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Early Detection of Heat Recovery Steam Generator Tube Leaks

A NACE International paper, HRSG Tube Failures: Prediction, Diagnosis and Corrective Actions highlights the challenge of these failures:

Many HRSGs were originally designed for base load operation. Recent economic factors have led to drastically increased cycling of many units. This has exacerbated the rate of failures.

These leaks can lead to forced outages that create associated costs including fines, lost time and replacement power purchases, which can exceed $100,000 USD per incident.

Heat recovery steam generator (HRSG) tubes are stressed by thermal expansion and contraction of the tubes from the combustion turbine exhaust gas. Optimizing steam temperature control can extend the life of the tubes. Monitoring the tubes with acoustic sensors can also help provide early warning to address leaks before they lead to unplanned downtime.

I connected with Emerson’s Juan Panama who explained that fatigue and stress cracking can occur in welds or the tubes themselves, or that poor welds could cause pinhole leaks. Leaks in the tubes themselves (outside of a weld area) are typically caused by corrosion processes, localized overheating, or incorrect metallurgy.

Juan noted that small super heater and re-heater tube leaks are difficult to detect, as no visible indications are present.

Without sensors to detect leaks, plant staff traditionally has checked by going into an outage and shutting down the HRSG (to allow steam to condense back into water to make leaks more visible), or if a leak is large enough by observing an increase in water usage at the control system. Continue Reading

Sizing and Selecting Control Valves for Desuperheaters

According to Wikipedia, superheated steam is steam:

…at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured.

It has:

…tremendous internal energy that can be used for kinetic reaction through mechanical expansion against turbine blades and reciprocating pistons, that produces rotary motion of a shaft. The value of superheated steam in these applications is its ability to release tremendous quantities of internal energy yet remain above the condensation temperature of water vapor; at the pressures at which reaction turbines and reciprocating piston engines operate.

With this property, superheated steam is used in many electrical power producing applications using steam turbines. Desuperheaters perform the function of reducing superheated steam temperature and recovering useful heat in the process.

Process Heating: Desuperheater Application Best PracticesIn a Process Heating magazine article, Desuperheater Application Best Practices, Emerson’s Mark Nord describes the role and importance of proper specification, installation and maintenance of control valves in this application.

Mark opens highlighting the growing operating challenges for power producers.

Increased cyclical operation, daily start-stop and faster ramp rates are required to ensure full-load operation, particularly at daily peak hours, and to maximize profit and plant availability. Changes resulting from environmental regulations and economics also are combining to alter the face of power production.

Steam is an important operational component and:

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Continuously Monitoring for Pipeline Corrosion

The need for pipelines to get oil & gas from producing regions to processing and distribution areas in the global supply chain continues to grow. Monitoring for corrosion is critical to prevent safety and environmental incidents.

This 3 ½ minute YouTube video, Roxar FSM Log 48 Area Corrosion Monitor shows how this technology permanently monitors a pre-defined pipeline area—improving pipeline integrity management (PIM) programs that traditionally rely on infrequent and offline techniques such as in-line inspection (ILI), direct assessment (DA) or hydrostatic pressure tests (HPT).

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Highly Accurate Continuous Emissions Monitoring

Continuous emission monitoring systems (CEMS) are used by manufacturers and producers that have combustion and other forms of emissions releases as part of their processes. These systems provide the information required for regulatory reporting.

A recent news announcement, Emerson’s New Hybrid Laser Process Gas Analyzer Reduces Costs for Continuous Emissions Monitoring, describes purpose-built technology to replace legacy measurement systems. At the heart of this CEMS is a high-accuracy Rosemount CT4400 Continuous Gas Analyzer that requires no consumables and minimizes maintenance.

In this news release, Emerson’s Paul Miller explained:

Our customers are looking for a better way to measure emissions without the on-going high costs or need for frequent calibration and complex sample preparation that requires NOx converters or ozone generators… The Rosemount CT4400 Continuous Gas Analyser gives them an answer to their exact requirements in a configuration they can just plug into their existing systems and be off and running – at a lower cost than previously possible. The reduced complexity of the system over what most companies are used to, results in higher reliability and analyser availability with a lot less personnel time required.

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Controlling Surge in Centrifugal Compressors

Centrifugal and axial compressors can experience a potentially destructive condition known as surge. Emerson’s Mikhail Ilchenko describes it for me.

Surge is a condition that occurs in compressors when the amount of gas they are trying to compress is insufficient for the size of the compressor and the blades lose their ability to transfer energy from the shaft to the fluid, causing a reverse flow of the gas. This condition can have catastrophic effects on the machine, so compressor manufacturers include anti-surge valves that recycle gas from the discharge to the suction when a low flow is detected. Usually compressors are designed such that these valves are only open on startup or under reduced rates.

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