Category: Forensic Engineering

Flash steam is a dynamic condition in which the water pressure suddenly drops below its boiling point and produces instant vaporization.

While the fluid experiences an abrupt expansion, the total enthalpy and temperature will remain the same. Most of the water molecules will absorb the excess of energy, as latent heat, and end up forming steam.

The resulting flow is a predominant mixture of vapor phase with a certain water content. This combination can be very abrasive due to the continuous liquid droplets’ impingement on the surrounding area.
This phenomenon can also lead to a variety of subsequent problems, such as mechanical vibrations, high noise, wearing or material erosion. These problems potentially contribute to an irreversible failure.

In addition, if the quality of the process water is poor, the pressure reduction would bring about precipitation of chemical compounds, deposition of impurities and generate further deterioration. Refer to the following post to find extra information about this type of failure, click here.

Valves are the most susceptible component for flash steam to occur. When a fluid passes through a control valve, the area restriction produces a significant depression and originate boiling. This thermodynamic transition forms vapor bubbles in the water that will continuously hit on the trim of control valves.

Since the volumetric flow rate must be maintained, the fluid velocity will inexorably increase and create even greater wearing effects.

For instance, in high pressure systems, superheated drain valves occasionally discharge into the low-pressure condensate flash tank. Under these conditions, the water flow normally exceeds sonic velocity and chokes at the flash steam zone. As a result, the rate of degradation accelerates exponentially.

The image shows the effects of impingement on the valve plug in a high pressure drain system. Notice the likeness in appearance to those effects that cause the presence of abrasive solids. The system did not contain any solid at all, however.

We took this photo during a startup of a power plant. Most of the high pressure drain valves were severely damaged. All of them presented the same corrosion pattern.

We managed to repair some valves and lapped the affected zone, but some other had to be inevitably replaced.
In the following link you can find other typical failures that come from flash steam and wrong designs. Cick here.

How do you solve this problem? Flashing is inherent to the process conditions. It is very important to have a deep understanding of the process operation. That will give you the perspective to foresee locations where flashing can be expected, thus, anticipate precise actions. While you cannot completely remove it, good design practices helps to prepare the system for such phenomenon and mitigate such consequences.

A correct material selection is critical to withstand both physical and chemical effects derived from flashing. Stellite is a cobalt base alloy (with around 30% chromium content) specially designed for wear and corrosion resistance. Though it is also suitable to perform perfectly at high temperature ranges. This material is commonly the preference for extreme service applications. In this practical case, the material of the plugs was 316SS. Did not have external cover of stellite or hardener trim shield to protect the seat surface. Despite the excellent resistant properties of 316SS, it was not enough to tolerate the conditions it was exposed to.

Another possibility to approach flashing erosion is to install a sacrificial orifice plate downstream from the valve to increase the backpressure. As a consequence, it would reduce the pressure drop across the valve and relax the service conditions. Flashing would probably still occur, but it would be relocated to the orifice plate area. In the event of a damaged orifice plate, you can easily replace them when necessary.

The plug can be protected with a drilled hole cage. This devise would reduce the size of the vapor bubbles, and therefore, minimize or even eliminate flashing.

Select a different valve design. For instantce, a multistate trim control valve or angle valves would allow to gradually dissipate the energy released during flashing. That it a positive way to control the degradation.

There is one more possible solution that entails the design of a reverse flow alignment plug which would displace the restriction area past the valve seat. This way the flashing and high velocity flows occurs right after the plug.

Flash steam happens when hot condensate pressure drops sharply below the vapor pressure of the liquid at that temperature. It is necessary to carry out a detailed analysis to identify any critical zone subject to experience flashing. In the event of flashing, first understand the nature of the problem, then prepare a comprehensive strategy plan according to the following strategies to control the degradation:

• Variation of process conditions to minimize or eliminate flashing
• Resistance materials to endure such conditions
• Flashing control by energy dissipation designs
• Flashing displacement by relocating the problem downstream to safer zones
• Valve Design Selection

Sometimes the best possible solution does not come from just one of the above strategies but requires the combination of several of them.

The main purpose of material selection is to reduce the cost of the design while meeting the operation performance target. However, due to the transient nature of commissioning, field activities can be significantly aggressive, and under some circumstances, can even exceed the operation conditions of in-line equipment. For this reason, it is very important to perform a pre-analysis of the commissioning strategy and identify the equipment that is going to see critical conditions, so that the appropriate material can be specified in each case.

This photo  was taken from the bottom of a diesel heater stack and illustrates the dramatic effects of acid dew point corrosion.

When sulfur-bearing fuel is burned in the combustion chamber, sulfur is converted to its dioxide (SO2) and trioxide (SO3) structure in the presence of other compounds such as CO2 and H2O.

Sulfur trioxide combines with water vapor to form gaseous sulfuric acid (H2SO4g). If temperature drops  below the acid dew point, liquid sulfuric acid will form and spread all over the metal surfaces of the equipment. Initially, sulfuric condensate won’t attack carbon steel.

According to the equilibrium curve H2O – H2SO4, this acid condensate is highly concentrated and its combination with iron will form a protective salt layer of iron sulfate, which prevents the development of corrosion.

The real problem comes when flue gas temperature falls below the dew point of water and dilutes the concentration of sulfuric acid. Under this condition, the acid dissociation evolves toward the formation of protons and will corrode the metal within a few hours.

From the engineering & design standpoint, this problem could have been avoided if the chemical composition of the fuel had been analyzed and therefore, a more resistant material for the stack had been selected.

When a steel surface is in contact with water under specific chemical and thermodynamic conditions, an oxide protective layer of magnetite (Fe3O4) is developed. Controlling the growth mechanism, the morphology and the solubility of such corrosion product it is possible to minimize the risk of failure and extend the life-time of the plant. The presence of O2 in the water chemistry is well known to be the most harmful corrosion catalyst.

For this purpose, oxide scavenger is generally dosed as protective agent which also contributes to build a film of magnetite. However, this common practice can be quite detrimental in the low temperature regions of the plant where the thermodynamic solubility equilibrium cannot be reached, and magnetite is been formed in the temperature range of maximum dissolution. As a result, single-phase FAC mechanism will lead to general corrosion.

Solution: keep O2 concentration high to change the Schikorr reaction kinetic to the formation of ferric oxide hydrate (FeOOH) which is stable at low temperature. The following photo shows the frontal baffle plate of a feedwater heater affected by flow-accelerated corrosion

Commissioning is likely the most critical stage in the life time of a power plant due to the transient conditions which boilers are subjected to.

From the first ramp-up to normal operation, the plant is exposed to a variety of transitory conditions, thermal expansions cycles, chemical excursion and equipment adjustments that potentially increase the chance of failure.

This photo illustrates the corrosion pattern of chloride attack on carbon steel and it was taken during an inspection carried out in a cooling water pump of a steam-water cycle.

When sulphur is present in natural gas, the average content is generally around 0.01% and it comes in the form of hydrogen sulfide.