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The piping supports upstream and downstream of the turbine were then inspected. These supports were not worked on before or during the outage. The supports appeared to be in good condition, but upon a closer inspection, the spring support on the inlet to the turbine was unloaded. The pipe support was marked with the design spring pre-load. The spring pre-load was then adjusted to the indicated value. The turbine was re-started and brought up to full speed. After this, all the turbine vibrations were under 2 mils and were stable. The turbine was then over speed tested, shut down and re-started. Again, the turbine vibrations were under 2 mils and stable.
A loose pipe support can cause excessive pipe strain, which can lead to seal rubs due to casing distortion. If the seal rubs are light, it is not uncommon that vibration problems only manifest themselves at higher speeds. If a vibration analyzer were available, seal rubs could have been identified by looking at the time-base vibration wave forms. Truncations in the time-based wave form indicate rubs. So, in this case, light
A loose pipe support also could have changed the stiffness of a non-rotating part of the turbine system and led to a structural resonance at about 4,000 rpm. This structural resonance could be in a segment of the steel base, a piping span, a bearing pedestal, etc. This non-rotating part vibrated, and this vibrated the turbine. Operating at a resonance magnifies the vibration. But, whatever the cause, stiffening the pipe supports corrected the vibration problem.
After several hours of running at constant vibration levels, there was a small, but noticeable increase in the vibration levels. For example, the exhaust end “X” vibration slowly increased from about 2.0 mils to 2.4 mils. During the troubleshooting process, the oil filter high point vent was opened and an excessive amount of air was bled from the filter. After the air was bled off, the vibrations fell back to a maximum of 2 mils or less. However, excessive air kept building up in the oil filter. The cause of the excessive air was investigated, and determined to be due to a leak in the main oil pump seal, or through one of the piping connections on the suction to the main oil pump.
The oil in the reservoir also appeared to be heavily aerated. Later on, when the turbine was shut down and just the auxiliary oil pump was run, there was no build up of air in the filter and the oil in the reservoir did not appear aerated. The main oil pump is mounted above the oil level and the suction operates in a vacuum. A spare oil pump was not available and a temporary bleed line from the top of the oil filter was added. Since this was done, the vibration has been very stable.
The trip/throttle valve actuator vibration was lower after the pipe support preload was increased, but it was still relatively high. This appeared to be due to a resonance due to the overhung actuator weight. As shown in Figure 5, a temporary wood support was added under the actuator. This eliminated the high actuator vibration at operating speed. Thus, not much stiffness was needed to correct the actuator vibration. A permanent support under the actuator is in the process of being added.
Excessive pipe strain can cause casing distortion, which can lead to seal rubs and high rotating machinery vibration. Something loose in the support structure, such as a loose pipe supports, also can cause a structural resonance, which can lead to rotating machinery vibration problems. In “Practical Solutions to Machinery and Maintenance Vibration Problems,” the author states that “… a non-rotating part may be the source for amplitude magnifying resonance in approximately 20 percent of all machines.” Rotating machinery vibrations also can be caused by subtle lubrication troubles, such as aerated oil. These are types of rotating machinery vibration problems that are caused by factors outside of the machine, and can be difficult to identify.
- Buscarello, Ralph T., Practical Solutions to Machinery and Maintenance Vibration Problems, Update International, 4th Edition, May 2002
Patrick J. Smith is lead machinery engineer at Air Products & Chemicals in Allentown, Pa., where he provides technical machinery support to the company’s operating air separation, hydrogen processing and cogeneration plants. You may contact him by e-mailing firstname.lastname@example.org.
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