CCUG/STUG tidbits: Shaft grounding, transformer bushings, valves

Shaft grounding, remote monitoring of data collected

Cutsforth New Shaft GroundingImportant to attend user group vendor fairs for two important reasons: It’s where the food and adult beverages are, and where you’ll almost always see something of value that you were not aware of. In the CCUG/STUG exhibit hall there were 68 exhibitors and an “ah ha” moment for the editors came at the Cutsforth booth where they learned of the company’s new shaft grounding system and companion monitoring system—perhaps a topic for discussion at the upcoming Generator Users Group meeting.

Recall that shaft voltage, left unchecked, can damage turbine/generator bearing surfaces, possibly causing a forced outage. In the extreme, the rotor could be damaged. Cutsforth’s shaft grounding and shaft contact assemblies, the editors were told, are designed to prevent this from happening. The latter, mounted on the shaft at the exciter end of the generator (figure), aides in early detection of changes in voltage—including short-duration transients.

The company’s shaft constant monitoring system takes grounding technology—grounding systems of various designs have been around forever—to the next level. It is designed to collect, store, and pass critical voltage information to plant personnel for analysis. Plus, the system provides waveform-level information when in-depth analysis is required. It supports common analog outputs, and can be configured for digital outputs to meet the varied communications needs of plants.

Even if not connected directly to the DCS, the system provides performance information on demand via the touch-screen monitor. Data can be downloaded via a USB port at the same monitor housing.

Protect against HV electrical failures

One of the open discussion topics at the CCUG meeting focused on HV electrical failures. A participating user mentioned the risks associated with COTA 750 and 900 bushings and the need consult Trench Group’s safety technical advisory immediately. Even though this advisory was issued in 2014, with the many staff changes experienced by the industry it is possible the document was overlooked.

The user stated that if you have these bushings you’re at risk. The failure type was said to be atypical in that these bushings fail at the internal connection causing catastrophic failure and complete loss of the transformer. Point was made that if the transformer is a GSU and there’s no spare, you’re out of service for nine to 16 months.

ASME code is a safety code, don’t circumvent it

Jorgen Gertz, a member of the ASME Boiler & Pressure Vessel Code committee and one of the industry’s leading experts on high-pressure piping systems, picked the CCUG meeting for his last presentation before retiring from Puffer-Swieven at the end of September. “Prevention of Water Hammer and Water Induction” was the perfect topic for both the systems engineers who attend the CCUG sessions and the turbine engineers who spend most of their time in STUG sessions (crossover between CCUG and STUG programs is allowed). Gertz’ thoughts on water hammer and water induction will be covered in a later issue of CCJ ONsite.

Access to Gertz gave the editors an opportunity to show him Fig 1 and ask, “What’s wrong with this picture?” The photo was given to the editors by a user participating in the walk-down of a generating plant offered for sale. No doubt the code was ignored, he said, adding that improperly installed pressure relief valves are not an unusual finding. Perhaps the most important point here is that isolation under a pressure relief valve is not allowed by the ASME code.

CCUG Valves Figs 1, 2

Next, Gertz pointed to what appears to be a fixed elbow immediately downstream of the valve. This is not recommended, Gertz said, because it can subject the valve to high stresses during thermal expansion and contraction. The suggested alternative to a hard connection is shown in Fig 2. For this arrangement it is important to allow sufficient space to prevent bottoming or side binding of the drip pan on the discharge pipe under maximum conditions of expansion.

He warned that when the valve is actuated during an over-pressure event, steam will escape through the clearance between the discharge stack and the drip pan until full flow is established. Back flow can occur if the stack is not sized correctly. Gertz also noted that back pressure must be included in calculations to determine the proper size of a pressure relief valve when its discharge is bolted to a fixed discharge stack.

Returning to Fig 1, Gertz said a drain at the low point on the discharge side of the valve was not in evidence. You don’t want water accumulating in that section of pipe, he added, particularly in cold climes. Fig 2 shows the drain at a low point on the valve body. An inspection port in the valve also is not visible, but it might be on the other side of the valve.

The bottom line: The isolation valve under the pressure relief valve in Fig 1 should be replaced with a spool piece.

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