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Butterfly Valves

As the name implies, these are valves, and, as such, provide isolation to a much higher degree than dampers.

Gas valves available from Flowseal Engineering include the following:

Butterfly Valves:

  • Concentric Butterfly Valves
  • Double Offset Butterfly Valves
  • Triple Offset Butterfly Valves

Line Blind Valves:

  • Goggle Valves
  • Swing-Type Line Blind Valves
  • Sliding-Type Line Blind Valves

Two Lever Disc Valves

Associated Terms for Swing Line Blind:

  • Spectacle Line Blind
  • Swing Blank
  • Line blind
  • Line Blind Valves
  • Line Blind Industrial Valves
  • Manual Line Blind
  • Figure 8 Valve
  • Mechanical line Blind
  • Manual Goggle Valve
  • Manual Line Blind
  • Total Isolation Valve
  • Soft Seated Line Blind
  • 100% Isolation Valve
  • Man Safe Isolation

Flowseal Goggle Valves are designed and fabricated to provide total and man safe isolation of gas pipelines in chemical, gas and industrial plants and are inspected, tested and certified in compliance with API (American Petroleum Institute) Standard 598.

Flowseal Goggle Valves can be supplied to suit a variety of conditions, from clean gas at ambient temperature to dirty and corrosive gas at high temperature, and can be of either the open or enclosed design.

Flowseal Goggle valves are designed for minimum maintenance, and that which is required is simple and can be performed whilst the system is online.

The following styles of goggle valves are available:

Automated Goggle Valve

The automated goggle valve is hydraulically operated and is recommended for larger sizes, or for applications where the operator would be at risk during manual operation. The clamping flanges are driven open and closed using a series of hydraulic cylinders, these cylinders span a metal expansion bellow, which allows the clamping flange to open and close. The traverse of the goggle plate is through a linear hydraulic cylinder whose stroke is exactly equal to the stroke of the goggle valve. This means that the end of a stroke on the cylinder in both directions is either the open or closed position of the valve, with no possibility of incorrect positioning or reliance on limit switches, etc.

Spiral Lock Goggle Valve

This consists of a manual slide-type goggle plate located between a pair of clamping flanges. The clamping flanges are spread apart and closed using a series of interconnected spiral cams, operated by a single lever or ¼ turn gearbox, depending on size.

It must be noted that all goggle valves when operated, allow the escape of gas to the atmosphere if under positive pressure, or suck air into the system if under negative pressure.

Goggle valves can be supplied as either “open” or “enclosed” units. The enclosed valves have a casing built around the goggle plate and clamping flanges, with a vent pipe attachment, which allows escaped gases to be ducted to a place of safety.

Swing Blanks

A swing blank consists of a swing plate with an open and a closed port (goggle plate), with a central pivot point located between two jackable companion flange sets, with swing bolts for fastening. Operation is manual and requires a degree of flexibility in the adjacent piping system to permit the jacking of the flanges before swinging the goggle plate.

General Design

Goggle valves can be fabricated for any pipe or duct size, to suit horizontal, angled or vertical pipes, with goggle plate operation horizontal or vertical.

On automated systems, hydraulics can be supplied to customer specifications. This can vary from a simple supply of directional valves, for the operation of the goggle valves, with hydraulic pressure supplied by the customer, to a fully automated system with a hydraulic power pack and PLC with all necessary interlocks, alarms and indications.

Associated terms for Goggle Valves:

  • Spectacle Line Blind
  • Spectacle Blind Valve
  • Mechanical Line Blind
  • Automated Mechanical Line Blind
  • Automated Goggle Valve
  • Fully Automated Goggle Valve
  • Hydraulically Operated Line Blind
  • Hydraulically Operated Goggle Valve
  • Electrically Operated Line Blind
  • Electrically Operated Goggle Valve
  • Total Isolation Valve
  • 100% Sealing Efficiency
  • Man-Safe Isolation
  • Gas-tight Valve
  • Enclosed Goggle Valve
  • Noxious Gas Isolation
  • Sulphuric Acid Isolation Valves
  • Dirty Gas valves
  • Spiral Lock Goggle Valve

Minimum Specifications


Materials of construction are selected for their suitability for the conditions of temperature and pressure, and the corrosive and abrasive environment of the system.


Frames are designed to accept a limited amount of axial, lateral and bending loading, however, we must be advised of valve orientation in the duct as well as goggle plate direction of travel, and all anticipated external loading.

The valves are then designed as supported or self-supporting units with external mounting or anchoring as required.

The flanges are pre-drilled to customer requirements.

The frames and housings are designed to withstand 1.5 times system pressure and design temperature as well as construction and transportation loadings.

Clamping Flanges

These are machined on the clamping faces and are either of a corrosion-resisting material or plated to prevent corrosion.

Goggle Plates

Goggle plate assemblies are designed for conditions of pressure, temperature and gas composition. Each ring is removable whilst the valve is in operation (on open valves) and seals are accessible for inspection or maintenance whilst online.

Goggle Plate Guides

These are designed to ensure the smooth and trouble-free operation of the valve. An important feature of the valves is that the guides, when unclamped, ensure that both upstream and downstream seals are clear of the clamping flanges before the goggle plate starts its traverse.

Hydraulic Clamping Cylinders

These are single acting spring return cylinders, with sufficient force in the springs to ensure sealing of the goggle plate under operating conditions. The springs used are Bellville washers and the design of the cylinders ensures that the washers can not be over-compressed, which would result in loss of clamping power.


The goggle plates are fitted with rubber “O” ring seals or other suitable seals, depending on operating conditions and temperature. The seals are fitted in machined grooves and are readily accessible for inspection and maintenance whilst in operation.

Limit Valve

A hydraulic valve is placed in the circuit, with a striker to detect that the clamping flanges are properly apart, before allowing fluid to the traverse cylinder, thus ensuring that the seals are clear of the clamping flanges before the goggle plate starts to move.

Where gas-tight operation is required, Two Lever Disc Valves, Double Offset, Butterfly or Knife Gate Valves provide an economical solution (however, these valves cannot be considered as man-safe).

Materials of construction and drive configurations are selected and designed to suit the application and customer requirements.

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Associated Terms for Butterfly Valves:

  • Double Offset Valve
  • Double Eccentric Valve
  • Soft Seated Valve
  • 100% Sealing Valve
  • Total Isolation Valve
  • 100% Isolation Valve
  • Sealing Class IV Valve
  • Gas Tight Valve
  • Energised Seal