MAST (Maximum Allowable Shaft Torque)

[ballvalveguy]

Does anyone know of an official test to determine the MAST of a ball valve? Should the test be done at the maximum temperature rating of the valve?

 

[valvit]

Most manufacturers will report MAST at Maximum Allowable Operating Pressure (MAOP). At maximum temperature rating, the MAOP is lower. If the valve MAST is lowest at maximum temperature and the coincident pressure, that is the value you should report.

Valvit

 

[TOVmania]

This is an open forum and, therefore, I understand that it can be seen as normal for someone who is not with an engineering degree to ask a question about a concept usually taught in ME 101 or 102.

Just one question Valvit, what does pressure has to do with MAST? They are connected in a way but in the way you have described.

 

[valvit]

TOV,

All valves must open or close with pressure in the pipeline. With the valve in the closed position, and full pressure (MAOP) on one side of the valve, and low to zero pressure on the other side, this is known as full differential pressure (aka delta-P). Most valves, when opening against the full differential pressure, will need to deliver the highest torque or thrust in order to open. At that point, at that highest pressure, is normally when the stem torque (or thrust) produces a stress that is closest to the maximum allowable stress in the drive train components...hence the name Maximum Allowable Stem Torque.

At lower differential pressures, the stem torque (or thrust) produces lower stresses in the drive train components, and therefore, are not the critical consideration.

Hope that helps.

Valvit

 

[TOVmania]

I was going to leave this thread as is, but I changed my mind.

ballvalveguy is asking how to test MAST and testing environment related to determining MAST value. I am not saying your initial statement is completely incorrect. What you explained is valve torque not MAST(Maximum Allowable Shaft Torque).

So here is my summary. Given that the material is compatible with the fluid,

1. In order to determine or compute MAST value of a shaft, engineers must start with design conditions including design pressure and design temperature. Usually maximum testing conditions, which is more severe than operating conditions. (Unlike what Valvit stated, it shall not be based on operating temperature or pressure.) The strength of material changes with temperature. Their relationship is inversely proportional, so one must be fully aware of the changes of mechanical properties of the material. Generally, the higher the temperature the lower the strength. However, this does not necessarily apply at cryogenic temperature and there is a guideline for low temp. application.

2. Geometry of shaft and design such as pins or key ways used for transferring torque from shaft to ball will affect the shaft strength.

3. Shaft diameter is another factor in determining MAST value of typical configuration. This relationship is proportional. Thicker the shaft, the higher the MAST.

4. MAST must be determined on the weakest part of the shaft, where the failure is most likely to occur.

Once the MAST value considering above design criteria is finalized,

1. It is time to compare with with valve torque values, which is what Valvit has described in the post.

2. These valve torques vary with temperature and pressure, and the values are specific to each design and quality of valve as well as their fluid controlling capability.

3. These values must be empirically acquired and statistically analyzed.

Now that one has the MAST value and valve torque figures, MAST value shall be sufficiently greater than required valve torque value under MAOP and operating temperature for proper sizing of operating element.

Testing MAST is mostly unnecessary. It may only be required if there isn't sufficient and reliable information to determine MAST value. Just verify it in stress simulation.

However, verify valve torque figures under stated condition to confirm that it still provide some safety factor against MAST value.

Thanks for reading my long post.