How to choose between External Trunnion and Internal Trunnion ball valves

[ksg1]

I have been asked to work on some trunnion valve requirements by my company. All our valves are with internal trunnions i.e. the trunnions are machined out of the ball casting/forging. In case of external trunnion this is not done and are relatively cheaper.
Can some one help on how the choice between either is done irrespective of rating part. My valve ratings are #150 & #300 with API 6D design.

 

[gerhardl]

ksg1,

I am not quite sure I understand your question correctly! Does your internal trunnion also have integrated trunnion blocks, that is no lower trunnion penetrating the body?

Any valve construction done after API 6D, and tested accordingly, will satisfy API 6D.

Apart from that any mechanical selecting process is of course and integrated anylysis of market/customers demands, technical solutions, production cost, maintaining cost and price/lifetime considerations.

Given more details and pointed questions it could possibly give you some tips, if you for instance are searching pros and cons for details in your construction, or tips on construction details.

 

[PeterIgg]

I think that the question relates to the style of trunnion valve. There is the Integral Trunnion design (ie Grove B5) and there is the Post Trunnion design (basically there is a hole in the ball top and bottom and the trunnion is inserted from outside the pressure shell).

The advantage of the integral trunnion design is that there will be no loads generated by pressure applied to the ball which are passed onto the stem - they are reacted by the trunnion plate bearing against the end adaptors. In the post style the stem and trunnion are effectively cantilevers and react the pressure generated loads. This can lead to premature stem seal failure if poorly designed. The other advantage of Integral Tunnion style is that there are fewer leak paths on the body as there is no need for a cover plate over the trunnion.

That said, the integral style trunnion is generally more readily available in the larger bore and higher pressure classes. You say that you are looking at Class 150/300 - my thoughts are that the benefits of the Integral style over the Post style are limited and that you will need to look at the relative costs to see if there is sufficient justification. The fact that many companies make both styles is testament to the fact that benefits of one over the other are not that significant for many applications.

 

[gerhardl]

I am aware of the differences and general pros and cons of the two different designs.

One part is the general differences as described by PeterIgg.

The other part is how to utilize this in a fabrication, marketing, practical application (fluid, flow, pressure, temperature, size) cost and lifetime environment etc. etc. For this the more specific situation has to be described.

General questions will give general answers.