Very Large Size Valves and Actuators – Design and Testing

Stefano Ragazzoni, Giacomo Campanelli Saipem S.p.A.

Fano (PU), Italy

Roberto Pozzati, Ivan Ambrosini Petrolvalves S.r.l.

Castellanza (VA), Italy

Trond Gjedrem Nord Stream AG Zug, Switzerland

ABSTRACT The Nord Stream Pipelines offshore the Baltic Sea have been designed for a life of 50 years. Special focus was put in the selection of the mainline actuated valves: The design of the pipelines isolation system was driven by the achievement of high levels of safety, reliability and availability. 48-in, Top-Entry metal-seated Ball Valves and Double-Expanding Gate Valves were selected. The supply of these valves involved very challenging developments for the design, manufacturing and testing of valves and actuators, also considering the demanding environmental conditions on the Russian side. This paper summarizes the most relevant issues related to the design and testing of valves and actuators.

INTRODUCTION The twin Nord Stream Pipelines, 48-in dia., 1,200+ km long, connecting Vyborg (Russia) to Greifswald (Germany) offshore the Baltic Sea, have been designed with the segmented pipeline concept (with Design Pressures of 221 barg to 177.5 barg), for a life of 50 years. Special focus was put at the early design stage in the selection of the mainline actuated valves, which have the functions of pipeline Pressure Protection (PPS), Emergency Shut-Down (ESD) and Isolation. The development of the isolation system of the pipelines was driven by the achievement of high levels of safety, reliability and availability over the system design life. 48-in, Top-Entry metal-seated Ball Valves (PPS & ESD) and Double-Expanding Gate Valves (Launch & Receive Traps isolation) were selected as the most appropriate valve types for their operational function. The main issues related to valve design were:

Achievement of very good metal-to-metal seal performance recovering the inherent massive deflections consequence of the valve size, combined with a high operating pressure.

Guarantee of long term operation, avoiding any need of maintenance shut-down.

This was obtained through:

Special design of seat-to-ball sealing surfaces to keep the lowest possible contact stress, reducing wear during operation.

Redundancy of pressure barriers both internally and toward the environment.

Low fatigue usage factor of all elements. Introduction of features to flush clean the sealing surfaces. Enhanced material selection to avoid any deterioration of critical

valve areas in long-term service. Valves were subject to Prototype Testing (including the application of large external bending moment, and the verification of valve maintainability under bending) and extensive Factory Acceptance Test (FAT). Double-acting spring-activated electro-hydraulic actuators (fail-to-close) were selected for the PPS & ESD valves. The valve design torque of 750,000+ Nm led to select an actuator torque of 1,250,000 Nm. The key issues for the selection and design of the actuators were high reliability and availability of the control system. The main design issues were:

Study of the best kinetic system to suit the valve torque requirements, limiting the actuator dimensions, and optimization of accumulators sizing.

Optimization of the spring-pack design for torque requirements. Material selection for low temperature application. Design for easy handling and installation. Design of the Hydraulic Power Unit (HPU) and control system to

suit the high reliability project requirements with fault tolerant control system in 2oo3 configuration.

Detailed fault analysis to ensure the required SIL 3 level. Design and manufacture of test equipment for Prototype Testing. Actuators were subject to extensive Prototype Testing (3,000 + 3,000 full cycles at the min and max design temperatures) and integrated FAT.

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Proceedings of the Twenty-second (2012) International Offshore and Polar Engineering ConferenceRhodes, Greece, June 17–22, 2012Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE)ISBN 978-1-880653-94–4 (Set); ISSN 1098-6189 (Set)

www.isope.org

VALVE DESIGN SELECTION Particular focus was put at the early design stage in the selection of the mainline actuated valves for PPS, ESD and trap isolation service. Ball Valves for PPS and ESD Service Top-Entry metal-seated valves (Fig. 1) were selected based on the following main considerations. Split body design would imply extremely long shut-down and

unsustainable related costs in case of any possible failure during operation.

Welded body design is relatively less expensive, but it is well known that this construction is not compatible with tight metal seal. Any valve maintenance would imply cutting the valve from the line with heavy implications in terms of pipeline outage.

Top Entry ball valve design provides reliable achievement of tight metal seal during manufacturing. The valve is fully maintainable on line: complete change-out of the internals can be run in few hours.

Valves for Trap Isolation Service The Double Expanding Gate Valve (DEGV), Fig. 1, was selected as this is the only valve providing within a unique valve body (and with just one operator) the same function and reliability of two valves in series: this minimizes the risk of failure (lower number of components, the failure of which could jeopardize safe operation). Long term experience with identical valves of comparable pressure class and size is excellent.

Fig. 1 Top-Entry Ball Valve and DEGV

BALL VALVE Overall View

The Nord Stream Top Entry Ball Valve (Figs. 1, 2) is an impressive structure of about 87,000 kg weight (actuator excluded). Body and Bonnet are cast one-piece elements; individual weight of rough cast pieces is respectively 44,300 kg (body) and 24,700 kg (bonnet). The ball is one forged piece of about 2 meter diameter and 16,400 kg weight.

Fig. 2 3D-Model of the Top-Entry 48-in Ball Valve All main valve components, due to the massive dimensions and stringent quality requirements, are at the top of manufacturing capability of qualified suppliers. Valve Features In order to guarantee 50 years operating life, all the measures have been taken to both reduce wear and to implement redundancy of pressure barriers. These include:

Flushing Ports, provided to inject cleaning fluid (light oil/kerosene) in the area of seat-to-ball metal-metal contact; any debris accumulation can be removed before the valve gets operated. In case of need, Emergency Sealant Media can also be injected through the same ports to mitigate any leaks. Flushing ports exhibit triple seal barriers towards the environment.

Fig. 3 Detail of seat and seat flushing port Quadruple Stem Seal

Primary Metal Gasket (® Infinity Seal) Secondary Thermoplastic Gasket Firesafe Graphite Gasket Emergency Sealant Injection

 

TOP ENTRY BALL VALVE 

DOUBLE‐EXPANDING 

GATE VALVE 

FLUSHING PORT 

INJECTOR

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Fig. 4 Details of Stem metal gasket, Stem Packing and Sealant Injection Triple Body-Bonnet Seal

Primary Metal Gasket Secondary Thermoplastic Gasket Emergency Sealant Injection (between primary metal and

secondary seal)

Fig. 5 Detail of Bonnet Seal assembly including Emergency Sealant Injection Double Piston Effect (DPE) Seats

With this design, each seat ring is ‘activated’ by any differential pressure existing across the seat itself, regardless where is the high-pressure side. This means that, should the upstream seat leak, the downstream seat gets immediately activated, providing the required tightness.

F1 = /4 (De2 – Dg2) p F2 = /4 (Dg2 – Di2) p

Fig. 6 Principle of DPE Seat Design Double Seat-to-Ball Seal

The flushing ports can also be used to inject sealant media, thus mitigating leaks trough unlikely damages of the primary metal seal.

Double Barrier on Drain/Vent Openings Both vent and drain openings are provided with two isolation valves in series.

Wear Resistant Dynamic Sealing Surfaces The whole ball and the mating seat surfaces are Tungsten Carbide Coated (300 μm). This coating is applied through a high velocity spray process (High Velocity Oxygen Fuel) that provides excellent

adhesion (> 60 MPa). The hardness of the layer overcomes 900 HV that renders the surface extremely resistant to abrasion. Seat and Ball sealing surfaces are then lapped together for a tight matching. Tungsten Carbide Coating is also applied on both faces of stem and metal gasket to avoid wear.

Inconel 625 Weld Overlay All areas where a deterioration of the surface quality may jeopardize the valve behavior are provided with corrosion-free Inconel 625 weld overlay (3 mm minimum thick). Typically, IN 625 WO is applied in: seat pockets, bonnet stem housing, ball surface (as corrosion protection of Tungsten Carbide Coating), body trunnion housing and any threaded connection through the valve body.

Valve Design Challenges The valve elements (both pressure boundary and internals) have been designed to minimize general stresses, local stresses and contact stresses to guarantee extended cyclic life under the worst predictable loading conditions, including pipeline loads. Special effort has been put to contain the inherent massive deflections consequence of the valve size, particularly where valve elements and components face sealing elements, i.e.: body-to-bonnet, bonnet-to-stem, body pockets-to-seat rings and seat ring-to-ball. Valve Pressure Boundary Design The Finite Element (FE) model used to evaluate stress and displacements includes the whole pressure containment, i.e.: Body, Bonnet, Bolts. ‘Non linear Contact Surfaces Technique’ was used to achieve a realistic behavior of each individual element.

Fig. 7 Finite Element Model of the Top-Entry Ball Valve Reinforcement ribs were introduced to limit deflections in the very critical seat pockets area. The shape of ribs was established through FE numerical simulation that involved a complex optimization process. The (achieved) goal was to keep the internal surface of the seat pockets as cylindrical as possible, regardless of the nature of applied loads (pressure, axial thrust and bending moment from line pipe). In this way, the best efficiency and reliability of seat gasket were guaranteed.

 

Infinity® Gasket

SEALANT INJECTOR

THERMOPLASTIC GASKET

FIRESAFE (GRAPHITE) RING

PRIMARY METAL GASKET

SEALANT INJECTOR

THERMOPLASTIC GASKET

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Fig. 8 FEA Model with detail of Reinforcing Ribs With the above model, a realistic and complete knowledge of stresses and relative movement between body and bonnet was achieved; the sealing capability of bonnet and stem gasket was evaluated and confirmed.

Fig. 9 Example of FEA result Ball and Seat Ring Design The FE model used to evaluate stresses and displacements includes the whole ball and (both) seats assemblies. Fig. 10 FEA of Ball and Seats Non Linear FE Technique has been used to design the assembly. As part of the evaluation of structural strength of the elements, a particular care was devoted to achieve acceptable and uniform bearing stresses all along the contact line between ball and seat ring. This was the result of an optimization process obtained balancing the mutual flexibility of the seat ring and the rigidity of the ball.

Fig. 11 Seat-to-Ball contact pressure analysis Drive Train Design Drive train elements were fully investigated to check their capability in long term operation.

                                                      Fig. 12 FEA of the Drive Train and detail of the stem spline connection

Fatigue Evaluation Fatigue cyclic life has been evaluated according to ASME Sect. VIII Div. 2 rules taking into account the worst loading combination for each valve component. Results confirm that the usage factors are widely on the safe side.

Valve Component Allow. Cycles No. Usage Factor (*) Body 2377 0.421 Bonnet 24900 0.040 Body Bolts 3060 0.327 Stem - 0.921

(*) Usage factor evaluated on the basis of a design cyclic life of 1000 cycles

DOUBLE EXPANDING GATE VALVE Overall View Each launch/receive pig trap of the Nord Stream pipelines is provided with one Double Expanding Gate Valve, which duty is to guarantee tight Double-Isolation-and-Bleed (DIB) capability between the trap and the line. The Double Expanding Gate is the only valve capable to guarantee the DIB function with a unique valve body. The valve itself is 8 meter high (10.4m with the electric actuator) and the weight is 92,500 kg. The Body and Bonnet are cast elements; individual weight of rough cast pieces is respectively 60,200 kg (body + bottom head) and 9,850 kg (bonnet). The double wedged gate is made of two low alloy forged pieces for a total weight of 17,400 kg. All main valve components, due the massive dimensions and stringent quality requirements, are at the top of manufacturing capability of qualified suppliers.

 

Reinforcing Ribs

    

 

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Fig. 13 3D-Model of the Double-Expanding Gate Valve

Valve Features In order to guarantee 50 years operating life, all the measures have been taken to both reduce wear and to implement, as far as possible, redundancy of pressure barriers. This includes: Flushing Ports, provided to inject cleaning fluid (light oil/kerosene)

in the area of seat-to-gate metal-metal contact: any debris accumulation can be removed before the valve gets operated. In case of need, Emergency Sealant Media can be injected through the same ports to  mitigate possible leaks. Flushing ports are provided with triple seal barriers towards the environment.

Fig. 14 Details of Seat Flushing Port and Stem Seal Quadruple Stem Seal

Thermoplastic Multiple Chevron Rings Fire-safe Graphite Rings Lip Seal Gasket Emergency Sealant Injector

Triple Body-Bonnet Seal Primary Metal Gasket Secondary Thermoplastic Gasket Emergency Sealant Injection (between primary metal and

secondary seal)

Pressure Activated (Fixed) Seats Proprietary design of fixed (screwed-in) seats provides excellent seat flexibility in ‘extension’ when the pressure energizes the seal, while the seat is extremely rigid in compression (to withstand the massive pressure thrust on gate segments). Thus the average contact pressure required to achieve tight seal is drastically reduced with evident benefits on cyclic life.

Fig. 15 Pressure-activated Seat

Double Seat-to-Gate Seal The flushing ports can also be used to inject sealant media, thus mitigating leaks trough unlikely damages of the primary metal seal.

Double Barrier on Drain/Vent Openings Both vent and drain openings are provided with two isolation valves in series.

Wear Resistant Dynamic Sealing Surfaces The whole Gate surfaces (including wedge internal parts) and the mating seat surfaces are Tungsten Carbide (TC) Coated. This coating is applied through a high velocity spray process (High Velocity Oxygen Fuel) that provides excellent adhesion (> 60 MPa). Hardness of the layer overcomes 900 HV that renders the surface extremely resistant to abrasion. Seat and Wedges sealing surfaces are then lapped together for a tight matching.

Inconel 625 Weld Overlay All areas where a deterioration of the surface quality may jeopardize the valve behavior are provided with corrosion-free Inconel 625 weld overlay (3 mm minimum thick). Typically, IN 625 WO is applied in: seat pockets, bonnet stem housing, gate surface (as corrosion protection of Tungsten Carbide Coating), and any threaded connection through the valve body.

Valve Design Challenges The valve elements (both pressure boundary and internals) have been designed to minimize general stresses, local stresses and contact stresses to guarantee extended cyclic life under the worst predictable loading conditions, including pipeline loads. Special effort has been put to contain the inherent massive deflections consequence of the valve size, particularly where valve elements and components face sealing elements, i.e.: body-to-bonnet, bonnet-to-stem, body pockets-to-seat rings and seat ring-to-wedges.

 

TC Coated Surfaces

 

   

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Valve Pressure Boundary Design The FE model used to evaluate stress and displacements includes the whole pressure containment, i.e.: Body, Bonnet, Bolts. ‘Non linear Contact Surfaces Technique’ was used to achieve a realistic behavior of each individual element.

Fig. 16 FEA Model of Double-Expanding Gate Valve Reinforcement ribs were introduced to limit deflections in the very critical seat pockets area. Shape of ribs was established through FE numerical simulation that involved a complex optimization process. The (achieved) goal was to keep the internal surface of seat pocket shoulders as flat as possible, regardless from the nature of applied load (pressure, axial thrust and bending moment from line pipe). In this way the best efficiency and reliability of seat-to-body seal was guaranteed.

Fig. 17 FEA model of valve body with reinforcing ribs With the above model, a realistic and complete knowledge of stresses and relative movement between body and bonnet was achieved; the sealing capability of bonnet and stem gasket was evaluated and confirmed.

Fig. 18 FEA results for valve body Gate and Seat Ring Design The FE model used to evaluate stresses and displacements includes the whole Gate segments and (both) seats assemblies. Non Linear FE Technique has been used to design the assembly.

Fig. 19 Gate-seat bearing stresses

Further to the evaluation of the structural strength of the elements, particular care was devoted to achieve acceptable and uniform bearing stresses all along the contact surface between the gate and the seat rings. This was the result of an optimization of seat seating profile as well as ring flexibility. Drive Train Design Drive train elements were designed to improve the fatigue life; their capability in long term operations was fully investigated.      

Fig. 20 FEA of Gate Segments and Drive Train

 

 

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Fatigue Evaluation Fatigue cyclic life has been evaluated according to ASME Sect. VIII Div. 2 rules taking into account the worse loading combinations for each valve component. Results confirm that the usage factors are widely on the safe side.

Valve Component Allow.Cycles No. Usage Factor (*) Body 1973 0.508 Bonnet 7242 0.365 Body Bolts 1600 0.625 Drive Train - 0.838

(*) Usage factor evaluated on the base of a design cyclic life of 1000 cycles MATERIAL SELECTION Material selection was aimed to guarantee 50 years of safe operation without any need to carry out significant maintenance. Metallic Materials Low Temperature Carbon Steel (LTCS) was selected for the

pressure containing barrier (body and bonnet) in virtue of its good mechanical strength and excellent low temperature toughness. All the body and bonnet interfaces with internals and/or gaskets were clad with Inconel 625 which exhibits excellent corrosion resistance.

All threads of body penetration are IN 625 clad. Ball is High Strength Carbon Steel fully IN 625 clad. Gate segments are high strength Low Alloy steel IN 625 clad on all

‘functional’ surfaces. Trim Material (seat rings, stem) is 25Cr Duplex Stainless Steel

which guarantees excellent resistance to corrosion as well as high mechanical strength.

Materials properties have been individually checked and certified according the rules of all applicable Standards and Codes.

Hardfacing Tungsten Carbide Coating is provided on all surfaces where metal-to-metal dynamic seal is required. Coating is performed by the HVOF process. Process qualifications include mechanical strength verifications and salt spray chemical attack. Plastic / Elastomeric Materials

All Thermoplastic materials used for the manufacture of gaskets are Explosive Decompression Resistant and all of them have been qualified according to Norsok M-710. Predicted operational life of thermoplastic gasket is:

PEEK > 50 years @ 160°C - 15% H2S Virgin PTFE > 280 years @ 180°C - 2% H2S Carbon filled PTFE > 100 years @ 180°C - 2% H2S

TESTING

Both Ball and Double Expanding Gate valves have been qualified through an extensive and very severe qualification procedure that involved High Pressure body test (both hydraulic and pneumatic), HP and LP seat test (including seat leak test, both hydraulic and pneumatic), cyclic tests (stroking test under Differential Pressure).

Fig. 21 Ball Valve Prototype Testing

Pressure test have been repeated while valve was subjected to massive external bending moment, cyclic tests (stroking test under Differential Pressure) were also repeated. Furthermore, calculated stresses and stresses measured through strain gauges in some significant areas have been compared. Readings confirmed good accuracy of the numerical simulations (FEA). All valves have then been subject to extensive FAT which included HP body pressure test, LP and HP seat pressure tests and seat tightness tests either hydraulic and pneumatic (Air LP, Nitrogen HP). Valve and actuator assembly have been tested to check safe/reliable operability under maximum predictable Differential pressure.

Bending Test Equipment

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Fig. 22 Gate Valve Prototype Testing QUARTER-TURN ELECTRO HYDRAULIC ACTUATOR Project requirement for an electro-hydraulic spring return actuator fail to close, due requested operating torques was not available on the market. The actuator design was developed taking into account the following minimum requirements:

a. Grant a safety factor of 1.5 against valve torques with spring and with minimum hydraulic pressure

b. Optimization of the actuator load transferring Kinematic and spring pack design, with reduction of actuator dimension and optimization of accumulators sizing

c. Be a compact and reliable design, quickly and easily removable from the valve and re-installable to minimize shut down time of the Nord Stream pipelines in case of actuator refurbishment

d. Material selection for low temperature application (-40°C design temperature)

e. Design of a power unit and control system to suit the high reliability project requirements with fault tolerant control system in 2-out-of-3 (2oo3) configuration.

f. Design and manufacture of equipment for actuator prototype testing and prototype test of the actuator at min and max design temperatures.

Actuator Design A preliminary study in collaboration with the spring manufacturers was conducted to establish if a spring return actuator was feasible for the requested operating torques (i.e. 1,250,000 Nm in open operation and 400,000 Nm in closing operation with spring extended). The following kinematic configurations for the actuator have been studied: single yoke and double yoke configuration. A single yoke configuration (Fig. 23) presents a center of gravity offset from valve center line, which results in a bending moment applied to the valve bonnet and valve stem.

Fig. 23 Single yoke actuator design – 1 spring pack and 1 cylinder

Fig. 24 Double yoke actuator design – 2 spring pack and 2 cylinders

For small valves and actuators, this effect may be neglected with respect to other design loads, but for this application this resulted in a bending moment of 312,000 Nm, with need to increase considerably the size of bolting between the valve bonnet and the actuator mounting spool. In addition, handling of single yoke actuator is not as easy as for a double yoke design, where the center of gravity is balanced and in the center of the mounting spool. The overall dimensions of the spring packs are reduced for a double yoke configuration with consequent advantages in compressing and assembling the springs, and realization of relevant equipment to compress them. At last, the total overall dimensions for the double yoke configuration are lower: 7.5 m, instead of 9 m for single yoke. Fig. 25 shows the final 3D model for the selected actuator with the double yoke configuration.

Fig. 25 Final 3D-Model of actuator for 48” PPS and ESD ball valves

The design continued selecting the asymmetric yoke angles and yoke arm to generate torque curves in open and close position tailored on valve resistant torques, minimizing actuator swept volumes and optimizing hydraulic operating pressures. These last parameters are dictating the sizing of the hydraulic accumulators for the power unit, and of the power unit main components such as hydraulic pumps, solenoid valves, oil reservoir size and flow regulating valves. As result of the analysis, the yoke geometry has been selected to obtain

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the following actuators torque curves, which in Figs. 26 and 27 are compared to the valve operating toques.

0

200000

400000

600000

800000

1000000

1200000

1400000

0 10 20 30 40 50 60 70 80 90

To

rqu

e (

Nm

)

Angle (°)

Spring Return Actuator OPEN Operation

Valve torque OPEN

Actuator torque OPEN

VALVE CLOSE VALVE OPEN

Fig. 26 Actuator vs valve operating torques to open

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

500000

0 10 20 30 40 50 60 70 80 90

To

rqu

e (N

m)

Angle (°)

Spring Return Actuator CLOSE Operation

Valve torque CLOSE

Actuator torque CLOSE

VALVE CLOSE VALVE OPEN

Fig. 27 Actuator vs valve operating torques to close

Springs number and size have been optimized to reduce dimensions and weight considering the following requirements: a) design for fatigue life of at least 10,000 cycles; b) design and material selection for operation at design operating

temperature of -40°C after being compressed for long period.

The solution adopted was to use 2 spring packs with 6 springs inside each pack. The total load given by the springs in compressed condition is 900 kN. In collaboration with the spring manufacturer, the springs design life has been calculated to be not less than 40,000 cycles according to EN 13906-1. Due to the weight of the actuator and the critical application of the valves, a system for installation of actuator on the valve, derived from subsea experience on retrievable actuators, has been studied to minimize actuator installation and removal time on the valve. The interface between actuator stem and valve stem is by means of joint, which is energized by a spring (Fig. 28). During the installation of the actuator on the valve, it is enough to couple the actuator output splined shaft to the valve actuator joint. The coupling can be connected in a unique position due to the particular geometry of the splined shaft. During alignment of the actuator shaft on to the joint, it is not possible to damage the joint since, due to the spring, the weight of the actuator is not directly applied to the joint. The pins have a conical shape helping rough alignment during deployment of the actuator on top of the spool. Once the actuator is

completely installed on the valve, the pins are capable to withstand at least 1.5 times the actuator torque, even without bolting installed. During test it has been proven that installation or removal of the actuator on valve can be performed in less than 30 minutes.

Fig. 28 Valve actuator mounting spool arrangement

Actuator Material Selection

All actuator materials have been selected considering design and temperature conditions, suitable for operation at -40°C. All pressure containing and torque retaining parts (i.e. cylinders, pistons, yoke, shafts) have been manufactured from forgings subjected to NDE including 3.2 certification for materials, ultrasonic examination and impact test at -50°C. Fabricated housing and spring containers have been manufactured in carbon steel plates suitable for low temperature and impact tested at -50°C. All structural welds have been subjected to liquid penetrant and radiographic examination. To improve safety during handling of the actuator, lifting lugs have been designed and machined directly on actuator body avoiding welding of additional plates, Fig. 29.

Fig. 29 Lifting lugs details Spring material has been selected considering safe operation at -40°C (which was demonstrated during qualification test), and to ensure relaxation (loss of spring load after cyclic test) less than 1.5% after 10,000 cycles and less than 4% at infinite time. Hydraulic fluid has been selected to be suitable for operating within design temperatures -40°C and +60°C without sensible variation of viscosity, to ensure stroking time repeatability at different ambient conditions. The compatibility between hydraulic fluid and actuator material has been verified. All seals have been selected to be suitable for at least -50°C and designed for long term operation. Power Unit and Control System Design

Power unit has been designed to accommodate the following requirements:

a) Accumulator sizing for two strokes of two actuators at the same time considering the worst condition temperature variations between -40°C and + 40°C and maximum and minimum operating actuator hydraulic pressure.

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b) Design of a power unit and control system to suit the high reliability project requirements with fault tolerant control system in 2oo3 configuration.

c) Design of and “environmental friendly” power unit avoiding any leakage of hydraulic fluid in ambient.

Accumulators have been sized for requirements at point a) above considering limitations for transportation and installation. This resulted in a compact accumulator skid of five piston type accumulators only, minimizing overall skid dimensions and possible leakage points, Fig. 30.

The Control System was completed with a double hydraulic block fault tolerant in 2oo3 configuration for each valve, to grant high reliability of the system and avoid undesired shut off of PPS and ESD valve due to failure of control system components.

The HPU has been provided with pressure transmitters in proper positions, to ensure that any failure on the control system components is detected during test of the valves.

SIL calculation have been performed to verify SIL requirements for the valve, actuator power unit system, and to establish test requirements and frequency for the valves, to keep high reliability levels for the complete system, and identify any component failures in due time.

Fig. 30 Accumulator Rack and HPU 3D-Models Actuator Prototype Test and Relevant Equipment

The requirement for qualification of hydraulic actuator was for 3,000 cycles at minimum temperature of -40°C and 3,000 cycles at high temperature of +60°C simulating the stroke of the valve. In addition, the performances of the actuator in terms of output torques have been recorded every 1,000 cycles to ensure that the actuator was not degrading, or subject to any undesired malfunction or damage. A test bench has been designed and manufactured, with the capability of building up a variable resistant torque to the actuator output stem simulating the resistant torque of the valve. Static torque delivered by the actuator in any position of the stroke was also measurable. Considering that the output torque of the actuator is 1,250 kNm, the test bench has been sized for 2,500 kNm static torque, Fig. 31. The resistant torque is generated displacing fluid from the cylinders full of oil to the empty cylinders during actuator stroke, building up pressure regulated by and electronic flow control valve. Controlled opening or closing of this valve allows to build up the required resistant load at

any point of the stroke. See Fig. 32 showing test bench working schematic.

Fig. 31 3D-Model of 2500 kNm test bench

Fig. 32 Test bench schematic The resistant torque is recorded by means of load cells on top of test bench recording counter reaction on actuator spool. The test bench has been integrated with a computerized system developed on purpose to perform automatic cycle test at high and low temperatures and recording all system parameters as hydraulic pressures, resistant torques applied to the actuator in any position, ambient temperature and actuator temperature. High and low temperatures have been reached installing an insulating box around the actuator and test bench. High temperature has been obtained by means of an industrial fan heater and low temperature by means of liquid nitrogen expansion in the insulated box. The temperatures were controlled within 2°C. Qualification tests of the actuator were completed and validated at the presence of third party inspection of Lloyd’s, SGS, and DNV. The qualification tests confirmed no significant variations of the actuator performance (torque and operating time). CONCLUSIONS The industry standard high-quality valve concept has been upgraded to suit the very demanding requirements of safety, reliability and durability of the Nord Stream Project. A new philosophy for actuator design has been developed and successfully tested, resulting in the world largest quarter turn actuator, single effect with spring return. The extensive Prototype and Factory Acceptance Tests confirmed the achievement of the primary objective of the design and manufacturing program of the 48-in valves and actuators for the Nord Stream Pipelines, namely a high reliability capable of guaranteeing the required long term operation of the system. The valves have successfully been installed and commissioned at site and are now in operation.

RESISTANT 

TORQUE PRESSURE

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