Pressure Seal Parallel Slide Gate Valve

A parallel slide gate valve closes with two flat discs, pressed apart by a spring or spreader, that slide down across two flat seat rings. In the open position both discs sit clear of the bore, so the passage is straight through and the pressure drop is the lowest of any isolation valve. In the closed position the spring holds light contact for the initial seal at low pressure, and the upstream line pressure then drives the downstream disc onto its seat for the primary seal. There is no wedge angle anywhere in the load path.

The pressure seal gate valve half of the name solves a different problem: how to keep a bonnet joint closed when the pressure acting on the bonnet area is enormous. Above about Class 600 the bolt circle needed for a conventional bolted bonnet becomes unreasonable in both size and preload, so the joint is inverted - the bonnet sits on a tapered seat and the line pressure pushes it harder into a metal gasket as pressure rises.

Together they occupy a specific place in a power plant: main steam isolation, hot reheat, turbine bypass, high-pressure boiler feedwater and HP header isolation, on clean superheated steam and treated water only. Below NPS 2 or DN 100 the economics change completely and the compact forged families take over - see the forged steel gate valve page for that side of the boundary.

Design Standard

Design Features

1. The advanced design concept eliminates leakage due to the damage of sealing surface caused by the sundries.
2. Significant security performance in high temperature applications.
3. The parallel disc designed to absorb thermally-induced body distortion, thereby making it ideal for high temperature steam or water supply piping.
4. Spring loaded disc not only assures contact between the disc and seat, but at all positions from open to close.The sealing between the disc and the seat is guaranteed under the equilibrate force of the spring between two discs and the valve service life is extended.
5. The parallel slide valve is free of jamming or seizure by temperature changing. At high pressure differential, the disc is pushed to the seat of lower pressure side,thus assures the sealing between the disc and seat by the pressure differential of the medium.
6. By-pass and drains can be supplied as required.

Pressure Seal Parallel Slide Gate Valve drawing

Material of Construction

Dimension & Weight

Gear Operation

Class 600

Class 900

Class 1500

Class 2500

Why Source From GNEE

We are a pressure seal parallel slide gate valve manufacturer working from Tianjin, and a pressure seal parallel slide gate valve supplier to more than 180 countries. Both halves of that sentence matter: we cast and machine, and we also consolidate a project's gate, globe and check valves into one shipment when a plant is buying a full valve package.

What every order includes, regardless of size:

The standard written down. Your quotation states the parent standard, the MSS SP-144-2020 edition, the style, and the end-to-end length - not just "API 600".

Four API 598 tests recorded. Shell, backseat, low-pressure closure and high-pressure closure, with the pressures printed on the report.

Material verified, not declared. PMI on every alloy body, so WC6 is WC6. A carbon steel substitution is the most expensive mistake in this product and the cheapest to catch.

PWHT chart and NDE reports in the pack, not on request after the fact.

Drawing review before casting. Face-to-face length, seat bore, bonnet taper and flange drilling are fixed at pattern stage; we review the drawing against your line list before release.

China pressure seal gate valve supply with export packing: flange faces protected, stem and handwheel braced, plates blocked, cases marked for line item.

OEM and ODM with your nameplate, tag numbers and document format.

Send these seven items and the quotation comes back with the standard cited, the test list, and a lead time rather than a price alone:

Size in NPS or DN, and pressure class

End connection: RF, RTJ or butt weld, plus the flange standard

End-to-end length if a spool already exists

Body material and design temperature, not just the class

Design pressure, and whether the duty is bidirectional

Bypass or equalizing arrangement, and whether it is scope of supply

Actuation: handwheel, gear operator with ISO 5210 top flange, electric or pneumatic, and the available power supply

Valve factory

 

 

 

 

Request a pressure seal parallel slide gate valve quote.

FAQ

What is the difference between a pressure seal and a bolted bonnet gate valve?

A bolted bonnet is sealed by bolt preload squeezing a gasket, so the bolts must always out-load the pressure pushing on the bonnet area. A pressure seal bonnet is sealed by that pressure itself: the bonnet sits on a tapered seat and internal pressure drives it up into a pressure seal gasket held by a retaining ring, so sealing force rises with line pressure. The bolted design stays practical to about Class 600; the pressure seal design is the normal choice from Class 900 upward and in large sizes, because the bolting needed above that would be impractically large. The trade is maintenance: a pressure seal bonnet cannot be opened in line.

Why does a parallel slide gate valve not suffer thermal binding?

Because it has no wedge angle. A wedge gate is driven between two inclined seats, so closing force is self-reinforcing; if the valve is closed hot and the body then contracts, the wedge is squeezed tighter and can seize or bend the stem. A parallel slide valve uses two flat discs against flat seats, closed only by a light spring plus line pressure on the downstream disc. Thermal movement slides the discs along the seat face instead of increasing an interference fit. The cost is lower seating force at low differential pressure, because there is no wedge to generate it.

Is a pressure seal gate valve an API 600 valve?

Not on the strength of API 600 alone. The standard's own title is Steel Gate Valves - Flanged and Butt-Welding Ends, Bolted Bonnets, and its scope is bolted-bonnet gate valves of NPS 2 and larger. A pressure seal bonnet is brought inside that framework by MSS SP-144-2020, which names API 600 as a parent standard and defines Style A (wall thickness, seat and stem reduced to ASME B16.34) and Style B (API 600 body with only the bonnet changed). A compliant purchase order therefore reads API 600 plus MSS SP-144 and states the style.

Which API 598 tests apply to a parallel slide gate valve?

API 598 Table 1 places parallel slide gate valves in the Globe and Parallel Slide Gate column, not the plain Gate column. In that column the shell test, backseat test, low-pressure closure test and high-pressure closure test are all required in the usual size and class ranges; a plain wedge gate valve has the high-pressure closure test as optional up to NPS 4 at Class 1500 and above NPS 4 at Class 600. So the test report for a parallel slide valve carries four tests, and a supplier offering only shell plus low-pressure closure has not tested to the column the valve belongs in.

What size and pressure class range is available?

MSS SP-144-2020 covers NPS 2 (DN 50) through NPS 50 (DN 1250) at Class 600, 900, 1500, 2500 and 4500. Our standard build is NPS 2 to 24 (DN 50 to 600) in Class 600, 900, 1500 and 2500, with larger sizes and Class 4500 quoted against a drawing. Class 150 and 300 are not offered in a pressure seal bonnet, because the internal pressure is too low to energise the gasket reliably; use a bolted bonnet at those classes.

Can the pressure seal gasket be reused?

No. The pressure seal gasket, often called a delta ring, is a single-use item: it is coined into the body and bonnet tapers and cannot be re-seated once disturbed. A replacement has to match the actual body and bonnet dimensions of that valve, which is why drawings or the serial number are needed rather than the size alone, and why lead time can run to weeks. Hold one spare per valve on critical main steam isolation duty, and inspect the body taper for damage before fitting, because a scored taper will not seal with a new ring.

What is pressure locking and how is it prevented?

Pressure locking is trapped liquid raising the pressure in the body cavity until it forces the discs hard against the seats. It happens when a valve is closed on liquid that then warms up, so the liquid expands with nowhere to go. Cavity pressure relief is provided by an internal equalizing hole in the downstream disc, an external equalizing line, or an external bypass with two bypass valves. The bypass also serves to equalize pressure across the disc before opening, which keeps the opening torque within the gear operator rating. It is a different failure from thermal binding and is fixed by a relief path, not by changing the disc design.

Which body material should be used for 540 C main steam?

ASTM A217 WC6 or ASTM A217 WC9, both rated for service up to 593 C, with a Stellite hardfacing on the seats. A216 WCB stops at 425 C and is not a main steam material. Above about 600 C the choice moves to ASTM A217 C12A, a 9Cr-1Mo-V casting taken to 649 C. A182 F22 is the forged equivalent of WC9 where a forged body is specified. A351 CF8M is a corrosion-resistant grade for feedwater and condensate rather than a high-temperature steam grade.

Can the valve be repaired in line?

No. The retaining ring has to be removed and the bonnet lifted out, which needs the line isolated, depressurised and, in steam service, cooled. That also means overhead clearance above the valve must be reserved at the piping design stage; insufficient clearance is the most common installation problem with this valve type. Seat lapping and gasket replacement are workshop jobs. Plan the maintenance access when the piping layout is frozen, not after the valve arrives.

Is a parallel slide gate valve suitable for throttling?

No. It is an isolation valve and should be operated fully open or fully closed. At a part-open position the two discs sit in the flow path, the seats erode and the discs can chatter. Where flow control is needed, specify a globe valve or a control valve in series rather than using this valve as a regulator. Its correct duty is main steam isolation, hot reheat, boiler feedwater and turbine bypass isolation, where the valve is either open or shut.

What is the face-to-face dimension?

Face-to-face and end-to-end dimensions follow ASME B16.10, and a parallel slide body uses the same gate valve table as a wedge body, so it can replace one directly. The length is not a single number per size and class: it changes with the end connection, because a ring joint flange is longer than a raised face flange, and with short or long pattern. As a reference point, a Class 1500 NPS 4 flanged gate valve is 546 mm (21.5 in). Order by end-to-end length, not by face-to-face, and hold +0/-0 mm tolerance on butt-weld ends.

What documents are supplied with each valve?

EN 10204 3.1 material certificates as standard, with 3.2 available when independent witnessing is required; the API 598 test report covering shell, backseat, low-pressure closure and high-pressure closure; NDE reports for casting repair areas; hardness results where NACE MR0175 is specified; a post-weld heat treatment chart where PWHT applies; and marking to MSS SP-25. For EU destinations the PED 2014/68/EU declaration and CE marking documentation are added. Third-party inspection by SGS, TUV, Bureau Veritas or Lloyd's Register is arranged against your ITP and must be scheduled to the production date, not after packing.

What is the minimum order quantity and lead time?

One piece is accepted, so a single valve can be bought for a maintenance job or for evaluation before a larger order. Common sizes in WCB and WC6 are held in stock and ship in about 7 days; made-to-order valves in WC9, C12A or non-standard end connections run about 20 to 25 days plus transit from Tianjin. Class 2500 and sizes above NPS 16 should be confirmed against the production schedule before the inquiry is closed.