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Isolation And Leakage Performance

A damper's isolating capability is the most frequently discussed means of evaluating damper performance.  However, is widely abused and there is no industry standard for its application.  Also, once a damper is installed, rarely if ever is leakage actually measured.

Most Engineers must design flue gas systems based on some type of leakage rate across dampers.  High leakage could effect the overall efficiency of the whole system.  An example would be scrubber bypass damper where leakage escapes directly up the stack and effects EPQA emission compliance.  With maintenance applications "Zero Leakage" is required, and is defines as having no flue gas leakage past the closed damper.  This is accomplished by using purge air to create a positive pressure barrier .  In recent years guillotine dampers have been used in place of double louver dampers.  Guillotines require less maintenance, have smaller seal air fans, and do not have a large pressure drop across the damper when open. 

A damper’s isolation capability can be stated in several ways.


This is the most common way to show performance.  The various leakage paths are added up to get a total area.  The theoretical leakage rate is then calculated.  Leakage is stated as either volume (CFM) or mass (Lb./Hr).  In both cases the actual density of the flue gas is a factor.  Flue gas density changes with temperature, and at higher temperatures the gas has a lower density causing higher leakage rates.  Pressure also is directly related to the amount of leakage.  At higher pressure there is more force to push the gas through the leak paths causing higher leakage.

When comparing leakage rates or requesting tighter sealing dampers it is common to state the percent of leakage compared to the total gas flow rate when fully open.  However, as shown on the attached example, the percent of leakage can vary greatly due to temperature and pressure although construction has not changed.

Area Blockage:

This is an older method of comparing isolating capabilities of dampers, where the percentage of total are blocked off when the damper is fully close is stated.  Although good for evaluating damper construction the area blockage method does not directly relate to leakage rates, since leakage will change with pressures and temperatures, while this leakage area may not.

This method is used by some people to give the impression that the damper is tighter sealing than an actual leakage rate would indicate.

When dealing with damper sealing performance the following is recommended:

  • Do not throw around raw percentage numbers as absolutes.  These numbers should only be used as rough comparisons of design alternatives.  For example, a structural seal guillotine may on average have a 1% leakage rate and a full seal guillotine may have a 1% leakage rate.

  • Since higher performance dampers usually cost more, you need to understand what type of performance is required.  Dampers used for controlling gas flow do not need a full set of seals, where dampers for isolating may.

  • When comparing leakage data from different vendors, we should recommend to our customers that they request a leakage calculation.  For equal comparisons the same temperature and pressure must be used.


The seal air system is sized to develop a pressure in excess of the normal system differential pressure.  To accomplish this a fan flow requirement must be accurately determined. 

Fan flow is determined using the general Bernoulli Equation and applying it to leakage based on mass flow through an orifice.  The amount of gas able to pass from one side of the damper to the other is called the leakage, Q.  When expressed in terms of standard cubic feet of air per minute the symbol is QSCFM.  The calculation of the leakage is based on the mass flow through an orifice.

Damper Don © 2009

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