Hydraulic Supply for Gas and Steam Turbines

RE 08126 Technical Information Edition: 2014-03

Contents

Introduction 2 System Overview 2 Circuit Description 2 Function 3 Main Components 3 - 4

RE 08126, Edition: 2014-03, Bosch Rexroth AG 2/4 Technical Information | Hydraulic Oil Supply for Gas and Steam Turbines

Tailor-made, reliable, low-maintainance – Hydraulic Oil Supply for Gas and Steam Turbines

Introduction System Overview

A gas or steam turbine utilizes two types of hydraulic The turbine oil hydraulic system consists of one or two systems: a cooling & unit and a control oil designs: two independent hydraulic systems or a single unit. They differ in the physical sizes pressure and flow. central system. The cooling & lubrication unit operates at low pressure Independent systems differ from central systems in the and supplies oil to reduce friction and wear on mechani- following ways: cal rotating elements of the turbine and generator. ▶▶ Two separate oil tanks While lubricating, the oil also absorbs heat produced by ▶▶ Use of different types of hydraulic fluids bearing friction and heat conduction along the turbine ▶▶ Return flow and filtration loop systems in the control shaft. This heat energy is carried by the oil and removed oil circuit from the system via a head exchanger. ▶▶ Possible differing locations of the hydraulic tanks The high pressure control oil unit supplies hydraulic power to the gas or steam valve , which controls Often the hydraulic units are designed with the top plate valve positions and regulates the turbine speed. The valve of the oil tank used as a mounting surface for most, if not position controls the flow of gas or steam supplying the all of the components including piping (see cover picture). turbine. The turbine speed controller provides the com- Other designs may mount key components on a separate mand signal to control these valves. frame which is mounted separately with other equipment. The following description provides an overview of the To achieve high availability, wear-critical and serviceable components are installed with redundant backups, gener- basic structure and function of these oil supply systems. ally consisting of and filtration. The description covers a central hydraulic unit with a When designing hydraulic systems, it is critical to single tank and an electrically driven main oil – not receive the specifications for the turbine and/or project. by the turbine. Redundant components are not shown for simplicity.

Circuit Description

Actuators (8) (16)

G (15)

(2) Turning gear Turbine Bearing Gear box Generator

(9) (10) (14) (7)

(4)

(11) (12) (13)

(1) (3) (18) (6) (17) (5)

Bosch Rexroth AG, RE 08126, Edition: 2014-03 Hydraulic Oil Supply for Gas and Steam Turbines | Technical Information 3/4

Function Main Components

During startup of a gas or steam turbine, proper sequenc- Main Oil Pump (11) ing of the pumps is required to ensure operational readi- During normal operation, the main oil pump provides the ness of the required control, regulating and safety total lubricating oil requirements for the turbine system. devices, and to also prevent damage of the installed General centrifugal pumps, as well as positive displace- equipment. ment pumps are used, which can be either gear or screw The hydraulic fluid (18) in the tank (4) can be preheated type pumps. using heater elements (6). When the fluid reaches an The main oil pump can be driven from an AC electric allowable start temperature, the start-up process of a motor or directly from the turbine using mechanical turbine can begin: coupling. When started, the control oil pump (1) fills the accumula- Auxiliary Oil Pump (12) tor (2) providing pressure for movement as The auxiliary oil pump is identical to the main oil pump. It required. is able to deliver the total lubricating oil requirements in Operation of the main oil pump (11), driven by an AC case of a failure of the main oil pump. This pump is driven motor, supplies lubricating fluid for all connected con- by an AC electric motor, and switches on when the sys- sumers. tem pressure of the main oil pump drops below a defined An auxiliary oil pump (12) is activated in the event of a value. failure of the main oil pump. When the main oil pump is driven directly from the tur- The emergency oil pump (13) is automatically started bine shaft , the pump speed is not high enough to pro- when the lubrication pressure drops below a specified duce a sufficient flow and pressure during start-up and limit. shutdown of the turbine. In these cases, the auxiliary oil The jacking oil pump (3) is used to pressurize the bear- pump is switched on to supplement the main oil pump. ings with high pressure and causes the turbine shaft Emergency Oil Pump (13) slightly the bearing race. In doing so, a thin film of lubri- The emergency oil pump is typically the same pump as cating oil provides a low friction layer between the bear- the auxiliary oil pump, but with a reduced input power. ing and the shaft. At this point, the hydraulic turning gear It is sized so that in the event of a failure, sufficient oil is starts to accelerate the turbine rotor. After a stable oil supplied to the bearings to allow the rotor to coast down layer has built up in the bearings, and ignition speed is to a stop. reached in , the turning gear is decoupled To allow for operation in the case of loss in the power from the turbine shaft and the jacking oil pump (3) is grid, this pump is driven by a DC electric motor, the switched off. The turbine then accelerates under its own power is supplied by a battery. power of gas or steam, until the desired speed is reached. The emergency oil pump is not intended for continuous As the lubricating oil is sent to the consumers, it passes operation. through a three-way valve (15) which, with its tempera- Jacking Oil Pump (3) ture dependent control function, modulates the amount The jacking oil pump is used in large turbine systems to of fluid that flows through the oil cooler (14). This is prevent metal-to-metal contact in the bearings during required to maintain oil temperature in its desired range. turbine start-up. This is accomplished by applying high- Because cleanliness of the oil has a large influence on the pressure oil to the turbine shaft bearings, raising the life of the turbine shaft bearings, the oil is drawn through shaft with the high-pressure oil. a wide-mesh suction strainer (not shown) at the inlet side If a hydraulic turning gear system is used, the turning the pump forced through a fine-mesh line-filter (8) before gear motor is supplied by oil from the jacking oil pump. reaching the bearings. Gear or pumps are typically used in this circuit. When using a fixed displacement lubricating oil pump, a Control Oil Pump (1) relief valve (16) is connected to the system at the filter The use of pressure compensated piston pumps is recom- outlet to maintain the required bearing lubrication pres- mended to supply high-pressure oil to the actuators. sure. The difference between the pump flow and the oil These pumps deliver the required flow and minimize flow used for lubrication returns through the relief valve power requirements. to the oil reservoir. (2) After the oil has passed through the oil channels, it flows A hydraulic accumulator is used to provide peak flow for via gravity through a return pipe to the reservoir, which is fast actuator motion as required by the processes. The located below the turbine axis. accumulator is sized so that in event of a power failure the “Emergency Shutdown Program” can be utilized without external power, to bring the turbine to a stop.

RE 08126, Edition: 2014-03, Bosch Rexroth AG 4/4 Technical Information | Hydraulic Oil Supply for Gas and Steam Turbines

Relief Valve (16) Three-Way Mixing Valve (15) A relief valve is recommended when fixed displacement Oil temperature is regulated by a temperature controlled pumps are used. Flow in excess of the current demand is three-way mixing or modulating valve. returned to the tank via the relief valve, which maintains Heater (6) constant pressure in the circuit. A reservoir immersion heater provides heating of the Duplex Low-Pressure Filter (8) hydraulic fluid at low ambient temperatures. The main Low-pressure filters with high nominal flow rates are used reason of the heater is to allow the turbine be started at a to remove particulate contamination from the lubricating moments notice. To prevent coking of the oil on the oil circuit. During turbine operation the filter elements heater surfaces, the heater’s power density must be can be changed as required by switching the flow to the properly chosen. other filter element. A contamination indicator provides Connections for Oil Conditioning System (17) indication of the filter’s condition. It is at the discretion of the turbine operator to use a Duplex High-Pressure Filter (9) mobile or stationary oil conditioning system. These The switchable double filters are designed for high pres- “offline conditioning system” can be connected to the sure and are required for the actuator servo controls. tank via special provided connection points. They remove particulate contamination from the high- Since particulate contamination is removed by the sys- pressure control circuit and are designed to allow for tem’s main filters, an oil conditioning system is used continuous usage during a filter element change. primarily to remove soluble impurities from the oil. High Pressure Filter (10) This is primarily introduced by condensation of A single high-pressure filter is used in the jacking oil humid air, or water migration through defective seals in circuit. Duplex filters are not required since this function an oil-water heat exchanger. An oil conditioning system only operates intermittently. Replacement of the filter can also remove organic compounds as well as oil which element can be performed when this circuit is not used. has oxidized and reacted with metallic elements within Oil Mist Separator (7) the system. A mist separator is located on the closed tank and gener- Most oil conditioning systems are equipped with a centri- ates vacuum in the tank, which continues along the main fuge or may employ vacuum distillation to condition the return line from the bearings. Oil mist, which is formed by oil. turbulent flow in the bearings is aspirated, and prevents Fluid (18) the formation of explosive oil vapor. Fluids used in lubrication circuits, are called turbine Tank (4) which do not contain zinc. They are characterized by a The capacity of lubricating oil tank can be up to one high aging resistance, an ability to separate air and water thousand gallons, or more and depends on the volume of and has fast defoaming rates. In addition to those already circulated oil. Sensors for oil level and temperature are mentioned fluid properties; antiwear, good lubricity, as mounted in the tank. The installation of tank baffles well as heat removal are desirable properties. The Fluid is increases the dwell time of the oil in the tank and pro- also responsible for the transport of wear particles to the motes deaeration of the oil. The manhole covers are filters and protection of surfaces against chemical corro- provided to allow access to the tank interior for cleaning sion. and inspection. In the control oil circuit, the oil should have good high- Oil Containment Tray (5) pressure characteristics suitable for use in control and An oil containment tray is mounted below the power unit regulation circuits. to prevent oil leakage into the environment. The contain- If a central hydraulic system is used, consideration must ment tray is present to contain accidental loss of oil as be given to the operation of the high-pressure control well as loss resulting from repairs and maintenance. circuit. Operation of this circuit with turbine oil as a fluid Oil Cooler (14) should be evaluated to ensure that the components will Oil-water coolers can be “shell and tube” type or plate- operate properly and without damage, due to the fluid type design, and are used to remove heat from the properties. hydraulic fluid. Air over oil coolers are also available.

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Bosch Rexroth AG, RE 08126, Edition: 2014-03