High Seat Torque

[Sid89]

We have a seat design for ball valve that produces high break torques. To save actuation costs, we need to redesign the seat geometry in order to reduce the torques. Is there any guideline or book for seat design that advises on seat geometry for low torques and at the same time not compromising its sealing function?

 

[SPDL310]

Sometime you can bring torques down by a geometry redesign. Quantifying the resilience of your seal ability will be costly, and/or take a significant period of time especially if there is a long history of field use for your old geometry. Most approaches will work on reducing the contact area, the contact force or some combination of the two. You may have an extremely inefficient design that has a lot of fat to trim, but unless that is the case you will most likely be robbing from Peter to pay Paul.

You will need to give some more detailed information about your valve to get a better response. Is this a trunnion or floating valve? Metal or soft seats? are you seeing high break torques under full differential pressure or at low pressure? What size is your valve? Any possibility of recommending a bypass line toe equalize pressure to allow for a cheaper actuator?

 

[Sid89]

@SPDL310 Thanks for your reply.

This is a 2" floating ball valve ANSI Class 1500 with soft seats (PEEK material). High torques are seen under full differential pressure as well as low pressure. Bypass line cannot be used so we must have to redesign the seat geometry.

 

[SPDL310]

Yeah no one is going to install a bypass for a 2" floater. I thought that you may have a issues with a big bore valve when you were talking about reducing the actuation cost. Here are a few things I can think of off the top of my head.

You can conduct testing to determine the coefficient of friction between your PEEK and ball material. You will want to be relatively consistent with your sample piece surface finishes. With that data you can determine if a surface finish, seat material or coating change will reduce the COF. You can work that back into a roughly proportionate decrease in torque.

You can perform a ball and seat contact FEA to determine you current ratio of seating stress to operating pressure. I cant give you any golden ratio here this is closely guarded proprietary information for every manufacturer. If it is looking high you can change your geometry to reduce contact area or seat piston seal diameter. you may be putting a lot of seating force into the seat before you ad any pressure if the break torque at low and high pressures are close together. Yo can try to lighten your ball by removing unneeded material. This will lead to less force required to support the ball.

You may be floating too far off center while seating. Being off center will lead to you ball creating a camming effect when rotating.

 

[Nick_Gray_88]

It seems like a common way aside this high-torque issue.
How do you know you have high torque? Is it restive to a competitors published data??
Your tolerance stack could be giving you issues, but probably not of a noticeable magnitude if you have enough float in other connections.
You can solve this all numerically without FEA, research some basic sealing theory and applying it to the seat design with some mechanics theory and you are sorted.
If you just throw it into FEA you’ll have no idea if the output is actually real unless you can get a feel for what it should be beforehand…