estimating air leakage

Section 9 - Technical Reference

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Estimating Air Leakage

Many applications do not require that air leakage be considered as a major determinant in the selection of a vacuum system. For example, many laboratory and industrial vacuum systems operate at moderate vacuum levels (15 – 22”HgV), and have very few components, vessels or valves, where air leakage can be introduced. In these types of systems, normal system demand approaches 100% of the total flow requirement. In some vacuum systems, however, the upstream piping includes numerous vessels, gages, valves, nozzles, pre-condensers, receivers or other components. Vessels with numerous penetrations (rotating mixers, reciprocating shafts, numerous sight glasses, etc) further increase the likelihood that air leakage will be an issue. These potential air leakage sites can result in significantly increased loads due solely to atmospheric air leakage. Where system vacuum levels are high, air leakage will be expanded many times, possibly causing air leakage to represent a significant portion of system demand. In a high vacuum system operating at 1 Torr (1 mmHgA), each SCFM of atmospheric air leaking into the system will expand by a factor of 760. Total air leakage of 0.125 lb/min would represent additional required capacity of 1241.55 ACFM, when expanded to 1 Torr. If anticipated system demand was 1000 ACFM (not including air leakage), the air leaking into the system would more than double the size of the vacuum system. Clearly, this cannot be ignored. In a laboratory vacuum system operating at moderately low vacuum level of 20”HgV (9.92”HgA), each SCFM of atmospheric air leakage will expand only by a factor of 3.02. In this case, the same air leakage of 0.125 lb/min would result in additional required capacity of only 4.93 ACFM. In a 1000 ACFM system, this amount could be considered negligible. The formula used to calculate ACFM of air leakage is:

2

1

29379

PP

mACFM ××= ,

where:

ACFM = additional capacity at system vacuum level m = mass flow of air leakage in lb/min P1 = atmospheric pressure (29.92”HgA, 760 Torr, etc) P2 = system vacuum level (same units as P1) (Note: pressure units must be expressed as absolute pressures)

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In the previous example of 0.125 lb/min air leakage @ 1 Torr, the calculations are as follows:

1760

29379125.0 ××=ACFM = 1241.55 ACFM @ 1 Torr

If the same mass flow of air leakage were to be calculated for a system operating at 50 Torr (approximately 2”HgA), the ACFM would be greatly reduced, in proportion to the new pressure ratio, as shown below:

50760

29379125.0 ××=ACFM = 24.83 ACFM @ 50 Torr

The following pages provide guidance to estimating air leakage in systems where air leakage may represent a significant portion of total system demand. As illustrated above, the higher the vacuum level, the more critical air leakage becomes. System tightness is an important consideration when selecting vacuum equipment for high vacuum applications.




MAXIMUM AIR LEAKAGE VALUES FOR COMMERCIALLY TIGHT SYSTEMS

SYSTEM VOLUME – CUBIC FEET (reprinted from HEI Standards for Steam Jet Vacuum Systems)

1. The air leakage rates indicated are guidelines only. In actual field practice, the air

leakage may be significantly larger or smaller depending upon the condition of sealing, maintenance factors and application.

2. For comparison purposes, alternate methods of air leakage estimation may be used.

3. Above chart reprinted from Heat Exchange Institute - Standards for Steam Jet

Vacuum Systems.

4. Select total system volume and vacuum level to determine estimate of air leakage for a “commercially tight” vacuum system.

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AIR

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Air Leakage Drop Tests When retrofitting or adding capacity requirements to a pre-existing vacuum system, it is often possible to perform an Air Leakage Drop Test to more accurately estimate air leakage. This provides a fairly accurate assessment of air leakage under current operating and maintenance conditions. To perform such a test, total system volume must be known (or easily estimated). This test is based on the fact that air leaks into the system at a constant rate as long as pressure within the system is less than 0.53 times atmospheric pressure (about 15”HgA). It is usually run with all agitators and other moving equipment in operation to duplicate leakage through seals and glands. System outlets and equipment being serviced should be isolated with tight isolation valves. Procedure:

1. Evacuate system to 13”Hg Absolute or lower, using the existing vacuum system. 2. Close off the isolating valve between the system and vacuum producer. 3. Record the elapsed time for pressure in system to rise from the initial pressure to a

higher observed pressure, (usually at least 1 – 2 “Hg). If leakage rate is low, a 1 to 2”Hg rise may take a long enough time to give an accurate estimate of leakage. However, stop the test before system pressure rises above 15”HgA.

4. Knowing system volume, (which must often be estimated, including volume of large piping runs), initial and final pressure, and elapsed time, the total leakage rate into the system may be calculated from the equation:

Leakage (lb/hr) = [(0.15) (Volume in Ft3) ( Pressure Rise in “Hg)] / (Time in minutes)

Example: V = Volume = 300 cu. Ft. P1 = Initial pressure in system = 5”HgAbs. P2 = Final pressure in system = 9”HgAbs T = time in minutes = 6.5 minutes

Leakage = (0.15) (300) (9-5) / (6.5) Leakage = 27.7 lbs/hr Leakage = 27.7 lb/hr / 60 min/hr = 0.46 lb/min.

From the mass flow of air leakage, ACFM of air leakage can be calculated, using normal system vacuum levels with the following formula (mentioned earlier):

2

1

29379

PP

mACFM ××=




Air Leakage Through Specific Components Another approach to estimating air leakage is to generate the Sum of estimated air leakage rates through the individual components of the vacuum system. The values in the following chart serve as a useful guide. Rates shown assume that average maintenance is routinely performed, and that the system was originally installed using good design and construction practices.

Specific Leak Rates for Rough Vacuum System Components

Component Leakage Rate (lb/hr per inch)

Static Seals Threaded connections 0.015 Conventional Gasket Seals 0.005 O-rings 0.002 Thermally cycled gasket seals Below 200oF 0.005 200- 400oF 0.018 Greater than 400oF 0.032 Rotary Seals Packing Glands 0.25 Mechanical Seals 0.10 Valves used to Isolate System Plug Cock 0.01 Ball Valve 0.02 Glove Valve 0.02 Gate Valve 0.04 Throttling Valves 0.25 Access Ports 0.020 Viewing Windows 0.015

(reprinted from Process Vacuum System Design & Operation by James L. Ryan and Daniel L. Roper)

Note: leakage rates above are lb/hr per inch of diameter.

What is the estimated leakage rate for a system with:

Component Calculation (2) ½” ball valves (2) (.5) (0.02) = 0.02 (4) 6” threaded connections (4) (6) (0.015) = 0.36

(1) 4” Access Port (1) (4) (0.020) = 0.08 (2) 2” Mechanical Seals (2) (2) 0.10) = 0.40

Leakage = 0.86 lb/hr


estimating air leakage

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