Troubleshooting vacuum systems: high horsepower motor trip-outs: improper vacuum levels, incorrect pump speeds, and faulty air/ water separation systems can cause motor trip-outs.

Editor's note: This is the second of a three-part series that covers troubleshooting paper machine vacuum systems. Part I was published in the September issue of Solutions! Part III, on hot pump operation and pump vibration, will be published in a future issue of Solutions!

When problems occur in a paper machine vacuum system, identifying the cause can be difficult. The symptoms--rather than the root cause--are often treated. This troubleshooting guide can help mill maintenance and production personnel identify vacuum problems and differentiate them from other system problems.

HIGH-HORSEPOWER MOTORS

How often do you get a call about a vacuum pump motor tripping out? Motor trip-outs happen for many reasons and only occasionally does replacing the pump correct the problem.

Troubleshooting should start with the operating conditions (or vacuum levels) and the pump speed. Is the pump operating above design vacuum levels? The vacuum gauge location is important in indicating true pump vacuum levels. Also, the vacuum gauge must be accurate. We recommended a 0-30-in. Hg vectored-only gauge, not a compound gauge that reads both vacuum and pressure. (Getting a positive pressure reading in a vacuum line on a paper machine is practically impossible.) Also, verify the accuracy of vacuum readings. Finally, operating with a higher vacuum level at the paper machine than at the vacuum pump is impossible. If a pressure drop in the piping exists, the highest vacuum level will be measured at the vacuum pump.

After determining the vacuum level, compare it to typical operating conditions. Higher vacuum levels usually cause higher horsepower requirements. Be certain that the selected drive motor allows the pump to operate at a full range of vacuum levels. Otherwise, the system requires a vacuum relief (in-bleed) valve to limit operating vacuum levels prevent motor overload.

PUMP SPEED

Determine that the measured pump speed is the same as design speed. Something as simple as having the wrong motor installed (such as 1800 rpm instead of 1200 rpm) can be the problem. This would more likely occur in a new system or after motor maintenance. Compare drive ratios of V-belts and gear reducers to the output speed or pump rpm. Drive manufacturers use the term exact ratio to determine the actual output, or driven speed. Also, with newel high-efficiency motors, the full-load speed is usually closer to the nominal rating of 1200 or 1800 rpm. For example, selecting a drive based on 1150 rpm (a common speed for older motors) and installing a new motor rated at 1190 rpm would yield a 3.5% increase in pump speed, with a comparable increase in horsepower.

WATER FLOW

Severe water overloading--caused by excess seal water or by problems with the paper machine vacuum service (conch, uhle box, etc.)--can trigger motor trip-outs. A liquid-ring vacuum pump is rated for a specific seal-water flow. Increasing this by 25-50% does not typically cause a power problem. Flows that are 2-3 times the rated flow usually cause motor overloads or belt drive failures unless the motors are oversized. Also, watch for sudden slugs that can make troubleshooting difficult.

High seal-water flows can occur for several reasons, including high seal-water pressure, lack of orifices, and worn spray nozzles. Typical seal-water pressure is 10-15 psig. Again, this pressure reading should be measured before the orifice and spray nozzle. As long as the orifices and spray nozzles are intact, seal-water pressure can be as high as 15-20 psig without causing problems. Beyond these pressures, excess water is wasted and contributes to power problems. Older vacuum systems are often have worn spray nozzles or nozzles have been removed and replaced with a straight pipe. The nozzle functions as an orifice, and 20+ years of continuous flow enlarges the nozzle and allows up to two times the desired flow.

Excessive flows (called carryover) from the paper machine are usually detectable by looking at the water discharging from the suspect vacuum pump (if the flow is visible). A cloudy water discharge from a vacuum pump using clear seal water is a good sign of carryover. Many vacuum systems have vacuum preseparators between the papermaking process and the vacuum pump(s). The separator removes water and contaminants from the air stream prior to the vacuum pump. These separators are common on flatboxes and uhle boxes and are also found on the couch or other suction rolls.

The type of suction device and machine speed determine the locations for preseparators. Any stationary vacuum or suction box--as opposed to a suction roll--should have a separator before the vacuum pump. These applications include flatboxes, transfer boxes, pickup shoes, and uhle boxes. Suction rolls, especially a couch or suction drum roll, should have preseparators when machine speeds are less than 1000 ft/min. At these speeds, the water removed under vacuum gets entrained into time roll and internal suction box and can 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.

AIR/WATER SEPARATION SYSTEMS

Understanding time application of air/water preseparation equipment also requires knowledge of proper piping methods and auxiliaries, such as seal tanks and low-net positive suction head (NPSH) removal pumps. Although a separator exists, the separated water must exit the system through a barometric seal pipe or low-NPSH pump. 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 (such as bottomfelt uhle boxes) may require a low-NPSH pump.

Air/water separation systems between the paper machine and the vacuum pump are extremely important and affect the vacuum pump operation. Some mills do not use any preseparation after the uhle boxes on the felts. Although water, chemicals, fiber, and felt hairs ideally should not pass through the vacuum pump, many liquid-ring pumps run for years with no preseparation equipment. One mill recently experienced trouble just after rebuilding a press. The machine in question did not have separators between the uhle boxes and vacuum pumps. The mill rebuilt the machine and added new vacuum pumps--without preseparators--for the uhle boxes. While water removal improved, the uhle box vacuum pumps continually tripped out their motors. The mill quickly added new separators and low-NPSH removal pumps to handle the greatly increased water flows from the felts; this solved the overload and trip-outs.

SLUGS AND PUMP OVERLOADS

The carryover problem sometimes causes water "slugs" due to vacuum piping pockets. This causes intermittent V-belt slipping on vacuum pump drives. Also, fluctuating loads can be measured at the drive motor. This usually shows up as a repeatable frequency, perhaps every 20-40 seconds. Removing the pockets from the piping or adding separation equipment can solve the problem.

Vacuum pump back pressure also causes pump overloading, typically when the vacuum pump operates with a discharge pressure of greater than 1 psig. Well-designed vacuum systems operate with a discharge pressure of less than 0.5 psig. MillS should design discharge systems for specific velocities without any "uphill" piping. Some mills may have added additional vacuum pumps to older vacuum systems without modifying the discharge system and piping. The additional vacuum pump(s) push more air through undersized piping, causing additional friction and increased vacuum pump back pressure.

Back pressure also occurs when the seal water leaving the pump is not removed from the discharge separator or vacuum pump sump at the same rate that it enters. Check the discharge separator for free flow to an open drain. Systems with a discharge sump must have the water level in the sump regulated to proper levels. 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.

A final cause of pump overloading is internal buildup due to lack of preseparation or from calcium carbonate scale deposits. The buildup produces internal back pressure and prevents seal water and air from exiting freely. Often the scale and buildup can be removed with a descaler Also, when using hard water, mills can add a chemical dispersant to the seal water to keep calcium carbonate in solution.

Dan Dunn is regional sales manager at Paperworx, a division of Nash Engineering, 140 Highland Ave., Trussville, AL 35173-1461. phone: [phone]