Some details of the intended service:
? high design pressure: >70 MPa (>10.2 ksi)
? bore sizes: 13 to 18cm (5" to 7")
? valves will have a geared operator, and be operated by ROV
? water depth is >1000m (>3,281 feet)
? the frequency of operation will be only a few times during the design life (~25 years).
? valve should be open for all of life, but could be closed at some point
? operational fluids will be raw, directly from a subsea well
If particulate is an issue, and you decide to use a ball valve, a metal-seated ball valve would be in order. Just a precaution with ball valves, if you were to close the valve and bring it to the surface at some point, the ball would trap the pressurized media. You may wish to consider a vented ball.
Corrosion and encrustation are big issues- it is a good idea to stroke the valve every few months to be assured it will close when required.
The ball valve can be re-opened if there exists a large pressure drop across across the valve , but a closed gate valve cannot be easily opened if a large DP exists.

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