ASME

Low Noise Solutions for Turbine Bypass to Air-Cooled Condensers

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Figure 1: ACC duct on a large combined cycle power station. The duct is long, large, and uninsulated.

Figure 2: Noise at the surface of the duct can propagate to nearby communities.

2 Air-Cooled Condenser Plants Demand Low- Noise Bypass Equipment

Introduction

In a power plant with an air-cooled condenser (ACC), steam is

carried from the steam turbine exhaust to the condenser via a

large, thin wall, uninsulated duct. Noise sources that discharge into

the ACC duct have much less attenuation than in a water-cooled

condenser. The ACC duct is typically external to the turbine building

and has a very large surface area. High noise levels at the ACC duct

surface can generate unacceptable noise levels at the plant boundary

and in neighboring communities.

This problem is especially important in combined cycle power

stations. Combined cycle power stations have 100% turbine bypass

systems. The combined steam flow and desuperheater cooling flow

from the bypass system discharges nearly 50% more mass flow

into the duct than the steam turbine, and at a higher enthalpy. This

large amount of mass flow is discharged into a dump device that is

much smaller than the steam turbine exhaust, concentrating noise

energy into a very small area. Single-stage control valves and dump

elements can generate external noise levels in excess of 130 dBA

at a distance of 1m from the ACC duct surface, and 75 dBA up to

a kilometer from the plant. With many combined cycle plants on

daily cycling, start-up noise can become a severe constraint in plant

operation.

Combined cycle power stations are also relatively compact, and are

much more likely to be sited in a sensitive environment than a large

coal-fired boiler. Plants with excessive noise levels may face financial

penalties and, in some cases, suspension of plant operation. Due to

the large size of the ACC duct, traditional noise treatment methods

like acoustic enclosures or insulation are impractical or insufficient.

The source noise must be treated in order to meet plant noise

requirements.

Complete Noise and Bypass System Specification

It is important to establish correct and complete noise specifications

for ACC systems. Almost all plants establish near field sound

pressure levels of 90 dBA for insulated pipes in order to provide a

safe working environment. In ACC plants the far field requirements

will usually dictate the near field requirements. Far field

requirements of 60 dBA at 400 feet from duct may require near

field requirements of 85 dBA at 3 feet from duct. Since the duct is

not insulated, the noise performance of the bypass system must be

significantly lower than is applied in conventional power stations.

Figure 3: Compact dump element with elliptical or “fish mouth” discharge. These designs generate large noise at the surface of the ACC duct.

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In a bypass system, there will be a variety of service conditions

corresponding to the different plant operating modes. Typical

operating modes include full-load trip, duct firing, cold start, and

hot start. The duration and frequency of these operation modes

varies significantly, and the far field noise requirements for the plant

may be different for each operating mode. The noise requirements

and operating conditions for the bypass system must be completly

defined and reviewed to insure that plant noise requirements are

met. The noise requirements and operating conditions also have

a significant effect on the cost, size, and complexity of the bypass

system design.

Sources of Noise in ACC Systems

The noise from the bypass system comes from two primary sources,

the steam bypass control valve and the final dump element that

discharges all steam flow and spraywater flow into the ACC duct.

The sound power and peak frequency of each source must be

controlled in order to reduce overall system noise.

The dominant source in large power stations is the final dump

element in the bypass to condenser systems. The most common

dump element designs feature a large array of 12 mm or 6 mm

drilled holes, densely packed on a flat circular plate, an elliptical

fish mouth device, or a dump tube (Figures 3 and 4). These designs

can generate noise levels in excess of 130 dBA at a distance of 1m

from the ACC duct surface. The large amount of concentrated sound

power creates vibration that can cause cracks in the duct walls and

dump element mounting ring (Figure 5).

The noise generated by the dump element at the ACC duct surface

can be significantly reduced by using a combination of smaller

orifice sizes and multi-stage pressure reduction. Smaller orifice

sizes shift the peak frequency of jets discharging from the dump

element. Multi-stage pressure reduction reduces the discharge

velocity of jets on the surface of the dump element. In some cases

we must apply both approaches in order to achieve the necessary

noise performance. DRAG® multi-stage technology provides the best

possible noise performance in bypass to condenser applications

(Figure 6).

Total ACC Noise is the Result of Many Individual Noise Sources

Figure 4: Compact dump tube.

Figure 5: Cracks at a lifting hub on the surface of an ACC duct. The cracks were generated by the high power, low frequency jet generated by a compact dump element.

Figure 6: Comparison of the sound power and frequency spectrum for three dump element technologies. The DRAG® resistor combines a multi-stage pressure letdown design with frequency shifting to reduce overall system noise.

Noise vs Freqency, Drag Resistor and Dump Tubes

40.0

60.0

80.0

100.0

120.0

140.0

160.0

10 100 1000 10000 100000Frequency, Hz

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DRAG Resistor Low Noise Dump Tube Compact Dump Tube

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4

Total System Design

The overlay on pages 8 and 9 shows an illustration of

a typical bypass system. The bypass system includes

many elements, including the steam bypass control

valve, diffusers, one or more desuperheaters, and the

final dump element. The total system design must be

reviewed to meet noise requirements. Noise sources

upstream of the final dump element will transmit

downstream into the ACC duct. The steam bypass

control valve and diffusers may require multi-stage

technology.

In bypass to condenser applications the temperature

after desuperheating is saturated because typically

Low Noise Performance Requires a Total System Solution

Figure 7: Comparison of far field noise performance for a CCPS with ACC duct. The first figure shows the noise field around a plant when the plant is in normal operation, with 85 dBA ambient noise level. The second figure shows the noise field around a plant when the bypass system is in operation. The bypass system generates 117 dBA at 1m from the duct surface, and significant far field noise.

design temperatures for ACC ducting is around 120C

(250F). To control steam enthalpy to conditions acceptable

for ACC, steam is saturated at the higher pressures existing

upstream of the dump device. These applications require

very large amounts of spraywater, and the source for this

is often cold water from the condensate extraction pumps

(CEP). The design of the desuperheater, the velocities in

the pipe system, and spraywater control logic must be

carefully made to ensure reliable operation. Bypass to

condenser applications require consideration of total system

design and more so in air-cooled condensers where noise

requirements, control and evaporation of spray water are

required to be more stringent.

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Benefits of DRAG® Multi-Stage Technology

CCI designs and manufactures a unique technology that provides

the best possible noise performance. This technology is available for

the steam bypass valve trim and for the final dump element.

The DRAG® design divides the flow through the control valve or

dump element into hundreds of multi-path multi-stage streams.

Each flow path consists of a specific number of right angle turns.

These flow paths establish a tortuous path, and each turn reduces

the pressure of the flowing medium. The pressure drop on the last

stage of a DRAG® disk is many times less than the pressure drop

on a single-stage orifice. With this technology we can specify the

necessary number of stages to achieve plant noise requirements. CCI

can provide this technology both within the control valve trim and

in the final dump element in the ACC duct.

The DRAG® resistor provides additional benefits in bypass to

condenser applications. The steam entering the condenser dump

element is typically wet steam, with 95% to 97% quality. Multi-

Stage conventional drilled hole dump devices are not recommended

as they will gradually be eroded by impinging high velocity wet

steam jets from the individual stages onto the material (diffuser) of

the next stage. DRAG® velocity control protects the dump element

from wet steam erosion, and stainless steel construction of the disks

ensures long service life. The DRAG® resistor also gives much greater

pipe and system design flexibility. The DRAG® resistor can provide

lower system noise with much higher inlet pressures. This gives

plant designers the flexibility to specify higher pressures and smaller

pipes sizes for the intermediate pipe between the bypass valve

and dump element. It also gives the bypass system designer more

flexibility to optimize system velocities for improved noise control

and desuperheating.

Special DRAG Hex Resistors

The DRAG® resistor disks for bypass to condenser applications

are assembled from hundreds of disk strips. The disk strips are

held together using a series of pins that cross link the strips. This

unique design provides the durability and toughness required to

withstand the dynamic forces that act on the resistor during a full-

load trip. The disks are manufactured from 12 chrome stainless

steel, which resists the thermal gradients and erosion from steam

quality variations associated with condenser discharge systems. The

disks use a special version of the DRAG® flow path that has been

optimized for discharge to the condenser applications.

Figure 8: Image of a typical DRAG® resistor for HRH bypass air-cooled condensers.

Figure 9: DRAG® multi-stage valve trim minimizes noise generation through velocity control.

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6 DRAG® Resistor – Dump Element Incorporating DRAG® Technology

Table 1: Standard DRAG® Resistor Configurations

Notes:The size of the DRAG® resistor may require the use of a bell housing to avoid excessive ACC duct blockage. - The bell housing diameters above assume that the DRAG® resistor is 100% contained in the bell housing and assumes an ACC duct pressure of 2 psia (.13 bara), and an enthalpy of 1170 BTU/lbm ( 2720 kJ/kg). - The bell housing diameter may be reduced if the DRAG® resistor is only partially contained.

HRH Bypass Steam Flow (excl spray water)

NominalDiameter (D

N)

Resistor Height (HR)

Max Resistor Diameter (DMAX)

Bell Housing Diameter

100000 - 300000 lbm/hr(45450 - 136360 mt/hr)

24” (61 cm)

33” (82 cm)

40” (102 cm)70” – 100”

(180 – 254 cm)

39” (99 cm)

47” (120 cm)

54” (137 cm)

175000 - 450000 lbm/hr(79545 - 204550 mt/hr)

30” (76 cm)

40” (102 cm)

44” (112 cm)85” – 125”

(216 – 318 cm)

48” (122 cm)

55” (140 cm

64” (163 cm)

300000 - 675000 lbm/hr(136360 - 306820 mt/hr)

36” (91 cm)

49” (125 cm)

51” (130 cm)105” – 150”

(267 – 381 cm)

57” (145 cm)

66” (168 cm)

76” (193 cm)

450000 - 900000 lbm/hr(181800 - 450000 mt/hr)

42” (107 cm)

57” (145 cm)

60” (154 cm)125” – 175”

(318 – 445 cm)

66” (168 cm)

76” (193 cm)

86” (219 cm)

Figure 10: Schematic of a standard DRAG® resistor and a typical bell housing assembly.

Small Diameter Drilled-Hole Technology

Small diameter drilled-hole valve trim and flow diffusers greatly minimize audible noise generation by breaking up large diameter jets and frequency shifting.

DRAG® Technology

CCI’s DRAG® multi-stage valve trim

minimizes noise generation through velocity control.

Alternate Configurations

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HRH Bypass Steam Flow (excl spray water)

NominalDiameter (D

N)

Resistor Height (HR)

Max Resistor Diameter (DMAX)

Bell Housing Diameter

100000 - 300000 lbm/hr(45450 - 136360 mt/hr)

24” (61 cm)

33” (82 cm)

40” (102 cm)70” – 100”

(180 – 254 cm)

39” (99 cm)

47” (120 cm)

54” (137 cm)

175000 - 450000 lbm/hr(79545 - 204550 mt/hr)

30” (76 cm)

40” (102 cm)

44” (112 cm)85” – 125”

(216 – 318 cm)

48” (122 cm)

55” (140 cm

64” (163 cm)

300000 - 675000 lbm/hr(136360 - 306820 mt/hr)

36” (91 cm)

49” (125 cm)

51” (130 cm)105” – 150”

(267 – 381 cm)

57” (145 cm)

66” (168 cm)

76” (193 cm)

450000 - 900000 lbm/hr(181800 - 450000 mt/hr)

42” (107 cm)

57” (145 cm)

60” (154 cm)125” – 175”

(318 – 445 cm)

66” (168 cm)

76” (193 cm)

86” (219 cm)

7Preferred System Configuration

Small Diameter Drilled-Hole Technology

Small diameter drilled-hole valve trim and flow diffusers greatly minimize audible noise generation by breaking up large diameter jets and frequency shifting.

DRAG® Multi-Stage Dump Device

CCI’s DRAG® multi-stage technology incorporated into a condenser dump device.

Total System Design

For every bypass system, CCI performs a complete system noise analysis using industry standard IEC & ISA calculation methods, optimizing system geometry and intermediate operating conditions to intelligently manage steam velocity and minimize noise generation in regions of area expansion.

Closed-Coupled Horizontal Piping Arrangement

Installing the bypass valve and desuperheater horizontially and close to the ACC duct eliminates the need for pipe elbows. This provides the simplest solution for system control and minimizes the risk of wet steam erosion.

DRAG® Technology

CCI’s DRAG® multi-stage valve trim

minimizes noise generation through velocity control.

SUMMARY

ACC plants can be a noise problem because:

n Turbine bypass systems dump into a large-diameter, uninsulated, thin-walled duct.

n They are commonly located very close to residential areas.

Total ACC noise is a product of many individual sources:

n Bypass valves

n Regions of area expansion

Low noise performance requires a total system solution:

n DRAG® Multi-Stage Valve Trim

n Small-Drilled-Hole Diffusers

n DRAG® Multi-Stage Dump Device

n Intelligently designed system geometryn Dump Devices

Alternate Configurations

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Two-Stage Desuperheating

In some situations, it is necessary to break up the

desuperheating into two separate stages. This is due to

the fact that turbine bypass systems, especially IP bypass

systems, operate with wet steam downstream of the

desuperheater. The system geometry determines if two-

stage desuperheating is necessary. This includes:

Systems with long outlet pipe runs: Long pipe runs

flowing wet steam lead to excess spraywater fallout and can

lead to a water hammer effect on the dump element.

Systems with pipe elbows: Pipe elbows not only increase

spraywater fallout, but are also very prone to erosion

caused by water droplets in the wet steam flow. In addition,

elbows located close to the dump element can lead to non-

uniform temperature gradients that can cause damage.

Two-stage desuperheating works by splitting the

desuperheating to maintain superheated steam in the

intermediate piping before the ACC duct. This minimizes

the risks associated with flowing wet steam. The remainder

of the spraywater is injected immediately before the

condenser dump element.

8Alternate Configuration: Two-Stage Desuperheating

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For more information, refer to the following

documents:

CCI Installation Guidelines

CCI Preventative Maintenance Program

Preventative Maintenance Program for Turbine Bypass Systems

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9Alternate Configuration: Two-Stage Desuperheating

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