Boiler start-up valve
Once-through boilers require a minimum evaporator flow to cool the boiler tubes during start-up (minimum evaporator flow for spiral wound evaporators is approximately 30%).
A recirculation system recirculates water out of the water separator back into the feedwater tank or to the economiser/evaporator to ensure the minimum evaporator flow. Different layouts of the recirculation system exist, recirculation of the water from the separator back to the feed water tank and water recirculation from the separator back to the economiser inlet with a recirculation pump. In addition to the above operating modes the start-up valves can be used during boiler-filling and evaporator (air) purging.
This operating mode typically requires high flow at low differential pressure.
- Solid tungsten carbide plug, seat and disk stack eliminates erosion and galling and extends performance life for maximum productivity
- Large flow passages to easily handle solids means continued reliable performance
- Multi-step letdown to control velocity, erosion, noise, and vibration for minimum down time and safety
- Increased rangeability due to a longer stroke means superior performance and control.
- Dual scrapers prevent solids from entering sealing areas for reduced maintenance.
- Venturi seat transition assures smooth flow into valve outlet and piping for problems downstream
- Easy maintenance, quick-change trim means no trim parts are welded or screwed into the body
- One choke body can handle changing wellhead conditions by swapping trim for less down time
- Up to 6 stages of DRAG® velocity control (with total velocity control trim disk stack)
- All trim seals are PTFE. PTFE is explosive decompression resistant
- Seat room purging – The trim is designed with a control edge upstream of the seat which shuts off the flow path for potentially damaging particles before the plug reaches the seat.
- Vena contracta separate from seat: In normal control mode the smallest flow area of the trim is downstream of the seat, keeping potential erosion away from the seat.
- Controlled gradual flow diversion: In the critical high velocity area, special care is taken to achieve a gradually controlled flow diversion preventing erosive water steam mixture hitting trim surfaces with high velocity.
- Anti-erosion coating on control surface: Proven, hard anti erosion coating to ensure long life time of the trim.
- Flashing water is highly erosive if it hits body or trim parts with high velocity. This must be avoided by design or the parts which are hit must be erosion resistant. An angle over-the-plug configuration with its straight outlet for the flashing steam is therefore the most favourable configuration. A rapid increase of flow diameter at the point where the medium expands, avoids erosion downstream of the valve. In the Z-shaped valve body, an outlet cage is needed to protect the body. It is recommended to have a straight pipe run downstream of the valve because the flashing water will also erode pipe bends.
- Boiler start-up valves often experience debris, because the water separator is typically the debris collector of the boiler. On the other hand the valve has to be absolutely tight for long operating periods. If the seat is only slightly damaged by debris which has been clamped between seat and plug, then continuous leakage of flashing water will quickly erode the seat further and increase the seat leakage. This will lead to severe damage of the valve. Therefore the trim design should prevent particles being trapped between seat and plug when the valve is closing.
- Light weight cast body
- Class V shut-off
- Graphite stem packing
- Forged design
- No trim in steam flow
- Tapered profile to minimise flow induced vibration
- Variable spray nozzle