VLB. Engineering GREAT Solutions. Steam Conditioning Valve

Transcription

1 VLB Engineering GREAT Solutions Steam Conditioning Valve

2 VLB VLB: Steam Conditioning Valve The VLB is an angle-style steam conditioning valve, used as high/low pressure turbine bypass or as a process valve. The VLB acts as a conduit for steam passing between two pressure systems. It is designed to reduce the pressure and temperature of the steam flowing from the higher pressure system to match the requirements of the lower pressure system. The valve has been equipped with a series of pressure reducing stages as well as a desuperheating stage close to its outlet. Regulation of the pressure is performed using a modulating plug, revealing a series of perforations in the valve bonnet as the valve opens. Regulation of the temperature is performed using an external spray water control valve. Spray water injection takes place in the valve outlet, using a series of mechanically atomising nozzles. Key features The bypass valve is connected in parallel with a steam turbine, acting as a secondary conduit between two adjacent pressure systems. Because the two pressure systems may feature widely divergent pressures and temperatures, the bypass valve must be capable of regulating these aspects in a controlled and predictable manner. > Upstream steam flow and downstream temperature is measured and used in regulating pass through and spray water injection. Steam passes through the extended drilled bonnet before being throttled though a cage type plug with multiple drilled holes which provides optimum characteristics for control of steam pressure > The extended bonnet prevents possible rotation forces caused by the steam flow, and acts as an extra pressure reducing stage. The extended bonnet also works as a strainer, protecting the valve seat and bonnet from damages > The plug slides in a hard surface hardened body covering/uncovering throttling holes, regulating the flow through the valve. In its closed position, the plug seats on the hard-faced seat, isolating the inlet from the outlet > Pressure reduction continues in the outlet, through the perforated cylinders. The cylinders increase the steam velocity and turbulence, improving the evaporation of the spray water > Spray water flow is controlled by an external water control valve before being injected into the steam flow > Homogenous forged body minimises thermal stress > Fully customisable inlet and outlet connections > Available in balanced tight design which reduces the required opening forces Pressure seal bonnet Pressure reducing pipes Plug / cage Spray water nozzle

3 Benefits > Allows controlled startup and shutdown of different loops in the power plant with minimum heat losses > Handles abnormal conditions such as rejection, turbine, pump or fan trips etc. in a manner to return the system to normal operation with minimum delay > Customised for each specific application > Fully machined circular section valve body reduces thermal stress, improves thermal cycling and increases the life time of the valve. > High performance and stable control despite pressure, flow and temperature transients > Keeps the steam in balance with load requirements e.g. in process industries > Reduced maintenance cost and downtime > Low noise > Customised inlet/outlet connections to suit customer steam pipes > Complies with the following standards: ASME, SIL, PED, IBR, CRN, FaMA, MoM, Gost, ISO 9001/14001/18001 Design > Pressure reduction and noise abatement The VLB features multi-stage pressure reduction with single-stage controllability. Through-put is controlled by a modulating plug. The initial pressure reduction takes place at the drilled bonnet cage. A second stage is positioned in the outlet, where the level of pressure reduction is finalised. Pressure reduction in both stages is achieved using perforated cylinders, where small holes are breaking the steam up into smaller fluid jets. This has the result of reducing noise as well as shaping the flow pattern for more efficient desuperheating downstream. > Desuperheating The VLB features a series of spray water evaporating nozzles inserted in its outlet. Spray water is supplied to the nozzles through a water pipe encircling the outlet. The pressure reducing pipes create a flow pattern which directs the flow of steam and spray water away from valve outlet walls, while increasing the velocity of steam passing by the spray nozzles. This gives efficient mixing of water and steam and is a key factor for desuperheating performance. > Construction The VLB features a fully forged valve body, designed to withstand rapid temperature changes. Inlets and outlets are circular in order to prevent asymmetrical stress patterns. The seat facing is made of a tough material with excellent sealing properties and good resistance against corrosion. Plug and stem are made of heat and corrosion resistant material, hardened in a unique process. Both inlet and outlet pipe diameters are designed to fit the steam pipes. The valve features a vacuum tight pressure seal bonnet, allowing easy maintenance and removes the need for tightening using screws. > Actuation The actuator is installed on top of the valve and fastened using a yoke. The yoke provides both rotation stop and a place for mounting limit switches. IMI CCI can supply pneumatic, hydraulic or electrical actuators sized to handle the forces needed to open and close the valve at the required stroke time, and to compensate for the forces caused by the system pressure pushing against the plug. > Design options IMI CCI has developed a series of design options for the VLB trim. Customisations can be accommodated based on customer demand, including preheating and drainage connections. K v K vs 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Valve stroke The image shows how at the first ~15% of the valve stroke, the change in flow coefficient increases only by 5%. After this point, it becomes linear Q 0 (l/h*1000) Capacity curves for a single nozzle with standard spring settings Δp (bar)

4 Applications > High pressure system - Pressure control / turbine bypass - Controlled pressure buildup in the boiler - Protection against exceeding the design pressure > Low pressure system - Controlled pressure buildup in the reheater - Pressure control / bypass of the intermediate and low pressure parts of the turbine. This helps avoid the release of safety valves and consequently helps to prevent large condensate losses. - Protection of the condenser in case of disturbances > Process industry - Controlled pressure in steam pipes to process, in parallel with the back pressure turbine - Provides the process with steam flow during startup of the turbine - Fast takeover of the entire steam flow from boiler to process in case the turbine stops/trips - Suitable with large quantities of cooling water Spray nozzle operating principle The atomising spray nozzle is housed inside a nozzle holder inserted into the pipe outlet. Water is routed through the pipe leg and the nozzle chamber before being supplied to the spray nozzle. > The nozzle itself has a spring loaded plug which extends as the pressure in the nozzle holder increases. The amount of water being injected by each nozzle is determined by a number of factors, including the diameter of the nozzle body opening, adjustment of the spring, and the pressure differential between the steam inside the desuperheater and the water pipeline. > The cooling water enters the inner nozzle chamber through a number of water channels. Water is rotated around the nozzle plug thanks to the special arrangement of the water channels. The plug and the seat are designed to create maximum water velocity at the nozzle edge point. The high velocity of the water when it leaves the nozzle guarantees fine atomisation, quickly evaporating the spray water. > In order to maintain a specific opening water pressure inside the inner nozzle chamber, the nozzle plug is preloaded by a spring. The force required to open the nozzle is set by the adjustment nut. > Spring strength and adjustment nut settings are chosen based on the pressure differential between the outlet steam and the water. Pressure inside the waterline must be high enough to prevent the water from flashing before exiting the nozzle chamber. > As the spray nozzles sprays perpendicular to the steam flow, the high relative velocity of water to steam creates an efficient secondary level of atomisation Pipe leg 2. Nozzle stud 3. Flange 4. Packing 5. Gasket 6. Nozzle holder 7. Spray water atomising nozzle

5 Configurations > Balanced Tight design (BTC) Selected when leakage tightness according to ANSI FCI 70-2/EN 1349 class V is required and pneumatic actuators preferred. Excellent selection also together with hydraulic and electric actuators. The pilot plug design makes it possible to reduce the actuating force by normalising the pressure in the valve and bonnet. > Balanced design (BC) Selected together with pneumatic, hydraulic or electric actuators where the leakage tightness according to ANSI FCI 70-2/EN 1349 class III or IV is acceptable. > Tight plug (TC) Selected when leakage tightness according to ANSI FCI 70-2 / EN 1349 class V is required. No pilot plug or pressure normalising. VLB-BTC VLB-BC VLB-TC Product specification Valve sizes 8-500mm seat diameter Other sizes available on request Pressure class ratings Up to ANSI-4500 (higher rating on request) Above DN400 Design temperature Up to 650 C Leakage class ANSI Class V for valves with tight plug (TC) Class III or IV for others. Rangeability 1-40 Regulatory standards ASME, PED, IBR, CRN, FaMA, MoM, Gost, ISO 9001/14001/18001, SIL SIL classification SIL level up to 3 achievable for both quick open and quick close depending on system configuration. Materials Forged material adapted to connecting pipe material. Example Steam turbine HP 2. Steam turbine LP 3. VLB HP bypass to cold reheat 4. VLB bypass to condenser 5. VST-SE bypass to auxiliary steam 6. VLB to auxiliary TT. Temperature transmitter PT. Pressure transmitter ZT. Position transmitter DCS. Customer DCS 5

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