I am looking for the proper type of valve to install to bleed pressure off water mains when they have to be taken down for maintenance. The specific instance is a 72" main with a 6" tee and valve piped to open air to release water to drop pressure to zero. The problem I see is with a large volume of water (big pipe at 100psi) moving past a valve as it's opened, it's likely to have some damage to the gate. I see that RW gate valves specifically say 'throttling' is not an intended use. Any suggestions on a readily available municipal water works valve for this application?

Throttling is typically defined as using a valve in a partly open position to adjust the flow in a pipe.  Usually on the discharge side of a pump.  This is generally a long term setting for the valve.  If you are just opening the valve fully, occasionally to blow off pressure, it shouldn't be a big deal.  The valve can probably handle this.  Make sure you open the valve fully open.

If you have seen this damage before, then avoid that type of valve and a plug valve could be used.  A plug valve will work well for this and should have no difficulty shutting tight.  However, it seems to me that the valve must have been left in a partially open position for quite a while.  It should have been opened fully.

It seems that most of the responses are from people that have not worked the field much. Stick with a gate valve, at the higher pressures, you might need to put in bypass line, as for the blowdown, a strainer and PR valve can be attached easly. Bleed down the line with the PR valve, then remove it. You need a gate valve for the blow off, for the duty rating and scouring effect.

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