Butterfly valves, ball valves and plug valves are frequently used as shutoff valves since they produce only negligible pressure losses, and thus energy losses, when fully opened. This is an important aspect in times of rising energy costs. These types of valves typically have a flow characteristic where the flow reacts to small positioning steps with over-proportionally large flow variations in the range of small flow cross sections.
This behaviour is reversed in the range of large flow cross sections, which means that even large positioning steps, and thus large variations of the cross section, will cause comparatively marginal flow variations. A typical example for this are valves with equal percentage characteristics where a defined positioning step results in a defined percentage variation of the cross section.
Efficient fast closing of shutoff valves with non-linear characteristic curves
For plants where large mass flows have to be controlled, this behaviour during the closing of the valve is very problematic due to the inertia, and the energy stored in it. In worst case, uncontrolled closing can cause a water hammer, i.e. a pressure peak which may lead to overstraining or even to the destruction of the valve. Pipe implosions can also be caused as a result of the vacuum which may occur upstream of the valve due to the un-retarded continued mass flow.
Really critical are output ranges <15% where almost 50% of the mass flow has to be retarded with less than a fifth of the open cross section. It is state of the art to close pipes, flanges and valves that are subject to water hammer in a controlled way using special mechanical-pneumatic devices or electric actuators, involving the use of additional pipe protection devices.
ABB’s new positioned
The ABB Positioner TZIDC Rev. 2.00 already offered a suitable way for the controlled, slow closing of such critical valves by means of a configurable set point ramp which internally converts step changes of the set point to an elongated linear set point variation. Since the set point ramp function also increases the stroking speed in output ranges without critical flow change, and thus pressure change, the secure closing in TZIDC Rev. 2.00 was however traded off for an unnecessarily long closing time.
The new version, TZIDC Rev. 3.00 now allows the combination of set point ramp with an output characteristic. The described combination allows a fast positioning of the valve in output ranges with uncritical pressure increase, combined with positioning in a manner which controls the pressure increase in the ‘critical’ output ranges.
Up until now, complex and thus expensive mechanical and pneumatic devices had to be used to ensure a controlled secure closing of shutoff valves with equal percentage characteristic. This function is now simply and efficiently realised via the digital positioner TZIDC. In addition, it integrates several options for the feedback of the current position, independently fed if required.
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