Why quality pays off

The failure of a valve in a cooling water system can have fatal consequences.

In cooling water circuits pumps, valves and their drives constantly regulate coolant flow and ensure continuous power plant operation. A failure of the pump doesn’t just represent a singular equipment failure, but has a direct impact on connected plant systems such as pump drives as well as indirectly connected systems such as condensers. An ultimately economic safeguard against (consequential) damages such as replacement investments in new pumps, motors or the costly start up of the systems lasting several days can, through structural measures in the planning phase, be largely if not entirely avoided.

What basic options are available to effectively and inexpensively protect the systems in cooling circuits?

During the pipe planning phase planning engineers have to explore the instruments and security systems in advance and to combine them optimally with one other in order to achieve the best possible protection at the lowest cost. By doing so, it is essential to consider the economic aspect in particular and to specifically employ redundant mechanisms only when necessary.

The example of a failure of the cooling circuit pump should demonstrate two essential safety systems: Following a short, system-dependent period, once the return flow of the medium begins, all currents should already be stopped in order to protect the pump and especially its drive from a reversal of motion against the operating direction of rotation. A reverse rotation caused by the reverse flow of water must be avoided at all costs to prevent the pump from being operated as a turbine. In particular, excessively high centrifugal forces and speeds can destroy the electric drive directly. A highly effective method of protection from rotational movement opposite the direction of operation is provided by special fittings in the form of non-return valves installed upstream from the pump. Medium or externally-controlled non-return valves are used here.

The latter are preferred for installation in conventional power plants, because system-specific properties can be accounted for. As an alternative to non-return valves, integrated freewheels (mechanical barriers against reverse rotation) in the pump unit provide efficient yet simple protection. Using these reverse rotation stops prevents a reversal of direction on the basis of a self-shifting clutch.

Modern, high quality pumps are equipped with such an automatic locking system. In contrast to non-return valves, which block the pipe completely, reverse rotation stops do not prevent any backflow of the medium; the water thus flows through the stationary pump, so plant systems upstream and downstream from the pump are not protected: the pipe system along with all plant components drains itself in an uncontrolled manner, in extreme cases, completely. A drained condenser in particular is undesired, because refilling the thousands of cooling tubes (15-30 mm in size and for the most part over 10 m long) in the tank may prevent rapid start up after the fault is remedied.

Complete and thus reliable protection is provided only by valves equipped, for example, with a hydraulic brake and lift unit, which gracefully shuts off the water completely. Another advantage of such a system is that with hydraulically controlled drives the closing time of the valve can be individually adjusted to the conditions of the whole system in order to safely and efficiently prevent additional damage such as water pressure surges due to the abrupt stopping of the pump. Cleverly combined, such a valve can perform several functions: as a non-return valve for pump drive protection, as a controlled shut-off valve to prevent pressure surges in the piping system, as a pump start-up valve for specifically starting up pumps within the characteristic curve as well as a shut-off valve to prevent backflow.

Compared to a purely medium-controlled non-return valve, in the VAG concept using a shut-off valve equipped with a brake and lift unit the hydraulic locking of the drop weight causes no energy to be expended for the open position of the shut-off valve, which significantly increases the energy efficiency of the system. With additional options such as a UVV safety lock the valve can also serve as a safety fitting in order to safely perform maintenance work in the pipe without an additional safety valve.

Combined with the hydraulic VAG HYsec Pro drive unit and the optional UVV safety lock, the double eccentric designed VAG EKN® Butterfly Valve combines all of these functions and has already proven itself in many power plants worldwide.