Break & Seating Torques, Are they Equal? 1

[Jerri76]

I will like to put this question out to all valve engineers, in hope of a valued explanation:

1. For a conventional butterfly v/v, is the breakaway & seating torques equal?

2. What equations/formulae can be used to relate the relevance of these 2 torques?

I will be grateful if there is anyone who will spare some time to reply my question.

Thank you for reading.

Jerri

 

[Jerri76]

I have with me here a set of formulae which relates to upstream & downstream break & seat torques:

For Shaft Downstream:

i. torque tp close v/v is given by:
Tseat= -Tpacking -Tseat -Thandwheel -deltaPmax(bearing torque factor + off balance torque factor)

ii. torque required to open v/v is:
Tbreak= +Tpacking +Tseat +Thandwheel +deltaPmax(bearing torque factor - off balance torque factor)

For Shaft Upstream:

i. torque tp close v/v is given by:
Tseat= -Tpacking -Tseat -Thandwheel -deltaPmax(bearing torque factor - off balance torque factor)

ii. torque required to open v/v is:
Tbreak= +Tpacking +Tseat +Thandwheel +deltaPmax(bearing torque factor + off balance torque factor)

Can anyone throw some light on it?

THANKS.

 

[kevinf]

Valve seating/unseating torques can only be estimated by emperical methods per vendor and per design and per model number of a valve. Manufacturing tolerances and material variations are involved. These will be rough estimates at best and as soon as the valve sits un-stroked for a while or is exposed to media the torque values can change by as much as 50%. If you are concerned about actuation power required, take the break-away or seating torque values stated by the manufacturer and add at least 50%. That should suffice for all commercial type applications, even for cooling tower seasonal changover for example.

 

[Fog1]

I agree with Kevinf on this one. Only the manufacturer can provide you with reasonably accurate breakout and seating torques, as these will all be design specific and established during test work.

Naturally the manufacturers will try and keep these values as low as possible but they will be different from valve design to valve design and seating material to seating material.

The seating and breakout torques are not necessarily the same indeed breakout can be time dependent, depending on the seat design and can increase with time.