When the silo is fully filled, then silomaxcapacity(s)=silocurrcapacity(s, t) and, of course silomaxcapacity(s)- silocurrcapacity(s, t)=0, right?

Then you have

silomaxcapacity(s) *(1-y(s, t)) >= 0

 -silomaxcapacity(s) *(1-y(s, t)) = 0

-silomaxcapacity(s) *(1-0) =-silomaxcapacity(s) silocurrcapacity(s, t)

y(s, t)=1 when silomaxcapacity(s)=silocurrcapacity(s, t)

Try

(Max-Current)+(Max*Y(s,t)) 0;

@przymeslaw yes! I am expecting 

y(s, t)=0 when silomaxcapacity(s)>silocurrcapacity(s, t)

y(s, t)=1 when silomaxcapacity(s)=silocurrcapacity(s, t) 

@Pete Bettinger 

in the second condition there is no equality sign. But in constraint programming is it allowed to use in this way?

if not, then y(s,t)=0 will satisfy both the constraints right?

@Pradyumna Krishnan, yes, you're right. If  there were (Max-Current)+(Max*Y(s,t)) >= 0 then for Current=Max both y(s,t)=0 and y(s,t)=1 would satisfy the two constraints at once. With > 0, as in Pete Bettinger's example this would be OK. However, > and < constraints are not valid in LP and in MILP, either. I don't see a way to avoid them.  I'm afraid that strict implementation of your conditions of switching may not be possible in terms of MILP i.e with = constraints only..

I can't help you with constraint programming.

Could you please mention the effect of y(s,t) to the objective value?

for example:

If y(s,t) = 1 is better than y(s,t) = 0, the constraint can be: Cur >= Max * y(s,t)

Otherwise, the constraint can be: Max - Cur >= 1 - y(s,t)

@ngoc anh dung Hello!

My objective is to maximise the conveying capacity q(silo) of particular recipe (pellets) to the silos. I have a binary variable silobinvar(s,t) which selects the silo to be filled. The recipe has to fill the silo fully upto maximum capacity. After that point, unloading of recipe has to be done.

In order to tell the program to start unloading, binary variable y(s,t) has to be turned on. But it is not related to my objective to increase the conveying capacity.  So I think I have explained you about the process.

Hope we can able to discuss about this.