that incident, the plant instituted a nozzle maintenance routine, which requires each desuperheater to be flow tested on a routine basis to discover and resolve issues before significant damage is incurred. When the remaining desuperheater nozzles were tested, six more of the 10 desuperheaters at this plant required repairs and/or replacement nozzles.

Now that the nozzles are routinely tested, failures are far reduced and desuperheater performance has dramatically improved. Most units continuously control within a few degrees of setpoint.

Conclusion

Desuperheater nozzles are simple devices, but they perform a critical role in protecting process plant equipment. Like any mission critical device, these nozzles should be flow tested and maintained to keep them operating at specification. It is relatively inexpensive to set up a nozzle testing program that pulls, tests and repairs or replaces nozzles on a routine basis, and ignoring these nozzles can result in extended outages and multimillion dollar repairs.

With the many other maintenance activities required in process plants, many companies find it is best to engage accredited service providers with the equipment and expertise to perform nozzle testing and address any issues. Desuperheaters can be dismantled and tested in local shops, or flow-tested onsite using portable test rigs. Accredited service providers can either repair damaged components, or they can replace nozzles as necessary to keep the equipment operating at specification. Availability of parts ensures fast turnarounds, keeping maintenance outages to an absolute minimum.

Desuperheater nozzles may not be complicated, but they are critical for process plant operation. Taking the time and effort to test and maintain them can pay huge benefits by avoiding equipment damage and unplanned outages.

_{Figures all courtesy of Emerson, except where noted.}