Based on writings of DOUGLAS F. SWEET, P.E.
How often do you get a call about the vacuum pump motor tripping out? It happens for a lot of reasons, and only occasionally does replacing the pump correct the problem. Again, to solve the problem, good troubleshooting skills must be employed and the questions of "what changed, and when did it change?" must be asked.
Two good starting points for determining horsepower requirements are the operating conditions vacuum levels) and pump speed (rpm). These are discussed in the following sections, along with the pump horsepower overloading caused by water overloading, backpressure, and internal buildup.
It is important to determine if the pump is operating at a point well above design vacuum levels. Remember, the vacuum gauge location must indicate actual pump vacuum levels. Also, the vacuum gauge must be accurate. The recom- mended gauge type is a 0-30 in. Hg vacuum (0-110 kPa vacuum) vacuum-only gauge, not a compound gauge that reads both vacuum and pressure.
Also, discussing vacuum gauges and their typical condition often leads you to believe that “a good gauge is still in the box.” Always be sure of the accuracy of vacuum readings. In addition, it is impossible to operate with a higher vacuum level at the machine than at the vacuum pump. If a pressure drop in the piping exists, the vacuum pump will be at the highest vacuum level. After determining the vacuum level, compare it to typical operating conditions. Higher vacuum
levels usually, but not always, cause higher horsepower requirements. Be sure the selected drive motor will allow the pump to operate at the full range of vacuum levels. Otherwise, a vacuum relief (in-bleed) valve will be required to limit operating vacuum levels.
Be sure the actual pump speed is the same as what was intended for the installation. Sometimes, something as simple as the wrong motor getting installed - 1,800 rpm vs. 1,200 rpm - can be the problem. This is more likely in a new system or after some motor maintenance.
Drive ratios of v-belts and gear reducers should be compared to the actual output speed or pump rpm. Drive manufacturers use the term "exact ratio” for determining the actual output, or driven speed, Also, with the newer, high efficiency motors, the full load speed is usually closer to the nominal rating of 1,200 rpm or 1,800 rpm. For example, selecting a drive based on 1,150 rpm (a common speed for older motors) and installing a new motor rated at 1,190 rpm would yield a 3.5% increase in pump speed, with a comparable increase in bhp.
Another common reason for high horsepower is severe water overloading. This can come from excess seal water or from the process. A liquid ring vacuum pump has a rating for a specific seal water flow and increasing this by even 25% or 50% does not typically cause a power problem. Flows that are two to three times the rated flow are most likely causing motors to overload, or belt drives to fail. Also, although Nash pumps can handle sudden slugs of water, they can be problematic. These can be intermittent, causing difficult troubleshooting.
High seal water flows are caused by several reasons, including high seal water pressure, lack of orifices, and worn spray nozzles (if the pump has them) - or all of the above. Typical seal water pressure is 10-15 psig (70-100 KPa). Again, this pressure reading should be before the orifice and spray nozzle. As long as the orifices and spray nozzles are intact, seal water pressure can be up to 15 or 20 psig (100-140 KPa) without difficulty. Beyond these pressures, excess water is only wasted and contributes to power problems.
Older vacuum systems are often found to have worn spray nozzles or nozzles that have been removed and replaced with a straight pipe. The nozzle functions as an orifice and more than 20 years of continuous flow will enlarge the nozzle and allow as much as two times the desired flow to pass.
Excessive flows, called carryover, from the process are usually detectable and can be resolved. The easiest way to detect carryover is to look at the water discharging from the suspect vacuum pump, if the flow is visible. Cloudy water discharging from a vacuum pump using clear seal water is a good sign of carryover.
Many vacuum systems, especially in paper mills, have vacuum pre-separators between the process and the vacuum pumps. The purpose of the separator is to remove water and contaminants from the air stream prior to the vacuum pump. Locations for pre-separators are determined by the type of suction device and machine speed. Any stationary vacuum or suction box should have a separator before the vacuum pump.
Also in paper mills, couch or suction drum rolls should have pre-separators on machine speeds below 1,000 rpm. At these speeds, the water removed under vacuum gets entrained into the roll and internal suction box, and this will flow to the vacuum pump. At higher speeds, the water slings out of the suction roll shell due to centrifugal force. Under some conditions, there can be significant flows of entrained water from suction rolls on twin wire formers at higher speeds.
With an understanding of the application of air/water pre-separation equipment, there must also be some knowledge of the proper piping methods and auxiliaries such as seal tanks and low NPSH removal pumps. Even though a separator exists, the separated water must exit the system through a barometric seal pipe or low NPSH pump. As discussed earlier, the seal pipe and seal tank can be used when there is sufficient elevation between the separator bottom and the liquid level in the seal tank. Vacuum systems with limited separator elevations may require a low NPSH pump. There is a significant amount of engineering applied to the design and installation of these systems, and this will not be covered here. However, the point is that air/water separation systems between the machine and the vacuum pump can be extremely important and affect the vacuum pump operation.
Sometimes, the carryover problem comes as slugs due to pockets in the vacuum piping. This causes intermittent slipping of the v-belts that drive the vacuum pumps. Also, the fluctuating loads can be measured at the drive motor. This usually shows up at a fairly repeatable frequency - for example, every 20 or 40 seconds. Solutions include removing the pockets from the piping or adding separation equipment.
Another cause of pump overloading is associated with vacuum pump backpressure. Backpressure occurs when the vacuum pump is operating with a discharge pressure of greater than 1 psig (7 kPag). Well designed vacuum systems operate with a discharge pressure of less than 0.5 psig (3.45 kPag). Proper discharge systems do not allow piping to run uphill and are designed for specific velocities. Older vacuum systems may have had additional vacuum pumps added to the system without modifying the discharge system and piping. The additional vacuum pump will push more air through undersized piping, causing additional friction and resulting backpressure on the vacuum pumps.
A second cause of backpressure occurs when the seal water leaving the pump is not removed from the discharge separator or vacuum pump sump at the same rate it entered. The discharge separator should be checked for free flow to an open drain. Systems with a discharge sump must have the water level in the sump regulated to hold it at proper 1evels. The first indication of high water levels in a sump or plugged water outlets in a discharge separator is water blowing out the exhaust stack.
Internal buildup within the pump Internal buildup within the pump is another cause of pump overloading. This can be from lack of pre-separation or from calcium carbonate scale deposits. These deposits usually occur on the pump rotor and within the discharge ports. This buildup causes internal backpressure and does not allow seal water and air to exit freely. Many times, the scale and buildup can be removed with a de-scaler while the pump is shut down. Also, in the event of hard water, a chemical dispersant can be added to the seal water to keep the scale constituents in solution, or a soft water source can be used.