We have an slow closing valve in the end of a vertical pump column which does not function correctly. The float is being kneaded. We are thinking to install an orifice plate in the flange of the base of the valve to reduce water hammer.This will help to solve the problem? Or it has another solution?

When the vertical pump starts, the air inside the column (above  the water level) will be forced by the water pumped  to escape by the slow closing valve- there is a check valve, just after the valve, retaining downstream water. The water velocity  in the column depends on the velocity of air exit. We are intending to reduce the velocity of air escape with an orificie before the valve (with a minor diameter than valve). Thus the water will arrive at the float  with lesser velocity and minor impact. (When the impact occurs the float is launched against the valve body  being damaged.

If your "slow closing" valve is a float controlled air vent, then an upstream orifice plate will work just fine on pressure rise. You also need to consider what will happen when the pump turns off. It may be necessary to let extra air in via a check valve, or something fancier, when the pump stops.
In fact you may want to review the entire system hydraulics as the existing valve may not be necessary if the air can be carried away with the water.
 
It may be worth considering that if the float is being deformed (kneaded?), then you have some fairly serious pressure spikes to deal with.
The normal ways of dealing with them are:
1) keep some air in the system to cushion the impact. This is what your orifice plate will do.
2) Stop the pressure rising so fast. This is what a pump control valve, and, to a lesser extent, a fixed bypass will do. Alternatives are variable speed drive ramp start and soft starter. With the soft starter you will find that everyone says they are great at the beginning, and then they get evasive when you request finalised speed/time information, so be careful before committing.

And it is normally best to have the check valve close to the pump discharge; not at the top of the line. Reduces vacuum hammer problems, and will reduce the amount of air getting in as the riser will not be under suction when the pump is off.

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