ACC Duct Dumping

(1)

ASME

Low Noise

Solutions for

Turbine Bypass

to Air-Cooled

Condensers

(2)

Lo

w N

o

is

e S

o

lu

ti

on

s fo

r T

u

rb

in

e B

yp

as

s

to A

ir

-C

oo

le

d C

on

d

en

se

rs

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.

(3)

Lo

w N

o

is

e S

o

lu

tion

s fo

r T

u

rb

in

e B

yp

as

s

to A

ir-C

oo

le

d C

on

d

en

se

rs

3

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 100000 Frequency, Hz Sound Power, dB

(4)

Lo

w N

o

is

e S

o

lu

ti

on

s fo

r T

u

rb

in

e B

yp

as

s

to A

ir

-C

oo

le

d C

on

d

en

se

rs

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.

(5)

Lo

w N

o

is

e S

o

lu

tion

s fo

r T

u

rb

in

e B

yp

as

s

to A

ir-C

oo

le

d C

on

d

en

se

rs

5

DRAG

®

_{Multi-Stage Technology}

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.

(6)

Lo

w N

o

is

e S

o

lu

ti

on

s fo

r T

u

rb

in

e B

yp

as

s

to A

ir

-C

oo

le

d C

on

d

en

se

rs

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)

Nominal Diameter (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.

(7)

Lo

w N

o

is

e S

o

lu

tion

s fo

r T

u

rb

in

e B

yp

as

s

to A

ir-C

oo

le

d C

on

d

en

se

rs

HRH Bypass Steam Flow (excl spray water)

Nominal Diameter (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) 7

Preferred 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 geometry

n Dump Devices

(8)

Lo

w N

o

is

e S

o

lu

tion

s fo

r T

u

rb

in

e B

yp

as

s

to A

ir-C

oo

le

d C

on

d

en

se

rs

8 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.

(9)

Lo

w N

o

is

e S

o

lu

tion

s fo

r T

u

rb

in

e B

yp

as

s

to A

ir-C

oo

le

d C

on

d

en

se

rs

9

For more information, refer to the following documents:

CCI Installation Guidelines CCI Preventative Maintenance Program Preventative Maintenance Program for Turbine Bypass Systems