Grounding brush discharge monitoring

In recognition of the possibility of static charge build up in condensing steam turbines, API 612 (2005) specifies that grounding brushes be installed. The electrical flow to ground through these brushes be monitored and useful information can be extracted.

This article carries excerpts from the paper, “Babbitted bearing health assessment” by John K Whalen of John Crane, Thomas D Hess of Chestnut Run, Jim Allen of Nova Chemicals and Jack Craighton of Schneider Electric.

Grounding brushes take current from the rotor to ground so that a charge does not build up on the rotor to the point where it discharges to ground though the best path possible – which is usually the closest point between the rotor and stator which is usually (hopefully) the point of minimum film thickness in a bearing. Typically this point of minimum film thickness is found in the active thrust bearing (as will be shown later). Shaft grounding brushes serve two purposes. The brushes are able to transmit modest amounts of stray current to ground for prevention of arc damage through parts of the machine (especially the bearings). The brushes also permit measurement of the shaft voltage and current, which allows assessment of the electromagnetic condition of the machine.

There are two types of electrical currents in rotating machines: electrostatic and electromagnetic. Electrostatic currents are primarily generated by impinging particles or droplet atomization (such as in wet stages of steam turbines) and can generally be handled with one grounding brush per shaft. Electrostatic currents usually cause minor damage, such as frosting of bearing babbitt, and the progress of damage is relatively slow. Electromagnetic currents are usually due to residual magnetism and/or stray currents created by electrical machines, such as generators, motors, and exciters. Events such as machinery rubs, improper welding, or lightning strikes can induce residual magnetism in the machine parts and cause the rotating machine to become a generator.

Electromagnetic currents can be extremely destructive. Severe damage can occur rapidly because of high current density. Damage such as burned areas and welding of components are possible. Monitoring to determine shaft current level and changes is performed using a shaft voltage current monitor (VCM). The VCM measures voltage and current through the grounding brushes. Ideally, the output of the VCM should be connected to a recording device or plant data historian so that trends of the voltage and current can be used to detect changes in shaft electrical properties.

If readings increase slowly, it could be an indication of deterioration of the electrical machinery (motor, generator, or exciter) or possible progressive self-magnetization of the machine. Sudden increases in current may indicate that self-magnetization has occurred, possibly due to the reasons previously mentioned.

If readings from the VCM decrease to zero, the grounding brush has most likely lost contact due to wear, or some other part of the brush circuit has opened. More detailed analysis of the shaft currents to determine cause of excessive shaft currents can be performed using an oscilloscope.

Source: https://www.turbomachinerymag.com/view/grounding-brush-discharge-monitoring

Comments

Top 8 Reasons for Mechanical Seal Failure and How to Prevent Them

Mechanical seals are critical components of pumps, responsible for maintaining a fluid-tight seal between the rotating shaft and the stationary pump housing. However, these seals can fail due to various factors, leading to leakage, reduced pump efficiency, and costly downtime. In this article, we will discuss the top reasons for mechanical seal failure in pumps and how to prevent them. 1-Improper Seal Selection Choosing the wrong mechanical seal can cause it to fail. Consider the following factors that can contribute to seal failure: • Chemical compatibility: All seal components, such as the seal faces and O-rings, must be compatible not only with the process fluid being pumped, but also with non-process fluids used for cleaning, steam, acid, and caustic flushes, etc. • Physical degradation: Using soft seal faces on abrasive liquids will not last. Shear-sensitive liquids, like chocolate, can break down and leave behind solids (such as cocoa powder) and force out liquids (like oil). • S...

[Read more...]

Dry Gas Seal Failure Modes

BY BHUSHAN NIKAM. Invented in the mid-20th century and typically equipped in process gas centrifugal, dry gas screw compressors and expanders, dry gas seals (DGS) are the preferred gas lubricated dry seal solutions available on the market. They have become the standard for new machines. DGS are robust, simple, consume less power, and are more efficient in reducing leakage than their predecessor. Various configurations such as tandem with and without an intermediate labyrinth ( Figure 1 ), single ( Figure 2 ), and double ( Figure 3 ) are available & shall be selected based on process requirements. In this article, we discuss the various DGS failure modes and how they should be addressed: PRESSURIZED HOLD/STANDBY Pressurized hold, also called settle-out condition, occurs when the compressor remains at a standstill, but the casing is pressurized. If an alternate process gas lacks sufficient pressure and flow, process gas enters the seal cavity through the process labyrinth ...

[Read more...]

Why Pump Shafts Often Break at the Keyway Area

By NTS Pump shaft failure can lead to significant downtime and repair costs in industrial plants. One of the most common locations for pump shaft failure is at the keyway area. In this article, we will explore the reasons why pump shafts often break at the keyway and what can be done to prevent such failures. The keyway is a high-stress point (weakest point) on the shaft, where a key is inserted to transmit torque between the shaft and the pump impeller or coupling. During operation, the keyway experiences cyclic loading that creates a bending moment in the shaft, which is concentrated in the keyway area. Over time, this cyclic loading can cause fatigue failure in the shaft material, leading to a fracture at the keyway. In addition to cyclic loading, other factors can contribute to shaft failure at the keyway. Improper keyway design or installation can lead to stress concentrations or inadequate clearance between the key and keyway . Misalignment or overloading can also cause ex...

[Read more...]

John Crane's Type 28 Dry Gas Seals: How Does It Work?

How Does It Work? Highest Pressure Non-Contacting, Dry-Running Gas Seal Type 28 compressor dry-running gas seals have been the industry standard since the early 1980s for gas-handling turbomachinery. Supported by John Crane's patented design features, these seals are non-contacting in operation. During dynamic operation, the mating ring/seat and primary ring/face maintain a sealing gap of approximately 0.0002 in./5 microns, thereby eliminating wear. These seals eliminate seal oil contamination and reduce maintenance costs and downtime. John Crane's highly engineered Type 28 series gas seals incorporate patented spiral-groove technology, which provides the most efficient method for lifting and maintaining separation of seal faces during dynamic operation. Grooves on one side of the seal face direct gas inward toward a non-grooved portion of the face. The gas flowing across the face generates a pressure that maintains a minute gap between the faces, optimizing flui...

[Read more...]

FACTORS IMPACTING COMPRESSOR SURGE

BY AMIN ALMASI. Surge can be a major challenge for turbo compressors. Operation in the surge area will result in instability, exposing the machine to destructive stresses and forces, high vibration, and even serious damage. Surge during shutdown (trip) has been reported for many turbo-compressors. This is particularly possible if the machine operates at high head and low flow, immediately before the trip, when the operating point can move toward the surge line and even pass it during coast-down (when the turbo-compressor reduces flowrate). When a turbo-compressor experiences a serious alarm, an emergency shutdown is usually initiated. But an immediate shutdown could result in a surge. In this case, the surge happens shortly after the shutdown (trip) and at a high energy level. This could be a surge at a high head (operating point could pass the surge line at high head). In many cases, there are advantages to not removing the driving power from the turbocompressor (tripping) immediately...

[Read more...]

Industrial Maintenance Solutions

Search This Blog