Taking the plunge for precise control

27 September 2011



Plunger valves allow for precise control of water supply systems, including hydro plants, writes Peter Oppinger


As early as the 1930s, the performance requirements of water supply systems and hydropower stations increased in such a way that pipeline diameters and pipeline lengths, as well as operating pressures and flow velocities, became considerably higher. The early shut-off devices used – mostly wedge gate valves – had to be optimised and adapted to meet the increased demands. Despite the fact that wider sealing rings were used and wedge guides were reworked or extended, in the end the gate valve was considered to be unsuitable for meeting new requirements.

At that time, the annular slide valve was considered a special type of valve. The round cross-section of the annular slide valve was transformed into an annular cross-section by an internal body, which turned into a round cross-section again at the outlet of the valve. Over the course of the years, the annular slide valve evolved into the annular piston valve and/or the plunger valve.

Plunger valves are special control valves, designed specifically for control tasks in water supply. Unlike gate valves and butterfly valves, plunger valves can meet the special requirements for controlled operation. Plunger valves are mainly used where volumetric flow rates need to be precisely apportioned or where wa-ter pressures have to be accurately controlled or reduced.

The plunger valve is a straightway control valve, i.e. it has an annular flow cross-section in any position. Inside the body, the plunger (also referred to as piston) is moved axially in the flow direction by a crank gear towards the sealing seat of the valve. Plunger valves are control and regulating devices which generate different pressure losses in pipeline systems by way of continuous narrowing towards the seat to change the flow rate depending on the regulating distance. Depending on the application, the nominal diameter of the valve has to be large enough to ensure that at the lowest pressure difference the highest required flow rate is achieved and/or that maximum differences in pressure are reduced without causing cavitation. In addition, any damage to the down-stream pipeline system or structure by vibration or cavitation must be prevented over the entire regulating dis-tance.

VAG has evolved the plunger valve, to make it suitable for control tasks in water supply systems. Today’s VAG RIKO Plunger Valve is available in almost all nominal diameters between DN 150 and 2000 and in pressure ratings ranging from PN 10 to 40. A great number of plunger valves are in use world-wide, even valves of sizes DN 600 and DN 800 with a nominal pressure of PN 100. In all sizes and pressure ratings, the compact body is made of premium-quality ductile cast iron EN–JS 1030 (GGG 40). Up to DN 600, the inner parts are made entirely of grade A2 stainless steel as standard. For all nominal diameters up to and in-cluding DN 2000, the piston is guided by bronze-welded longitudinal guides. This ensures optimal longitudi-nal guiding and thus the backlash-free sliding of the plunger with very low operation torques at the same time.

The body consists of one piece and the inner body is connected to the outer body by ribs. On the inlet side, the inner body has a spherical shape. The design of the annular space results in a continuous narrowing towards the seat. The piston or plunger is moved by an internal crank drive. New sealing systems for the body seat, the plunger and the shaft bearings have been developed to provide absolute corrosion protec-tion, excellent performance and a long service life. Due to the fully enclosed bearing, no fluid can penetrate into the bearing, to ensure high durability. The sealing of the shaft was adopted from the proven sealing prin-ciple of the VAG EKN Butterfly Valve, i.e. the shaft is sealed by a double O-ring seal. Inside the valve, be-tween the body and the plunger, a quadring seal sealing on four sides is used as a double seal. It is resis-tant to twisting and ensures permanent tightness even under high stress cycles. The valve seal at the seal-ing seat was arranged in the no-flow zone in a way that prevents stones and pebbles from causing damage to the valve seal and minimises wear of the seal. All seals are approved to W 270, which allows their unre-stricted use with drinking water.

Planning and design

Plunger valves are modulating valves which, according to the throttling principle, generate pressure losses in pipelines to change the flow rate according to a desired characteristic depending on the regulating distance. The nominal diameter of the plunger valve and its regulating behaviour are always dependent on each other. However, they can only be determined one after another and then have to be adjusted to each other. Their regulating behaviour is always influenced by the resistances present in the entire plant. This means that the longer the pipeline, the higher the resistance, and the larger the total of the resistances present in the plant is in relation to the resistance of the open plunger valve, the lower the throttling effect will be.

Outlet type / Seat

Due to its linear control characteristic, the VAG RIKO Plunger Valve ensures excellent cavitation behaviour and very little pressure loss at the same time. Its outlet type is variable and, coming as a kit, allows the change of the valve characteristic. This is an essential advantage as the valve can be adapted to changed operation conditions even after its delivery. Depending on the application and the operation specifications, different types of outlets such as standard seat ring, orifice cylinder or slotted cylinder as well as various cus-tomised cylinders to prevent cavitation are available.

UseCAD design software

The individual design of the control valve and its various outlet types can be realised using VAG’s proprietary UseCAD software tool. The specifications needed for reliable design include the dynamic pressure upstream and downstream of the control valve as well as the desired flow rates Qmin and Qmax. This allows the accurate determination of the control behaviour. Cavitation-free control behaviour should always be the goal.

A highlight of the most recent version – VAG UseCAD 6.1 – is the automatic generation of 3D images of vari-ous types of valves. The -D solid models generated with VAG UseCAD 6.1 can be exported into all common 3D formats such as DWG 3D, IGES 3D, SAT 3D or STEP 3D. Special attention has been paid to the so-called critical valve components as their movability can have a decisive influence on the alignment of the plant.

Besides the 3-D generation the VAG UseCAD 6.1 supports all areas of conventional valve planning and supplies valve knowledge in a condensed form. The electronic valve catalogue provides extensive technical information. Besides technical data sheets, sales texts and valve symbols, the VAG UseCAD also supplies 2D projections of the entire range of valves for conventional -D planning.

The function of pressure loss calculation rounds off the valve catalogue. This additional program allows the calculation of the pressure losses developing during flow – over the entire range of various pipeline ele-ments. With its user-friendly addition of the valves and pipeline components used, such as elbows or divid-ers, the program calculates the resulting pressure loss. The results can be exported and can thus be adopted into the user documentation fast and easily.

Fields of application

VAG plunger valves are suitable for use with raw water, drinking water and cooling water with temperatures of usually 50° C. The main fields of application of plunger valves include: conveyance of water in catchment basins and dams; bypass lines of hydropower stations; long-distance pipelines; water treatment in water-works; water supply in pump stations; supply control of elevated tanks; drinking water networks; and cooling water circuits of industrial and power plants.

Peter Oppinger, VAG Armaturen GmBH. Email: [email protected] www.vag-armaturen.com



VAG Armaturen's work at hydro plants: case study - Rur Dam Schwammenauel, Germany

The Rur dam Schwammenauel in the North Eifel mountains has a capacity of 203Mm3, making it one of Ger-many's largest dams. It catches the rainfall in the headwaters of the Rur.
At the project, steel pipes and bifurcations were built into the last 50m of the tunnel to the dam's bottom out-let. A total of three pipes (2 x DN 2000 and 1 x DN 2300) ran through the valve house, which is on the downstream toe of the dam. The two smaller pipes served as bottom outlets and flowed into the adjacent stilling basin. The biggest of the three pipes, which was used as a spillway, ran above ground for 80m to the power station.
The two Ring Valves that regulated the water flow from the bottom outlet pipes and all of the pipesââ"šÂ¬Ã¢"žÂ¢ pipe burst safety systems that were installed in the valve house had the same nominal dimensions. The over 60-year-old valves were showing increasing signs of wear and were becoming increasingly fragile. Mid-2002, the bottom outlet operations had to be cut back. Inspections and tests followed. The ensuing market analysis and feasibility study were presented at the beginning of 2004. The result: the valves in both bottom outlets and the spillway had to be replaced.
VAG-Armaturen was chosen to replace the valves with Hollow Jet Discharge Valves because they met the output capacity requirements and also close better and more reliably during emergency shut-offs. The techni-cal requirements were discussed in detail during several on-site visits: the dimensions, the material for each of the components, the coating system for the inside and outside of each of the valves and the actuation re-quirements.
The inside of the new pipe section was reduced to connect the Hollow Jet Discharge Valve to the pipe burst safety system, which was connected directly to the existing pipeline with a short DN 2000 to DN 1800 tran-sition adaptor. Due to the space constraints in the valve house, VAG produced a short, welded, DN 2000 Butterfly Valve as a pipe burst safety device.
The Ring Valve is at its widest in the middle, and gets smaller towards the outlet and the water side of the valve house wall. The diameter of the Hollow Jet Discharge Valve, on the other hand, steadily increases up to 3.6m. These new dimensions meant that work needed to be carried out on the water side of the valve house wall and the bridge crane's runway girders. Old pictures showed that the old valves had been trans-ported and fitted in the skeleton valve house. The current gate was too small and the crane system too low, which meant that structural changes were necessary.
The construction of the hydraulic steel structure started on 13 June 2005. The work was carried out by Hein-rich Scheven Anlagen- und Leitungsbau in Erkrath. VAG delivered all of the required valves.
So that the construction could start on the hydraulic steel structure, the downstream bottom outlet tunnel was closed with a Roller Gate and sealed off from the dam. The 7000m3 of water that remained in the tunnels were drained through the old bottom outlet valves and the tunnels physically inspected. The old valves were then removed.
In order to dispense water from the dam at short notice, the bottom outlet valves and the power station pipe-lines were removed at different times. Because of the space constraints, the two new VAG Hollow Jet Dis-charge Valves were placed in front of the end of the two pipe sections.
Only then could the remaining pipe segments and valves for the bottom outlet pipes and the ventilation pipes for the Hollow Jet Discharge Valves be lined up and installed.
The abutment wall for the Hollow Jet Discharge Valve was repaired before the work started on the power station pipe. This guaranteed that a pipe would always be available to dispense water during the construc-tion work.
The two guide pipes were flanged onto the Hollow Jet Discharge Valves from the outside and the actuators installed. The old DN 2300 pipe burst safety valve was removed from the power station pipe. To install the new valve and the additional dismantling joint, the pipeline was separated on both sides with a cutting torch and prepared for the welding of two DIN flanges. The welding joints were X-rayed and the installation suc-cessfully completed.
The bottom outlet tunnel was flooded and the Roller Gate removed. The pipes and valves were again sub-mitted to a leak and function test at the beginning of September so that the serviceability and the higher ca-pacity of the Hollow Jet Discharge Valves could be verified against the calculations.



Valves Valves


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