Diverter Flap Valve

Most gas handling systems have a requirement to change the gas flow direction to allow access to critical components, without shutting down the whole plant. With traditional dampers there is the danger of an upset, or explosion, if the two dampers do not act simultaneously. The FOURESS Diverter Valves was developed to handle such a situation in gas-turbine installations which incorporates a heat recovery boiler. Subsequently the concept has been used in a wide variety of industrial applications, such as refinery heat recovery plants, pollution control and nuclear ventilation systems and D.G Sets.

Why a Diverter ?

• Improve boiler efficiency by reducing heat losses through a dump stack or by-pass.
• Permit ‘On-load’ maintenance of waste heat boilers, gas turbines and fans.
• Ensure maximum plant safety by preventing both gas paths from being shut-off simultaneously.
• Increase plant efficiency by minimizing back pressure through the Isolator.
• Increase plant flexibility by allowing easy and safe switching between boilers, and/or turbines.

Features

FOURESS Diverters Key Features

• Floating ‘Blade’ assembly
• Insulation on both outer ‘Blade’ surfaces
• Minimal differential expansion in the ‘Blade’
• Pressure assisted ‘Blade’ seals
• High Basic sealing efficiency
• Integrally stiffened Plenum
• Suitable for external or internal insulation
• Pressure assisted metal shaft seals
• Self aligning bearings
• Completely external drive system

Configurations

Diverter Valves can be used where the gas is either to be directed to alternate outlets or enters the system from alternate inlets. In both cases there are two basic configurations.

Economic Evaluation

Diverter Valves maximize plant efficiency by reducing by-pass heat losses and having a minimum back pressure effect. For a typical gas turbine installation, compared to louvre dampers, the increased cost (if any) will be recovered in under two years. This is without assigning any value to the increases safety or the advantage of being able to undertake maintenance of the heat recovery steam generator without shutting down the turbine to insert a blanking plate.

Regulation

FOURESS Diverter Valves are designed with a sufficiently high natural frequency so that they can be held indefinitely, under normal conditions, at any intermediate position. Thus they can be used to regulate the gas flow, even at full load. The graph shows the flow characteristic for a typical gas turbine waste heat boiler application. The precise curve for each case is however dependent on the system layout and equipment parameters.

Features

FOURESS FLEXIBLE METALLIC SEAL

The FOURESS seal consists of a thin metal leaf strip

• Predeflected by a bias spring.
• The seal strip is pressed against the landing surface.
• Forming a flat 20mm wide sealing surface around the blade periphery. The operational deflection of the sealing element is about 17mm and it can tolerate up to 12mm misalignment, without any decrease in sealing efficiency. In the open position the seal is prevented from fluttering by a support plate.
• Against which the seal strip is held by the bias spring.

ALTERNATIVE SEALING SYSTEMS
( applicable to either blade side )

Simplex: A sealing efficiency of 99.98%, or better, on cross-sectional area, is attained by a single row of sealing elements around the periphery.

Duplex : To attain access to an isolated section, without the need for a blanking plate, 100% isolation can be provided using a double row of seals. In the interspace a peripheral air barrier is created by a small fan, ensuring that any gas leakage is positively barred. In the event of a pressure surge or fan failure, the fan ducting acts as a vent to atmosphere, enabling personnel to evacuate the isolated section.

Twinseal : As an alternative, with non-toxic gases, the space between the seals can be vented to atmosphere when the isolator is closed.

INSULATED DESIGN

Blade : Heat conducted through an uninsulated steel blade represents energy losses by radiation and convection. The basic design of the isolator enables insulation to be provided on either, or both sides of the blade. This is always fitted on the cold face, to avoid the danger of distortion

Plenum : The plenum chamber, which is stiffened to support the valve but not other structures, is an integral part of the unit. It can be designed for external insulation or be supplied with its own internal lining.

INTERNAL INSULATION

Internally lined Diverters are typically fitted with ceramic fiber insulation mounted on stainless steel pins at 300mm centers. The pins are attached to the body using manual metal arc welding ( MMAW ) as this is more reliable than stud welding and preferred by most gas turbine manufacturers. The insulation is then clad with 1.6mm [3] thick stainless steel, which ahs the advantage of a lower expansion rate than austenitic stainless steels, fitted so as to prevent buckling of the material during operation.

In addition, to reduce differential expansion problems, the seal landing bars on internally insulated Diverters, are bolted, not welded, to the outer frame. Small seals are then used to bridge the expansion gaps between the various segments.

HIGH TEMPERATURE BEARINGS

This specially designed gun metal bearing

• Consists of a number of replaceable segments mounted in a concentric ring which, in turn, is connected to a mounting plate by a flexible stainless steel diaphragm.
• This allows the central bearing system to deflect angularly and provides the self aligning capability.
• The shaft seal consists of two split annular flexible metallic discs. These, mounted in a machined housing fixed to the inside of the frame are held in place by a stainless steel plate. This enables the gland seal to float and accommodates shaft movement.

ACTUATION

FOURESS Diverter Valves can be operated electrically, pneumatically, hydraulically or manually. They are supplied with single or double drive depending on the torque required to move the blade. The drive systems, which have no internal linkages, are externally mounted so as to be accessible at all times.

SINGLE MODULE-MULTIPLE MODULE

For easy transportation and installation, individual diverter value modules are limited to approximately 3000x6000x3000mm. Where gas flow conditions require larger units these are created by combining a diverter valve module with individually operated single flap modules or having two or more diverter valve modules in parallel.

ORIENTATION

FOURESS Diverter Valves are suitable for installation in horizontal, vertical and inclined ducts. The main shaft can, depending on the duct layout, be arranged for any required orientation.

Gas Conditions

FOURESS Diverter Valves are capable of being designed to withstand temperatures and differential pressure up to 725OC and 2000mm H2O. Gas velocity is normally limited to approximately 30 m/s, however, with special arrangements velocities up to 45 m/s can be handled.

CONTROL
Limit switches, pairs, are supplied to control the terminal positions of the isolator blade. This facilitates interlocking with other electrical controls. For units with DUPLEX sealing system a pressure sensing device is supplied, which can be used to trigger an alarm, in the event of an air barrier failure or pressure surge in the duct.

DUST CONTROL
Where dust accumulation could interfere with proper closure of the isolator it is possible to incorporate preventive arrangements, such as dust clearance doors, hoppers, or an air purge system.

How Tight is Gas-Tight ?

The whites squares in the graph represent apertures equivalent to the sum of leaking areas of equipment with the claimed relevant sealing efficiencies on cross-sectional area. However, even this does not tell the whole story. Infect, percentage volumetric leaking is equal to approximately 3 to 6 times the percentage leakage area equivalent. Thus a 99% tight damper will allow through 3 to 6 % of the total gas flow.

The fields of application for FOURESS Isolators extends to many industries and activities, including :

Fouress Isolators are being used for the isolation of :

Material Specification