Double Block and Bleed

How does it work: Double Block-and-Bleed & Double Isolation-and-Bleed Introduction

 There is a lot of misunderstanding misunderstanding surrounding surrounding the term double block-and-bleed block-and-bleed (DBB) as used to describe valve functionality. functionality. It seems almost every valve buyer and manufacturer has a dierent idea of what the term means for valve selection which can result in the wrong s!eci"cations or valve ty!e. #ome of this confusion in the oil and gas industry stems from the fact that there are two credible credible sources that de"ne the term dierently. $%nother !oint of confusion comes when many !eo!le use the term double block-and-bleed when they really want a valve with ca!abilities of double isolation-and-bleed (DIB)& 'on anson ameron*s Director of %!!lications +ngineering e,!lains. These dierence dierences s in de"nitions and terms are im!ortant when it comes to which valve ca!ability to use in what ty!e of system. $henever a customer says they want DBB it is mainly based on the a!!lication and their !ast e,!erience with what they consider DBB valves to be& says ike int/ ameron*s District anager for +ngineered 0 1rocess 2alves. $Based on customers* !referred !referred valve ty!e and a!!lication we can then oer ball gate or e,!anding !lug valve o!tions with DBB ca!abilities.&  The most basic thing that a user is looking for when when they s!ecify a double blockblockand-bleed valve is a com!act valve or valve system s ystem that !rovides more reliable isolation in critical areas than a standard single valve would. This smaller system or single valve unit serves to reduce the installation foot!rint saves on e,tra !i!ing re3uirements re3uirements and reduces weight in critical areas. These all a ll save s!ace time and cost. DBB and DIB Definitions

 There are are two entities in the 4nited 4nited #tates that de"ne DBB 5 the %merican %merican 1etroleum 1etr oleum Institute (%1I) and the 6ccu!ational #afety and ealth %dministration (6#%). %1I is a trade organi/ation o rgani/ation that acts as an advocate a dvocate as well as researcher for %merica*s oil and natural gas industry outlining many industry guidelines for safe o!eration. 6#% is the branch of the 4nited #tates De!artment of 7abor that is charged with enforcing health and safety legislations outlining legislation that !rotects both !eo!le and our environment. %ccording to %1I 8D $#!eci"cation for %ccording fo r 1i!eline 2alves& 2alves& standards a double blockand-bleed valve is a $single valve with two seating surfaces that in the closed !osition !rovides a seal against !ressure from both ends of the valve with a means of venting9bleeding the cavity between the seating surfaces.& surfaces .& %1I also

notes in this de"nition that this valve does not !rovide !ositive double isolation when only one side is under !ressure. In contrast the 6#% :ederal regulation describes DBB as $the closure of a line duct or !i!e by closing and locking or tagging two in-line valves and by o!ening and locking or tagging a drain or vent valve in the line between the two closed valves.& %1I*s DBB de"nition does not achieve the same level of isolation as the 6#%*s de"nition. %1I allows DBB valves to be one single valve with two unidirectional seats while the 6#% standard can only be achieved with two se!arate valves with a method to bleed !ressure in between. There are some valves that utili/e a twin valve design. By combining two valves into one body a twin valve design reduces weight and !otential leaks !aths while meeting the 6#% re3uirements for double block-and-bleed. 2alve associations usually choose to follow either %1I*s or 6#%*s de"nition but some have created their own handbook with their own de"nitions for industry terms DBB included. :or e,am!le the British 2alve 0 %ctuator %ssociation (B2%%) de"nes double block-and-bleed as $a manifold that combines one or more isolation valves usually ball valves and one or more bleed9vent usually a needlestyle global valve into one assembly for interface with other com!onents (e.g. !ressure measurement transmitters !ressure gauges and switches).& B2%% like %1I says that for DBB ca!abilities only one valve is re3uired not a system. %ccording to B2%% $DBB valves re!lace the !revious traditional techni3ue em!loyed by !i!eline engineers to create a double block-and-bleed con"guration in the !i!eline usually by fabricating three valves using ;anges
seats do not rely on an outside mechanism to relieve !ressure. 6n the other hand a DIB valve utili/es one or two bidirectional seats. The valve !rovides double isolation from !ressure at both ends of the valve but there is usually one o!erational drawback a DIB valve cannot relieve body cavity !ressure !ast the seats meaning its seats are not self-relieving. hen using a DIB valve an e,ternal relief system is necessary to relieve !ressure buildu!. % DBB or DIB valve can !rovide isolation in both the u!stream and downstream directions even in high-!ressure or high-tem!erature situations. Isolation is im!ortant in cases where leakage through a valve could have ma=or conse3uences. 6nce the ;uid is isolated the bleed mechanism can drain the area between the two valves or two seating surfaces. This is im!ortant for maintenance and9or integrity check situations where leakage can be monitored. Applications and Benefits of DBB and DIB

Both DBB and DIB valves save s!ace reduce the need for costly multivalve systems and !rovide /ero leakage ca!abilities from u!stream to downstream. The valve also allows for an integrity check of seat seals. In addition both valves can be used in a variety of a!!lications and markets such as 7>? !etrochemical transmission and storage natural gas industrial !rocesses mainline and manifold valves in li3uid !i!elines and re"ned !roducts transmission lines. DBB and DIB valves are used in a!!lications where critical isolation is needed to ensure that leakage does not occur. De!ending on the a!!lication and ty!e of service a wide range of DBB or DIB valve ty!es and o!tions are available to "t s!eci"c needs. :or e,am!le in li3uid service near waterways or munici!alities double-e,!anding gate valves with DBB ca!abilities like ameron@s WKMA Pow-Real! "ate #al#es are !referred for critical isolation because they !rovide a tight mechanical seal u!stream and downstream simultaneously which is normally unaected by !ressure variations or vibrations. $hen I teach our sales force and customers about double block-and-bleed I use the %1I 8D de"nition& says ameron*s gate valve !roduct manager %nthony arrea. $In my !resentations I e,!lain that a DBB valve is a valve with two seats and when closed will seal against !ressure from both ends of the valve allowing the body cavity to be bled.& %nother a!!lication in which DBB and DIB valves are used is the meter calibration market. +very closed valve in the meter system must seal dro! tight. +ven a small leak will cause errors in the meter calibration and the incorrect meter factor will !ersist until the ne,t !roving o!eration. This can cost end users huge sums of money. hoosing the correct %1I veri"ed DBB or DIB valve can hel! ensure correct calibration almost every time.

 The mechanical wedge action of a double block-and-bleed !lug valve like ameron@s $%%RA' (A'(% A )win eal! com!resses both the u!stream and the downstream seals "rmly against the valve body needing no hel! from the line !ressure to aect a !ositive seal. Double block-and-bleed !lug valves which are used a great deal in the transmission and storage markets !rovide consistent and !rovable /ero leakage of various ;uids. *onclusion

$hen determining whether to use a DBB or DIB valve and which de"nition to follow %1I or 6#%& says anson $it is im!ortant to have a clear understanding of the similarities and dierences of the ty!es of block-and-bleed valves and the s!eci"c a!!lication the valve will !lay in. %dditionally it is im!ortant that the features re3uired for isolation are fully tested during factory acce!tance testing of  the valve.& #ince DBB and DIB have become such generic terms in the industry it is im!ortant to take into consideration the a!!lication media and various environmental challenges when choosing the a!!ro!riate solution and valve ty!e.  The following table will be !art of an Informative %!!endi, in the ne,t edition of %1I 8D which is currently out for ballot. The table has been augmented with a column showing where the valve ty!es com!ly with 6#% regulations. Greenhalgh, Martin. Valve and Actuator User' Manual. 6th ed. Oxfordshire: British Valve & Actuator Association, !"!. #rint. 

(al#e )+pe

ealin" Arran"e,ent

 Two block valves %ny valves with bi-directional sealing with bleed between #lab and9or 1ressure energi/ed 5 downstream through-conduit sealing only9",ed seats () gate #lab and9or 1ressure energi/ed 5 u!stream and through-conduit downstream sealing () gate (DIB-)

BlockandBleed Ces

Double Double Isolation-and- HA Double Block-andBleed Block-andBleed Bleed Ces Ces Ces

>o ()

>o

>o

>o

 Ces

Ces (E)

Ces (E)

>o

 Trunnion mounted ball valve  Trunnion mounted ball valve (DIB-)

4!stream sealing !ressure energi/ed seats both self- relieving ()

 Ces

(F)

>o (G)

>o

4!stream and downstream sealing !ressure energi/ed e,am!le two bi-directional sealing seats ()

 Ces

(F)

(8)

>o

 Trunnion mounted ball valve (DIB-)

4!stream and downstream sealing !ressure energi/ed e,am!le one bidirectional and one unidirectional sealing seat ()

 Ces

(F)

6nly if the bidirection seat is on the downstream side (8)

>o

:loating ball valve 1lug

+,!anding 1lug (DIB-) +,!anding ?ate (DIB-)

1ressure energi/ed

>o ()

>o

>o

>o

1ressure energi/ed downstream sealing >o () ()

>o

>o

>o

echanically energi/ed

Ces

Ces

Ces

>o

echanically energi/ed

Ces

Ces (E)

Ces (E)

>o

otes: 1. The terms upstream and downstream refer to the pressure source and open end/equipment respectively and do not refer to flow direction. 2. Not possible to bleed from valve body, but bleed may be in downstream pipework/pipeline 3. ependin! on detail desi!n of the valve, some valves can have preferred sealin! direction and/or a specified sequence of operation. ". ependin! on detailed desi!n #. ownstream seat can provide a second barrier at pressures below the cavity relievin! pressure, but will not provide a hi!h pressure barrier. $. ependin! on detailed desi!n and ability to achieve testin! per section 13