DPE Seat Ball valves 1

[tigeralum]

Does API 6D specify a test procedure for DPE seat Ball valves?
How are DPE seats different from DIB seats?
Appreciate your comments.

 

[valvit]

Often there is confusion regarding DPE (double piston effect) seats, and DBB (double block and bleed), and we now have DIB-1 and DIB-2 in Annex B of ISO 14313/API 6D in the mix.

Much of the confusion comes from the use of the word “double” and that fact that all of these terms have something to do with the seats.

The ISO/API definition of DBB reads as follows: “single valve with two seating surfaces that, in the closed position, provides a seal against pressure from both ends of the valve with a means of venting/bleeding the cavity between the seating surfaces”. The definition for DIB reads as follows: “single valve with two seating surfaces, each of which, in the closed position, provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the seating surfaces”. The definition of DIB isn’t very clear all by itself, so it’s best to read it concurrently with the requirements of the DIB testing in Annex B for clarification.

A DPE seat, is a seat design, where the location of the seals on the back side of the seat are located in such a way that the seat can use pressure from either the pipeline or the valve cavity as a source of pressure to provide a piston force to effect a seal. Conversely, a SPE (single piston effect, or self-relieving) seat has the seals on the back side located in such a way that the piston force can only come from the pipeline direction. If the body cavity pressure exceeds the pipeline pressure (no more than 1,33X the valve pressure rating per ISO/API) a SPE seat will release pressure to the pipeline side. A SPE seat will release body pressure, even if it doesn’t exceed the valve pressure rating, but if there is a differential pressure between the pipeline and the body cavity, where the body cavity is at a higher pressure. The amount of differential pressure at which the seat will relieve varies by valve model and manufacturer.

DPE is synonymous with “bi-directional”, and SPE is synonymous with “uni-directional” as defined by ISO 14313/API 6D.

All this being said...there is no such thing as a DIB seat. DIB stands for Double Isolation and Bleed, and it is the manner by which DPE and SPE seats are tested to ensure proper function.

DIB-1 is a test performed on a valve where both seats are uni-directional (DPE) and a DIB-2 test is performed on a valve with one seat uni-directional (SPE) and one seat bi-directional (DPE). Both DIB-1 and DIB-2 are optional tests, and must be requested in order for the test to be performed. So, either a DIB-1 or a DIB-2 test will test the effectiveness of a DPE seat.

 

[superweo]

The answer of camvalve is exhaustive except for one point.
In fact a SPE valve is not unidirectional as both seats are designed to release the overpressure.
On the contrary if a ball valve is designed with one seat SPE and the other seat DPE, then the valve becomes unidirectional as the downstream seat (DPE) acts as a second pressure barrier and the upstream seat (SPE) acts as a safety relief valve.
So SPE upstream and DPE downstream.

Ciao

 

[valvit]

Apologies for any confusion...my reference to uni-directional and bi-directional are to ISO 14313/API 6D Clauses B.11 and B.12 where the document refers to the individual seat design, not the valve design.

I express caution to presuming a valve with a DPE seat downstream and SPE upstream as to only configuration. Many end-users will install the DPE seat in the direction of higher risk...to protect personnel or equipment, and this can be either upstream or downstream.