Low vacuum levels are one of the more common problems/symptoms experienced in vacuum systems. The word "symptom" is really emphasized here, because the difficulty is rarely caused by the vacuum pump alone. However, the vacuum pump often gets changed out, only to discover the problem still exists.
First, recognize that vacuum levels are a measurement of resistance to airflow, where the airflow is induced by the vacuum pumps and the resistance is the various dewatering processes and system piping. Changes in resistance to airflow are caused by various process variables, including machine geometry and machine speed.
The following sections discuss some typical causes of low vacuum levels, including:
Open valve in the vacuum line or header; plugged screens at the vacuum pump inlet; uncovered barometric seal leg from a pre-separator; and low seal water flow at the vacuum pump.
Many plants, especially paper mills, elect to keep startup screens in place at the vacuum pump inlet. This is not a problem, as long as the screens are of a substantial material. Also, it is important to locate the pump vacuum gauge directly into the vacuum pump inlet, below the screen, since there is usually a tapped connection at that point. This allows you to observe any increase in the vacuum at the pump compared to the level at the paper machine. The screens may have a tendency to blind over debris, which acts as a throttling valve at the vacuum pump. This causes a high vacuum reading at the pump and a low vacuum reading at the process.
For example, a paper mill was ready to replace a uhle box because of what appeared to be poor sealing of the uhle box cover and inadequate end deckles. The "symptom" was a reading of only 6 in. Hg vacuum (-20 kPag) at the uhle box. There was no vacuum gauge at the vacuum pump, but the cast iron inlet flange and proximity beneath the flange was sweating and about 4.4°C colder than the ambient temperature. A gauge was installed in the pump inlet, on the pump side of the screens, and 235 mbar abs. was measured. It was discovered that the inlet screens had been forgotten, and one particular screen was nearly blinded over. The cold temperature was due to the refrigeration effect from the rapidly expanding air across the plugged screen. The screen was cleaned off and the uhle box vacuum returned to satisfactory levels.
Low vacuum can be caused when the vacuum level exceeds the limits of a vacuum pre-separator, seal leg, or seal tank system in systems that have such equipment. High vacuum levels can draw all of the water out of the seal tank, leaving the seal piping open to the atmosphere. Vacuum pre-separators operate with either a barometric seal leg (pipe) or with a low NPSH removal pump. The seal leg is the simplest design, but failure to correctly design, install, and operate this simple system will result in perpetual vacuum system problems, so a few basic engineering practices must be followed:
Significantly reduced seal water flow can result in lower pump capacity. Vacuum pumps require 10-l5 psig (70-100 KPa) seal water at a point measured upstream of an orifice. However, a plugged orifice may not let the proper seal water flow pass, even when the correct pressure is indicated. Remember: Pressure does not indicate flow!
Note: Hot seal water - 43° to 49°C and higher - also causes reduced vacuum pump capacity. This is most often a system design issue and is not addressed here.