900/1500LB Ball Valves

We are creating a spec for a customer based off of the PIP specs. They are requesting that we add Ball Valves to the 900 & 1500 specs. The PIP version does not have ball valves in them...my question is why. Is there some reason that ball valves shouldn't be used at this LB rating? There has to be some reason behind not using the ball valves.

What is the Commodity?
Sour Service. Water with H2S

- What does the "PIP Spec" have as their basic shut-off (Block) valve?
Gate Valve

- Why does your Client want Ball Valves?
The only answer i can get is "because that's the way we have done it before"

- What is the difference in cost between the PIP block valve and the Ball valve?
Cost is not an issue.

- Have you and your Client considered all of this?
Client seems kinda of ignorant.

The only advantage that I see is its quick ON / OFF action and so the decision could be an outcome of HAZOP study. But there could be more, what does the P&ID indicate for Water with H2S service?

Looking at the piping arrangement point of view, the ball valves might have been choosen bacause the gate valves tenads to leakage when place in vertical position. Since, H2S is highly hazardous service, Ball Vlave might have been requested by the client depending upon previous experience. However Ball Vlaves at this rating must have to be metal seated.