Performance Evaluation Techniques for Paper Machines Vacuum

TIP 0404-55 ISSUED - 2001 2001 TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.

Performance evaluation techniques for paper machine vacuum systems Scope This document describes general techniques for the evaluation of performance of vacuum systems on paper machines. Its intent is to give paper makers a procedure for seeking and eliminating eliminating bottlenecks in vacuum systems and implementing optimum vacuum performance by red ucing operating costs and increasing productivity.

Definitions 1.  ACFM : Actual Cubic Feet per Minute. T he measurement of vacuum pump volumetric flow. 2. Orifice Test : Vacuum pump testing method. 3.  Dwell Time: The amount of time it takes for the felt to travel from the leading edge to the trailing edge of a suction box slot.

Materials The following materials are recommended for following the guidelines: Flat plate orifice testing equipment, described in TIP 0420-12 Tape measure

Safety precautions

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Evaluating Pump Tests Pump test results should be compared to original capacity at the optimum vacuum level for each application. By dividing the actual tested capacity, m 3 /hr (ACFM), by the capacity, m3 /hr (ACFM), shown on the pump manufacturer’s performance curve, the % of original capacity is obtained. For example, if a pump designed to have a capacity of 5,950 m3 /hr (3,500 ACFM) at 380 mm Hg (15 inches Hg) vacuum only tested at 5,100 m3 /hr (3,000 ACFM) at 380 mm Hg (15 inches Hg) vacuum, this pump is said to be at eighty-six percent (86%) of original capacity at that vacuum level. Capacity comparisons are made at the actual rotational speed (RPM) of the pump. Calculating Vacuum Pump Operating Costs The two primary operating costs associated with vacuum pumps are energy and seal water. Use Equation 1 to calculate annual energy costs for vacuum pump operation. Equation 1: $Ea = kW x $UR x hr x day where: $Ea = Annual energy cost kW = pump kilowatt usage from curve $UR = utility rate at mill in $/kW hr hr = operation hours per day day = operation days per year

$Ea = BHP x 0.746 x $UR x hr x day where: $Ea = Annual energy cost BHP = pump horsepower from curve $UR = utility rate at mill in $/kW hr hr = operation hours per day day = operation days per year

To calculate the annual seal water costs for vacuum pump operation, use Equation 2. Equation 2: $SWa = l/min x 60 x $SWr x hr x day where: $SWa = Annual Seal Water Cost l/min = seal water flow $SWr = seal water cost rate, $/1000 liters hr = operation hours per day day = operation days per year

$SWa = gpm x 60 x $SW r x hr x day where: $SWa = Annual Seal Water Cost gpm = seal water flow $SW r = seal water cost, $/1000 gallons hr = operation hours per day day = operation days per year

After calculating the energy and seal water costs for each vacuum pump, they can be summed to give the total cost

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Equation 4: $SWwa = $SWa x (1-Eff) where: $SWwa = annual wasted seal water cost $SWa = annual seal water costs from Eq.2 Eff = vacuum pump efficiency from pump test More detail is available regarding vacuum pump operating cost in “The Full Operating Costs of Liquid Ring Vacuum Pumps” located in the 1995 TAPPI Engineering Conference Proceedings. Suction (Uhle) Box Sizing Properly sized felt suction or uhle boxes are critical to insure that felt de-watering is maximized. There are two sizing parameters that should be evaluated: box diameter and slot width. Box Diameter The velocity of the water/air mixture flowing through the felt suction box should be below 1,065 m/min (3,500 ft/min). By knowing the airflow, m3 /hr (ACFM), supplied from the vacuum pump, minimum box diameters can be calculated using Equation 5: Equation 5: Minimum Box Diameter (mm) =

3  flow( m  / hr ) 19.89

Minimum Box Diameter (inch) =

 flow( ACFM )

0.052

Slot Width Suction box slot widths should be sized to maintain a dwell time of 2 to 4 milliseconds. Dwell time is the amount of  time that it takes for the felt to travel across the slot. Maintaining a dwell time within this range will allow enough time for the water in the felt to travel through the felt and into the suction box. If the dwell time is greater than 4 milliseconds, felts will tend to drag across the box and could experience premature wear. Calculating correct slot width sizing base on a specific dwell time is done using Equation 6: Equation 6:

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Cylinder Machine

Application Suction Drum Sheet Boxes Suction Press Felt Suction Box

Purpose (with vacuum) Consolidate and strengthen sheet by water removal Consolidate sheet by water removal Dewater sheet with pressing and vacuum Dewater felt for cleaning

Vacuum Levels (HgV) 510-560 mm (20-22 inches) 510-560 mm (20-22 inches) 255-510 mm (10-20 inches) 205-455 mm (8-18 inches)

To properly size vacuum pumps for their respective applications, TAPPI’s technical information paper “Paper Machine Vacuum Selection Factors (TIP 0502-01)” can be used. These factors (ACFM/in2) are used to calculate the total airflow (ACFM) that is required for most wet end application. To calculate the total ACFM requirement of  each application, the vacuum factor (ACFM/in 2) is multiplied by the total open area (in 2) of the application. For example, suppose a couch roll application on a 200 inch wide SBS paperboard machine having single 6 inch box has a vacuum factor of 7 ACFM/in2 (found in TIP 0502-01). The total required capacity for that suction box is: 6 in x 200 in x 7 ACFM/in2 = 8,400 ACFM @ 20”HgV (14,274 m3 /hr @ 560 mm HgV). Adjustments should always be made for inlet vapor and seal water temperatures. Your vacuum pump supplier will be able to provide these correction factors for their specific vacuum pumps. For applications not located in the TIP, contact your vacuum pump supplier for assistance. Process Piping Piping diameters and layouts are selected based on air flow and vacuum pump requirements. It is important that process piping be sized properly to prevent excessive line losses and vacuum pump operating problems. Sizing and layout recommendations for the different areas of the vacuum system are discussed here. Inlet Piping from Machine to P re-separator The piping from the paper machine to the vacuum pre-separators will be carrying air with large volumes of water. Because of this, all piping in this area should be horizontal or downhill to the separator. Uphill piping can result in large vacuum losses along with vacuum fluctuations. The velocity of the air with entrained water should be kept below 1,067 m/min (3,500 ft/min). Equation 7 can be used to calculate the minimum pipe diameter. Equation 7: 3

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Equation 9: Minimum Inlet Separator Diameter (mm) =

3  flow( m  / hr )

Minimum Inlet Separator Diameter (inch) =

 flow( ACFM )

92.83

0.245

The rule of thumb for sizing minimum separator height is, height > 2 x diameter. Separator suppliers should be contacted to verify that the chosen separator is able to separate the amount of water being pulled from the process. Barometric Droplegs Once the process air and water have been separated, the water must be removed from the separator with either a barometric dropleg or water removal pump. The barometric dropleg must be long enough to obtain a standing water level that has enough static head to overcome the vacuum level being pulled by the vacuum pump. If not, the water will be pulled up the dropleg, fill the separator, and pulled over into the vacuum pump. The barometric dropleg length is calculated using Equation 10: Equation 10: Minimum barometric dropleg length (m) = (0.0136 m x vac) + 0.9 m where vac = vacuum level, mm Hg Minimum barometric dropleg length (feet) = (1.13 ft. x vac) + 3 ft where vac = vacuum level, inches Hg The length calculated is the distance from the bottom of the separator to the overflow drain in the seal pit. The drop-leg diameter should be sized to maintain a water velocity below 2.44 m/sec (8 ft/sec). Equation 11 is used the calculate the drop-leg pipe diameter: Equation 11: Minimum barometric dropleg diameter (mm) =

 flow(l / min)

8.7

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separators, the diameter should be sized to reduce the air/water velocity below 750 fpm. To calculate the minimum inlet separator diameter, use Equation 12: Equation 12: 3

Minimum Discharge Separator Diameter (mm) =

m  / hr  1000



vac )

1000  92.83

Minimum Discharge Separator Diameter (inch) =

 ACFM 

30

vac)

30 0.245

Again, the rule of thumb sizing for minimum separator/silencer height is, height > 2 x diameter. Separator suppliers can offer various levels of separation efficiency and silencing capabilities when specifying separator/silencers. They should be contacted to select a specific silencer that best fits the application. Summary

The techniques discussed in this TIP will help to generalize the overall condition and design of a vacuum system. There are many other aspects involved in vacuum system design that are not covered in this paper. Topics such as seal water and felt conditioning systems are detailed in other TAPPI Technical Information Papers. The following outline summarizes the design parameters and equations discussed in the body of the paper. The figure at the end of  the paper also shows many of the guidelines that have been defined. 1.

Vacuum Pump Testing: see TIP 0420-12

2.

Vacuum Pump Operating Costs:

Annual Wasted Energy Cost, Equation 1 $Ea = kW x $UR x hr x day $Ea = BHP x 0.746 x $UR x hr x day Annual Seal Water Cost, Equation 2 $SWa = l/min x 60 x $SW x hr x day $SWa = GPM x 60 x $SW x hr x day 3.

Wasted Operating Costs:

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Vacuum System Piping:

a.

From Machine to Pre-Separator: Maximum Air/Water Velocity = 1,065 m/min (3,500 ft/min) Minimum Pipe Diameter (mm) =

b.

3  flow( m  / hr ) 19.89 , Equation 7

Minimum Pipe Diameter (in) =  flow( ACFM ) 0.052 From Pre-Separator to Vacuum Pump: Maximum Air Velocity = 1,677 m/min (5,500 ft/min) Minimum Pipe Diameter (mm) = Minimum Pipe Diameter (in) =

7.

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3  flow( m  / hr ) 12.53 , Equation 8  flow( ACFM )

0.033

Pre-separators

Maximum Internal Velocity = 228 m/min (750 ft/min) Minimum Inlet Separator Diameter (mm) = Minimum Inlet Separator Diameter (in) =

3  flow( m  / hr )  flow( ACFM )

92.83 , Equation 9

0.245

Minimum Height > 2 x Diameter 8.

Barometric Drop-legs

Minimum barometric dropleg length (m) = (0.0136 m x vac) + 0.9 m, Equation 10 Minimum dropleg length (feet) = (1.13 ft. x vac) + 3 ft Maximum drain water velocity = 2.44 m/sec Maximum drain water velocity = 8.0 ft/sec Minimum barometric dropleg diameter (mm) = Minimum barometric dropleg diameter (inches) = 9.

Seal Pits

Seal Pit Volume = 2.5 x Dropleg Volume.

 flow(l / min)  flow( gpm)

8.7 , Equation 11 0.051

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Additional information Effective date of issue: January 19, 2001. Task Group Members: Jeff Pappalardo, Chairman James Stein Richard Reese Jeffrey Reese

References 1. 2. 3. 4.

Pappalardo, J.P., Tappi 1995 Engineering Conference Proceedings, “The Full Operating Costs of Liquid Ring Vacuum Pumps.” TAPPI, Tappi 1999 Technical Information Papers, “Guidelines for Measurement of Vacuum Pump Air Flow.” TAPPI, Tappi 1998 Technical Information Papers, “Paper Machine Vacuum Selection Factors.” Smith, G.F., “The Machine Vacuum System – How to Get the Most Out of This Versatile Papermaker’s Tool.”

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Maximum Velocity

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5,500 ft/min

 Minimum Pipe Diameter 

 ACFM  x 0.033

Air-flow to Vacuum Pump

Max. Velocity

Inlet Separator Maximum Velocity

3,500 ft/min

 Minimum Pipe Diameter 

 Min . D

Discharge Separator

750 ft/min

 ACFM  x 0.245

D

H>2xD

 ACFM  x 0.052

D

Air/Water From Process

H

Water to Seal Max. Velocity

L L (ft) = (1.13 ft/”HgV x Vacuum “HgV) + 3 ft.

D

 Min . D

X 6 in.

Max. Velocity

8 ft/sec

Min. D

GPM x 0.051

750 ft/min

ACFM x [(30 - Vac) 30] x 0.245 H>2xD

•Seal Pit Volume = 2.5 x dropleg Volume •Height X is used in volume calculation •Length from dropleg to bottom of seal pit > 6 inches