bypass system

ANSI/ISA–77.13.01–1999 (R2008)

Fossil Fuel Power Plant Steam Turbine Bypass System

Reaffirmed 30 December 2008

Copyright International Society of Automation Provided by IHS under license with ISA Licensee=SK Engineering & Construction/5984034001, User=kumar, vikas

Not for Resale, 02/18/2014 21:25:24 MSTNo reproduction or networking permitted without license from IHS

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Copyright 2008 ISA. All rights reserved.

ANSI/ISA–77.13.01–1999 (R2008) Fossil Fuel Power Plant Steam Turbine Bypass System

ISBN: 978-1-934394-74-8

Copyright © 2008 by ISA. All rights reserved. Not for resale. Printed in the United States of America. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of the Publisher.



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Preface

This preface, as well as all footnotes and annexes, is included for information purposes and is not part of ANSI/ISA–77.13.01–1999 (R2008).

This document has been prepared as part of the service of ISA towards a goal of uniformity in the field of instrumentation. To be of real value, this document should not be static but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms and asks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alexander Drive; P. O. Box 12277; Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288; E-mail: [email protected].

The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Units (SI) in particular, in the preparation of instrumentation standards. The Department is further aware of the benefits to USA users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, this Department will endeavor to introduce SI-acceptable metric units in all new and revised standards, recommended practices, and technical reports to the greatest extent possible. Standard for Use of the International System of Units (SI): The Modern Metric System, published by the American Society for Testing & Materials as IEEE/ASTM SI 10-97, and future revisions, will be the reference guide for definitions, symbols, abbreviations, and conversion factors.

It is the policy of ISA to encourage and welcome the participation of all concerned individuals and interests in the development of ISA standards, recommended practices, and technical reports. Participation in the ISA standards-making process by an individual in no way constitutes endorsement by the employer of that individual, of ISA, or of any of the standards, recommended practices, and technical reports that ISA develops.

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EVEN IF ISA IS UNAWARE OF ANY PATENT COVERING THIS DOCUMENT, THE USER IS CAUTIONED THAT IMPLEMENTATION OF THE DOCUMENT MAY REQUIRE USE OF TECHNIQUES, PROCESSES, OR MATERIALS COVERED BY PATENT RIGHTS. ISA TAKES NO POSITION ON THE EXISTENCE OR VALIDITY OF ANY PATENT RIGHTS THAT MAY BE INVOLVED IN IMPLEMENTING THE DOCUMENT. ISA IS NOT RESPONSIBLE FOR IDENTIFYING ALL PATENTS THAT MAY REQUIRE A LICENSE BEFORE IMPLEMENTATION OF THE DOCUMENT OR FOR INVESTIGATING THE VALIDITY OR SCOPE OF ANY PATENTS BROUGHT TO ITS ATTENTION. THE USER SHOULD CAREFULLY INVESTIGATE RELEVANT PATENTS BEFORE USING THE DOCUMENT FOR THE USER’S INTENDED APPLICATION.

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THE USER OF THIS DOCUMENT SHOULD BE AWARE THAT THIS DOCUMENT MAY BE IMPACTED BY ELECTRONIC SECURITY ISSUES. THE COMMITTEE HAS NOT YET ADDRESSED THE POTENTIAL ISSUES IN THIS VERSION.



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ANSI/ISA–77.13.01–1999 (R2008) - 4 -


The following people served as members of ISA77.13:

NAME COMPANY

J. Schleis, Chairman Wood Group Turbine Control Services, Inc. W. Holland, Managing Director Southern Company E. Adamson Invensys C. Fernandez-Varela Comision Federal de Electricidad H. Foreman Power Consultants K. Gabel Centerior Energy M. Kuffer NEOTECHA AG D. Lee ABB Automation, Inc. H. Maxwell Bechtel Power Corporation I. Mazza Trasferimento Di Tech G. Mookerjee U.S. Department of Energy R. Papilla Edison O&M Services K. Schoonover Con-Tek Valves, Inc. L. Stratton Control Components, Inc. I. Verhappen MTL Instrument Group D. Younie Wood Group Turbine Control Services, Inc. The following people served as voting members of ISA77:

NAME COMPANY

G. McFarland, Managing Director Emerson Process Mgmnt Power & Water Sol. D. Roney, Chair URS – Washington Division L. Altcheh Israel Electric Corporation S. Alvarez Compania Inspeccion Mexicana J. Batug PP&L, Inc. D. Christopher Consultant G. Cohee Applied Control Systems D. Crow Invensys R. Eng Hitachi Power Systems America A. Gavrilos ABB, Inc. J. Gillman JFG Technology Transfer LLC W. Hocking Invensys Process Systems W. Holland CH2M Hill R. Hubby Consultant R. Johnson Consultant D. Lee ABB Automation, Inc. G. Mookerjee Detroit Edison Company J. Olson Tennessee Valley Authority P. Reeves TXU Sandow M. Skoncey First Energy Generation Corporation T. Stevenson Constellation Energy C. Taft Consultant A. Zadiraka The Babcock & Wilcox Company



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This standard was approved for publication by the ISA Standards and Practices Board on 30 December 2008.

NAME COMPANY

T. McAvinew, Chair Jacobs Engineering Group M. Coppler Ametek, Inc. E. Cosman The Dow Chemical Co. B. Dumortier Schneider Electric D. Dunn Aramco Services Co. J. Gilsinn NIST W. Holland Consultant E. Icayan ACES, Inc. J. Jamison Consultant K. Lindner Endress & Hauser Process Solutions AG V. Maggioli Feltronics Corp. A. McCauley, Jr. Chagrin Valley Controls, Inc. G. McFarland Emerson Process Management R. Reimer Rockwell Automation N. Sands E I du Pont H. Sasajima Yamatake Corp. T. Schnaare Rosemount, Inc. J. Tatera Consultant I. Verhappen MTL Instrument Group R. Webb Consultant W. Weidman Parsons Energy & Chemicals Group J. Weiss Applied Control Solutions LLC M. Widmeyer Consultant M. Zielinski Emerson Process Management



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CONTENTS

1 Scope ...................................................................................................................................................9

2 Purpose ................................................................................................................................................9

3 Definitions.............................................................................................................................................9

4 Bypass systems..................................................................................................................................11

4.1 General requirements ....................................................................................................................11

4.2 Elements ........................................................................................................................................12

4.3 Capacity .........................................................................................................................................13

4.4 Design requirements ......................................................................................................................13

4.5 Turbine bypass instrumentation.....................................................................................................21

4.6 Control and logic requirements ......................................................................................................23

4.7 Alarm requirements........................................................................................................................25

4.8 Operator interface ..........................................................................................................................26

Annex A — References...............................................................................................................................29

Annex B — The use of bypass systems .....................................................................................................31

Annex C — Valve life expectancy...............................................................................................................33

Annex D — Some typical high- and low-pressure turbine bypass valve ....................................................34

Annex E — Figures .....................................................................................................................................35



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1 Scope

This standard covers the design requirements and operator interface for steam turbine bypass systems for drum and once-through steam generators and combined cycle plants. Hardware configurations are suggested to obtain the minimum design requirements to obtain a safe and operable system. Both fixed percentage bypass and variable pressure systems are covered.

It is applicable to boilers with steam capacities of 200,000 lb/hr (25 kg/s) or greater.

2 Purpose

This standard establishes the minimum requirements for design specifications to implement steam turbine bypass systems and hardware configurations for drum and once-through, fossil fuel power plant boilers.

The turbine bypass system should provide for cold start-up, warm start, hot restart, load rejection, turbine shutdowns, and unit trips. The system shall be designed to provide pressure, temperature, and flow control of steam around and through the turbine by controlling each bypass valve, isolation valve, and associated desuperheater. The desuperheating function may be integral with the bypass valve. The turbine bypass system does not interface with the turbine control and supervisory system. The turbine bypass system is set to maintain steam pressure, and any coordination with the turbine is through interaction with the process as the turbine demands more or less steam. There is no direct interconnection of control systems.

3 Definitions

The following definitions are included to clarify their use in this standard and may not correspond to the use of the word in other texts. For other definitions, see ISA-51.1, Process Instrumentation Terminology.

3.1 alarm: an indication used to alert an operator about an abnormal operating condition.

3.2 automatic tracking: the action of a control system to automatically track a setpoint or the process variable without any other corrective mechanisms.

3.3 boiler: the entire vessel in which steam or other vapor is generated for use external to itself, including the furnace, consisting of waterwall tubes; the firebox area, including burners and dampers; the convection area, consisting of any superheater, reheater, and/or economizer sections as well as drums, generating tubes, and headers.

3.4 condenser backpressure elements: a multiple breakdown diffuser, normally installed in the steam condenser neck, used to generate a positive back pressure upstream of the condenser vacuum and to reduce the kinematic energy of steam from an external source other than the turbine exhaust.

3.5 controller: any automatic, semi-automatic, or manual device or system of devices used to regulate the boiler turbine, or any other equipment within defined parameters. If automatic, the device or system responds to variations in temperature, pressure, water level, flow, or other control variables.

3.6 differential producer: a measuring element that is inserted in a process flow path and used to create a pressure drop that is proportional to the square of the volumetric flow rate.



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3.7 fail safe: the capability to go to a predetermined safe state in the event of a specific malfunction.

3.8 fault tolerant: built-in capability of a system to provide continued, correct execution of its assigned function in the presence of a hardware and/or software fault.

3.9 integral windup: the saturation of the integral controller output, in the presence of a continuous error, which may cause unacceptable response in returning the process to its setpoint within acceptable limits of time and overshoot.

3.10 load: a device that receives power or that power which is delivered to such a device, as in the rate of output, lb/hr (kg/s) of steam or megawatts (kilowatts) of electrical generations.

3.11 logic system: decision-making logic equipment with its associated power supplies, I/O hardware, and sensing devices.

3.12 mode (submode): a particular operating condition of a control system, such as manual, automatic, remote, or coordinated.

3.13 redundant (redundancy): the duplication or repetition of elements in electrical or mechanical equipment to provide alternative functional channels in case of failure of one channel.

3.14 severe duty valve: a mission-critical valve, typically seeing high-pressure drop service, which may see cavitating or flashing fluids, or if not properly designed, may see early trim erosion, vibration, or excess noise.

3.15 shall, should, and may: the word “SHALL” is to be understood as a REQUIREMENT; the word “SHOULD” as a RECOMMENDATION; the word “MAY” as a PERMISSIVE, neither mandatory nor recommended.

3.16 steam quality: the ratio of the vapor’s mass to the mixture’s mass.

3.17 turbine: a machine that converts energy from a moving fluid into rotating mechanical energy that drives a load. In a power plant, a turbine converts energy in the steam into mechanical energy to drive an electric generator (the mechanical load).

3.18 turbine governor valves: the primary control valves used to regulate the flow of steam through the turbine during normal operation.

3.19 turn-down ratio: the ratio from maximum operating to minimum operating conditions, providing a controllable or measurable span. The device must perform over this range.



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4 Bypass systems

4.1 General requirements

The steam turbine bypass system requirements are defined for those components and logic systems necessary to handle steam to and around the turbine.

The design of systems to prevent water damage to steam turbines is covered by the American Society of Mechanical Engineers (ASME) standard TDP-1.

The steam should be of a minimum quality of 92 percent to avoid impingement and corrosion.

Figures E.1 and E.2 show a typical turbine steam bypass system to assist in explaining the design requirements of this clause. Other figures show specific areas to help clarify the text materials.

This standard will cover the following functions:

a) Matching with an acceptable difference the metal to steam temperatures before steam is admitted to the turbine

b) Handling the difference between the generated and consumed steam flows during transient conditions

4.1.1 High-pressure (HP) bypass system

The HP bypass system shall fulfill the following requirements:

a) Control the pressure of the steam bypassing the HP turbine

b) Control the pressure of the main steam from the boiler

c) Control the flow and temperature of steam through the cold reheat line to cool the boiler reheater tubing

d) Control the flow of steam through the main steam line to cool the boiler final superheater in case of sliding pressure operation

e) Prevent lifting of main steam and hot reheat safety valves during transient operations

4.1.2 Intermediate-pressure (IP) and low-pressure (LP) bypass system

The IP and LP bypass system shall fulfill the following requirements:

a) Control the pressure of the steam bypassing the IP and LP turbines

b) Control the pressure and temperature of the hot reheat steam from the boiler

c) Prevent the lifting of hot reheat safety valves during transient operations

d) Protect the condenser against excessive pressure, temperature, and steam kinematic energy



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4.2 Elements

The elements that make up the steam turbine bypass system are those added over the elements required for a system without the ability to bypass steam around the HP, IP, and LP turbines. Therefore, those elements that are added due to the bypass are shown in figures E.1 and E.2.

4.2.1 HP bypass elements

a) HP bypass control valve (HPB) (see notes 1 and 3)

b) Before HP bypass control valve, block valve (B) (see note 3)

c) HP bypass desuperheater (DES) (see note 1)

d) HP bypass desuperheater spray water control valve (SPV) (see notes 2 and 3)

e) HP bypass desuperheater spray water block valve (B) (see note 3)

f) HP turbine cold reheat, non-return valve (NRV)

g) HP turbine bypass control system

h) HP turbine bypass instrumentation

i) HP turbine reverse flow valve (see note 4)

NOTE 1 — The pressure control valve and desuperheater may be combined into one HP turbine mainstream bypass pressure control and desuperheating valve as shown in figure E.2.

NOTE 2 — The desuperheater may have an integral spray water control valve.

NOTE 3 — Block valves are usually required to assure that leakage does not occur through the control valve. This function could be integrated in a composite control valve, provided the block valve’s purpose is not compromised.

NOTE 4 — The reverse flow valve may be necessary to prevent excessive windage heating of the HP turbine blades during a hot restart if initial loading is accomplished using the LP/IP sections.

4.2.2 IP and LP elements

a) IP/LP bypass control valve (IP/LPB) (see notes 1 and 3)

b) Before IP/LP bypass control valve, block valve (B) (see note 3)

c) IP/LP bypass desuperheater (DES) (see note 1)

d) IP/LP bypass desuperheater spray water control valve (SPV) (see notes 2 and 3)

e) IP/LP bypass desuperheater spray water block valve (B) (see note 3)

f) Condenser back pressure elements (I)

g) IP/LP pressure bypass control system

h) IP/LP bypass instrumentation



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NOTE 1 — The pressure control valve and desuperheater may be combined into one IP and LP turbine hot reheat bypass pressure control and desuperheating valve as shown in figure E.2.

NOTE 2 — The desuperheater may have an integral spray water control valve.

NOTE 3 — Block valves are usually required to assure that leakage does not occur through the control valve. This function could be integrated in a composite control valve, provided the block valve’s purpose is not compromised.

4.3 Capacity

The steam flow capacity of the bypass system is governed by a number of other variables in the overall steam system. These are

a) heat distribution in the boiler;

b) turbine rotor diameter;

c) condenser internals;

d) startup, loading, unloading, and shutdown practices and requirements for the unit;

e) safety considerations; and

f) economics.

4.3.1 System size

There are a number of possibilities to size the bypass system. In this document the bypass system size or capacity to fulfill the two functions previously defined in 4.1 are as follows:

a) A bypass system that matches the steam-to-turbine metal temperatures should be sized for 15 percent of maximum continuous rated (MCR) flow at valves wide open. This system reduces the startup time by about 30 minutes.

b) A bypass system that handles the difference between the generated and consumed steam flows during upset or transient conditions should handle 40 percent of MCR flow at valves wide open or should have a greater size range.

c) A bypass system that keeps the steam generator running at full load without blowing the safety valves in case of a turbine or generator trip at full load should handle 100 percent of MCR flow at valves wide open.

4.4 Design requirements

The design requirements for each of the elements are as follows.

4.4.1 HP and IP/LP turbine bypass control valve

a) Design criteria The design criteria for the bypass system valves shall be specified as shown in tables 4.4.1(a) and 4.4.1(b).



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Table 4.4.1(a) — HP turbine bypass control valve

Severe duty, pressure type or pressure- and temperature-reducing type (see note 2)

Operating Conditions The coordinated inlet and outlet pressure, temperature, and flow conditions for all distinct service situations

Inlet and Outlet Design Pressure

The maximum design pressure for the inlet and outlet

Inlet and Outlet Design Temperature

The maximum design temperature for the inlet and outlet

Inlet Pipe Size (Internal Diameter [ID]) and Material

The same as the piping just before the valve

Outlet Pipe Size (Internal Diameter [ID]) and Material

The same as the piping just after the valve

Noise Level The desired maximum noise level at 1.0 meter (3.28 feet) from the valve (see ISA-75.07-1997)

Shutoff Leakage Class The desired shutoff class at the design pressure and temperature rating of the valve — usually Class V of ANSI/FCI 70.2

Travel Time The desired minimum travel time from any position including fully closed at the maximum operating pressure

Fail State The HP bypass valve is normally a fail-closed valve (see note 1.)

Modulating Time The maximum time for full stroke operation

Turndown Ratio The full range of coordinated operating parameters of flow, pressure, and temperature

Quick Opening Time The travel time for a valve that is provided with quick opening devices

NOTE 1 — If the applicable pressure vessel code permits, this valve can also be used as a fail-open safety relief. An “HP to condenser” valve is preferably fail closed.

NOTE 2 — If the control valve has an integral water injection system, the average droplet size and distribution and the evaporation length (90% of total droplet mass) shall be provided by the manufacturer.



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Table 4.4.1(b) — IP and LP turbines hot reheat bypass control valve

Severe duty, pressure type or pressure- and temperature-reducing type with stem sealing for condenser vacuum if required (see note 2 from previous page)

Operating Conditions The coordinated inlet and outlet pressure, temperature, and flow conditions for all distinct service situations

Inlet and Outlet Design Pressure The maximum design pressure for the inlet and outlet

Inlet and Outlet Design Temperature

The maximum design temperature for the inlet and outlet


The same as the hot reheat piping just before the valve


The same as the hot reheat piping just after the valve

Noise Level The desired maximum noise level at 1.0 meter (3.28 feet) from the valve (see ISA-75.07-1997)

Shutoff Leakage Class The desired shutoff class at the desired design pressure and temperature rating of the valve — usually Class V of ANSI/FCI 70.2

Travel Time The desired minimum travel time from any position, including fully open, at the maximum operating pressure

Fail State The IP and LP turbines bypass valve is preferably a fail-closed design

Modulating Time The maximum time for full-stroke operation


Condenser Back Pressure The maximum back pressure for valve operation at a specified pressure, temperature, and Flow

NOTE — If the code permits, this valve can also be used as a fail-open safety relief. An “HP to condenser” valve is preferably fail closed.

b) Control valve actuator The type, motive power, material, function, and design of the actuator desired to meet the requirements of the specific control valve should be either specified or left to the manufacturer.

c) Control valve position indication The number, type, and operation of position switches and transmitters should be specified.

d) Stem, trim, packing, and seat The material and design of the stem, trim, packing, and seat shall be specified or left to the manufacturer. The design and materials shall be suitable to resist mechanical, thermal, and fluid induced wear.

e) Valve stem vacuum sealing connection (applies to IP and LP bypass control valve if required by specific design) The size, type, and materials for the connection should be specified or left to the manufacturer.

f) End-connection preparation The end preparation for the valves should be specified with the weld-end or flange details.



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g) Cycles The number of cycles per month and modes of operation along with the temperature gradients should be specified.

h) Trim and valve outlet kinematic energy Trim and valve outlet velocities shall be specified to reduce noise, vibrations, and erosion.

4.4.2 HP and IP/LP bypass block valve

a) Design criteria The design criteria for the HP and IP/LP bypass block valves shall be specified as shown in table 4.4.2.

Table 4.4.2 — Bypass block valves

These are non-pressure-reducing type valves (see note 1).

Operating Pressure Same as control valve

Operating Temperature Same as control valve

Design Pressure Same as control valve

Design Temperature Same as control valve

Before Block Valve Inlet and Outlet Pipe Size (Internal Diameter [ID]) and Material

Same as or larger than the inlet of the control valve

Shutoff Leakage Class Shall be per MSS SP-61, Class V

Travel Time Required minimum and maximum travel time from one end to the other shall be specified

Fail State Follow control valve fail state

NOTE — Block valves are usually required to assure that leakage does not occur through the control valve. This function could be integrated in a composite control valve provided the block valve’s purpose is not compromised.

b) Actuator The type, motive power, material, and design of the actuator desired to meet the opening and closing time should be specified or left to the manufacturer.

c) Block valve position indication The number, type, and operation of position limit switches and transmitters should be specified.

d) Stem, disk, packing, and seat The material and design of the stem, disk, packing, and seat should be specified or left to the manufacturer. The design and materials shall be suitable to resist mechanical, thermal, and fluid induced wear.



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e) Stem vacuum sealing connection (intermediate and low pressure bypass block valves only, if required) The size, type, and materials for the connection to the valves should be specified for vacuum sealing or left to the manufacturer.

f) End-connection preparation The end preparation should be specified with the weld-end or flange details.

g) Pressure loss The bypass block valves should be specified to have a low pressure loss and minimum flow disturbance to minimize operational instability of the downstream piping and valves.

4.4.3 Desuperheater

The design criteria for the main steam and hot reheat steam desuperheaters shall be specified as shown in table 4.4.3. The desuperheater may have an integral spray water control valve. The design parameters that affect the time/distance required for evaporation of the water spray include

a) the nozzle design and associated droplet size distribution;

b) the relative location of associated nozzles and their direction of spray into the superheated steam flow path;

c) the amount of initial and final superheat available in the steam/water mixture;

d) the steam and droplet velocity and degree of turbulence in the mixing region;

e) the amount and temperature of the injected water; and

f) the water surface tension and viscosity.

Table 4.4.3 — Desuperheater

Body Material The body material shall be specified for each desuperheater

Internal Components The material for the internal components shall be specified or left to the manufacturer

Type The type of desuperheater shall be specified

Mounting The position in the bypass piping shall be specified

Operating Conditions The operating, design, and boiler hydro test pressures and temperatures shall be specified

Temperature Control Range The temperature reduction differential at various steam flow and pressure conditions shall be specified

Average Droplet Size and Distribution

The average droplet size and distribution shall be provided by the manufacturer for each set of operating conditions

Evaporation Length (90% of Total Droplet Mass)

The evaporation length shall be provided by the manufacturer for each set of operating conditions

End Connection Preparation The end preparation shall be specified, along with the weld-end or flange details



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4.4.4 Desuperheater spray water control valve

a) Design criteria The design criteria for the desuperheater spray water control valve shall be specified as shown in table 4.4.4.

Table 4.4.4 — Desuperheater spray water control valve

This is a pressure-reducing-type valve.

Operating Conditions The coordinated inlet and outlet pressure, temperature, and flow conditions for all distinct service conditions

Design Pressure The maximum design pressure for the system The maximum design pressure for the system

Design Temperature The maximum design temperature for the system


The same as the piping just before the valve


The same as the piping just after the valve

Noise Level The desired maximum noise level at 1.0 meter (3 feet) from the valve (see ISA-75.07-1997)

Shutoff Leakage Class The desired shutoff class at the desired design pressure and temperature rating of the valve—usually Class V of ANSI/FCI 70.2

Travel Time The desired minimum travel time from any position, including fully open, at the maximum operating pressure

Fail State The valve shall be coordinated with the fail state of the HP bypass and IP/LB bypass valves.

Modulating Time The maximum time for full-stroke operation


b) Control valve actuator

The type, motive power, material, function, and design desired to meet the requirements of the control valve should be either specified or left to the manufacturer.

c) Control valve position indication The number and type of position limit switches and transmitters should be specified.

d) Stem, trim, packing, and seat The material and design of the stem, trim, packing, and seat shall be specified or left to the manufacturer. The design and materials shall be suitable to resist mechanical, thermal, and fluid induced wear.

e) End-connection preparation The end preparation for the valves should be specified along with the weld-end or flange details.



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4.4.5 Desuperheater spray water block valve

a) Design criteria The design criteria for the desuperheater spray water block valve shall be specified as shown in table 4.4.5.

Table 4.4.5 — Desuperheater spray water block valve

These valves are non-pressure-reducing type (see note 1).

Operating Pressure Same as control valve

Operating Temperature Same as control valve

Design Pressure Same as control valve

Design Temperature Same as control valve

Before Block Valve Inlet and Outlet Pipe Size (Internal Diameter [ID]) and Material

Same as for the inlet of the control valve

Noise Level Same as control valve

Shutoff Leakage Class Shall be per MSS SP-61

Travel Time Required minimum and maximum from one end to the other shall be specified

Fail State Follow control valve fail state

NOTE — The desuperheater spray water block valve is required for water induction prevention. This isolation function may be integrated into a composite control valve provided that the block valve’s protection function is not compromised.

b) Actuator The type, motive power, material, and design of the actuator desired to meet the opening and closing time should be specified or left to the manufacturer.

c) Block valve position indication The number, type, and operation of position limit switches and transmitters should be specified.

d) Stem, disk, packing, and seat The material and design of the stem, disk, packing, and seat should be specified or left to the manufacturer. The design and materials shall be suitable to resist mechanical, thermal, and fluid induced wear.

e) End-connection preparation The end preparation should be specified, along with the weld-end or flange details.

f) Pressure loss The bypass block valves should be specified to have a low pressure loss and minimum flow disturbance to minimize operational instability of downstream piping and valves.



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4.4.6 Cold reheat non-return valve

a) Design criteria The design criteria for the cold reheat non-return valve shall be specified as shown in table 4.4.6.

Table 4.4.6 — Cold reheat non-return valve

This is a non-pressure-reducing-type valve.

Operating Pressure Same as HP bypass control valve

Operating Temperature Same as HP bypass control valve

Design Pressure Same as HP bypass control valve

Design Temperature Same as HP bypass control valve

Internal Diameter Same as for cold reheat piping

Noise Level Same as for HP bypass control valve

Shutoff Leakage Class Shall be per MSS SP-61

Travel Time Required minimum from one end to the other shall be specified

Material Required materials shall be specified

b) Actuator

The type, motive power, material, and design of the actuator desired to meet the opening and closing time should be specified or left to the manufacturer.

c) Non-return position indication The number, type, and operation of position limit switches should be specified.

d) Shaft, disk, packing, and seat The material and design of the shaft, disk, packing, and seat should be specified or left to the manufacturer. The design and materials shall be suitable for the mechanical, thermal, and fluid induced wear.

e) Counterweight The size, type, and materials for the counterweights should be specified or left to the manufacturer.

f) End-connection preparation The end preparation should be specified along with the weld-end or flange details.

g) Pressure loss The valve should be specified to have a very low pressure loss and minimum flow disturbance to minimize operational instability of the downstream piping and valves.

4.4.7 Desuperheater spray water filter

If the desuperheater spray water control valve does not include filtering, a dual element, full flow filter should be specified. Differential pressure detection, measurement, and alarm should be included.



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4.4.8 Condenser element

The design criteria for the condenser pressure element to withstand impingement and erosion from the wet steam should be specified as follows (see EPRI Report CS-2251):

a) Steam quality (The minimum expected quality of the steam should be specified.)

b) Steam dump condenser entry (The preferred location for the bypass steam should be left to the condenser manufacturer.)

c) The desired level of backpressure to be generated at a specific pressure, temperature, and flow condition.

4.5 Turbine bypass instrumentation

The design criteria for the turbine bypass high, intermediate, and low pressure instrumentation should be specified as follows.

4.5.1 Instrumentation components

Instrumentation components furnished with the equipment shall be in accordance with the following articles and shall be constructed to withstand high vibration and high temperatures encountered in the actual service. Explosion-proof construction shall be furnished where required by applicable code.

4.5.1.1 Limit switches

Limit switches, except those integrally mounted on motor-operated valves, shall be specified.

4.5.1.2 Pressure elements

Pressure elements shall be specified.

4.5.1.3 Temperature elements

Temperature elements shall be specified.

4.5.1.4 Solenoid valves

Solenoid valves shall be specified. Valves shall be selected based on body construction, trim materials, packing, and internal arrangements suitable to the application. Solenoid enclosures shall be NEMA 4 unless otherwise required. Solenoid coils shall be Class H, high-temperature construction or Class F, as applicable, and shall be suitable for continuous duty.

4.5.1.5 Pressure gauges

Gauges for control air supply and signal pressures integral to the instrument shall be in accordance with the manufacturer’s standards. All other gauges shall be specified as needed.

4.5.1.6 Thermometers

Thermometers for local mounting shall be specified as needed.



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4.5.1.7 Thermowells

Fluid system temperature sensors shall be equipped with thermowells. The thermowell’s design shall be certified acceptable for the maximum conditions of temperature, pressure, type of fluid, and fluid velocity by methods described in ASME Performance Test Code 19.3. Thermowells shall be welded or threaded and, if threaded, constructed to allow seal welding after installation. Threaded thermowells shall be a minimum of 3/4 inch NPT. Thermowell insertion length shall be specified.

4.5.1.8 Test wells

Test wells shall meet all the criteria for material, design, construction, and certification stipulated for thermowells.

4.5.1.9 Vibration transducers

A non-contact, vibration pick-up system for monitoring valves, steam lines, and the condenser inlet should be considered.

4.5.1.10 Position transmitters

Position transmitters have to be able to withstand the high vibration, high temperatures, and humidity.

4.5.2 Instrument installation

Instruments should be installed as close as is practical to the source of the measurement, with consideration given to excessive vibration, temperature, and accessibility for periodic maintenance. Recommendations for the location of instrument and control equipment connections can be found in the publication, Recommendations for Location of Instrument and Control Connections for the Operations and Control of Watertube Boilers, by the Measurement Control and Automation Association (MCAA).

Thermowell installation for temperature measurements shall meet the requirements of ANSI/ASME B31.1, ASME Code for Pressure Piping.

Thermowell installation shall consider location, mounting, and velocity criteria in making a proper interface with the process.

4.5.3 Process measurements

Process measurements for steam turbine bypass controls are listed. For location of these measurements, refer to figures E.1 and E.2.

4.5.3.1 Main steam pressure

A pressure measurement taken at the turbine inlet is required for HP turbine steam bypass control.

4.5.3.2 Main steam temperature

A temperature measurement taken at the turbine inlet is required for operator information.

4.5.3.3 First stage pressure

A pressure measurement taken at the turbine first stage (impulse chamber) is required for IP/LP turbine steam bypass control.

4.5.3.4 First stage inner metal temperature



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A temperature measurement taken as near as possible to the inner metal surface of the first stage is required for operator information.

4.5.3.5 Reheat outlet temperature

A temperature measurement taken at the outlet of the reheater superheater section is required for IP/LP turbine steam bypass control.

4.5.3.6 Reheat innerbowl temperature

A temperature measurement taken as near as possible to the inner metal surface of the turbine reheat bowl is required for operator information.

4.5.3.7 HP bypass steam temperature

A temperature measurement taken downstream of the HP bypass desuperheater is required for HP turbine steam bypass control.

4.5.3.8 Reheat outlet pressure

A pressure measurement taken at the outlet of the reheater superheater section is required for IP/LP turbine steam bypass control.

4.5.3.9 LP bypass steam temperature

A temperature measurement taken downstream of the LP bypass desuperheater is required for IP/LP turbine steam bypass control.

4.5.3.10 Condenser pressure

A pressure measurement taken in the condenser is required for IP/LP turbine steam bypass control.

4.5.3.11 Condenser temperature

A temperature measurement taken in the condenser is required for IP/LP turbine steam bypass control.

4.6 Control and logic requirements

The function of the turbine bypass system is to take the boiler energy output and generate the demand for pressure and temperature reduction to simulate the high pressure, intermediate pressure, and low pressure stages of the turbine in an efficient and stable manner. This subclause addresses the means for controlling the difference between the generated and consumed steam flow and steam temperature for HP bypass control and IP and LP bypass control.

4.6.1 System design requirements

The turbine bypass control’s logic system design shall be fault tolerant. The designer shall recognize the failure behavior of components when designing a fail-safe system.

The logic system design shall include diagnostics to monitor and alarm any microprocessor component failures, including processor, data transfer, I/O, and power supplies.

Control access to the turbine bypass’s decision-making logic shall be (1) prohibited while fuel is being fired in the boiler and (2) protected from unauthorized changes. Decision-making logic includes, but is not limited to, maintenance interface, test and bypass functions, alarms, and I/O configuration.



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Pneumatic and single loop controllers may be used in the control system design.

4.6.2 HP bypass controls

4.6.2.1 HP bypass control valve

Single-element, high-pressure bypass control is the minimum control strategy required to regulate the steam pressure leaving the boiler. Referring to figure E.3, main steam pressure is measured and compared to a setpoint, with the results used to regulate the high-pressure control valve. The steam pressure setpoint is limited within a minimum and maximum value and ramped smoothly to prevent process upsets. The valve is not permitted to open if the desuperheater spray water pressure is insufficient, if no water is present, or if the block valve is not fully open.

For redundancy and added safety, a two-out-of-three or one-out-of-three pressure-monitoring safety system should be used.

4.6.2.2 HP bypass desuperheater spray water valve

Single-element, HP bypass steam temperature control is the minimum control strategy required to regulate the steam temperature leaving the HP turbine bypass. Referring to figure E.3, HP bypass steam temperature is measured and compared to a setpoint. The results are used to regulate the HP desuperheater spray water control valve.

4.6.2.3 HP bypass and desuperheater spray water block valve(s) logic

Provisions shall be made to override the HP bypass demand and block valve sequence logic and to open the control and block valve in the event of

a) a turbine trip;

b) a generator breaker open; or

c) an operator open request.

Provisions shall be made to override the HP bypass demand and block valve sequence logic and to close the control and block valve in the event of an operator close request.

A sequence control logic shall be provided for the block valve(s) operation to preserve its tight shutoff ability. The block-valve logic shall open the block valve to its fully open position prior to the initial opening of its modulating control valve. The block-valve logic shall close the block valve after its modulating valve is fully closed.

4.6.3 IP and LP bypass controls

4.6.3.1 IP and LP bypass control valve

Single-element IP and LP bypass control is the minimum control strategy required to regulate the steam pressure leaving the boiler’s reheater section. Referring to figure E.4, reheat outlet steam pressure is measured and compared to a setpoint with the results used to regulate the IP and LP pressure control valve. The steam pressure setpoint is programmed based on the turbine’s first stage pressure and is limited within a minimum and maximum value. The valve is not permitted to open if the desuperheater spray water pressure is insufficient, if no water is present, or if the block valve is not fully open.

4.6.3.2 IP and LP bypass desuperheater spray water valve



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Single-element, IP and LP bypass steam temperature control is the minimum control strategy required to regulate the steam temperature leaving the LP turbine bypass. Referring to figure E.4, LP bypass steam temperature is measured and compared to a setpoint with the results used to regulate the IP and LP desuperheater spray water control valve. The LP setpoint is derived from the IP and LP bypass control valve position, the reheater outlet pressure, and the reheater outlet temperature.

If the distance between the spray water injection and temperature measurement device is too short to evaporate the water, a heat balance calculation can be carried out to position the water valve.

A one-out-of-three or a two-out-of-three pressure-monitoring safety system should be used for redundancy and safety.

4.6.3.3 IP and LP bypass control and block valve(s) logic

Provision shall be made to override the IP and LP pressure bypass demand and block valve sequence logic and to close the control and block valve in the event of

a) condenser high pressure;

b) condenser high temperature;

c) hotwell high level; or

d) desuperheater spray water low pressure.

A sequence control logic shall be provided for block valve(s) operation to preserve its tight shutoff ability. The block valve logic shall open the block valve to its fully open position prior to the initial opening of its modulating control valve. The block valve logic shall close the block valve after its modulating valve is fully closed.

4.6.4 Automatic tracking

Automatic tracking shall be provided such that any control mode transfer is accomplished without process upset.

4.6.5 Integral windup prevention

Means shall be provided with the HP and IP/LP bypass flow and temperature control strategy to prevent integral windup of the feedback controller when the primary regulating device is at a limit (fully open or fully closed).

4.7 Alarm requirements

Minimum alarm requirements shall include the following information:

a) High condenser pressure

b) High condenser temperature

c) High condenser level

d) Low HP spray water pressure

e) Low LP spray water pressure



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f) Loss of control power

g) Loss of final drive power

h) Control loop trip-to-manual

i) Control and bypass valve open/close abnormal status

j) High HP outlet temperature

k) High LP outlet temperature.

4.8 Operator interface

4.8.1 Operator information

The following information used in the HP and LP/IP bypass control system shall be made available to the operator:

a) Main steam pressure

b) Main steam temperature

c) First stage pressure

d) First stage inner metal temperature

e) Reheat outlet temperature

f) Reheat inner bowl temperature

g) HP bypass steam temperature

h) RH outlet pressure

i) LP bypass steam temperature

j) Main steam pressure setpoint

k) HP bypass steam temperature setpoint

l) LP bypass steam pressure setpoint

m) LP bypass steam temperature setpoint

n) All alarms

o) Automatic/manual control loop status

p) Reheat outlet minimum pressure setpoint

q) Control and bypass valve (open/closed status).

In addition to the above, valve position(s) should be made available to the operator.



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4.8.2 Operator control functions

The control system shall include capabilities for the automatic/manual control of each individual, final device.

The control system shall include capabilities for the operator to control/adjust the main steam pressure setpoint, main steam pressure rate-of-change, and the HP bypass steam temperature setpoint.

Consideration should be given to setpoint limits that would be accessible to the operator.

The control room operator shall receive indications and have control functions for the bypass system and process interfaces.



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Annex A — References

AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI)

ANSI/FCI 70-2 Control Valve Seat Leakage, 2006

Available from: ANSI 25 W. 43rd Street, 4th Floor New York, NY 10036 Tel.: (212) 642-4900

AMERICAN SOCIETY OF MECHANICAL ENGINEERS (ASME)

ASME PTC19.3-1991 Safety Standard for Compressors for Process Industries

ANSI/ASME B31.1-2007 Power Piping

ANSI/ASME TDP-1-1998 Recommended Practice for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation

Available from: ASME International Three Park Avenue New York, NY 10016-5990 Tel.: (800) 843-2763

ELECTRIC POWER RESEARCH INSTITUTE (EPRI)

Report CS-4810 Turbine and Superheater Bypass Evaluation (dated October 1986)

Report CS-2251 Recommended Guidelines for the Admission of High Energy Fluids to Steam Surface Condensers (dated February 1982)

Available from: EPRI 3420 Hillview Ave. P.O. Box 10412 Palo Alto, CA 94304-139 Tel: (800) 313-3774

INTERNATIONAL ELECTROTECHNICAL COMMISSION (IEC)

ISO/IEC Directives, Part 2 Rules for the Structure and Drafting of International Standards

Available from: IEC P.O. Box 131 3, rue de Varembe CH-1211 Geneva 20 Switzerland Tel: 41 22 919 0211



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ANSI/ISA–77.13.01–1999 (R2008) – 30 –


ISA

ISA-5.1-1984 (R1992) Instrumentation Symbols and Identification

ISA-51.1-1979 (R1993) Process Instrumentation Terminology

Available from: ISA 67 Alexander Drive P.O. Box 12277 Research Triangle Park, NC 27709 Tel: (919) 549-8411

MANUFACTURERS STANDARDIZATION SOCIETY OF THE VALVE AND FITTINGS INDUSTRY, INC. (MSS)

MSS SP-61, 2003 Manufacturers Standardization Society ⎯ Valve and Fitting Industry Pressure Testing of Steel Valves

Available from: MSS 127 Park St. NE Vienna, VA 22180 Tel.: (703) 281-6613

MEASUREMENT CONTROL AND AUTOMATION ASSOCIATION (MCAA)

Recommended Instrument Connections: SAMA/ABMA/IGCI Recommended Instrument Connections. Jointly published by Scientific Apparatus Makers Association, American Boiler Manufacturers' Association, and Industrial Gas Cleaning Institute. [Washington, D.C.] : Scientific Apparatus Makers Association, 1981. NOTE — This document is no longer in print, and no longer supported by the Scientific Apparatus Makers Association, but available from ISA with permission from MCAA. Available from: MCAA 2093 Harper's Mill Road

P.O. Box 3698 Williamsburg, VA 23187

Tel: (757) 258-3100

http://www.measure.org



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Annex B — The use of bypass systems

The steam generator output can be run down in ten minutes or less to a house load value of approximately 10 to 20 percent without creating excessive temperature gradients when bypass systems are used.

The pressure setpoint must follow the actual steam pressure value at the superheater outlet during variable pressure operation. This setpoint can be changed automatically or manually.

The quick opening signal for the HP bypass valve can be initiated as a function of the control position of the turbine stop valve(s) or the generator trip signal.

Bypass steam flow is used as a feedforward signal for the temperature control system. The bypass valve position indexed by the steam pressure is indicative of this steam flow. For 40-percent bypass systems and larger, an HP bypass steam flow measurement may be needed for the control strategy that regulates the boiler’s feedwater demand. If steam flow is measured by the turbine’s first stage pressure (steam flow index), then an HP bypass flow nozzle is required to measure the bypass steam flow, or a characterized bypass valve position indication may be used. If steam flow is measured before the main steam stop valve, then an HP bypass flow nozzle is not required.

The European Boiler Codes allow omitting the conventional safety valves and operating the system with the HP bypass valves only. The American Boiler Codes require conventional safety valves even with the bypass valves.

All valves in the bypass system are usually equipped with a control system, power unit, and actuators that permit operating valves of any size full stroke in two seconds or less.

The following list highlights some particular advantages of correctly sized and designed bypass valves:

a) Cold start For the boiler, especially in supercritical units, the bypass system allows enhanced operation of the furnace, primary and secondary superheaters, flash tank, and main and reheater steam lines in the early steam water cycle. This improves the system’s steam purity before starting the turbine. It further reduces the start-up times drastically. The turbine can be started from the turning gear and can reach the rated speed in 15 to 30 minutes, provided the turbine rotor temperature is above the fracture appearances transition temperature. The bypass operation may take approximately 2.5 to 3.5 hours. The steam flow through the superheater and reheater enhances the tube cooling effect and may allow the steam generator to operate with a higher increased furnace firing rate. Rotor bore temperatures should follow the turbine manufacturer’s temperature gradients.

b) Warm start The advantages of the bypass system mentioned under cold start also apply to this mode, where the casing temperature of the HP turbine is usually above 212°F (100°C). The bypass valves allow the operator to optimally match the steam to the metal temperatures under all speed conditions.



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c) Hot start Minor disturbances that caused the unit to trip will usually involve a hot restart. Many of the advantages mentioned earlier apply to this mode of operation. The bypass has the ability to closely match the metal temperature of the heavy turbine parts, and this makes it unnecessary to go through tedious cool down and rewarming procedures.

d) Partial- or full-load rejection and quick restart In the case of a partial- or full-load rejection, the bypass valves have to open immediately. The bypass control system opens the bypass valves to the same degree the turbine control valves were before they closed. The turbine can shut down slowly and prepare to restart. Protective systems should be provided to trip the boiler when HP bypass or LP bypass valves fail to open and when insufficient cooling steam flow passes through the superheater or reheater. The European Boiler codes require variable pressure safety valves on the reheater outlet and omission of the safety valves on the reheater inlet.



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Annex C — Valve life expectancy

There are basically two major valve design standards in use: ANSI B-16.34 in the Americas and DIN 3840 (German) in Europe. The first takes the “cookbook approach” by providing easy-to-use tables for determining the required wall thicknesses. The latter requires the engineer to perform an accurate stress analysis by using the so called “pressure area comparison method” (the flow area versus the pressure containing section area). This approach is similar to ASME Section 3, Nuclear Valves.

DIN 3840 requires the designer to select the correct material strength value and apply a safety factor as a function of the material characteristics (type and condition) and the “expected lifetime.” In turbine bypass valves, this is often a P22 Chrome-Molybdenum Alloy Steel (CrMo) forging with certification 3.1A or 3.1B, according to DIN 50049, and the creep rupture value at the design temperature (example 1000°F) for an expected lifetime of 100,000 or 200,000 hours. Please note that the safety factor is 1.0 in case of 200,000 hours.

A designer knows much better where the products’ limits are when using DIN 3840. DIN results in thinner walls. Thus steady state thermal stresses will be lower. However, the maximum thermal stress, which impacts the cycle life of the valve, will occur during the thermal transient as the valve heats up on opening.

These maximum thermal transient stresses are essentially independent of the wall thickness for the range of metal thicknesses used in Steam Turbine Bypass valves. The presence of water spray inside the valve body would significantly increase the maximum thermal stresses present and would result in a reduced cyclic life.

DIN penalizes the use of cast bodies instead of forgings by a higher safety factor, not only for the design but also for the testing. Paragraph 7.5 of DIN 3840 briefly mentions “Additional Stresses” that must be considered. The German TRD (and AD [non-fired piping]) Boiler rules provide the guidelines to calculate those. ASME Section 1, in turn, requires loadings other than pressure and static head. But no rules for this have been set.

The advantage of using DIN 3840 is the implicit time element that goes into the analysis and makes a statement about the expected lifetime.

ASME Code Case 1331 provides guidelines for the calculations of the cyclic lifetime of, for example, turbine bypass valve bodies (low cycle fatigue). These values have been used extensively over many years and are particularly important for the design of HP bypass and trip valves with very short stroking times.

Utility people generally expect a turbine bypass to last 20 to 25 years. This typically is used to translate into 10,000 to 12,000 total cycles. Today, bypass systems in combined cycle plants can be exposed to a much more cyclic service, but, generally, they also are operating at lower pressures and temperatures, although recent developments have design temperatures up to 1100°F.



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Annex D — Some typical high- and low-pressure turbine bypass valve

High Pressure Turbine Bypass Valve

Life Expectancy: 20 - 25 Years

Size and ANSI Class: 12" x 18" ANSI 1500 - 4500#

Maximum Design Pressure: 6812 PSIG @ 100°F (38°C) (cold working)

Body Material: ASTM/ASME SA182-F22 or SA217-WC9

Disk Material: Hardened heat resistant

Maximum Temperature: 1050°F (566°C)

Stem Material: Hardened heat resistant

Seat Material: Hardened heat resistant

Packing and Trim Materials: Service conditions of valve

Seat Leakage: ANSI/FCI 70-2 (see reference in annex A)

Low Pressure Turbine Bypass Valve

Life Expectancy: 20 - 35 Years

Size and ANSI Class: 20" x 30" ANSI 900#

Maximum Design Pressure: 2250 PSIG @ 100°F (38°C) (cold working)

Body Material: A217-WC6/WC9 or SA182-F11/F22

Disk Material: Hardened heat resistant

Maximum Temperature: 932 – 968°F (500 – 520°C)

Stem Material: Hardened heat resistant

Seat Material: Hardened heat resistant

Packing and Trim Materials: Service condition of the valve

Seat leakage: ANSI/FCI 70-2 (see reference in annex A)



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Annex E — Figures

Figure E.1 — Turbine steam bypass system with separate pressure reducing valves and desuperheaters



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Figure E.2 — Turbine steam bypass system with combined pressure reducing and desuperheating valves



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Figure E.3 — HP turbine steam bypass control



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Figure E.4 ⎯ IP/LP turbine steam bypass control



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Developing and promulgating sound consensus standards, recommended practices, and technical reports is one of ISA’s primary goals. To achieve this goal the Standards and Practices Department relies on the technical expertise and efforts of volunteer committee members, chairmen and reviewers.

ISA is an American National Standards Institute (ANSI) accredited organization. ISA administers United States Technical Advisory Groups (USTAGs) and provides secretariat support for International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO) committees that develop process measurement and control standards. To obtain additional information on the Society’s standards program, please write:

ISA Attn: Standards Department 67 Alexander Drive P.O. Box 12277 Research Triangle Park, NC 27709

ISBN: 978-1-934394-74-8



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bypass system

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