anoopbindal,

I am used to water seal vessels being vertical in flare systems. Remember that the purpose of the water seal vessel is to:

1. Create slight pressure in the flare system.
2. Prevent air ingress into the system upstream of the seal vessel.
3. Prevent flashback from flare to flare header.

For this reason the vessel needs to be quite "tall" to accommodate the dip legs so is normally vertical. (I have personally not seen horizontal ones). A horizontal vessel that achieves the above three things would also be acceptable but probably much more expensive due to very large diameter.

The liquid knock out drums on the flare system however can be either horizontal or vertical. Which to use is a purely economic decision based on the layout of the system (slope of flare header) and the amount of liquid expected in an upset/emergency situation.

I agree yours with respect to flare water seal drum to be vertical, but for flare KO Drum, we used to use it (and we recommend) to be horizontal to allow enough resident time to damp (or at least to minimize) the pulsation of gases (coming from downstream of safety valves) and to allow for depositing of liquid droplets accompanied with flare gases.