Typical (average) MTBF for Actuated Ball Valve

[Orga78]

Hi,

This post is related to the previous thread "ball valve & actuator assy in vertical orientation".

I'd like to get some inputs on a typical MTBF for a ball valve in 110 bar, 80 degC operating conditions, vertical mounted.

Thanks!

 

[gerhardl]

MTBF (= mean time between failure?)

Any mechanical device, made up of several components, will obviously depend on the quality of each component, nothing better than the weakest link and weakest part of any component.

Any comparison between similar (but not in all parts equal) products, could give an indication, but nothing more.

The best practical advice would be to check and test your product at the time for normal production equipment check for other production equipment. If really critical for the plant, and high cost of unplanned stop, it would be reasonable to have one complete unit (actuator inclusive) in stock as spare, and change to the new at regular inspection stops. You can then check and repair the one taken out separately.

For theoretical information it would probably interesting to have a look at the European view:

https://en.wikipedia.org/wiki/Safety_integrity_level

or this forum thread408-180990

 

[Orga78]

Thanks gerhardl.

Yes, MTBF I meant is Mean Time Before Failure.

We've just got 3 occurrences with failed ball valve (stem tilted due to side load, exerted by heavy actuator), and all occurrences are days apart. Thanks for your input on the risk assessment for the failure, and the need to look at our spare parts strategy.

 

[gerhardl]

Hello Orga78,

I see now the connection with your thread408-428919.

With your post above describing incidents 'days apart' you have either (if this is different, but similar position for more than one valve) a systematic failure after x hours in process, or (if repeated for one valve after repair) already weakened components not changed, contributing to new failure.

The cause of failure for ball-valves with actuators, not including failures of the actuators itself, is most often wear of sealing, often given by extreme fluid conditions with abrasive actions and deposits.

A 'normal' failure sequence is, depending on application, somewhere between three months and factory's lifetime, average perhaps 1-3 years up to 10-25 years.

Both type of repeated failures will show you nothing but weak, not correctly selected and/or supported equipment for your purpose. 'Tilted stem' may be caused by heavy weight, but also tells you that some of the stem construction or bearing is too weak.

Sidewise mounting can contribute to abrasive wear of sealings/bearings for the stem.

Check that the ballvalve you use have solid stem bearing, solid stem, supported with bearings top and bottom, extra sealing rings to prevent fluid reaching the stem bearings etc. In addition check that the supplier says OK to sidewise mounting, preferably with references,and this in addition to aligned support for pipeline, valve and actuator.

Note: a too stiff connection between actuator and valve, can give stem and bearing/sealing damage if unlinear forces occur.