Through Conduit Slab Gate Valve

API 6D classifies a gate valve by its closure member rather than by its marketing name: a closure of one piece is a slab gate, and two or more pieces make an expanding gate. The slab is a single plate, ground flat on both faces, with a circular bore through its centre that matches the pipeline bore. Open it fully and the bore lines up with the pipe; close it and the plate drops between two seat rings. That is the whole of the through conduit idea, and it is why a pipeline pig, a scraper or a hot tap cutter runs through an open valve without meeting an edge.

Two consequences follow. The seat faces sit outside the flow stream while the valve is open, so produced sand and mill scale do not wash across the sealing surface the way they do on a wedge gate valve. And the flow path has no pocket where solids can settle, which is why this design appears on crude, slurry and pigged gas lines rather than on clean plant service where a cheaper valve would do. It is also the valve normally written into a pig launcher and receiver schedule, because the pig has to enter the barrel without meeting a seat edge on the way. What the design does not do is remove the body cavity. With the plate down, the volume between the two seats is a closed chamber, and everything in the seat and cavity sections below follows from that.

The names in this family are loose. Through conduit gate valve, slab gate valve and pipeline gate valve all describe the same API 6D article, and none of them describes a parallel slide gate valve, which carries two independent discs and belongs to API 600 plant service. That distinction matters more than it looks: both bodies are dimensioned to the same ASME B16.10 gate valve table, so they fit the same pipe spool, but only the slab gate can be pigged, and only the slab gate is bought against pipeline duty rather than against a bonnet joint rating.

Design Standard

Design Features

1. Valve with soft & metal seats. In case of soft seat damaged, the metal to metal seat will act as a secondary seal.
2. The seat with self pressure relieving function to prevent the pressure increasing too high in cavity.
3. O-rings and / or spring preloaded force offer a bi-directional sealing of the valve and are easy for operation.
4. Seat sealing surface is located outside the fluid, which will be free from erosion,and can be cleaned up during discs movement. 

Through Conduit Slab Gate Valve drawing

Material of Construction

Dimension & Weight

Class 150

Class 300

Class 600

Class 900

Valve factory

 

 

 

 

Why GNEE and How to Order

We cast, machine and test this valve, and we test every one of them before it is painted. What that means in practice is a test report with your serial number on it, not a certificate of compliance with a batch number.

• Shell and seat test on every valve, recorded per serial number, with the medium and the hold time on the report
• Third-party inspection to your ITP at the production stage, not after packing
• Seat and stem sealant injection fittings, machined in, not added with a drill after assembly
• Wooden crate packing for NPS 24 and above, with flange face protection and the stem blocked against movement in transit
• Spares kit with the first order on request: seat inserts, O-rings, packing and bonnet gasket

To quote, send us these seven items: size and pressure class, body and trim material, seat type and required leakage rate, end connection and face-to-face dimension, operating temperature and medium, actuation, and the certificate and inspection requirement. If you have the line list or the general arrangement drawing, send that instead and we will read the specification off it. Enquiries reach an engineer within one working day at

sale@gneetube.com or +86 15824687455.

FAQ

Is a through conduit gate valve the same as a slab gate valve?

Yes. Through conduit describes the flow path, slab describes the closure member: a single plate with a bore through it. API 6D classifies the closure as one piece, which is a slab-gate valve, or two or more pieces, which is an expanding-gate valve. Pipeline gate valve is the third name for the same article. Some catalogues also use through conduit for the expanding type, which is why the closure drawing, not the name, is what you should read.

Does a slab gate valve have a body cavity?

It does. The conduit is continuous only while the valve is open. Once the plate is down between the seats, the volume between those seats is a closed chamber, and it fills with line fluid. That chamber is what makes double block and bleed possible, and it is also what can be over-pressured by thermal expansion of trapped liquid. Pages that describe this design as having no body cavity are describing the open position only.

Can a slab gate valve be double isolation and bleed?

Not with the seats that make it self-relieving. A single piston effect seat is pushed onto the plate by line pressure and lifted off it by cavity pressure, so the same geometry that relieves the cavity destroys the second independent barrier. Double isolation and bleed needs a double piston effect seat on both sides, which seals against cavity pressure and therefore has no relief path, so an external cavity relief valve becomes mandatory. Double block and bleed is the standard capability of a stock slab gate valve; DIB is a special order with a different seat.

What pressure class and size range does a slab gate valve cover?

We supply NPS 2 to 48 (DN 50 to DN 1200) in ASME Class 150 to 2500. The practical ceiling narrows as the class rises: Class 2500 is normally made to order above NPS 12. API 6D itself covers NPS 1/2 to 60 in Class 150 to 2500, so a size outside our range is a sourcing question rather than a standard question.

What is the maximum temperature of a slab gate valve?

The seat insert sets it, not the body. A reinforced PTFE seat runs to about 200 degrees Celsius, nylon to about 93 degrees Celsius, PEEK to about 260 degrees Celsius, and a metal seat with hardfacing or tungsten carbide to about 593 degrees Celsius. Above the insert limit the valve is still pressure tight but stops being bubble tight, which in liquid service is the failure you notice. State the operating temperature with the medium when you enquire, because the answer changes the seat material and the price.

Which fire test standard applies to a gate valve?

API 6FA. It covers valves in the API 6A and API 6D family, which is where a multi-turn gate valve sits. API 607 covers quarter-turn valves and valves with nonmetallic seats, so it is the correct citation for a ball or butterfly valve and the wrong one for a gate valve. API 6FD covers check valves. A fire certificate is a type test of a design, size range and pressure class, so check that the report brackets the valves you are buying.

Can a slab gate valve be used for throttling or flow control?

No. A partly open slab puts the bore edge into the flow stream, and the resulting jet erodes the seat face and sets up vibration that will loosen the seat and damage the stem seal over time. The valve is an on-off isolation device and should be specified that way. If the line needs regulation, that is a separate control valve; if it needs frequent cycling, a trunnion mounted ball valve does the job with less wear.

What is the difference between a slab gate valve and a parallel slide gate valve?

A slab gate has one plate with a bore through it and sits in an API 6D pipeline valve. A parallel slide gate has two separate discs held apart by a spring against two seat rings, and it belongs to API 600 plant service, where the pressure class ceiling is set by the bolted bonnet joint rather than by pipeline duty. Both use the same ASME B16.10 gate valve face-to-face table, so they look interchangeable on a drawing and are not interchangeable in function: only the slab gate is piggable.

Slab gate vs expanding gate: what is the difference?

The expanding gate is two pieces with a wedge action between them. At the end of the closing stroke the segments spread and press mechanically against both seats, so the seal does not depend on line pressure and holds at low differential pressure. The slab is one piece and relies on spring load plus line pressure, so it seals well at high differential pressure and relies on the springs when the differential is small. The expanding design costs more, needs a longer stroke and cannot be self-relieving in the same way.

What documents are supplied with each valve?

Material test reports to EN 10204 3.1 for the body, bonnet and trim, a hydrostatic shell and seat test report with the pressures and hold times recorded against the serial number, a dimensional inspection report, and a certificate of conformity. Third-party inspection to your ITP is arranged at the production stage rather than after packing, because a witnessed test has to happen while the valve is still on the test bench. EN 10204 3.2 is available when the inspector is your nominated body.

What is the minimum order quantity and lead time?

One valve. Stock sizes in A216 WCB, Class 150 to 600, ship in about seven days from our Tianjin warehouse. Cast and machined to order, the normal cycle is 20 to 25 days for NPS 2 to 24 in carbon steel, and four to six weeks once alloy body material, metal seats or an actuator is added. Large sizes above NPS 30 and Class 1500 valves are scheduled individually because the body casting is the constraint.

What warranty do you give?

Eighteen months from shipment or twelve months from installation, whichever comes first, covering manufacturing defects in the body, closure and seat assembly. Wear parts are excluded: seat inserts, O-rings, stem packing and gaskets are consumables and are supplied as a spares kit. If a valve fails inside the warranty period we ask for the serial number, the test report and photographs before agreeing the disposition, and we replace or repair rather than argue about the cause first.

Can you manufacture to our drawing or our brand?

Yes, for both. We work from your general arrangement drawing, your material specification and your test requirement, and we return a drawing for approval before casting. OEM nameplates, your tag numbering and your painting system are all possible, and the marking can follow API 6D or your own marking specification. What we will not do is stamp a standard we have not tested to: every valve is designed and tested to API 6D, and we do not claim an API Monogram.

Slab gate valve vs ball valve: which should I choose for a pipeline?

Start with how often the valve has to move. A slab gate valve is the cheaper full bore valve above NPS 20 and holds a seat for years without being cycled, which is what a mainline block valve does. A trunnion mounted ball valve turns in a quarter revolution, seals more tightly at low differential pressure and needs a smaller actuator, so it wins wherever the valve is operated often or has to close on a trip signal. If the line has to be pigged, both will pass a pig; if it carries abrasive solids, the ball valve has the advantage because its seat does not slide across the closure.