POWER March 2013 - 18

Adaptive Brush
Seal Solutions for
Air Preheaters
design approach should be verified and revised, if necessary,
by ensuring that line pressure after elevation and frictional
change does not drop below the saturation pressure.
The pressure loss caused by increasing elevation (upwardrising
pipe) is ideally minimized by placing all HP heaters
side by side on the same elevation in the plant and the
low-pressure (LP) heaters side by side on a lower elevation.
However, the deaerator must be installed at a much higher
elevation because of the boiler feed pump's net positive
suction head requirements. Therefore, when the lowest HP
heater drains to the deaerator, the drain line pressure change
due to elevation becomes significant. For example, the lowest
pressure HP heater must drain from a low elevation (say,
7 meters) to the deaerator at a higher elevation (say, 30
meters), thus making the elevation difference (30 - 7 = 23
meters) very significant.
Elevation differences are particularly important at part-load
operation, when the differential operating pressure between the
HP heater and the deaerator decreases. In this case, the level
control valve should be located so that the line pressure does
not drop below the saturation pressure for all operating cases
upstream of the level control valve. Otherwise, the control valve
flow capability will be significantly reduced and the potential for
erosion in the piping system will increase.
XtraSeal HT
High performance. Long life.
Sealeze's patent-pending brush-based
sealing system delivers reliability and
extended functional service life.
Replace seals less often
* Consistent effective sealing, 2-5 times longer than
standard seals
* Inherent high elasticity allows dissipation of stress
under deformation; reducing drag, wear and sudden
failures.
More effective than other seal
* Conforms to sealing surface
* Each ilament is lexible, conforming to varying gap
sizes and surface irregularities for a continuous seal
* Thousands of ilaments, nestle tightly together
creating a
high integrity seal. Under pressure these ilaments
pack even tighter.
Calculate Pressure at Drain Piping Exit Downstream
of Level Control Valve
The drain piping downstream of the level control valve can handle
single-phase flow or two-phase flow, or the initial part of the
drain flow can be single phase, changing to two phase farther
downstream.
In all cases, the pressure drop in the piping can include significant
static head, which must be considered in determining
the control valve outlet pressure (P2
). Of course, the static head
is much larger for single-phase flow due to the presence of the
fully liquid column downstream of the control valve.
Single phase is promoted by subcooling condensate in the
upstream heater drain cooler. In drain piping downstream of the
control valve, single-phase flow is indicated when critical exit
pressure is found to be higher than the saturation pressure.
The pressure in the drain piping exit downstream of the control
valve depends on whether this section has single-phase,
1. Go with the flow. A typical feedwater heater with condensate
drain lines is shown. Courtesy: Bechtel Power Corp.
Steam in
Feedwater
out
For more than 25 years, Sealeze has helped design and manufacturing
engineers design solutions.
800.787.7325
18
e-mail: industrial@sealeze.com
www.sealeze.com
ISO 9001 Certiied
Condensate
drain line
CIRCLE 12 ON READER SERVICE CARD
www.powermag.com
POWER | March 2013
Feedwater in
Channel and
channel cover
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POWER March 2013

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