20 inche gate valve with SOLID WEDGE 2

99% of the time a flexible wedge can be substituted for a solid wedge without an issue. The only service I have run across where a customer truly did not want a flexible wedge was a ash type service where they had had problems with the ash compacting and sticking inside the flex wedge groove.

Even in that case they still wanted flex wedges in some of the larger lines and they installed steam purges on the valve body to occasionally blast clean the wedge groove.

The purpose of having flexible wedge is to allow for compensation for any angular misalignment between your disc and your seat. If you were to have a solid wedge, and an angular misalignment of 1 degree, for a 20" valve, this is a significant reduction in seating contact, which could potentially result in leakage.

For smaller valves, it is easier to achieve the seat to disc alignment, thus a solid wedge is possible.

My advice, let them know a flexible wedge is the better option and the potential for future maintenance headaches. If they do not agree with you, then give them what they're paying for.